Advanced Textiles for Wound Care

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Woodhead Publishing in Textiles: Number 85

Advanced textiles for wound care

Edited by S. Rajendran

Oxford Cambridge New Delhi

© 2009 Woodhead Publishing Limited The Textile Institute and Woodhead Publishing The Textile Institute is a unique organisation in textiles, clothing and footwear. Incorporated in England by a Royal Charter granted in 1925, the Institute has individual and corporate members in over 90 countries. The aim of the Institute is to facilitate learning, recognise achievement, reward excellence and disseminate information within the global textiles, clothing and footwear industries. Historically, The Textile Institute has published books of interest to its members and the textile industry. To maintain this policy, the Institute has entered into partnership with Woodhead Publishing Limited to ensure that Institute members and the textile industry continue to have access to high calibre titles on textile science and technology. Most Woodhead titles on textiles are now published in collaboration with The Textile Institute. Through this arrangement, the Institute provides an Editorial Board which advises Woodhead on appropriate titles for future publication and suggests possible editors and authors for these books. Each book published under this arrangement carries the Institute’s logo. Woodhead books published in collaboration with The Textile Institute are offered to Textile Institute members at a substantial discount. These books, together with those published by The Textile Institute that are still in print, are offered on the Woodhead web site at: www.woodheadpublishing.com. Textile Institute books still in print are also available directly from the Institute’s website at: www.textileinstitutebooks.com. A list of Woodhead books on textile science and technology, most of which have been published in collaboration with The Textile Institute, can be found at the end of the contents pages.

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Contributor contact details xi Woodhead Publishing in Textiles xv Preface xxi

Part I The use of textiles in particular aspects of wound care 1

1 Wound management and dressings 3 S. Ather and K. G. Harding, Cardiff University, UK 1.1 Introduction 3 1.2 Types of wound 3 1.3 Mechanism of wound healing 4 1.4 Factors affecting wound healing: why wounds fail to heal 11 1.5 Wound healing: treatment options 13 1.6 Future trends 17 1.7 Conclusions 18 1.8 References 18

2 Testing dressings and wound management materials 20 S. T. Thomas, formerly of Surgical Materials Testing Laboratory, Medetec, UK 2.1 Introduction 20 2.2 The need for laboratory testing 21 2.3 Fluid-handling tests 23 2.4 Low-adherence tests 36 2.5 Conformability tests 37 2.6 Microbiological tests 38 2.7 Odour control tests 42

2.8 Biological tests 44 2.9 References 45

3 Textile materials and structures for wound care products 48 B. S. Gupta, North Carolina State University, USA, and J. V. Edwards, United States Department of Agriculture – Agricultural Research Service, USA 3.1 Introduction 48 3.2 The role of wound dressings 49 3.3 Categorization of wounds 50 3.4 Minor wounds 51 3.5 Healing mechanisms 53 3.6 Wound dressings 55 3.7 Types of dressings available 60 3.8 Bandages 70 3.9 Materials used in dressings and bandages 71 3.10 Textile processes involved in formation of dressings and bandages 79 3.11 Acknowledgement 92 3.12 References 92

4 Interactive dressings and their role in moist wound management 97 C. Weller, Monash University, Australia 4.1 Introduction 97 4.2 Normal wound healing 98 4.3 Wound characteristics 100 4.4 Dressings 102 4.5 Interactive wound dressings 105 4.6 Future trends 110 4.7 Conclusions 111 4.8 Sources of further information and advice 112 4.9 References 112

5 Bioactive dressings to promote wound healing 114 G. Schoukens, Ghent University, Belgium 5.1 Introduction 114 5.2 Physiology of wound healing 115 5.3 Principles and roles of bioactive dressings 117 5.4 Types and structures of bioactive dressings 118

5.5 Example of bioactive dressing: di-O-butyrylchitin (DBC) 127 5.6 Future trends 144 5.7 Acknowledgements 146 5.8 References 146

6 Advanced textiles for wound compression 153 S. Rajendran and S. C. Anand, University of Bolton, UK 6.1 Introduction 153 6.2 Elastic compression bandages 154 6.3 Venous leg ulcers 155 6.4 Venous leg ulcer treatment 157 6.5 Applications of bandages 163 6.6 Present problems and novel bandages 165 6.7 Three-dimensional spacer compression bandages 169 6.8 Conclusions 175 6.9 References 175

7 Antimicrobial textile dressings in managing wound infection 179 Y. Qin, Jiaxing College, China 7.1 Introduction 179 7.2 Topical antimicrobial agents in wound care 181 7.3 Main types of antimicrobial wound dressings 183 7.4 Wound dressings containing silver 187 7.5 Applications of modern antimicrobial wound dressings containing silver 190 7.6 Future trends 193 7.7 Sources of further information and advice 195 7.8 References 195

8 Novel textiles in managing burns and other chronic wounds 198 H. Onishi and Y. Machida, Hoshi University, Japan 8.1 Introduction: current practice in the management of deep skin wounds or ulcers 198 8.2 Normal treatment options for deep skin wounds or ulcers 201 8.3 Novel wound dressings for managing deep skin wounds or ulcers 205 8.4 Future trends 212

8.5 Sources of further information and advice 215 8.6 References 215

Part II Types of advanced textiles for wound care 221

9 Drug delivery dressings 223 P. K. Sehgal, R. Sripriya and M. Senthilkumar, Central Leather Research Institute, India 9.1 Introduction 223 9.2 Wounds: deﬁ nition and types 224 9.3 Wounds which require drug delivery 226 9.4 Delivering drugs to wounds 231 9.5 Types of dressings for drug delivery 235 9.6 Applications of drug delivery dressings 240 9.7 Future trends 244 9.8 Conclusions 246 9.9 References 247

10 The use of ‘smart’ textiles for wound care 254 J. F. Kennedy and K. Bunko, Advanced Science and Technology Institute, UK 10.1 Introduction 254 10.2 Basic principles and types of smart textiles 255 10.3 Characteristics of smart textiles 256 10.4 Textiles in control of exudate from wounds 262 10.5 Examples of ‘smart’ textiles for wound care 265 10.6 Response of dressings to bacteria 267 10.7 Future trends 268 10.8 Sources of further information and advice 271 10.9 References 272

11 Composite dressings for wound care 275 M. Joshi and R. Purwar, Indian Institute of Technology Delhi, India 11.1 Introduction 275 11.2 Deﬁ nition of composite dressings 276 11.3 Structure of composite dressings 277 11.4 Materials and textile structures used in composite dressings 279

11.5 Types of composite dressings 284 11.6 Trends in composite dressings: embroidery technology 286 11.7 Conclusions 288 11.8 References 288

12 Textile-based scaffolds for tissue engineering 289 M. Kun, C. Chan and S. Ramakrishna, National University of Singapore, Singapore 12.1 Introduction: principles of tissue engineering 289 12.2 Properties required for ﬁ brous scaffolds 290 12.3 Materials used for scaffolds 293 12.4 Relationship between textile architecture and cell behavior 294 12.5 Textiles used for tissue scaffolds and scaffold fabrication 298 12.6 Applications of textile scaffolds in tissue engineering 303 12.7 Future trends 308 12.8 Sources of further information and advice 310 12.9 References 312

(* = main contact) Chapter 3

Chapter 1 Professor Bhupender S Gupta* Department of Textile Engineering, Shahzad Ather and Keith G Chemistry & Science Harding* College of Textiles Wound Healing Research Unit North Carolina State University Department of Surgery Raleigh, NC 27695-8301 Cardiff University USA Cardiff E-mail: [email protected] UK Dr J Vincent Edwards E-mail: [email protected] USDA-ARS Chapter 2 Southern Regional Research Center Dr Stephen Thomas 1100 Robert E Lee Blvd MEDETEC New Orleans 1 Radyr Farm Road LA 70124 Radyr USA Cardiff E-mail: [email protected]. CF15 8EH gov UK E-mail: [email protected] [email protected]

Chapter 4 Chapter 7 Carolina Weller Dr Yimin Qin Department of Epidemiology and Biochemical Materials Research Preventative Medicine and Development Center School of Public Health and Jiaxing College Preventative Medicine 56 Yuexiu Road South Monash University Jiaxing 314001 Level 3 Burnet Building, DEPM Zhejiang Province The Alfred China 89 Commercial Road E-mail: [email protected] Melbourne Vic 3004 Chapter 8 Australia Hiraku Onishi* and Yoshiharu E-mail: Carolina.Weller@med. Machida monash.edu.au Department of Drug Delivery Chapter 5 Research Hoshi University Professor Gustaaf Schoukens 2-4-41, Ebara Ghent University Shinagawa-ku Faculty of Engineering Sciences Tokyo 142-8501 Department of Textiles Japan Technologiepark 907 E-mail: [email protected] B-9052 Zwijnaarde (Gent) Belgium Chapter 9 E-mail: gustaaf.schoukens@ Dr P K Sehgal*, Dr R Sripriya and UGent.be Dr M Senthilkumar Chapter 6 Bioproducts Laboratory Central Leather Research Institute Dr S Rajendran* and S C Anand Adyar, Chennai 600 020 Centre for Materials Research and Tamil Nadu Innovation India University of Bolton E-mail: [email protected] Bolton BL3 5AB UK E-mail: [email protected]

Chapter 10 Chapter 11 John F Kennedy* and Katarzyna M Joshi* and Roli Purwar Bunko Department of Textile Technology Advanced Science and Technology Indian Institute of Technology Institute Delhi 5 The Croft, Buntsford Drive New Delhi 110016 Stoke Heath, Bromsgrove India Worcestershire E-mail: [email protected] B60 4JE [email protected] UK

E-mail: [email protected] Chapter 12 Formerly of Ma Kun, Casey K. Chan and Chembiotech Laboratory Seeram Ramakrishna* University of Birmingham Division of Bioengineering & Dept. Research Park of Mechanical Engineering, Vincent Drive Faculty of Engineering Birmingham National University of Singapore B15 2SQ Singapore 119077 UK E-mail: [email protected]

1 Watson’s textile design and colour Seventh edition Edited by Z. Grosicki 2 Watson’s advanced textile design Edited by Z. Grosicki 3 Weaving Second edition P. R. Lord and M. H. Mohamed 4 Handbook of textile fi bres Vol 1: Natural fi bres J. Gordon Cook 5 Handbook of textile fi bres Vol 2: Man-made fi bres J. Gordon Cook 6 Recycling textile and plastic waste Edited by A. R. Horrocks 7 New fi bers Second edition T. Hongu and G. O. Phillips 8 Atlas of fi bre fracture and damage to textiles Second edition J. W. S. Hearle, B. Lomas and W. D. Cooke 9 Ecotextile ’98 Edited by A. R. Horrocks 10 Physical testing of textiles B. P. Saville 11 Geometric symmetry in patterns and tilings C. E. Horne 12 Handbook of technical textiles Edited by A. R. Horrocks and S. C. Anand 13 Textiles in automotive engineering W. Fung and J. M. Hardcastle 14 Handbook of textile design J. Wilson 15 High-performance fi bres Edited by J. W. S. Hearle 16 Knitting technology Third edition D. J. Spencer

17 Medical textiles Edited by S. C. Anand 18 Regenerated cellulose fi bres Edited by C. Woodings 19 Silk, mohair, cashmere and other luxury fi bres Edited by R. R. Franck 20 Smart fi bres, fabrics and clothing Edited by X. M. Tao 21 Yarn texturing technology J. W. S. Hearle, L. Hollick and D. K. Wilson 22 Encyclopedia of textile fi nishing H-K. Rouette 23 Coated and laminated textiles W. Fung 24 Fancy yarns R. H. Gong and R. M. Wright 25 Wool: Science and technology Edited by W. S. Simpson and G. Crawshaw 26 Dictionary of textile fi nishing H-K. Rouette 27 Environmental impact of textiles K. Slater 28 Handbook of yarn production P. R. Lord 29 Textile processing with enzymes Edited by A. Cavaco-Paulo and G. Gübitz 30 The China and Hong Kong denim industry Y. Li, L. Yao and K. W. Yeung 31 The World Trade Organization and international denim trading Y. Li, Y. Shen, L. Yao and E. Newton 32 Chemical fi nishing of textiles W. D. Schindler and P. J. Hauser 33 Clothing appearance and fi t J. Fan, W. Yu and L. Hunter 34 Handbook of fi bre rope technology H. A. McKenna, J. W. S. Hearle and N. O’Hear 35 Structure and mechanics of woven fabrics J. Hu 36 Synthetic fi bres: nylon, polyester, acrylic, polyolefi n Edited by J. E. McIntyre 37 Woollen and worsted woven fabric design E. G. Gilligan 38 Analytical electrochemistry in textiles P. Westbroek, G. Priniotakis and P. Kiekens

39 Bast and other plant fi bres R. R. Franck 40 Chemical testing of textiles Edited by Q. Fan 41 Design and manufacture of textile composites Edited by A. C. Long 42 Effect of mechanical and physical properties on fabric hand Edited by Hassan M. Behery 43 New millennium fi bers T. Hongu, M. Takigami and G. O. Phillips 44 Textiles for protection Edited by R. A. Scott 45 Textiles in sport Edited by R. Shishoo 46 Wearable electronics and photonics Edited by X. M. Tao 47 Biodegradable and sustainable fi bres Edited by R. S. Blackburn 48 Medical textiles and biomaterials for healthcare Edited by S. C. Anand, M. Miraftab, S. Rajendran and J. F. Kennedy 49 Total colour management in textiles Edited by J. Xin 50 Recycling in textiles Edited by Y. Wang 51 Clothing biosensory engineering Y. Li and A. S. W. Wong 52 Biomechanical engineering of textiles and clothing Edited by Y. Li and D. X-Q. Dai 53 Digital printing of textiles Edited by H. Ujiie 54 Intelligent textiles and clothing Edited by H. Mattila 55 Innovation and technology of women’s intimate apparel W. Yu, J. Fan, S. C. Harlock and S. P. Ng 56 Thermal and moisture transport in fi brous materials Edited by N. Pan and P. Gibson 57 Geosynthetics in civil engineering Edited by R. W. Sarsby 58 Handbook of nonwovens Edited by S. Russell 59 Cotton: Science and technology Edited by S. Gordon and Y-L. Hsieh 60 Ecotextiles Edited by M. Miraftab and A. Horrocks

61 Composite forming technologies Edited by A. C. Long 62 Plasma technology for textiles Edited by R. Shishoo 63 Smart textiles for medicine and healthcare Edited by L. Van Langenhove 64 Sizing in clothing Edited by S. Ashdown 65 Shape memory polymers and textiles J. Hu 66 Environmental aspects of textile dyeing Edited by R. Christie 67 Nanofi bers and nanotechnology in textiles Edited by P. Brown and K. Stevens 68 Physical properties of textile fi bres Fourth edition W. E. Morton and J. W. S. Hearle 69 Advances in apparel production Edited by C. Fairhurst 70 Advances in fi re retardant materials Edited by A. R. Horrocks and D. Price 71 Polyesters and polyamides Edited by B. L. Deopora, R. Alagirusamy, M. Joshi and B. S. Gupta 72 Advances in wool technology Edited by N. A. G. Johnson and I. Russell 73 Military textiles Edited by E. Wilusz 74 3D fi brous assemblies: Properties, applications and modelling of three-dimensional textile structures J. Hu 75 Medical textiles 2007 Edited by J. Kennedy, A. Anand, M. Miraftab and S. Rajendran 76 Fabric testing Edited by J. Hu 77 Biologically inspired textiles Edited by A. Abbott and M. Ellison 78 Friction in textile materials Edited by B. S. Gupta 79 Textile advances in the automotive industry Edited by R. Shishoo 80 Structure and mechanics of textile fi bre assemblies Edited by P. Schwartz 81 Engineering textiles: Integrating the design and manufacture of textile products Edited by Y. E. El-Mogahzy

82 Polyolefi n fi bres: industrial and medical applications Edited by S. C. O Ugbolue 83 Smart clothes and wearable technology Edited by J. McCann and D. Bryson 84 Identifi cation of textile fi bres Edited by M. Houck 85 Advanced textiles for wound care Edited by S. Rajendran 86 Fatigue failure of textile fi bres Edited by M. Miraftab 87 Advances in carpet technology Edited by K. Goswami

It is apparent that quality of life is a key issue in the healthcare of people. Textile materials play an important and crucial role in designing appropriate structures for the healthcare of people and medical companies. With the increasing threat from new strains of bacteria and viruses and the growing problems such as Deep Vein Thrombosis (DVT) and leg ulcers, it is vital that new or enhanced medical devices should be developed to cope with the situation. The market potential for medical textile products is considerable. The UK has one of the largest medical device markets in the world. The market is dominated by the National Health Service (NHS), accounting for approximately 80% of healthcare expenditure. There is a considerable high market potential for advanced wound dressings. The wound care industry generated between US$3.5 and 4.5 billion for the period between 2003 and 2006, mostly from the USA and Europe. The wound care market is predicted to grow to US$12.5 billion in 2012 and the global advanced wound care segment is the fastest growing area with growth of 10% a year. In Europe the advanced wound care market is expected to grow by an average of 12.4% a year to US$1.23 billion in 2010. The growth forecast for antimicrobial wound dressing is 25.9% per annum. In the USA alone there are over 100 000 surgeries performed daily which can result in surgical wounds. Ageing population creates increased demand for all types of surgical intervention. In 1991 the estimated annual cost of treating pressure ulcers in the UK was over £750 million, and in 2004 the total costs were £1.4–£2.1 billion or 4% of the total NHS budget. The annual cost of treating diabetic foot ulceration accounts for 5% of the total NHS budget in the UK. The annual cost for treating venous leg ulceration in Britain is £650 million and in the USA it is around $1 billion. While traditional dressings currently dominate the wound care market, raising awareness of the clinical beneﬁ ts provided by advanced wound dressings is bound to widen their uptake. Continued research and development into developing high-tech wound dressings that fulﬁ l the principal

xxi

essential requirements of keeping the wound moist to accelerate healing, being nonadherent to wound bed and having antibacterial and antiodour properties not only promotes wound healing with special reference to diffi cult-to-heal wounds but also reduces the treatment cost considerably which, in turn, has a direct impact on the economy. Wound management is a global problem and the need for a comprehen- sive book which links textiles and wounds for better wound management has been felt for a long time. This interdisciplinary state-of-the art book has been designed to meet the growing challenges in advanced wound care management. The chapters are carefully written by multinational authors who have vast experience in medical and/or textile disciplines. During editing I found that the chapters not only provide a wealth of information on wound management but also problem-solving techniques. The book is organised into two parts. The chapters in Part I address the principles and physiology of wounds and wound healing, and how textiles play a vital role in managing acute and chronic wound healing. Part I emphasises the fact that wound healing depends not only on medication but also on the use of proper dressing techniques and suitable dressing materials. Dressings vary with the type of wound and wound management, and no single dressing is universally applicable in enhancing wound healing. The role of interactive dressings, bioactive dressings, antimicrobial dressings and special dressings for managing burn wounds is critically discussed in Part I. In addition, a chapter in Part I demonstrates that textile bandages are the only treatment option to enhance the healing of diffi cult-to-heal venous leg ulcers. The testing and characterisation of wound dressings are also critically reviewed taking account of the real laboratory scenario. The high-tech dressings such as drug delivery dressings, temperature control dressings, smart dressings and composite dressings are focused in Part II. A unique last chapter covers the cultivation of human organs and body parts on textile scaffolds. Each chapter includes a wealth of bibliographical information which can serve as a ‘ready reckoner’ for fi nding additional information in specifi c subject area. Efforts have been exerted to edit the chapters to be easily readable by medical professionals, textile scientists and researchers as well as wound dressing manufacturers. This book provides readers with much needed information in the interdisciplinary subject areas of nursing and textiles. I am deeply indebted to the authors of this publication and have no doubt that their contribution will be a useful resource document making a greater contribution to this emerging discipline.

S. Rajendran The University of Bolton

S. ATHER and K. G. HARDING, Cardiff University, UK

Abstract: The various types of wounds and their mechanisms of healing are described and factors affecting the management of wound healing are outlined. For chronic wounds, a number of factors when present in combination lead to the non-healing of wounds. Wound management should therefore be multifactorial and aim at correcting the underlying abnormalities. Options for treatment are described with no single treatment being universally effective owing to the multiple molecular and cellular events involved so that a combination of different therapies is required. Future trends include application of gene therapy and stem cell therapy.

Key words: wound healing, wound management, chronic wounds.

1.1 Introduction A wound is deﬁ ned as a break in the epithelial integrity of the tissues. This disruption can be deeper and involve subepithelial tissues including dermis, fascia and muscle. They can be caused accidentally, intentionally or be a part of a disease process.1 A wound is caused by physical trauma where the skin is torn, cut or punctured (an open wound), or where a blunt force trauma causes a contusion (a closed wound). The history of wound care spans from prehistory to modern medicine and has evolved from simple wound covers ranging from vinegar-soaked dressings, through topical antibiotics to topically applied growth factors.2 Even during early historical periods several factors were noted that speeded up or assisted the process of healing. The necessity for hygiene, the prevention of bleeding and, later on, the germ theory of disease paved the way for modern wound management.

1.2 Types of wound Wounds can be classiﬁ ed in many ways, by acute or chronic, by cause (e.g., pressure, trauma, venous leg ulcer, diabetic foot ulcer), by the depth of tissue involvement, or other characteristics such as closure (primary or secondary intention).

1.2.1 Acute wound An acute wound is deﬁ ned as a recent wound that has yet to progress through the sequential stages of wound healing.3 An acute wound is acquired as a result of an incision or trauma and heals in a timely and orderly manner. Surgically created wounds include all incisions, excisions, and wounds that are surgically debrided. Surgical wounds include all skin lesions that occur as a result of trauma (e.g. burns, falls), as a result of an underlying condition (e.g. leg ulcers), or as a combination of both.

1.2.2 Chronic wounds Wounds that fail to heal in an anticipated time frame and orderly fashion and often recur are considered chronic.3 Venous leg ulcers, pressure ulcers and diabetic foot ulcers are some examples of chronic wounds.

1.2.3 Open and closed wounds Wounds are also differentiated as open or closed wound types:

Open wounds: examples include incision or incised wounds, laceration, abrasions, punctured wounds and penetrating wounds, Closed wounds: examples include contusions, haematoma and crush injuries.

1.3 Mechanism of wound healing The aim of wound healing is homeostasis and restoration of tissue integrity. It is a well-orchestrated and complex process which is triggered by tissue injury and ends by regeneration or repair. Typically healing is divided into categories based on the anticipated nature of the repair process (Fig. 1.1).

1.3.1 Healing by primary intention Wound edges are approximated with sutures, staples or adhesive within hours of its creation with no defect. This enables closure to occur quickly with minimal tissue needed to repair the defect and minimal scarring.

1.3.2 Healing by secondary intention The wound is left open and no formal closure is done. Healing occurs by epithelialisation and contraction, e.g. healing associated with a large and/or deep wound in which the tissue edges cannot be approximated. The

Differential wound healing

Wound healing

Scarless Scarring Non-healing

Foetal skin, Adult skin Chronic wounds oral mucosa

Excessive scarring

Hypertrophic scars, keloid scars 1.1 Differential wound healing.

size of the gap determines the degree of new tissue matrix and epidermal surface needed for complete closure.4

1.3.3 Delayed primary/tertiary healing Wound closure is delayed for several days; this is usually employed for infected wounds. Irrespective of the cause, wounds heal in a very similar fashion. Studying this process and how to optimise this remains the central focus of attention for the clinicians. It is a dynamic and interactive process that involves a variety of blood and parenchymal cells, extracellular matrices and soluble mediators. During this process, wound healing passes through four phases of haemostasis, inﬂ ammation, proliferation and remodelling. These phases are clinically indistinct and overlap in time. Tissue injury sets in motion a cascade of cellular and biochemical activities which leads to healing of the wound. In the following sections, stages in the process of wound healing are described (Fig. 1.2).

1.3.4 Haemostasis The fi rst step in the process (immediate up to 2–4 h) of infl ammation is haemostasis, which is characterised by vasoconstriction and coagulation. It starts soon after injury and is usually completed within the fi rst few hours. Injury to the tissues causes disruption of blood vessels and lymphatics exposing the platelets to fi brin and collagen. This activates the

Wound biology

Injury

Platelet activation

Fibrin clot Macrophages

Provisional Fibroblast Growth factors matrix Proteases ↓ Cellular density ECM production Re-epithelialisation ↓ Blood supply Migration ↑ Contraction Granulation ↑ Collagen orientation tissue ↑ Epithelial thickness Scar formation/ maturation 1.2 Biology of wound healing.

platelets and complement cascade. Platelets also interact with the injured tissue, causing the release of thrombin, which converts soluble, circulating fi brinogen to fi brin, which in turn traps, and activates platelets and forms the physical entity of the hemostatic ‘plug’.5 The activated platelet releases cytokines and growth factors including thromboxane A-2 and serotonin which are important infl ammatory mediators and also cause vasoconstriction The clot also serves to concentrate the elaborated cytokines and growth factors including platelet-derived growth factor (PDGF) and transforming growth factor (TGF) β1.6 Coagulation leads to hemostasis, which initiates healing by leaving behind messengers that bring on an infl ammatory process. Defi ciency of clotting factors (Factor VII. IX, XII) leads to impaired wound healing.7

1.3.5 Infl ammation The stage of infl ammation starts soon after haemostasis (immediate up to 2–5 days) and is usually completed within the fi rst 48 to 72 h but it may last as long as 5 to 7 days.8 The initial vasoconstriction is followed by vasodilatation and increased vascular permeability in response to histamine and other vasoactive mediators.

Role of neutrophils The net result of this change in vascular permeability is an inﬂ ux of polymorphonuclear cells (PMN) and monocytes in the injured area in a protein-

Phases of wound repair II Cell proliferation I Inflammation and matrix deposition III Matrix remodelling

Fibroplasia Angiogenesis Re-epithelialization Extracellular matrix synthesis -Collagens -Fibronectin Extracellular matrix -Proteoglycans synthesis, degradation and remodelling Granulocytes Bleeding ↑ Tensile strength Maximum response Phagocytosis Coagulation ↓ Cellularity Platelet activation ↓ Vascularity Complement activation Macrophages Cytokines

0.1 0.3 1 3 10 30 100 300 Days after wounding (log scale) 1.3 Wound biology: phases of wound repair.

rich fl uid. Neutrophils phagocytise debris and bacteria, they also kill bacteria by releasing caustic proteolytic enzymes and free radicals in a process called ‘respiratory burst’.9 The surrounding tissue matrix in unwounded tissue is protected by protease inhibitors which can be overwhelmed and penetrated if the infl ammatory response is extremely robust leading to damage to normal tissue. Unless stimuli for neutrophil recruitment persist at the wound site, the neutrophil infi ltration ceases after a few days, they undergo apoptosis and are engulfed and degraded by macrophages.10

Macrophages Macrophages start appearing in the wound two days after the injury and dominate the wound cell population over the next few days. Beside resident macrophages, the majority of macrophages at the wound site are recruited from the blood. Monocytes extravasate from the blood vessel, become activated and differentiate into mature tissue macrophages. Macrophages are crucial to wound healing and perform a number of functions. They act as antigen-presenting cells and remove debris and dead cells by phagocytosis. Perhaps their more important role in the process of healing is synthesis of numerous potent growth factors, such as TGF-β, TGF-α, basic ﬁ broblast growth factor (bFGF), platelet-derived growth factor, and vascular endothelial growth factor, which promote cell proliferation and the synthesis of extracellular matrix molecules by resident skin cells.11 These factors also help in angiogenesis, migration and activation of

ﬁ broblast thus setting the stage of proliferation.12 It has been shown experi- mentally that macrophage depletion using antisera results in a signiﬁ cant delay in healing.13

Role of inﬂ ammatory mediators Inﬂ ammatory mediators play a central and major role in the process of wound healing. They include a collection of soluble factors present either in plasma in an inactive form or released by damaged and nearby cells and leukocytes in an attempt to control the damage and initiate healing.

Mechanisms of infl ammatory resolution Infl ammation performs several important functions. It clears the wound of infectious organism and debris, and brings about a change in the micro-environment of the wound to set the stage for proliferation. However, successful repair after injury requires resolution of the infl ammatory response. The mechanisms controlling this down-regulation of the infl ammatory response are poorly understood and for years it was thought that the infl ammatory response would just ‘burn itself out’. Recent evidence, however, suggests that this process is organised as a series of reactions to produce stop signals referred to as ‘check point controllers of infl ammation’.14 Lipoxins and aspirin-triggered lipoxins are the stop signals for infl ammation. Autocoids also display potent anti-infl ammatory actions and are termed resolvins.15 Down-regulation of pro-infl ammatory mediators and the reconstitution of normal microvascular permeability, which contributes to the cessation of local chemoattractants, synthesis of anti- infl ammatory mediators, apoptosis, and lymphatic drainage also play their role. An excessive or prolonged infl ammatory response results in increased tissue injury and poor healing.

1.3.6 Proliferation This phase starts around the second or third day after injury and continues for up to 3 or 4 weeks. This is marked by the appearance of ﬁ broblasts in the wound and overlaps with the inﬂ ammatory phase. As in other phases, the changes in this phase do not occur in a series, but overlap in time.

Granulation tissue formation Fibroblasts start to appear in the wound from the third to fourth day and their numbers peak between the seventh and fourteenth days. They migrate from the wound margins using the ﬁ brin-based provisional matrix created

during the infl ammatory phase of healing. Under the infl uence of bFGF, TGF-β, and PDGF secreted by macrophages they proliferate and synthe- sise glycosaminoglycans and proteoglycans, elastins and fi bronectin, the building blocks of the new extracellular matrix of granulation tissue, and collagen. As the number of macrophages diminishes, fi broblasts themselves begin to secrete bFGF, TGF-β, and PDGF. They also begin producing keratinocyte growth factor and insulin like growth factor I. After secretion of collagen molecules, they are organised in the form of collagen fi bres which are then cross-linked into bundles. Collagen gives the wound its tensile strength and, in addition, cells involved in infl ammation, angiogenesis, and connective tissue construction attach to, grow and differentiate on the collagen matrix laid down by fi broblasts.16 Collagen deposition increases the tensile strength of the wound. Initially collagen levels in the wound increase, but later on homeostasis is reached as the collagen is also being degraded by collagenases.

Angiogenesis Angiogenesis accompanies the fi broplasia phase and is essential to scar formation. Endothelial cells located at intact venules are stimulated by vascular endothelial growth factor (VEGF) which is secreted mainly by keratinocytes at the wound edge and also by macrophages, fi broblasts and platelets in response to hypoxia and the presence of lactic acid. Endothelial cells originating from parts of uninjured blood vessels develop pseudopo- dia and push through the extracellular matrix (ECM) into the wound site. They produce the degradation agents including plasminogen activator and collagenase and invade the wound by the enzymatic degradation of fi brin clot once the new granulation tissue (i.e., extracellular matrix, collagen, capillaries) is laid down.17 Through this activity, they establish new blood vessels which later on join to form capillary loops and establish blood fl ow in the wound. Cells, when adequately perfused, stop producing angiogenic factors, and migration and proliferation of endothelial cells is reduced.17 Eventually, blood vessels that are no longer needed die by apoptosis; this explains the change in color seen in scar tissue as it matures.

Epithelialisation The initial event in epithelialisation is migration of undamaged epithelial cells from the wound margins. Keratinocytes at the wound edges are stimulated by EGF and TGF-α produced by activated platelets and macrophages,15 they proliferate and begin their migration across the wound bed within 12 to 24 h after injury.18 The ﬁ rst step of migration involves separation of the keratinocytes from each other and their anchors to the cell

basement membrane.19 The process of migration continues until the migrating cells from opposing sides of the wound touch each other. At the point of contact, migration ceases in a process known as contact inhibition.19 Once this process is complete, keratinocytes stabilise them by forming fi rm attachments to each other and the new basement membrane.20 All these changes in the wound lead to the formation of granulation tissue which consists of infl ammatory cells, fi broblasts and new vasculature in a hydrated matrix of glycoproteins, collagen and glycosaminoglycans, the components of a new, provisional ECM. The provisional ECM is different in composition from the ECM in normal tissue and includes fi bronectin, collagen, glycosaminoglycans and proteoglycans.21

Contraction About a week after the injury, the fi broblast differentiates into myofi broblasts, pulls the edges of the wound together and the wound begins to contract.22 Contraction peaks at 5 to 15 days post wounding and continues even after the wound is completely re-epithelialised.23 Contraction reduces the size of the wound and, thus, reduces the amount of ECM needed to fi ll the wound24 and facilitates re-epithelialisation by reducing the distance which migrating keratinocytes must travel. At the end of the granulation phase, fi broblasts begin to undergo apoptosis, converting granulation tissue from an environment rich in cells to one that consists mainly of collagen.23

1.3.7 Maturation and remodelling Maturation and remodelling of the collagen into an organised and well- mannered network is the fi nal stage of the healing process (from day 8 up to 2 years). If this is compromised, then the wound’s strength will be greatly affected. On the other hand, excessive collagen synthesis can lead to the formation of a hypertrophic scar or keloid. The maturation phase can last for two years or longer, depending on the size of the wound and whether it was initially closed or left open. This phase is characterised by the removal of type III collagen and its replacement by mature type I collagen. There is a rapid production of type I collagen but there is no net gain as the old collagen is also being degraded by collagenases. New collagen fi bres are rearranged, cross-linked, and aligned along tension lines but they can never become as organised as the collagen found in uninjured skin.25 The second characteristic feature of this stage is programmed cell death or apoptosis and, thus the number of cell types such as macrophages, keratinocytes, fi broblasts, and myofi broblasts is reduced.18,20 Remodelling is regulated by fi broblasts through the synthesis of ECM components and MMPs that control cell

differentiation.26 All of these changes produce a cell-deﬁ cient environment with excessive connective tissue. Blood vessels that are no longer required die by apoptosis and the remainder acquire a basement membrane and become relatively impermeable. All of these factors lead to the increase in tensile strength, decrease in erythema and scar tissue bulk, and the ﬁ nal app earance of the healed scar.

1.4 Factors affecting wound healing: why wounds fail to heal In most cases, wound healing is a natural, uneventful process which leads to the restoration of tissue integrity. But, in some cases, wounds fail to heal and become a complex medical problem requiring specialised care and treatment. If a wound has not improved signiﬁ cantly in four weeks, or if it has not completed the healing process in eight weeks, it is considered a chronic, non-healing wound. Wound healing is dependent on the interaction of different cells, mediators and growth factors. Alterations in one or more of these components may account for the impaired healing observed in chronic wounds. Chronic wounds may be arrested in any of the healing phases but, most commonly, disruption occurs in the inﬂ ammatory or proliferative phase27 with the accumulation of excessive extracellular matrix and matrix metalloproteinases such as collagenase and elastase, which result in premature degradation of collagen and growth factors.28 An optimum microenvironment and the absence of cytotoxic factors are essential for healing of wounds. Many local and systemic factors have been implicated in the delayed healing of wounds.

1.4.1 Local factors Infection Infection is the commonest local cause for delayed wound healing. Bacteria delay wound healing by activating the alternative complement pathway and exaggerating and prolonging the inﬂ ammatory phase of wound healing. The bacteria, themselves elaborate toxins and proteases, also compete for oxygen and nutrients, and this ultimately damages the cells.

Tissue ischaemia Local hypoxia is detrimental to cellular proliferation, resistance to infection and collagen production. This may be the result of foreign bodies, infection, suture material, or the presence of peripheral vascular disease.

Poor surgical technique Proper tissue handling and closure with appropriate sutures is very important. Wound healing is affected if the tissues are devitalised, strangulated with sutures or improperly debrided.

Others Formation of haematomas, presence of foreign bodies and mechanical pressure are some of the other local factors in the pathophysiology of delayed wound healing.

1.4.2 Systemic factors Ageing Healing in the elderly is generally delayed but the ﬁ nal result is qualita- tively similar in elderly people. There are many physiological changes associated with ageing which can lead to delayed healing. Reduced skin elasticity and collagen replacement inﬂ uence healing. Reduced immunity and other chronic diseases can also affect the healing process.

Nutritional status Defi ciency of various nutrients and vitamins can affect the wound healing process. Proteins are required for all the phases of wound healing and are particularly important for collagen synthesis. In protein defi ciency states, cellular and humoral immune responses are blunted, fi broplasia and all aspects of matrix formation are delayed. Glucose balance is essential for wound healing and it provides the energy required for cell function. Insulin may act as a fi broblast growth factor and its defi ciency leads to suppression of collagen deposition in the wound.29 Defi ciency of fatty acids can also impair healing.

Vitamins Vitamin A defi ciency has been associated with slowed re-epithelisation, decreased collagen synthesis and stability and an increased susceptibility to infection. Vitamin C (ascorbic acid) is an essential cofactor during collagen bio- synthesis. In scurvy, the collagen formed is unhydroxylated, relatively unstable and subject to collagenolysis. Vitamin K defi ciency results in a defi ciency in the production of the clotting factors (factors II, VII, IX and X) that are vitamin K dependent

resulting in bleeding diathesis, hematoma formation and secondary detrimental effects on wound healing. Iron is required to transport oxygen. Other minerals like zinc and copper are important for enzyme systems and immune systems. Zinc deﬁ ciency contributes to disruption in granulation tissue formation.

Underlying diseases Diabetes, arthritis, renal disease, heart disease, cancer, immune disorder, lung disease blood disorders and surgery all affect the process of wound healing.

Medication Anti-inﬂ ammatory, cytotoxic, immunosuppressive and anticoagulant drugs all reduce healing rates by interrupting cell division or the clotting process.

1.5 Wound healing: treatment options 1.5.1 Basic care Wound repair requires the timed and balanced activity of inﬂ ammatory, vascular, connective tissue, and epithelial cells and their mediators. It is important to provide an environment that is conducive to wound healing, to treat the underlying origin of the wound, and to correct associated abnormalities.

1.5.2 Wound dressings Dressings do not heal wounds; properly selected dressings do, however, promote healing and prevent further harm to the wound. Wound dressings are passive, active or interactive.30 Passive dressings simply provide cover while active or interactive dressings are believed to be capable of modifying the physiology of the wound environment. Interactive dressings include hydrocolloids, hydrogels, alginates and foams. An ideal dressing should maintain a moist environment at the wound interface and act as a barrier to micro-organisms. Commonly available dressings include.

Alginate These dressings are highly absorbent and are composed of calcium and sodium salts of alginic acid, obtained from seaweed. They are useful in

medium to heavily exuding wounds and are also good for bleeding wounds. Examples include Kaltostat, Sorbsan and Algisite.

Hydrogels Hydrogels have a high water content which creates a moist wound surface and helps in the debridement of wounds by hydration and promotion of autolysis. As absorption of exudate is poor, they can cause maceration. Examples include Aquafoam, Intrasite, Nu-Gel, Purilon and Sterigel.31

Hydrocolloids Hydrocolloids are composed of a matrix of cellulose and other gel-forming agents, including gelatin and pectin. These dressings promote autolysis and aid granulation. Examples include sheets such as Alione, Combiderm and Duoderm; paste such as GranuGel; and hydroﬁ bre such as Aquacel and Versiva.31

Semipermeable ﬁ lms Semipermeable dressings are good for low to medium exuding wounds. Examples include Opsite, Flexiguard, Tegaderm, Melﬁ lm and Bioclusive.

Foam dressings Foam dressings are useful for moderately exudating wounds as they prevent ‘strike through’ of exudate to the wound surface. They also provide cushion and support to the wound. Examples include Allevyn, Lyofoam, Tielle plus and Biatin Adhesive.

Antimicrobial dressings Antimicrobial dressings are good for infected wounds especially in diabetics. Examples include Acticoat, Aquacell Ag, Arglaes, Inadine and iodoﬂ ex.

1.5.3 Bioengineered skin Bioengineered skin is generally divided into: 1. permanent, such as autografts and 2. temporary, such as allografts (including de-epidermised cadaver skin and in vitro reconstructed epidermal sheets), xenografts (i.e. conserved pig skin) and synthetic dressings

Allogenic grafts are produced from neonatal ﬁ broblasts and keratinocytes. Available in the form of dermal, epidermal or composite grafts, they are better than traditional skin grafts as they are non-invasive, do not require anaesthesia and avoid potential donor site problems.

Epidermal grafts Available in both autograft and allogenic forms, epidermal grafts include Epicell, Laserskin, CellSpray, Bioseed-S and LyphoDerm.27 They are useful for coverage of large skin defects with acceptable cosmetic results and are indicated for burns and leg ulcers. Their main disadvantages include fragility and difﬁ culty in handling owing to a lack of backing material. They are unsuitable for deep wounds as they only provide temporary cover. They are most successful when placed on a dermal bed.

Dermal grafts Dermal grafts are either cellular or acellular, and allogenic in nature, and, hence, available for immediate use. Products include Integra, Alloderm, Biobrane, Transcyte and Dermagraft.27 They are indicated for burns, deep wounds, and for cosmetic procedure. However, they cannot be generated in large quantities and are susceptible to infections.

Composite grafts Composite grafts are bilayered skin grafts and contain epidermal and dermal components. Apligraft is a commercially available product and is indicated for diabetic and venous ulcers. It lacks skin adnexal structures but produces all the cytokines and growth factors that are produced by normal skin. Allergy to bovine collagen, limited shelf-life and infection can limit their use.27

1.5.4 Non-surgical innovations Vacuum assisted closure Vacuum-assisted closure (VAC) therapy entails placing an open-cell foam dressing into the wound cavity and applying a controlled sub- atmospheric pressure. This produces negative pressure in the wound, leading to improved blood ﬂ ow and oxygenation. It also helps in removing excessive ﬂ uid and slough.32 This stimulates granulation tissue formation, wound contraction and early closure of wound.

Intermittent pneumatic compression This is an effective treatment for chronic ulcers on legs with severe oedema. It provides compression (at 20–120 mm Hg) at preset intervals.33 It improves lymphatic and venous ﬂ ow and helps in the healing of chronic ulcers.

Hyperbaric oxygen Hyperbaric oxygen is thought to expedite healing as it has been shown that ischaemic lesions heal less well34 and certain growth factors do not work in hypoxic conditions.35 Debate continues on its value in certain wound conditions.

Other therapies Laser, ultrasound, hydrotherapy, versijet, electromagnetic therapy and electrotherapy are some other therapies which are used to stimulate healing.

1.5.5 Drug therapy Drugs affect wound healing by assisting or interfering with speciﬁ c phases. Drugs can reduce peripheral vascular resistance, reduce blood viscosity and cause local or systemic vasodilatation leading to improved tissue perfusion and oxygenation. Currently available drugs include pentoxifylline, which decreases platelet aggregation leading to decreased viscosity and improved capillary microcirculation. It is useful in patients with chronic venous ulcers who cannot tolerate compression. Iloprost a vasodilator and prostacyclin ana- logue is good in the treatment of arterial and vasculitic ulcers.33 Calcium channel blockers and glyceryl trinitrate (GTN) ointment have also been used as vasodilators in cases of vaculitic ulcers caused by Raynaud’s disease and ischaemic ulcers, respectively.

1.5.6 Growth factors Growth factors are soluble signalling proteins which infl uence wound healing through their inhibitory or stimulatory effect during different stages of the wound healing process. Produced by different cells they act on infl ammatory cells, fi broblasts, and endothelial cells to direct the processes involved in wound healing. Recombinant human-platelet derived growth factor-bb (rhPDGF-BB, Becaplermin) is the only FDA-approved

growth factor available for clinical use. In clinical trials, this has been shown to increase the incidence of complete wound closure and decrease the time to achieve complete wound healing. Basic ﬁ broblast growth factor (available commercially in Japan) stimulates endothelial cell migration and proliferation. Its topical application leads to faster granulation tissue formation and epidermal regeneration in burn wounds. However, this effect is not seen in diabetic foot ulcers. Preclinical trials have shown promising results for epidermal growth factor and keratinocyte growth factor in venous ulcers and ﬁ broblast growth factor platelet derived growth factor (PDGF) for pressure ulcers. Despite promising preclinical data, results of the clinical trials are disap- pointing. Inherent instability of these proteins in the hostile environment of the wound makes them ineffective. Time of application, dosage, mode of delivery or the combination of the growth factors may be incorrect and further evaluation is required.

1.6 Future trends 1.6.1 Gene therapy To be clinically effective a high concentration of growth factors is needed, which requires frequent and high dosing, but it is prohibitively expensive. Introduction of the gene rather than the product (growth factor) is thought to be cheaper and more efﬁ cient in treating non-healing wounds. It can lead to a sustained local availability of these proteins and can be cost effective. The technology to introduce genes through physical or biological vectors has existed for some time. Long-term expression of the therapeutic gene remains a challenge but only a transient gene expression is required for wound repair. Phase I studies are being conducted at the moment and their results will dictate any further course of action in this ﬁ eld.

1.6.2 Stem cells therapy Stem cells which are thought to be present in every tissue are characterised by their prolonged self-renewal capacity and by their asymmetric replication. There is potential that stem cells may reconstitute dermal, vascular and other elements required for optimum wound healing. Han et al.36 showed the potential of human bone marrow stromal cells to accelerate wound healing in vitro by measuring the amount of collagen synthesis and the levels of basic ﬁ broblast growth factor. Though the technique is still in its infancy, Badiavas et al.37 have shown that direct application of autologous bone marrow and its cultured cells may accelerate the healing process.

1.7 Conclusions Chronic wounds do not have a unique defect but a number of factors, which, when present in combination, lead to the non-healing of wounds. Wound management should therefore be multifactorial and aim at correcting the underlying abnormalities. No single treatment is universally effective owing to multiple molecular and cellular events involved and a combination of different therapies is required.

1.8 References

1. robson mc, Wound infection: a failure of wound healing, caused by an imbalance of bacteria. Surg Clin North Am 1997; 77:637–50. 2. janis je and attinger ce, Current concepts in wound healing. Plast Reconstr Surg 2006; 117(7 Suppl):4S–5S. 3. attinger ce, janis je, steinberg j, schwartz j, al-attar a and couch k, Clinical approach to wounds: debridement and wound bed preparation including the use of dressings and wound-healing adjuvants. Plast Reconstr Surg 2006 Jun; 117(7 Suppl):72S–109S. 4. iocono ja, ehrlich hp and gottrup f et al., The biology of healing. In: DL Leaper and KG Harding, Editors, Wounds: biology and management, Oxford University Press, Oxford, England (1998), pp. 12–22. 5. kerstein md, The scientifi c basis of healing. Adv Wound Care 1997; 10(4):30–36. 6. singer aj and clark ra, Cutaneous wound healing, N Engl J Med 1999; 341:738–746. 7. beck e, duckert f and ernst m, The infl uence of fi brin stabilizing factor on the growth of fi broblasts in vitro and wound healing. Thromb Diath Haemorrh 1961; 6:485. 8. haas af, Wound healing, Dermatol Nurs 1995; 7:28–34. 9. greenhalgh dg, The role of apoptosis in wound healing. The International J Biochem Cell Biol 1998; 30(9):1019–1030. 10. martin p and leibovich sj, Infl ammatory cells during wound repair: the good, the bad and the ugly. Trends Cell Biol 1998; 15(11):599–607. 11. dipietro la and polverini pj, Role of the macrophage in the positive and negative regulation of wound neovascularisation. Am J Pathol 1993; 143:678–684. 12. witte m and barbul a, Role of nitric oxide in wound repair. Am J Surg 2002; 183:406. 13. leibovich sj and ross r, The role of the macrophage in wound repair. Am J Pathol 1975; 78:71–100. 14. trengove nj, stacey mc and macauley s, Analysis of acute and chronic wound environment: The role of protease and their inhibitors. Wound Repair Regen 1999; 7:442. 15. lawrence w and diegelmann r, Growth factors in wound healing. Clin Dermatol 1994; 12:157. 16. ruszczak z, Effect of collagen matrices on dermal wound healing. Adv Drug Deliv Rev 2003; 55(12):1595–1611.

17. greenhalgh dg, The role of apoptosis in wound healing. Int J Biochem Cell Biol 1998; 30(9):1019–1030. 18. iocono ja, ehrlich hp and gottrup f et al., The biology of healing. In: DL Leaper and KG Harding, Editors, Wounds: Biology and Management, Oxford University Press, Oxford, England (1998), pp. 12–22. 19. garrett b, Re-epithelialisation, J Wound Care 1998; 7:358–359. 20. clark raf, Wound repair: Overview and general considerations (ed 2). In: RAF Clark, Editor, The Molecular and Cellular Biology of Wound Repair, Plenum Press, New York, NY (1995), pp. 3–50. 21. ruszczak z, Effect of collagen matrices on dermal wound healing. Adv Drug Del Rev 2003; 55(12):1595–1611. 22. eichler mj and carlson ma, Modeling dermal granulation tissue with the linear fi broblast-populated collagen matrix: A comparison with the round matrix model. J Dermatol Sci 2005; 41(2):97–108. 23. stadelmann wk, digenis ag and tobin gr, Physiology and healing dynamics of chronic cutaneous wounds. Am J Surg 1998; 176(2):26S–38S. 24. calvin m, Cutaneous wound repair, Wounds 1998; 10:12–15. 25. lorenz hp and longaker mt, Wounds: biology, pathology, and management. Stanford University Medical Center, 2003. 26. streuli c, Extracellular matrix remodeling and cellular differentiation, Curr Opin Cell Biol 1999; 11:634–640. 27. enoch s, grey je and harding kg, Recent advances and emerging treatments. BMJ Apr 2006; 332:962–965. 28. diabetes care, american diabetes association. Consensus development conference on diabetic foot wound care. 1999; 22:1354–1360. 29. eaglestein wh and mertz pm, ‘Inert’ vehicles do affect wound healing. J Invest Dermatol 1980; 74:90. 30. hanson c, Interactive wound dressings. A practical guide to their use in older patients. Drugs Aging 1997; 11:271–284. 31. jones v, grey je and harding kg, Wound dressings. BMJ Apr 2006; 332:777–780. 32. fl eck tm, fl eck m, moidl r, czerny m, koller r, giovanoli p, hiesmayer mj, zimpfer d, wolner e and grabenwoger m, The vacuum-assisted closure system for the treatment of deep sternal wound infections after cardiac surgery. Ann Thoracic Surg 2002, 74(5):1596–1600. 33. enoch s, grey je and harding kg, Non-surgical and drug treatments. BMJ Apr 2006; 332:900–903. 34. jonsson k, hunt tk and mathes sj, Oxygen as an isolated variable infl uences resistance to infection. Ann Surg 1988; 208(6):783–787. 35. wu l et al., Effects of oxygen on wound responses to growth factors: Kaposi’s FGF but not basic FGF stimulates repair in ischemic wounds. Growth Factors 1995; 12(1):29–35. 36. han sk et al., Potential of human bone marrow stromal cells to accelerate wound healing in vitro. Ann Plast Surg 2005; 55 :414–419. 37. badiavas ev and falanga v, Treatment of chronic wounds with bone marrow- derived cells. Arch Dermatol 2003; 139(4):510–516.

S. T. T HOM AS, formerly of Surgical Materials Testing Laboratory, Medetec, UK

Abstract: The development of modern dressings is briefl y reviewed and a description is given of how new test methods and specifi cations have evolved to characterise various key aspects of the performance of the products concerned. For thousands of years man has applied a variety of materials to his wounds to control bleeding and promote healing. For much of that time many of the materials used were largely those that occurred naturally, or which were developed and used primarily for another purpose. Only in the last thirty to fi fty years have large number of increasing complex products been developed specifi cally for application to open wounds of all types. These new dressings vary widely in composition and construction and, as a result, new families of performance-based test methods and specifi cations have been developed in order in order to characterise their performance.

Key words: wound dressings, wound management, ﬂ uid handling, hydrocolloids.

2.1 Introduction The use of materials to cover or treat wounds stretches back into antiquity but the use of standards and specifi cations to characterise these materials is relatively new. The fi rst standards that were used to characterise dressings were very simple and concentrated primarily upon the structure rather than the function of the products concerned. As new, ever-more sophisticated dressings were introduced, there was an increasing need for test systems to demonstrate that these materials perform in a consistent manner and delivered the performance claimed for, and expected of them. Whilst it is undoubtedly true that ultimately it is how a product performs clinically that will determine its acceptability and commercial success, well-designed laboratory tests can provide a useful performance indicator, particularly in comparative terms. In this chapter, the need for dressing standards is discussed and how these standards have evolved, and briefl y outlines test methods that can be

used to assess key aspects of the performance of many different types of products.

2.2 The need for laboratory testing Laboratory tests for dressings are required for a number of reasons: • To demonstrate compliance with national or international standards or specifi cations. • To ensure product meets ‘in-house’ manufacturing standards. • To facilitate comparisons with competitive products. • To generate data to support allocation of shelf life (stability/storage). There are essentially three types of standards or specifi cations. • Structural standards, which defi ne the structure and/or composition of a product. • Performance standards, which describe one or more key aspects of the function of a dressing. • Safety standards, designed to ensure that a product, when used appropriately, is unlikely to adversely affect the health or wellbeing of the individual to whom it is applied.

2.2.1 The development of dressings For centuries mankind had little option but to apply readily available natural substances to his wounds to staunch bleeding, absorb exudate or promote healing. Initially, these would have consisted of simple materials such as honey, animal oils or fat, cobwebs, mud, leaves, moss or animal dung applied in the crude form in which they were found, but later these and other ‘raw materials’ began to be combined together, either to make them easier to handle, or to improve their clinical effectiveness. Whilst most of these early preparations were probably of little or no value, others, such as honey, used alone or mixed with oils or waxes, undoubtedly con- ferred some real clinical benefi ts to the user. Up to the end of the 19th century, whenever dressings were required to cover wounds, absorb exudate or remove blood during a surgical procedure, practitioners of the time used whatever materials were to hand, often recycling old pieces of cloth or linen fabric for this purpose. This was sometimes fi rst unravelled to form short ends of thread called ‘charpie’, or the surface was scraped with knives to produce ‘soft lint’, a soft fl uffy material not dissimilar to absorbent cotton (cotton wool) which could be used to pack cavities and soak up exudate or blood. The fi rst product to be manufactured commercially for use as a dressing was absorbent (sheet) lint. This was formed by scraping sheets of old linen

with sharp knives to raise a ﬁ brous ‘nap’ on one surface, increasing the absorbency of the cloth but decreasing its tensile properties. Initially a manual process, a lint-making machine was developed in the ﬁ rst half of the 19th century which used new, specially woven, fabric for the purpose. This resulted in a more consistent product with, it is assumed, a considerably lower bioburden! Originally lint was formed from linen but this was eventually replaced by cotton by the middle of the 20th century.

2.2.2 The fi rst standards for dressings As new types of dressings were developed and the production of surgical materials became more mechanised, it became necessary to develop formal standards to ensure that these were consistently produced to an agreed of level of quality. The fi rst of these appeared in two supplements to the British Pharma- ceutical Codex (BPC) of 1911 and these were later incorporated into the 1923 edition of this publication. Over 80 products were described, the majority of which consisted of cotton fi bre, both medicated and unmedi- cated, and a variety of cotton fabrics together with a few more complex products such as emplastrums (plasters) and oiled silk. The BPC remained the principal source of standards for surgical dressings within the United Kingdom for over 50 years, but, in 1980, these were transferred to the British Pharmacopoeia (BP). When, as a result of European legislation, dressings became classifi ed as Medical Devices, monographs for these materials were subsequently omitted from the BP.

2.2.3 The importance of performance-based specifi cations The early pharmacopeial monographs consisted almost entirely of structural specifi cations supplemented by limit tests for potential contaminants. Whilst such standards undoubtedly have a value as quality control checks to ensure that products which have previously been shown to meet a specifi c clinical need are produced in a consistent way from a range of well-characterised materials, they do not facilitate comparisons between the performances of different types of dressings. Their proscriptive and infl exible nature also prevents or delays the introduction of new and more innovative products which may be structurally different from the standard materials. Recognising these limitations, in the early 1990s, the Surgical Dressings Manufacturing Association (SDMA) set up a series of working groups, comprising technical staff from the industry and the NHS, to devise a new family of performance-based test methods and specifi cations. These were based on a number of instances upon work that had been pioneered within the Surgical Materials Testing Laboratory (SMTL), an NHS facility that specialised in testing wound dressings and other medical disposables for the

NHS in Wales. This group published test methods for bandages, alginates, ﬁ lms, hydrocolloids and hydrogels, many of which subsequently became incorporated into the BP and/or adopted as European Standards. The types of tests required to characterise the performance of a dressing should be determined by the nature and condition of the wound to which the product is to be applied. The functions of a dressing have been described more fully in the past,1 but for the purpose of this chapter the principal requirements are summarised below. • Exudate management/environmental control • Control or prevention of infection • Provide a bacterial barrier • Odour control • Low-adherence • Freedom from toxicity.

2.3 Fluid-handling tests The effective management of the moisture content of a wound and the surrounding skin is perhaps the most important requirements of any dressing system. In the case of exuding wounds, this implies the removal of excess wound fl uid, but, in dry or lightly exuding wounds, the dressing may be required to conserve moisture in order to maintain the exposed tissue in the optimum state of hydration to facilitate epithelialisation or promote autolytic debridement. The ability to control the loss of moisture from a wound is commonly determined by the moisture vapour permeability of the dressing or dressing system. Because excessive exudate can cause maceration of the peri- wound skin, which in turn can lead to infection, considerable attention has been given by the industry to the development of highly absorbent products that are able to prevent fl uid from spreading over the surrounding healthy tissue. Some dressings, such as hydrophilic polyurethane fi lms, are very permeable to water vapour and thus permit the passage of a signifi cant quantity of the aqueous component of exudate from the wound to the environment by evaporation. In practice, however, most permeable products are unable to cope with the volume of fl uid that is produced by heavily exuding leg ulcers, burns or malignant wounds. In such situations, products that have the ability to absorb or otherwise retain signifi cant quantities of liquid are required, although many also combine this absorptive function with a signifi cant degree of moisture vapour permeability. These two values determine the ability of a dressing to cope with wound exudate and are described as its fl uid handling capacity (FHC).2 Numerous different tests have been described to characterise the fl uid handling properties of dressings, which vary from simple ‘dunk and drip’

tests to more sophisticated techniques in which a suitable test ﬂ uid is applied to a sample of dressing under controlled conditions, some of which have been incorporated in the European Standard (BS EN 13726-1)3 described below.

2.3.1 Free swell absorptive capacity This, the fi rst standard test described in BS EN 13726-1, measures the uptake of fl uid by fi brous dressings such as those made from alginate fi bre presented either in sheet or rope form. The dressing is placed in a Petri dish together with a quantity of test solution equivalent to 40 times the weight of the test sample, and held for 30 min at 37 °C after which it is gently removed from the dish, allowed to drain for 30 s and reweighed. The absorbency is then expressed as the mass of solution retained per 100 cm2 (for sheet dressings) or per gram of sample for cavity dressings. Unless otherwise stated, all absorbency tests are performed using ‘Test solution A’, a mixture of sodium chloride and calcium chloride solutions containing 142 mmol of sodium ions and 2.5 mmol of calcium ions as the chloride salts. This solution has an ionic composition similar to human serum or exudate. The presence of both sodium and calcium ions is required as these both have a marked effect upon the gelling characteristics of alginate fi bres. It will immediately be seen that a serious criticism of this method is that the alginate is tested in the absence of any pressure, which means that the results obtained bear little relation to the volume of exudate that the dressing will take up under normal conditions of use. (This point is returned to later.) In the presence of sodium ions, alginate fi bres absorb fl uid and swell, sometimes taking on a gel-like appearance. The degree of swelling and dispersion is determined by the chemical structure, ionic content and method of preparation of the dressing. Some alginate products, having a high mannuronic acid content, appear to form an amorphous mass, whilst others with a high guluronic acid content tend to retain their structure and swell to a much lesser degree.4 These properties are quite important as they determine how the dressing will perform when introduced into a wound and BS EN 13726-1 describes two simple tests that help to characterise the alginate and determine its dispersion/solubility.

2.3.2 Fluid-handling capacity This test, also described within BS EN 13726-1, and based on a method published previously,5 provides information on the amount of test ﬂ uid

both retained and transpired by products such as hydrogel and hydrocolloid sheets and other dressings made from foam which incorporate an integral waterproof backing layer. The method is not suitable for testing fi brous products or permeable absorbents. The test involves the use of a simple piece of apparatus known as a Paddington Cup. This is described in detail in the standard, but, essentially, it consists of a cylinder with an internal cross-sectional area of 10 cm2 having a fl ange at each end. To one end of the cylinder is fi tted an annular ring, the internal diameter of which is identical to that of the cylinder, and, to the other, a solid plate, which can be clamped into position forming a watertight seal. A piece of dressing under examination is cut to shape and clamped between the annular ring and one of the fl anges. The cylinder together with all the associated parts is then weighed. Approximately 20 ml of test fl uid is added to the cup and the plate clamped in position. The cup is then weighed again before being placed in an incubator capable of maintaining the internal temperature and humidity within specifi ed limits (37 ± 1) °C and relative humidity <20% throughout the test. After a predetermined period (usually 24 h) the cylinder is reweighed, the plate removed and the excess fl uid is allowed to escape after which the cylinder is reweighed once again. From these weighings, it is possible to calculate the amount of fl uid lost through the back of the dressing by evaporation during the period of test, and the weight of fl uid retained within its structure. The sum of these two values represents the Fluid Handling Capacity of the dressing. By way of illustration, Tables 2.1–2.3 contain the published results of fl uid handling tests performed on 12 hydrocolloid dressings over 24, 48 and 96 h.5 Individually the tables illustrate the differences between products at each time point, but a comparison of the results achieved with individual products at the two time points clearly demonstrates how the products vary in terms of their rate of hydration. Some products reach full absorbency after 24 h, others are still absorbing after 96 h. These differences clearly have potentially important clinical implications for the use of the products concerned.

2.3.3 Moisture vapour transmission rate (MVTR) This same apparatus described in BS EN 13726-1 may be used in a similar although less complicated fashion, to determine the MVTR of permeable fi lm dressings. This method is described in BS EN 13726-2.6 Although the offi cial method specifi es that the apparatus shall be incubated with the solution in contact with the test sample, it is also possible

Table 2.1 Fluid-handling characteristics of hydrocolloid dressings after 24 h*

Moisture Weight FHC vapour loss absorbed [g (s.d.)] [g (s.d.)] [g (s.d.)]

Tegasorb Thin 0.58 (0.03) 2.89 (0.26) 3.47 (0.26) Tegasorb 0.60 (0.22) 4.40 (0.11) 5.01 (0.26) Cutinova Hydro 0.67 (0.03) 2.53 (0.05) 3.21 (0.05) Askina Bioﬁ lm Transparent 0.27 (0.02) 0.24 (0.07) 0.51 (0.08) Askina Transorbent 3.35 (0.51) 0.80 (0.05) 4.16 (0.55) Comfeel Plus Plaques Biseautees 1.33 (0.05) 1.53 (0.04) 2.86 (0.06) Comfeel Plus Transparenter 0.65 (0.10) 3.98 (0.12) 4.63 (0.16) Comfeel Plus 0.49 (0.05) 3.27 (0.09) 3.76 (0.11) Flexibler (sample 1) Comfeel Plus 0.77 (0.12) 3.33 (0.06) 4.10 (0.11) Flexibler (sample 2) Varihesive E 0.03 (0.01) 1.94 (0.11) 1.97 (0.12) Granuﬂ ex 0.03 (0.02) 1.75 (0.13) 1.78 (0.14) Hydrocoll 0.50 (0.04) 5.62 (0.11) 6.12 (0.12) Algoplaque 0.22 (0.45) 1.32 (0.14) 1.54 (0.38)

* Reproduced with permission from: A comparative study of the properties of twelve hydrocolloid dressings http://www.worldwidewounds.com/1997/july/ Thomas-Hydronet/hydronet.html.

Table 2.2 Fluid-handling characteristics of hydrocolloid dressings after 48 h*

Dressing Moisture Weight FHC vapour loss absorbed [g (s.d.)] [g (s.d.)] [g (s.d.)]

Tegasorb Thin 1.46 (0.07) 3.70 (0.07) 5.17 (0.08) Tegasorb 2.27 (1.06) 5.25 (0.39) 7.52 (0.69) Cutinova Hydro 1.92 (0.02) 2.87 (0.04) 4.80 (0.06) Askina Bioﬁ lm Transparent 0.38 (0.02) 0.12 (0.14) 0.50 (0.13) Askina Transorbent 3.90 (0.38) 0.75 (0.03) 4.64 (0.38) Comfeel Plus Plaques Biseautees 2.28 (0.21) 4.39 (0.05) 6.67 (0.19) Comfeel Plus Transparenter 3.02 (0.21) 1.68 (0.12) 4.70 (0.28) Comfeel Plus Flexibler (sample 1) 1.55 (0.06) 3.73 (0.09) 5.28 (0.06) Comfeel Plus Flexibler (sample 2) 2.21 (0.12) 3.45 (0.14) 5.66 (0.16) Varihesive E 0.13 (0.03) 2.77 (0.10) 2.90 (0.11) Granuﬂ ex 0.10 (0.01) 2.86 (0.08) 2.96 (0.08) Hydrocoll 1.06 (0.05) 6.46 (0.21) 7.52 (0.25) Algoplaque 0.17 (0.03) 2.81 (0.09) 2.98 (0.07)

* Reproduced with permission from: A comparative study of the properties of twelve hydrocolloid dressings http://www.worldwidewounds.com/1997/july/ Thomas-Hydronet/hydronet.html.

Table 2.3 Fluid-handling characteristics of hydrocolloid dressings after 96 h*

Dressing Moisture Weight FHC vapour loss absorbed [g (s.d.)] [g (s.d.)] [g (s.d.)]

Tegasorb Thin 3.45 (0.11) 3.59 (0.48) 7.04 (0.51) Tegasorb 5.50 (0.75) 5.57 (0.66) 11.06 (0.19) Cutinova Hydro 4.04 (0.07) 2.95 (0.05) 6.99 (0.05) Askina Bioﬁ lm Transparent 0.79 (0.01) 0.19 (0.16) 0.98 (0.17) Askina Transorbent 9.89 (0.73) 0.82 (0.05) 10.72 (0.75) Comfeel Plus Plaques Biseautees 6.41 (0.31) 4.45 (0.10) 10.85 (0.31) Comfeel Plus Transparenter 9.31 (1.01) 1.45 (0.22) 10.77 (0.81) Comfeel Plus Flexibler (sample 1) 4.28 (0.75) 4.03 (0.24) 8.31 (0.77) Comfeel Plus Flexibler (sample 2) 5.67 (0.58) 3.69 (0.14) 9.36 (0.51) Varihesive E 0.91 (0.18) 3.35 (0.11) 4.25 (0.19) Granuﬂ ex 0.63 (0.47) 3.35 (0.36) 3.98 (0.13) Hydrocoll 2.19 (0.11) 5.02 (0.69) 7.19 (0.76) Algoplaque 0.44 (0.03) 3.81 (0.19) 4.25 (0.20)

* Reproduced with permission from: A comparative study of the properties of twelve hydrocolloid dressings http://www.worldwidewounds.com/1997/july/ Thomas-Hydronet/hydronet.html.

to undertake the test with the cylinder inverted so that the dressing is not in direct contact with the liquid but exposed only to moisture vapour. This is important because the permeability of some types of polyurethane ﬁ lm will increase dramatically whilst in contact with liquid but revert back to previous values when this is removed – a characteristic that has obvious and important implications for its use as a wound dressing when in contact with intact peri-wound skin.2

2.3.4 Dynamic MVTR The fl uid handling and moisture vapour transmission methods described previously, provide a single value for the amount of fl uid which is lost through the back of the dressing during the test period. Whilst this may be suitable for fi lm dressings, the permeability of which remain relatively consistent throughout the test period, the test has serious limitations for products such as hydrocolloids in which the permeability of the dressing changes with time as the adhesive mass on the wound contact surface gradually becomes hydrated. This problem can be overcome by a modifi cation to the standard test which has been described previously.5 A Paddington Cup is set up as

described and placed upon the pan of a top loading balance located in a controlled humidity cabinet which is connected to a data logger. In this way, the weight of the Paddington Cup can be continuous monitored over the test period, and, from the recorded data, the change in moisture vapour transmission rate may be determined. Depending upon the product this may be virtually zero for a number of hours, gradually increasing to reach a steady state some time during the following 24–48 hours.5,7 The weight of fl uid absorbed by the dressing during this time is determined as previously described. Using this method, it is also possible to demonstrate the marked change in permeability that occurs when a hydrophilic polyurethane membrane is brought into contact with liquid. Two similar absorbent polyurethane foam dressings with semipermeable fi lm backing layers were tested as described and after a 6-h incubation with the dressings uppermost, i.e. not in contact with the test solution, the chambers were inverted so that the test materials were allowed to become wet. The results, shown in Fig. 2.1, show a marked increase in the permeability of one product but not the other. This change in permeability has important implications for the relative fl uid-handling properties of the dressings when applied to heavily exuding wounds.

12 ) 2

4 Loss of moisture vapour (g/10 cm 2

0 024681012141618202224 Time (h) 2.1 Effect of inversion of the test chamber after 6 h on the moisture vapour transmission rate of two ﬁ lm-backed dressings.

2.3.5 Shortcomings of existing test methods

Although methods described thus far facilitate comparisons of the fl uid- handling properties of similar products within specifi c groups and provide useful quality control tests to confi rm consistency of production, they do not permit direct comparisons between different groups of dressings such as alginates and hydrocolloids. They also cannot be used to investigate the performance of the different dressings when subjected to pressure – a major factor in the treatment of venous leg ulcers where dressings are often subjected to pressures as high as 40 mm Hg. Some dressings, such as foam sheets are like bath sponges, capable of taking up large volumes of fl uid, but unable to retain this under even light pressure. Much research has therefore been devoted to maximising the performance of foam dressings by casting the foam from hydrophilic polymers and/or the inclusion of super-absorbents within the porous structure of the foam itself. These developments have resulted in the formation of a family of products that are among the most absorbent and widely used dressings available. Test systems are therefore required to compare these different dressings under varying levels of pressure in simulated clinical conditions. Over the years, numerous methods for measuring the absorbency of wound dressings have been described, most of which consist of variations on a common theme. The dressing under examination is placed on a simple wound model, which usually consists of a metal or acrylic plate with a small hole or depression in the centre. A weight is then applied to the back of the dressing to simulate the pressure applied by a bandage and test fl uid is applied to the dressing through the plate by means of a peristaltic pump or syringe driver. In some test systems, the fl uid is not actively pumped into the dressing but is absorbed by the dressing itself from some form of constant head apparatus.8 A particular example of such a test system is the ‘demand wetability’ apparatus that was developed to investigate how fi bre pre-treatments and alignment can infl uence the uptake of water.9 For a surgical dressing, the absorbent capacity is generally taken to be that volume of fl uid taken up by the time at which strike-through occurs. Strike-through is defi ned as the point at which absorbed fl uid reaches the outer surface or edge of a dressing and this may be determined in a number of different ways. In one early system devised by SMTL,10 the distribution of liquid absorbed by the dressing was monitored electronically by means of sharp steel spikes which penetrated into the body of the dressing. The disruption to the structure of the test sample caused

by the spikes cast doubt on the validity of this method and so it was abandoned. Many of the other test systems developed previously also suffered from a series of disadvantages: • They can be diffi cult to automate and therefore may not always be suitable for running unattended over extended periods. • The application of a weight and/or strike-through detector on the back of the dressing prevents the loss of moisture vapour, an important mechanism by which some dressings cope with exudate production. • If moisture vapour transmission is to be measured, it is not possible to apply a weight or strike-through detector on the back of the dressing to simulate the effect of pressure. • The use of a positive feed system for the application of test fl uid can produce anomalous results. For example, a hydrocolloid dressing that is fi rmly stuck to the acrylic sheet may appear to be capable of absorbing large volumes of test solution when in fact this is simply being trapped under the dressing in the form of a huge bubble. • Most simple test systems provide a single value for the total absorbency of a dressing with little indication of how the product performs over a specifi ed period of time (dynamic performance testing). • Many wound models are not suitable for testing hydrogels or packing materials. • They do not readily enable predictions to be made of clinical wear times of different types of dressings.

2.3.6 Development of the Wrap rig Research undertaken in the SMTL, led to the development of a new test system (called the Wrap rig), which facilitates comparison of the ﬂ uid handling properties of most types of dressings irrespective of their structure and composition even whilst under compression.

Design requirements of a test system In order to address some of the shortcomings of test methods described previously and produce a system that more closely approximated to the clinical situation, a series of key design criteria for the new model were derived previously.11 • Fluid should be provided to the test sample by some form of pump or other suitable positive ﬂ ow device. A passive uptake technique is not acceptable.

• The fl uid should not be presented to the test sample under excessive pressure. Previous test systems have, in effect, injected fl uid into the dressing under pressure, although there is no evidence that this occurs in wounds. • There must be some suitable method for controlling the temperature of the system for the duration of the test, to reproduce the environmental conditions in the wound. • The test should provide some indication of the dynamic performance of the dressing, measuring its fl uid-handling capacity profi le over time, and not just a single total absorbency fi gure. • The equipment should be capable of delivering test solution at a range of different fl ow rates, so that the effect of different rates of exudate can be examined. • The apparatus should indicate when either vertical or lateral strike- through has occurred. • The apparatus should be suitable for testing a wide range of different types of dressings to permit direct comparison of the results. Previous methods were frequently dedicated to one type of technology, such as alginates. • The equipment should be compatible with a range of different test solutions. • Where appropriate, the apparatus should permit the application of varying loads to the test samples in order to determine the effect of external pressure. • The apparatus should permit the measurement of moisture vapour transmission by the dressing as an integral part of the test. • The test should be easy to perform and provide results that can be reproduced within and between laboratories. • The equipment should not be excessively expensive to produce. • The test should, ideally, provide an indication of the wear time of a dressing in normal clinical use. In addition to the primary design criteria, a number of additional features were identifi ed, which, although not essential, are considered desirable features in a wound model. • The apparatus should withstand sterilisation or disinfection. • The apparatus should permit analysis of wound fl uid that has been in contact with the dressing to measure changes in ionic composition or changes in concentration of solutes such as proteins caused by selective absorbtion by the dressing. • The apparatus should permit microbiological examination of the local environment during the course of the test.

2.2 The Wrap rig.

• The test equipment should allow the pressure beneath the dressing to be varied if this is considered desirable. A prototype test system was developed based upon these criteria.11 Following some early validation work, some minor modiﬁ cations were made and a new version of the rig was produced; the use of this rig has been described in a subsequent publication which also clearly illustrated the effects of compression upon dressing performance.12 Essentially the apparatus consists of a number of separate components.

The wound model The Wrap rig model consists of a two-part stainless-steel plate (the ‘wound bed’), mounted on a Perspex table (Fig. 2.2). An electronically controlled heating mat beneath the steel plate keeps the plate and test sample under examination within a narrow temperature band. The central section of the plate is milled from solid stainless steel and includes a shallow circular recess bearing two ports on opposite ends of a 15-mm-long shallow channel. An inlet hole 3 mm in diameter and a outlet hole 7 mm in diameter permit the introduction and unimpaired exit of test solution. The diameter and depth of the circular recess is sufﬁ cient to accommodate two thin absorbent pads that ensure the effective transfer of liquid from the channel to the dressing above. Test ﬂ uid, introduced by means of a syringe pump, travels along the narrow channel and passes out through the second port, falling verti-

cally down through a short, wide-bore tube. This tube discharges into a receiver placed on the pan of an electronic balance. The liquid in the receiver is covered with a layer of oil to prevent loss by evaporation. The balance is connected to an electronic data capture device that records changes in the balance reading at predetermined intervals throughout the period of the test. A dressing sample typically measuring 10 cm × 10 cm is secured on to the test plate which is set to a predetermined temperature. Test fl uid applied to the test rig passes along the open channel some or all of which will be taken up by the dressing. Any unabsorbed fl uid continues to pass along the channel until it falls through the outfl ow pipe into the receiver, causing a change in the balance reading. The amount of fl uid that accumu- lates in this way is inversely proportional to the absorbency of the dressing. A highly absorbent dressing may take up all the liquid that is applied to it, while less absorbent products will absorb only for a short time or take a little while to reach maximum absorbency. During a test, therefore, the maximum weight of fl uid that can be taken up by a dressing is determined by the fl ow rate of the syringe pump. As this test system is designed to simulate the normal use of a dressing, it is important to ensure that the test conditions employed are as clinically relevant as possible, particularly in relation to the production of exudate. Previous studies have found that a heavily exuding wound typically produces around 5 ml per 10 cm2 per 24 h,13 but in the presence of infection this value can easily double.14 Although it is possible to run the test with these fl ow rates, for most tests the syringe pump is normally set to deliver a nominal 1 ml h−1 as this value provides a reasonable compromise between clinical relevance and a need to keep testing times to a minimum for practical reasons. When testing cavity wound dressings, alginate fi bre or hydrogel dressings, a simple modifi cation is made to the apparatus. A piece of stainless tube is fi xed inside the recess in the centre of the plate to form a chamber into which the dressing is placed. Although strike-through measurements are not appropriate with such dressings, it is possible to apply pressure to cavity dressings such as alginate packing by means of a weighted stainless- steel piston, which forms a sliding fi t in the tube. In all other respects, the test procedures remain the same. When testing hydrogel dressings, the open end of the chamber is sealed with a piece of aluminium foil held in place with impermeable plastic tape to prevent evaporation. The ability of this test system to differentiate between different hydrocolloid dressings is shown in Fig. 2.3. All the products, which were tested in the same way, are superfi cially similar in appearance but as the fi gure clearly indicates, marked differences exist in their ability to absorb and retain test solution.

8 7 6 5 4 3

Fluid uptake (g) 2 1 0 01020304050 Time (h) 2.3 Fluid handling properties of four hydrocolloid dressings measured using the Wrap rig.

The pressure/vertical strike-through plate As previously discussed, the application of a weight to the back of the dressing to simulate pressure produced by compression bandages has the undesirable effect of occluding the back of the dressing and preventing the transpiration of moisture vapour, an important part of the function of the dressing. This problem was overcome in this model in a novel way by combining in one piece of apparatus, the functions of strike-through plate, moisture-absorbing-unit, and pressure plate. A stainless-steel box, one surface of which consists of a coarse stainless-steel mesh supported inter- nally by pillars attached to the inner face of the other surface to impart structural rigidity, can be ﬁ lled with silica gel and when placed upon the test sample with the mesh surface downwards it fulﬁ ls three functions: • It permits the application of pressure to the dressing without occluding the outer surface. • It provides a humidity gradient across the dressing to facilitate passage of moisture vapour. • It acts as an electrical contact to detect strike-through.

Recording strike-through A method of recording strike-through was devised that proved to be relatively easy and trouble free. This consisted of a Psion Organiser II (Model XP) ﬁ tted with a Digitron Model SF10 Datalogger unit. The SF10 unit plugs into the top of the Psion II, and is a four-channel unit capable of taking up to four external probes to measure temperature, pressure, relative humidity or voltage. For the purpose of this study, a voltage probe,

capable of recording from 0–2500 mV, was used. A 1.5-V (AA sized) alkaline cell was used to supply the necessary electromotive force. Initial experience with this equipment suggested that the combination provided a simple and reproducible technique for measuring strike-through, recording values that changed instantly from 0 mV to in excess of 1000 mV when strike-through occurred. At this point, the contribution made by the loss of moisture vapour to the ﬂ uid-handling properties of the sheet dressings may be determined by measuring the change in weight of the silica gel in the strike-through plate.

Lateral strike-through detectors Lateral strike-through may be detected by the use of four brass strips, typically 90 mm × 20 mm × 3.5 mm thick, connected with short lengths of ﬂ exible cable. The under-surface of each strip is covered with a layer of insulating tape to prevent it from making electrical contact with the stainless-steel plate. In use, the strips, which are connected to the strike- through detector, can be pushed gently against the exposed edges of the dressing to detect any moisture that appears at the edge of the dressing. The new test rig is currently being evaluated by a multidisciplinary group comprising representatives from most European dressings companies, in addition to others with an interest in dressing design or performance. This group, which was originally funded by a research grant from the Engineer- ing and Physical Sciences Research Council (EPSRC), is concerned with the development and validation of methodologies for medical device evaluation.

2.3.7 Fluid-affi nity test Some hydrogel dressings, both in sheet and amorphous form, have the ability to absorb or donate liquid according to the condition of the underlying tissue. This means that they can absorb fl uid from a heavily exuding wound or donate moisture to dry or devitalised tissue to promote autolytic debridement. Test systems are therefore required which can be used to assess both these properties. The material in question is placed in contact with other gels made from varying concentrations of agar or gelatine representing a spectrum of tissue types, and the transfer of liquid to or from the test sample to the standard gels is measured by recording any change in weight of the sample. A fairly basic version of this technique was described in three early publications15–17 but this was gradually refi ned and used as the basis of a standard method described in BS EN 13726-1.

2.4 Low-adherence tests 2.4.1 Importance of low-adherence The results of a international survey,18 identifi ed that pain and trauma were ranked as the most important factors to consider when changing dressings. A total of 3918 clinicians who responded to a written questionnaire, considered that pain was most commonly associated with dressings drying out or adhering to the wound bed, factors that were also considered to be responsible for wound trauma. Some products like alginates, hydrogels or silicone products show little propensity to adhere to granulating wounds whilst other such as simple gauze or non-woven fabrics perform particularly badly in this context. It is generally believed that there are two mechanisms of adherence. The fi rst is the inherent ‘stickness’ of serum which acts like a simple adhesive that forms a bond between two opposing surfaces. The second mechanism is a little more complex, involving the penetration into the dressing of serous fl uid containing cellular debris which dries to form a solid ‘scab’ on the wound surface but which also incorporates some of the structural elements of the dressing which acts like reinforcing bars in concrete. As a result when the dressing is removed the scab, together with underlying new epithelium, is forcibly removed leading to rewounding and delayed healing. Many dressings have therefore been developed with a wound- contact surface that is designed specifi cally to reduce adherence, examples of which have been described previously.19

2.4.2 Measurement of adherence potential Predicting the way in which dressings will perform clinically in terms of adherence has proved unexpectedly diffi cult, as no standard laboratory test system has been described for this purpose. In 1982, a method was devised by SMTL in which a cold-cure silicone rubber material was applied onto the surface of a test sample using a plastic former to control the area of application.10 Once the rubber had set, the dressing was removed from the former containing the silicone block by means of a tensiometer to record the applied force, using a 180° peel. Although no absolute values could be applied as limits, the test system was used to rank products in order of their adherence potential. In a later modifi cation to this test, an aqueous solution of gelatine was used to replace the silicone as this more closely represents the in vivo situation. This procedure is still used as a non-offi cial ‘in-house’ test.

2.5 Conformability tests

Dressings applied around joints or to other areas of tissue that are subject to movement or distortion must, to some degree, accommodate this movement without causing excess pressure, or in the case of adhesive products, shearing forces that can cause skin trauma. Whilst products such as bandages tend to accommodate changes in body geometry fairly readily, products such as hydrocolloids, semipermeable fi lm dressings and self-adhesive island dressings can sometimes cause clinical problems.20 A test designed to assess the extensibility and permanent set conformability of primary wound dressings by measuring its extensibility and permanent set is described in BS EN 13726-4. In this test, strips of dressing 25 mm wide with an effective test length of 100 mm are extended by 20% in a constant rate of traverse machine at 300 mm min−1. The maximum load is recorded and the sample is held in this position for 1 min. It is then allowed to relax for 300 s before being remeasured. Further samples taken from a direction perpendicular to the fi rst are also tested in a similar way to account for ‘directionality’ in structure of the material. The standard requires that the test report records the maximum load, the extensibility and the permanent set which are calculated using the formu- lae provided. An alternative test method, which eliminates the problems of directionality by extending a sample in all directions at once has been developed using a modifi cation of the Apparatus for the Measurement of Waterproofness described in the BP 1993. This consists of a chamber, open at one end, bearing a fl ange with an internal diameter of 50 mm. A retaining ring with the same internal diameter as the hole in the fl ange is mounted over the open end of the cylinder which can be lowered down onto the fl ange by means of a screw thread. A sample of the dressing under examination is placed on the fl ange and held fi rmly in place by means of the retaining ring. During the course of this test, air is slowly forced into the chamber by means of a large syringe. The resultant rise in the pressure within the chamber causes the dressing to expand and form a hemisphere which gradually increases in size until the upper surface of the dressing comes into contact with a marker placed 20 mm above the dressing surface at the start of the test. This pressure reading is then recorded by means of a transducer. In this test, the conformability of a dressing is considered to be inversely proportional to the pressure required to deform it by a predetermined amount and is represented by the mean infl ation pressure of the samples examined. Results of the test performed on a range of hydrocolloid dressings are shown in Table 2.4.

Table 2.4 Conformability of hydrocolloid dressings*

Dressing Mean inﬂ ation pressure [mm Hg (s.d.)]

Tegasorb Thin 107 (4.6) Tegasorb 187 (32.0) Cutinova Hydro 105 (3.7) Askina Bioﬁ lm Transparent 103 (5.7) Askina Transorbent † Comfeel Plus Plaques Biseautees 164 (11.8) Comfeel Plus Transparenter ‡ Comfeel Plus Flexibler (sample 1) 166 (13.0) Comfeel Plus Flexibler (sample 2) 161 (8.9) Varihesive E 156 (4.6) Granuﬂ ex 162 (5.3) Hydrocoll 194 (20.4) Algoplaque 154 (17.3)

* Reproduced with permission from: A comparative study of the properties of twelve hydrocolloid dressings http://www.worldwidewounds.com/1997/july/ Thomas-Hydronet/hydronet.html. † Dressing is permeable to air and therefore could not be tested. ‡ Dimensions of dressing were too small for testing.

2.6 Microbiological tests 2.6.1 Bacterial barrier properties Wounds of all types represent a potential source of cross-infection, particularly if they are infected with antibiotic resistant organisms such as MRSA. It is therefore important to ensure that wounds, in so far as is possible, are isolated from the environment to prevent the ingress or egress of pathogenic micro-organisms. Many dressings therefore consist of (or include in their construction) a layer which prevents the transmission of micro-organisms into or out of a wound. Most commonly, this layer consists of a piece of cast polyurethane ﬁ lm but sometimes closed cell foams are used for this purpose. The ability of a bacterium to pass through a dressing is determined by the presence of a liquid pathway. For dry wounds, a thick layer of absorbent cotton or gauze may be sufﬁ cient to prevent contamination, but, as soon as this becomes wet, the barrier properties are lost and the dressing becomes useless in this regard. It follows, therefore, that a test system for dressings designed to provide an effective bacterial barrier is required which provides the ‘worst-case’ scenario, and once again a method has been devised within SMTL that has gained widespread acceptance.

A sterile dressing is aseptically clamped between two sterile ﬂ anged hemispheres (closures of reactions vessels are typically used) such that the dressing is maintained in the vertical plain. A microbiological liquid nutrient medium is introduced into both chambers one side of which contains a heavy inoculum of a suitable test organism. The apparatus is then incubated for an appropriate period after which the chamber containing the previously sterile nutrient solution is examined for evidence of growth. If no growth is visible, the dressing is considered to represent an effective bacterial barrier but, if growth is detected, samples are plated out to conﬁ rm that it is the test organism not extraneous contamination from a failure in aseptic technique. This test is currently being evaluated by the industry before being formally proposed as a new European Standard.

2.6.2 Antibacterial properties Some dressings contain agents that have intrinsic antimicrobial activity such as antibiotics, antiseptics, silver ions or materials which possess a sig- niﬁ cant osmotic pressure capable of inhibiting bacterial growth. In clinical practice, these materials are released from the dressing to exert an antibacterial effect. Such dressings are often recommended or promoted for the treatment or prevention of soft tissue infections. Other products contain antimicrobials that are immobilised or ﬁ xed within the structure or the wound contact surface of the dressing, i.e. not released into the local wound environment. These materials are claimed simply to prevent the proliferation of micro-organisms within the dressing itself. They have no direct effect upon the wound and as such are more suited for preventing cross- infection than for treating existing wound infections.

Tests for immobilised antimicrobial agents A number of methods have been described for evaluating the antimicrobial properties of immobilised antimicrobial agents. A quantitative procedure for the evaluation of the degree of antibacterial activity of ﬁ nishes on textile material has been published by the American Association of Textile Chemists and Colorists.21 According to this method, swatches of test and control material are inoculated with the test organisms using sufﬁ cient test material to absorb the total volume of inoculum (1 ± 0.1 ml) leaving no free liquid. After incubation, the samples are eluted with a suitable extractant containing a neutralising agent and the number of viable organisms present in this solution is determined using standard microbiological techniques.

Whilst this method may be appropriate for testing a homogeneous mass of material, it is not suitable for evaluating a structured product such as a dressing that has several different components of which only one, the wound contact layer for example, may have antimicrobial activity. Any bacterial suspension that is taken up by the non-medicated part of the dressing will remain unaffected by the antibacterial fi nish on the medicated portion leading to anomalous results. This defi ciency in the method is noted in a later test published by ASTM International.21 This describes an alternative method that ensures good contact between the bacteria and treated substrate by constant agitation of the test specimen in a bacterial suspension for a specifi ed contact time. The suspension is then serially diluted and the number of remaining viable organisms is determined and compared with values obtained using an appropriate control or untreated sample. This method also includes a procedure to ensure that the antimicrobial activity detected is not caused by leaching of the active ingredient as follows. An extract of the dressing is placed into an 8-mm hole in an agar plate inoculated with a test organism and the presence of any zone of inhibition around the well indicates the presence of leaching, thus rendering the test invalid. An alternative test, devised within the SMTL, graphically demonstrates the ability of a dressing to kill or inhibit the growth of micro-organisms that come into contact with it and thereby prevent the transfer of contaminated material into or out of a wound. In this test an agar plate has two channels cut out of it as to effectively form two separate agar areas in the Petri dish. One of the agar blocks is sterile; the other is inoculated with the test organism. A strip of dressing under examination is placed on top of the two blocks forming a bridge. Sterile water is place in the channel on the outer side of the contaminated agar to increase the water content of the gel and provide a ‘driving force’ to encourage the movement of moisture from the contaminated agar along or through the dressing to the sterile agar on the other side of the second channel. The Petri dish is incubated as normal with the dressings in place after which it is examined to detect the presence of growth around the margin of the test sample on the sterile agar surface. This test determines the ability of bacteria to survive on the dressing surface and migrate along it from the contaminated agar to the sterile agar. A positive result in this test suggests that it is possible that micro-organisms could be transported laterally out of a contaminated wound onto the surrounding skin, or potentially move in the opposite direction from the intact skin into the wound itself. No growth on the sterile agar suggests that the dressing does indeed have the ability to kill or prevent the growth of bacteria that come into contact with it.

Tests for antimicrobial agents released from dressings For products that are designed to kill bacteria within the wound, alternative test systems are required based upon the following considerations. The ability of an antimicrobial dressing to exert a beneﬁ cial clinical effect is dependent upon three factors:

• The nature and spectrum of activity of the agent concerned • The concentration of the material present in the dressing • The release characteristics (does the material actually get into the wound?).

In its simplest form, the test can consist of the application of a piece of dressing applied to an agar plate that has previously been seeded with the test organism. Antibacterial activity is indicated by the presence of a halo or zone of inhibition around the sample, the size of which is determined at least in part by the concentration and solubility of the active ingredient. Such tests are easy to perform and can involve the use of different test organisms.22–25 A possible criticism of this type of simple test is that the moisture content of the agar may be insuffi cient to facilitate extraction of the active agent from the dressing, or that the affi nity of the dressing for moisture may be such that it effectively retains the moisture within its structure and thus limits the amount that is released onto the agar to exert an inhibitory effect. It is also possible that the active agent may require the presence of sodium or calcium ions normally present in exudate or serous fl uid to release or activate the biocidal agent within the dressing. Both of these problems may be overcome by a modifi cation to the method in which a well is cut in the agar plate into which the dressing sample is inserted. The residual volume within the well is then fi lled with Solution A (used in absorbency testing) or, for research purposes, calf or horse serum. Alternatively, it is possible to make an extract of the dressing sample using an appropriate solution and place this into the well. A further test involves the incubation of a piece of dressing with a suitable volume of a bacterial suspension containing a known number of micro-organims. Following incubation, the dressing sample is extracted with an appropriate recovery medium and a total viable count performed on the extractant to determine the decrease in the number of viable organisms present. This test has the advantages that it can be conducted over various time intervals, and that it also provides a quantitative result. The tests have been described in detail previously in laboratory-based comparisons of silver containing dressings.26,27 Unlike previous test systems, which gained fairly rapid acceptance by the industry enabling them to become adopted as offi cial standards, it is

more difﬁ cult to reach a consensus on tests for antimicrobial activity. Whilst there is a reasonable prospect of achieving agreement on the experimental techniques which can be used, there remains a problem in the interpretation of the results, the levels of activity that are required clinically, and, therefore, the limits which should be attached to each method.28–31

2.7 Odour control tests Certain types of wounds such as pressure ulcers, leg ulcers and fungating (cancerous) lesions produce noxious odours which, even in moderate cases, can cause signiﬁ cant distress or embarrassment to a patient and their relatives.32

2.7.1 Causes of wound odour The smell from a wound is caused by a cocktail of volatile agents that includes short-chain organic acids (n-butyric, n-valeric, n-caproic, n- heptanoic and n-caprylic) produced by anaerobic bacteria,33 together with a mixture of amines and diamines such as cadaverine and putrescine that are produced by the metabolic processes of other proteolytic bacteria. Organisms frequently isolated from malodorous wounds include anaerobes such as Bacteroides and Clostridium species, and numerous aerobic bacteria including Proteus, Klebsiella and Pseudomonas spp.

2.7.2 Management of wound odour The most effective way of dealing with malodorous wounds is to prevent or eradicate the infection responsible for the odour but, if for some reason this is not possible, it may be necessary to address the problem by some other means. In 1976, Butcher et al.,34 described the use of a charcoal cloth produced by carbonising a suitable cellulose fabric by heating it under carefully controlled conditions. The small pores formed in the surface of the ﬁ bres greatly increase their effective surface area and, hence, their ability to remove unpleasant smells, as it is believed that the molecules that are responsible for the production of the odour are attracted to the surface of the carbon and are held there (adsorbed) by electrical forces. Since 1976, a number of odour-absorbing dressings containing activated charcoal have been produced commercially, some of which are intended to be placed in direct contact with the wound whilst other products are designed as secondary dressings which are placed over a primary dressing, but beneath the retaining dressing or bandage.

2.7.3 Testing odour absorbing dressings Despite the relatively widespread use of odour-absorbing dressings, however, little objective comparative data is available on their odour- and fl uid-handling characteristics. A number of possible test systems have been described,35–37 but some of these have only considered the effi cacy of the dressing in the dry state, the fl uid-handling properties of the dressing being considered separately.38 This may be important because the presence of liquids, particularly those containing organic solutes, may have implications for the performance of the activated charcoal, competing for active sites with the molecules responsible for the odour and thus reducing its effectiveness. Previous studies have used both chemical,35–37 and biological materi- als39,40 to test the effi cacy of odour-adsorbing dressings. The former technique is often favoured as the effi ciency of the dressing can be determined using standard analytical techniques such as gas–liquid chromatography. Determination of dressing performance using biological materials is currently restricted to more subjective methods of assessment such as the use of a human test panel. Lawrence et al.,40 adopted this second approach when they compared the odour-absorbing properties of fi ve dressings containing activated charcoal with that of a cotton gauze swab, acting as a control. A more objective test system that could be used to compare the ability of different dressings to prevent the passage of a volatile amine when applied to a wound model under simulated ‘in-use’ conditions was devised by SMTL. This apparatus consists of a stainless-steel plate bearing a central recess (50 mm diameter × 3 mm deep) into which is inset a removable perforated stainless-steel disc and a disposable Millipore pre-fi lter. Fitted to the stainless-steel plate is an airtight Perspex chamber with inlet and outlet ports (Fig. 2.4). The dressing under examination is placed over the recess and sealed around the edges with impermeable plastic adhesive tape.

2.4 Apparatus for testing odour-absorbing properties of wound dressings.

The test solution, consisting of Solution A to which is added 2% diethylamine and 10% newborn bovine serum, is applied to the dressing through the perforated plate at a rate of 30 ml h−1 by means of a syringe pump. The concentration of diethylamine in the air in the perspex chamber is constantly monitored using a Miran 1B2 portable ambient analyser (Quantitech Ltd, Milton Keynes) and the measured values recorded electronically using a datalogger. The test is continued until the concentration of diethylamine present in the air above the dressing has risen to approximately 15 ppm. By relating the ﬂ ow rate of test solution to the rate at which wounds normally exude, it is possible to use the apparatus to estimate the useful life of a dressing in the clinical environment. The test system was evaluated in an independent study which compared the odour-absorbing properties of eight different dressings.41 The authors concluded that although there were still shortcomings associated with the use of the technique related to the application and orientation of the test sample, it appears to be the best way of objectively ascertaining quantitative comparable data on the odour-absorbing properties of different dressing products.

2.8 Biological tests Because dressings come into intimate contact with damaged tissue, blood or body ﬂ uids, it is important to ensure that they are free from any agents that can adversely effect wound healing or otherwise cause an adverse reaction within the wound. The standard approach to testing medical devices is described in BS EN ISO 10993. This standard is divided into 18 parts, each of which describes a particular type of test or procedure that may be relevant to speciﬁ c types of medical devices. For topical wound dressings the most relevant parts are as follows.

2.8.1 Part 5: Tests for in vitro cytotoxicity This part of the standard42 describes test methods to assess the in vitro cytotoxicity of materials using techniques in which cultured cells (typically L-929 mouse ﬁ broblasts) are either exposed to an extract of the test sample, or brought into intimate contact with the sample itself using an agar diffusion or ﬁ lter diffusion method. Cytotoxicity is graded on a four- point scale from non-toxic to severely cytotoxic according to the damage occasioned to the cell system used.

2.8.2 Part 10: Tests for irritation and sensitisation This part of the standard43 describes a technique in which extracts of the dressing are injected subcutaneously into multiple sites on the backs of

rabbits following which the injection sites are examined visually for evidence of irritation (erythema and oedema) immediately and after 24, 48 and 72 h. The sensitisation potential is determined by intradermally injecting and occlusively patch testing multiple sites on 10 guinea pigs. The treated sites are examined visually for evidence of a skin reaction after 24, 48 and 72 h.

2.9 References 1. thomas s. Wound management and dressings. London: Pharmaceutical Press, 1990. 2. thomas s. Fluid handling properties of Allevyn Dressing. Wound management communications 2007; http://www.medetec.co.uk/Documents/ Fluid%20handling%20properties%20of%20Allevyn%20foam%20dressing 24-4-07.pdf. 3. BS EN 13726-1:2002 Test methods for primary wound dressings. Part 1: Aspects of absorbency. London: BSI, 2002. 4. thomas s. Observations on the fl uid handling properties of alginate dressings. Pharm. J 1992; 248:850–851. 5. thomas s, loveless p. A comparative study of the properties of twelve hydrocolloid dressings. World Wide Wounds 1997; http://www.worldwidewounds. com/1997/july/Thomas-Hydronet/hydronet.html. 6. BS EN 13726-2:2002 Test methods for primary wound dressings. Part 2: Moisture vapour transmission rate of permeable fi lm dressings. London: BSI, 2002. 7. thomas s, loveless p. Moisture vapour permeability of hydrocolloid dressings. Pharm J 1988; 241:806. 8. williams aa. Profi ciency of contemporary wound dressings. In: Turner TD, Brain KR, eds. Surgical dressings in the hospital environment. Cardiff: Surgical Dressings Research Unit, UWIST, Cardiff, 1975; 163–179. 9. harrison g, d’silva ap, horrocks ar, rhodes d. An apparatus to measure the water absorption properties of fabrics and fi bre assemblies. Med Text 1997; 96:134–140. 10. thomas s, dawes yc, ha np. Wound dressing materials – testing and control. Pharm J 1982; 228:576–578. 11. thomas s, fram p. The development of a novel technique for predicting the exudate handling properties of modern wound dressings, 2001. 12. thomas s, fram p, phillips p. The importance of compression on dressing performance. World Wide Wounds 2007; http://www.worldwidewounds.com/2007/ November/Thomas-Fram-Phillips/Thomas-Fram-Phillips-Compression- WRAP.html. 13. lamke lo, nilsson ge, reithner hl. The evaporative water loss from burns and water vapour permeability of grafts and artifi cial membranes used in the treatment of burns. Burns 1977; 3:159–165. 14. thomas s, fear m, humphreys j, disley l, waring mj. The effect of dressings on the production of exudate from venous leg ulcers. Wounds 1996;8(5): 145–149. 15. thomas s, hay np. Assessing the hydro-affi nity of hydrogel dressings. J Wound Care 1994; 3(2):89–92.

16. thomas s, hay p. Fluid handling properties of hydrogel dressings. Ostomy Wound Manage 1995; 41(3):54–56, 58–59. 17. thomas s, hay np. In vitro investigations of a new hydrogel dressing. J Wound Care 1996; 5(3):130–131. 18. moffatt c. The principles of assessment prior to compression therapy. J Wound Care 1998; 7(7):suppl 6–9. 19. thomas s. Low-adherence dressings. J Wound Care 1994; 3(1):27–30. 20. ravenscroft mj, harker j, buch ka. A prospective, randomised, controlled trial comparing wound dressings used in hip and knee surgery: Aquacel and Tegaderm versus Cutiplast. Ann R Coll Surg Engl 2006; 88(1):18–22. 21. ASTM E 2149-101 Standard Test Method for determining the antimicrobial activity of immobilised antimicrobial agents under dynamic contact conditions. Pennsylvania: ASTM International, 2001. 22. AATCC TEST METHOD 147-1998 Antibacterial Assessment of Textile Materials: Parallel Streak Method. North Carolina: American Association of Textile Chemists and Colorists, 1999. 23. thomas s, russell ad. An in vitro evaluation of Bactigras, a tulle dressing containing chlorhexidine. Microbios Lett 1976; 2:169–177. 24. thomas s. An experimental evaluation of a chlorhexidine medicated tulle gras dressing (letter). J Hosp Infect 1982; 3:399–400. 25. thomas s, dawes c,y ha np. Improvements in medicated tulle dressings. J Hosp Infect 1983; 4:391–398. 26. thomas s, mccubbin p. A comparison of the antimicrobial effects of four silver-containing dressings on three organisms. J Wound Care 2003; 12(3): 101–107. 27. thomas s, mccubbin p. An in vitro analysis of the antimicrobial properties of 10 silver-containing dressings. J Wound Care 2003; 12(8):305–308. 28. lansdown ab, jensen k, jensen mq. Contreet Foam and Contreet Hydrocol- loid: an insight into two new silver-containing dressings. J Wound Care 2003; 12(6):205–210. 29. lansdown ab. Silver-containing dressings: have we got the full picture? J Wound Care 2003; 12(8):317; author reply 317–318. 30. thomas s, mccubbin p. Silver dressings: the debate continues. J Wound Care 2003; 12(10):420. 31. thomas s, ashman P. In vitro testing of silver containing dressings. J Wound Care 2004; 13(9):392–393. 32. van toller s . Psychological consquences arising from the malodours produced by skin ulcers. Proceedings of 2nd European Conference on Advances in Wound Management 1992, Harrogate: 70–71. 33. moss cw, dees sb, guerrant go. Gas chromatography of bacterial fatty acids with a fused silica capillary column. J Clin Microbiol 1974; 28:80–85. 34. butcher g, butcher ja, maggs fap. The treatment of malodorous wounds. Nurs Mirror 1976; 142:76. 35. schmidt rj, shrestha t, turner td. An assay procedure to compare sorptive capacities of activated carbon dressings: the detection of impregnation with silver. J Pharm Pharmacol 1988; 40:662–664. 36. shaw rj, maher ga. A test method for quantifying absorptive capacities of intact charcoal-containing wound management products. Proceedings of the

7th European Conference on Advances in Wound Management, Harrogate 1977. 37. griffi ths b, jacques ej, jones sa. Determination of malodour reduction performance in various charcoal containing dressings. Proceedings of the 7th European Conference on Advances in Wound Management, Harrogate, UK 1997, Harrogate, UK. 38. griffi ths b, jacques ej, waring mj, bowler p, bishop s. Investigation into the fl uid-handling characteristics of various charcoal containing dressings. Proceedings of the 7th European Conference on Advances in Wound Manage- ment, Harrogate, UK 1997, Harrogate, UK. 39. myles vm, griffi ths b, bishop s. An investigation into the selective adsorption of malodour molecules onto charcoal containing dressings. Proceedings of the 7th European Conference on Advances in Wound Management, Harrogate, UK 1997, Harrogate, UK. 40. lawrence c, lilly ha, kidson a. Malodour and dressings containing active charcoal. Proceedings of the 2nd European Conference on Advances in Wound Management 1992, Harrogate, UK: 70–71. 41. lee g, anand sc, rajendran s, walker i. Effi cacy of commercial dressings in managing malodorous wounds. Br J Nurs 2007; 16(6):S14, S16, S18–20. 42. bs en iso 10993-5:2003 Biological evaluation of medical devices. Test methods for primary wound dressings. Part 5: Tests for in vitro cytotoxicity. London: BSI, 1999. 43 bs en iso 10993-5:2003 Biological evaluation of medical devices. Part 10: Tests for irritation and sensitization. London: BSI, 1999.

B. S. GU P TA, North Carolina State University, USA, and J. V. EDWARDS, United States Department of Agriculture – Agricultural Research Service, USA

Abstract: A thorough understanding of the healing mechanism and dressing requirements for different types of wound repair is necessary before optimum products based on textiles could be engineered. In this chapter, ﬁ rst a general overview of the wounds and their healing, and of the available dressings and bandages, is presented. This is followed by a detailed discussion of the polymer and ﬁ ber materials of which the dressings are composed and of the textile processes that are used in forming the wound care products.

Key words: wounds, wound healing strategies, dressings, bandages, dressing polymers and ﬁ bers, textile processes.

3.1 Introduction In a human’s fast-paced life, characterized by external hazards and physiological neglects, physical injury of one or the other form is a commonly encountered event. It must be attended to in order to protect body health, function and appearance. The primary procedure used is the application of a dressing, which, in the main, protects the site against external assaults and aids in generating the needed physiological environment for efﬁ cient repair. The dressing can be as simple as a strip of plain textile, or as complex as an engineered composite that contains layers of different geom- etries and reactive materials, including medicines. The type of dressing depends on the type and the condition of the wound. Wound repair may be simple and inconsequential and performed by the patient itself or it may be complex enough to warrant surgery and hospitalization. An effective treatment requires a thorough understanding of wound types and healing mechanisms and knowledge of the interventions that are available and would ideally assist in the repair process. A major increase in the understanding of the requirements for a dressing emerged in the 1970s when the pioneering work of Winter1 showed that the wounds healed faster and more satisfactorily if the environment at the site was kept moist. Until then, the primary function of a dressing was

considered as one or more of absorbing the exudate, keeping the wound dry, and protecting it against external pathogens as well as further injury. The structure was also required to be comfortable. With the progress already made in terms of the understanding of the healing steps and requirements for different types of wounds, a wide variety of dressings are now available. A minor wound may be defi ned as one that is not chronic and also not seriously acute. However, if not attended to properly, the same wound could become infected, enlarged and acute enough to require advanced dressing and medical procedure. The fi rst line of treatment of a minor wound is a passive dressing, which is product that is textile in origin and may be made by a weaving, knitting or non-woven process. The dressing can also be a solid fi lm, cast directly from a polymer. If there is bleeding and the potential for swelling, a bandage may be required in conjunction with absorbent gauze in order to exert transverse pressure and control both hemorrhage and edema. Like the passive dressings, the bandages are also primarily textile structures that are made by one of the fabric-forming technologies. In this chapter, after a general review of wounds, healing, and dressing requirements, a brief description of the various types of dressings and the bandages available to choose from is given. A more in-depth coverage of some of the advanced dressings, touched on in the chapter, can, however, be found in other chapters of the book. In the second half of this chapter, a detailed discussion of the fi ber materials and the textile structures used in producing the dressing and bandage products is presented.

3.2 The role of wound dressings Skin is the largest organ of the body and has multiple components and, therefore, functions: the epidermis, which is the outer layer and composed of dead cells, is hydrophobic and responsible for protecting against the environment;2 the dermis, or the middle layer, which is made up of living cells with a network of blood vessels and nerves, is responsible for register- ing external stimulus, i.e. touch/feel, and thermal regulation of the enclosed body; and the subcutaneous layer, which is mainly made up of fat, is responsible for insulating the body against shock. The cells on the surface are constantly replaced by those below, causing the top layer to slough off. The repair of an epithelial wound then is essentially a scaling up of this process by the use of interventions. Much has been learnt during the past half a century about wounds, the healing process and the nature of the products required to successfully treat the lesion. A wet environment, composed of isotonic saline and wound ﬂ uids, has been shown experimen- tally to be most favorable for rapid healing of wounds. The inﬂ ammatory and proliferative phases of dermal repair in healing are also accelerated.3

It was realized that bacteria did not generate in the wound but were acquired from an external source; thus simply washing with soap and water alleviated much of the risk of infection. Use of a protective covering for- mulated the simple but effective aseptic means for the treatment. An essential part of any wound management is wound dressing, important considerations for which are the extent to which it restricts evaporation of water from the wound surface, buffers pain and trauma, manages exudates and protects against bacterial invasion. Although wound healing which is the stated part of a wound management protocol has been described in recorded history, our understanding of its basic principles has grown more in the past half century than in the preceding two millennia.4 The recent outstanding growth in our knowledge about healing is highly promising and has already led to introduction of new and exciting concepts, novel therapeutic modalities, and innovative wound management products. As new materials are discovered, new dressings emerge which promise to play an active role in modifying healing of all types of wounds.5,6 As the products become more sophisticated, they also tend to become more ‘wound-speciﬁ c’. In order to select an optimum treatment for a wound and for different stages during healing, it is important that we understand the types of wounds we encounter, the sequence of events that take place during repair, and the treatment options we have available to choose from.

3.3 Categorization of wounds Wounds have been categorized in many different ways, but they all refl ect commonly the differences in the required treatment, and the expected time and prospects of healing. The classifi cation of wounds recognizes the type of injury (blunt contusion, sharp laceration, thermal, chemical, etc.), the extent of tissue loss, and the presence of infection, foreign bodies, and underlying structural injuries (fracture of bone, exposure of vital parts such as tendons and blood vessels). A general classifi cation of wounds is as follows: 1. Wounds with no tissue loss. 2. Wounds with tissue loss. This generally includes three types of wounds; those that are (a) caused by burning, trauma or abrasion; (b) the result of secondary events involving chronic ailments, as, for example, venous statis, diabetic ulcers, and pressure sores; or (c) induced as a part of the treatment of the wound itself, as, for example, the wound arising at the donor site for skin grafting, or the wound due to derma-abrasion.

From the standpoint of the extent of injury, a wound may also be classiﬁ ed in terms of the layers involved: 1. The superﬁ cial wounds, involving only the epidermis. 2. The partial thickness wounds, involving also the dermis. 3. The full thickness wounds, involving, additionally, the subcutaneous fat or deeper tissue. Most wounds can also be broadly characterized as acute and chronic. In the former type, that may or may not have tissue loss, healing tends to proceed through a timely and orderly reparative process. In the chronic wounds, on the other hand, healing has failed to proceed through this process or it has proceeded without establishing a sustainable anatomical and functional result. The chronic wounds are classically subdivided into venous statis ulcers, pressure ulcers, and diabetic ulcers. To a lesser extent, the traumatic wound with extensive cutaneous loss that has not been replaced for some reasons also may fall in the category of chronic wounds.4,7

3.4 Minor wounds A wound that has no tissue loss and is not chronic can be classifi ed as minor with high prospects of healing with minimum scarring. Minor wounds often occur as the result of unanticipated trauma and may include injuries, such as lacerations, abrasions and blisters, and more serious wounds such as skin tears and bites. In many instances, such as superfi cial wounds, the skin may only require protection from further injury and can be treated at home with due regard given to the possibility that infection may be present or could arise. Infection is usually one of the biggest risks for minor traumatic wounds. A visual check for the presence of foreign material, its removal and careful cleansing may precede the application of a wound dressing. If, on the other hand, the wound is deep, as for example caused by penetration, then the possibility that an underlying structure may have been damaged needs to be considered. Some of the causes of minor wounds are:8 • Lacerations. This wound occurs when soft body tissue has become torn and it is often irregularly shaped and jagged. It is highly common for this type of wound to be contaminated with debris and or bacteria by the object that caused it. • Abrasions or grazes. These are more exactly superfi cial wounds in which the top layer of the skin is damaged or removed, e.g. by the skin sliding across a rough surface. Small blood vessels may become visible and bleed. These injuries often contain dirt and gravel. Abrasions are

considered the most common type of wound, and perhaps the least dangerous.9 • Blisters. These are usually the result of friction between the top two layers of the skin. Puncturing the blister, draining the fl uid and removing the top layer often allow the area to heal more quickly. In many cases, the blister will burst of its own accord. In both instances, a protective dressing is required. • Cut (incision). Such wounds usually have clean edges which are the result of surgery, or injury caused by a sharp-edged object. Since blood vessels are cut straight across, there can be profuse bleeding. Among all types of wounds, incisions are the least likely to become infected, because the abundance of fl owing blood serves to protect against pathogens fi nding their way in. • Puncture. A puncture wound typically occurs when the skin is pierced by a cylindrical object such as a needle or a nail. These wounds can be dangerous as one cannot easily identify the depth to which the puncture has reached; they can be particularly dangerous if the wound is located on the abdomen or thorax.9 With this type of wound, bleeding will occur, in a similar way to the wound caused by a knife. Figure 3.1 depicts the difference between a laceration and a puncture wound.10 • Penetration. A penetration is a type of puncture but the damage is deeper such as it happens when a knife or bullet enters the body. • Bites. These may be human or animal and are of special concern, especially if caused by an animal, as bacteria from the mouth can enter and

(a) (b) 3.1 Wound types: a) laceration, b) puncture.9

result in an increased risk of tetanus and infection. Most animal bites are sustained from pets, usually dogs, and can cause abrasions, deep scratches, and lacerations as well as puncture wounds. Cat bites are considered more serious due to the high incidence of infection.

3.4.1 General treatment strategy for minor wounds Cleaning the wound and the surrounding skin is usually the ﬁ rst stage in treating a minor wound. This step removes debris and other foreign material which, if left, could cause infection. Abrasions require thorough irrigation as dirt is frequently embedded in the ruptured skin. An antiseptic solution may be used to cleanse the wound. Clean surgical wounds that have been sutured simply require the cleaning of old blood before the application of a dry dressing. In some cases it may be necessary to debride the wound before proceeding; in others, repair to underlying structures may need to be addressed before applying a dressing. Wounds greater than 6–8 h old have an increased risk of infection. In all cases of traumatic injuries, the patient’s tetanus status needs to be assessed for coverage. Following this, an assessment of the wound in terms of the location, size and depth and any additional trauma to the underlying structures needs to be determined. Animal bites need to be monitored for 24–48 h for signs of infection. After thorough cleansing and assessment, a choice of dressing is then made, which may be a simple low-adherent or an advanced multi-layer composite to not only protect the wound but also to absorb blood or exudates and keep the wound moist. A detailed discussion of dressings is given later in this chapter.

3.5 Healing mechanisms The various phases involved in wound healing are seen in all wounds irrespective of whether it is a carefully opposed, clean, incised, wound that is healed by primary intention or one that is not opposed, has tissue loss, or is infected and exuding and is healed by secondary intention. The length of each phase varies with wound type and extent, and can be manipulated to promote or delay healing. The body’s natural healing process can be broken down into several steps: hemostasis, inﬂ ammation, proliferation, and maturation or remodeling. Immediately after injury, the platelets from the severed blood vessels begin to aggregate and form a platelet plug. This reduces bleeding. A clot forms in the opening of the wound, which dehydrates in contact with the air and forms a scab. In the second phase, neutrophils, monocytes and macrophages emerge, which tend to demolish or debride any devitalized

tissue and foreign bodies present, such as bacteria. The phagocytes act to clear debris and destroy the ingested material.11 In the third stage, new vessels are formed which carry the oxygenated blood to the site bed. The fi broblast cells lay down a network of collagen fi bers surrounding the neo-vasculature of the wound. In the fi nal stage, the process of remodeling of the collagen fi bers laid down in the proliferation phase occurs, and this may take a long time (Fig. 3.2).12 A problem arises in the last steps of the healing process for large or cavity wounds as the body is not able to completely seal the site with a scab-like formation. The cavity must be plugged with an appropriate dressing to assist in the process. Depending on the type of wound, healing treatment is considered to be by one of three processes: primary, secondary, or tertiary (often called delayed primary).6 Healing by primary intention occurs in most surgical

Skin surface Red blood cell Fibrin Wound Platelet

PMN Epidermis and dermis of skin Macrophage

(a) Injury (b) Coagulation

TGF-β PDGF

Macrophage PMN

(d) Late inflammation (48 h) (c) Early inflammation (24 h)

Collagen Fibroblast

(e) Proliferation (72 h) (f) Remodelling (weeks to months) 3.2 Phases of cutaneous wound healing (a) injury, (b) coagulation, (c) early inﬂ ammation (24 h), (d) late inﬂ ammation (48 h), (e) proliferation (72 h), (f) remodelling (weeks to months).12

wounds in which the edges have been adequately approximated. Such wounds are usually clean and heal rapidly. Large wounds with signiﬁ cant tissue loss are allowed to heal by secondary intention. Such wounds are often encountered following massive trauma, surgical ablation of large tumors, or deep burns. Tertiary or, more appropriately, delayed primary healing is induced by reconstruction using skin grafts or ﬂ aps. Tissue gaps and poorly approximated edges will ultimately heal by secondary intention accomplished by re-epithelialization from the wound edges and mostly by wound contraction. Clearly, primary or delayed primary wound healings are by far preferable and superior to secondary healing.

3.6 Wound dressings 3.6.1 Historical The history of surgical wound management indicates how, with research and understanding of wounds and their healing, dressings have evolved over time and set the criteria for the design of new and better dressings and effective wound management. Until the research by Winter1 which illustrated the benefi ts of a moist environment for healing in the second half of the twentieth century, relatively few advances had taken place in wound care management. Since the early nineteenth century, advances in wound treatment occurred largely due to experience gained in military surgery. These observations brought out the benefi ts of a sterile environment in healing and led to clean and sterile gauze replacing the non- sterile products. In 1880, Joseph Gamgee developed the famous composite dressing consisting of absorbent cotton or rayon fi ber enclosed in a retaining sleeve.13 The dressings used were sometimes medicated with iodine or phenol. Gauzes impregnated with paraffi n were introduced as non-adherent dressings for the treatment of burns and other similar wounds. Medicated versions of these, the so-called ‘tulle gras’ were subsequently introduced and some of these are still being used. The work of Winter led to the development of a whole range of new dressings: fi lms, gels, foams, polysaccharide materials and chitosan. These have revolutionized the treatment of wounds of all types. Some of the requirements for ideal wound dressing mentioned are:14–17 1. Absorbing exudates and toxic components from the wounds surface 2. Maintaining a high humidity at the wound/dressing interface 3. Allowing gaseous exchange 4. Providing thermal insulation 5. Protecting the wound from bacterial penetration 6. Being non-toxic 7. Easily removable without causing trauma to the wound.

Properties that were added later include (1) having acceptable handling qualities, and (2) being sterilizable and comfortable.9 Having acceptable handling qualities means that product will not tear easily and disintegrate into wound.

3.6.2 Case for moist environment The most signiﬁ cant advance in wound care resulted from the work of Winter1 which showed that the occluded wounds, i.e. those in a moist environment, healed faster than the dry wound. An open wound, which is exposed to air, dehydrates and results in the formation of a scab or a scar. The latter forms a mechanical barrier against migrating epidermal cells, causing them to move through a deeper level of tissue, retarding healing (Fig. 3.318). A moist environment prevents the formation of scab and allows the cells to move unhampered.19 If exudate is present, such as from an ulcer, it should be absorbed by the dressing, so that it does not solidify in the wound.

3.6.3 Dynamic nature and requirements Each wound is different from any other even on the same individual, occurring because of the same reason, and in about the same region. Although it may fall in a given category, it still itself remains unique. This is because a wound is inﬂ uenced by so many variables within the host and the environment that each will act independent of all others.20 This is exacerbated by the facts that both the wounds and the hosts are dynamic and constantly change. Thus, there is the ongoing challenge faced in selecting the wound care dressing at each stage for each different person, and this requires a frequent reassessment of the lesion. Surgical wound assessment is an ongoing nursing responsibility that should be conducted every time a

Occlusive dressing No dressing

Normal scab

Moist exudate Epidermis Dry exudate Wound Wound surface surface Dry dermis

3.3 Healing under a moist environment.18

dressing is changed, until the healing is complete. Assessment tools are available that determine the following aspects:21,22 • Type of wound – superﬁ cial or cavity. • Age of wound – fresh, days, weeks, dehisced (split along a natural line). • Stage of healing – granulating, epithelializing. • Progress of wound – healing, deteriorating, become necrotic, infected or static.

3.6.4 General classifi cation Broadly, dressings may be classifi ed as (1) passive, (2) interactive, and (3) bioactive, based on the nature of dressing action required. However, as illustrated above, the concept of wound occlusion to promote moist healing has probably impacted dressing design as much as any other development over the last thirty years. Wound occlusion does require careful regulation of the moisture balance at the site with vapor permeability helping the dressing to stay within its absorption limit. Thus, occlusive dressing types have been developed depending on the nature of the wound and the accom- panying exudate. The theory of moist wound healing has led to approximately eight or nine separate types of materials and devices (Table 3.1), useful for different treatment indications. Each material type that represents these distinct groups has molecular and mechanical characteristics that confer properties to promote healing under specifi cally defi ned clinical indications. For example, it has been recommended that wounds with minimal to mild exudate be dressed with hydrocolloid, polyurethan, and saline gauze, and wounds with moderate to heavy exudate be dressed with alginate dressings. Dressings may also be selected based on wound tissue color, infection, and pressure ulcer grade.23 When taken together, the combined properties of the dressing materials given in Table 3.1 would constitute an ideal dressing. Improvements in dressings that function at the molecular or cellular level to accelerate healing or monitor wound function are included among the ideal characteristics and may be termed interactive and intelligent materials, respectively. For example, a dressing that removes harmful proteases from the wound to enhance cell proliferation is an example of an interactive product. A dressing having a detection device in the material signaling ‘time- to-change,’ because the material has reached its capacity for deleterious protein levels, or reached a pH or temperature imbalance, may be termed ‘intelligent’. Currently, there is not a universal dressing that will work for all wound types. Therefore, a dressing for a wound should be chosen on a case by case basis. When choosing a product, there are several factors to

Dressing and ﬁ ber Description Properties Indications type

Thin fi lms Semipermeable, polyurethane Permeable to water and oxygen Minor burns, pressure areas, membrane with acrylic adhesive providing a moist environment donor sites, post-operative wounds Sheet hydrogels Solid, non-adhesive gel sheets that Carrier for topical medications. Light to moderately exudative consist of a network of cross- Absorbs its own weight of wound wounds. Autolytic linked, hydrophilic polymers exudate. Permeable to water debridement of wounds. which can absorb large amounts vapor, and oxygen, but not to Stage II and III pressure of water without dissolving or water and bacteria. Wound sores. losing structural integrity. Thus, visualization they have a fi xed shape. Hydrocolloids Semipermeable polyurethane fi lm Impermeable to exudate, micro- Shallow or superfi cial wound in the form of solid wafers; organisms, and oxygen. Moist with minimal to moderate contain hydroactive particles as conditions produced promote exudate. sodium carboxymethyl cellulose epithelialization which swells with exudate or forms a gel. Semipermeable Soft, open cell, hydrophobic, Permeable to gases and water Used for leg and decubitus foam polyurethane foam sheet vapor, but not to aqueous ulcers, sutured wounds, 6–8 mm thick. Cells of the solutions and exudate. Absorbs burns and donor sites. foam are designed to absorb blood and tissue fl uids while the liquid by capillary action. aqueous component evaporates through the dressing. Cellular debris and proteinaceous material is trapped. Amorphous Similar in composition to sheet Gels clear, yellowish, or blue from Used for full-thickness wounds hydrogel hydrogels in their polymer and copper ions. Viscosity of the gel to maintain hydration. It water make-up. Amorphous gels varies with body temperature. may be used on infected are not crosslinked. They usually Available as tubes, foil packets, wound or as wound fi ller contain small quantities of added and impregnated gauze sponges ingredients such as collagen, alginate, copper ions, peptides, and polysaccharides

© 2009 Woodhead Publishing Limited Fillers Calcium alginate which consists of Heavily exudating wounds, Heavily exudating wounds an absorbent fi brous fl eece with including chronic wounds as leg sodium and calcium salts of ulcers, pressure sores, fungating alginic acid (ratio 80 : 20). carcinomas. Wounds containing Dextranomer beads consist of soft yellow slough, including circular beads, 0.1 to 0.3 mm in infected surgical or post-traumatic diameter, when dry. The bead is wounds a three-dimensional crosslinked dextran, and long-chain polysaccharide Contact layer Greasy gauzes consisting of tulle The dressing which is porous non- Shallow or superfi cial wounds dressings (tulle gauze and petroleum jelly. absorbent and inert is designed to with minimal to moderate gauze with Silicone-impregnated dressing allow the passage of wound exudate petroleum jelly) sheet consists of an elastic exudate for absorption by a transparent polyamide net secondary dressing impregnated with a medical- grade crosslinked silicone Gauze packing Cotton gauze used both as a Cotton gauze may be wetted with For chronic wounds it fi lls primary and secondary wound saline solution to confer moist deep wound defects and is dressing. Gauze is manufactured properties. Possesses a slight useful over wound gel to as bandages, sponges, tubular negative charge, which facilitates maintain moist wound; bandages and stockings. uptake of cationic proteases. needs to be packed lightly. Improvement in low-linting and Absorbent and elastic for mobile May traumatize wound if absorbent properties. Gauze is body surfaces allowed to dry still a standard of care for chronic wounds Wound vacuum Polyurethane foam accompanied Wound fi lled with foam and sealed Deep wound to stimulate the assisted closure by vacuum negative pressure in with a fi lm. Vacuum is obtained growth of granulation tissue the wound bed over wound

consider, such as: (1) location, (2) size, (3) wound depth, (4) exudates amount, (5) infection, (6) frequency and diffi culty of dressing change, (7) cost, and (8) comfort. Traditional products like gauze and tulle that account for the largest market segment are passive products. Interactive materials composed of polymeric fi lm products are mostly transparent, permeable to water vapor and oxygen but impermeable to bacteria. These fi lm products are recommended for low exuding wounds. Bioactive dressings are ones that deliver substances active for wound healing; either by delivery of bioactive compounds or the dressing is constructed from a material having endogenous activity. These materials include proteoglycans, collagen, non- collagenous proteins, alginates or chitosan.

3.7 Types of dressings available The range of dressing products is so large that there is potential for con- fusion when deciding which one to use. No single product is suitable for all types of wound and, when deciding which dressing to use, it is important to assess the wound and the stage of healing. Speciﬁ c objectives must be identiﬁ ed; for example, if a wound is sloughy, the prime objective will be to de-slough by absorbing exudate; if the wound is clean and granulating, the aim will be to provide a moist environment to aid in healing. If the wound has a large cavity, it must be plugged. Based on considerations such as these, the available range of wound care materials can be described as below.24–28

3.7.1 Gauze dressings A gauze is a traditional dressing and is still one of the most widely used product. In general, it is used for many purposes: skin preparation, cleansing, wiping, absorption, and protection. It can be either woven or non- woven. Cellulose fi bers (cotton and rayon) are the typical materials used in making gauze. Synthetic fi bers, in particular polyester, are also employed to modify properties and reduce cost. If woven, the pattern used is plain but it can vary from loose to tight. The non-woven dressings are highly homogeneous and soft. Figure 3.4 shows a comparison between a woven gauze and a non-woven sponge. Gauze comes in pad, strip, roll, and ribbon form and is easy to handle; it packs easily which makes it a good choice when the wound is located in hard to reach areas. Gauze can be dry, moist, or impregnated. There are some disadvantages associated with the use of gauze: it is not a good thermal insulator and, when removed, it can cause damage to the wound. The damage occurs because the fi bers of the gauze get embedded into the wound exudates and when removed it often peels off some of the newly

3.4 Woven and non-woven gauze.29

formed epithelium. Impregnation of the fabric in a suitable compound alleviates this difﬁ culty.

Dry gauze Dry gauze, which is perhaps the most widely used material in home care, is used as a cover, as a means to prevent contamination, or to trap and lessen exudate. It is used as a primary absorbent dressing on a wound with a high amount of exudate. It is also used on closed wounds to prevent infection or additional trauma. The main problem encountered with gauze is that it tends to adhere to the wound and when removed from a large lesion may pull some of the newly formed tissues with it. If not being used for mechanical debridement as described below, it should be moistened before removal if it appears to be dry and adherent to the wound.

Moist gauze This type of product is used to help maintain a moist environment and most often to promote granulation or protect a granulating wound, which is one of the ﬁ rst visual signs of healing.30 It has been noted that ‘wet-to-dry’ and ‘wet-to-moist’ gauze dressings are often used in practice in a way that makes them indistinguishable.31 In fact, it should be noted, they have two different end purposes. Wet-to-dry dressings have been traditionally used as a means of mechanical debridement. The Agency for Healthcare Research and Quality has promoted the use of these for the debridement function. However, there are a variety of alternative methods for debridement, for example surgical debridement (when such is possible) and debridement using proteases, including collagenase-based and papain/ urea-based formulations. The wet-to-dry gauze method for mechanical debridement is a controversial method as it is known to cause pain to the patient on removal. On the other hand, the wet-to-moist gauze is used for creating moist wound healing. For treatment of open ulcerating wounds,

gauze is made moist by soaking in normal saline. Normal saline dressings, which are still a standard of in-home and nursing home care, appear to act as an osmotic dressing. It has been shown that the osmolarity, sodium and chloride concentrations in dressings, placed on chronic wounds, remain relatively isotonic with time. The reason for this is that the dressing as a result of evaporation increases its tonicity. This draws ﬂ uid from the wound into the product so that a dynamic equilibrium occurs and the dressing regains isotonicity. Thus, the dressing remains functional for as long as it removes ﬂ uid from the wound.

Impregnated gauze Further work has been done to ﬁ x the limitations of gauze by impregnating it with substances that promote wound healing. Substances currently used include; hydrogels, saline, and antimicrobial agents, as well as a wide variety of other materials, including parafﬁ n wax. One such gauze, known for its vast improvement, is referred to as ‘Smart Gauze’, and has been found to be both super-absorbent as well as non-adherent to the tissue. Both the moist and the impregnated gauzes fall into the category of advanced occlusive dressings.

3.7.2 Impregnated dressings With the work of Lister in 1867, in which bandages were impregnated with carbolic acid, the use of antiseptic treatment arose, and, shortly thereafter, Joseph Gamgee produced the ﬁ rst composite product containing cotton or viscose ﬁ ber medicated with iodine.32 Some of these dressings are still in use. These materials include gauzes and non-woven sponges, ropes and strips saturated with either saline, hydrogel, or other wound-healing promoting compounds, including peptides, polysaccharides, alginate, copper ions, and collagen. They are non-adherent and require a secondary dressing.

3.7.3 Transparent fi lm dressings Films are homogeneous materials, which come in different thicknesses and consist of a polymer sheet that has one adhesive side. Film dressings are acceptable coverings for superfi cial wounds; being impermeable to liquids, water and bacteria, but permeable to moisture vapor and atmospheric gases, makes them suitable for use as occlusive structures. Because of their transparent nature, the fi lms facilitate visual inspection without having to remove the dressing. They are applied to partial thickness wounds with very little or no exudate. They are also used to manage intravenous (IV) sites, lacerations, abrasions and second-degree burns.

3.7.4 Composite dressings These dressings combine physically different components into a single product and provide multiple functions, such as bacterial barrier, absorption, and adhesion. The dressings, comprising multiple layers, have a semi- adherent or non-adherent contact layer that covers the wound and may include an adhesive border. The inner is the contact layer that is designed to accept the ﬂ uid and allow it to pass into the layer above where it is absorbed and held. The outer most layer is designed for protection and to secure the dressing to the skin. They are used as primary or secondary dressing; the latter, for example, for daily applications of creams, ointments etc. An example of how different types of carbohydrate combinations, involving cellulose, alginate and other components, can be designed into composite dressings is shown in Fig. 3.5.

3.7.5 Biological dressings In 1975, Rheinwald and Green33 developed a method that made it possible to cultivate human keratinocytes so that a 1–2 cm2 keratinocyte cultured graft could be generated in about 3 weeks. This work paved the way for the eventual development of skin substitutes and biomaterials with wound interactive properties and biological activity, which have progressed

Carbohydrate-based dressing

Hydrogels and hydrocolloids Odour-absorbing dressings

dressing

dressings Gauze and contact layer

Fillers and sugar-based Cotton dressing Carbohydrate-based Alginates and Charcoal cloth hydrogels and Crosslinking dextranomer beads/ dressing carboxymethyl cellulose chemistry sucrose and honey

Composite dressings that enhance the properties of both alginate and cotton while incorporating protease- sequestering properties 3.5 Various materials from carbohydrate sources present in wound dressings may be combined to form composite dressings with enhanced properties.

from the mid-1990s through the present. Biological dressings are derived from a natural source. Collagen dressings, derived from bovine, porcine, or avian sources, fall in this group. All these products are meant to accelerate healing. The biological dressings are available in many forms, including gels, solutions or semi-permeable sheets. While gels and solutions can be applied directly to the wound surface and covered with a secondary dressing, the sheet form can be simply used as a membrane and left in place for undisturbed healing. Biological dressings are indicated to be used for partial thickness wounds such as burns, abrasions, donor sites, skin tears, etc. Skin substitutes that also fall under this category are being increasingly used. These contain both cellular and acellular components that appear to release or stimulate important cytokines and growth factors that have been shown to be associated with accelerated healing.34 Some basic materials may also play a role in up-regulating growth factor and cytokine production and blocking destructive proteolysis. In this regard, the biochemical and cellular interactions that greatly promote healing have only recently been elucidated for some of the occlusive dressings described in Table 3.1. Some carbohydrate-based dressings stimulate growth factors and cytokine production. For example, certain types of alginate dressings have been shown to activate human macrophages to secrete pro-infl ammatory cytokines.35 Interactive dressing materials may also be designed with the purpose of either entrapping or sequestering molecules from the wound bed and removing the components responsible for deleterious activity from it as the product is removed, or stimulating the production of benefi cial growth factors and cytokines through unique material properties. They may also be employed to improve recombinant growth factor applications. The impetus for material design of these dressings derives from advances in the understanding of the cellular and biochemical mechanisms underlying healing. With the knowledge of the interaction of cytokines, growth factors and proteases in acute and chronic wounds,35–38 the molecular modes of action have been elucidated for dressing designs as balancing the biochemical events of infl ammation. The use of polysaccharides, collagen, and synthetic polymers in the design of new fi brous materials that optimize wound healing at the molecular level has stimulated research on dressing material interaction with wound cytokines,34 growth factors,39–41 proteases,42–45 reactive oxygen species,46 and extracellular matrix proteins.45

3.7.6 Absorptive dressings Absorptive dressings are usually multilayer wound dressings which provide either a semi-adherent quality or a non-adherent layer. They are combined with highly absorptive layers of ﬁ bers such as cotton, rayon, etc. These

dressings are designed in a way so as to minimize its adherence to the wound so that the secondary trauma is as less as possible. These dressings are generally used as primary or secondary dressings for surgical incisions, lacerations, abrasions, burns, skin grafts or any draining wound.

3.7.7 Alginate dressings These advanced absorbent dressings are non-woven, non-adherent, pads and ribbons composed of natural polymers derived from brown seaweed. Alginates provide many benefi ts for use as dressings: they swell and retain large amounts of water (100 to over 1000% of their dry mass), thus providing an optimal moist environment; the act of swelling and diffusion throughout the gel allows the wound exudates to be absorbed, which, in turn, speeds up healing; and, because of the wet structure, the dressing does not stick to the wound bed and cause secondary trauma upon removal. The alginate dressing can be applied either pre-wetted, to supply a desic- cated wound with moisture, or dry, to aid in the absorption of exudates. They must, however, be used with a secondary dressing. As for the mechanism, when a dry mass of the material is applied to the site, it begins to absorb exudates during which a reversal in the ion- exchange process (from calcium ion in the dressing to sodium ions in the blood and exudates) occurs. This transforms the water-insoluble calcium alginate into water-soluble sodium alginate, thus absorbing a large amount of fl uid.47 The moist gel formed fi lls and covers the wound. The process is also said to make the dressing an excellent hemostatic agent, thus promoting clotting.48 For dressings, alginates are made as ropes for packing deep wounds and as sheets for treating shallow wounds. Alginate dressings act as a antimicrobial by absorbing micro-organism-infected exudates, which are then removed when the dressing is changed.47

3.7.8 Chitosan dressings Chitosan is well known for its haemostatic properties and its bacteriostatic and fungistatic behaviors, all of which are particularly useful for wound treatment. As a haemostat, chitosan helps in natural blood clotting and in blocking of nerve endings, thereby reducing pain. The polymer gradually depolymerizes to release N-acetyl-β-d-glucosamine, which initiates ﬁ broblast proliferation, helps in ordered collagen deposition, and stimulates an increased level of natural hyaluronic acid synthesis at the wound site. These processes aid in increased wound healing and decreased scar formation. Figure 3.6 gives a schematic representation of the mechanism by which chitosan works. It shows that chitosan, which is a polymeric amine, becomes

3.6 Haemostatic properties of chitosan.49

positively charged when wet and attracts the negatively charged ions in blood and exudates.49 Such attraction allows the material to clot blood quickly and also to act as an antibacterial agent on account of it attracting the negatively charged particles, including bacteria (Fig. 3.6), which are then removed at the dressing change.47 An increase in healing rate of as much as 75% has been reported.48 Because of the similarities in the chemical structures between chitosan and cellulose, the two are frequently mixed at both the polymer and the fi ber levels. Such blending allows a manufacturer to engineer products with desired properties at lower cost. An example of a commercial product is 25% chitosan and 75% rayon. The improved fi ber properties obtained allow the material to be converted into dressing by knitting, weaving or one of the available non-woven processes or by casting the polymer into a fi lm. The functionality of the polymer also allows it to have built-in compounds that provide additional antimicrobial, and even deodorizing, characteristics.50 One of the heaviest uses of chitosan is in making dressings for use by military in the combat zone. The claim of a product is that it will bond with the blood in 1 to 5 min and will also form an adhesive-type structure to protect the wound until access to a medical facility becomes available. Such quick response is needed in managing combat related injuries, as nearly 80% of all battlefi eld deaths result from bleeding in less than 10 min.49 More specifi cally, chitosan has been developed as a sponge for use in controlling lethal extremity arterial hemorrhages in arteries.51

Chitosan is among a number of currently deployed haemostatic agents used within the armed services and has recently been contrasted with other materials for its efﬁ cacy in this application.52,53 Chitosan-based haemostatic products have also been compared with the more highly ordered crystalline structures of glycosaminoglycan materials54 to learn more about the relative roles that carbohydrate structure and charge play in eliciting haemostatic activity. In addition to its use in fabric or ﬁ lm form, the material may also be incorporated into dressings in the form of powder and beads, and it has been grafted onto cotton to study its antimicrobial and haemostatic activities.

3.7.9 Chitosan/alginate bicomponent fi ber dressings Some wound products even combine chitosan and alginate to form a fi ber.55 With these bicomponent fi bers (Fig. 3.756), the positively charged chitosan on the outside attracts the negatively charged microbes on the skin, promoting antimicrobial activity. The alginate polymer inside is still able to absorb the exudates. Thus, a non-toxic, biocompatible, absorbent and antimicrobial wound dressing could be made that combined the best attributes from both polymers to generate a faster healing product for large exuding wounds.

3.7.10 Hydrocolloid dressings Hydrocolloid dressings can have various compositions, but the most common composition has a backing (outer layer) of either a vapor- permeable ﬁ lm or thin sheet of foam on which a mixture of sodium carboxymethyl cellulose, elastomers, adhesives and gelling agents are coated. Once the dressing is ﬁ xed on the wound, the warmth softens the lining of the dressing providing a gel cover for the wound bed. Hydrocolloid dressings are able to absorb a minimal amount of wound exudate and once its capacity is reached the remainder may leak out from under the dressing, termed ‘strike-through’. The strike-through can be fast and,

Chitosan Alginate

3.7 Core-sheath alginate/chitosan.56

therefore, these dressings are best suited for rehydrating dry black/ brown necrotic tissues and wounds containing dry yellow slough. The hydrocolloid dressings are available in many shapes and some also have an additional adhesive border to prevent leakage or sliding of the dressing over the wound.

3.7.11 Hydrogel dressings There are two types of hydrogel dressings.

Amorphous gels Amorphous gels donate moisture to a dry wound and are, therefore, used for rehydrating dry necrotic or sloughy tissues and keeping granulation tissues moist – in much the same way as do the hydrocolloids. If the wound is sloughy and wet, this dressing will not be suited for the application. The amorphous gels come in a variety of forms and some have hydrocolloid or alginate added in an attempt to make them better at debriding or coping with wet wounds. The amorphous gels require a secondary dressing, usually a vapor-permeable fi lm. The viscosity of amorphous hydrogels contributes to its function, and its ability to maintain integrity after absorbing wound fl uid determines its role in moist wound healing. Less viscous types liquify after absorbing small amounts of exudate, and thereby add fl uid to the wound. The more viscous types, on the other hand, maintain their structure and form a protective barrier over the site, thereby sequestering wound fl uid and increasing the bioavailability of the exudate constituents, including proteases, for autolytic debridement and wound repair.

Non-amorphous gels The non-amorphous gels provide a moist interface at the wound bed but do not donate as much water as do the amorphous gels and are, therefore, not the ﬁ rst choice for rehydrating a wound. However, they do provide a moist cover for granulating wounds. They are very soothing to wounds, making the dressings particularly suitable for use on superﬁ cial burns, including those caused by radiotherapy reactions. As they are gentle on removal, they are also suitable for use on easily damaged skin. The sheet gels come in adhesive and non-adhesive versions.

Foam dressings Foam dressings usually consist of coatings of foamed solutions of polymers on sheets. The foam has small and open cells capable of holding ﬂ uids. Their absorption capacity depends on the thickness of the layer, and the material. Foams are permeable to gases and water vapor, but not to aqueous

solutions and exudate. Foams absorb blood and tissue ﬂ uids while the aqueous component evaporates through the dressing. Cellular debris and proteinaceous material are trapped in the material. These dressings are generally used on partial and full thickness wounds.

3.7.12 Antimicrobial dressings Almost any type of dressing, i.e. sponge, gauze, ﬁ lm, or absorptive, can be made to have antimicrobial properties by incorporating agents such as silver and iodine. Silver is easily incorporated into chitosan and alginate products, thus greatly enhances their antimicrobial protection for the wound. A unique beneﬁ t of using silver in dressing is that it is highly effective in minute amounts (∼1 ppm).47

3.7.13 Silicone dressing These are atraumatic dressings (they do not cause trauma to newly formed tissue or tissue in the peri-wound area) based on soft silicones, which are a particular family of solid silicones, that are soft and tacky, and which, therefore, tend to adhere to dry surfaces. A silicone dressing consists of a contact layer that is coated with silicone. These materials are inert and their main attribute is that they can be removed from a sensitive wound without causing trauma. The exudates can also be absorbed but this is accomplished by using an absorbent dressing that is given a coating of silicone. These dressing are particularly recommended for use as the ﬁ rst-line prophylactic treatment against development of hypertrophic scar and keloid after surgery.57

3.7.14 Categories based on the management of moisture The dressings discussed above can be further grouped into three main types:16

Dressings that absorb exudates Absorbent dressings have a very high capacity for holding ﬂ uid. Hence, for a wound generating high levels of exudate, absorbent dressings will require fewer changes within a set period of time. Two of the materials that can support this function are the alginate and the foam products.

Dressings that maintain hydration As a wound heals more, its exudate generation becomes less. This is when the wound starts to granulate or ﬁ ll in with new connective tissue. When exudate levels decrease, it is not advisable to use absorbent dressings as

they may result in the dehydration of the tissues. In such a situation, all that is required is to maintain the hydration level. The hydrocolloid and the ﬁ lm products are suited for this application.

Dressings that donate moisture When wounds are completely dry, they become covered by a layer of dead tissue, which needs to be removed to allow the wound to heal optimally. Often such tissues are removed by autolysis debridement which is slow digestion of the dead cells by enzymes. In such cases, maintaining a moist environment helps the process. The dressing should actively add moisture to the wound, and this is best performed by a hydrogel material.

3.8 Bandages The functions of bandages are to (1) hold a dressing in place, (2) apply compression in some applications, such as to arrest bleeding in heavily hemorrhaging wounds or to treat varicose vein or leg ulcers, (3) immobilize fractures, (4) tie anesthetic tubes, and (5) hold cuffs, masks and other parts of textiles worn by a medical person for personal protection, safety or ease of mobility. Commonly, bandages performing a load-bearing function are made up of knitted or woven textiles, some containing elastomeric threads for required stretch and recovery force. Many types of products are available, including relatively inextensible, highly extensible, adhesive/cohesive, tubular, and medicated paste bandages.58 The non-extensible bandages, essentially woven with open weave for breathability and low weight, are used for holding an absorbent pad or dressing in place at sites where stretch is not required, for example fi nger, arm or lower leg, tying anesthetic tubes and drains in position, and holding up a surgeon’s trousers. The extensible bandages are used for retaining a dressing and applying compression for control of edema and swelling in the treatment of venous disorders of the lower limb. The bandage should keep the dressing in close contact with the wound, and not inhibit movement or exert signifi cant pressure that causes pain or restricts blood fl ow. The compression dressing falls in a class by itself and is used primarily for treatment of varicose vein ailment. The optimum pressure needed varies with condition and place on the leg. The highest pressure is required at the ankle and the lowest on the upper thigh. Stockings in different sizes and having various mechanical properties are now available to fi t a range of patients and provide gradient pressures of magnitudes suited for different specifi c categories of venous disorders. The pressures used are as low

as 14 and as high as 40 mm Hg. Mathematical analysis gives the bandage pressure P (Pa) on the surface as a function of the bandage tension T (N)– itself a function of the extension and the elastic modulus–, the radius of curvature of the limb R (m) and the width of the bandage W (m) as: T P = [3.1] RW

As the effects of superimposed layers are additive, a conﬁ guration involving two turns of a bandage will essentially double the pressure. Because the bandage pressure is inversely proportional to R, wrapping a bandage at a given tension T will give the highest pressure at the ankle, where R is lowest, medium pressure at the calf, and the lowest pressure at the thigh. Adhesive bandages are made of woven cotton or rayon fabric that is coated with a suitable adhesive. Highly twisted or crepe yarns in warp provide a degree of stretch and elasticity to the bandage that can serve as a structure for treatment of varicose veins and for immobilizing orthopedic fractures resulting from sport and other injuries. Cohesive bandages combine some of the characteristics of ordinary stretch bandage with those of adhesive products. Whilst they do not adhere to the skin, a special coating on the surface enables layers to adhere to each other, thus preventing slippage and untying during use, including sports activity. The simplest form of a tubular bandage consists of a knitted tube of a lightweight fabric. These are used under orthopedic casts, or placed over arms or legs that are covered with a medication. In summary, it is clear that bandages are more directly textile structures that supplement dressings in treatment of wounds, both external and internal.

3.9 Materials used in dressings and bandages A number of polymeric materials are used as fi lms, fi bers and other structures for developing wound-dressing products. Some of the primary materials employed are cotton, rayon, polyester, nylon, polyolefi ns, acrylic, polyurethan, chitosan and alginate. No doubt some other materials have been considered, for example recombinant silk, but for economic or technical reasons, they have either not passed beyond the experimental stage or are used in a limited way. A brief introduction to the chemical nature, the physical structure and the properties of the materials used in wound management products follows. An understanding of these can provide the manufacturer with a means of selecting the most appropriate material for a dressing for each different application.

3.9.1 Cotton The fi ber that has been historically the most highly used in construction of dressings, absorbent pads, and bandages, is cotton, which still accounts for a signifi cant volume of wound-care products in the world. The chemical structure of one repeat unit of the cellulose chain is shown in Fig. 3.8. Each repeat has three hydroxyl groups that are capable of linking with neighboring chains by hydrogen bonds. These groups, when free or weakly bonded, also attract and bond with water. The oxygen bridges between the repeat units allow chains to bend and twist, making the polymer fl exible. The chains are quite long, and the fi ber has over 60% crystalline structure. The most commonly used cotton in wound products is short (<2.5 cm). The cross-section is kidney bean shaped, i.e. relatively fl at, but it becomes round when swollen with aqueous fl uids. Once the waxes and impurities, normally present on raw fi ber, are removed by a chemical scouring/ bleaching step, the fi ber surface becomes greatly hydrophilic and instantly wettable. Under ambient conditions, the material has a moisture regain (mass of water absorbed per dry mass of fi ber, expressed as percentage) of about 8%, but when soaked can take up to about 30% water and swell signifi - cantly. One unique characteristic of cotton that separates it from other fi bers is that it becomes stronger when wet. This makes the fi ber preferred for use in towels, pads, sponges, swabs and many other absorbent wound care and surgical products, where mechanical integrity along with absorption are important. With an abundance of functional groups on the chains, the structure can be chemically modifi ed to incorporate additional hydrophilic groups to enhance the fi ber’s absorptive properties, or therapeutic agents for imparting antibacterial or healing characteristics. The highly absorbent carboxymethyl cellulose (CMC) is a cellulosic material (usually based on wood fi bers) that has been grafted with groups such as acrylic acid and which allows the structure to swell enormously and absorb many times the fi ber’s weight in water.

3.9.2 Rayon The fi ber, although chemically similar to cotton (Fig. 3.8), differs from it in physical structure: rayon’s molecular weight is about one-fi fth and crystallinity about one-half of that of cotton. These differences make rayon relatively weaker and more extensible, but more absorbent (about two times) than cotton. Thus, under ambient conditions, the fi ber absorbs about 14% moisture, and when soaked, can swell and absorb almost 70% by weight water. The fi ber, however, becomes signifi cantly weaker when wet and, therefore, requires care when used directly on open wounds. One of the major applications of the fi ber has been in disposable absorbent

CH2OH H OH O H H O OHH Cellulose (cotton, rayon, lyocell) OH H H H O H OH CH2OH H O () Elastomer OOCCNNR' R O n CCNNR' O O H RO – aliphatic structure (n ~ 10–30) R' – ring structure

CH2OH H NH2 O H H O OHH Chitosan OH H H H O H CH OH NH2 2 O O

Polyester (PET) O CH2 CH2 O C C n

Polyamide O O C () NH CH2 5 NH C Nylon 6 n

O O C ()CH C ()CH Nylon 66 2 4 NH 2 6 NH n H H Polyacrylonitrile C C H CN n

Polyolefin H H C C UHMWHD polyethylene HH n

CH3 CH3 C CH C CH Polypropylene (iPP) 2 2 H H n

OH OH O OH O O O O Alginic acid HO HO nO O OH m 3.8 Molecular structures of a ﬁ ber.

pads, sponges, and sanitary napkins. For use in absorbent dressings, usually a multilayer structure with rayon sandwiched between a permeable wound contact layer and an outer ﬁ lm layer is preferred. Being endowed by even more free functional groups than cotton, desirable compounds can be grafted readily to the material for speciﬁ c uses.

3.9.3 Polyester Polyester is one of the most versatile of the manufactured fi bers that fi nds applications in many categories of textile products and is one of the widely used synthetic fi bers in medical products. It is also the fi ber frequently selected for combining with cotton and rayon in developing needled and spunlaced non-wovens for dressings. The chemical constitution of the commonly used material, poly(ethylene terephthalate) or PET, is shown in Fig. 3.8. The fi ber has an aromatic component and an aliphatic sequence. Although the polymer lacks strong functional groups, the molecules, when drawn, pack closely and lead to a semi-crystalline mechanically strong and thermally stable fi ber. Because of the lack of polar groups, the fi ber has a low attraction for water (moisture regain ∼0.4% under normal conditions); this makes the material largely hydrophobic. A number of variations of the basic repeat are available but they vary primarily in terms of the proportion of the aromatic and the aliphatic components and, as a result, lead to fi bers with different physical and mechanical properties. Some low molecular weight aliphatic polyesters are used as low melt adhesives for binder applications. Aliphatic polyesters capable of hydrolytic degradation are also used in the manufacture of bioabsorbable products, in particular surgical sutures. The polymer can be melt-extruded both as a fi lm and a fi ber, the latter in different cross-sectional shapes and sizes to provide materials with various surface, physical, and mechanical properties.

3.9.4 Nylon Although many different types of nylons can be produced, the two widely used are nylon 66 and nylon 6 (Fig. 3.8). The chains, being void of aromatic compounds, lead to fi bers that have low modulus and high extensibility. The presence of amide groups in the chains, however, allows hydrogen bonding between NH and CO groups of adjacent chains; this gives the fi ber excellent mechanical and thermal stability. The amide groups in the chains also attract water, thus making the fi ber reasonably hydrophilic (moisture regain 4%) and wettable. Being one of the most stretchable and elastic of the common textile materials, in addition to being acceptably hydrophilic, nylon is a preferred material for the manufacture of tubular bandages and stretchable pressure hosiery for treatment of venous leg ulcers.

3.9.5 Polyolefi ns The two materials of interest are polyethylene and polypropylene (Fig. 3.8). The one based on the simplest of the hydrocarbon polymers, polyethylene, can be produced in the extended form using the gel spinning process. This leads to the extended chain structures with ultra high modulus and tenacity (HMPE) suitable for use in many high-performance technical textiles. In the non-extended folded chain form, the polymer does not have adequate mechanical properties to lead to fi bers, but the material can be made into fl exible fi lm or low melt adhesive or wax. The second material is polypropylene which is also made into fi ber and fi lm. For making fi bers, the chains must be in a stereo regular form. The olefi n fi bers have the lowest density (0.91–0.96 g cm−3) of all fi brous materials. Both materials being strictly hydrocarbons are truly hydrophobic and, therefore, do not wet. They also have very low surface energy (∼24 mN m−1), which, among textile polymers, is only higher than that for polytetrafl uoroethylene (∼12 mN m−1). Accordingly, olefi ns generally lead to low- adhesion fi lm products, which are used for developing low-adhesion contact layers for dressings. The fi bers also have low melting points (<175 °C), which vary with chain length and, in the case of polypropylene, with tactic- ity. Accordingly, olefi ns are frequently used as low-melt coatings on regular fi bers or as binder fi bers for making fabrics by the non-woven methods. In dressings, the polymer fi nds extensive use in the development of contact layer fi lms.

3.9.6 Acrylic The chemical structure of the basic polymer is illustrated in Fig. 3.8. The pendant acrylonitrile groups (C≡N) are highly polar and lead to extensive bonding between chains. Because of this, a second component, up to about 15% by weight, which is another vinyl monomer, e.g. methyl acrylate or vinyl acetate, is inserted into chains to improve polymer solubility for extrusion into fi ber. The fi ber, with only about 2% moisture regain, has limited interaction with aqueous fl uids. A major attribute of the fi ber that particularly suits its use in dressings is its naturally high resistance to colonization of micro-organisms.

3.9.7 Elastomeric fi bers The unique characteristic of elastomeric materials, whether used as shaped fi bers or fi lms, is that they are soft and highly stretchable and elastic. The synthetic elastomers are linear block copolymers containing soft amorphous sections that provide stretch and hard crystalline components that

3.9 Illustration of ﬁ ne structure in elastomeric ﬁ bers, composed of blocks of crystallizable hard segments and non-crystallizable coiled soft segments.

act as tie points and hold the structure together in a memory-endowed mechanically stable material. These polymers are usually known as the segmented polyurethanes, whose general chemical structure is given in Fig. 3.8. The hard segments of neighboring chains tend to associate with each other and crystallize, while the soft segments remain largely coiled and unassociated (Fig. 3.9). Stretch of the order of 100% or more is common. The material being elastic and having the ability to return back to its original shape and size, its major application is in developing products that are required to exert transverse pressure of the required level for treatment of skin disorders. Thus, using elastomeric ﬁ bers in a bandage, one can keep an absorbent dressing pressed on a puncture wound to stop bleeding, or in hosiery one can apply desired pressures on different parts of leg to treat venous disorders.

3.9.8 Chitosan Chitosan is a natural biopolymer and is derived from chitin. The latter is the second most abundant natural polysaccharide on earth, with cellulose being the ﬁ rst. It is found in the shells of crabs, shrimps, prawns and other crustaceans. While chitin itself can be used in dressings, it is its deacetylated derivative chitosan that is used extensively in this application. Figure 3.10 shows the difference between chitin and chitosan, where the acetyl group is eliminated and replaced with an amino group.59 The degree of substitution varies from product to product but is typically about 80%.60 The modiﬁ cation makes the material a poly(β-(1-4)-2- amino-2-deoxy-d-glucopyranose), which is a polycationic biopolymer that is also found in some insects and species of fungi.48 The deacetylation process provides open groups that cause the chemical and biochemical reactivity of chitosan to be relatively much greater and is one reason why

OH OH OH O O O O O O HO HO HO NH NH NH C=O C=O C=O CH CH CH3 3 3 Chitin

OH OH OH O O O O O O HO HO HO

NH2 NH2 NH2 Chitosan

OH OH OH O O O O O O HO HO HO OH OH OH Cellulose 3.10 Chitin, chitosan and cellulose.

this form is used more frequently. Another reason is that chitosan is more soluble than chitin. Chitosan-based composite fi lms are gaining attention for their ability to stop bleeding in wounds and promote healing. Chitosan is biodegradable, biocompatible, non-toxic, and has great antibacterial properties. Its properties make it an excellent choice for many medical applications. As a fi ber or fi lm it is used for wound care, and, in microcapsule or bead form, it is utilized for drug delivery.61 For use as a fi lm, that has some mechanical integrity, chitosan is mixed with another polymer.62

3.9.9 Alginate Alginate is a natural polysaccharide which is derived from brown seaweeds.63–65 It has been used for a variety of purposes, ranging from thickening agents in food, to pharmaceutical additives. It is a block copolymer made up of 1,4-linked b-d-mannuronic acid (M) and a-l-guluronic acid

COO– COO– OH OH O O O OH OH O O O OH O OH O O OH OH COO– COO– GG GG

– –OOC OH OOC OH O O O HO O HO O HO O O OH –OOC MM M

OH –OOC HO O O O HO – O OOC HO O OH O O HO COO– MMG 3.11 Mannuronic (M) and guluronic (G) block structures in alginate polymers.65

(G) residues (see Fig. 3.11 for stereochemical structures). There are three block structures present: M/M blocks, G/G blocks and M/G blocks. There are a variety of seaweeds that alginate is derived from and each species has a different composition of the acids. These result in a range of properties found in the material. Those containing a higher percentage of gluronic acid tend to be stronger and stiffer than the ones containing a higher percentage of the mannuronic acid. The latter, however, tend to have better swelling properties. In order to achieve alginate fi bers from seaweeds, sodium alginate must be extracted from crushed and washed raw material. The sodium alginate is soaked in water where a viscous solution is formed which is then extruded into a calcium chloride bath. In the process, the sodium ions are replaced by the calcium ions. After washing and drying of the gel, the insoluble calcium alginate fi bers remain.66 Webs containing the fi bers may be composed by fi rst airlaying or carding-cum-crosslapping and then needle punching, to make the dressing. Fibers may also be converted into knitted or woven fabrics or assembled in a rope form. When the calcium alginate dressing is used on a wound, a reverse ion-exchange process takes place. The sodium ions in the blood and wound exudates are exchanged with the calcium ions in the alginates.66 Moisture is necessary for the alginate dressings to work properly, which is why they will not be benefi cial on dry wounds and are used mainly on heavily exuding wounds.

During production, other desired molecules can be introduced resulting in altered physical and chemical properties. For instance, silver, chitosan, and other antimicrobial molecules can be integrated either physically during ﬁ ber formation or chemically by grafting them onto the alginate chains.67 Molecules are also frequently added to improve the mechanical properties of the dressing. The ﬁ bers produced can be processed to form a non-woven mesh by one of the methods available.

3.10 Textile processes involved in formation of dressings and bandages 3.10.1 Introduction Dressings are made from polymers and fi bers in a variety of types. The form in which these materials are used may be fi lm, rope, ribbon or fabric. The latter are made by a weaving, knitting, braiding or a number of non-woven processes. Unless the product required is primarily load bearing, the fabric used is most likely the non-woven. The latter is usually soft and bulky and is porous and absorbent. Non-wovens are also homogeneous and most economical to produce. Films used as dressings are also non-wovens, but they are cast or extruded directly from polymer. These are thin and fl exible but usually not as three-dimensionally drapeable and conformable as are the woven and the knitted fabrics. The same limitation also applies to the regular non-woven fabrics, i.e. they lack drape and conformability. A description of the methods of forming fabrics for dressings is given below. Since the non-woven fabrics are the largest used in forming dressing products and there are several technologies involved in making these fabrics, a more detailed description is presented of these processes and the structures made from them.

3.10.2 Yarns Fibers or fi laments obtained from extrusion of polymers are twisted (for short fi bers) or entangled (for continuous fi laments) to form a continuous thread that has mechanical integrity enough to support stresses imposed on them during their weaving, knitting, or braiding into fabrics. A typical short fi ber or staple yarn will contain about 100 individual fi bers in the cross-section and have a diameter of the order of 0.2 mm. These one- dimensional structures are therefore quite strong and fl exible.

3.10.3 Woven fabrics A signiﬁ cant fraction of gauze fabrics are based on woven structures. These fabrics are manufactured on a loom that has a running sheet of yarns,

Warp cross-section 3.12 Plain weave and leno weave.

known as warp, within which are inserted cross-threads, known as weft, at right angles. The woven fabrics used in wound management applications have the simplest structure called plain weave, shown in Fig. 3.12. Woven fabrics are relatively inextensible but dimensionally very stable structures with porosity that can be varied from nearly zero value to very high value (gauze, cheese cloth). A major problem with the woven fabric is the tendency of threads to unravel at the cut edge. A special weave know as ‘Leno’ in which two warp threads twist around a weft reduces this tendency.68

3.10.4 Knitted fabrics Knitted fabrics are categorized as either ‘weft’ or ‘warp’ constructed. Of the two, the faster and the more economical to produce is the weft knitted fabric that can be accomplished with a single package of yarn. The warp knitting process, on the other hand, requires a warp beam, i.e. a running sheet of yarns as does the weaving process. Loops are formed transversely in the case of the weft knitting and essentially vertically in the case of the warp knitting (Fig. 3.13). Simplest or plain weft knits tend to be very extensible and dimensionally unstable. One could improve on these properties by using additional yarns that interlock the loops. In contrast, the warp knitted structures are basically more interlocked and dimensionally stable. The knitted fabrics are more ﬂ exible, compliant and conformable than the woven fabrics, and one of them, the warp, also does not unravel as easily when cut to ﬁ t an application. A major limitation of knitted fabrics in some applications is porosity which, owing to the nature of the loop structure, tends to be high. The knitted nets and bandages can be draped more effectively than woven materials over areas that require three-dimensional conformability. One of the primary applications of knitted structures in wound care is in compression hosiery used for treatment of venous stasis.

Face Back wale wale

1 х 1 rib Half-tricot structure 3.13 Weft knit and warp knit structures.

3.10.5 Braided fabrics Although not used as extensively as the other two types in wound care, the braided fabrics are unique structures that are particularly suited for producing small diameter tubular and mechanically stable rope structures, the latter suitable for packing cavity wounds. The braids (Fig. 3.14) resemble woven fabrics except that the crisscrossing is not at right angle. One can have ﬂ at braids and tubular braids, the latter with and without a core. The braids are mechanically stable structures like the woven, but they are more ﬂ exible and conformable than the woven. The criss-crossing pattern in braids also leads to porosity.

3.10.6 Non-woven fabrics As a major function of dressings is to provide a soft and resilient hand, absorb and retain exudates, if present, and serve as a protective covering, non-woven structures suit the application and ﬁ nd extensive utilization in the products. Such structures have been used as the facing, as well as the entire absorbent pad of a wide variety of dressings. These fabrics (Fig. 3.15) differ from woven and knitted in that the former are more homogeneous, softer, and more resilient, than the latter. The processes involved are also faster and more economical. A reason for the latter is that the fabric is made directly from the ﬁ bers, without the intermediate step involving the yarns, or even from the polymer itself. Many different types of processes are involved in the manufacture of non-wovens used in wound care products. These lead to different structures

(a) (b) 3.14 Braided structures a) without and b) with core.

3.15 Non-woven fabric.69

and properties and, therefore, suit different functions of a dressing. Accord- ingly, it will be useful to include here an introduction to the different processes involved and the structures produced by them. Considering that there is a high similarity between the design of an advanced multi-layer composite dressing and an engineered absorbent sanitary product (napkin, diaper, or an incontinence pad), a highly relevant but detailed discussion of non-woven processes, structures produced by them, materials used in producing them, and the properties obtained, can be found in a chapter, titled ‘non-wovens in absorbent materials,’ by Gupta and Smith.70

A non-woven fabric consists of a web based on fi bers or fi brils. A binder may be incorporated to hold fi bers more fi rmly and provide mechanical integrity and strength as required. The manner in which the fi bers are assembled into a web and bonded has a profound effect on the web properties. A web structure can range from the extremes of a highly oriented fi ber confi guration produced by the carding process to one of completely random arrangement obtained from the air laying or spunbonding process. An intermediate arrangement obtained by cross-lapping can also be developed. The bonding of fi bers is accomplished either mechanically through entanglements, chemically through use of a binder, or thermally through melting and fusing, or using a combination of these methods. A non-woven web is generally considered ‘fi nished’ as soon as the fi bers are assembled into an appropriate structure and bonded by one of the methods mentioned. It is possible, however, to give additional chemical or mechanical treatment to the bonded web, which can have an important infl uence on web characteristics. These include, as examples, a chemical repellent, wetting or antimicrobial treatment, and mechanical embossing, aperturing or glazing treatment.

Uni-directional dry form process This is a carding process in which fi bers are teased, combed and oriented by wires and formed into a web a few fi bers thick, with the fi bers predominantly oriented along the longitudinal direction. A structure based on such a web will usually be characterized by good tensile properties in the longitudinal or the machine direction, but poor properties in the transverse direction. Cross-lapping of the web is done to alleviate this defi ciency and also to increase its weight. The cross-laying (Fig. 3.16) is done in a continuous manner, often onto another carded uni-directional web. The result is a fi ber web with one layer having a uni-directional orientation in the machine direction and the other succeeding layers having a uni- directional web oriented at an angle close to but less than 90° to the machine direction. By proper selection and juxtaposition of the layers of different webs, it is possible to control the orientation in the fi nal web.

Random laid or air laid process In this process, individualized fi bers are suspended into an air stream, and allowed to be deposited on a moving perforated belt or drum (Fig. 3.17). The web obtained has fi bers distributed randomly. In addition to the typical textile length fi bers (0.9–2 in.), very short wood pulp fi bers (∼0.3 in.) can also be used in this method. The process of deposition prompts some fi bers

Carded web Feed conveyor

Upper conveyor belt Web

Lower conveyor belt

Delivery conveyor

3.16 Carding cross-lapping process.71

Air

Suspended fibers

Random web

Suction

Apron

3.17 Random air laying process.72

to be lifted out of the X–Y plane and become partially oriented in the Z direction. This results in a web having higher bulk and larger void volume, which enhances the web’s ﬂ uid absorbing and holding capability.

Chemical and thermal bonding of webs A bonding method frequently employed for the dry form web is chemical, involving an acrylic or a latex binder.73 The proprietary formulation will include other ingredients as well, which are a catalyst (for cross-linking), a viscosity modiﬁ er, a defoaming agent, and a surfactant. The resin add-on

can vary over a wide range, but the typical value will lie in the 10–25% range. The binder can be applied by a variety of procedures, including saturation of fabric, spraying the web, or printing binder on the web with a roll. The saturation method is normally not considered for absorbent materials, as the binder covers essentially all fi ber surfaces. Spray bonding with careful adjustment of parameters can provide bonding at just the cross-over points, leaving the rest of the web material largely binder free. The placement of the binder resin can be even more fi nely controlled by print bonding, in which the binder is positioned on the web in a discrete predetermined pattern utilizing engraved rolls74 or a rotary screen. More recently, the use of binder in the form of foam has been utilized.75 This leads to a reduction in the drying cost of the printed web, as the binder is diluted with air rather than water. A modifi cation of the bonding process has also been used, which is suited in particular for the contact layer of the dressing. This involves the use of thermal techniques for bonding.76,77 The fabric contains a blend of low melt fi bers (polyethylene and polypropylene) and other fi bers, natural or man-made, and is bonded with heat using engraved hot rolls. Another version of the thermal method involves the use of bicomponent fi bers that have a higher melting core with a lower melting sheath. When heat is employed by embossed calendar rolls or by hot air, the sheath layers bond.

Spunbond process A polymer melt is extruded through a large number of spinnerets to form numerous continuous fi laments. These fall onto a moving conveyer and form a web by overlapping with each other. Fibers are oriented randomly, which is achieved by oscillating spinnerets, fi eld-induced by electrical charges, and/or the controlled fl ow of air. Webs may be bonded by thermal or chemical means over their entire area to produce a thin, smooth, fabric. Such a structure is stiff, but bonding at strategic points can be used to produce a softer and more fl exible fabric. The polymers used in spunbonding are mainly polyester, nylon and polypropylene. Although made from a hydrophobic polymer, the web having a high porosity lends itself to use as a wound contact layer. Print bonding used for dry form non-wovens can also be utilized in bonding these webs.78 The amount of bonding area for a typical spunbond fabric ranges from about 10 to 35%.

Needlepunch process In the needlepunch process, in which primarily the carded- cum-crosslapped or random laid webs are used, the bonding is achieved by the penetration of a set of barbed needles through the structure (Fig. 3.18). The barbs are designed such that, as a needle penetrates, groups

Stripper plate

Fibers

Bed plate

3.18 Needlepunch process with magniﬁ ed view of the needling action.71,72

of fi bers from the top layer of the web are engaged and driven through the thickness of the web. As the needle retracts, the fi bers are released from the one-way barbs. This results in mechanical entrapment and orientation of the portions of fi bers in the thickness direction. By control of the needle penetration and the repetition of the needling action from the opposite side of the fabric, a three-dimensional, mechanically entangled network can be achieved.79 The process tends to consolidate the web but creates oriented capillaries or channels along the thickness. This imparts resiliency to the web and the capability to effi ciently imbibe and hold large volume of fl uid. There is a minimum amount of fi ber that is required for effective mechanical engagement. Consequently, most needle-felt structures have a weight of 60 g m−2 or more.

Wet laid process The wet laid non-woven process bears a considerable resemblance to the paper manufacturing process. Short fi bers (generally one-quarter inch or less) are suspended in an aqueous slurry along with minor amounts of other constituents. The slurry is subjected to strong agitation in order to cause the fi bers to be uniformly distributed. The slurry is conveyed to a moving wire belt, where the liquid drains through and the fi bers are left in the form of a mat in largely random confi guration. The web is then removed from the conveyor, taken through a bonding process, dried and then wound up onto a roll. In paper manufacturing usually 100% pulp (∼3 mm) is used and bonding by hydrogen linkages is extensive, which leads to the typical thin and stiff structure. In the wet laid non-woven process, on the other hand, compositions can range from sizeable wood pulp content to a completely synthetic

ﬁ ber furnish. Also, in these structures, usually the ﬁ bers used are longer and crimped or convoluted, and the frequency of bonding is lower. This leads to a fabric that is relatively bulkier and softer. If pulp is used as one of the constituents, its fraction and, therefore, the resulting hydrogen bonding, are small. An external binder is often employed to increase strength while maintaining bulk. Bonding is usually achieved by either the chemical or the thermal methods described earlier.

Spunlace process This process, which also leads to mechanical bonding, utilizes high-energy, closely spaced, water jets that emerge from an injector and impinge on a web substrate and entangle loose arrays of fi bers (Fig. 3.19).80,81 The process is claimed to have the capability of producing a variety of surface and fabric patterns from many different precursor webs made from essentially any fi ber that is not too stiff or brittle to bend or move. Figure 3.20 shows a spunlaced web made from a blend of polyester and rayon fi bers. The injectors are positioned above the moving backing belt or rotating drums, which are perforated and carry the unbonded web through the unit.82,83 The impaction of water jets forces fi bers onto and into the backing belt, giving the fi ber web surface the texture of the surface of the belt. If the backing belt has large holes, an apertured web is obtained; if it has very fi ne holes, a web approach- ing a non-apertured structure is obtained. This versatility associated with the confi guration of the supporting belt provides the capability to produce a wide range of surface and structural features in a spunlace fabric. Wood pulp/polyester fi ber blended spunlace fabrics were originally the largest volume of products produced by the process; however, other

High water pressure

Water jet head

Base web

Entangled web

Web support

Drum

Air and water return 3.19 Spunlace process.71

3.20 Spunlace web from rayon and polyester.

spunlace fabric types have become signifi cant commercial successes, including those based on 100% bleached cotton and its blend with synthetic fi bers. In addition to drylaid and wetlaid precursors, a direct-laid web from the spunbonding or meltblowing process has also been used, but usually as part of a multilayer construction. Composite fabrics have been produced by hydroentangling a drylaid or a wetlaid staple fi ber web superimposed on a direct-laid fabric that serves as a reinforcing scrim.84 This variant is especially useful in producing a composite of a lightweight spunbound fabric of olefi n or polyester around whose fi laments a wood pulp layer has been entangled. This can provide a strong yet absorbent fabric from relatively low-cost raw materials. Modifi cations of the hydroentangling system have also been used. One such process involves fi ber entanglement at a relatively low level, which is still suffi cient to convey some mechanical integrity; this is then supplemented by a limited amount (1–5%) of external latex binder, which conveys additional strength and integrity to the structure.85 In another modifi cation, the low level of fi ber entanglement is supplemented by adding a small amount (∼15%) of thermoplastic binder fi ber. After drying the fabric, additional heat is applied to melt and thus activate the latter. A polyolefi n bicomponent fi ber is used for this process, although other binder fi bers can also be employed. This same technique, involving addition of a low melt fi ber, is now used at times to enhance the physical properties of needle punched fabrics as well.

A vast majority of spunlace fabrics are produced in basic weights ranging from about 20 g m−2 to about 100 g m−2. A web of weight less than 20 g m−2 does not develop enough integrity unless an auxiliary means of bonding or very low denier ﬁ bers providing high number of ﬁ bers are used. Webs weighing more than about 100 g m−2 are usually too heavy to be penetrated by water jets, unless very high energy levels are used.

Meltblown process Meltblowing is a unique, one-step process in which the melt of a polymer emerging from orifi ces is blown into super fi ne fi bers by hot, high-velocity air. A molten polymer is blown into ultrafi ne fi bers and collected on a rotary drum or a forming belt with a vacuum underneath the surface to form a nonwoven web. Two hot air streams at near sonic velocity at the polymer exit attenuate the extruded stream of polymer (Fig. 3.21). A typical meltblown web contains fi bers of a range of sizes (Fig. 3.22); however, typically, the fi bers are 1–10 μm in diameter. The fi bers are tacky and tend to stick to each other and take the shape of the apron or object on which they are collected. It is recognized that the fi bers are weak

Heated air Polymer

Screw extruder Gear Collecor

Die body 3.21 Schematic of a meltblowing process.

3.22 Micrograph of a meltblown fabric showing variable ﬁ ber size.86

and the web lacks uniformity. This currently restricts the application of meltblown structures to non-load-bearing products. However, a meltblown web may be combined with a web made by another method and form a composite that not only has improved mechanical properties but also other desirable physical properties. Such products contain ﬁ bers of different types and sizes, which can lead to novel structures with properties not possible from a single material or process.87 Although polypropylene has been the material used most widely, other polymers have also been used successfully, these being polyethylene, polyester, and nylon. Essentially, any thermoplastic polymer, including biodegradable, can be used.

Polymer web process These processes essentially lead to sheet materials such as thin foams, plastic nets,82,88 perforated ﬁ lms,83,89 and similar materials. The methods of manufacturing these miscellaneous materials are as diverse as their natures. They frequently arise from modiﬁ ed plastic processing techniques and often represent the combination of technologies. In general, the products are not hydrophilic and, hence, not inherently absorbent. Because of their structure and characteristics, however, these materials can often substitute for a component of an absorbent product, including non-adherent contact layer and protective outer layer of a dressing.

Advanced composites from combination of technologies Attractive composites can be made by a combination of airlaid and spunlace processes. One example is infusing pulp by airlaying onto a regular cotton or rayon web during hydroentangling. Such a product has similar or superior absorbency of 100% cotton or rayon fabric but at a fraction of the cost. A typical blend for pulp and ﬁ ber will be 50/50. Such a structure can be used as the absorbent layer in a dressing. Another structure that has received great emphasis during the past two decades is one that combines spunbond and meltblown processes in producing a composite fabric. Examples of these are the spunbond (SB)/meltblown (MB), known as SM, and the SB/MB/SB or SMS composites. Often two layers of meltblown are sandwiched between two of spunbond to lead to SMMS. The production and properties of these are particularly enhanced by the use of polypropylene/polyethylene bicomponent materials in the preparation of MB webs.90,91 The spunbond fabric, combined with ultra- lightweight meltblown fabric, is suited for use as facing for absorbent products (Fig. 3.23). Most interesting of the composite structures, however, that can lend to direct use as dressings, are the cellulose-centered non-wovens that are capable of being produced on an integrated line.70,92 These are laminates

that have cotton, or another cellulosic ﬁ ber web, sandwiched as a core between two layers of meltblown and/or spunbond webs, with the size and the characteristics of each of the three adjusted to suit the application. For developing such products, the absorbent material used could be cotton or rayon of regular length, fed from a pre-formed roll, or pulp of short length, deposited directly from an airlaying system on the site in coordination with the formation of the spunbond/meltblown webs (Fig. 3.24).

3.23 Micrograph of a polypropylene SMMS fabric.91

Spunbond extrusion Cotton nonwoven Cotton centered system spunbonded/meltblown composite

Heated calender rollers

Melt blown fabric 3.24 Preparation of cotton-centered composite with spunbond and meltblown webs forming the outer surfaces.

3.11 Acknowledgement The ﬁ gures included in this chapter were put together by Mr Venugopal Boppa, a graduate student in Textile Engineering at the North Carolina State University, Raleigh, NC 27695-8301. The authors express their thanks to Mr Boppa for this valuable assistance.

3.12 References

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C. W ELLER, Monash University, Australia

Abstract: Technological advances in the development of interactive wound dressings are reviewed. An important original work was that of George Winter, who demonstrated the value of a moist wound environment in wound healing. Recent advances in wound management incorporate new technologies that interact with the wound at a cellular level rather than simply reducing moisture loss. The balance of moisture is critical to healing and this principle has been the driving force in the development of products that are currently available such as hydrogels, hydrocolloids, alginates, and foams and ﬁ lms. Many interactive dressings are able to work actively with wound properties such as wound exudate, tissues, cells and some growth factors to enhance healing.

Key words: moist wound healing, tissue repair, interactive dressings, hydrogels, hydrocolloids, alginates.

4.1 Introduction As with many aspects of medicine, the fi eld of wound management has had an interesting journey. We have come a long way since the 16th century when Ambroise Pare travelled with the French army as a barber–surgeon. To counteract the purported poisonous effects of gunpowder, the wounds of injured soldiers were scalded with boiling oil. A shortage of boiling oil at the Battle of Turin forced Pare to concoct another therapeutic intervention for the doomed soldiers. He mixed a portion of egg yolks, turpentine, and oil of roses to help heal gun power wounds (an ancient roman turpentine remedy) and found, to his surprise, that the soldiers deprived of boiling oil not only survived but their wounds healed more quickly and with less pain. Pare had created the fi rst clinical trial in wound care (Baxter 2002). Pare may have, unknowingly, also concocted one of the fi rst interactive moist wound dressings. Since the 16th century, there have been many periods of enlightenment in the area of wound management. Through World War I, the task of changing dressings was the realm of physicians and medical students. In the 1930s, the dressing changes were performed by nurses. For the next 40

years, the mainstay dressings were gauze, cotton wool pads, paraffi n impregnated gauze and absorbent cotton. A major development in moist wound healing was the original work by George Winter which demonstrated the value of a moist wound environment in wound healing, although the concept of moist wound healing did not begin to receive serious consideration until the 1970s and 1980s. Our understanding of the function of wound healing has increased remarkably since Winter’s work ‘Formation of a scab and the rate of epithelialisation of superfi cial wounds in the skin of young domestic pigs’ was published in Nature (Winter 1962). This work was the foundation of the concept of moist wound management as it showed that wounds that heal under moist conditions healed 50% faster than those wounds that healed in dry, open to air, conditions. Moist wound healing not only facilitates fragile epithelial cells to migrate more freely it also enhances viability of the epithelial cells as the cells are protected from dehydration and scab formation. Epithelialisation of the wound therefore occurs more quickly if a moist wound environment is maintained (Weller 2005). Before the work of Winter and the human validation of Winter’s research (Hinman and Maibach 1963), practitioners commonly accepted the notion that wound healing depended on maintaining a dry wound bed, The scab has, in the past, been described as ‘nature’s dressing’. Today, most clinicians are more likely to accept that a moist wound environment facilitates the healing of wounds and promotes the growth of new tissue. Recent advances in wound management incorporate new technologies that interact with the wound at a cellular level rather than simply reducing moisture loss (Leaper et al. 2002). A balanced moist wound environment aids cellular growth and collagen proliferation within a non-cellular matrix. The balance of moisture is critical to healing and this principle has been the driving force in the development of products that are currently available. These are hydrogels, hydrocolloids, alginates, and foams and fi lms. Nowadays, many of these dressings also incorporate antibacterials in their delivery systems (Leaper et al. 2002).

4.2 Normal wound healing The healing of a wound requires a well-orchestrated integration of complex biological and molecular events of cell migration, cell proliferation and extracellular matrix (ECM) deposition. In cutaneous injuries that heal readily and do not have an underlying pathophysiological defect (acute wounds), the main aim of the body is to achieve a quick repair expending the least amount of energy. This often means such wounds will heal with a scar and no regeneration (Falanga 2005). In wounds with underlying

pathophysiological abnormalities such as venous leg ulcers, diabetic foot ulcers and full thickness burn injury evolutionary adaptations may not have occurred and impaired healing is the result.

4.2.1 Phases of wound healing The basic elements of wound healing, with information mainly derived from experimental wounds in animals, cannot be separated and categorised in a clear defi nitive way. It is useful to divide the repair process into four overlapping phases of coagulation, infl ammation, migration– proliferation (including matrix deposition) and remodelling (Falanga 2005), each phase being modulated by different cytokines and growth factors (Leaper et al. 2002). The infl ammatory phase of wound repair starts almost immediately after wounding and lasts for up to several days when symptoms such as oedema, erythema, heat and pain are well known. The initial response to injury is vasoconstriction which stops blood loss and platelet activation promotes fi brin clot formation. The fi brin plug consists of platelets enmeshed in fi brinogen, fi bronectin, vitronectin and thrombospondin which not only forms a temporary wound cover but also protects against bacteria. Substances released by platelets include a wide range of growth factors including platelet-derived growth factor (PDGF) and transforming growth factor. These, as well as other growth factors have an early role in cell recruitment and ECM formation. Injury to the wound creates a hypoxic episode owing to blood vessel damage. Hypoxia increases keratinocytes migration, early angiogenesis, proliferation of fi broblasts and the synthesis of crucial growth factors and cytokines including PDGF and vascular endothelial growth factor. Once the infl ammatory phase is almost complete, wound contraction begins. Formation of ECM, proteins, angiogenesis, contraction and keratinocytes migration are important elements of these phases. The balance between contraction and wound closure depends on the depth and location of the wound and the presence of complications such as infection which could impair healing (Falanga 2005). Acute wounds progress through the linear process of overlapping biological and molecular events, whereas chronic wounds are often in different phases at the same time and progression may not occur in synchrony (Falanga 2005). This ‘normal’ wound healing model has helped us understand the basic biology of tissue repair and use that knowledge when the healing process is impaired. We now know that chronic wounds contain decreased levels of some intrinsic growth factors, including PDGF, basic fi broblast factor, extracellular glycoprotein (ECG) and Transforming Growth Factor beta (TGFβ), compared with acute wounds (Harding et al. 2002).

4.3 Wound characteristics

The type, size, and depth of wound will all have important implications for healing events at the cellular and molecular level. In acute wounds and surgical wounds, there is usually less tissue damage and the wound will close by primary intention. Wounds that are restricted to the superfi cial layer of the dermis still have some keratinocytes in the hair follicles in the wound bed and therefore can heal from the edges as well as from within the wound bed (Enoch et al. 2006b). Determining wound duration, size and depth and wound bed condition are important aspects to consider in wound assessment. Specifi c wound characteristics correlate with healing. Patients with a large wound area (>2 cm2), an ulcer of long duration (>2 months), and ulcer depth (exposed tendon, ligament or bone) were three important factors that would delay healing (Margolis et al. 1999; Margolis et al. 2002; Margolis et al. 2004). As research and understanding improves at a cellular level, we are better able to assist the body not only by covering the wound to protect it but also by providing wound dressings to aid the healing process. Normal wound healing processes require restoration of epithelisation and collagen formation. The fi rst occurs by migration and proliferation of keratinocytes from the wound edges and by differentiation of stem cells from remaining hair follicle bulbs. The second occurs by infl ux of growth factors secreted by macrophages, platelets and fi broblasts, by fi broblast proliferation and subsequent synthesis and remodelling of collagenous dermal matrix. However, for full thickness burn injuries and chronic wounds such as pressure ulcers, venous ulcers and diabetic foot ulcers, these processes are damaged. Inter- active wound dressings have been developed to improve healing in these chronic conditions (Sibbald et al. 2003). The time it takes for a chronic wound to heal will vary due to the idio- syncratic nature of each wound and inherent complex factors, which may impede healing. If a wound is slow to heal, there may be an underlying reason for non-healing. This may because of vascular disease or mechanical injury such as trauma or pressure. The cause will need to be identifi ed and addressed if the wound is to be healed. For venous ulcers, we would not only consider the dressing choice but we would also need to use compression bandaging in order for the wound to heal. Controlling factors affecting healing requires an understanding of both intrinsic and extrinsic factors and addressing these where possible to improve healing. Physical factors such as diabetes mellitus, obesity, venous insuffi ciency, decreased perfusion, malnutrition, old age, organ failure, sepsis, and restrictions on mobility have an impact on healing (Phillips et al. 2000). Correcting underlying pathology and any co-morbidities is

central to managing the wound. The inﬂ ammatory process is important in acute wound healing and if disturbed will affect healing and increase the risk of wound chronicity. Immunosuppressive states and the use of immunosuppressive drugs such as corticosteroids or methotrexate that are known to affect the immuno-inﬂ ammatory response all adversely affect wound healing and increase the risk of sepsis. (Burns and Pieper 2000, Troppmann et al. 2003). Tissue repair research and advances in moist wound healing pharmaceuticals have been pivotal in improving wound dressing technology and facilitating the moist wound healing principles (Schonfeld et al. 2000). Different products are able to simplify debridement by aiding autolysis and wound hydration, facilitate the wound cleansing process, aid in the granulation and epithelialisation process and in some instances allow growth factors, cells and enzymes to be activated to an optimal level for healing to take place. Wound bed preparation is paramount to improved healing. An important factor in wound bed preparation is the maintenance of moisture balance, which often involves exudate management. Failure to manage exudate adequately can expose the peri wound skin to toxic wound exudate that may impede the healing process. The TIME (Table 4.1) principle of wound bed preparation was developed by an International Advisory Board on wound bed preparation (Schultz et al. 2004; Schultz et al. 2003). The TIME principle is based on four factors: wound factors, clinical action factors, suggested product selection and the healing outcome.

Table 4.1 TIME wound bed preparation

Description Factors Clinical action

Tissue non-viable Slough or necrotic Remove defective tissue tissue present (surgical or autolytic debridement) Inﬂ ammation and/or Increased exudate, Remove or reduce bacterial infection surface discoloration load (antimicrobials, or increased odour debridement of devitalised tissue) Moisture imbalance Heavy exudate, risk of Restore moisture balance maceration or dry (absorb exudate or add wound bed – risk of moisture to dry wounds) desiccation Edge of wound not Chronic wound with Address T/I/M factors advancing prolonged inﬂ ammation

Moist wound healing processes that have been researched and determined to be signiﬁ cantly more effective than the dry healing of the past have improved healing outcomes. However, it has become evident that moist wound healing alone may not be sufﬁ cient to improve healing outcomes. Equally as important is a holistic approach to wound care. A holistic approach addresses the whole patient in a medical, environmental and social context. Four simple actions underlie moist wound management practice: 1. Determine aetiology of the wound; 2. Identify any co-morbidities or complications that may contribute to wound recurrence or delay healing; 3. Assess the status of the wound and select dressing product; and 4. Review management plan with patient.

4.4 Dressings An ideal dressing not only maintains a moist environment but also: • absorbs excess exudate and prevent maceration of surrounding skin and prevent the wound from desiccation;

• allows gaseous exchange so that oxygen, water vapour and CO2 may pass in and out of the dressing; • provides thermal insulation to maintain the optimum wound core temperature, and • provides a barrier to bacteria to minimise contamination of the wound. The interactive ideal dressing should also be: • free of toxic or particulate components, • atraumatic on removal, • comfortable and conformable, and • cost effective. (adapted from Ovington and Pierce 2001)

4.4.1 Choosing the appropriate dressing product As wounds are dynamic and undergo different phases of healing, the choice of dressing will change with each phase (Cutting and White 2002). Dressing choice is based on three simple criteria: the colour of wound bed tissue, the depth of the wound and the level of exudate. Dressing choice can be based on the colour, depth and exudate (CDE) rule as shown in Table 4.2.

Table 4.2 Wound assessment and dressing selection

Colour Exudate Aim and dressing selection

Pink Nil Maintain moist environment, protect and insulate. Foams, thin hydrocolloids, thin hydroactives, fi lm dressings and simple non-adherent dressing such as the modern tulles will provide the necessary cover. Red unbroken Low Prevent skin breakdown. Thin hydrocolloids or fi lm dressings provide protection. Red High Maintain moist environment, absorb exudate and promote granulation and epithelialisation. Foams, alginates and hydroactive dressings help control exudate. Hydrocolloids as paste or powder for deeper areas. Red Low Maintain moist environment and promote granulation and epithelialisation. Hydrocolloid, foams, sheet hydrogels and fi lm dressings will maintain a moist environment. It is possible to use a combination of amorphous hydrogels with a foam cavity dressing in deeper wounds. Yellow Low Remove slough absorbs exudate and maintain a moist environment. Hydrogel in particular will re-hydrate the slough. Hydrocolloids will also aid in autolysis. Yellow High Remove slough and absorb exudate Hydrocolloids as paste or powder for the deeper wounds. Alginates will aid in the removal of the slough and absorb the exudate. Black Low Rehydrate and loosen eschar. Surgical debridement is the most effective method of removal of necrotic material; Dressings can enhance autolytic debridement of the eschar. Amorphous hydrogels, hydrocolloid sheet. Green High Absorb infected exudate. Hypertonic saline, silver products, cadexamer iodine.

4.4.2 Colour The Red wound is a granulating wound with new tissue fi lling the defi cit and possibly with some islets of epithelium present. The aim is to absorb any excess exudate, maintain a moist environment and protect the wound. The Yellow wound contains a level of slough. This is non-viable tissue that must be removed or healing will not take place. The methods of removal are surgical, re-hydration with dressings such as hydrogels or hydrocolloids. The aim is slough removal by re-hydration and absorption of exudate. The Pink wound is in the fi nal stages of healing with new epithelium covering the wound. The aim is to protect this very delicate tissue, prevent the wound from drying out so as to maintain a moist environment, and insulate the wound. The Black wound has an outer layer of thick hard eschar; this must be removed to commence the healing process. The fastest and most effective method is by surgical removal. The use of dressings such as hydrogels to aid autolytic debridement will at best be slow. The Green wound term is often used to describe an infected wound. The use of topical antibiotics is generally discouraged. The most appropriate treatment is to control high exudate levels, protect surrounding skin from toxic wound exudate, identify micro-organisms by wound biopsy and treat the client with systemic antibiotics.

4.4.3 Depth The wound may be superﬁ cial, partial thickness, deep or a cavity. The product choice will depend on the shape, location and type of wound. The shape and depth of the wound will vary depending on underlying aetiology. For example, venous ulcers are often superﬁ cial and highly exudating whereas arterial ulcers are deeper and have a ‘punched out’ appearance with little exudate. In some instances, the wound may have undermined edges and a careful assessment is paramount to ensure that the dressing product chosen will be easily removed and prevent residual product from being left in the cavity.

4.4.4 Exudate Exudate is produced from ﬂ uid that has leaked out of blood vessels and closely resembles blood plasma. Fluid leaks from capillaries into tissue at a rate that is determined by the permeability of the capillaries

and the hydrostatic and osmotic pressures across the capillary walls. Most wounds will contain some exudate; this will vary from very little to copious levels. In a healing wound, exudate production generally reduces over time. In a chronic wound that is not healing, exudate production may continue and be excessive due to ongoing infl ammatory processes (WUWHS). Studies suggest that wound fl uid from acute wounds may have a benefi - cial effect on wound healing, whereas that of chronic wounds may inhibit healing. Any unexpected change in exudate characteristics may indicate a change in wound status and may indicate the presence of infection. Careful monitoring of the exudate can provide information for the application of systemic and local therapies. Individual wound care products have specifi c functions which relate to the volume, viscosity and nature of the exudate and these should guide skin care and dressing selection (Vowden and Vowden, 2003). The choice of both primary and secondary dressing will depend on the exudate level and the depth of the wound (White and Cutting 2004). The balance of moisture in the wound environment can be maintained primarily by applying newer generation interactive wound dressings.

4.5 Interactive wound dressings Modern interactive dressings facilitate wound healing by altering the environment and interacting with the wound surface to optimise the healing process. Many interactive dressings have the ability to work actively with wound properties such as wound exudate, tissues, cells and some growth factors to enhance healing. Interactive dressings have different properties and vary in the ability to absorb exudate (Bale et al. 2001). Selecting the most appropriate dressing to treat the wound can be complicated and understanding form and function is often a challenge for busy clinicians (Carville 2006). Effective wound management requires knowledge of the process of tissue repair and the knowledge of the properties of the dressings available. Dressing selection is only one part of a holistic wound management plan, it is necessary to assess the patient’s concerns before assessing the wound and choosing the dressing (Schultz et al. 2003). Interactive dressings alter the wound environment and interact with the wound surface to optimise healing. Interactive dressings use the environment provided by the body to encourage normal healing. Bioactive dressings stimulate the healing cascade. Interactive dressing form, function, properties and some of the indications of the commonly used dressing groups in clinical practice are outlined in Table 4.3.

Dressing form Brand names Properties and indications

Semi-permeable fi lms Opsite (Flexigrid, Flexifi x, Post-op), Non-absorbent, semi-permeable, waterproof, Tegaderm, Bioclusive, Cutifi lm, allow wound visualisation. Superfi cial burns, Polyskin, Hydrofi lm and Aqua grazes, closed surgical incisions, small skin Protect Film tears and IV sites. May be used as a secondary dressing. Contraindications: Known infection or fragile skin. Foams Allevyn (adhesive, cavity), Lyofoam Protects wound and periwound. Thermal (Flat, Extra, C, T, A), Curafoam, insulation. Useful for moderately to heavily Hydrosorb, Tegafoam, exudating wounds. Not effective for wounds Permafoam, Truefoam and with dry eschar. Superfi cial and cavity Cavicare wounds, Venous ulcers (with compression), pretibial lacerations, infected ulcers, skin tears, pressure ulcers, skin grafts or donor sites and pilonidal sinuses. May require secondary dressing. Alginates Kaltostat, Sorbsan, Algoderm, Need wound exudate to function. Curasorb, Comfeel Seasorb and Most suitable for heavily exudating wounds Algisite M. Melgisorb such as venous ulcers, pressure ulcers and dehisced abdominal wounds. Forms moist gel in wound. Highly absorptive. Fills dead space.

© 2009 Woodhead Publishing Limited Hydrocolloids Duoderm (Extra thin, CGF, paste), Useful for light to moderately exudating wounds Comfeel (ulcer dressing, that would beneﬁ t from autolytic transparent, contour dressing), debridement. Leg ulcers, pressure ulcers, Tegasorb, Replicare, Combiderm burns and donor sites. Thin sheets are useful and Hydrocoll over suture lines and IV sites. Do not use occlusive sheets on infected wounds. Reduces pain. Hydrogels Intrasite Gel (amorphous Absorbency of hydrogels is limited and best unpreserved). Intrasite used in minimally exudating wounds or Conformable (gauze dehydrated wounds such as minor burns, impregnated). Solosite Gel grazes/lacerations, donor sites and pressure (amorphous preserved). Solugel ulcers. Soothing and reduces pain. Rehydrates (amorphous preserved and dry wound. Comes in amorphous gel, sheets, unpreserved). Duoderm Gel and freeze dry forms. Indications for the (amorphous unpreserved). Nu-gel thicker gel products include protection of (sheet hydrogel). Comfeel Purilon exposed tendon and/or bone from Gel (amorphous). Curaﬁ l dehydrating and rehydrating eschar before (amorphous). Curagel (sheet debridement. The thinner gel products are hydrogel). Aquaclear (sheet useful for soothing burns and acute chicken- hydrogel). Hypergel (hypertonic pox lesions. saline) (amorphous). Steri Gel (amorphous). Second skin (sheet hydrogel)

4.5.1 Semi-permeable fi lm dressings Film dressings are adhesive, thin transparent polyurethane, which are permeable to gas but impermeable to liquid and bacteria. Films are elastic, conformable and transparent allowing inspection of wound. As fi lms are non-absorbent, they are not suitable for exudating wounds, although island dressings with a central non-stick pad are available and can absorb slightly more exudate that the simple fi lms. The three factors that most drastically affect the pattern, speed and quality of healing are dehydration of exposed tissues, the status of the blood supply bringing oxygen and nutrients to the area, and sepsis. Wounds exposed to the air lose water vapour, the upper dermis dries and healing takes place beneath a dry scab. Covering a wound with an occlusive dressing prevents scab formation and radically alters the pattern of epidermal wound healing (Winter 2006). Film dressings are indicated as primary dressings in minor burns, simple abrasions and lacerations. It is also common to use fi lms as a post-operative layer over dry sutured wounds. Films can also be used as secondary dressings to waterproof a primary dressing such as foam. Films can be used as a protective layer on skin that is prone to superfi cial pressure ulcers, although incorrect removal of fi lm dressings when applied to fragile skin may cause trauma to surrounding skin so they should be used with care. Films should be discontinued if the level of exudate results in pooling under the dressing as not only is the dressing choice incorrect but it may also cause maceration to the surrounding skin and increase the risk of infection. Films may stay in place for up to one week and frequency of change may be dependent on position of wound, type and size of wound.

4.5.2 Foam dressings Foam dressings are made from polyurethane, which may in some cases have been heat-treated on one side to create a semi-permeable membrane. This allows the passage of exudate through the non-adherent, semi-permeable surface into the insulating foam. These open-cell sheets with varying cell sizes come in single, double or multilayer products. Foams have several advantages; they are highly absorbent, cushioning and protective, and in sulate and conform well to body surfaces. Foams facilitate a moist wound environment and absorb excess exudate to decrease the risk of maceration. Foam dressings are also available with charcoal impregnation for malodorous wounds. Foam wound cavity dressings reduce dead space in the wound, conform to wound shape and absorb large amounts of exudate therefore reducing the need for frequent dressing changes, although cavity foam dressings

require secondary dressings that adds to cost. Foams are generally non- adhesive and require a secondary dressing or tape/bandage to keep in place. Depending on the level of exudate, foams can be left in place for up to 7 days (Carville 2006). A prospective open-label study to assess the clinical performance of foam dressings in chronic wounds found a signiﬁ cant decrease in the level of exudate from the wound bed, the proportion of patients with peri-wound skin problems and the percentage of patients with wound pain. Foam dressings are indicated in venous ulcers with high exudate levels (Zoellner et al. 2006).

4.5.3 Alginate dressings Alginate may be either calcium or calcium/sodium salts of alginic acid. These products are highly absorbent, biodegradable dressings derived from seaweed. An active ion exchange of calcium ions for sodium ions at the wound surface forms soluble sodium alginate gel that provides a moist wound environment. The polysaccharides are made up as textile fi bre or as freeze-dried sheets. Calcium dressings need exudate from the wound to function and therefore are not suitable for dry wounds or wounds with hardened eschar. When placed into an exudating wound, the sodium ions present in the wound fl uid displace the calcium and forms sodium alginate. The fi brous nature of most alginates can leave residual fi bres in the wound if there is insuffi cient wound exudate to gel the fi bres. This may precipitate an infl ammatory reaction as it stimulates a foreign body response. Caution is also needed when using alginate rope dressings into very deep or narrow sinuses, as complete removal can be diffi cult. Studies have shown that some calcium alginate products promote haemostasis in bleeding wounds owing to the active release of calcium ions that aids the clotting mechanism (Segal et al. 1998). The calcium ions released by this type of dressing are a natural co-factor in the coagulation therapy. Alginate dressings are available in sheet, ribbon or rope form in various sizes and require a secondary dressing. Alginates are used for wounds with moderate to heavy exudate, haemostasis post-debridement or biopsy.

4.5.4 Hydrocolloid dressings Hydrocolloids are moisture-retentive dressings, which contain gel-forming agents such as sodium carboxymethylcellulose and gelatin. Many products combine the gel-forming properties with elastomers and adhesives are applied to a carrier such as foam or ﬁ lm to form an absorbent, self- adhesive, waterproof wafer. In the presence of wound exudate hydrocolloids absorb liquid and form a gel, the properties of which are determined

by the nature of the formulation (Heenan 2007). In sheet form, the polymer outer layer can be either semi-occlusive or occlusive. Hydrocolloid interaction debrides by autolysis and can reduce dressing frequency by up to 7 days’ wear time depending on amount of exudate and the type of the hydrocolloid product (Segal et al. 1998). The product is also available in paste and powders for increased absorption and to decrease dead space in the wound cavity. Generally, hydrocolloids are not recommended on clinically infected wounds due to the semi-occlusive nature of the dressing. There have been reports of hypergranulation with prolonged use of hydrocolloids in moderate to highly exudating wounds, so wound tissue assessment is paramount when applying hydrocolloids over a period of months so that the product can be discontinued before hypergranulation occurs. Hydrocolloid dressings are indicated for chronic venous ulcers, pressure ulcers, burns, partial thickness wounds, diabetic foot ulcers.

4.5.5 Hydrogel dressings Hydrogel dressings are semi-occlusive and composed of complex hydro- phyllic polymers with a high (90%) water content. As the name implies, hydrogels are designed to hydrate wounds, re-hydrate eschar and aid in autolytic debridement. Hydrogels are insoluble polymers that expand in water and are available in sheet, amorphous gel or sheet hydrogel- impregnated dressings. Hydrogels provide a moist environment for cell migration and absorb some exudate. Autolytic debridement without harm to granulation or epithelial cells is another advantage of hydrogel dressings. Hydrogels are recommended for wounds that range from dry to mildly exudating and can be used to degrade slough on the wound surface. Hydrogels have a marked cooling and soothing effect on the skin, which is valuable in burns and painful wounds. In addition to their use in wounds, the thin hydrogels are helpful in the management of chicken pox and shingles. The viscosity varies between products, Purilon and IntraSite gel are two of the thickest gels available to help the product stay in the cavity of the wound and Solugel and Solosite are two of the thinnest for easy spread over a larger area (Carville 2006). Some amorphous gels contain propylene glycol that can cause allergic reactions in elderly skin. Amor- phous hydrogels are applied liberally onto or into a wound and covered with a secondary dressing such as foam or ﬁ lm. Hydrogels can remain in situ for up to 3 days. Hydrogels are indicated in dry, sloughy wounds with mild exudate, partial thickness wounds.

4.6 Future trends Novel techniques are being developed to improve healing in difﬁ cult- to-treat wounds. Emerging dressing types include bioactive dressings and

tissue engineered skin substitutes. Randomised controlled clinical trials to examine safety and effi cacy of many of the new materials are lacking and clinical uptake has been slow (Begg et al. 1996; Enoch et al. 2006b). When choosing the optimum treatment for patients, it is important to take into account ease of use, patient satisfaction and cost effectiveness (Carville 2006). Research into genetic coding mechanisms to identify factors infl uencing wound healing will be an area to monitor. Several markers that could form the basis of new diagnostic tools for use in wound care are currently under investigation, ongoing research is likely to discover others. Some markers under current investigation are: platelet-derived growth factor, sex steroids, thyroid hormones, immunohistochemical markers, infl ammatory mediators such as cytokines and interleukins to monitor healing status. Nitric oxide could be clinically useful in predicting wound outcomes and interventions to alter wound nitric oxide levels may be of benefi t. These markers may be able to aid the clinician in choosing appropriate dressing products depending on the phase of the wound, tissue colour, depth of wound and exudate level (Enoch et al. 2006a). Current areas of research include regulation of target genes of cells involved in wound healing. New methods of delivery of cell products to the wound bed include stem cells that are capable of differentiating into essential cells involved in wound healing. To date, there is only experimental evidence for gene and stem cell therapy in the treatment of chronic wounds (Enoch et al. 2006b). The cost-effectiveness of new treatments compared with standard care must also be considered, not only in terms of direct treatment costs, but in terms of length of initial hospital stay, requirements for home and community care, secondary dressing needs, and the quality of the overall outcome.

4.7 Conclusions Technological advancement in interactive dressing development has grown since Winter’s research. Availability of different types of wound products has increased remarkably in the last decade. New wound dressing technologies continue to be developed to improve healing in difﬁ cult to treat wounds (Embil et al. 2000). Wound care practitioners have at their disposal an extensive range of interactive dressings. Emerging dressing types include bioactive dressings and tissue-engineered skin substitutes. Wound dressings have come a long way since Pare’s effort in the battle of Turin. Tissue repair research and advances in moist wound healing pharmaceuticals has been pivotal in the improvement of wound product technology. This review has characterised the different types of established interactive dressings. For new dressing types, ‘bioactivity’ appears to be the way forward in maintaining a moist healing environment, offering antimicrobial properties and cellular interactions.

4.8 Sources of further information and advice European Wound Management Association (EWMA) World Union of Wound Healing Societies (WUWHS) WUWHS Principles of best practice: wound exudate and the role of dressings. A consensus document EWMA position document 2005: Identifying criteria for wound infection. EWMA position document 2004: Wound bed preparation in practice. EWMA position document 2002: Pain at wound dressing changes Cochrane wound group www.cochranewounds.org

4.9 References

bale, s., baker, n., crook, h., rayman, a., rayman, g. and harding, k. g. (2001). Exploring the use of an alginate dressing for diabetic foot ulcers. J Wound Care, 10(3), 81–84. baxter, h. (2002). How a discipline came of age: a history of wound care. J Wound Care, 11(10), 383–386, 388, 390. begg, c., cho, m., eastwood, s., horton, r., moher, d., olkin, i., et al. (1996). Improving the quality of reporting of randomized controlled trials. The CONSORT statement. JAMA, 276(8), 637–639. burns, j. and pieper, b. (2000). HIV/AIDS: impact on healing. Ostomy Wound Manage, 46(3), 30–40, 42, 44; quiz 48–39. carville, k. (2006). Which dressing should I use? It all depends on the ‘TIMEING’. Aust Fam Physician, 35(7), 486–489. cutting, k. f. and white, r . j. (2002). Maceration of the skin and wound bed. 1: Its nature and causes. J Wound Care, 11(7), 275–278. embil, j. m., papp, k., sibbald, g., tousignant, j., smiell, j. m., wong, b., et al. (2000). Recombinant human platelet-derived growth factor-BB (becaplermin) for healing chronic lower extremity diabetic ulcers: an open-label clinical evaluation of efﬁ cacy. Wound Repair Regen, 8(3), 162–168. enoch, s., grey, j. e. and harding, k. g. (2006a). ABC of wound healing. Non-surgical and drug treatments. BMJ, 332(7546), 900–903. enoch, s., grey, j. e. and harding, k. g. (2006b). Recent advances and emerging treatments. BMJ, 332(7547), 962–965. falanga, v. (2005). Wound healing and its impairment in the diabetic foot. Lancet, 366(9498), 1736–1743. harding, k. g., morris, h. l. and patel, g. k. (2002). Science, medicine and the future: healing chronic wounds. BMJ, 324(7330), 160–163. heenan, a. (2007). Alginates: an effective primary dressing for exuding wounds. Nurs Stand, 22(7), 53–54, 56, 58. hinman, c. d. and maibach, h. (1963). Effect of air exposure and occlusion on experimental human skin wounds. Nature, 200, 377–378. leaper, et al. (2002). Growth factors and interactive dressings in wound repair, EWMA Journal, 2(2), 17–23.

margolis, d. j., berlin, j. a. and strom, b. l. (1999). Risk factors associated with the failure of a venous leg ulcer to heal. Arch Dermatol, 135(8), 920–926. margolis, d. j., bilker, w., santanna, j. and baumgarten, m. (2002). Venous leg ulcer: incidence and prevalence in the elderly. J Am Acad Dermatol, 46(3), 381–386. margolis, d. j., knauss, j. and bilker, w. (2004). Medical conditions associated with venous leg ulcers. Br J Dermatol, 150(2), 267–273. ovington, l. g., pierce, b. (2001). Wound dressings: form, function, feasibility and facts. In: Krasner, D, Rodeheaver G, Sibbald G (Eds). Chronic wound care: a clinical sourcebook for healthcare professionals. Wayne, PA: Management Publications Inc., 311–319. phillips, t. j., machado, f., trout, r., porter, j., olin, j. and falanga, v. (2000). Prognostic indicators in venous ulcers. J Am Acad Dermatol, 43(4), 627–630. schonfeld, w. h., villa, k. f., fastenau, j. m., mazonson, p. d. and falanga, v. (2000). An economic assessment of Apligraf (Graftskin) for the treatment of hard-to-heal venous leg ulcers. Wound Repair Regen, 8(4), 251–257. schultz, g. s., barillo, d. j., mozingo, d. w. and chin, g. a. (2004). Wound bed preparation and a brief history of TIME. Int Wound J, 1(1), 19–32. schultz, g. s., sibbald, r. g., falanga, v., ayello, e. a., dowsett, c., harding, k., et al. (2003). Wound bed preparation: a systematic approach to wound management. Wound Repair Regen, 11 Suppl 1, S1–28. segal, h. c., hunt, b. j. and gilding, k. (1998). The effects of alginate and non-alginate wound dressings on blood coagulation and platelet activation. J Biomater Appl, 12(3), 249–257. sibbald, r. g., orsted, h., schultz, g. s., coutts, p. and keast, d. (2003). Preparing the wound bed 2003: focus on infection and inﬂ ammation. Ostomy Wound Manage, 49(11), 23–51. troppmann, c., pierce, j. l., gandhi, m. m., gallay, b. j., mcvicar, j. p. and perez, r. v. (2003). Higher surgical wound complication rates with sirolimus immuno- suppression after kidney transplantation: a matched-pair pilot study. Transplan- tation, 76(2), 426–429. vowden, k. and vowden, p. (2003). Understanding exudate management and the role of exudate in the healing process. Br J Community Nurs, 8(11 Suppl), 4–13. weller, c. d., sussman, g. (2006). Wound dressings update. Journal of Pharmacy Practice and Research, 36(4), 318–324. white, r. j., cutting, k. f. (2004). Maceration of the skin and wound bed by indication. In: White, R. J. (Ed), Trends in wound care III, Quay Books, 23–29. winter, g. d. (1962). Formation of the scab and the rate of epithelization of super- ﬁ cial wounds in the skin of the young domestic pig. Nature, 193, 293–294. winter, g. d. (2006). Some factors affecting skin and wound healing. J Tissue Viability, 16(2), 20–23. zoellner, p., kapp, h. and smola, h. (2006). A prospective, open-label study to assess the clinical performance of a foam dressing in the management of chronic wounds. Ostomy Wound Manage, 52(5), 34–36, 38, 40-32.

G. SCHOU K ENS, Ghent University, Belgium

Abstract: The role of bioactive dressings derived from natural resources in managing both acute and chronic wounds is discussed. Bioactive dressings deliver substances active in wound healing either by delivery of bioactive compounds or by being constructed from materials having endogenous activity. These materials include hydrocolloids, hydrogels, alginates, collagens, honey dressings, chitosan, chitin, derivatives from chitosan or chitin and biotextiles. The application of dibutyrylchitin (DBC) derived from chitin is extensively reviewed. The O-butyrylation of chitin or chitosan is an important modiﬁ cation of chitin which increases the biochemical activity of chitin or chitosan. Future developments in bioactive wound dressing materials based on chitin, chitosan and their derivatives are indicated using the increased knowledge and understanding of the action of these dressings in wound healing.

Key words: bioactive dressing, wound healing, alginate, hydrocolloids, chitin.

5.1 Introduction Bioactive dressings are dressings which deliver substances active in wound healing; either by delivery of bioactive compounds or constructed from materials having endogenous activity. These materials include hydrocolloids, alginates, collagens, chitosan, chitin, derivatives from chitosan or chitin and biotextiles. These bioactive dressings belong normally to the FDA dressing category of interactive wound and burn dressings. Wound healing is a natural restorative response to tissue injury. Healing is the interaction of a complex cascade of cellular events that generates resurfacing, reconstitution, and restoration of the tensile strength of injured skin. Healing is a systematic process, traditionally explained in terms of three classic phases: infl ammation, proliferation, and maturation or remodelling. In another scheme, the wound healing process can be divided into four continuous phases, namely haemostasis, infl ammation, proliferation and remodelling. A clot forms and infl ammatory cells debride injured tissue during the infl ammatory phase. Epithelialisation, fi broplasia, and angiogenesis occur during the proliferative phase. Meanwhile, granulation tissue forms and the wound begins to contract. Finally, during the maturation phase, collagen forms tight cross-links to other collagen and, with

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protein molecules, increases the tensile strength of the scar. The role of bioactive dressings derived from natural resources in managing both acute and chronic wounds is discussed in this chapter and the application of dibutyrylchitin (DBC) derived from chitin is extensively reviewed.

5.2 Physiology of wound healing 5.2.1 Inﬂ ammatory phase The early events of wound healing are characterised by the inﬂ ammatory phase, a vascular and cellular response to injury. An incision made through a full thickness of skin causes a disruption of the microvasculature and immediate hemorrhage. Following incision of the skin, a 5- to 10-min period of vasoconstriction ensues, mediated by epinephrine, norepine- phrine, prostaglandins, serotonin, and thromboxane. Vasoconstriction causes temporary blanching of the wound and functions to reduce hemorrhage immediately following tissue injury, aids in platelet aggregation, and keeps healing factors within the wound.

5.2.2 Proliferative phase Formation of granulation tissue is a central event during the proliferative phase. Infl ammatory cells, fi broblasts, and neovasculature in a matrix of fi bronectin, collagen, glycosaminoglycans, and proteoglycans comprise the granulation tissue. Granulation tissue formation occurs 3–5 days following injury and overlaps with the preceding infl ammatory phase.

5.2.3 Epithelialisation Epithelialisation is the formation of epithelium over a denuded surface. Epithelialisation of an incisional wound involves the migration of cells at the wound edges over a distance of less than 1 mm, from one side of the incision to the other. Incisional wounds are epithelialised within 24 to 48 h after injury. This epithelial layer provides a seal between the underlying wound and the environment. The process begins within hours of tissue injury. Epidermal cells at the wound edges undergo structural changes, allowing them to detach from their connections to other epidermal cells and to their basement membrane.

5.2.4 Collagen The synthesis and deposition of collagen is a critical event in the proliferative phase and to wound healing in general. Collagen consists of three

polypeptide chains, each twisted into a left-handed helix. Three chains of collagen aggregate by covalent bonds and twist into a right-handed super- helix, forming the basic collagen unit. A striking structural feature of collagen is that every third amino acid is glycine. This repeating structural feature is an absolute requirement for triple-helix formation. Collagen is rich in hydroxylysine and hydroxyproline moieties, which enable it to form strong cross-links. Defi ciencies of oxygen and vitamin C, in particular, result in underhydroxylated collagen that is less capable of forming strong cross-links and, therefore, are more vulnerable to breakdown. Approximately 80% of the collagen in normal skin is type I collagen; the remaining is mostly type III. In contrast, type III collagen is the primary component of early granulation tissue and is abundant in embryonic tissue. Collagen fi bres are deposited in a framework of fi bronectin. Collagen remodelling during the maturation phase depends on continued collagen synthesis in the presence of collagen destruction. During remodelling, collagen becomes increasingly organised. Fibronectin gradually disappears, and hyaluronic acid and glycosaminoglycans are replaced by proteoglycans. Type III collagen is replaced by type I collagen. Water is resorbed from the scar. These events allow collagen fi bres to lie closer together, facilitating collagen cross-linking and ultimately decreasing scar thickness.

5.2.5 pH of wounds Surface wounds develop, according to Leveen and others, a respiratory

alkalosis owing to escape of CO2 from the wound surface into the air. Therefore, the pH of surface wounds is in the region of pH 8. This alkalinity produces an inappropriate shift of the oxyhemoglobin–hemoglobin dissociation curve and reduces the available oxygen supply to the tissues. In contrast to alkalinisation of the wound, acidiﬁ cation was found to increase

the partial pressure of oxygen (pO2) of surface wounds by appropriate shift in the oxyhemoglobin–hemoglobin dissociation curve (Bohr effect). In addition, signifi cant quantities of ammonia were found on most surface wounds further contributing to the alkalinity of the wound. The concentrations of ammonia found in surface wounds were histotoxic. This histotoxicity is pH dependent. Ammonia is non-toxic in acid medium because of neutralisation. A number of chemicals suitable for clinical acidifi cation of wounds have already been investigated. A prerequisite is the prolonged chemical acidifi cation of clinical wounds. Acetic acid solutions do not maintain acidity for periods longer than one hour. The histotoxicity of ammonia provokes the slough found in infections caused by some potent urease-producing organisms. Chemical acidifi cation of wounds is most effective in minimising the toxicity of the ammonia formed by urease- producing organisms.

5.3 Principles and roles of bioactive dressings

From 1946, after the Second World War, experiments were carried out with a number of dressings based on products derived from polyamide or polyethylene, and, later on, polyurethane fi lm dressings. Later on, other interactive and bioactive dressings were developed such as impregnated gauzes, foam dressings, hydrogels, alginates, hydrocolloids, hydrofi bres, odour-absorbing dressings and, more recently, bioactive dressings based on chitin or chitosan and their derivatives. The most signifi cant advance in wound care came from Winter’s study1,2 in the 1960s, which showed that occluded wounds healed much faster than dry wounds and a moist wound healing environment optimised the healing rates. He demonstrated that, when wounds on pigs are kept moist, epithelialisation is twice as rapid as on wounds allowed to dry by exposure to air. Later Hinman and Maibach3 confi rmed his work on human beings in 1963. An open wound that is directly exposed to air will hydrate and a scar is formed. This forms a mechanical barrier to migrating epidermal cells, which are then forced to move into a deeper level of tissue, thus, prolonging the healing process. Moist healing prevents the formation of scar as the dressing absorbs wound exudate secreted from the ulcer. The ability of bioactive dressings to absorb exudates and provide a moist environment is a very important characteristic of these dressings. Another important aspect of these bioactive dressings is their ability to neutralise the wound alkalinity and to restore the natural pH of the wound or the skin. This requires the presence of an acidic carboxylic group (—COOH) in these bioactive dressings or the formation of acidic compounds owing to the hydrolysis of the applied dressings. These bioactive dressings are also affected by classical hydrolysis or are degraded by the action of enzymes, like hydrolases, lysozymes and/or matrix metalloproteinases. Non-healing chronic wounds differ from acute wounds signifi cantly. Whereas healing acute wounds have low levels of protein-degrading enzymes, exudates from non-healing chronic wounds contain elevated levels of proteases, like matrix metalloproteinases and elastase.4–6 More- over, the concentrations of pro-infl ammatory cytokines,7 as well as reactive oxygen species8 are signifi cantly higher, compared with the concentrations ⋅− in acute wounds. Reactive oxygen species, such as superoxide radicals (O 2 ) and hydroxyl radicals (OH⋅), as well as reactive nitrogen species, such as nitric oxide (NO⋅) arise from infl ammatory cells.9–11 An imbalance between reactive oxygen and nitrogen species and the antioxidant defence mechanism of a cell, leading to an excessive production of oxygen metabolites, leads to condition of ‘oxidative stress’. Overproduction of reactive oxygen and nitrogen species results in an imbalanced oxidant/antioxidant static in wounds and especially in chronic wounds.8,11,12 Owing to the resulting

disproportion between the degradation and remodelling processes, chronic wounds persist in the infl ammatory phase of the normal healing process for months or even years. The reduction in reactive oxygen and reactive nitrogen species in the wound fl uid seems to be a suitable way to stop the infl ammation process and diminished epithelisation and to support the normal healing process. The antioxidant capacity of bioactive dressing materials may also be an important characteristic for wound healing. The chemical structure of the bioactive dressing materials is important for this antioxidant capacity. Some bioactive dressing materials contain a large number of functional groups, such as —OH, —COOH, and others, which allow various interactions with other molecules via hydrogen bonds and electrostatic interactions. As studies with oxidised, phosphorylated, carboxymethylated and sulphonated cotton gauze have shown, the chemical modifi cation of the wound dressing leads to a decreased activity of proteases, such as matrix metalloproteinases and elastase, in chronic wound fl uids.13 A possible explanation is that the negatively charged groups of the gauze interact with arginine side chains on the surface of elastase. Studies of the topical application of compounds with free radical scavenging properties on patients have shown to signifi cantly improve healing and protect tissue from oxidative damage. However, a too high concentrated application of antioxidants may result in toxic response in the wound. To obtain good results on wound healing, a slow release of the antioxidant is required. Current concepts of modern wound management are focused on a moist wound environment. This is benefi cial for tissue remodelling (re-epithelialisation) owing to a good control of the humidity of the wounds, together with a better control of the pH of the wounds and the oxygen permeability and concentration in the wounds. In addition to these aspects and others, such as biocompatibility, biodegradability, and fl uid absorption, the understanding of the antioxidant capacity of the bioactive dressing materials is important for improved wound healing.

5.4 Types and structures of bioactive dressings 5.4.1 Alginates Wound dressing based on alginic material is well known, in the literature as well as from a commercial point of view, in wound management.14 Alginates are linear copolymers of β-(1-4)-linked d-mannuronic acid and α-(1-4)-linked l-guluronic acid units, which exist widely in many species of brown seaweeds. Alginate ﬁ bres can be prepared by extruding solutions of sodium alginate into a calcium salt solution bath or an acidic solution to produce the corresponding calcium alginate or alginic ﬁ bres, respectively.

Typically, the degree of crystallinity of the alginate fi bres is of the order of 30% with a degree of water absorption of 90% after immersion in water. Fibres of this type have been used extensively in wound dressing applications owing to their excellent biocompatibility, non-toxicity, and potential bioactivity. Alginate fi bres, typically as a calcium salt, interact with the wound exudates to form a moist gel, as a result of the ion exchange between the calcium ions in the fi bre and the sodium ions in the exudates.15 The gel-forming property of alginate eliminates fi bre entrapment in the wound and helps in removing the dressing without much trauma, thus, reducing the pain experienced by the patient during dressing changes.16 Such gela- tion provides the wound with a moist healing environment, which promotes healing and leads to better cosmetic repair of the wound. The gel-forming property provides a moist environment that leads to rapid granulation and re-epithelialisation.1 In a controlled clinical trial, a signifi cant number of patients dressed with calcium alginate was completely healed at day 10. In another study with burn patients, calcium alginate signifi cantly reduced the pain severity and was favoured by the nursing personnel because of its ease of care. This in situ generation of a moist healing environment and the consequent high absorbency of the alginate dressings are two of the outstanding properties, which make the alginate dressing one of the most versatile wound dressings available today. In addition, alginate-containing dressings have been demonstrated to activate macrophages within the chronic wound bed and generate a pro-infl ammatory signal which may initiate a resolving infl ammation characteristic of healing wounds.17 Therefore, many commercially available wound dressings used in wound management contain calcium alginate fi bres. The alginate dressings currently available on the market have mainly been manufactured from fi bres of calcium alginate or sodium–calcium alginate. Depending on the quantity of the mannuronic or guluronic acid derivatives in the fi bre-forming material, the fi bres change into gel form to varying degrees.18,19 Modern active dressings, besides their function of providing a moist wound environment, should also be adapted to the stage of wound healing and, on this basis, capable of stimulating the granulating process or protecting against the damage of a newly formed tissue.20 A possible advantage of alginate fi bres is that they are relatively easy to modify by incorporating appropriate metal ions, microelements or other biologically active substances that accelerate the healing process or have bacteriostatic properties.21 As an alternative to standard alginate dressing, new alginate dressings that have been trialled are said to have superior properties, e.g., zinc alginate fi bres. The replacement of sodium ions with zinc ions during solidifi cation allows zinc alginate fi bres with suitable properties for medical

applications to be prepared. Zinc alginate fi bres show high moisture absorption, an acceleration of the wound-healing process and antibacterial character resulting from the presence of zinc ions. These fi bres can be used for the manufacture of dressings to be used for wounds in subsequent healing stages. The bacteriostatic effects of zinc can be increased or extended by incorporating typical bactericidal or fungicidal agents into fi bres during the spinning stage. In addition to zinc ions, silver ions have been found to have antibacterial effects on some microbes. The silver salt is normally seen as the most effective antimicrobial agent in the treatment of burn patients.22 The so-called antibacterial fi bres can be obtained by treatment with an aqueous solution of silver nitrate. The calcium alginate fi bres were converted into calcium/ silver alginate fi bres by the treatment with silver nitrate. The calcium alginate fi bres have little antibacterial activity, but the calcium/silver alginate fi bres have good antibacterial activity toward Staphylococcus aureus.23 Various alginate wound dressing materials have been commercially utilised and reviewed in the literature. Some commercial alginate wound dressing materials include: Algisite® (Smith & Nephew), Kaltostat® (ConvaTec), Tegagen ® HG or HI (3M Health Care), Comfeel SeaSorb® (Coloplast AS), AlgiDERM (Bard), Algosteril® (Johnson & Johnson), CarraSorb H® (Carrington), CURASORB® (Kendall), Dermacea® (Sherwood-Davis & Geck), Fybron® (B. Braun), Gentell® (Gentell), Hyperion Advanced Alginate Dressing® (Hyperion Medical Inc.), Kalginate® (DeRoyal), and Maxorb® (Medline).

5.4.2 Hydrocolloids Hydrocolloids, fi rst patented in 1967,24 were originally used in stoma care. Later on, hydrocolloids were also used in practice for both acute and chronic wounds.25 According to Cockbill and Turner,26 classical hydrocolloids are supposed to consist of 40% polyisobutylene, 20% sodium carboxymethylcellulose, 20% gelatine and 20% pectin. The hydrocolloid dressing absorbs the wound fl uid and as a result changes into a jelly-like mass. The outside of the dressing is covered with a polyurethane foam or fi lm which enables the exchange of water vapour and protects the wound against contamination from the outside. In addition to this sheet form, hydrocolloids are also available as a paste or granules which are used to fi ll up the deep wounds.26 Hydrocolloids are said to aid the healing by creating a moist environment and, through an intensifi cation of the autolysis process, advance debridement.26 They are supposed to be comfortable, and time- and money-saving because dressing changes are less frequently required. The reasons for these frequently cited characteristics are not always based on research evidence.

Amorphous hydrogels are a category of products used for debridement. These gels function by keeping the wound moist and thereby enhancing autolytic debridement of necrotic tissue by enzymes generated in the body by infl ammatory cells. A too aggressive debridement can impair the healing as some of the active components may be cytotoxic.27 Often polyols are used for amorphous hydrogels as bacteriostatic agents, but are known for being potential allergenic. Some of the amorphous hydrogels may contain from 15 to 20 wt% of propylene glycol and from 2 to 4.5 wt% of sodium carboxymethylcellulose. The more recently developed amorphous hydrocolloids are based on water-insoluble, water-swellable cross-linked cellulose derivatives, an alginate and distilled water. These amorphous hydrocolloids contain between 3.3 and 4.8 wt% of cross-linked carboxymethylcellulose, between 0.3 and 0.53 wt% of alginate and the rest distilled water.28 These hydrocolloids show excellent debriding and good absorption properties, they speed up debridement of necrotic tissue in chronic wounds, and they speed up the healing of the wound without the use of propylene glycol. In experiments on wound healing, the amorphous hydrocolloids accelerate epithelialisation of clean superfi cial wounds by more than 20% compared with the control. Hydrocolloids are frequently used in the treatment of pressure ulcers. For this application, hydrocolloids are more effective than gauze dressings with regard to the number of healed wounds,29 the reduction of the pressure ulcer dimensions,30 the time needed for dressing changes,30–32 the absorption capacity, the pain during dressing changes33 and the side-effects.31 Compared with other bioactive dressings, such as alginate dressings or biosynthetic dressings, hydrocolloids are signifi cantly less effective.34 Commercial hydrocolloid dressings include Comfeel Contour® (Colo- plast AS), Granufl ex® (ConvaTec), Tegasorb ® (3M Health Care), and Duoderm® (ConvaTec).

5.4.3 Hydrogel Traditionally, wounded tissues have been covered with various ointments and gauzes. However, these treatments do not shield the wound from external contamination and do not sufﬁ ciently retain moisture. The gauzes often become embedded in the wound tissue causing damage and pain when wound dressings are changed. A more recent approach has been to use various forms of hydrogels as wound dressings. The term hydrogel refers to water-absorbing gel substances of varying rigidity. Hydrogels of least rigidity are often administrated by syringe while more rigid hydrogels are packed in sterile sachets opened immediately before applying the hydrogel to the wound.

Most of the hydrogel wound dressings are hydrogels based on algi- nates35–38 and alginate hydrogels can be obtained by cross-linking the alginate polymers. Such hydrogels contain large amounts of water without dissolution. Applications with the hydrogels of alginates include cell or tissue encapsulation, tissue engineering, drug delivery and hydrogel wound dressings. Most frequently, the basic structure of the hydrogel wound dressing is a matrix composed of either sodium or calcium alginate or a combination of both. Upon treating alginate with sodium or calcium ions, insoluble ﬁ bres are formed. This alginate matrix imparts a cohesive structure to the dressing and gives the hydrogels mechanical properties suitable for protecting a wound while remaining ﬂ exible. The amount of alginate incorporated in the hydrogels varies from 1 to 4 wt% according to the type used. The hydrogels are loaded with salt components, obtained by absorbing salts from sodium, magnesium, silver or other therapeutically acceptable salts into the hydrogel. These salts have advantageous disinfectant properties and various wound healing properties. For example, a 4 wt% sodium chloride solution consisting of sterilised sea salt is loaded in the hydrogel.39 This gives the hydrogel anti-bacterial properties and makes use of the known therapeutic value of sterilised seawater solutions on open wounds.40 Some commercial hydrogel dressing materials are Granugel® (Con- vaTec), Intrasite Gel® (Smith & Nephew), Nu-Gel® (Johnson & Johnson) and Sterigel® (Seton).

5.4.4 Collagen Among the bioactive and biocompatible wound-dressing materials, collagen is probably the most promising skin substitute or wound-dressing biomaterial, owing to minimal inﬂ ammation, cytotoxicity and its property to promote cellular growth.41 Collagen is the predominant extracellular protein in the granulation tissue of healing wound and a rapid increase in the synthesis of this protein in the wound area occurs soon after an injury.42 In addition to providing strength and integrity to a tissue matrix, collagen also plays a dual role in haemostasis.43 Collagen has characteristics suitable for medical application, such as biodegradability and weak antigenicity, and has been used over the years in the preparation of resorbable surgical sutures and wound-dressing materials.44 The role of collagen at each phase of wound healing is well understood and appreciated. The major applications of collagen-based dressings introduced to date include, collagen ﬁ lms, collagen gels and collagen sponges45. Most of the commercially available skin substitutes are collagen based, ranging from the acellular dressings like BioBrane®

(Smith & Nephew)46 or Integra® (IntegraTM)46,47 to the autologous keratinocyte-loaded counterparts Apligraf® (Novartis)45,48 or OrCell® (Ortec International Inc.).48 However, all suffer from relative rapid biodegradation in vivo, frequently with a non-synchronised dermal regeneration rate.49 Maintenance of structural and functional properties of the bioactive dressing material whilst maintaining cell compatibility until the healing has been completed is a necessary and challenging task to be addressed. Chemical collagen cross-linking agents have negatively affected cell viability and physical methods of cross-linking have proved insufﬁ - cient50,51 to maintain structural integrity in vivo.51

5.4.5 Honey dressings For centuries, honey has been used as an effective remedy for wounds, burns and ulcers. In recent years, there has been renewed interest in the medicinal properties of honey. Honey is most commonly used as a topical antibacterial agent to treat infections in a wide range of wound types. These include leg ulcers, pressure ulcers, diabetic foot ulcers, infected wounds resulting from injury or surgery and burns. Honey is used when conventional antibacterial treatment with antibiotics and antiseptics is ineffective. Honey promotes rapid healing with minimal scarring. Honey can be used as a fi rst aid treatment for burns as it has anti- infl ammatory activity. Honey is composed primarily of sugars and water, on average, 80 wt% sugar and 17 wt% water. The primary sugars are fructose (38%) and glucose (31%). Honey also contains acids, on average 0.57 wt%. The high acidity of honey owing to the presence of acids plays an important role in the system which prevents bacterial growth. The pH of honeys may vary from approximately 3.2 to 4.5, with an average value of 3.9, making it inhospitable for attack by most bacteria. The high acidity of honey neu- tralises also the high pH value of wounds, especially of burned wounds, and stimulates the rapid healing of these wounds. It has long been known that honey possesses antimicrobial properties that make it suitable for use in treating a range of infections and skin disorders. This antimicrobial activity can be attributed to a number of factors such as the natural presence in honey of hydrogen peroxide, its high saccharide content which tends to dehydrate bacteria by osmosis and its high acidity. Honey can also contain an enzyme, glucose oxidase, which reacts with glucose to produce hydrogen peroxide and gluconic acid, both of which have an antibacterial effect. In recent years, a lot of research was published about the clinical effects of topical honey in superfi cial burns and wounds.52 Important results from this research were the measures of wound-healing time and

of infection rates. Generally, a positive effect was observed on these two parameters. If the mean time to wound healing was 13.5 days in conventionally treated patients, the wound healing occurred in 9 days for honey- treated patients. Also, the number of infected wounds decreased from 12 to 5.5%. In another study, a comparison was made between honey and silver sulfadiazine on burn wound healing. Healing was faster with honey, 90% were healed after 14 days, than with silver sulfadiazine where only 52% were healed after 14 days. These results render honey suitable for use in dressing of wounds. However, an obvious problem associated with the use of honey in such circumstances is that it is a rather runny substance. On an exuding wound, it rapidly becomes unevenly liqueﬁ ed and runs off the wound. Attempts have been made to overcome this problem by combining honey with other materials like viscosity-enhancing additives or the use of absorbent material such as existing bandages or gauzes. New solutions are proposed such as the use of a three-layer structure.53 A possible dressing is composed of non-woven calcium alginate fabric impregnated with a mixture of sodium alginate and honey, so as to form a wound-contacting layer of the sodium alginate/honey mixture, an intermediate layer of the fabric impregnated with the sodium alginate/honey mixture and a backing layer of the fabric.

5.4.6 Chitin, chitosan and derivatives Chitin, a naturally abundant mucopolysaccharide, and the supporting material of crustaceans, insects, etc., is well known to consist of 2-acetamido-2-deoxy-β-d-glucose through a β (1→4) linkage. Chitin is the second most abundant organic compound in nature after cellulose. Chitin is widely distributed in marine invertebrates, insects, fungi and yeast. However, chitin is not present in higher plants and higher animals. Generally, the shell of selected crustacean was reported to consist of 30–40% protein, 30–50% calcium carbonate and calcium phosphate, and 20–30% chitin. Chitin is widely available from a variety of sources, among which the principal one is waste from shellfi sh such as shrimps, crabs and crawfi sh. It also exists naturally in a few species of fungi. In terms of its structure, chitin is associated with proteins and, therefore, is high in protein content. Chitin fi brils are embedded in a matrix of calcium carbonate and phosphate that also contains protein. The matrix is proteinaceous, where the protein is hardened by a tanning process. Studies demonstrated that chitin represents 14–27% and 13–15% of the dry weight of shrimp and crab processing wastes, respectively. Its immuno- genicity is exceptionally low, in spite of the presence of nitrogen. It is a highly insoluble material resembling cellulose in its solubility and low

chemical reactivity. It may be regarded as cellulose with hydroxyl at position C-2 replaced by an acetamido group. Like cellulose, it functions naturally as a structural polysaccharide. Chitin is a white, hard, inelastic, nitrogenous polysaccharide and the major source of surface pollution in coastal areas. Chitin is made up of a linear chain of acetylglucosamine groups, while chitosan is obtained by removing sufﬁ cient acetyl groups

(—COCH3) from the molecule to make it soluble in most dilute acids. Chitosan is the N-deacetylated derivative of chitin, although this N- deacetylation is almost never complete. A clear nomenclature with respect to the degree of N-deacetylation has not been deﬁ ned between chitin and chitosan.54,55 The structures of cellulose, chitin and chitosan are shown in Fig. 5.1.

CH2OH CH2OH H O H O H H OH H O O OH H H H H OH n H OH Cellulose

CH2OH CH2OH H O H O H H OH H O O OH H H H H n NHCOCH3 H NHCOCH3 Chitin

CH2OH CH2OH H O H O H H OH H O O OH H H H H n NH2 H NH2 Chitosan 5.1 Structures of cellulose, chitin and chitosan.

As seen in Fig. 5.1, the only difference between chitosan and cellulose

is the amine (—NH2) group in the position C-2 of chitosan instead of the hydroxyl (—OH) group in cellulose. However, unlike plant fi bre, chitosan possesses ionic charges, which gives it the ability to chemically bind with negatively charged fats, lipids, cholesterol, metal ions, proteins and macromolecules. In this respect, chitin and chitosan have attained increasing commercial interest as suitable resource materials owing to their excellent properties including biocompatibility, biodegradability, adsorption, and ability to form fi lms, and to chelate metal ions. Chitin and chitosan are of commercial interest due to their high percentage of nitrogen (6.89%) compared with synthetically substituted cellulose (1.25%). This makes chitin a useful chelating agent.54 As most modern polymers are synthetic materials, their biocompatibility and biodegradability are much more limited than those of natural polymers such as cellulose, chitin, chitosan and their derivatives. However, these naturally abundant materials also exhibit a limitation in their reactivity and processability.56,57 In this respect, chitin and chitosan are recommended as suitable functional materials, because these natural polymers have excellent properties such as biocompatibility, biodegradability, non-toxicity and adsorption properties. Recently, much attention has been paid to chitosan as a potential polysaccharide resource.58 Although several efforts have been reported to prepare functional derivatives of chitosan by chemical modifi cations,59–61 very few attained solubility in general organic solvents62,63 and some binary solvent systems.64–66 Chemically modifi ed chitin and chitosan structures resulting in improved solubility in general organic solvents have been reported by many workers.67–76 Most of the naturally occurring polysaccharides, e.g. cellulose, dextran, pectin, alginic acid, agar, agarose and carragenan, are neutral or acidic in nature, whereas chitin and chitosan are examples of highly basic polysaccharides. Their unique properties include polyoxy salt formation, ability to form fi lms, chelate metal ions and optical structural characteristics.62 Like cellulose, chitin functions naturally as a structural polysaccharide, but differs from cellulose in its properties. Chitin is highly hydrophobic and is insoluble in water and most organic solvents. It is soluble in hexafl uoro- isopropanol, hexafl uoroacetone, and chloroalcohols in conjugation with aqueous solutions of mineral acids67 and dimethylacetamide containing 5% lithium chloride. Chitosan, the deacetylated product of chitin, is soluble in dilute acids such as acetic acid and formic acid. Recently, the gel-forming ability of chitosan in N-methylmorpholine N-oxide and its application in controlled drug release formulations has been reported.73–75 The hydrolysis of chitin with concentrated acids under drastic conditions produces relatively pure d-glucosamine. Unlike most polysaccharides, chitosan possesses charge because of deacetylation and makes the polymer soluble in water. However, the degree of solubility depends on the extent of acetylation.76

The nitrogen content of chitin varies from 5 to 8% depending on the extent of deacetylation, whereas the nitrogen in chitosan is mostly in the form of primary aliphatic amino groups. Chitosan, therefore, undergoes reactions typical of amines, of which N-acylation and the Schiff reaction are the most important. Chitosan derivatives are easily obtained under mild conditions and can be considered as substituted glucans. N-Acylation with acid anhydrides or acyl halides introduces amido groups at the chitosan nitrogen. Acetic anhydride affords fully acetylated chitins. Linear aliphatic N-acyl groups above propionyl permit rapid acetylation of hydroxyl groups. Higher benzoylated chitin is soluble in benzyl alcohol, dimethylsulfoxide, formic acid and dichloroacetic acid. The N-hexanoyl, N-decanoyl and N-dodecanoyl derivatives have been obtained in methanesulphonic acid.77,78 The presence of the more or less bulky substituent weakens the hydrogen bonds of chitosan; therefore N-alkyl chitosans swell in water in spite of the hydrophobicity of the alkyl chains, but they retain the fi lm-forming property of chitosan.54 The basic structural element of chitin is N-acetylglucosamine, which initiates fi broblast proliferation, helps in ordered collagen deposition, and stimulates an increased level of natural hyaluronic acid synthesis at the wound site. It helps in faster wound healing and scar prevention. Chitosan is known in the wound-management fi eld for its haemostatic properties. Further, it also possesses other biological activities and affects macrophage function to result in faster wound healing. It also has an aptitude to stimulate cell proliferation and histoarchitectural tissue organisation. The biological properties including bacteriostatic and fungistatic properties are particularly useful for wound treatment. Like alginate material, there is also number of references on chitosan in wound treatment. They showed that chitosan facilitated rapid wound re-epithelialisation and the regeneration of nerves within a vascular dermis. Early returns to normal skin colour at chitosan-treated areas were also demonstrated. Treatment with chitin and chitosan demonstrated a substantial decrease in treatment time with minimum scar formation on various animals. The preparation of water-soluble chitosan for delivering an anticancer agent has been patented.79

5.5 Example of bioactive dressing: di-O-butyrylchitin (DBC) 5.5.1 General description of DBC An original method of synthesis of di-O-butyrylchitin (DBC), the soluble derivative of chitin, was worked out at the Technical University of Łódz´, Poland.65,66 The proposed method applied to chitin of different origin (crab, shrimp and krill shells, and insect chitin) gave products of deﬁ nite chemical

O O O O O C C O C

C3H7 C H C H O O 3 7 O 3 7 O O O

NH NH NH O OO CO CO CO CO CO CO

C H CH3 CH3 CH3 3 7 C3H7 C3H7 5.2 Chemical structure of dibutyrylchitin (DBC) after complete O-butyrylation (degree of substitution = 2).

structure with a degree of esterifi cation very close to two (Fig. 5.2). DBC is readily soluble in common organic solvents and has both fi lm and fi bre-forming properties.67–72 Such properties of DBC created the possibility of manufacturing a wide assortment of DBC materials suitable for medical applications in the form of fi lms, fi bres, non-woven, knitted materials, and woven fabrics. It was also stated that the treatment of fi nished materials made from DBC, carried out under mild alkaline conditions, led to chitin regeneration without destroy- ing their macrostructure.72 Moreover, the regeneration of DBC to chitin resulted in improving the mechanical properties of the newly obtained materials containing regenerated chitin (RC).74,75 Thus, O-butyrylation of chitin, preparation of several forms of dressings from DBC, and then returning to pure chitin in the process of alkaline hydrolysis of DBC materials, gives the possibility of practically unlimited manufacturing of DBC and chitin-based dressing materials, which are comfortable and easy in use. The fi rst investigations of biological properties of DBC and RC materials, carried out in vitro and in vivo in accordance with the European standards EN ISO 10993 (‘Biological evaluation of medical devices’), showed good biocompatibility of both polymers75,77,78,80,81 and their ability to accelerate wound healing.74,82 Recent investigations published by Muzzarelli et al.83 confi rms the biocompatibility of DBC. The presented results indicated that DBC is not cytotoxic for fi broblasts and keratinocytes. The fi rst clinical investigations into medical properties of DBC have been carried out at the Polish Mother’s Health Institute in Łódz´, Poland. DBC samples under investigation have been used in the form of non-woven materials made at Technical University of Łódz´ using krill chitin as a source of DBC. Results of their clinical investigations were presented at the 6th International Conference of the European Chitin Society84 and published.85 Further results have been received using non-woven dressing

materials with DBC that was obtained from shrimp shell chitin.86-91 Combined results of clinical investigations of DBC non-woven dressing materials are used and explained in this chapter.

5.5.2 Biological properties of DBC: in vitro and in vivo assays The realisation of clinical investigations was preceded by the determination of biological properties of DBC fi bres and wound dressings in the form of neutral polypropylene sheets covered bilaterally by DBC fi lms. The investigations were carried out in accordance with the requirements of the standard EN ISO 10993 (‘Biological evaluation of medical devices’). They included investigation of cytotoxic effects of aqueous extracts of DBC fi bres and wound dressings coated with DBC, investigation of haemolytic effects of aqueous extracts of DBC fi bres, fulfi lment of intracutaneous reactivity tests of aqueous extracts, evaluation of local tissue reactions after implantation of DBC fi bres into the gluteal muscles of the Wistar rats and peritoneal cavity of mice BALB/C. Moreover, the investigation of the tissue regeneration process after coverage of damaged rabbit skin with DBC type wound dressings was completed. In order to defi ne the infl ammatory and immunomodulation effects of DBC-containing wound dressings, an assessment of their infl uence on the induction of cytokines and on the synthesis of nitrogen oxides of human leukocytes was conducted. Evaluation of the infl uence of DBC for activation of the blood coagulation system was completed in the latter research. Fibres and wound dressings were sterilised by ethylene oxide before investigation. From the results of biological investigation, the following conclusions were drawn: • Tests of aqueous DBC extracts from fi bres performed in vitro proved that cytotoxicity and haemolytic effects were not found.77,81 • After animal tests conducted in vivo on rabbits, no reaction resulted from intradermal application of DBC fi bres.77,81 • No intensifi cation of the tissue reaction was found after implantation of DBC fi bres into the gluteal muscles of the Wistar rats83 or into the peritoneal cavity of mice BALB/C.77 The results of microscopic obser- vation were the same as those obtained in investigation of the reactions in the surrounding of the Maxon surgical threads.77 • Tests of IL1β and IL6 interleukin levels in the peritoneal fl uids of the experimental mice BALB/C have not proved the existence of statistically essential differences when comparing two animal groups: the group represented by animals with DBC fi bres inserted into the peritoneal cavity and those with peritoneal-implanted Maxon surgical threads.77

• It was confi rmed that the polypropylene nonwoven materials coated with DBC do not demonstrate cytotoxicity and primary irritation effects, do not cause an increase of the activities of TNF-α, IFNs or the nitrogen oxide level, and have an active positive infl uence on the wound healing process.74,81 • DBC materials showed the activity of blood procoagulation and have no infl uence on the activity of the plasma protein coagulation system.92 Based on these results, the Ethics Committee of the Polish Mother’s Health Institute has given approval for a project on the application of DBC in surgical paediatric patients.

5.5.3 Rationale for the provision of butyrate Short-chain fatty acids, especially butyrate, play a central metabolic role in maintaining the mucosal barrier in the gut. A lack of butyrate, leading to endogenous starvation of enterocytes, may be the cause of ulcerative colitis and other infl ammatory conditions. The main source of butyrate is dietary fi bre, but it can also be derived from structured biopolymers like DBC. Butyrate has been shown to increase wound healing and to reduce infl ammation in the small intestine.93 In the colon, butyrate is the dominant energy source for epithelial cells and affects cellular proliferation and differentiation by as yet unknown mechanisms. Recent results suggest that the luminal provision of butyrate may be an appropriate means to improve wound healing in intestinal surgery and to ameliorate symptoms of infl ammatory diseases. It was also suggested that butyrate may inhibit the development of colon cancer.94,95 Butyrate has a relatively short metabolic half life. The half-life of butyrate in plasma is extremely short, as concentrations of butyrate in plasma peaked between 0.25 and 3 h after application and had disappeared by 5 h after the application.

5.5.4 Interaction between DBC and enzymes The enzymic susceptibility of DBC and RC and the consequences on their polymer structure is an important factor in its use as a bioactive wound dressing material. When dealing with DBC, the chemical nature of the modiﬁ ed chitin, an ester, suggests that it would be susceptible to enzymic attack by lipases insofar as the removal of butyryl groups is concerned.

Characteristics of used DBC ﬁ bres Average results of elemental analysis of DBC gave: C 55.80% (calculated 55.98%), H 7.51% (calculated 7.29%) and N 4.10% (calculated 4.08%) and

suggested that the degree of esterifi cation was very close to 2; intrinsic viscosity 1.41 dL g−1 in acetone at 25 °C; intrinsic viscosity 2.17 in dimethylacetamide at 25 °C; weight average molecular weight c. 170 kDa; degree of crystallinity (when in form of fl akes) 47.2%. The fi bres were prepared by wet spinning, by using a 16% solution of DBC in dimethylformamide; the nonwovens were prepared from 6-cm fi bres by the needle-punching technique. The 1H NMR spectrum of DBC was recorded using a Brucker AM

400 spectrometer. Deuterated chloroform, CDCl3 was used as a solvent. In the 1H spectrum both substituted hydroxyl groups in the secondary position 3, at 5.0 ppm, and in the primary position 6, at 4.3 ppm, gave

well separated signals. This applies to both protons in CH3 and CH2 groups. The proton signals of the N-acetyl group of the original chitin are present in the spectrum as a singlet placed at 1.9 ppm. The intensity

ratio of signals of the CH3 group, coming from the butyryl substituent, to the signal intensity of N-acetyl group, derived from the spectra, amounts to 1 : 2. According to NMR investigation, the reaction product is dibutyryl-

chitin. Both signals of amide group: CH3—CO at 2.0 ppm and NH at 6.1 ppm indicate that more than 90% of N-acetyl groups of the original chitin are present in DBC.

Preparation of regenerated chitin fi bres (RC) The DBC fi bres were immersed in 1.25 m aqueous NaOH solution at 50 °C and allowed to be hydrolysed for defi nite periods of time up to 150 min; then the samples were washed several times with water to remove alkali, dried and weighed; then they were treated with acetone to extract unre- acted DBC and dried and weighed again.

Treatment with lipases Lipase solutions were prepared by dissolving the desired lipase (100 mg) −1 in 100 ml of a 0.5 g l NaN3 aqueous solution, ﬁ nal pH 6.0. The samples (c. 200 mg) were introduced in 40 ml of enzyme solution and kept on a rolling bar machine for 4 weeks. The lipases (EC 3.1.1.3) were crude porcine pancreas lipase L-3126 (PPL), and wheat germ lipase L-3001 (WGL). The results showed that DBC is not degraded by lipases.

Susceptibility of dibutyrylchitin to enzymic hydrolysis DBC does not seem particularly prone to enzymic hydrolysis; in fact it is not hydrolysed by lysozyme and it is not degraded by lipases. Lysozyme and lipase being the main enzymes that could release oligomers of

N-acetylglucosamine from chitin-based textiles and non-wovens used in wound management, it seems that the biochemical signifi cance of DBC is low. Collagenase and amylase do not appear to be able to remove any signifi cant portion from the fi bres, but it should be said that collagenase has a certain tendency to adhere to the fi bres while exerting poor activity on them. Even crude cellulase and pectinase preparations exerted very little hydrolytic activity on dibutyrylchitin over the extended period of 48 days at 25 °C. It can thus be added that dibutyrylchitin seems to be relatively resistant to biodegradation. This resistance to biodegradation will be further discussed in the chapter describing the clinical experiments. These clinical experiments indicate a complete biodegradation of DBC when applied as bioactive material in wound dressings.

5.5.5 Live/dead assays The live/dead assays were indicative of full biocompatibility of fi broblasts with the DBC fi lm: the viability values for the cells grown in regular multiwell plates ranged from 95.5 to 75.6%, close to those for DBC-lined plates (93.0–72.8%) in the 24–72 h period. For the cell proliferation assay, the percentages of cells positive to fl uo- rescein over the 24–72 h period were similar in the two cases: regular multiwell plates (20.6–53.3%) and DBC-coated multiwell plates (22.7–60.0%). The Neutral Red Retention and the Lactate Dehydrogenase spectropho- tometric assays provided results in line with those already illustrated: there was no signifi cant difference between the Neutral Red release rate for reference fi broblasts and DBC-exposed fi broblasts.

5.5.6 Haemocompatibility The contact angle value for untreated human blood decreased with time from the initial value of 60° to 42° in 21 min. The contact angle values for heparinised blood were slightly but signiﬁ cantly lower, owing to addition of heparin, and decreased with time at a similar rate, from 53° to 42°. The relatively modest values for the angles observed indicated that DBC is wettable by human untreated and heparinised blood. However, DBC is less wettable than glass. On glass slides, the contact angle decreased from 23° to 16° for heparinised blood in 17 min. The fact that the drops of untreated blood deposited on DBC ﬁ lm maintained their shape after 21 min is indicative of the absence of thrombogenic or coagulation-enhancing capacity in DBC. The values for saline (9 g l−1 NaCl) were the following: DBC 75°, chitin 56°, and chitosan 64°.

5.5.7 Radical scavenging activity of DBC DBC was submitted to the action of free radicals in order to detect its ⋅ scavenging activity. The radicals were benzyl radical C6H5CH2 and ada- ⋅ mantyl radical C9H13 , both generated in situ by photoactivation from stable precursors. Owing to their very short life, the radicals were stabilised with the spin trap dimethylpyrroline N-oxide (DMPO). DBC solutions in dichloromethane containing a precursor of benzyl radicals were exposed to sunlight or UV under inert atmosphere. The spectra were recorded with a Bruker EPR spectrometer in aqueous acetonitrile–DMPO solution. The NMR spectra obtained on the chromatographic fractions from a silica gel column showed the formation of adducts of DBC and benzyl group. Similar preliminary results were obtained with the adamantyl radical. The EPR spectra recorded in aqueous acetonitrile–DMPO solution for adamantyl radicals, the same with added DBC and for irradiated DBC in the absence of radical precursors, show that:

• DBC alone does not produce radicals upon irradiation. • DBC is a free radical scavenger because it modiﬁ es the radical species of the adamantane type. • DBC forms adduct with the benzyl radicals.

These results are in agreement with published results that ﬁ nd that oli- gomeric species of chitosan exhibit radical scavenging activity towards a number of radicals (other than those used here). Normally, the radical scavenging activities of chitin or chitosan are strongly dependent on the degree of acetylation. Chitosan with the highest degree of deacetylation is characterised by the highest radical scavenging properties, owing to the high number of free amino groups. The scavenging mechanism of chitosan on free radicals may be related to the fact that free radicals can react with

the residual amino groups (NH2) to form stable macromolecule radicals + and the NH2 groups can form ammonium groups (NH3 ) by absorbing hydrogen ions from the solution. The positive results of DBC as a free radical scavenger and antioxidant is the result of a totally different mechanism because most (89%) of the

NH2 groups are acetylated in the used DBC. It is likely that the free radi-

cals are reacting with the CH2 groups of the O-substituted butyryl groups. These radical-containing CH2 groups are stable for a long time and are consequently neutralised by reaction with other compounds containing radicals. The free radical scavenger action and antioxidant capacity of DBC is related to the presence of the butyryl groups and their stable radical substances. This antioxidant capacity of DBC can be an important factor for non- healing acute wounds. Whereas healing acute wounds have low levels of

protein-degrading enzymes, exudates from non-healing chronic wounds contain elevated levels of proteases, like matrix metalloproteinases and elastase.4–6 Moreover, the concentrations of pro-infl ammatory cytokines,7 as well as reactive oxygen species,8 are signifi cantly higher, compared with the concentrations in acute wounds. Reactive oxygen ⋅− ⋅ species, such as superoxide radicals (O2 ) and hydroxyl radicals (OH), as well as reactive nitrogen species, such as nitric oxide (NO⋅) arise from infl ammatory cells.9–11 Overproduction of reactive oxygen and nitrogen species results in an imbalanced oxidant/antioxidant static in wounds and especially in chronic wounds.8,11,12 Owing to the resulting disproportion between degradation and remodelling processes, chronic wounds persist in the infl ammatory phase of the normal healing process for months or even years. The reduction in reactive oxygen and reactive nitrogen species in the wound fl uid by the free radical scavenger action of DBC seems to be a logical way to stop the infl ammation process and diminished epithelialisation and to start the normal healing process.

5.5.8 Conclusions of the biochemical tests All biochemical tests described indicated that DBC is biocompatible for human fi broblasts and keratinocytes. The SEM data on the colonisation of non-woven DBC by rat fi broblast-like cells confi rmed the biocompatibility of DBC that seems also endowed with radical scavenging properties. These results are in line with those already published on other chitins and chitosans: for instance, keratinocytes were co-cultured with fi broblasts in chitosan–gelatin scaffolds to construct an artifi cial bilayer skin in vitro. The haemocompatibility assessment based on contact angle analysis indicated that DBC does not promote coagulation or thrombus formation when in contact with human blood.

5.5.9 In vitro cytotoxicity tests The objectives of the in vitro cytotoxicity tests were: 1. To evaluate the cytotoxic effect of materials involved in the production of DBC. 2. To evaluate the fi nal products (non-wovens or knitted materials). The cell cultures used were: primary obtained fi broblasts, human 3T3 cells, and human lymphocytes. In conclusion, the fi nal non-woven materials obtained from dibutyrylchitin showed that cells grown in contact with the material showed good cell viability. It seemed that they were not adhesive for the cells. Based on

the cytotoxicity tests and on the physical properties of the materials, a selection of non-wovens was prepared in order to perform in vivo animal experiments: DBC and cellulosic ﬁ bers containing 4 wt% DBC.

5.5.10 In vitro and in vivo investigations The main aim of research on wound-dressing material made from DBC is to understand the mechanism of wound healing by DBC-based material. The in vitro and in vivo investigations were carried out in co-operation with the Medical Academy in Wroclaw, Poland. The experiments conducted investigated main biomedical characteristics like cytotoxicity, intradermal reactivity and immunological response, as well as biodegradability. Results showed that dibutyrylchitin has similar properties to chitin or RC, and all requirements described by European Standards are fulfi lled. Clinical investigation conducted in the Polish Mother Memorial Hospital shows a positive infl uence of DBC fabric on wound healing. The main aim of this research was to determine the mechanism of wound healing under DBC application and to compare it with the mechanism of wound healing using RC and chitosan products. The infl uence of DBC implants on the formation of the intercellular matrix of connecting tissue (collagen and glycosaminoglycans) and granular tissue in wound model was investigated. In addition, the reaction of the whole organism and thermoregulation of the body was examined. Investigations were carried out on 42 male Wistar rats, with mass 250 g ± 30 g. Animals were divided into six groups (of seven animals each): 1. Control: polypropylene mesh implanted subcutaneously. Size of implant 2 × 3 cm; 2. DBC A9: polypropylene meshes connected with DBC nonwovens were implanted. The surface density of the non-woven was 40 g m−2; fi bres were formed from DBC polymer with an intrinsic viscosity of 1.75 dl g−1; 3. DBC A8: polypropylene meshes connected with DBC non-wovens were implanted. The surface density of non-woven was 40 g m−2; fi bres were formed from DBC polymer with an intrinsic viscosity of 2.08 dl g−1; 4. Copolymer: polypropylene meshes connected with DBC/RC non- wovens were implanted. The surface density of non-woven was 40 g m−2; fi bres were formed from DBC polymer with intrinsic viscosity 1.75 dl g−1 and hydrolysed to 50% of RC; 5. RC: polypropylene meshes connected with RC non-wovens were implanted. The surface density of non-woven was 40 g m−2; fi bres were

p<0.002 3000 p<0.002 2500 p<0.03

dry tissue 2000 –1 1500 1000 500 0 GAG in µ g mg

Chitin Control DBC-A8 DBC-A9 Chitosan Copolymer 5.3 Concentration of glycosaminoglycans (GAG) in granular dry tissue.

formed from DBC polymer with intrinsic viscosity 1.75 dl g−1 and regenerated to chitin (RC); and 6. Chitosan: polypropylene meshes connected with chitosan non-wovens were implanted. The surface density of nonwoven was 40 g m−2. The most important results are hereby summarised. The content of glycosaminoglycans in granular tissue was signifi cantly higher in wounds with DBC. In addition, an increase in the content of glycosaminoglycans was observed for RC and copolymer. For wounds with implanted chitosan, the content of glycosaminoglycans was signifi cantly lower (Fig. 5.3). The polymers examined did not infl uence the total collagen in the wound but DBC A9, copolymer and chitosan did reduce the soluble collagen. In addition, the tendency for reducing content of soluble collagen in the wound is observed with DBC A8 and RC groups (Fig. 5.4). The amount of total collagen is not reduced, therefore we can conclude that the examined polymers signifi cantly improve the quality of collagen in the wounds. The following conclusions were obtained from these experiments: 1. Dibutyrylchitin (DBC) exerts not only local but also general effects on the animal organism (infl uence on thermoregulation); 2. Similar phenomena were seen in animals treated with RC. In the wounds, several benefi cial effects of implanted inserts containing DBC dressings were seen: increase of granulation tissue weight, anti- edematic properties, elevation of the glycosaminoglycans content and improvement of collagen quality in the wounds; and 3. The results of the present experiments and clinical studies strongly support the suggestion that DBC is a good dressing material that improves the healing process.

p<0.05 450 p<0.05 400 p<0.05 350 300 250 200 150 100 50 0 Soluble collagen µ g 100 mg wet tissue

Chitin Control DBC-A8 DBC-A9 Chitosan Copolymer 5.4 Concentration of soluble collagen.

5.5.11 In vivo animal experiments The healing of surgical wounds after DBC implants in rats followed the regular course; thus, the presence of DBC did not delay the early stages of the healing process. Clear indications of resorption were obtained. The role of DBC is essentially conﬁ ned to imparting better handling and mechanical resistance; it does not seem that DBC has any relevant role in promoting the ordered regeneration of the wounded tissues, because it is hardly susceptibile to enzymic hydrolysis by lysozyme, lipase, collagenase and amylase. The information that DBC is also poorly degraded by other enzymes, such as raw cellulase and pectinase might be important in other ﬁ elds: DBC appears to be particularly resistant in the environment, taking into account that chitin and chitosan were recognised as promptly degradable by both enzyme preparations, and in particular by Aspergillus niger pectinase. Additional experiments indicated that when DBC is put into an alkaline medium, a slow hydrolysis of the DBC took place until the medium is returned to a neutral pH. DBC may be an important material to control and regulate the pH of the wound during the healing process. As already mentioned, pH is an important parameter for wound healing and the reduction of pH to neutral constitutes an important positive effect of DBC on wound healing. The objectives of the in vivo animal experiments were to evaluate the in vivo behaviour of the material with regard to:

• anti-inﬂ ammatory properties, • their wound healing properties, • their biodegradation. To investigate the wound healing efﬁ ciency of DBC dressing materials, the dressings were placed on the wound bed of critical size skin defects. Twelve Wistar Han rats (six male, six female, 60 days old) were used. A histological evaluation of the wound area and surrounding healthy tissue at different time points post-injury was performed and the results for the various potential bioactive dressing materials are presented.

Cellulose fi bres containing 4 wt% DBC Day 4 post-injury: A thick crust, composed of fi bres and blood, covered the wound area. In two rats, the epithelium started to re-grow from the wound edges, in the third, there was no formation of new epithelium. Many neutrophils were present underneath the scab and between the fi bres. There was no formation of new connective tissue. Day 7 post-injury: A thick scab was present. The epithelium started to renew in two out of the three rats. There were still infl ammatory cells present, but no formation of connective tissue. Day 14 post-injury: Almost the same as at day 7. Day 35 post-injury: A thin scab covered the wound. No (complete) re- epithelialisation and practically no new connective tissue formation was noticed. The material was still present in the wound site. Many infl ammatory cells were observed.

Non-woven DBC Day 4 post-injury: The epithelium was starting to renew and was covered with scab. The formation of connective tissue had not started yet, and some neutrophils were present underneath the scab and between the dressings. Day 7 post-injury: The epithelium was restoring. Scab stayed present. The connective tissue was starting to be formed and some of the DBC was reabsorbing. Day 14 post-injury: A thin scab covered the newly formed epithelium. The connective tissue is composed of dense collagen bundles, with a lot of blood vessels. The DBC material was being reabsorbed. Day 35 post-injury: All scab had disappeared. The epithelium showed invaginations in the underlying connective tissue. Dense collagen bundles, a lot of blood vessels and some skin appendages appeared in the connective tissue. There were no signs of inﬂ ammation and the DBC seemed completely reabsorbed.

SeaSorb alginate dressing Day 4 post-injury: A thick scab was present at the wound site. In one out of the three rats, the epithelium started to re-grow. There was no formation of connective tissue. Neutrophils were observed around the alginate material. Day 7 post-injury: A thin scab covered the newly forming epithelium. Some infl ammatory cells could be seen around the alginate. At the wound site of two rats, new connective tissue started to be formed. Day 14 post-injury: Re-epithelialisation was completed and a thin scab rested on it. In two rats, dense collagen and blood vessels appeared in the connective tissue above the alginate layer. Day 35 post-injury: A thin and fl at epithelium covered the former wound area. All scab was removed, and there were no signs of infl ammation. The connective tissue was dense, with some blood vessels and skin appendages appeared at the former wound edges. The alginate layer was not reabsorbed during these 35 days.

Open wound Day 4 post-injury: The epithelium was starting to re-grow at the wound edges. A thick layer of scab spanned the wound site. Underneath the scab, neutrophils were present. There was no formation of new connective tissue. Day 7 post-injury: The epithelium was restored, but scab remained. New connective tissue was forming. Day 14 post-injury: No scab was noticed on the thin and ﬂ at epithelium. The connective tissue was dense with some blood vessels. No indications of inﬂ ammation. Day 35 post-injury: Same observations as at day 14 post-injury. The connective tissue is dense, with not a lot of blood vessels and no skin appendages: presence of scar tissue.

5.5.12 Conclusions of the in vivo experiments The wounds covered with the DBC dressing and the SeaSorb alginate showed the best wound healing: dense collagen in the connective tissue with a lot of blood vessels and some skin appendages at day 35 post-injury. The main difference was that the DBC material was degraded (complete absorption of the material was noticed after 35 days). The DBC-material could stay on the wound during the healing process, while this alginate should be removed after a couple of days. Owing to this biodegradable property, DBC-based dressing has advantages compared with other

100

10 Time (day)

1 020406080100 Absorption (%) 5.5 Time for complete absorption of chitin ﬁ bres into wounds.

available wound-dressing materials. In contrast with the laboratory tests of DBC with different enzymes to study the enzymatic hydrolysis of DBC, DBC is completely degraded in this application of wound dressing material. This difference will be discussed later on, after the description of the clinical experiments. The best choice for solubility and bioactive dressing materials based on acetylated chitosan seems to be around 50% substitution. This follows also from the absorption of chitin or chitosan fi bres into wounds as function of the degree of substitution as represented in Fig. 5.5. The fast absorption, in vivo, of chitin fi bres into treated wounds is obtained with chitin containing 50% acetylglucosamine. These fi bres are completely absorbed in 1 week against 4 weeks for the used chitin fi bres with 90 to 100% acetyl groups. The complete absorption of the chitin fi bres into the wounds is a result of the combined action of two enzymes, lipases for the hydrolysis and lysozymes for the breakdown of the hydrolysed chitins into their basic monomers or oligomers. This time for complete absorption of the chitin fi bres corresponds with the time necessary for the complete absorption of the DBC-based fi bres.

5.5.13 Results of clinical investigations The surgical staff of the Department of Paediatric Surgery was provided with a number of DBC petals for medical application. It has been planned to apply DBC in the group of 29 children with following indications: • burns, • wounds of various aetiology, • bed sores – in patients immobilised for long periods regardless of reason, • lesion of parietes as a result of multifactorial processes (e.g. sepsis).

(a) (b) 5.6 Lower limb burn injury (a) before treatment and (b) healing nearly completed.

The fi rst group of patients comprised 10 persons suffering from burns. Nine of them were aged 7–40 months and the mean age in this group was 16 months. The total area of thermal burns of patients range from 5 to 20%. The depth of burns was classifi ed in each case as 2a. All the burns healed up within 1–2 weeks after DBC petal application. The healing processes are documented in Fig. 5.6a and b. One patient who was a 17-year-old boy suffered from electric burn of class 3, locally to the left forearm. After removal of necrotic tissue, a DBC petal was applied as a pre-treatment before surgery – split-thickness skin transplantation. The result of the healing process was very good. The second group of patients suffered from wounds other than burns with various aetiology such as: post-operative, post-traumatic and post- traumatic/post-operative. Five patients complained of post-operative wounds. In the post-traumatic group, there were fi ve patients. Three of them sustained injury to the digits with partial necrosis of soft tissues. After removal of necrotic tissue, the DBC petal was applied and there was no need of further surgical interference. The other two children had extensive wounds to the limbs and skin loss owing to necrosis. The petals of DBC were applied on the raw surface as a pre-treatment before surgery. In one patient, the wound healed up spontaneously and rapidly and he avoided surgery. In the other, the split-thickness skin transplant was performed. The healing processes of these groups were very good. Four patients from the post-traumatic/post-operative group sustained severe injuries to the lower limbs resulting in open fractures of the tibia with loss of soft tissue. All were repeatedly operated by orthopaedic sur- geons. As a consequence, residual chronic ulcers with tibial bone exposed were seen in all cases. The application of DBC petals resulted in the healing of wounds in two cases.

The third group of patients is related to various entities leading to skin/ epidermis loss. The local treatment with the use of DBC petals was introduced in a 1.5-year-old child with foci of necrosis of the skin in the course of general sepsis. As a result of removal of the necrotic tissue and application of DBC petals, healing of the wound was achieved. The last group of patients suffered from bedsores. Extensive chronic bedsores were infected and covered with pus. The pieces of DBC did not stick to the bedsore and were ﬂ owing freely on its surface, making no contact with the tissue. The conservative treatment was discontinued. DBC-supported wound dressings for pre-clinical studies were prepared by using chitosan, chitin and chlorhexidine together with DBC non-wovens. A total of 80 pieces (10 × 10 cm) of DBC non-wovens were treated with chlorhexidine, forwarded for gamma-ray sterilisation and then tested in the Clinics of Surgery in Rome, Italy. These new dressings were found most useful in all cases treated, leading to healing of a variety of wounds.

5.5.14 Discussion and conclusions of the clinical trials As far as thermal burn patients are concerned, the depth of the wound was evaluated as 2a in all cases. This means there was no necrotic tissue and no indication for operative treatment. In all cases, healing was quick and uneventful, and in no case did infection develop. Regarding the only child with an electric burn, the fi rst step in the procedure was the removal of necrotic tissue. The application of DBC, as preparation for surgery, and the operation itself followed. The non-septic wounds are by defi nition an indication for surgical treatment. As a matter of fact, all patients presented were primarily treated this way. In three of them, after plastic operation, small unhealed patches with granulation were found. No signs of infection and no necrotic tissue were indicated. All these places healed up soon after the application of DBC. Another four children suffered mechanical injury, and the wounds were complicated by the presence of necrotic tissue. The fi rst stop in all cases was the removal of necrosis followed by the use of DBC. Two of them were considered as candidates for further surgery, but this was the case only in one patient. Surprisingly, the other one healed up soon enough to avoid surgery. The greatest surprise was the uneventful progress, which took place by two children repeatedly operated for open, complicated leg fracture. The wounds were clean, without necrosis but with exposed bone in the centre. In spite of this, they healed up quickly. The third such patient hopefully will be going the same way. DBC was also applied in three other children with various entities. They all have two things in common: the lesion of the skin/epidermis and the absence of necrosis and infection.

In all cases, DBC dressings have been applied to the clean wound, and not removed until the end of the healing process, while DBC has been disintegrated in the area of the wound. No other medical products have been applied for the wound healing. The results were at least promising but too few cases have been observed until now, and the aetiology was diverse. It turned out that, in another suggested indication for DBC application, in the cases of bedsore, no beneﬁ t was seen. However, extensive bedsores are always connected with infections; suppuration, non-viable tissue, and indications arise for surgery rather than for conservative treatment. The observations presented are preliminary and further evaluation is necessary. Summarising, it is possible to conclude: • Preliminary results of DBC application are highly promising: DBC seems to promote the healing of wounds. • Selection of patients for this treatment has to be meticulous: the tissues to be covered with DBC should be viable, possible without infection. Otherwise, the results are doubtful as in the case with bedsores. • Further randomised trials with referential groups should be completed to obtain evidence-based proofs of beneﬁ cial effects of DBC wound dressings.

5.5.15 Biodegradation of DBC in wound dressings Research results show that matrix metalloproteinases are present at elevated levels during early wound healing and suggest that matrix metalloproteinases may play a signiﬁ cant role in wound healing. The matrix metalloproteinases are a family of zinc metalloendopeptidases that can collectively cleave all components of the extracellular matrix. Early research work on the porcine skin model96,97 identiﬁ ed changes in matrix metalloproteinases during wound healing. Collagenase activity was high early after wounding and declined with post-operative time, returning to baseline levels when epithelialisation was complete. Gelati- nase activity followed a similar pattern. The activity of matrix metalloproteinases returns to normal as healing progresses.98 A known enzyme that catalyses the deacetylation of N-acetylglucosamine to form glucosamine and acetate is a zinc-dependent enzyme,99 UDP-3-O- (R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC). LpxC functions via a metalloprotease-like mechanism by using zinc-bound water as nucleophile. In clinical experiments with DBC-based wound dressings, matrix metalloproteinases probably catalyse the deacetylation of DBC in the presence

of zinc-bound water. The ﬁ nal result of this deacetylation reaction is the formation of O-butyrylated chitosan. As it is known from other research results that O-butyrylated chitosan is soluble in water,100 a possible explanation for the complete resorption of DBC during the wound-healing experiments is the deacetylation of DBC by the action of matrix metalloproteinases in combination with zinc-bound water and the formation of a complete water-soluble derivative, O-butyrylated chitosan. This water- soluble derivate is completely absorbed in the wound and probably hydrolysed afterwards by other enzymes. The clinical experiments with DBC-based bioactive wound-dressing materials elucidate a totally different action of wound healing and possible other approaches for bioactive wound-dressing materials.

5.6 Future trends The role of bioactive dressing materials is to deliver substances active in wound healing. Most of the commercial available so-called bioactive wound dressings are based on the principle of occluding the wounds, usually by preventing the formation of a scab by the action of absorbing the wound exudate secreted from the ulcer. These dressings maintain a moist environment on the wound, and prevent exposure to air and dehydration. In addition, they make it possibile to control the pH of wounds, mostly owing to their intrinsic acid nature, and the partial pressure of oxygen. Some of the commercially available bioactive wound-dressing materials contain bioactive substances such as calcium, zinc or silver ions. Increas- ingly common is the use of silver-based antiseptics. The new, advanced dressings are impregnated with silver-based antiseptics linked to their broad-spectrum activity and their lower propensity to induce bacterial resistance than antibiotics. Chitosan, chitin and their derivatives have been studied widely as bioactive wound-dressing materials, but wound-dressing materials based on these products are still not as fully commercially available as other bioactive dressing materials, such as alginates. It is likely that the future trends in bioactive wound dressing materials will be the use of materials based on chitin, chitosan and their derivatives. A better knowledge and understanding of the action of these dressings in wound healing will be the basis for future developments of more bioactive dressing materials. As described, some of these materials are completely absorbed in the healing wound, must not be changed during the healing process, are characterised by an excellent antioxidant activity and, some of them, have a good anti-bacterial activity. So, there are many possibilities for future developments of effective bioactive dressing materials. Owing to their complete resorption into the healing wounds, the use of powder instead of ﬁ bres or ﬁ lms is possible

and this can be an important factor for the price of the bioactive dressing materials and their fi nal cost in the treatment of wounds. In addition, the use of these materials can be expanded to use by private individuals in addition to the classical clinical use. Examples of such bioactive dressing materials are dressings made from blends of chitosan and alginate,101 blends of alginate and water-soluble chitin.23 Flexible, thin, transparent, novel chitosan-alginate polyelectrolyte complex membranes caused an accelerated healing of incision wounds in a rat model compared with conventional gauze dressing. Water-soluble chitin, or half-deacetylated chitin, can be prepared from chitosan by N-acetylation with acetic anhydride. Alginate and water-soluble chitin blend fi bres can be obtained by solution spinning into a coagulation bath. There is good miscibility between alginate and water-soluble chitin. The introduction of water-soluble chitin into the fi bres improves water-retention properties of the blend fi bres compared with pure alginate fi bre. The use of these blends based on alginate and chitin or chitin derivatives may be a further and next step in the development of new bioactive dressing materials. Another type of experimental bioactive dressing material is based on a blend of collagen and chitosan.102 Wound dressing materials were obtained from these blends by electrospinning by using poly(ethylene oxide) as a third component. Poly(ethylene oxide) was necessary to improve the processability of the blends and to obtain fi bres by electrospinning. Electrospinning is a simple and effective method for preparing nanofi bres with diameters ranging from 5 to 500 nm. Wound dressings from electrospun fi bres potentially offer many advantages over conventional processes. With its huge surface area and microporous structure, the electrospun fi bres could quickly interact with the enzymes present in the wound exudate. From animal studies, the membrane produced from the electrospun collagen/chitosan/poly(ethylene oxide) fi bres was better than gauze and commercial sponge in wound healing. The future development will be the production of nanofi bres from chitin derivatives, by electrospinning of its aqueous solutions or ethanol solutions, or by the direct production of nanoparticles from those derivatives. This will probably increase the biochemical activity of those derivatives. As also demonstrated by the example, the O-butyrylation of chitin or chitosan is an important modifi cation of chitin and increases the biochemical activity of chitin or chitosan. Even water-soluble derivatives of chitin or chitosan can be obtained by O-butyrylation. Probably, an optimum choice of the acetylation degree of the starting chitin for O-butyrylation is still necessary to increase the bioactive action of those compounds. The increased biochemical activity of chitin derivatives by O-butyrylation is also supported by the wound-healing activity of an ointment containing dibutyryl cyclic adenosine monophosphate (DBcAMP).103 Not only the

degree of O-butyrylation, but also the degree of N-acetylation or N- butyrylation and the porosity of the materials may be very important in order to obtain bioactive dressing materials with still better wound-healing properties.104–106

5.7 Acknowledgements The study on DBC has been supported by the European Commission under the Project CHITOMED, QLK5-CT-2002–01330. My thanks go to all companies, universities and collaborators incorporated in that research project and their references are cited in the publications and references concerning DBC.

5.8 References 1. winter, g.d., ‘Formation of the scab and the rate of epithelialization of superfi cial wounds in the skin of the young domestic pig’ Nature, 193 (1962) 293–294. 2. winter, g.d., scales j.t., Nature, 197 (1963) 91–92. 3. hinman, c.d., maibach, h., Nature, 200 (1963) 377–378. 4. trengrove, n.j., stacey, m.c., macauley, s., bennett, n., gibson, j., burslem, f., murphy, g., schultz, g., ‘Analysis of the acute and chronic wound environment: the role of proteases and their inhibitors’, Wound Repair Regen, 7 (1999) 442–452. 5. yager, d.r., nwomeh, b.c., ‘The proteolytic environment of chronic wounds’, Wound Repair Regen, 7 (1999) 433–441. 6. barrick, b., campbell, e.j., owen, c.a., ‘Leukocyte proteinases in wound healing roles: roles in physiologic and pathologic processes’, Wound Repair Regen, 7 (1999) 410–422. 7. harris, i.r., yee, c.e., walters, c.e., cunliffe, w.j., kearney, j.n., wood, e.j., ingham, e., ‘Cytokine and protease levels in healing and non-healing chronic venous leg ulcers’, Exp Dermatol, 4 (1995) 342–349. 8. james, t.j., hughes, m.a., cherry, g.w., taylor, p.t., ‘Evidence of oxidative stress in chronic venous ulcers’, Wound Repair Regen, 11(3) (2003) 172–176. 9. zabucchi, g., bellavite, p., berton, g., dri, p., ‘Free radicals generation by the infl ammatory cells’, Agents Actions Suppl, 7 (1980) 159–166. 10. efron, d.t., most, d., barbul, a., ‘Role of nitric oxide in wound healing’ Curr Opin Clin Nutr Metab Care, 3 (2000) 197–204. 11. moseley, r., stewart, j.e., stephens, p., waddington, r.j., thomas, d.w., ‘Extra- cellular matrix metabolites as potential biomarkers of disease activity in wound fl uid: lessons learned from other infl ammatory diseases?’ Br J Dermatol, 150 (2004) 401–413. 12. rasik, a.m., shukla, a., ‘Antioxidant status in delayed healing type of wounds’, Int J Exp Pathol, 81 (2000) 257–263. 13. edwards, j. v., yager, d.r., cohen, i.k., diegelmann, r.f., montante, s., bertoniere, n., bopp, a.f., ‘Modifi ed cotton gauze dressings that selectively

absorb neutrophil elastase activity in solution’, Wound Repair Regen, 9 (2001) 50–58. 14. paul, w., sharma, c.p., ‘Polysaccharides: biomedical applications’ in Ency- clopedia of surface and colloid science, Marcel Dekker Inc., NY, 2004. 15. qin, y.m., agboh, c., wang, x.d., gilding, k., ‘Alginate fi bers and dressings’, Text Mag, 25(4) (1996) 22–25. 16. ‘Bioactive products’ in Developments in wound care, PJB Publications Ltd., (1994) 43–60. 17. thomas, a., harding, k.g., moore, k., ‘Alginates from wound dressings activate human macrophages to secrete tumour necrosis factor-alpha’, Biomaterials, 21 (2000) 1797. 18. qin, y., agboh, c.h., wang, x., ‘Alginate fi bres’, Chem Fibres Int, 46 (1996) 272. 19. kang, h.a., shin m. s., yang j.w., ‘Preparation and characterization of hydro- phobically modifi ed alginate’, Polym Bull, 47 (2002) 429–435. 20. petkow, l., grkiewicz-petkow, a., ‘Modern dressing materials in treating chronic wounds and ulceration of calfs, especially using hydrocolloid dressing’, Phebol Rev, 10(4) (2002) 101–105. 21. girrbach, u., ‘Bioaktive Textilien: Trend oder gimmick?’, Int Text Bull, 2 (2003) 34. 22. klasen, h.j., ‘A historical review of the use of silver in the treatment of burns. II. Renewed interest for silver’, Burns, 26 (2000), 131–138. 23. fan, l., du, y., zhang, b., yang, j., cai, j., zhang l., zhou, j., ‘Preparation and properties of alginate/water-soluble chitin blend fi bres’, J Macromol Sci, A, 42 (2005) 723–732. 24. chen, j.l., ‘Bandage for adhering to moist surfaces’, Patent US3339456 (1967). 25. dealey, c., The care of wounds: a guide for nurses, 3rd Edn., Blackwell Publishing, Oxford (2005). 26. cockbill, s.m.e., turner, t.d., ‘The development of wound management products’, Chronic wound care: a clinical source book for healthcare professionals, Krasner DL, Rodeheaver GT and Sibbald RG eds, pp. 233–248. 27. ponec m., haverkort m., soei y.l., kempenaar j., bodde h., ‘Use of human keratinocyte and fi broblast cultures for toxicity studies of topically applied compounds’, J Pharm Sci, 79(4) (1990) 312–316. 28. wulff t., aagren s.p.m., nielsen s., Patent US6201164 to Coloplast A/S, March 13, 2001. 29. hollisaz, m.t., khedmat, h., yari, f., ‘A randomized clinical trial comparing hydrocolloid, phenytoin and simple dressings for the treatment of pressure ulcers’, Dermatol, 4 (2004) 18. 30. alm, a., hornmark, a.m., fall , p.a., linder, l., bergstrand, b., ehrnebo, m., madsen, s.m., setterberg, g., ‘Care of pressure sores: a controlled study of the use of a hydrocolloid dressing compared with wet saline gauze com- presses’, Acta Dermato-Venereol Suppl, 149 (1989) 1–10. 31. neill, k.m., conforti, c., kedas, a., burris, j.f., ‘Pressure sore response to a new hydrocolloid dressing’ Wounds: A compendium of clinical research and practice, 1 (1989), 173–185. 32. kim, y.c., shin, j.c., park, c.i., oh, s.h., choi, s.m., kim, y.s., ‘Effi cacy of hy drocolloid occlusive dressing technique in decubitus ulcer treatment a comparative study’, Yonsei Med J, 37 (1996) 181–185.

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73. struszczyk, h., ciechan´ska, d., wawro, d., ste˛ plewski, w., krucin´ska, i., szosland, l., van de velde, k., kiekens, p., ‘Some properties of dibutyrylchitin fi bres’. (2004) Proceedings of 6th International Conference of the Euro- pean Chitin Society, EUCHIS’04, Poznan´, Poland. 74. szosland, l., cisło, r., krucin´ska, i., paluch, d., staniszewska-kus´, j., pielka, s., solski, l., z˙ywicka, b., ‘Dressings made from dibutyrylchitin and chitin accelerating wound healing’, (2002), Proceedings of International Conference MEDTEX’2002, Łódz´, Poland. 75. włochwicz, a., szosland, l., binias, d., szumilewicz, j., ‘Crystalline structure and mechanical properties of wet-spun dibutyrylchitin fi bres and products of their alkaline treatment’, J Appl Pol Sci, 94 (2004) 1861–1868. 76. rajendran, s., anand, s.c., ‘Developments in medical textiles’, Text Prog, 32 (2002) 1–37. 77. staniszewska-kus´, j., paluch, d., szosland, l., kołodziej, j., staniszewska- kus´, j., szymonowicz, m., solski, l., ‘A biological investigation of dibutyrylchitin fi bres’, Eng Biomater, II(7–8) (1999) 52–60. 78. paluch, d., pielka, s., szosland, l., staniszewska-kus´, j., szymonowicz, m., solski, l ., z˙ywicka, b., ‘Biological investigation of the regenerated chitin fi bres’, Eng Biomater, 12 (2000) 17–22. 79. nah, j.w., jung, t.r., jang, m., jeong, y., ‘Water soluble chitosan nanoparticles for delivering an anticancer agent and preparing method thereof’, Patent application US2006/0013885, January 2006. 80. paluch, d., szosland, l., staniszewska-kus´, j., solski, l., szymonowicz, m., ge˛ barowska, e., ‘The biological assessment of chitin fi bres’, (2000), Polym Med, 30(3–4), 3–31 (Source: Scopus database). 81. szosland, l., krucin´ska, i., cisło, r., paluch, d., staniszewska-kus´, j., solski, l., szymonowicz, m., ‘Synthesis of dibutyrylchitin and preparation of new textiles made from dibutyrylchitin and chitin for medical applications’, 2001. Fibres Text East Eur, 9(34) (2001) 54–57. 82. pelka, s., paluch, d., staniszewska-kus´, j., zywicka, b., solski, l., szosland, l., czarny, a., zaczyn´ska, e., ‘Wound healing accelerating by a textile dressing containing dibutyrylchitin and chitin’, Fibres Text East Eur, 11 (2003) 79–84. 83. muzzarelli, r.a.a., guerrieri, m., goteri, g., muzzarelli, c., armeni, t., ghiselli, r., cornellisen, m., ‘The biocompatibility of dibutyryl chitin in the context of wound dressings’ Biomaterials, 26 (2005) 5844–5854. 84. chilarski, a., szosland, l., krucin´ska, i., błasin´ska, a., cisło, r., ‘Non- wovens made from dibutyrylchitin as novel dressing materials accelerating wound healing’ (2004) Proceedings of 6th International Conference of the European Chitin Society, EUCHIS’04, Poznan´, Poland. 85. chilarski, a., szosland, l., krucinska, i., błasinska, a., cisło, r., ‘The application of chitin derivatives as biological dressing in treatment of thermal and mechanical skin injuries’, Annual Pediatr Traum Surg, Div Pediatr Traum Surg, 8(XXXII) (2004) 58–61. 86. terbojevich, m., cosani, a., ‘Molecular weight determination of chitin and chitosan’, (1997), Chitin handbook 87–101, Muzzarelli R.A.A., Peter M.G. (eds.), European Chitin Society, Ancona, Potsdam. 87. szumilewicz, j., szosland, l., ‘Determination of the absolute molar mass of chitin and dibutyrylchitin by means of size exclusion chromatography coupled

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103. zhou, l.j., ono, i., ‘Stimulatory effects of dibutyryl cyclic adenosine monophosphate on cytokine production by keratinocytes and ﬁ broblasts’ Br J Dermatol, 143 (2000) 506–512. 104. schoukens, g., krucinska, i., blasinska, a., chrzanowski, m., szosland, l., kiekens, p., ‘Review of techniques for manufacturing dibutyrylchitin nonwoven biomaterials’ General lecture, 2nd International technical Textiles Congress, Instanbul/Turkey, (2005) July 13–15. 105. schoukens, g., ‘Bioactive materials from butyrylated chitosan’ 11th Annual Seminar and Meeting Ceramics, Cells and Tissues, October 2–5 (2007), Faenza (Italy). 106. schoukens, g., kiekens, p., krucinska, i., ‘New bioactive textile dressing materials from dibutyrylchitin’ 3rd International Technical Textiles Con- gress, December 1–2 (2007), Istanbul (Turkey).

S. RAJENDRAN and S. C. ANAND, University of Bolton, UK

Abstract: The application of compression textiles in managing venous leg ulcers is discussed. The classiﬁ cation of compression bandages, merits and limitations of the current compression therapy regimen, and the research into the development of novel orthopaedic padding and compression bandages are highlighted. The role of compression therapy in the treatment of oedema, varicose veins and deep vein thrombosis (DVT) is outlined. Research and development of a novel single-layer 3D spacer bandage, which replaces the existing multilayer bandage regime, is discussed.

Key words: wound, venous leg ulcer, padding bandage, compression bandage, 3D spacer bandage.

6.1 Introduction It has been predicted that there is a substantial market potential for advanced wound dressings. The woundcare industry generated between US$3.5 and 4.5 billion for the period from 2003 to 2006, mostly from the US and Europe. The European advanced wound management market was valued at US$544.4 million in 2004 and is expected to grow by an average of 12.4% a year to US$1.23 billion in 2010. The forecast for annual growth is between 10 and 15% in 2012. An ageing population creates an increased demand for ulcer treatment. The pressure ulcer treatment accounts for 4% of the National Health Service (NHS) annual budget. The annual cost of treating diabetic foot ulceration accounts for 5% of the total NHS budget. The treatment of venous leg ulcers creates considerable demands upon healthcare professionals throughout the world. The total cost to the NHS for venous leg ulcers treatment is about £650 million per annum, which is 1–2% of the total healthcare expenditure. Costs per patient have recently been estimated to be between £1200 and £1400. In the US, venous leg ulcers affect 3.5% of people over the age of 65 and the estimated annual cost is from $1.9 to $2.5 billion. In the EU, the annual cost for treating patients with venous leg ulcers accounts for 1–2% of the overall healthcare expenditure. In Australia, around 1% of the adult population suffer from venous ulceration.

153

Venous ulcers are the most common type of chronic leg ulceration. Chronic ulcers are deﬁ ned as those lasting six weeks or more.1 In the UK alone, about 1% of the adult population suffers from active ulceration during their life time.2 Approximately 400 000 patients have the initial symptoms of leg ulcers and 100 000 have open leg ulcers that require treatment.3 About 80% of patients who have leg ulceration suffer with a venous ulcer. Some patients may have more than one episode of venous ulceration with estimated recurrence rates ranging from 6 to 15%.4 The prevalence of leg ulcers increases with age affecting 1.69% of patients aged between 65 and 95 years. The incidence rate for patients in this age group is estimated at 0.76% for men and 1.42% for women.5 It has been established that compression therapy, by making use of compression bandages, is an efﬁ cient treatment for healing various leg ulcers, despite surgical strategies, electromagnetic therapy and intermittent pneumatic compression.

6.2 Elastic compression bandages Bandages can be used for many purposes and include retention, support and compression: • Retention bandages are used to retain dressings in the correct position. • Support bandages provide retention and prevent the development of a deformity or change in shape of a mass of tissue due to swelling or sagging. • Compression bandages are employed mainly for the treatment of leg ulcers and varicose veins. Elastomeric compression bandages made with rubber were fi rst used in the late 19th century. However, these have now been replaced by lighter, stronger, more comfortable and washable bandages made from Lycra or other elastane fi bres. Modern bandages are either woven or knitted and are designed to provide prescribed levels of compression in accordance with specifi ed performance-based standards (Table 6.1).

Table 6.1 Types of bandages

Bandage Commercial name Remark

Retention bandage Slinky®, Stayform®, Exerts very little K-Band®, Easiﬁ x®, pressure on a limb Slinky®, Crinx®, Tensoﬁ x® Support bandage Crepe BP®, Elastocrepe® Prevents formation of oedema and supports joints

Table 6.1 Continued

Bandage Commercial name Remark

Compression Exert various pressure, bandages according to the type on a limb Light compression J-Plus®, K-Crepe® Gives sub-bandage (3a) pressures of 14– 17 mm Hg at the ankle Moderate Veinopress®, Granuﬂ ex Sub-bandage pressures compression (3b) adhesive 18–24 mm Hg Compression® High compression Setopress®, 25–35 mm Hg (3c) Tensopress ® Surepress® Extra high Bilastic Forte®, Blue line Up to 60 mm Hg compression (3d) webbing® Paste bandages Zincaband®, Tarband®, Woven cotton fabric Quinaband®, impregnated with a Icthaband® medicated cream or paste. Used for the treatment of eczema and dermatitis Tubular bandages Elasticated Tubifast®, Tubigrip® Dressing on awkward sites Foam padded Netelast® Provides padding and Tubipad ® protection against physical damage

6.3 Venous leg ulcers 6.3.1 Venous leg ulcers: problem It is important that the arterial and venous systems should work properly without causing problems to blood circulation around the body. Pure blood fl ows from the heart to the legs through arteries taking oxygen and food to the muscles, skin and other tissues. Blood then fl ows back to the heart carrying away waste products through veins. The valves in the veins are unidirectional which means that they allow the venous blood to fl ow in an upward direction only. If the valves do not work properly or there is not enough pressure in the veins to push back the venous blood towards the heart (chronic venous insuffi ciency, CVI), the pooling of blood in the veins takes place and this leads to higher pressure on the skin. Because of high pressure and lack of availability of oxygen and food, the skin deteriorates and eventually the ulcer occurs. The high venous pressure causes oedema followed by tissue breakdown. The initial indications of venous leg ulcers

are oedema, swollen veins (varicose veins), stasis eczema, fi brosis, lipodermatosclerosis, atrophie blanche, ankle fl are and blood clots in veins (deep vein thrombosis, DVT). DVT is a growing problem for passengers on long- haul fl ights. DVT blocks blood from fl owing towards the heart. Venous ulcers appear in the gaiter area of the lower limb between the ankle and mid-calf. They can vary in size ranging from very small to large ulcers that extend beyond the gaiter area. The wound is characteristi- cally shallow, irregular in shape, and has sloping well-defi ned borders. Typically, the skin surrounding the wound is thickened and hyper- pigmented indicating lipodermatosclerosis.6 With chronic ulcers a yellow– white exudate is observed signifying the presence of slough. A shiny appearance indicates a fi brinous base, which inhibits new tissue formation and wound healing. Varicose veins and ankle oedema often accompany a venous ulcer.7 Approximately 80% of patients who have a venous leg ulcer suffer from some form of discomfort, while 20% experience severe or unremitting pain.8

6.3.2 Diagnosis of venous leg ulcers The diagnosis of lower limb ulceration must start by determining the patient’s full clinical history together with a physical examination of the condition. It is essential to identify possible risk factors that could cause ulceration or impact on the treatment of the ulcer. These risk factors could include arterial disease, trauma and malignancy.9 A number of non-invasive test methods are available to the clinician for investigating the cause of leg ulceration and venous insufﬁ ciency. These test methods help to assess the arterial and venous circulation of the patient and can provide information on the location of blood reﬂ ux or an obstruction within the veins.

Doppler ultrasonography Doppler ultrasonography is used to measure the ankle-to-brachial blood pressure index (ABPI) of the patient. The ultrasound technique produces a signal that identiﬁ es the presence of blood ﬂ ow within the arteries. The ABPI is obtained by measuring the systolic blood pressure within the dorsalis pedis or posterior tibial artery of the lower limb and the ipsilateral brachial artery of the arm.10 The ratio between the ankle systolic pressure and the brachial systolic pressure provides the ABPI value. Measurement of the ABPI is important in order to exclude arterial disease as the cause of ulceration or as a possible risk factor that might inhibit treatment. An ABPI of >0.80 leads to diagnosis of a venous leg ulcer, whereas patients with the ABPI of <0.92 indicates the presence of arterial disease.11 An

index of about 0.8 or below is generally considered indicative of signiﬁ cant arterial disease. These patients should be excluded from high-compression bandage therapy since its use could lead to further ulcer complications or even limb amputation.12 The Doppler ultrasonic measurement technique produces elevated readings when diagnosing patients that may have diabetes and other conditions with calciﬁ ed arteries.13

Ultrasound scanning Colour duplex ultrasound scanning is currently the technique of choice in order to assess the venous system of the lower limb. The technique combines ultrasound imaging with pulsated Doppler ultrasound and provides detailed anatomic information of the superfi cial, deep, and perforating venous systems. It can identify specifi c veins in which blood refl ux occurs or obstructions which may be contributing to venous hypertension.

Photoplethysmography and air plethysmography Photoplethysmography and air plethysmography are simple tests designed to evaluate calf muscle dysfunction and degree of venous refl ux. The techniques are used to observe the change in blood volume within the lower limb before and after exercise. Application of a tourniquet to restrict blood fl ow within the superfi cial system allows the deep venous system to be assessed for a potential obstruction. Invasive venous tests such as ascending and descending phlebography are also used to assess venous insuffi ciency. Phlebography combines electromagnetic radiation (X-rays) and fl uorescent materials to provide a technique that allows the veins to be clearly visualised. These immunofl uorescence methods can detect venous outfl ow obstructions, provide information of valvular incompetence, and also highlight the presence of pericapillary fi brin.14 Phlebography is usually used before a patient undergoes valvular surgery.

6.4 Venous leg ulcer treatment It should be stated that venous leg ulcers are chronic and there is no medication to cure the disease other than the compression therapy. A sustained graduated compression mainly enhances the ﬂ ow of blood back to the heart, improves the functioning of valves and calf muscle pumps, reduces oedema and prevents the swelling of veins. Mostly elderly people are prone to develop DVT, varicose veins and venous leg ulcers. Venous leg ulcers are the most frequently occurring type of chronic wound accounting for 80 to 90% of all lower extremity ulceration and compression remains the

mainstay of treatment.15 Compression treatment has been extensively covered in a Cochrane review.16

6.4.1 Compression bandages Compression bandaging is the ‘Gold standard’ for managing venous leg ulceration and treating the underlying venous insuffi ciency.17 The main function of a compression bandage is to exert external pressure onto the leg, and this is determined by its elastic properties. A recent study investigated the degree of pressure required to narrow and occlude the superfi cial and deep veins of the calf when a subject is in different body positions. For compression therapy to be effective, it has to exceed the hydrostatic pressure within the veins in order to narrow the vessels and achieve a subsequent increase in blood fl ow. Initial narrowing of the veins occurred at a pressure of 30–40 mm Hg in both the sitting and standing positions and complete occlusion occurred at 20–25 mm Hg (supine position), 50–60 mm Hg (sitting position), and at 70 mm Hg (standing position).18 In a further study, Partsch19 compared the different haemodynamic effects that are achieved when using compression stockings and compression bandages. The study concluded that compression stockings which exert external pressures of up to 40 mm Hg are effective in increasing blood fl ow velocity (supine position), and reducing oedema after extended periods of sitting and standing. In addition, short-stretch and multi-layered compression bandages which exert pressures of over 40 mm Hg reduce venous hypertension during walking and improve the venous pumping function.

Classifi cation Compression bandages are mainly classifi ed as elastic and non-elastic. Elastic compression bandages (Table 6.2) are categorised according to the level of pressure generated on the angle of an average leg. Class 3a bandages provide light compression of 14–17 mm Hg, moderate compression (18–24 mm Hg) is imparted by class 3b bandages, and 3c type bandages impart high compression between 25 and 35 mm Hg.20 The 3d type extra- high-compression bandages (up to 60 mm Hg) are not often used because the very high pressure generated will reduce the blood supply to the skin. It must be stated that approximately 30–40 mm Hg at the ankle which reduces to 15–20 mm Hg at the calf is generally adequate for healing most types of venous leg ulcers.21 Compression stockings provide support to treat DVT and varicose veins, and to prevent venous leg ulcers. They are classifi ed as light support (Class 1), medium support (Class 2) and strong support (Class 3).22

Table 6.2 Elastic Bandage Classiﬁ cation

Class Bandage type Bandage function

1 Lightweight conforming Apply very low levels of sub-bandage pressure and are used to hold dressings in place 2 Light support Apply moderate sub-bandage pressure and are used to prevent oedema or for the treatment of mixed aetiology ulcers 3a Light compression Exert a pressure range of 14–17 mm Hg at the ankle 3b Moderate compression Exert a pressure range of 18–24 mm Hg at the ankle 3c High compression Exert a pressure range of 25–35 mm Hg at the ankle 3d Extra high compression Exert a pressure of up to 60 mm Hg at the ankle

Compression hosiery Elastic stockings are used for the treatment of DVT which is associated with a risk of pulmonary embolism and post thrombotic syndrome (PTS). 23 The recent introduction of two-layered high-compression hosiery kits may have provided an alternative solution for the treatment of venous ulceration since they are easier and safer to apply than traditional compression bandages and improve patient concordance.24 The hosiery kits consist of two knee-high garments; a light compression (10 mm Hg) understocking and a Class 3 compression-hosiery overstocking providing 25–35 mm Hg. The understocking is applied on to the leg ﬁ rst and owing to its smooth surface allows the overstocking to slip over it for ease of application. Com- pression stockings (antiembolism stockings) are the most commonly available and accepted methods for DVT treatment. Compression hosiery contains elastomeric yarns that are capable of recovering their size and shape after extension giving similar performance properties to long-stretch compression bandages. There are three classiﬁ - cation Standards for graduated compression hosiery: the British Standard,25 French Standard,26 and German Standard.27 Attempts were made to produce a European Standard (draft ENV 12718:2001) but consensus could not be achieved and consequently the Standard was cancelled.28 The above Standards generally classify compression hosiery according to the level of pressure exerted around the ankle (Table 6.3).25–27 Patients are advised to wear elastic stockings every day after the ulcer has healed in order to prevent recurrence.29 However, in everyday practice

Table 6.3 European Classiﬁ cation of Compression Hosiery

Class Support British Standard French Standard German Standard BS 6612:1985 ASQUAL RAL-GZ 387:2000

1 Light 14–17 mm Hg 10–15 mm Hg 18–21 mm Hg 2 Medium 18–24 mm Hg 15–20 mm Hg 23–32 mm Hg 3 Strong 25–35 mm Hg 20–36 mm Hg 34–46 mm Hg 4HeavyNot reported>36 mm Hg >49 mm Hg

patients are reluctant to wear compression hosiery on a long-term basis.30 A systematic review concluded that there is circumstantial evidence to suggest that compression hosiery does reduce ulcer recurrence but there is no strong evidence to support this. In addition, high-compression stockings may be more effective than moderate compression in preventing ulcer recurrence.31

Compression system Compression can be exerted to the leg either by a single-layer bandage or multilayer bandages. In the UK, a four-layer bandaging system is widely used whilst in Europe and Australia the non-elastic two-layer short stretch bandage regime is the standard treatment. A typical four-layer compression bandage system comprises of padding bandage, crepe bandage, high- compression bandage and cohesive bandage. Both the two layer and four layer systems require padding bandage (wadding or orthopaedic wool) that is applied next to the skin and underneath the short stretch or compression bandages. A plaster type non-elastic bandage, Unna’s boot is favoured in the USA. However, compression would be achieved by three-layer dressing that consists of Unna’s boot, continuous gauze dressing, followed by an outer layer of elastic wrap. It should be realised that Unna’s boot, being rigid, is uncomfortable to wear and medical professionals are unable to monitor the ulcer after the boot is applied. Unna’s Boot provides a high working pressure when the calf muscle contracts, but very little pressure while the patient is at rest.32 The high working pressure serves to increase blood fl ow, while the low resting pressure facilitates deep venous fi lling. The Unna’s Boot is only effective in ambulatory patients and requires constant re-application as leg volume decreases owing to a reduction in oedema. Used widely in the USA, the Unna’s Boot system is uncomfortable to wear because of its rigidity and is both expensive and diffi cult to apply.

A multi-centre study compared the venous ulcer healing rates between Unna’s Boot and CircAid® in 38 patients.33 The time to heal for the Unna’s Boot group of patients was 9.69 ± 3.28 weeks in comparison to the CircAid® device group which was 7.98 ± 4.41 weeks. The study data supported a trend towards more rapid ulcer healing in the CircAid® device group but the results did not reach statistical signiﬁ cance owing to the small number of patients studied. Short-stretch bandages function in a similar manner to the rigid/inelastic Unna’s Boot. They consist of 100% high twisted cotton yarns and are applied onto the limb at full extension. Unlike elastic bandages, short-stretch bandages ﬁ rmly hold the calf thereby providing a high working pressure when the patient walks.34

6.4.2 Padding bandages (orthopaedic wool or wadding) Padding bandages play a signiﬁ cant role in the successful treatment of venous leg ulcers. A variety of padding bandages are used beneath the compression bandage system as padding layers in order to evenly distribute pressure and give protection. They absorb high pressure created at the tibia and ﬁ bula regions. It will be noticed that the structure of a padding bandage is regarded as an important factor in producing a uniform pressure distribution. Research has shown that the majority of the commercially available bandages do not provide uniform pressure distribution.35,36 A padding at least 2.5 cm thick is placed between the limb and the compression bandage to distribute the pressure evenly at the ankle as well as the calf region. Wadding helps to protect the vulnerable areas of the leg from the high compression levels required along the rest of the leg.37 Padding can also be used to reshape legs which are not narrower at the ankle than the calf. It makes the limb more like a cone-shape so that the pressure is distributed over a pressure gradient with more pressure at the foot and less at the leg. Generally, the longer a compression bandage system is to remain in place, the greater is the amount of padding needed. An ideal padding bandage should be: • lightweight and easy to handle; • soft and impart cushioning effect to the limb; • capable of preventing tissue damage; • capable of distributing pressure evenly around the leg; • a good absorbent and have good wicking properties; • comfortable and should not produce irritation or any allergic reaction to the skin on prolonged contact; • easily tearable by hand; and • cheap.

6.4.3 Ideal compression bandages It should be noted that compression bandages may be harmful if not applied properly. They provide high tension as well as high pressure. A thorough assessment involving several criteria is therefore essential before applying a compression bandage on a limb. For example, it is important to consider the magnitude of the pressure, the distribution of the pressure, the duration of the pressure, the radius of the limb and the number of bandage layers. The ability of a bandage to provide compression is determined by its construction and the tensile force generated in the elastomeric ﬁ bres when extended. Compression can be calculated by Laplace’s Law, which states that the sub-bandage pressure is directly proportional to the bandage tension during application and the number of layers applied but inversely proportional to limb radius.38 Sub-bandage pressure is a function of the tension induced into the compression bandage during application. Applying the bandage with a 50% overlap effectively produces two layers, which generates twice the pressure. When a compression bandage is applied at a constant tension on a limb of increasing circumference, it will produce a sub-bandage pressure gradient with the highest pressure exerted on the ankle. The sub-bandage pressure will increase for people with smaller ankles. The ability of a bandage to maintain sub-bandage pressure is determined by the elastomeric properties of the yarns, the fabric structure, as well as the ﬁ nishing treatments applied to the fabric. The structure of a compression bandage is regarded as an important factor in producing a uniform pressure distribution. An ideal compression bandage should: • provide compression appropriate for the individual; • provide pressure evenly distributed over the anatomical contours; • provide a gradient pressure diminishing from the angle to the upper calf; • maintain pressure and remain in position until the next change of dressing; • extend from the base of the toes to the tibial tuberosity without gap; • function in a complimentary way with the dressing; and • possess non-irritant and non-allergenic properties.

6.4.4 Ideal bandage pressure Compression bandages are mostly used during the initial therapy phase where the aim of treatment is to reduce oedema and overcome venous insuffi ciency. A number of different types of compression bandage systems are commercially available and, as discussed, the bandages are classifi ed as either rigid/inelastic, short-stretch, long-stretch, or multilayered. The type of fabric construction infl uences the degree of extensibility that the bandage

will have. At some point, the bandage will not be able to extend or stretch any further (lock-out) under a predetermined tension. Evidence suggests that a sub-bandage pressure of 35–40 mm Hg at the ankle, which gradually reduces to 17–20 mm Hg at the knee, is required to overcome venous hypertension and successfully treat venous leg ulcers.39 A recent study investigated the degree of pressure that is required to narrow and occlude leg veins when a subject is in different body positions. The authors found that initial narrowing of the veins occurred at a pressure of 30–40 mm Hg in both the sitting and standing positions. Complete occlusion of the superﬁ cial and deep leg veins occurred at 20–25 mm Hg (supine position), 50–60 mm Hg (sitting position), and at 70 mm Hg (standing position).18

6.5 Applications of bandages The elastic properties of the bandages help to provide a high recoiling force, which serves to increase venous fl ow and reduce venous hypertension. In addition, they conform easily around the lower limb and allow for frequent dressing changes. Skill is required to apply compression bandages at the correct tension and to avoid excessive sub-bandage pressures.40 Application of high sub-bandage pressure on patients with any type of micro-vascular disease can lead to further occlusion and pressure necrosis of these vessels.41 Some manufacturers supply compression bandages with a series of geometric markers printed onto the bandage surface. The markers assist in the application of a predetermined level of compression by visually distorting when the bandage is stretched to a specifi c tension. For example, printed rectangles become squares when the correct bandage tension is reached. In a multilayer bandaging system, three or four layers of different types of bandage are used to provide external compression. A multilayer system may include a combination of nonwoven padding bandage, inelastic creep bandage, elastic compression bandages and cohesive (adhesive) bandage. The different properties of each bandage type contribute to the overall effectiveness of the bandage system. The elastic bandage component provides sustained compression while the cohesive bandage offers rigidity thereby enhancing calf muscle pump function. The four-layer high compression system developed by a clinical group at Charing Cross Hospital (London) has gained wide acceptance for use in UK hospitals. The four- layer system was developed specifi cally to incorporate different bandage types and properties in order to overcome the clinical issues of exudate, protection of bony prominences, and the ability to sustain sub-bandage pressure over a period of time.42 In addition, the system was designed to apply the required 40 mm Hg of pressure at the ankle, overcome dispro- portionate limb size and shape, and to remain in position on the leg without

slippage. Application of the four-layer system involves ﬁ rst applying a padding bandage layer from the base of the toes to just below the knee. A crepe bandage is applied next followed by an elastic compression bandage. Finally, a cohesive layer is applied in order to add durability and to complete the overall pressure proﬁ le. Examples of different types of compression bandages, cohesive bandages, padding bandages, and multilayer compression systems are shown in Table 6.4. A multilayer high compression bandage system has been shown to provide a safe and effective treatment option for uncomplicated venous leg ulcers. Ulcer healing rates of up to 70% at twelve weeks have been obtained.43 The four-layer bandaging technique has been shown to heal chronic ulcers that have failed to respond with traditional adhesive plaster bandage systems.44 A recent review on compression therapy for venous leg ulcers concluded that a multilayer compression system is more effective than low compression or single-layer compression.45

Table 6.4 Illustration of bandages used in compression therapy

Bandage name Function Manufacturer

Tensopress Type 3c long stretch bandage Smith + Nephew Setopress Type 3c long stretch bandage Medlock Medical SurePress Type 3c long stretch bandage ConvaTec Adva-co Type 3c long stretch bandage Advancis Medical Dauerbinde K Long stretch bandage Lohmann + Rauscher Silkolan Type 2 short stretch bandage Urgo Limited Tensolan Type 2 short stretch bandage Smith + Nephew Comprilan Type 2 short stretch bandage Smith + Nephew Actiban Type 2 short stretch bandage Activa Healthcare Actico (Cohesive) Type 2 short stretch bandage Activa Healthcare Rosidal K Type 2 short stretch bandage Lohmann + Rauscher Co-Plus Cohesive bandage Smith + Nephew Tensoplus Cohesive bandage Smith + Nephew Coban Cohesive bandage 3M Surepress Padding bandage ConvaTec Soffban Padding bandage Smith + Nephew K-soft Padding bandage Urgo Limited Softexe Padding bandage Medlock Medical Advasoft Padding bandage Advancis Medical Flexi-ban Padding bandage Activa Healthcare Cellona Padding bandage Lohmann + Rauscher Ultra-soft Padding bandage Robinsons Healthcare Ortho-band Padding bandage Millpledge Healthcare Formﬂ ex Padding bandage Lantor (UK) Limited Profore Multilayer compression system Smith + Nephew Proguide Multilayer compression system Smith + Nephew Ultra Four Multilayer compression system Robinsons Healthcare System 4 Multilayer compression system Medlock Medical K-four Multilayer compression system Urgo Limited

6.6 Present problems and novel bandages During the past few years, there have been increasing concerns relating to the performance of bandages especially pressure distribution properties for the treatment of venous leg ulcers. This is because compression therapy is a complex system and requires two or multilayer bandages, and the performance properties of each layer differ from those of other layers. The widely accepted sustained graduated compression mainly depends on the uniform pressure distribution of different layers of bandages in which textile fi bres and bandage structure play a major role. The padding bandages commercially available are nonwovens that are mainly used to distribute the pressure, exerted by the short stretch or compression bandages, evenly around the leg. Otherwise higher pressure at any one point not only damages the venous system but also promotes arterial disease. Therefore, there is a need to distribute the pressure equally and uniformly at all points of the lower limb and this can be achieved by applying an effective padding layer around the leg beneath the compression bandage. In addition, padding bandages should be capable of absorbing the high pressure created at the tibia and fi bula regions. Wadding also helps to protect the vulnerable areas of the leg from generating extremely high pressure levels as compared with those required along the rest of the leg. The research carried out at the University of Bolton involving 10 most commonly used commercial padding bandages produced by major medical companies showed that there are signifi cant variations in properties of commercial padding bandages,35,36 more importantly the commercial bandages do not distribute the pressure evenly at the ankle as well as the calf region (Fig. 6.1). In addition, the integrity of the non-woven bandages is also of great concern. When pres-

60.00

50.00

40.00 PB1 PB2 PB3 30.00 PB4 PB5 PB6 20.00 PB7 PB8 PB9 10.00 PB10 Measured pressure (mm Hg)

0.00 2.93 5.86 8.79 11.72 14.6517.58 20.5123.4426.3729.22 32.1535.0838.0140.9443.8746.8049.73 52.6655.6658.5259.99 Applied pressure (mm Hg) 6.1 Pressure distribution of commercial padding bandages.

sure is applied using compression bandages, the structure of the non-woven bandages may collapse and the bandage would not impart a cushioning effect to the limb. Comfort and a cushioning effect are considered to be essential properties for padding bandages because they stay on the limb for several days. Twelve padding bandages, which consisted of single-component fi bres, binary blends and tertiary blends incorporating polyester, bicomponent fi bres and natural fi bres such as cotton and viscose, have been designed and developed at the University of Bolton (Table 6.5). The salient properties of the developed bandages are: • all the developed padding bandages possess suitable bulkiness; • none of the bandages has lower tensile strength or breaking extension that hinders its performance characteristics as an ideal padding bandage; • the tear resistance of bandages, except 100% hollow viscose (NPB5) is high and this means that the bandage cannot be easily torn by hand after wrapping around the leg. However, making perforations at regular intervals across the bandage facilitates easy tearing; • the absorption of solution containing Na+ and Ca2+ ions (artifi cial blood) is signifi cantly high, irrespective of fi bre type and structure; • the rate of absorption of all the developed bandages is also high; and • the pressure distribution of all the novel bandages is good up to 60 mm Hg (Fig. 6.2). In the UK, multilayer compression systems are recommended for the treatment of venous leg ulcers.46 Although multilayer compression bandages are more effective than single-layer bandages in healing venous leg ulcers,45 it is generally agreed by clinicians that multilayer bandages are too bulky for patients and the cost involved is high. A wide range of compression bandages is available for the treatment of leg ulcers but each of them has a different structure and properties and this infl uences the variation in performance properties of bandages. In addition, long stretch compression bandages tend to expand when the calf muscle pump is exercised, and the benefi cial effect of the calf muscle pump is dissipated. It is a well- established practice that elastic compression bandages that extend up to 200% are applied at 50% extension and at 50% overlap to achieve the desired pressure on the limb. It has always been a problem for nurses to exactly stretch the bandages at 50% and apply without losing the stretch from ankle to calf, although there are indicators for the desired stretch (rectangles become squares) in the bandages. Elastic compression bandages are classifi ed into four groups (Table 6.3) according to their ability to produce predetermined levels of compression and it has always been a problem to select the right compression bandage for the treatment. The

Identiﬁ cation Product Fibre type Fibre dtex; length Blend ratio Structure code (mm) (%)

NPB1 Single component Polyester 3.3;40 100 Needlepunched (both sides) NPB2 Single component Polyester (bleached) 5.3;60 100 Needlepunched (both sides) NPB3 Single component Hollow polyester 3.3;50 100 Needlepunched (both sides) NPB4 Single component Viscose 3.3;40 100 Needlepunched (both sides) NPB5 Single component Hollow viscose 3.3;40 100 Needlepunched (both sides) NPB6 Single component Lyocell 3.3;38 100 Needlepunched (both sides) NPB7 Binary blends Polyester/viscose 3.3;40/3.3;40 75 : 25 Needlepunched (both sides) NPB8 Binary blends Polyester/viscose 3.3;40/3.3;40 50 : 50 Needlepunched (both sides) NPB9 Binary blends Polyester/viscose 3.3;40/3.3;40 25 : 75 Needlepunched (both sides) NPB10 Binary blends Polyoleﬁ n/viscose 2.2;40/3.3;40 20 : 80 Needlepunched (both sides) and thermal bonded NPB11 Tertiary blends Polyester/viscose/cotton 3.3;40/3.3;40/1.8;22 33 : 33 : 33 Needlepunched (both sides) (bleached) NPB12 Tertiary blends Polyester/viscose/polyoleﬁ n 3.3;40/3.3;40/2.2;40 60 : 25 : 15 Needlepunched (both sides) and thermal bonded

60.00

50.00 NPB1 NPB2 NPB3 40.00 NPB4 NPB5 NPB6 30.00 NPB7 NPB8 NPB9 20.00 NPB10

Measured pressure (mm Hg) NPB11 NPB12 10.00

0.00 2.93 5.86 8.79 11.72 14.6517.58 20.5123.4426.37 29.2232.1535.0838.0140.9443.8746.8049.7352.6655.6658.5259.99 Applied pressure (mm Hg) 6.2 Pressure distribution of novel padding bandages.

inelastic short stretch bandage (Type 2) system, which has started to appear in the UK market, has the advantage of applying at full stretch (up to 90% extension) around the limb. Short stretch bandages do not expand when the calf muscle pump is exercised and the force of the muscle is directed back into the leg which promotes venous return. The limitations of short stretch bandages are that a small increase in the volume of the leg will result in a large increase in compression and this means the bandage provides high compression in the upright position and little or no compression in the recumbent position when it is not required. During walking and other exercises the sub-bandage pressure rises steeply and while at rest the pressure comparatively drops. Therefore, patients must be mobile to achieve effective compression and exercise is a vital part of this form of compression. Moreover, the compression bandage is not in contact with the skin when there is a reduction in limb swelling because the short stretch bandage is inelastic, and it has already been stretched to its full. The application of a multilayer bandage system requires expertise and knowledge. Nurses must undergo signiﬁ cant practice-based training in order to develop appropriate bandage application skills needed for multilayer compression system. Successful bandaging relies upon adopting good technique in both stretching the bandage to the correct tension and ensuring proper overlap between layers. In addition, nurses need to have knowledge of the different performance properties of each bandage within the multilayer system, and how the performance each bandage combines is to achieve safe and adequate compression. The ability of multilayer bandage systems to maintain adequate compression levels for up to one week has

reduced the necessity for frequent dressing changes and has therefore, decreased treatment costs. However, the cost of a multilayer compression system is still relatively high owing to the requirement for a specifi c bandage for each layer. Tolerance to multilayer compression system is generally good but non-compliance in some patients often results in prolonged or ineffective treatment. Some patients are unable to wear footwear due to the bulkiness of multilayer compression regime. These patients often refuse treatment since the requirement to remain house-bound is totally unac- ceptable. At night patients fi nd compression bandages too uncomfortable and often remove them in order to sleep. Since the application of multilayer compression systems is complex most patients are unable to re-apply the bandages themselves. In order to address some of the problems mentioned above, a novel non- woven vari-stretch compression bandages (NVCB) has been designed and developed at the University of Bolton. The principal features of the NVCB are:47,48 • novel non-woven technology was used to develop the variable compression bandages. It should be mentioned that no non-woven compression bandages are listed in Drug Tariff. In the UK, the availability of wound dressings and bandages for use in patients’ homes is dictated by the Drug Tariff; • the performance and properties of the novel bandages are superior to existing multilayer commercial compression bandages. This fulfi ls the requirement of ideal variable pressure from ankle to below knee positions of the limb for the treatment of venous leg ulcers; and • vari-stretch non-woven bandages also meet the standards and the toler- ances stipulated by BS 7505.

6.7 Three-dimensional spacer compression bandages Recently, spacer technology has been increasingly used to produce three- dimensional materials for technical textiles sectors such as the automotive, medical, sports and industrial market. The spacer technology is fl exible, versatile, cost effective and an ideal route to produce 3D materials for medical use. It is identifi ed that spacer is the right technology to produce novel compression bandages that meet the prerequisites of both ideal padding and compression bandages. The main reasons for the current interest in 3D spacer fabrics for producing novel compression bandages are several-fold. In 3D spacer fabrics, two separate fabric layers are combined with an inner spacer yarn or yarns using either the warp knitting or weft knitting route (Fig. 6.3). The two layers can be produced from different fi bre types such as polyester, polyamide, polypropylene, cotton, viscose,

6.3 A spacer structure.

lyocell and wool and can have completely different structures.49 It is also possible to produce low modulus spacer fabrics by making use of elastic yarns. Elastic compression could be achieved by altering the fabric structure. It should be mentioned that 3D structure allows greater control over elasticity and these structures can be engineered to be uni-directional, bi-directional and multi-directional. Uni-directional elasticity is one of the desired properties for compression bandages. The three-dimensional nature of spacer fabrics makes them ideal for application next to the skin because they have desirable properties that are ideal for the human body.50 The 3D fabrics are soft, have good resilience that provides a cushioning effect to the body, are breathable, and are able to control heat and moisture transfer.49 For venous leg ulcer applications, such attributes, together with improved elasticity and recovery, promote faster healing. It must be stated that 3D spacer fabrics can also be produced using double-jersey weft knitting machines.49 The main advantages of weft-knitted spacer fabrics over warp-knitted fabrics include cost effectiveness because there is no need to prepare a number of warp beams and spun yarns, and coarser count hairy yarns can be used on weft-knitting machines. Because of the problems associated with the currently available bandages for the treatment of venous leg ulcers as discussed in section 6.6, it is vital to research and develop an alternative bandaging regime that meets all the requirements of an ideal compression system. The research and development programme currently in progress at the University of Bolton has the ultimate aim of developing a single-layer compression-therapy regime for the treatment of venous leg ulcers. The research programme imposes signiﬁ cant challenge in developing 3D spacer

bandages for compression therapy. There is no doubt that there would be substantial savings for the NHS in the UK and other health services in the world because the ultimate goal of the research programme is to replace the multilayer bandages with a single-layer bandage. A single-layer system simpliﬁ es and standardises the application of compression, is more patient friendly, reduces the nursing time, and signiﬁ cantly decreases the treatment cost.

6.7.1 Effect of pressure transference of spacer bandages Four spacer fabrics identiﬁ ed as black (1), white (2), white (3) and blue (4) were used to study the pressure transference at various pressure ranges. Four padding bandages (PB1a to PB4a) recently available at Drug Tariff were also used for comparison. Pressure transference apparatus and an extension test rig were used to study the pressure transference of spacer bandages both at unrestrained and stretch conditions. It can be observed in Fig. 6.4 that the pressure transference of different spacer bandages at any one point varies, and it mainly depends on the structure and ﬁ bre content of the material. It is interesting to note that spacer bandages distributed the applied pressure more uniformly around the leg than did commercial padding bandages (Fig. 6.5). For instance, the white (2) spacer bandage absorbed an applied pressure of 43.9 mm Hg and a pressure of transfer 2 mm Hg at one point. In other words, the absorbed pressure of 41.9 mm Hg is uniformly

12.00

10.00

8.00 Black (1) White (2) 6.00 White (3) Blue (4) 4.00

Measured pressure (mm Hg) 2.00

0.00 7.3 14.6 21.9 29.3 36.6 43.9 51.2 58.6 Applied pressure (mm Hg) 6.4 Pressure transference of spacer bandages (relaxed).

30 PB1a PB2a PB3a 20 PB4a Measured pressure (mm Hg)

0 0 2.94 5.9 8.81 11.8 14.7 17.6 20.6 23.5 26.4 29.3 32.2 35.2 38.1 41.1 44 46.9 49.9 52.8 55.7 58.6 60.1 Applied pressure (mm Hg) 6.5 Pressure transference of commercial padding bandages.

distributed inside the fabric structure which is one of the essential requirements for venous leg ulcer treatment. On the other hand, the commercial padding bandage (PB4a) absorbed 43.9 mm Hg and transferred 35 mm Hg at one point (Fig. 6.5) and this means the bandage distributed only 8.9 mm Hg uniformly inside the structure. The higher output pressure from the bandage at one point is undesirable and may slow down and/or block the blood fl ow in arteries. Figures 6.6 to 6.9 represent the pressure transference of spacer bandages at known pressures under extension up to 120%. It is noticed that an increase in applied pressure does not infl uence the pressure transference at any one point and the variation is marginal in all the samples. This affi rms that these spacer fabrics can be used as ideal padding bandages and, by controlling the tension, it will be possible to generate the required pressure for the treatment of venous leg ulcers. As discussed in section 6.4.3, the pressure generated on the limb by a bandage is directly proportional to the tension of the bandage and the number of layers but inversely proportional to the width of the bandage and the circumference of the limb. This Laplace’s concept is being applied in the research and development programme into the mathematical modelling of spacer bandages to achieve the required pressure mapping for the treatment of venous leg ulcers at the University of Bolton.

7.00

6.00

5.00

4.00 7.3 (mm Hg) 14.6 (mm Hg) 3.00 21.9 (mm Hg) 29.3 (mm Hg) 2.00 36.6 (mm Hg) 43.9 (mm Hg)

Pressure transference (mm Hg) 51.2 (mm Hg) 1.00 58.6 (mm Hg)

0.00 0 10 20 30 40 50 60 70 80 90 100 110 120

Fabric extension (%) 6.6 Effect of extension on pressure transference of spacer bandages – black (1).

4.50

4.00

3.50

3.00

2.50 7.3 (mm Hg) 2.00 14.6 (mm Hg) 21.9 (mm Hg) 1.50 29.3 (mm Hg) 36.6 (mm Hg) 1.00 43.9 (mm Hg) Pressure transference (mm Hg) 0.50 51.2 (mm Hg) 58.6 (mm Hg) 0.00 0 102030405060708090100 Fabric extension (%) 6.7 Effect of extension on pressure transference of spacer bandages – white (2).

6.00

5.00 7.3 (mm Hg) 4.00 14.6 (mm Hg) 21.9 (mm Hg) 29.3 (mm Hg) 3.00 36.6 (mm Hg) 43.9 (mm Hg) 2.00 51.2 (mm Hg) 58.6 (mm Hg)

Pressure transference (mm Hg) 1.00

0.00 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140150 Fabric extension (%) 6.8 Effect of extension on pressure transference of spacer bandages – white (3).

10.00

9.00

8.00

7.00

6.00 7.3 (mm Hg) 5.00 14.6 (mm Hg) 4.00 21.9 (mm Hg) 29.3 (mm Hg) 3.00 36.6 (mm Hg) 43.9 (mm Hg)

Pressure transference (mm Hg) 2.00 51.2 (mm Hg) 1.00 58.6 (mm Hg)

0.00 0102030405060708090 Fabric extension (%) 6.9 Effect of extension on pressure transference of spacer bandages – blue (4).

6.8 Conclusions Compression-delivery systems have an important role in the treatment of venous leg ulceration since they can reduce venous refl ux, increase venous and arterial blood fl ow, improve microcirculation, and reduce ankle oedema. There is a variety of compression bandaging systems available, the advantages and disadvantages of each are constantly reviewed and debated. In the UK, for example, a four-layer bandaging system is popular and has been shown to provide a safe and effective treatment option. Other countries, such as the USA, prefer the rigid characteristics of the Unna’s Boot. The success of any compression bandage system relies upon the skill and expertise of the clinician applying it. Moreover, the effective management of venous leg ulcer involves careful selection of bandages to reverse the venous blood fl ow back to the heart. The contribution of padding as well as compression bandages in healing the ulcer is signifi cant. The advantages and limitations of the existing two-layer and four-layer bandaging regimens have been discussed. It is obvious that the pressure transference of commercial padding bandages varied and none of the padding bandages investigated satisfi ed the requirements of an ideal padding bandage. On the other hand, the novel padding bandages exhibited a uniform pressure distribution around the leg. The paper also demonstrated the need for developing a single-layer bandaging regime for the benefi t of the elderly and to cut the cost of treatment. The use of 3D spacer technology has been investigated and the results affi rmed that spacer bandages would be utilised to design and develop a single-layer system that could replace the currently used cumbersome four-layer system. A suitable spacer structure can combine the desirable attributes of both the padding and two-dimensional compression bandages into one composite three-dimensional structure.

6.9 References

1. reichenburg, j., davis, m. Venous Ulcers. Semin Cutan Med Surg 2005; 24(4): 216–226. 2. martson, w.a., carlin, r.e., passman, m.a., farber, m.a., keagy, b.a., parent iii, f.n. Healing rates and cost effi cacy of outpatient compression treatment for leg ulcers associated with venous insuffi ciency. J Vasc Surg 1999; 30(3): 491–498. 3. sarkar, p.k., ballentyne, s. Management of leg ulcers. Postgrad Med J 2000; 76: 674–682. 4. nicholaides, a.n. Investigation of chronic venous insuffi ciency: a consensus statement (France, March 5–9, 1997). Circulation 2000; 102(20): E126– E163.

5. margolis, d.j., bilker, w., santanna, j., baumgarten, m. Venous leg ulcer: incidence and prevalence in the elderly. J Am Acad Dermatol 2002; 46(3): 381–386. 6. naschitz, j.e., yeshurun, d., misselevich i., boss j.h. The pathogenesis of lipodermatosclerosis: facts, uncertainties and theories. J Eur Acad Dermatol Venereol 1997; 9(3): 209–214. 7. zimmet, s.e. Venous leg ulcers: modern evaluation and management. Dermatol Surg 1999; 25(3): 236–241. 8. franks, p.j., moffatt, c.j., bosanquet, n., oldroyd, m., greenhalgh, r.m., mccollum, c.n. connolly, m. Community leg ulcer clinics: effect on quality of life. Phlebology 1994; 9(2): 83–86. 9. london, n.j.l., donnelly, r. Ulcerated lower limb. BMJ 2000; 320: 1589–1591. 10. sibbald, r.g. Venous leg ulcers. Ostomy Wound Manage 1998; 44(9): 52–64. 11. sumner, d.s. Non-invasive assessment of peripheral arterial occlusive disease. In: Rutherford, K.S, editor. Vascular Surgery (3rd edition) Philadelphia: WB Saunders, 1998; 41–60. 12. callam, m.j., ruckley, c.v., harper. dale, j.j. Chronic ulceration of the leg: extent of the problem and provision of care. Br Med J (Clin Res Ed) 1985; 290(6485): 1855–1856. 13. mcguckin, m., stineman, m., goin, j., williams, s. Draft guideline: diagnosis and treatment of venous leg ulcers. Ostomy Wound Manage 1996; 42(4): 48–54. 14. valencia, i.c ., falabella, a., kirsner, r.s. eaglstein, w.h. Chronic venous insufﬁ ciency and venous leg ulceration. J Am Acad Dermatol 2001; 44(3): 401–424. 15. blair, s.d., wright, d.d.i., backhouse, c.m., riddle, e., mccollum, c.n. Sustained compression and the healing of chronic ulcers. BMJ 1988; 297: 1159–1161. 16. cullum, n., nelson, e.a., ﬂ etcher, a.w., sheldon, t.a. Compression for venous leg ulcers. Cochrane Database Syst Rev 2004; (2): CD000265. 17. choucair, m., phillips, t.j. Compression therapy. Dermatol Surg 1998; 24(1): 141–148. 18. partsch, b., partsch, h. Calf compression pressure required to achieve venous closure from supine to standing positions. J Vasc Surg 2005; 42(4): 734–738. 19. partsch, h. Do we still need compression bandages? Haemodynamic effects of compression stockings and bandages. Phlebology 2006; 21(3): 132–138. 20. thomas, s. Bandages and bandaging: the science behind the art. Care Sci Pract 1990; 8(2): 56–60. 21. simon, d. Approaches to venous leg ulcer care within the community: compression, pinch skin grafts and simple venous surgery. Ostomy Wound Manage 1996; 42(2): 34–40. 22. anon. The complete Scholl guide to health care for legs. Luton: Scholl, 1996. 23. hach-wunderle, v. düx, m., hoffmann, a., präve, f., zegelman, m., hach, w. Treatment of deep vein thrombosis in the pelvis and leg. Deutsch Ärztebl 2008; 105: 23–34.

24. polignano, r., guarnera, g., bonadeo, p. Evaluation of SurePress comfort: a new compression system for the management of venous leg ulcers. J Wound Care 2004; 13(9): 387–391. 25. british standards institution. Specifi cation for graduated compression hosiery. BS 6612:1985; London: BSI, 1985. 26. certifi cat de qualite-produits. Referentiel technique prescrit pourles ortheses elastiques de contention des membres. 1999; Paris: ASQUAL. 27. deutsches institut für gütesicherung und kennzeichnung. Medizinische Kompressionsstrümpfe RAL-GZ 387. 2000; Berlin: Beuth. 28. clark, m., krimmel, g. Lymphoedema and the construction and classifi cation of compression hosiery. Lymphoedema Framework. Template for Practice: compression hosiery in lymphoedema. 2006; London: MEP Ltd. 29. williams, c. Leg ulcer after care: the role of compression hosiery. Br J Nurs 2000; 9(13): 822–828. 30. vowden, k.r., vowder, p. Preventing venous ulcer recurrence: a review. Int Wound J 2006; 3(1): 11–21. 31. nelson, e.a., bell-syer, s.e.m., cullum, n.a. Compression for preventing recurrence of venous ulcers. Cochrane Database System Rev 2000; (4): CD002303. DOI: 10.1002/14651858.CD002303. 32. partsch, h. Compression therapy of the legs: a review. J Dermatol Surg Oncol 1991; 17(10): 799–805. 33. depalma, r.g., kowallek, d., spence, r.k. Comparison of costs and healing rates of two forms of compression in treating venous ulcers. Vasc Surg 1999; 33(6): 683–690. 34. hampton, l. Venous leg ulcers: short stretch bandages for compression therapy. Br J Nurs 1997; 6(17): 990–998. 35. rajendran, s., anand, s.c. Design and development of novel bandages for compression therapy. Br J Nurs (Tissue Viability Supplement) 2003; 12(17): S20–S29. 36. rajendran, s., anand, s.c. Development of novel bandages for compression therapy, Wounds UK 2002, Harrogate, 19–20 November 2002. 37. gibson, b., duncan, v., armstrong, s . Know how: ischaemic leg ulcer. Nurs Times 1997; 93(36): 34–36. 38. moffat, c., harper, p. Leg ulcers. Churchill Livingstone, Edinburgh, 1997. 39. stemmer, r. Ambulatory-elasto-compressive treatment of the lower extremities particularly with elastic stockings. Derm Kassenarzt 1969; 9: 1–8. 40. logan, r.a., thomas, s., harding, e.f., collyer, g. A comparison of sub-bandage pressures produced by experienced and inexperienced bandagers. J Wound Care 1992; 1(3): 23–26. 41. simon, d.a., freak, l., williams, i.m. et al. Progression of arterial disease in patients with healed venous ulcers. J Wound Care 1994; 3(4): 179–180. 42. moffatt, c.j., dickson, d. The Charing Cross high compression four-layer bandaging system. J Wound Care 1993; 2(2): 91–94. 43. nelzen, o., bergqvist, d., lindhagen, a. Leg ulcer etiology – a cross sectional population study. J Vasc Surg 1991; 14(4): 557–564.

44. buchbinder, d., mccullough, g.m., melick, c.f. Patients evaluated for venous disease may have other pathological considerations contributing to symptom- atology. Am J Surg 1993; 166: 211–215. 45. cullum, n., nelson, e.a., ﬂ etcher, a.w. sheldon, t.a. Compression for venous leg ulcers. Cochrane Database System Rev 2001; (2): CD000265. DOI:10.1002/ 14651858.CD000265. 46. nhs centre for review and dissemination. University of York. Compression therapy for venous leg ulcers. Effect Healthcare 1997; 3(4): 1–12. 47. rajendran, s., anand, s.c. The contribution of textiles to medical and healthcare products and developing innovative medical devices. Indian J Fibre Text Res 2006; 31: 215–229. 48. rajendran, s., anand, s.c. Challenges in development of woundcare medical devices, FiberMed 06, Tampere, Finland, 7–9 June, 2006. 49. anand, s.c. Spacers – at the technical frontier. Knit Int 2003; 110: 38–41. 50. anon. Spacer fabric focus. Knit Int 2002; 109: 20–22.

Y. QIN, Jiaxing College, China

Abstract: The causes and consequences of clinical wound infection are reviewed and the various types of antimicrobial materials that have been used to manage wound infection are summarised. Methods of incorporating antimicrobial materials into textile dressings are discussed and the applications of modern silver-containing antimicrobial wound dressings are highlighted.

Key words: wound care, silver, antimicrobial materials, textile dressings, infection.

7.1 Introduction Wounds are defi ned as skin defects caused by mechanical, thermal, electrical or chemical injuries, or by the presence of an underlying medical or physiological disorder. A wounded skin is typically associated with the loss of the normal skin function, such as its ability to serve as a barrier to invasion by bacteria. Many types of wounds also exude a large amount of fl uid, which together with the warm body temperature and rich nutritional components, serve as the ideal place for bacterial growth, leading eventually to wound infection and cross-infection in hospital wards. These infections complicate patient illness, cause anxiety, increase patient discomfort and, in the worst cases, can lead to death of the patient. Depending on the level of bacteria colonisation, wound infection can be classifi ed into the following stages:1–3 • wound contamination: the presence of bacteria within a wound without any host reaction; • wound colonization: the presence of bacteria within the wound which multiply or initiate a host reaction; • critical colonization: multiplication of bacteria causing a delay in wound healing, usually associated with an exacerbation of pain but with no overt host reaction; • wound infection: the deposition and multiplication of bacteria in tissue with an associated host reaction.

179

Of the various types of bacteria that can cause wound infection, Staphy- lococcus aureus is a Gram-positive bacterium that exists as a skin com- mensal, with the ability to cause a wide range of infections from localised skin eruptions to life-threatening conditions such as bacteraemia, endocar- ditis and pneumonia. S. aureus is one of the most common causes of hospital-acquired infections and its pathogenicity is caused by the production by the organism of coagulase, an enzyme that clots plasma and thus inhibits host defence mechanisms. In recent years, methicillin- resistant S. aureus (MRSA) has attracted much attention in the medical fi eld because of its deadly nature. Since it was fi rst reported in the UK in the 1980s, many different strains of MRSA have been found, affecting a large number of individuals in many different healthcare settings. The degree to which people are affected ranges in severity from simple wound colonisation, which does not need to be treated aggressively, to systemic infection such as bronchopneumonia, which may be fatal. The cost of managing the problems associated with infection is considerable. According to a report by the National Audit Offi ce, at any given time, about 9% of hospital patients have a nosocomial infection, i.e. a hospital- acquired infection, costing the National Health Service as much as £1bn per annum and contributing to the death of an estimated 5000 people each year.4 In 1995, the US Offi ce of Technology Assessment reported that antibiotic-resistant infections caused by six species of bacteria in US hospitals cost the country at least $1.3bn (£709m) a year. The report of the UK working party on hospital infection,5 and, more recently, the report by the National Audit Offi ce,6 recommended that, despite practical problems, where infection control facilities may be inadequate or in situations where MRSA has become endemic, active intervention to prevent the further spread of the organism is of benefi t and should be encouraged. Wound dressings that readily permit strike- through or shed fi bres on removal should be avoided as they may transmit contaminated particles that could easily be carried around the room on air currents, contaminating adjacent surfaces. The use of irrigant solutions to remove adherent dressings may also increase the potential for infected material to be transferred from the wound to the surrounding area, either in droplets that bounce off the wound surface when a jet of solution is applied with force by means of a syringe or even in a gentle trickle that runs down the patient’s leg. Semi-permeable dressings such as fi lms, fi lm-foam combinations and hydrocolloids, which effectively seal off the peri-wound area, may help to prevent the passage of contaminating organisms both into and out of a wound.7 However, the use of these products depends on whether their fl uid-handling characteristics and performance are appropriate to the condition of the wound and the amount of exudate produced.

The most effective way to control the spreading of bacteria from wound sites is to incorporate antimicrobial agents in wound dressings. Over the years, many antimicrobial materials have been used in wound management. These include chlorhexidine, honey, hydrogen peroxide, iodine, proﬂ avine, silver and many other novel materials.

7.2 Topical antimicrobial agents in wound care For patients with infected wounds, two basic treatment protocols are available for medical practioners, viz, systemic administration of antibiotics and topical application of antimicrobial agents on the wounded area directly. Whilst systemic antibiotics is outside the scope of the present review, over the years, many topical antimicrobial agents have been used to treat wound infection. These are brieﬂ y summarised below.

7.2.1 Chlorhexidine Chlorhexidine is available as the diacetate, digluconate or dihydrochloride, with the digluconate form most frequently used in wound management. Chlorhexidine was discovered in 1946 and introduced into clinical practice in 1954.8 It has rapid, bactericidal activity against a wide spectrum of non- sporing bacteria by damaging outer cell layers and the semi-permeable cytoplasmic membrane to allow leakage of cellular components. It also causes coagulation of intracellular constituents, depending on concentration.9 It is widely used as an antiseptic in handwashing and as a surgical scrub, but, in wounds, its application has been limited largely to irrigation.

7.2.2 Honey Honey has been used in the wound management practice for a long time and many therapeutic properties have been attributed to honey, including antibacterial activity and the ability to promote healing.10 Evidence of antibacterial activity is extensive, with more than 70 microbial species reported to be susceptible.11 The use of honey for infected wounds is increasing in popularity and a number of dressings or preparations containing it are now available; some of these have been shown to possess good antimicrobial activity against a wide range of pathogenic organisms, including resistant strains.10,12

7.2.3 Hydrogen peroxide Hydrogen peroxide has been widely used as an antiseptic and disinfectant. A 3% solution has most often been used to clean wounds. It is a clear,

colourless liquid that decomposes in contact with organic matter. It has a broad spectrum of activity against bacteria, with greater effect on Gram- positive species than Gram-negatives. Hydrogen peroxide functions as an oxidising agent by producing free radicals that react with lipids, proteins and nucleic acids to affect cellular constituents.

7.2.4 Iodine Iodine is an element that was discovered in 1811. It is a dark violet solid that dissolves in alcohol and potassium iodide. Its ﬁ rst reported use in treating wounds was by Davies in 1839,13 and later it was used in the Ameri- can Civil War. Early products caused pain, irritation and skin discoloura- tion, but the development of iodophores (povidone iodine and cadexomer iodine) since 1949 yielded safer, less painful formulations. Povidone iodine is a polyvinylpyrrolidone surfactant/iodine complex (PVP-I); cadexomer iodine is composed of beads of dextrin and epichlor- hydrin that carry iodine. Both release sustained low concentrations of free iodine whose exact mode of action is not known, but involves multiple cellular effects by binding to proteins, nucleotides and fatty acids. Iodine is thought to affect protein structure by oxidising S–H bonds of cysteine and methionine, reacting with the phenolic groups of tyrosine and reacting with N–H groups in amino acids, such as arginine, histidine and lysine, to block hydrogen bonding. It reacts with bases of nucleotides to prevent hydrogen bonding, and it alters the membrane structure by reacting with C=C bonds in fatty acids.14 Iodine has a broad spectrum of activity against bacteria, mycobacteria, fungi, protozoa and viruses.

7.2.5 Profl avine Profl avine is a brightly coloured acridine derivative that was extensively used during the Second World War in the treatment of wounds.15 Modern use is as a prophylactic agent in surgical wounds packed with gauze soaked in profl avine hemisulfate solution. It is an intercalating agent that inhibits bacteria by binding to DNA and prevents unwinding before DNA synthesis.

7.2.6 Silver Silver has a long history as an antimicrobial agent,16 especially in the treatment of burns. Metallic silver is relatively unreactive, but in aqueous environments silver ions are released and antimicrobial activity depends on the intracellular accumulation of low concentrations of silver ions. These avidly

bind to negatively charged components in proteins and nucleic acids, thereby effecting structural changes in bacterial cell walls, membranes and nucleic acids that affect viability. In particular, silver ions are thought to interact with thiol groups, carboxylates, phosphates, hydroxyls, imidazoles, indoles and amines either singly or in combination, so that multiple deleterious events rather than speciﬁ c lesions simultaneously interfere with microbial processes. Hence, silver ions that bind to DNA block transcrip- tion, and those that bind to cell surface components interrupt bacterial respiration and adenosine triphosphate (ATP) synthesis.17 In wound care, silver has been utilised in several formulations such as silver nitrate, silver sulfadiazine (SSD) and other types of silver-containing compounds. Silver nitrate is no longer widely used, but SSD and silver- releasing dressings remain popular. When introduced in 1968,18 SSD was recommended as a topical treatment for the prevention of pseudomonad infections in burns, but it has since been demonstrated to possess broad- spectrum antibacterial, antifungal, and antiviral activity.19–21 Other agents that have been recommended for the treatment of MRSA infections include tea tree oil22 and gentian violet ointment.23 Extracts of tea are also said to have inhibitory effects on MRSA.24

7.3 Main types of antimicrobial wound dressings In developing wound dressings with antimicrobial function, it is important to note that, in addition to its antimicrobial functions, wound dressings have many other performance criteria, such as the need to protect the wound surface, the ability to absorb wound exudates, and the ease of application and removal. Traditionally, wound dressings are made from textile materials since they share the same functional requirement of being able to protect the underlying object. Over the years, various methods have been used to combine antimicrobial agents with base materials. These methods are described below.

7.3.1 Antimicrobial creams and ointment Antimicrobial agents can be combined with a suitable thickening and gelling agent to prepare creams and ointment with antimicrobial properties. For example, Betadine Solution and Betadine Cream contain 10 and 5% povidone iodine, respectively. It has been shown that these products are effective against MRSA.25 In another example, antimicrobial silver ions can be combined with a hydrogel. SilvaSorb Silver Antimicrobial Wound Gel is an amorphous gel wound dressing for use in moist wound care management. Its SilvaSorb

MicroLattice® technology maintains an optimally moist wound environment by either absorbing slight drainage or donating moisture while it delivers antimicrobial ionic silver. SilvaSorb Silver Antimicrobial Wound Gel can act as an effective antimicrobial barrier.

7.3.2 Textile fabric soaked with antimicrobial agents A simple way to prepare an antimicrobial wound dressing is to soak a traditional dressing in a hydrogel containing antimicrobial agents. For example, some parafﬁ n gauze dressings are available medicated with antibiotics or antimicrobial agents for the treatment or prevention of infection: Sofra-Tulle from Hoechst contains framycetin, Bactigras from Smith and Nephew Medical Ltd and Serotulle from Seton Healthcare contain 0.5% chlorhexidine acetate, and Inadine (Johnson and Johnson Medical Ltd), which is similar in appearance to the other medicated tulle products contains povidone iodine in a polyethylene glycol base on a knitted viscose fabric. Urgotul SSD dressing comprises a polyester mesh impregnated with carboxymethylcellulose, vaseline and silver sulfadiazine (3.75%). Silver sulfadiazine is composed of sulfonamide, which is bacteriostatic, and silver, which is bactericidal. Its mechanism of action results from the synergetic activity of the sulfonamide and silver components, which inhibit the replication of bacterial DNA.26

7.3.3 Antimicrobial-coated textile substrate Antimicrobial agents can be coated onto the surface of a textile substrate to produce antimicrobial wound dressings. For example, Acticoat consists of two layers of a silver-coated, high-density polyethylene mesh, enclosing a single layer of an apertured nonwoven rayon and polyester fabric. These three components are ultrasonically welded together to maintain the integrity of the dressing while in use. Silver is applied to the polyethylene mesh by a vapour deposition process which results in the formation of microscopic crystals of metallic silver. Upon activation with water, Acticoat provides a rapid and sustained release of silver ions within the dressing and to the wound bed for three or seven days. Silverlon is a knitted fabric dressing that has been silver-plated by means of a proprietary autocatalytic electroless chemical (reduction–oxidation) plating technique. This technique coats the entire surface of each individual ﬁ bre from which the dressing is made, resulting in a very large surface area for the release of ionic silver.

7.3.4 Textile fi bres containing antimicrobial agents Antimicrobial compounds can be mixed with the spinning solution during the spinning process to produce fi bres with antimicrobial properties. For example, AlphaSan RC5000 is a silver sodium hydrogen zirconium phosphate with an average particle size of about 1 μm. It consists of a three-dimensional, repeating framework of sodium hydrogen zirconium phosphate, with many equally spaced cavities containing silver. Silver (at 3.8% by weight) provides the main antimicrobial properties, while the framework matrix acts to distribute silver evenly (without clumping or pooling) throughout the individual fi bres where the AlphaSan particles are added. When AlphaSan RC5000 is mixed with sodium alginate solution, the fi ne particles can be evenly distributed in the spinning solution under a high rate of shearing. Because the particles are very fi ne, they can be suspended uniformly while the solution is extruded to form fi bres. Since the sodium hydrogen zirconium phosphate framework prevents the silver ions from oxidising the alginate, this type of silver- containing alginate fi bre remains white even after sterilisation through irradiation.27 Figure 7.1 shows the photomicrograph of alginate fi bre with the Alphasan RC5000 particles uniformly distributed inside the fi bre structure.

Silver-containing particles

7.1 Photomicrograph of alginate ﬁ bre with the Alphasan RC5000 particles uniformly distributed inside the ﬁ bre structure.

7.3.5 Textile composite containing antimicrobial fi bres Antimicrobial fi bres can be mixed with traditional textile materials to produce antimicrobial wound dressings. For example, Silvercel combines the potent broad-spectrum antimicrobial action of a silver-coated nylon fi bre with the enhanced exudates-management properties of alginate fi bres. Because of the sustained release of silver ions, the dressing acts as an effective barrier and helps reduce infection. As is shown in Fig. 7.2, the antimicrobial properties are built-in through the use of X-STATIC silver-coated fi bres blended into the nonwoven structure. Silvercel dressing has been proven effective in vitro against 150 clinically isolated micro-organisms, including antibiotic-resistant strains.

7.3.6 Other novel methods Actisorb Silver 220 consists principally of activated carbon impregnated with metallic silver, produced by heating a specially treated ﬁ ne viscose fabric under carefully controlled conditions. The carbonised fabric is enclosed in a sleeve of spun-bonded nonwoven nylon, sealed along all four edges, to facilitate handling and reduce particle and ﬁ bre loss. When applied to a wound, the dressing adsorbs toxins and wound degradation products as well as volatile amines and fatty acids responsible for the production of wound odour. Bacteria present in wound exudate are also attracted to the surface of the dressing where they are killed by the antimicrobial activity of the silver.

X-Static fibre

Alginate fibre

7.2 Photomicrograph of Silvercel wound dressing.

Aquacel consists of a ﬂ eece of sodium carboxymethylcellulose ﬁ bres containing 1.2% ionic silver. In the presence of exudate, the dressing absorbs liquid to form a gel, binding sodium ions and releasing silver ions.

7.4 Wound dressings containing silver 7.4.1 Antimicrobial properties of silver Silver has broad-spectrum antimicrobial properties and has a low level of toxicity to human body. In recent years, as bacterial resistance to antibiotics becomes common and as more and more attention is being paid to cross-infection in hospital wards, wound dressings with antimicrobial properties are becoming increasingly popular. Silver is gaining importance as an effective antimicrobial component of advanced wound dressings. The antimicrobial action of silver products has been directly related to the amount and rate of silver released and its ability to inactivate target bacterial and fungal cells. In various laboratory and clinical studies, it has been found that metallic silver does not possess signifi cant antimicrobial potency whilst silver ions are highly antimicrobial.28 The oligo- dynamic microbiocidal action of silver compounds at low concentrations probably does not refl ect any remarkable effect of a comparatively small number of ions on the cell, but rather the ability of bacteria, trypanosomes and yeasts to take up and concentrate silver from very dilute solutions.29 Therefore, bacteria killed by silver may contain 105–107 Ag+ per cell, the same order of magnitude as the estimated number of enzyme-protein molecules per cell.30 Chemically, metallic silver is relatively inert but its interaction with moisture on the skin surface and with wound fl uids leads to the release of silver ion and its biocidal properties. Silver ion is a highly reactive moiety and avidly binds to tissue proteins, causing structural changes in bacterial cell walls and intracellular and nuclear membranes.31 These lead to cellular distortion and loss of viability. Silver binds to and denatures bacterial DNA and RNA, thereby inhibiting replication.32 A recent study demonstrated the inhibitory action of silver on two strains of Gram-negative Escherichia coli and Gram-positive S. aureus. It found that exposure to silver nitrate led to the formation of electron-light regions in their cytoplasm and condensation of DNA molecules.33 Granules of silver were observed in the cytoplasm, but RNA and DNA damage and protein inactivation seemed to be the principal mechanisms for bacterio- stasis. Silver-related degenerative changes in bacterial RNA and DNA, mitochondrial respiration and cytosolic protein led to cell death.

The action of silver ion on cell walls is illustrated by reference to the yeast Candida albicans. Silver has been shown to inhibit the enzyme phosphomannose isomerase (PIM) by binding cysteine residues.34 This enzyme plays an essential role in the synthesis of the yeast cell wall, and defects led to the release of phosphate, glutamine and other vital nutrients. Recent studies suggest that the microbicidal action of silver products is partly related to the inhibitory action of silver ion on cellular respiration and cellular function.35 The exact nature of these silver radicals is not clear but Ovington36 noted that nanocrystalline silver products (Acticoat, Smith and Nephew) can release a cluster of highly reactive silver cations and radicals, which provide a high antibacterial potency on account of unpaired electrons in outer orbitals. Silver and silver radicals released from Acticoat also cause impaired electron transport, bacterial DNA inactivation, cell membrane damage, and binding and precipitation of insoluble complexes with cytosolic anions, proteins and sulfydryl moieties.

7.4.2 Types of silver compounds used in wound dressings Silver is a group 11 element (formerly group Ib) of the Periodic Table and exists as two isotopes, 107Ag and 109Ag, in approximately equal proportions. In solution, silver exhibits three oxidation states, viz, Ag+, Ag2+ and Ag3+, each capable of forming inorganic and organic compounds and chemical complexes. Compounds involving Ag2+ or Ag3+ are unstable or insoluble in water. The silver compounds used in wound dressings can be divided into three groups: • Elemental silver, e.g. nanocrystalline particles or foil. • Inorganic compounds/complexes, e.g. silver nitrate, silver sulfadiazine, silver oxide, silver phosphate, silver chloride or a silver zirconium compound. • Organic complexes, e.g. colloidal silver preparations, silver zinc allan- toinate or silver proteins. Colloidal silver solutions were the most common delivery system before 1960. It is in the form of charged pure silver particles (3–5 ppm) held in suspension by a small electric current, where the positively charged ions repel each other, hence making it possible for the particles to remain in solution when applied topically to a wound. Although it is highly bacterio- cidal with no resistance, since the solutions are unstable when exposed to light, colloidal silver offers little practical value. When the silver ions are complexed to small proteins to improve stability in solution, they become more stable. However, they are also much less antibacterial than pure ionic silver.

Table 7.1 Silver compounds used in various wound dressings

Manufacturer Name of the product Silver compound used

Argentum Silverlon Metallic silver Smith and Nephew Acticoat Metallic silver Medline SilvaSorb Silver chloride Convatec Aquacel Ag Silver chloride Medline Arglaes Silver calcium phosphate Coloplast Contreet Silver ammonium complex Johnson and Johnson Actisorb Silver 220 Silver carbon

In the 1960s, various silver salts were developed. The salts AgCl, AgNO3,

and Ag2SO4 act as stable delivery systems for silver ions. Silver nitrate was the most widely used compound but it is dangerous to use in concentrations exceeding 2%. An aqueous 0.5% silver nitrate solution is the standard solution for treating burns and infected wounds. However, nitrate is toxic to wounds and cells and appears to decrease healing. It is also unstable in light. Various studies have shown that pure silver ions and radicals produce the best antimicrobial results as well as optimising the wound-healing environment. As a consequence, the silver salts and complexes used today were developed to maintain a sustained release of silver ions. A typical silver-containing compound is Alphasan RC5000 developed by Milliken. As a zirconium phosphate based ceramic ion-exchange resin containing silver, Alphasan is effective against a range of micro-organisms that can cause undesirable effects. The material is widely used in Europe, Japan, and the United States, and it has been approved by the US FDA for contact applications. Table 7.1 summarises the various silver compounds used in silver-containing wound dressings.

7.4.3 Methods of incorporating silver into wound dressings The silver-containing wound dressings currently available vary considerably in their overall structure, and in the concentration and formulation of the silver compounds used. Overall, silver ions can be attached to wound dressings by four basic methods: • Physical treatment of the base material. In this method, the ﬁ bre or fabric can be coated with metallic silver; • Chemical treatment of the base material. In this method, the ﬁ bre or fabric can be treated with silver-containing solutions, whereby silver ions can be attached to the wound dressing through ion exchange;

Table 7.2 Typical silver contents of silver-containing wound dressings

Proprietary name Ag content (mg 100 cm−2)

Silverlon 546 Calgitrol Ag 141 Acticoat 105 Contreet Foam 85 Contreet Hydrocolloid 32 Aquacel Ag 8.3 SilvaSorb 5.3 Actisorb Silver 220 2.7 Arglaes powder 6.87 mg g−1

• Blending. Fine particles of the silver compounds can be blended with the base material; • Blending of silver-containing fi bres with other fi bres. This method is used in the production of Silvercel where alginate fi bres are blended with the silver coated X-static fi bres. Because of the differences in the types of silver compounds and techniques in applying them to the wound dressings, different silver-containing wound dressings have considerably different silver contents. Table 7.2 shows the typical silver contents of the silver-containing wound dressings.37–39

7.5 Applications of modern antimicrobial wound dressings containing silver In the management of infected wounds, modern silver-containing wound dressings have become widely used. Depending on the characteristics of wounds, the profi les for silver-containing wound dressings can be divided into three main types: • Products having a high silver content that give a rapid release of silver ions and are designed for wounds with heavy exudate and bacterial colonisation; • Products that maintain a more modest silver-release pattern, where silver ion is released over several days. These are claimed to be suffi - cient for moderate to severe pathogenic bacterial populations. The non-silver components of these dressings are attuned to wound bed management, i.e., exudate control, debridement of wound debris and management of the wound environment; • Products with a low silver content, which may be suffi cient for low- grade infections in chronic wounds but are more appropriately used as a barrier to infection in acute wounds, burns and surgical injuries.

Figure 7.3 shows the silver concentration in the contact solution after the silver-containing dressings are placed in contact with wound fl uid over various periods of time. It can be seen that Silverlon and Acticoat release more silver ions than other types of products. Both the Silverlon and Acticoat products are coated with a high level of metallic silver. The high level of silver release is needed since both products are intended for burn wounds where the prevention of infection is an important consideration. In other types of dressings, the silver ions act as an antimicrobial agent to control bacteria growth and prevent cross-infection. As is shown in Fig. 7.4, with silver-containing alginate wound dressings, the dressing fi rst absorbs exudate from the wound bed into the dressing structure. As the dressing becomes wet, silver ions are activated and liberated into the contacting solution, exerting an antimicrobial effect on the bacteria. In these cases, the rate of silver release can be signifi cantly lower than in burn wound management. Lansdown et al.40 made a sequential microbiological examination of wound swabs, wound exudate and wound scale from seven patients with chronic wounds. After determining the silver content using atomic absorption spectrometry, they found that:

60 −1 50 Silverlon Acticoat 40 SilvaSorb Arglaes 30 Aquacel Ag concentration, mg l

20 Silver

0 0 20 40 60 80 Time (h) 7.3 Silver concentration in the contact solution after the silver- containing dressings are placed in contact with wound ﬂ uid for various periods of time.

Wound exudate with bacteria Non-woven alginate with silver

Fibre swelling

Bacteria trapped in non-woven alginate

Release of silver ions

Bacteria Ag Ag Ag Ag killed with Ag Ag Ag Ag Ag Ag silver ions Ag

7.4 Mechanism of action for silver-containing alginate wound dressings.

• All silver released into the wound bed was absorbed by wound exudate or debris (wound scale, etc); • Silver uptake by wound exudate is approximately proportional to its viscosity (protein content); • Silver absorbed into the wound bed may be released into the exudate for several weeks following the termination of silver therapy; • The amount of silver released from the dressings is closely related to the amount of moisture absorbed; • Wounds treated with silver do not attain a germ-free status, suggesting that silver-resistant organisms such as S. aureus and Pseudomonas aeruginosa may contribute to a delay in healing. In a detailed study of the performances of various silver-containing wound dressings, Thomas38 found that, when testing against S. aureus: • Acticoat exhibited a marked bactericidal effect within 2 h; • Contreet-H had an inhibitory effect; • Actisorb Silver 220 appeared to prevent the proliferation of the organism within the dressing after a minimum contact time of 4 h. Actisorb Silver 220 removed the organisms from the suspension and bound them to the surface of the charcoal ﬁ bres. These organisms remain viable for many hours until they are progressively inactivated by the silver ions in the dressing. The inner core, which contains the silver, forms an effective barrier, whereas the outer nylon sleeve does not.

While total silver content is important, other factors also inﬂ uence the ability of a dressing to kill micro-organisms. These include the distribution of the silver within the dressing (whether it is present as a surface coating or is dispersed through the structure), its chemical and physical form (whether it is present in a metallic, bound or ionic state) and the dressing’s afﬁ nity for moisture – a prerequisite for the release of active agents in an aqueous environment. Products in which the silver content is concentrated on the dressing surface rather than ‘locked up’ within its structure performed well, as did those in which silver was present in the ionic form. An in vitro study41 compared the antimicrobial properties of Acticoat with a solution of silver nitrate and cream containing silver sulfadiazine against 11 antibiotic multi-resistant clinical isolates. Acticoat was the most effective at killing the organisms. A later study compared the activity of the same dressings against a spectrum of common burn wound fungal pathogens and showed that the silver-coated membrane provided the fastest and broadest-spectrum fungicidal activity.42 Yin et al.43 compared the antimicrobial activity of Acticoat with silver nitrate, silver sulfadiazine and mafenide acetate in order to determine their minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and zone of inhibition. Although mafenide acetate produced the greatest zone of inhibition, the MBC of the product was higher than its MIC, indicating that it had a bacteriostatic rather than a bactericidal action. In contrast, the MICs and MBCs of the silver-containing products were very similar, indicating that their activity is essentially bactericidal. The authors showed that, although the MIC values for the three silver preparations were very similar when calculated in terms of their silver content, Acticoat acted more rapidly than the other two products, perhaps because the metallic silver on the surface of the dressing forms a reservoir of silver ions, which are released continuously and are therefore always available for bacterial uptake.

7.6 Future trends In developing antimicrobial textile dressings to manage wound infection, it should be noted that since wound dressings are directly in contact with broken skin, there are strict requirement for the type of applicable antimicrobial materials. These materials should meet the following requirements: • Be non-toxic and safe to use on broken skin; • Do not cause allergic reaction; • Do not develop bacteria resistance; • Have a broad-spectrum antimicrobial effect; • Have sustained antimicrobial effect during the useful lifetime of the material

In this respect, organic antimicrobial compounds commonly used for antimicrobial textiles have limited use in wound management, since, in most cases, they are not suitable for use on broken skin. In recent years, two classes of materials have become increasingly used in the wound- management industry as novel materials for the manufacture of wound dressings with antimicrobial function. These are brieﬂ y discussed below.

7.6.1 Metal ions with antimicrobial properties In recent years, silver has been successfully used as an effective antimicrobial agent for wound management and silver-containing wound dressings are now widely used thoroughout the world. In addition to silver, other metal ions have also proven effective in preventing bacterial growth. In particular, zinc and copper ions, whilst being non-toxic to human, can be easily attached to wound dressings through chelation with fi bres containing amine groups. In this respect, chitosan fi bres treated with zinc and copper compounds have the combined antimicrobial properties of the chitosan as well as the metal ions. Experimental results have shown that the zinc and copper containing chitosan fi bres have excellent antimicrobial effi cacy. As can be seen in Table 7.3, when tested against C. albicans, the original chitosan produced a reduction in bacterial count of 78.6%, the respective fi gures for the copper and zinc containing fi bres were 96.2 and 97.7%.44

7.6.2 Naturally occurring antimicrobial materials Melaleuca alternifolia oil, also called tea tree oil, has demonstrated promising effi cacy in treating wound infections. Tea tree oil has been used for centuries as a botanical medicine, and has only in recent decades surfaced in the scientifi c literature as a promising adjunctive wound treatment. Tea tree oil is antimicrobial and anti-infl ammatory, and has demonstrated its ability to activate monocytes. There are few apparent side effects to using tea tree oil topically in low concentrations, with contact dermatitis being the most common. Tea tree oil has been effective as an adjunctive therapy

Table 7.3 The antimicrobial effect against Candida albicans by different types of chitosan ﬁ bres

Samples Bacteria count (cfu ml−1)Reduction (%)

Control 5.4 × 103 n/a Chitosan fi bre 1155 78.6 Cu(II) chitosan fi bre 208 96.2 Zn(II) chitosan fi bre 123 97.7

in treating osteomyelitis and infected chronic wounds in case studies and small clinical trials.45 It is reported that, of the various naturally occurring antimicrobial compounds, tea tree oil is effective against skin infections; honey can be used for wound infections; mastic gum can be used for Helicobacter pylori gastric ulcers and cranberry juice for urinary tract infections. Many infections may prove amenable to safe and effective treatment with non- antibiotic naturally occurring compounds.46

7.7 Sources of further information and advice For more information on wound care and wound management products, the readers should consult the following references. 1. A prescriber’s guide to dressings and wound management materials, VFM Unit, Welsh Ofﬁ ce Health Department, 1997. 2. G. Bennett and M. Moody, Wound care for health professionals, Chapman and Hall, London, 1995. 3. C. Dealey, The care of wounds, Blackwell Science Ltd, Oxford, 1994. 4. D.J. Leaper and K.G. Harding (eds), Wounds: biology and management, Oxford University Press, 1998. 5. M. Morison, C. Moffatt, J. Bridel-Nixon and S. Bale (eds), Nursing management of chronic wounds, Mosby, London, 1997. 6. S. Thomas, Wound management and dressings, The Pharmaceutical Press, London, 1990. 7. J. Wardrope and J.A.R. Smith, The management of wounds and burns, Oxford University Press, Oxford, 1992.

7.8 References

1. ayton m. Wound care: wounds that won’t heal. Nurs Times, 1985, 81(46): suppl 16–19. 2. falanga v, grinnell f, gilchrest b, et al. Workshop on the pathogenesis of chronic wounds. J Invest Dermatol, 1994, 102(1): 125–27. 3. kingsley a. A proactive approach to wound infection. Nurs Stand, 2001, 15(30): 50–54. 4. national audit offi ce. The management and control of hospital acquired infection in acute NHS trusts in England. London: Stationery Offi ce, 2000; 121 (HC 230 Session 1999–2000). 5. duckworth g, cookson b, humphreys h, et al. Revised guidelines for the control of methicillin-resistant Staphylococcus aureus infection in hospitals. British Society for Antimicrobial Chemotherapy, Hospital Infection Society and the Infection Control Nurses Association. J Hosp Infect, 1998, 39(4): 253–290. 6. national audit offi ce. Improving patient care by reducing the risk of hospital acquired infection: a progress report. London: Stationery Offi ce, 2004; (HC 876 Session 2003–2004).

7. hutchinson j j, lawrence j c. Wound infection under occlusive dressings. J Hosp Infect, 1991, 17(2): 83–94. 8. russell a d. Introduction of biocides into clinical practice and the impact on antibiotic-resistant bacteria. J Appl Microbiol, 2002, 92 Suppl: 121S–135S. 9. mcdonnell g, russell a d. Antiseptics and disinfectants: activity, action, and resistance. Clin Microbiol Rev, 1999, 12(1): 147–179. 10. molan p c. The role of honey in the management of wounds. J Wound Care, 1999, 8(8): 415–418. 11. molan p c. The antibacterial activity of honey. Part 1. Its use in modern medicine. Bee World, 1992, 80(2): 5–28. 12. moore o a, smith l a, campbell f, et al. Systematic review of the use of honey as a wound dressing. BMC Complement Altern Med, 2001, 1(1): 2. 13. davies j. Selections in pathology and surgery. Part II. London: Longman, Orme, Browne, Greene and Longmans, 1839. 14. gottardi w. Iodine and iodine compounds. In: Block S, editor. Disinfectants, sterilisation and preservations (3rd edition). Philadelphia, USA: Lea Febinger, 1983. 15. mitchell g a g, buttle g a h. Profl avine in closed wounds. Lancet, 1943, ii: 749. 16. klasen h j. Historical review of the use of silver in the treatment of burns. I. Early uses. Burns, 2000, 26(2): 117–130. 17. trevors j t. Silver resistance and accumulation in bacteria. Enzyme Microb Technol, 1987, 9: 331–333. 18. fox c. Topical therapy and the development of silver sulphadiazine. Surg Gynecol Obstet, 1968, 157: 82–88. 19. wlodkowski t j, rosenkranz h s. Antifungal activity of silver sulphadiazine. Lancet, 1973, 2(7831): 739–740. 20. speck w t, rosenkranz h s. Letter: Activity of silver sulphadiazine against dermatophytes. Lancet, 1974, 2(7885): 895–896. 21. rahn r o, setkiw j k, landry l c. Ultraviolet irradiation of nucleic acids complexed with heavy metals. III. Infl uence of Ag+ and Hg+ on the sensitivity of phage and of transforming DNA to ultraviolet radiation. Photochem Photobiol, 1973, 18: 39–41. 22. carson c f, cookson b d, farrelly h d, et al. Susceptibility of methicillin- resistant Staphylococcus aureus to the essential oil of Melaleuca alternifolia. J Antimicrob Chemother, 1995, 35(3): 421–424. 23. saji m. Effect of gentian violet against methicillin-resistant Staphylococcus aureus (MRSA). Kansenshogaku Zasshi, 1992, 66(7): 914–922. 24. yam t s, hamilton-miller j m, shah s. The effect of a component of tea (Camellia sinensis) on methicillin resistance, PBP2’ synthesis, and beta- lactamase production in Staphylococcus aureus. J Antimicrob Chemother, 1998, 42(2): 211–216. 25. goldenheim p d. In vitro effi cacy of povidone–iodine solution and cream against methicillin-resistant Staphylococcus aureus. Postgrad Med J, 1993, 69(Suppl 3): S62–65. 26. meaume s, senet p, dumas r. Urgotul: a novel non-adherent lipido-colloid dressing. Br J Nurs, 2002, 11: 42–50. 27. qin y, groocock, m r. PCT WO/02/36866A1, May 2002.

28. lansdown a b g. Physiological and toxicological changes in the skin resulting from the action and interaction of metal ions. CRC Crit Rev Toxicol, 1995, 25: 397–462. 29. charley r c, bull a t. Bioaccumulation of silver by a multispecies population of bacteria. Arch Microbiol, 1979, 123: 239–244. 30. clarke a j. General pharmacology. In: Heffter, A. (ed.). Handbuch der experi- mentelle Pharmakologie. Ergänzung, Vol. 4. Berlin: Springer, 1937. 31. ovington l g. Nanocrystalline silver: where the old and familiar meets a new frontier. Wounds, 2001, 13: (suppl B), 5–10. 32. modak s m, fox c l. Binding of silver sulfadiazine to the cellular components of Pseudomonas aeruginosa. Biochem Pharmacol, 1973, 22: 2391–2404. 33. feng q l, wu j, chen g q, et al. A mechanistic study of the antibacterial effect of silver ions on Escherischia coli and Staphylococcus aureus. J Biomed Mat Res, 2000, 52: 662–668. 34. wells t n, scully p, paravicini g, et al. Mechanisms of irreversible inactivation of phosphomannose isomerases by silver ions and fl amazine. Biochemistry, 1995, 34: 7896–7903. 35. demling r h, disanti l. Effects of silver on wound management. Wounds, 2001, 13: (Suppl A), 5–15. 36. ovington l g. Nanocrystalline silver: where the old and familiar meets a new frontier. Wounds, 2001, 13: (suppl B), 5–10. 37. thomas s, mccubbin p. A comparison of the antimicrobial effects of four silver-containing dressings on three organisms. J Wound Care, 2003; 12(3): 101–107. 38. thomas s, mccubbin p. An in vitro analysis of the antimicrobial properties of 10 silver-containing dressings. J Wound Care, 2003, 12(8): 305–308. 39. lansdown a b, williams a. How safe is silver in wound care? J Wound Care, 2004, 13(4): 131–136. 40. lansdown a b g, williams a, chandler s, et al. Silver absorption and antibacterial effi cacy of silver dressings. J Wound Care, 2005, 14(4):155–160. 41. wright j b, lam k, burrell r e. Wound management in an era of increasing bacterial antibiotic resistance: a role for topical silver treatment. Am J Infect Control, 1998, 26: 572–577. 42. wright j b, lam k, hansen d, et al. Effi cacy of topical silver against fungal burn wound pathogens. Am J Infect Control, 1999, 27: 344–350. 43. yin h q, langford r, burrell r e. Comparative evaluation of the antimicrobial activity of ACTICOAT antimicrobial barrier dressing. J Burn Care Rehabil, 1999, 20: 195–200. 44. qin y, zhu c, chen j, liang d, wo g. Absorption and release of zinc and copper ions by chitosan fi bers. J Appl Polym Sci, 2007, 105(2): 527–532. 45. halcon l, milkus k. Staphylococcus aureus and wounds: a review of tea tree oil as a promising antimicrobial. Am J Infect Control, 2004, 32(7): 402–408. 46. carson c f, riley t v. Non-antibiotic therapies for infectious diseases. Commun Dis Intell, 2003, 27 Suppl: S143–146.

H. ONISHI and Y. MACHIDA, Hoshi University, Japan

Abstract: Recent developments in the theory of wound healing and in the use of wound dressings to manage skin wounds, including wounds caused by exogenous factors like skin defect and endogenous factors such as diabetes, are described. Results of recent studies on novel dressings based on the latest materials science and textiles technology are discussed with respect to their characteristics, usefulness and advantages.

Key words: wound dressing, burn, pressure ulcer, skin defect, diabetes, textiles, wound healing, deep wound, chronic wound.

8.1 Introduction: current practice in the management of deep skin wounds or ulcers 8.1.1 Management of burns, pressure ulcers and skin defects The concept of wound healing has changed recently. Before the concept of moist wound healing was generally accepted, disinfection, bandaging, leaving scar tissue and keeping a wound area dry were regarded as the best ways of promoting wound healing. However, it was then realized that drying the wound tended to delay its healing (Winter and Scales, 1963). Keeping wound areas moist, with a cover such as polyethylene ﬁ lm, was found to be more effective for faster healing, and since then moist wound healing has been accepted as a general concept of wound healing (Horn- castle, 1995; Bryan, 2004; Brett, 2006). Thus, ﬁ rstly, wound washing or debridement is performed, and then treatment with sutures, dressings, etc. is started. Many dressings have been developed for the management of various skin wounds, such as burns, skin defects, and pressure ulcers (Kaya et al., 2005; Martineau and Shek, 2006). These have been used for the protection of wounds, removal of exudates, prevention of infection, promotion of the

198

rate of healing, etc. (Seaman, 2002; Fletcher, 2003). In moist wound healing, the dressings have to keep the wound area under moist (not wet) conditions. Furthermore, they must protect the wound from contamination and infection, which are the greatest potential problems in moist wound healing. In particular, hematoma, excessive exudates and necrotic tissue often cause infection and delay wound healing in severe or chronic wounds (Caputo et al., 1994; Stotts and Hunt, 1997; Mi et al., 2002; Dowsett, 2004). The choice of dressing, therefore, is very important. Antimicrobial agents are generally useful to protect the wound from infection, although they must be used with care because their application may sometimes be unfavorable owing to toxic side effects (Simons and Morales, 1980; Lans- down, 2006; Aoyagi et al., 2007; Atiyeh et al., 2007). A combination of wound dressings and pharmaceutical agents is often utilized as a benefi cial approach in the treatment of deep wounds. The process of wound healing is shown in Fig. 8.1. In severe or chronic wounds, the dermis and panniculus adiposus are damaged, and the muscle and bone sometimes invaded. Since deep dermal wounds and damage extending to the panniculus adiposus lack hair follicles, which are important for fast re-epithelialization, the healing proceeds gradually. Re- epithelialization is initiated only from the edge of the wound. Wound healing is completed through a necrosis and infection stage, an agglutina- tion period, a proliferation period of granulation and re-epithelialization, and a fi nal remodeling stage (Gabbiani et al., 1971; Rovee et al., 1972; Clark et al., 1995; Clark, 1996). Immediately after the production of the wound, a blood clot covers the damaged site, and leukocytes, macrophages, etc., associated with infl ammation, gather there in the necrosis and infection stage. In severe and chronic wounds, hematoma, exudates and necrotic tissues are fairly

Epidermis

Dermis

Panniculus adiposus

(a) Hair follicle (b) 8.1 Wound healing process in (a) non-deep and (b) deep wounds (re-epithelialization → ; granulation Ö).

extensive. These are often subject to infection, deteriorating the condition of the wound. At this stage, therefore, the removal of excessive exudates and necrotic tissue is important, and antimicrobial agents may be needed to prevent deterioration of the wound. Washing and debridement of the wound surface are important to promote the rate of healing. Keeping the wound moist is necessary in order not to inhibit the healing process in the wound. A few days after the production of the wound, fi broblast cells proliferate by responding to various cell growth factors excreted in the exudates. The cells produce extracellular matrices, such as collagen, which bury the wound area. This granulation step is a very important repairing process. Angiogenesis is also important because it plays an important role in the supply of oxygen and nutrients, which are necessary in the proliferation stage. At the same time, re-epithelialization is an essential step in the completion of wound healing. In severe or chronic wounds, which lack hair follicles, re-epithelialization starts at the edge of the epidermis. The wound is fi lled with granulation tissue, which shrinks gradually with healing, and is then covered by the process of re-epithelialization. The removal of contamination and infection, and the maintenance of moist conditions, are important to achieve effi cient granulation and re-epithelialization (Rovee et al., 1972; Brett, 2006). Dressings specifi cally designed to create such conditions are chosen. Some growth factors or pharmaceutical agents can be utilized to promote granulation and re-epithelialization. Finally, remodeling continues after the wound has closed up. Dermal remodeling reduces the scar, which is very important for cosmetic reasons.

8.1.2 Management of chronic leg ulcers and diabetic foot wounds Burns or pressure ulcers are mainly produced by exogenous factors. Endog- enous factors, on the other hand, are essentially associated with skin defects in leg ulcers and diabetic foot wounds. These are very complicated ulcers and wounds that are chronic in most cases and diffi cult to heal. Venous or arterial occlusion can cause ischemic defects in the related area, leading to poor blood circulation. This results in poor nutritional conditions in the periphery of the legs, including the skin, making these areas vulnerable to infections (Nelson et al., 2004). Poor healing may necessitate leg amputation. In the treatment of leg ulcers, it is very important to improve ischemic defects fi rst. This improvement, including elastic compression around the ulcer area, appears to be useful in the healing process for leg ulcers. The combination of dressing application and medical compression seems to be useful in the treatment of venous leg ulcers (Koksal and Bozkurt, 2003). Diabetic foot ulcers are also signifi cantly related to ischemic defects (Jude et al., 2007). Diabetics can often develop foot ulcers owing to ischemic

defects of the peripheral blood vessels, which, like leg ulcers, are often chronic and difﬁ cult to heal. These ulcers are often accompanied by infections and the ischemic defects delay healing of the diseased sites. In the treatment of diabetic foot ulcers, the most important factor is to improve the diabetic conditions. In addition, the application of dressings to the ulcers appears to be effective. The combination of dressings and antibacterial agents seems to be useful. For example, use of hyaluronate and an iodine complex produced signiﬁ cant improvement in ulcers of this type (Sobotka et al., 2006, 2007). Furthermore, topical treatment with dressings containing epidermal growth factor (EGF) showed positive effects in promoting the healing of chronic diabetic foot wounds (Hong et al., 2006). In the treatment of these chronic wounds, it seems necessary to keep the wound or ulcer in moderately moist conditions; extreme moist conditions can increase the infection. It is therefore suggested that wound dressings with or without bioactive agents are useful to enhance the healing of chronic leg ulcers or diabetic foot ulcers. For the treatment of the deep wounds or ulcers, this chapter examines wound dressings and composites containing pharmaceutical agents – some commercially available and some not – and compares them with common dressings.

8.2 Normal treatment options for deep skin wounds or ulcers Moist wound healing is generally accepted today and the dressings used are based on this concept, although extreme moist conditions are not appropriate for healing chronic ulcers, such as leg or diabetic foot ulcers. The maintenance of wet conditions around the wound provides circumstances that allow biologically active agents, such as growth factors and cytokines, to be kept in the wound area (Cross and Mustoe, 2003; Akita et al., 2006). Cell migration is allowed in wet conditions, but not in dry conditions. Contamination with scar tissue, caused in dry conditions, can inhibit the process of proliferation, resulting in delayed healing. Dressings designed to fulﬁ ll these conditions have recently been developed. However, moist healing can also lead to a deterioration of wound conditions, such as infection, particularly in deep skin wounds. For the treatment of deep skin wounds, therefore, some pharmaceutical agents are often necessary in addition to dressings, in order to support wound healing.

8.2.1 Topical pharmaceutical agents Topical pharmaceutical agents used in deep skin wounds are shown in Table 8.1. In the infection and necrosis stage, contamination, infection and

Table 8.1 Topical formulations for the treatment of severe skin wounds

Classiﬁ cation Formulation (commercial Active ingredient name)

Debridement Bromelain ointment Bromelain agents Elase Fibrinolysin, deoxyribonuclease Francetin.T.Powder Trypsin, fradiomycin sulfate Incarnant Fiblast spray Trafermin agent Prostandin ointment Alprostadil–alfadex Olcenon ointment Tretinoin tocoferil Actosin ointment Bucladesine sodium U-Pasta Kowa ointment Sucrose, povidone iodine Reﬂ ap Lysozyme hydrochloride Solcoseryl ointment Solcoseryl Antibacterial Erythrocin Erythromycin agents Achromycin ointment Tetracycline hydrochloride Chloromycetin cream Chloramphenicol Gentacin Gentamicin sulfate Terramycin ointment Oxytetracycline with polymixin B hydrochloride, Polymixin B sulfate Baramycin ointment Bacitracin, fradiomycin sulfate Geben cream Sulfadiazine silver Fucidin leo ointment Sodium fusidate Cadex ointment Iodine, Cadexomer

scar tissue are obstacles to the subsequent healing process (Wysocki, 2002). Contamination must be removed by washing because foreign substances lead to foreign-body reactions. Drugs containing enzymes that hydrolyze biogenic substances are commercially available for this purpose. These allow the degradation of proteins and nucleic acids in the exudates and their easy removal, resulting in better conditions at the wound surface (Levine et al., 1973; Schwarz, 1981). Antimicrobial agents are necessary to suppress possible infection. Various kinds of antimicrobial agents are used, and dosage forms of ointment and cream are commercially available (Breloff and Caffesse, 1983; Lee et al., 1984; Yoshida et al., 1997). Silver sulfadiazine (Geben cream) is often used because it is widely effective against Gram positive and Gram negative bacteria and fungi. These agents are useful to treat the wound in the early stage. However, prolonged use may prevent the subsequent healing process, resulting in delayed healing. After cleaning the wound surface with debridement agents and antimicrobial drugs, the promotion of proliferation, granulation and re- epithelialization are important in order to accelerate the rate of wound

healing. Pharmaceutical agents and growth factors are included in the promoting agents, but caution should be exercised when using them (Kawa- bata et al., 2002; Kakigi et al., 2005; Asai et al., 2006). For example, excessive granulation caused by overuse can delay the completion of wound healing or produce an ugly scar. Although these agents are effective in improving the severe situation of wounds, caution has to be exercised in applying them, and overuse must be avoided in order to reduce side effects.

8.2.2 Currently used dressings As shown above, the misuse or overuse of pharmaceutical agents can become an obstacle to wound recovery. The concept of moist wound healing indicates that it is better to provide the conditions to help the wound tissue to recover naturally (Bryan, 2004). The use of dressings in the treatment of wounds is based on this concept. Dressings have therefore been developed to fulfi ll these conditions (Seaman, 2002). In fact, when there is infection or necrotic tissue in the wound, dressings are applied after cleaning the wound surface to remove these obstacles. It is possible to use a combination of dressings with pharmaceutical agents. Various commercially available dressings are shown in Table 8.2, and many are useful for deep skin wounds. A polyurethane (PU) fi lm is often used as a thin dressing (Holland et al., 1985; Cosker et al., 2005). Since gases such as oxygen, and water vapor can pass through a PU fi lm, but liquids such as water and exudates cannot, this fi lm allows wet conditions for wounds. Although a PU fi lm can be left covering a light wound for a long time, prolonged use is diffi cult owing to the generation of excessive exudates. Changing the fi lm is, therefore, required when wound conditions deteriorate. Chitin fi lms are used in the form of non-woven textile, cotton-like sheets or sponge forms, and are available for the treatment of various skin wounds, including deep skin ulcers (Su et al., 1997, 1999). As chitin fi lms allow liquids such as exudates to pass through them, some dressings are needed to keep the wound under wet conditions. Chitin fi lms are biocompatible and biodegradable, although the degradation process is lengthy. Beschitin W is a typical thin chitin fi lm that is very useful in the treatment of light or moderate burns. Beschitin W-A is a cotton-like sheet, thicker than Beschitin W, which can be applied to deep wounds, i.e. to wounds extending to the panniculus adiposus, such as pressure ulcers. Beschitin F is a sponge type of chitin, 2 mm thick, that is available for the treatment of deep skin wounds, especially wounds up to muscle or bone. These must be applied to the wound after cleaning by washing or debridement, since contamination, excessive exudates or scar tissue threaten to cause infection. Chitin

Table 8.2 General wound dressings for the treatment of severe skin wounds

Degree of wound severity Material Commercial name

Transparent occlusive Polyurethane fi lms Opsite cover Tegaderm Wound reaching into Chitin fi lm Beschitin W dermis Hydrocolloid dressing DuoActiveET Tegasorb light Absocure-surgical Hydrogel Viewgel NU-GEL Wound reaching into Hydrocolloid dressing Comfeel panniculus adiposus DuoActive DuoActiveCGF Absocure-wound Tegasorb Hydrogel Jelliperm Intrasite gel GranuGEL Chitin fi lm Beschitin W-A Alginate Kaltostat Sorbsan Algoderm Kurabio-AG Hydrofi ber Aquacel Hydropolymer Tielle Polyurethane foam Hydrosite Hydrosite AD Wound reaching into Polyurethane foam Hydrosite cavity muscle or bone Chitin fi lm Beschitin F

fi lms provide wet conditions to the wound and, furthermore, have the ability to promote the granulation process. Like PU fi lms, these dressings need to be changed when wound conditions deteriorate. Composite fi lms containing hydrocolloids are also used in the treatment of wounds of varying degrees. This type of fi lm is usually composed of a hydrocolloid-containing matrix and a waterproof sheet (Agren and Everland, 1997; Sugihara et al., 2000). Water is absorbed by the hydrocolloids and wet conditions are maintained around the wound. The ulcer dressing Comfeel is relatively thick (1.1 cm), and superior to the thin type of hydrocolloid-containing composite fi lm in the absorption of exudates. Hydrogels are usually used in combination with a non-woven sheet. Their function is the same as that of composite fi lms containing hydrocolloids.

Textiles made from calcium sodium alginate perform the excellent function of signifi cant absorption of exudates and hemostasis (Gilchrist and Martin, 1983; Vanstraelen, 1992). These can be applied to deep skin wounds, that is, to wounds extending to the panniculus adiposus. Both ribbon- and sheet-types of these textiles are available. The ribbon-type is suitable for the treatment of pockets in the wound. These dressings need to be changed before deterioration of the gel state. A textile of carboxymethylcellulose sodium (CMC) is used in a similar manner to calcium sodium alginate textiles (Piaggesi et al., 2001; Walker et al., 2003). The main characteristic of the CMC textile is its signifi cant absorption of exudates. The CMC textile is changed before deterioration of the gel state by the absorption of exudates. Hydropolymers and PU foams are useful for the treatment of deep skin wounds (Rubin et al., 1990; Berry et al., 1996; Diehm and Lawall, 2005). Hydropolymers can be applied to deep skin wounds, that is, to wounds extending to the panniculus adiposus. With regard to PU foams, special forms are made for the wound cavity or sacral region. They can be used for deep wounds extending to the muscle and bone and need to be changed before they are completely fi lled with exudates.

8.3 Novel wound dressings for managing deep skin wounds or ulcers Although the widely used dressings described above are useful in treating various wounds, it is difﬁ cult to remove completely the possibility of infection and delay of wound healing. These problems tend to occur in severe wounds accompanied by excessive exudates, infection and the contamination of necrotic tissues. Novel wound dressings have, therefore, been developed. This section examines some examples of novel wound dressings, including novel textiles and the combination of dressings and bioactive substances.

8.3.1 Novel wound dressings made only of polymers Nanofi brous polyurethane membrane As stated above, PU fi lms are a typical dressing used to protect wounds and keep moist conditions around the wound as an occlusive dressing (Martin et al., 1992; Cross and Mustoe, 2003). Although water vapor and oxygen can permeate PU fi lms, exudates can accumulate in severe wounds, leading to infection or delayed healing. In this case, wound dressings with higher permeability are desirable to give protection from infection and hydration. An electrospinning technique has recently been utilized,

enabling the production of polymer nanofi bers in order to manufacture nanofi brous textile (Khil et al., 2003). The nanofi brous PU fi lm consists of PU fi ber a few hundred nanometers in diameter, and with ultra-fi ne porous structure. The nanofi brous PU fi lm was non-toxic, controlled water vapor well, exhibited excellent oxygen permeation and promoted drainage of fl uids due to high porosity. Furthermore, the superfi ne structure allowed no invasion of exogenous bacteria. Comparison was made between the effects of the nanofi brous PU fi lm and the control PU fi lm (Tegaderm®) on a full-thickness wound in guinea pigs. The dermis of the wound covered with Tegaderm® showed a fairly long-lasting infl ammatory state, while in the case of the nanofi brous PU fi lm, the re-epithelialization rate was increased and the dermis well organized. Histological studies confi rmed that the re-epithelialization rate was accelerated in the nanofi brous PU fi lm. This nanofi brous PU fi lm prepared by electrospinning can, therefore, be proposed as a valid wound dressing.

Cross-linked alginate dressings Non-woven calcium alginate fi ber dressings have been used on deep skin wounds. Kaltostat and Sorbsan are typical non-woven fi ber alginate dressings and provide wounds with good absorption of the exudates and an occlusive environment. However, these dressings have been shown to cause cytotoxicity and foreign-body reaction owing to remaining dressing debris, leading to severe chronic infl ammation (Matthew et al., 1995; Suzuki et al., 1998). Non-woven fi ber alginate dressings are biodegraded, but the degradation rate is very slow. Suzuki et al. showed that calcium alginate fi bers remained after implantation in the muscle of rabbits, and severe chronic infl ammation continued (Suzuki et al., 1999). This was considered to be caused by foreign-body reaction to the debris of dressings. However, an alginate gel prepared by cross-linking with ethylenediamine, using the amide coupling reagent water-soluble carbodiimide, showed no cytotoxicity and reduced foreign-body reaction to a great extent (Suzuki et al., 1999). In experiments using pigs, cross-linked alginate gel showed a signifi cantly higher wound closure rate in a full-thickness wound than Kalto- stat and Sorbsan (Fig. 8.2). Furthermore, the biodegradation rate of crosslinked alginate gel was much faster than Kaltostat and Sorbsan, i.e. it was absorbed completely within a few months without infl ammation. Since the crosslinked alginate gel has a non-fi brous structure, it is considered to be easily biodegraded because of its high-swelling with biological fl uid. The crosslinked alginate gel appears to overcome the disadvantages of the conventional dressings Kalostat and Sorbsan. However, weakness of the gel strength may be a drawback of crosslinked alginate gel, and this needs to be improved.

Wound closure after 15 days (%)

90 92 94 96 98 100

Crosslinked alginate gel

Kaltostat

Sorbsan

8.2 Wound closure rate after 15 days. Mean ± SD (n = 10) (Suzuki Y et al., 1999).

Fibroin/alginate sponge Much attention has recently been paid in material sciences to a protein: silk fi broin. In particular, it has been found that silk fi broin is an invaluable biosynthetic material in the fi eld of biomedical engineering and wound healing (Yeo et al., 2000; Min et al., 2004a). Silk string has been used as a suture since ancient times, and silk is therefore widely known as a safe material. Silk fi broin is very biocompatible and its safety and usefulness have been demonstrated in various studies, showing that its fi lm is very effective in wound healing and exhibits high compatibility as a vascular graft, and its sheet and sponge forms can function as a good support for the proliferation of fi broblasts, epithelial cells, etc. Silk fi broin products act as a safe and strong support structure, and promote granulation tissue proliferation and re-epithelialization (Min et al., 2004b; Gobin, 2005). Alginate, on the other hand, has the ability to absorb wound exudates effectively, and alginate dressings provide an appropriately moist environment to the wound. Given the advantages of silk fi broin in the promotion of proliferation and alginate’s maintenance of a moist environment, it is suggested that their combined use is useful in wound healing. A sponge made by blending silk fi broin and alginate (SF/AA-S) was prepared by mixing the solution and subsequent lyophilization. SF/AA-S was compared with silk fi broin sponge (SF-S) and alginate sponge (AA-S) for wound closure rate, granulation and re-epithelialization following the treatment of full-thickness wounds in rats (Roh et al., 2006). The rate of wound size reduction was signifi cantly faster in SF/AA-S than in the control (Nu Gauze), SF-S and AA-S. The area of new epithelialization tissue was the largest in SF/AA-S, which showed a signifi cantly better effect on

0.16 ) 2 0.14

0.12

0.1

0.08 Epithelialization (mm

0.06 Control AA-S SF-S SF/AA-S 8.3 Effect of wound dressings on re-epithelialization. Mean ± SE (n = 6) (Roh et al., 2006).

re-epithelialization than SF-S and AA-S (Fig. 8.3). The area of collagen deposition in the granulation tissue was signifi cantly increased in SF-S, AA-S and SF/AA-S compared with the control, but no signifi cant difference was observed between the treated groups. The acceleration of re- epithelialization by SF/AA-S was also confi rmed by the increase in proliferating cell nuclear antigen expression. Thus, SF/AA-S showed signifi cant synergic wound healing effects as compared with SF-S and AA-S, mainly associated with the promotion of re-epithelialization rather than collagen deposition. SF/AA-S may be useful clinically.

Chitin/chitosan-blended ﬁ lm Chitin is biocompatible and biodegradable and has the ability to promote wound healing (Ervin, 1976; Nishimura et al., 1984). Chitin is useful as an excellent biosynthetic material for the treatment of wounds of varying degrees of severity, and is used clinically in non-woven sheet and spongy forms, etc. These forms absorb exudates from the wound and provide a properly moist environment around the wound, but hardly exhibit any antimicrobial effects. Therefore, frequent changes of the forms are necessary when exudate accumulation or infectious states are observed. Chito- san, on the other hand, synthesized by the alkaline deacetylation of chitin, shows antimicrobial activity to various bacteria (Seo et al., 1992). Further- more, chitosan exhibits good biocompatibility and appropriate moisturiz- ing effects (Gobin et al., 2005). Chitosan is far superior to chitin in these areas. Biomaterials made of chitosan have recently been utilized as a good scaffold to facilitate tissue regeneration. Chitin/chitosan-blended preparations are, therefore, considered potentially useful in treating skin wounds. Chitin-, chitosan-, and chitin/chitosan ﬁ lms, called CN-F, CA-F and CN/ CA-F, respectively, were prepared by casting techniques, using various

kinds of chitin and chitosan. They were compared for in vitro characteristics such as swelling and strength, and their in vivo effects were examined for wound states and the rate of reduction of the wound area, using rats with full-thickness burn wounds (Tachihara et al., 1997a). CN-F exhibited a small absorption of water, but CA-F made of chitosan with a moderate degree of deacetylation showed a high absorption of water. CN/CS-F exhibited medium water absorption. CN-F was rigid in vitro and in vivo, and lacked the ability to absorb the exudates. CA-F made of chitosan with moderate deacetylation degrees showed good absorption of exudates, but their strength was not maintained in the in vitro swelling study. However, these disadvantages were improved in CN/CA-F. Although CA-F made of chitosan with a high deacetylation degree exhibited fairly good absorption of exudates, the rate of reduction of the wound area was not promoted. On the other hand, CN/CA-F made using chitosan with moderate deacetylation degrees was fairly fl exible and adaptable to the wound surface, and showed a signifi cantly faster rate of reduction of the wound area. In addition, the effect of CN/CA-F on wound states and reduction of the wound area was better than with Beschitin W. CN/CA-F showed fairly good absorption of exudates and fairly good maintenance of fl exibility and structure strength, leading to a good environment for tissue recovery. Further- more, improved wound states, such as a reduction of pus, were observed in CN/CA-F, which was considered to be associated with chitosan’s antimicrobial effects. The inhibition of infection resulted in better wound states and promotion of the rate of wound healing. CN/CA-F is considered to be an excellent dressing for the treatment of severe burn wounds.

8.3.2 Novel wound dressings loaded with bioactive substances PVA sponge containing chito-oligosaccharide Chito-oligosaccharides (COS), obtained by the hydrolysis of chitin and chitosan, exhibit biological functions such as immunostimulating action, and these biological properties are dependent on the number of sugars contained as well as N-acetyl-COS (Suzuki et al., 1986; Tokoro et al., 1988, 1989). COS consisting of 5–6 sugars shows high biological activity. COS is attracting attention as a new bioactive substance and is expected to exhibit effective and high interaction with tissue because it has a much lower molecular weight than chitosan. You et al. developed polyvinyl alcohol (PVA) sponge containing COS and examined its efﬁ cacy in wound healing (You et al., 2004). Sponges with various PVA/COS ratios, called PVA/ COS-S, were prepared by mixing in aqueous solution and subsequent lyophilization. PVA/COS-S inhibited the growth of bacteria, while simple

PVA sponge (PVA-S) showed no antimicrobial activity. PVA/COS-S released COS gradually over several days. The rate of wound healing was examined, using rats with full-thickness wounds. When compared with cotton gauze (the control), PVA-S and PVA/COS-S showed faster closure of the wound. PVA/COS-S was more effective than PVA-S, and PVA/ COS-A with a higher ratio of COS showed a faster rate of reduction of the wound area. COS-loaded PVA sponge can easily be prepared and is considered a useful formulation for wound treatment because of its high degree of effectiveness.

Poly(l-leucine) sponge loaded with silver sulfadiazine Various kinds of dressings or formulations have been developed for the treatment of wound healing, but infection is still a serious problem in clinical use. In particular, excessive exudates and the appearance of pus occur frequently in severe or chronic wounds. In these cases, quick drainage of the exudates and the supply of antimicrobial agents may be needed in addition to ensuring the moist environment. Poly(l-leucine) is insoluble in aqueous conditions, less toxic and slowly biodegradable. These characteristics may be appropriate for impregnating pharmaceutical agents and supplying them gradually to the wound. A poly(l-leucine) sponge containing silver sulfadiazine (AgSD), called PL/AgSD-S, was prepared by mixing and subsequent lyophilization (Kuroyanagi et al., 1991, 1992). The release of AgSD from the sponge was examined in vivo using mice with full- thickness dorsal skin wounds: that is, the remaining amount of AgSD was measured after application to the wound. The AgSD was gradually released over one week. Its in vivo antibacterial activity was examined using mice with full-thickness dorsal skin wounds, inoculated with bacteria such as Staphylococcus aureus or Pseudomonas aeruginosa. Twelve hours after inoculation, the formulation was applied to the wound and the number of bacteria was counted. Compared with the non-treated group, the sponge suppressed the growth of S. aureus or P. aeruginosa. This study demonstrated that the AgSD-loaded poly(l-leucine) sponge would be useful in the treatment of bacteria-infected wounds. When this sponge was clinically tested using patients with burn wounds or pressure ulcers, it showed excellent protection and suppression of infection. Ointments and creams are still used in the treatment of severe wounds such as serious burns and pressure ulcers, but they require frequent change of the formulation and gauze used as a cover, resulting in a burden for the patients. However, dressings such as PL/AgSD-S are easy to handle, provide wound protection and good suppression of infection, and result in an enhanced quality of life (QOL) for patients. Various other types of dressing containing AgSD were also

reported to show these advantages (Tachihara et al., 1997b; Cho et al., 2002). These formulations are considered useful for the treatment of severe or chronic wounds.

Chitosan/polyurethan fi lms containing minocycline Chitin/chitosan-blended fi lms have also been shown to be effective for the treatment of severe burn wounds (Tachihara et al., 1997a). In that study, the fi lms made of chitosan with a high degree of deacetylation were also found to be more effective than chitin fi lm and gauze alone. However, these blended or chitosan fi lms did not necessarily demonstrate good wound states in terms of the elimination of pus, etc. Minocycline (MC) is widely and highly effective in the suppression of bacteria in skin ulcers (Shigeyama et al., 2001). In fact, attempts have also been made to develop topical formulations containing MC (Shigeyama et al., 1999, 2001). However, these semi-solid formulations require frequent changing and multiple washing of the wound, leading to pain or burden for the patients. Dressings are considered to be better for usability and QOL. Therefore, the combination of dressings and some antimicrobial agents was suggested to improve effi cacy. Chitosan/PU fi lms were produced, called CA/MC-F, in which MC powder was sandwiched between chitosan fi lm, controlling the release of MC, and the PU fi lm (TegadermTM) acting as a backing fi lm, (Aoyagi et al., 2007). The CA/MC-F, composed of chitosan with an 83% deacetylation degree and a small amount of MC (2 mg), called CA83/MC-F, showed a gradual drug release over 2 days in vitro and in vivo. CA83/MC- F, CA83 fi lm, MC ointment, Geben cream and Beschitin W were applied to full-thickness burn wounds in rats in the early stage (Days 2 and 4), and the reduction rate of the wound area and wound states were compared between the different formulations. CA83/MC-F and CA83 fi lm showed a faster reduction rate of the wound area than MC ointment, Geben cream and Beschitin W (Fig. 8.4). When the wound underwent complete occlusion, the wound states deteriorated owing to excessive exudates. Appropri- ate drainage seemed to be required to promote wound healing. When a greater amount of MC (10 mg) was contained, the sandwich fi lm showed a worse effect because MC remained too long on and in the wound. Pro- longed use and overuse of MC appeared to worsen wound states and delay wound healing. CA83 fi lm and CA83/MC-F containing 2 mg of MC exhibited improved wound states and promoted wound closure rate when it was applied in conditions allowing appropriate drainage. Given the effect on wound states, CA83/MC-F may be more effective than CA83 fi lm. Chitosan/PU fi lm containing MC is suggested as a useful formulation for the treatment of severe burn wounds.

Control 100 CA83/MC-F CA83 film Beschitin W 75 MC ointment Geben cream

50 Wound area(%) 25

0 0 51015

Time (days) 8.4 Change of wound area after application of preparations. Mean (n = 3) (Aoyagi et al., 2007).

8.4 Future trends Many dressings other than the novel dressing formulations described above have also been developed recently. Materials which not only provide appropriate environments but also function well to promote wound healing will become important in obtaining more effective dressings or formulations. Technical innovation and the discovery of better functional substances are being actively progressed. Some interesting current studies in the development of novel dressings or formulations are described below.

8.4.1 Nanofi brous non-woven matrices In the fi eld of textiles, an electrospinning method is one of the technical innovations for the production of nanofi bers (Greiner and Wendorff, 2007). In addition to the nanofi brous PU membrane referred to above, nanofi - brous non-woven textiles have been manufactured with chitin, collagen and silk fi broin (Min et al., 2004c; Noh et al., 2006; Roh et al., 2006). These biosynthetic polymers are considered invaluable owing to their promotion of tissue generation and wound healing. Chitin nanofi brous non-woven matrices (Ch-N) and conventional fi brous nonwoven membrane Beschitin W (Ch-M) were compared with respect to their scaffold characteristics for tissue regeneration and wound healing, that is, for cell attachment and the spreading of keratinocytes and fi broblasts (Noh et al., 2004). The Ch-N consisted of electrospun chitin fi bers of a few hundred nanometers

(a) 100 μm (b) 100 μm 8.5 Structures of (a) chitin nanoﬁ ber (Ch-N) and (b) Beschitin W (Ch-M) (Noh et al., 2006).

diameter, while the Ch-M was composed of approximately 10-μm-thick chitin microfi bers (Fig. 8.5). The Ch-N showed faster biodegradation than Ch-M, Ch-N promoted cell attachment and spreading of keratinocytes and fi broblasts, more than Ch-M did, and Ch-N coated with type I collagen promoted cellular response. These results suggest that Ch-N would be useful for wound healing. Collagen nanofi brous matrices were also fabricated by the electrospinning technique and found to be effective for wound healing. Furthermore, silk fi broin nanofi brous non-woven matrices, produced by the same technique, showed good cell attachment and spreading of keratinocytes and fi broblasts. Thus, nanofi brous non-woven matrices manufactured with these biocompatible biosynthetic polymers are considered a very effective candidate as a novel dressing for the treatment of severe wounds, because they can provide a similar structure and function to those of the natural extracellular matrix (ECM). They are now expected to offer very high potential as a technique for recovery from severe wounds, and will be examined actively in the future.

8.4.2 Bioactive dressings A further trend may consist of a combination of dressings and bioactive substances including pharmaceutical agents. N-(2,2,2-Triﬂ uoroethyl)-8- methoxy-4-methyl-2-benzazepin-3-one was found to promote the repair of skin wounds by facilitating epithelial cell migration, and this 2-benzazepine derivative is a potential new drug for the treatment of such wounds

(Matsuura et al., 2007). Antibiotics are still useful in suppressing infection. Minocycline-based formulations or silver-based dressings have also been examined, as described above. Furthermore, composites made from a polymer matrix and a controlled release system, such as collagen sponges with gentamycin-loaded poly(dl-lactide-co-glycolide) (PLGA) micropar- ticles, have been developed to achieve both promotion of wound healing and suppression of infection (Schlapp and Friess, 2003; Friess and Schlapp, 2006). Recently, bioactive peptides or proteins, importantly related to the promotion of wound healing, have shown a high potential for clinical use. For example, since the basic fi broblast growth factor (bFGF) promotes the proliferation and growth of fi broblast cells and vascular endothelial cells, it is useful to accelerate wound healing (Yamamoto et al., 2000; Huang et al., 2006). These functions can operate effi ciently when used with dressings that provide appropriate environments for the wounds. A gelatin sponge dressing containing bFGF-loaded gelatin microspheres (GM- bFGF) showed more sustained release of bFGF, as compared with a gelatin sponge containing free bFGF (GS-bFGF) (Fig. 8.6), and a greater degree of reduction in the wound area in pigs with full-thickness skin defects (Huang et al., 2006). The dressing was biocompatible and did not cause foreign-body reaction. The newly formed dermis exhibited almost the same structure as that of normal skin. Dressings loaded with bioactive proteins are suggested as useful and novel wound formulations. Although the use of bioactive proteins is diffi cult owing to problems such as the determination of optimal concentrations, they are certain to be useful for wound

3 GS-bFGF 2 GM-bFGF bFGF released ( μ g) 1

0 0123456 7 Time (days) 8.6 Release of bFGF from GS-bFGF (sponge containing free bFGF) and GM-bFGF (sponge containing microspheres loaded with bFGF) (Huang et al., 2006).

healing and will be examined actively in the future. In addition, pharma- cologically active natural substances such as polysaccharides have received much attention due to their multi-functionality and low toxicity. Polysac- charides isolated from the fungus Phellinus gilvus were also found to be effective in promoting wound healing (Bae et al., 2005). Natural bioactive substances will be useful candidates for dressing/bioactive substance composites in the treatment of severe burn wounds.

8.5 Sources of further information and advice Dressings and formulations relating to the healing of severe or chronic wounds cover various fi elds of science, such as medical science, biomedical engineering, biomaterials, cellular biology, microbiology, pharmacology and pharmaceuticals. Information on dressings and pharmaceutics that have already been offi cially approved can be obtained from package inserts, brochures, publications and websites made publicly available by adminis- trative organisations. These provide information on their application and regulation of use. Practical guides, patents and papers issued by companies or manufacturers are also very useful for obtaining detailed information. News about medicines, industries and the development and research of companies also help us to obtain current or future information. Papers from universities and research institutes provide concepts, methodologies and the evaluation of individual research studies. Many books and reviews about wound healing have recently been published. Biomaterial science is probably at the center of this fi eld, but various sciences are considered fundamentally or practically to promote the development of dressings and formulations for the treatment of severe wounds. Furthermore, it must not be forgotten that, along with the development of materials, dressings and formulations, the establishment of their proper use, depending on wound states and patient conditions, is also of great importance.

8.6 References

agren m s, everland h, ‘Two hydrocolloid dressings evaluated in experimental full-thickness wounds in the skin’, Acta Derm Venereol, 1997 77(2) 127–31. akita s, akino k, imaizumi t, tanaka k, anraku k, yano h, hirano a, ‘The quality of pediatric burn scars is improved by early administration of basic ﬁ broblast growth factor’, J Burn Care Res, 2006 27(3) 333–8. aoyagi s, onishi h, machida y, ‘Novel chitosan wound dressing loaded with minocycline for the treatment of severe burn wounds’, Int J Pharm, 2007 330(1–2) 138–45. asai j, takenaka h, katoh n, kishimoto s, ‘Dibutyryl cAMP inﬂ uences endothelial progenitor cell recruitment during wound neovascularization’, J Invest Derma- tol, 2006 126(5) 1159–67.

atiyeh b s, costagliola m, hayek s n, dibo s a, ‘Effect of silver on burn wound infection control and healing: review of the literature’, Burns, 2007 33(2) 139–48. bae j s, jang k h, park s c, jin h k, ‘Promotion of dermal wound healing by polysaccharides isolated from Phellinus gilvus in rats’, J Vet Med Sci, 2005 67(1) 111–4. berry d p, bale s, harding k g, ‘Dressings for treating cavity wounds’, J Wound Care, 1996 5(1) 10–7. breloff j p, caffesse r g, ‘Effect of Achromycin ointment on healing following periodontal surgery’, J Periodontol, 1983 54(6) 368–72. brett d w, ‘A review of moisture-control dressings in wound care’, J Wound Ostomy Continence Nurs, 2006 33(6 Suppl) S3–8. bryan j, ‘Moist wound healing: a concept that changed our practice’, J Wound Care, 2004, 13(6) 227–8. caputo g m, cavanagh p r, ulbrecht j s, gibbons g w, karchmer a w, ‘Assessment and management of foot disease in patients with diabetes’, N Engl J Med, 1994 331(13) 854–60. cho y s, lee j w, lee j s, lee j h, yoon t r, kuroyanagi y, park m h, pyun d g, kim h j, ‘Hyaluronic acid and silver sulfadiazine-impregnated polyurethane foams for wound dressing application’, J Mater Sci Mater Med, 2002 13(9) 861–5. clark r a, nielsen l d, welch m p, mcpherson j m, ‘Collagen matrices attenuate the collagen-synthetic response of cultured fi broblasts to TGF-beta’, J Cell Sci, 1995 108(Pt 3) 1251–61. clark r a (1996), ‘Wound repair – overview and general considerations’, in Clark R A, The molecular and cellular biology of wound repair (2nd ed.), New York, Plenum Press, 3–50. cosker t, elsayed s, gupta s, mendonca a d, tayton k j, ‘Choice of dressing has a major impact on blistering and healing outcomes in orthopaedic patients’, J Wound Care, 2005 14(1) 27–9. cross k j, mustoe t a, ‘Growth factors in wound healing’, Surg Clin North Am, 2003 83(3) 531–45. diehm c, lawall h, ‘Evaluation of Tielle hydropolymer dressings in the management of chronic exuding wounds in primary care’, Int Wound J, 2005 2(1) 26–35. dowsett c, ‘The use of silver-based dressings in wound care’, Nurs Stand, 2004 19(7) 56–60. ervin v, ‘The effect of chitin powder on the healing of skin wounds’, Magy Trau- matol Orthop Helyreallito Seb, 1976 19(2) 108–14. fl etcher j, ‘Managing wound exudate’, Nurs Times, 2003 99(5) 51–2. friess w, schlapp m, ‘Sterilization of gentamicin containing collagen/PLGA microparticle composites’, Eur J Pharm Biopharm, 2006 63(2) 176–87. gabbiani g, ryan g b, majne g, ‘Presence of modifi ed fi broblasts in granulation tissue and their possible role in wound contraction’, Experientia, 1971 27(5) 549–50. gilchrist t, martin a m, ‘Wound treatment with Sorbsan – an alginate fi bre dressing’, Biomaterials, 1983 4(4) 317–20. gobin a s, froude v e, mathur a b, ‘Structural and mechanical characteristics of silk fi broin and chitosan blend scaffolds for tissue regeneration’, J Biomed Mater Res A, 2005 74(3) 465–73.

greiner a, wendorff j h, ‘Electrospinning: a fascinating method for the preparation of ultrathin fi bers’, Angew Chem Int Ed Engl, 2007 46(30) 5670–703. holland k t, harnby d, peel b, ‘A comparison of the in vivo antibacterial effects of “OpSite”, “Tegaderm” and “Ensure” dressings’, J Hosp Infect, 1985 6(3) 299–303. hong j p, jung h d, kim y w, ‘Recombinant human epidermal growth factor (EGF) to enhance healing for diabetic foot ulcers’, Ann Plast Surg, 2006 56(4) 394–8; discussion 399–400. horncastle j, ‘Wound dressings. Past, present, and future’, Med Device Technol, 1995 6(1) 30–4, 36. huang s, deng t, wu h, chen f, jin y, ‘Wound dressings containing bFGF- impregnated microspheres’, J Microencapsul, 2006 23(3) 277–90. jude e b, apelqvist j, spraul m, martini j, silver dressing study group, ‘Prospec- tive randomized controlled study of Hydrofi ber dressing containing ionic silver or calcium alginate dressings in non-ischaemic diabetic foot ulcers’, Diabet Med, 2007 24(3) 280–8. kakigi a, sawada s, takeda t, ‘The effects of basic fi broblast growth factor on postoperative mastoid cavity problems’, Otol Neurotol, 2005 26(3) 333–6. kawabata h, kamada t, takatsuka y, takeuchi s, suzuki s, makino t, utsunomiya a, ‘Successful treatment for leg ulcers due to hydroxyurea in a patient with chronic myelogenous leukaemia’, Haematologia (Budap), 2002 31(4) 369–72. kaya a z, turani n, akyüz m, ‘The effectiveness of a hydrogel dressing compared with standard management of pressure ulcers’, J Wound Care, 2005 14(1) 42–4. khil m sa, ch d i, kim h y, kim i s, bhattarai n, ‘Electrospun nanofi brous polyurethane membrane as wound dressing’, J Biomed Mater Res B Appl Biomater, 2003 67(2) 675–9. koksal c, bozkurt a k, ‘Combination of hydrocolloid dressing and medical compression stockings versus Unna’s boot for the treatment of venous leg ulcers’, Swiss Med Wkly, 2003 133(25–26) 364–8. kuroyanagi y, kim e, shioya n, ‘Evaluation of a synthetic wound dressing capable of releasing silver sulfadiazine’, J Burn Care Rehabil, 1991 12(2) 106–15. kuroyanagi y, kim e, kenmochi m, ui k, kageyama h, nakamura m, takeda a, shioya n, ‘A silver-sulfadiazine-impregnated synthetic wound dressing composed of poly-l-leucine spongy matrix: an evaluation of clinical cases’, J Appl Biomater, 1992 3(2) 153–61. lansdown a b, ‘Silver in health care: antimicrobial effects and safety in use’, Curr Probl Dermatol, 2006 33 17–34. lee a h, swaim s f, yang s t, wilken l o, ‘Effects of gentamicin solution and cream on the healing of open wounds’, Am J Vet Res, 1984 45(8) 1487–92. levine n, seifter e, connerton c, levenson s m, ‘Debridement of experimental skin burns of pigs with bromelain, a pineapple-stem enzyme’, Plast Reconstr Surg, 1973 52(4) 413–24. martin p, hopkinson-woolley j, mccluskey j, ‘Growth factors and cutaneous wound repair’, Prog Growth Factor Res, 1992 4(1) 25–44. martineau l, shek p n, ‘Evaluation of a bi-layer wound dressing for burn care. II. In vitro and in vivo bactericidal properties’, Burns, 2006 32(2) 172–9. matsuura k, kuratani t, gondo t, kamimura a, inui m, ‘Promotion of skin epithelial cell migration and wound healing by a 2-benzazepine derivative’, Eur J Pharmacol, 2007 563(1–3) 83–7.

matthew i r, browne r m, frame j w, millar b g, ‘Subperiosteal behaviour of alginate and cellulose wound dressing materials’, Biomaterials, 1995 16(4) 275–8. mi f l, wu y b, shyu s s, schoung j y, huang y b, tsai y h, hao j y, ‘Control of wound infections using a bilayer chitosan wound dressing with sustainable antibiotic delivery’, J Biomed Mater Res, 2002 59(3) 438–49. min b m, lee g, kim s h, nam y s, lee t s, park w h, ‘Electrospinning of silk fi broin nanofi bers and its effect on the adhesion and spreading of normal human keratinocytes and fi broblasts in vitro’, Biomaterials, 2004a 25(7–8) 1289–97. min b m, jeong l, nam y s, kim j m, kim j y, park w h, ‘Formation of silk fi broin matrices with different texture and its cellular response to normal human keratinocytes’, Int J Biol Macromol, 2004b 34(5) 281–8. min b m, lee g, kim s h, nam y s, lee t s, park w h, ‘Electrospinning of silk fi broin nanofi bers and its effect on the adhesion and spreading of normal human keratinocytes and fi broblasts in vitro’, Biomaterials, 2004c 25 (7–8) 1289–97. nelson e a, iglesias c p, cullum n, torgerson d j, venus i collaborators, ‘Randomized clinical trial of four-layer and short-stretch compression bandages for venous leg ulcers (VenUS I)’, Br J Surg, 2004 91(10) 1292–9. nishimura k, nishimura s, nishi n, saiki i, tokura s, azuma i, ‘Immunological activity of chitin and its derivatives’, Vaccine, 1984 2(1) 93–9. noh h k, lee s w, kim j m, oh j e, kim k h, chung c p, choi s c, park w h, min b m, ‘Electrospinning of chitin nanofi bers: degradation behavior and cellular response to normal human keratinocytes and fi broblasts’, Biomaterials, 2006 27(21) 3934–44. piaggesi a, baccetti f, rizzo l, romanelli m, navalesi r, benzi l, ‘Sodium carboxyl-methyl-cellulose dressings in the management of deep ulcerations of diabetic foot’, Diabet Med, 2001 18(4) 320–4. roh d h, kang s y, kim j y, kwon y b, young kweon h, lee k g, park y h, baek r m, heo c y, choe j, lee j h, ‘Wound healing effect of silk fi broin/alginate-blended sponge in full thickness skin defect of rat’, J Mater Sci Mater Med, 2006 17(6) 547–52. rovee d t, kurowsky c a, labun j, downes a m (1972), ‘Effect of local wound environment on epidermal healing’, in Maibach H, Rovee D T, Epidermal wound healing, Chicago, Year Book Medical Publishers, 159–181. rubin j r, alexander j, plecha e j, marman c, ‘Unna’s boot vs polyurethane foam dressings for the treatment of venous ulceration. A randomized prospective study’, Arch Surg, 1990 125(4) 489–90. schlapp m, friess w, ‘Collagen/PLGA microparticle composites for local controlled delivery of gentamicin’, J Pharm Sci, 2003 92(11) 2145–51. schwarz n, ‘Wound cleansing with the enzyme combination fi brinolysin/deoxyribonuclease’, Fortschr Med, 1981 99(25) 978–80. seaman s, ‘Dressing selection in chronic wound management’, J Am Podiatr Med Assoc, 2002 92(1) 24–33. seo h, mitsuhashi k, tanibe h (1992), ‘Antibacterial and antifungal fi ber blended by chitosan’, in Brine C J, Sandford P A, Zikakis J P, Advances in chitin and chitosan, London, Elsevier Applied Science, 34–40.

shigeyama m, ohgaya t, kawashima y, takeuchi h, hino t, ‘Mixed base of hydrophilic ointment and purifi ed lanolin to improve the drug release rate and absorption of water of minocycline hydrochloride ointment for treatment of bedsores’, Chem Pharm Bull, 1999 47(6) 744–8. shigeyama m, ohgaya o, takeuchi h, hino t, kawashima y, ‘Formulation design of ointment base suitable for healing of lesions in treatment of bedsores’, Chem Pharm Bull, 2001 49(2) 129–33. simons j j, morales a, ‘Minocycline and generalized cutaneous pigmentation’, J Am Acad Dermatol, 1980 3(3) 244–7. sobotka l, velebný v, smahelová a, kusalová m, ‘Sodium hyaluronate and an iodine complex – Hyiodine – new method of diabetic defects treatment’, Vnitr Lek, 2006 52(5) 417–22. sobotka l, smahelova a, pastorova j, kusalova m , ‘A case report of the treatment of diabetic foot ulcers using a sodium hyaluronate and iodine complex’, Int J Low Extrem Wounds, 2007 6(3) 143–7. stotts n a, hunt t k, ‘Pressure ulcers. Managing bacterial colonization and infection’, Clin Geriatr Med, 1997 13(3) 565–73. su c h, sun c s, juan s w, hu c h, ke w t, sheu m t, ‘Fungal mycelia as the source of chitin and polysaccharides and their applications as skin substitutes’, Bioma- terials, 1997 18(17) 1169–74. su c h, sun c s, juan s w, ho h o, hu c h, sheu m t , ‘Development of fungal mycelia as skin substitutes: effects on wound healing and fi broblast’, Biomaterials, 1999 20(1) 61–8. sugihara a, sugiura k, morita h, ninagawa t, tubouchi k, tobe r, izumiya m, horio t, abraham n g, ikehara s, ‘Promotive effects of a silk fi lm on epidermal recovery from full-thickness skin wounds’, Proc Soc Exp Biol Med, 2000 225(1) 58–64. suzuki k, mikami t, okawa y, tokoro a, suzuki s, suzuki m, ‘Antitumor effect of hexa-N-acetylchitohexaose and chitohexaose’, Carbohydr Res, 1986 151 403–8. suzuki y, nishimura y, tanihara m, suzuki k, nakamura t, shimizu y, yamawaki y, kakimaru y, ‘Evaluation of a novel alginate gel dressing: cytotoxicity to fi broblasts in vitro and foreign-body reaction in pig skin in vivo’, J Biomed Mater Res, 1998 39(2) 317–22. suzuki y, tanihara m, nishimura y, suzuki k, yamawaki y, kudo h, kakimaru y, shimizu y, ‘In vivo evaluation of a novel alginate dressing’, J Biomed Mater Res, 1999 48(4) 522–7. tachihara k, onishi h, machida y, ‘Evaluation of fi lms of chitin, chitosan and chitin–chitosan mixture as dressings for dermal burn wounds’, Yakuzaigaku, 1997a 57(1) 40–49. tachihara k, onishi h, machida y, ‘Preparation of silver sulfadiazine-containing spongy membranes of chitosan and chitin–chitosan mixture and their evaluation as burn wound dressings’, Yakuzaigaku, 1997b 57(3) 159–167. tokoro a, tatewaki n, suzuki k, mikami t, suzuki s, suzuki m, ‘Growth-inhibitory effect of hexa-N-acetylchitohexaose and chitohexaose against Meth-A solid tumor’, Chem Pharm Bull, 1988 36(2) 784–90. tokoro a, kobayashi m, tatewaki n, suzuki k, okawa y, mikami t, suzuki s, suzuki m, ‘Protective effect of N-acetyl chitohexaose on Listeria monocytogenes infection in mice’, Microbiol Immunol, 1989 33(4) 357–67.

vanstraelen p, ‘Comparison of calcium sodium alginate (KALTOSTAT) and porcine xenograft (E-Z DERM) in the healing of split-thickness skin graft donor sites’, Burns, 1992 18(2) 145–8. walker m, hobot j a, newman g r, bowler p g, ‘Scanning electron microscopic examination of bacterial immobilisation in a carboxymethyl cellulose (AQUACEL) and alginate dressings’, Biomaterials, 2003 24(5) 883–90. winter g d, scales j t, ‘Effect of air drying and dressings on the surface of a wound’, Nature, 1963 197 91–2. wysocki a b, ‘Evaluating and managing open skin wounds: colonization versus infection’, AACN Clin Issues, 2002 13(3) 382–97. yamamoto m, tabata y, kawasaki h, ikada y, ‘Promotion of fi brovascular tissue ingrowth into porous sponges by basic fi broblast growth factor’, J Mater Sci Mater Med, 2000 11(4) 213–8. yeo j h, lee k g, kim h c, oh h y l, kim a j, kim s y, ‘The effects of PVA/chitosan/ fi broin (PCF)-blended spongy sheets on wound healing in rats’, Biol Pharm Bull, 2000 23(10) 1220–3. yoshida t, ohura t, sugihara t, yoshida t, ishikawa t, homma k, kouraba s, kimura c, murazumi m, ‘Clinical effi cacy of silver sulfadiazine (AgSD: Geben cream) for ulcerative skin lesions infected with MRSA’, Jpn J Antibiot, 1997 50(1) 39–44. you y, park w h, ko b m, min b m, ‘Effects of PVA sponge containing chitooligo- saccharide in the early stage of wound healing’, J Mater Sci Mater Med, 2004 15(3) 297–301.

P. K. SEHGAL, R. SRIPRIYA and M. SENTHILKUMAR, Central Leather Research Institute, India

Abstract: A description of wound types is presented and the wounds that need drug delivery dressings are identifi ed. Both acute and chronic wounds require drugs for controlling infection during healing. Healing of such wounds needs proper management mediated through the concept of delivering drug to the wound site. The properties of drug delivery dressings are described. Some dressings control the transport of biological fl uids, whereas others offer haemostatic properties leading to control of infection and improved healing. With the advent of tissue engineering technology and advances in micro and nanotechnology, gene delivery and stem cell research, it is possible to design and construct an environment-sensitive dressing tailor-made to the specifi c wound type leading to its complete healing without any adverse reactions.

Key words: acute wounds, chronic wounds, drug delivery dressings, infection control, healing.

9.1 Introduction The administration of topical medications to wound sites is one of the most documented areas in medical history.1 Conventional wound therapy comprised primarily of various ointments, local antimicrobial agents, and sterile bandages.2 In the early 1980s, very few dressing types were available; they comprised mainly traditional dressings, paste bandages and similar preparations. During the mid-1980s modern wound management products began to emerge and hospitals across the world started using them in a small way. Today, their uses have increased manifold. The principal function of a wound dressing is to protect the wound from infection and dehydration and to provide an optimum healing environment. The choice of a wound dressing is dependent on the cause, presence of infection, wound type and size, stage of wound healing, cost and patient acceptability. One dressing type may not be appropriate for all wounds. An ideal wound dressing should have a host of advantages which includes rapid and cosmetically acceptable healing to prevent or combat infection, inducing haemostasis, non-toxic, non-allergic and non-sensitizing nature of the material to absorb wound exudates and wound odour, to provide debridement action and thermal insulation and to allow gaseous and ﬂ uid exchange.

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In addition to this, it should be cost-effective with a long shelf-life and provide maximum comfort to the patients.3 Wound dressings are mainly employed to prevent bulk loss of tissue and they are effective against trauma, chronic wounds such as chronic burn wounds, and diabetic, decubitus and venous stasis ulcers. Where there is infection, the dressings applied without the use of a drug may not be effective because the infectious organisms preferentially target the wound beneath the dressing materials and elicit serious infections requiring removal of the dressing for further treatment and healing.4–7 Treatment of these wounds requires the suppression and control of bacterial growth. Such wounds require topical antimicrobial treatment with a dressing for proper management and healing. Here, the role of drug delivery dressings can play an important role in the effective healing and proper management of wounds. Current efforts in the area of drug delivery in wound dressings include the development of targeted delivery in which the drug shows activity at the wound site. Sustained release formulations in which the drug is released over a period of time in a controlled manner are effective in such cases. Localized drug delivery technologies are emerging as a way to target an optimum dose of a bioactive substance precisely where it is needed, rather than distributing excessive and unnecessary drug over the wound and avoiding excessive drug spreading throughout the body via the systemic circulation. Targeting an optimum dose can be especially useful for drugs with a narrow therapeutic index, i.e. the difference between the dose at which the drug becomes therapeutically active and the dose at which undesired side effects can occur. Dressings can be signifi cantly more effective and safer than their intravenous (IV) or orally administered counterparts, particularly with respect to unwanted side effects. The interest in formu- lated dosage forms, where the drug release can be controlled, has increased steadily during the last fi ve decades. In most cases, the purpose is to make a product that maintains a prolonged therapeutic effect at a reduced dosing frequency.8 In addition to improved effi cacy and safety, the frequency of administration can be decreased with sustained delivery, thereby improving patient compliance. Controlled release systems are particularly useful for drugs which have relatively short half-lives and require a high frequency of administration in conventional dosage forms.

9.2 Wounds: deﬁ nition and types A wound is a type of physical trauma wherein the skin is torn, cut or punctured to create an open wound or where blunt force trauma causes a contusion to create a closed wound. In pathology, a wound speciﬁ cally refers to a sharp injury which damages the dermis of the skin.

The types of open wound are incisions or incised wounds; incisions which involve only the epidermis are classifi ed as cuts, rather than wounds. Lacerations are irregular wounds caused by a blunt impact to soft tissue which lies over hard tissue. Abrasions (grazes) are a superfi cial wound in which the topmost layers of the skin (the epidermis) are scraped off, often caused by a sliding fall onto a rough surface. Puncture wounds are caused by an object puncturing the skin, such as a nail or needle. Penetration wounds are caused by an object such as a knife entering the body. Gunshot wounds are caused by a bullet or similar projectile driving into or through the body. Since the open wound is a disruption of normal anatomic structure and function,9 it can be classifi ed in many ways. We may call it an acute or chronic wound depending upon its nature or duration or type. By defi nition, an acute wound is acquired as a result of trauma or an operative procedure and proceeds normally in a timely fashion along the healing pathway with least external manifestations without complications.10 Surgically created wounds include all incisions, excisions, and wounds that are surgically debrided and non-surgical wounds include all skin lesions that have occurred as a result of trauma (e.g. burns, falls) and such acute wounds are usually successfully managed with local wound care. Wounds that fail to heal in the anticipated time frame and often recur are considered chronic wounds. These wounds present major challenges to healthcare professionals and have serious consequences for the patient’s quality of life. These wounds are visible evidence of an underlying condition such as extended pressure on the tissues, poor circulation, or even poor nutrition. Pressure ulcers, venous leg ulcers, and diabetic foot ulcers are examples of chronic wounds. Successful management of chronic wounds demands treatment of the whole body of the person who is suffering with such wounds. It involves meticulous local wound care, an understanding of the need to diagnose the reason for the specifi c wound and to treat the underlying cause, a working knowledge of modern wound dressings, and correction and management of the patient’s underlying conditions for effective recovery and management. Wound depth is classifi ed by the initial level of tissue destruction evident in the wound: superfi cial, partial thickness, or full thickness.11 Superfi cial wounds involve only the epidermis, partial-thickness wounds involve only epidermis and dermis, and full-thickness wounds involve the subcutaneous fat or deeper tissue. Before any medical or paramedical evaluation or intervention is attempted, an open wound is considered as minor if it is superfi cial and away from natural orifi ces, if there is only minor bleeding and if it is not caused by a tool or an animal. Any other wound should be considered as severe. For severe open wounds, there is a risk of blood loss which could lead to shock and an increased chance of infection as bacteria may enter a wound from surrounding tissue or air. Owing to the risk of infection, the

wound should be kept clean, and closed if possible until professional help is available. Closed wounds have fewer categories, but are just as dangerous as open wounds. The types of closed wounds are contusions (more commonly known as bruises), caused by blunt force trauma that damages tissue under the skin; haematoma (also called a blood tumour), caused by damage to a blood vessel that in turn causes blood to collect under the skin; and crushing injuries, caused by a great or extreme amount of force applied over a long period of time. Wounds are also classiﬁ ed as: wounds without tissue loss (e.g. in surgery, cuts, incisions) and wounds with tissue loss, such as burn wounds, wounds caused as a result of trauma, abrasions or as secondary events in chronic ailments, e.g. venous stasis, diabetic ulcers or pressure sores and iatrogenic wounds such as skin graft donor sites and dermabrasions.

9.3 Wounds which require drug delivery Restoration of tissue continuity after injury is a natural phenomenon. Infection, quality of healing, speed of healing, fl uid loss and other complications that enhance the healing time represent a major clinical challenge. Acute wounds are expected to heal within a predictable time frame, although the treatment required to facilitate healing will vary according to the type, site, and depth of a wound. The primary closure of a clean, surgical wound would be expected to require minimal intervention to enable healing to progress naturally and quickly. However, in a more severe traumatic injury such as a burn wound or gunshot wound, the presence of devitalized tissue and contamination with viable (e.g. bacterial) and non-viable foreign material is likely to require surgical debridement and antimicrobial therapy to enable healing to progress through a natural series of processes, including infl ammation and granulation, to fi nal reepi- thelialization and remodelling. In marked contrast, chronic wounds are most frequently caused by endogenous mechanisms associated with a pre- disposing condition that ultimately compromises the integrity of dermal and epidermal tissue.12 Pathophysiological abnormalities that may predis- pose to the formation of chronic wounds such as leg ulcers, foot ulcers, and pressure sores include compromised tissue perfusion as a consequence of impaired arterial supply (peripheral vascular disease) or impaired venous drainage (venous hypertension) and metabolic diseases such as diabetes mellitus. Advancing age, obesity, smoking, poor nutrition, and immuno- suppression associated with diseases such as AIDS, hepatitis A and B, or cancer, or with drugs (e.g. chemotherapy or radiation therapy) may also exacerbate chronic ulceration. Pressure or decubitus ulcers have a different aetiology from other chronic wounds in that they are caused by sustained

external skin pressure, most commonly on the buttocks, sacrum, and heels. However, the underlying pathology often contributes to chronicity and, in this situation, pressure sores, like all chronic wound types may sometimes heal slowly and in an unpredictable manner. Chronic wounds involving progressively more tissue loss give rise to the biggest challenge to wound- care product researchers. The second major challenge is the prevention of scarring, keloid formation or contractures and a cosmetically acceptable healing. In these cases, drug delivery dressings prove beneﬁ cial for effective and proper wound management.

9.3.1 Wound infections Infection has been defi ned as the deposition and multiplication of organisms in a tissue with an associated host reaction. If the host reaction is small or negligible, then the organism is described as colonizing the wound rather than infecting it. Whether a wound becomes infected or not is determined by the host’s immune competence and the size of the bacterial inoculum. With normal host defences and adequate debridement, a wound may bear a level of 100 000 (105) micro-organisms per gram of tissue and still heal successfully. Beyond this number, a wound may become infected. It is well documented that, if a wound becomes infected, the normal healing is disrupted as the infl ammatory phase becomes chronic, disrupt- ing the normal clotting mechanisms and/or promoting disordered leukocyte function. These factors together or independently, prevent the development of new blood vessels and formation of granulation tissue. The production of destructive enzymes and toxins by mixed communities of organisms may also indirectly affect healing. Thus, it is critical to prevent infections which may occur by normal skin wound contaminants. A prolonged infl ammatory response results in the release of free radicals and numerous lytic enzymes that could have a detrimental effect on cellular processes involved in wound healing. Proteinases released from a number of bacteria are known to affect growth factors and many other tissue proteins that are necessary for the wound healing process.13,14 The increased production of exudates that often accompanies increased microbial load has been associated with the degradation of growth factors and matrix metalloproteinases (MMPs) which subsequently affect cell proliferation and wound healing.15 Some bacteria are rapidly able to form their own protective microenvironment (biofi lm) following their attachment to a surface and the ability of the host to control these organisms is likely to decrease when this biofi lm community matures. In addition, within a stable, biofi lm community, interactions between aerobic and anerobic bacteria would be likely to increase their net pathogenic effect, enhancing their potential to cause infection and delay healing.

Beta haemolytic streptococci (Streptococcus pyogenes), enterococci (Enterococcus faecalis) and staphylococci (Staphylococcus aureus/MRSA) are the gram positive cocci and Pseudomonas aeruginosa, Gram-negative aerobic rod are the potential wound pathogens. Enterobacter species, Escherichia coli, Klebsiella species and Proteus species are the gram- negative facultative rods and Bacteroides and Clostridium anaerobes are classiﬁ ed as wound pathogens. Yeasts (Candida) and Aspergillus are fungi which are responsible for wound infection.

9.3.2 Acute wounds Acute soft tissue infections Cutaneous abscesses, traumatic wounds, and necrotizing infection are classifi ed as acute soft tissue infections. S. aureus is the single causative bacterium in approximately 25 to 30% of cutaneous abscesses16,17 as shown by microbial investigations, and has been recognized as being the most frequent isolate in superfi cial infections seen in hospital accidents and emergency departments.18 Contrary to this, studies have revealed the presence of polymicrobial aerobic–anaerobic microfl ora in approximately 30 to 50% of cutaneous abscesses,19 50% of traumatic injuries of varied aetiology,20 and 47% of necrotizing soft tissue infections.21 Necrotizing soft tissue infections occur with different degrees of severity and speed of progression; they involve the skin (e.g. clostridial and non-clostridial anaerobic cellulitis), subcutaneous tissue to the muscle fascia (necrotizing fasciitis), and muscle tissue (streptococcal myositis and clostridial myonecrosis). S. aureus has been described as being the single pathogen in two patients with rapidly pro- gressing necrotizing fasciitis of the lower extremity,22 and, in a study of necrotizing fasciitis in eight children,23 the presence was reported of pure S. pyogenes in two patients and a mixed predominance of Pepto- streptococcus spp., S. pyogenes, B. fragilis, C. perfringens, E. coli, and Prevotella spp. in the others. Potentiation of infection by microbial synergistic partnerships between aerobes, such as S. aureus and S. pyogenes, and non-sporing anaerobes has been recognized in various types of non-clostridial cellulitis and necrotizing fasciitis.24 The classifi cation of necrotizing soft tissue infections is complex and based on the assumed causative micro-organism(s), the initial clinical fi ndings, the type and level of tissue involved, the rate of progression, and fi nally the type of therapy required.25 However, this classifi cation of such infections serves little clinical purpose because the prognosis and treatment are the same and, consequently, differentiation is required only between pure clostridial myonecrosis21 as it involves muscle invasion and is associated with a higher mortality rate and other non-muscle associated soft tissue infections.

Bite wound infections The reported infection rate for bite wounds in human ranges from 10 to 50% depending on the severity and location of the bite. About 20% of dog bites and 30 to 50% of cat bites become infected.26 In a study reported by Brook,27 74% of the animal bite wounds inﬂ icted on 39 humans contained, predominantly, a polymicrobial aerobic–anaerobic microﬂ ora, comprising S. aureus, Peptostreptococcus spp., and Bacteroides spp. The majority of bite wounds harbour potential pathogens many of which are anaerobes. The common anaerobes in animal bite wounds are Bacteroides, Prevotella, Porphyromonas, and Peptostreptococcus spp.28 and the less common potential pathogens such as Pasteurella multocida, Capnocy- tophaga canimorsus, Bartonella henselae, and Eikenella corrodens.29

Burn wound infections Infection is a major complication in burn wounds, and it is estimated that up to 75% of deaths following burn injury are related to infections.30 Exposed burnt tissue is susceptible to contamination by micro-organisms from the gastrointestinal and upper respiratory tract;31 many studies have reported the prevalence of aerobes such as P. aeruginosa, S. aureus, E. coli, Klebsiella spp., Enterococcus spp., and Candida spp.32 In other studies involving more stringent microbiological techniques, anaerobic bacteria have been shown to represent between 11 and 31% of the total number of microbial isolates from burn wounds.33 Predominant anaerobic microﬂ oras in burn wound isolates are Peptostreptococcus spp., Bacter- oides spp., and Propionibacterium acnes.34 Mousa33 also reported the presence of Bacteroides spp. in the wounds of 82% of patients who developed septic shock and concluded that such micro-organisms may play a signiﬁ - cant role in burn wound sepsis.

9.3.3 Chronic wounds A chronic wound can be deﬁ ned as a wound in which the normal process of healing has been disrupted at one or more points during the phases of haemostasis, inﬂ ammation, proliferation, and remodelling of a wound. Diabetic, decubites and venous ulcers are the most common types of chronic wounds.

Diabetic ulcers These are the most common cause of foot and leg amputation. In patients with type I and type II diabetes, the incidence rate of developing foot ulcers is approximately 2% per year. The average cost for 2 years of treatment is $27,987 per patient.35 The diabetic foot ulcer is mainly neuropathic in

origin, with secondary pathogenesis being a blunted leukocyte response to bacteria and local ischaemia owing to vascular disease. These wounds usually occur on weight-bearing areas of the foot. Plantar ulcers associated with diabetes mellitus are susceptible to infection due to the high incidence of mixed wound microﬂ ora36 and the inabil- ity of the polymorphonuclear neutrophils (PMNs) to deal with invading microorganisms effectively.37 As in most wound types, S. aureus is a prevalent isolate in diabetic foot ulcers, together with other aerobes including S. epidermidis, Streptococcus spp., P. aeruginosa, Enterococcus spp., and coliform bacteria.38 With good microbiological techniques, anaerobes have been isolated from up to 95% of diabetic wounds,39 the predominant isolates being Peptostreptococcus, Bacteroides, and Prevotella spp.40 In view of the polymicrobial nature of diabetic foot ulcers, Karchmer and Gibbons41 suggested that the treatment could be based on a better understanding of the general microbiology of these wounds rather than deﬁ ning the causative microorganism(s).

Decubitus (pressure) ulcers Decubitus ulcers develop as a consequence of continued skin pressure over bony prominences; they lead to skin erosion, local tissue ischaemia, and necrosis, and those in the sacral region are particularly susceptible to faecal contamination. The wound tends to occur in patients who are unable to reposition themselves to off-load weight, such as paralyzed, unconscious, or severely debilitated persons. Approximately 25% of decubitus ulcers have underlying osteomyelitis42 and bacteraemia is also common.43 One of the few reported acknowledgements of the role of polymicrobial synergy in chronic wound infection was made by Kingston and Seal,44 who com- mented that, since the bacteriology of decubitus ulcers is similar to that of some of the acute necrotizing soft tissue infections, the anaerobic and aerobic bacteria involved are likely to contribute to the deterioration of a lesion. The opportunity for microbial synergy in many decubitus ulcers was demonstrated by Brook,45 who reported mixed aerobic and anaerobic microﬂ ora in 41% of 58 ulcers in children; S. aureus, Peptostreptococcus spp., Bacteroides spp. (formerly members of the B. fragilis group), and P. aeruginosa were the predominant isolates.

Venous ulcers In the venous ulcer, chronic passive venous congestion of the lower extremities results in local hypoxia. One current hypothesis of the pathogenesis of these wounds includes the impediment of oxygen diffusion into the tissue across thick perivascular ﬁ brin cuffs. Another belief is that

macromolecules leaking into the perivascular tissue trap growth factors needed for the maintenance of skin integrity. Additionally, the fl ow of large white blood cells slows down owing to venous congestion and occluding capillaries thus becoming activated and fi nally damaging the vascular endothelium leading to ulcer formation. Venous ulcers are the most common form of leg ulcers. Up to 80% of leg ulcers are the result of chronic venous hypertension, most commonly caused by valvular incompetence.46 The microfl ora of chronic venous leg ulcers is frequently polymicrobial, and anaerobes have been reported to constitute approximately 30% of the total number of isolates in non-infected wounds.47 Although S. aureus is the most prevalent potential pathogen in leg ulcers,48 Bowler and Davies49 reported a signifi cantly greater frequency of anaerobes (particularly Pep- tostreptococcus spp. and pigmenting and non-pigmenting gram negative bacilli) in clinically infected leg ulcers than in non-infected leg ulcers (49 versus 36% of the total numbers of microbial isolates, respectively). The same investigators also suggested that aerobic–anaerobic synergistic interactions are likely to be more important than specifi c micro-organisms in the pathogenesis of leg ulcer infection; this mechanism is not widely recognized in the management of surgical50 and chronic wound infections.

9.4 Delivering drugs to wounds Once the type of wound is identifi ed and if it is classifi ed as a wound requiring a drug, an appropriate drug that can be effectively or selectively localized on and in the diseased tissue should be the next logical step that requires immediate attention for the recovery of the patient. Delivery of most drugs, whether by oral administration or through injection, follows what is known as fi rst order kinetics. Here, initial high concentrations of the drugs in blood are obtained, followed by an exponential fall in concentrations. This is problematic because therapeutic effectiveness will not ensue once drug concentrations fall below certain levels. Furthermore, some drugs are toxic at high concentrations in blood and it is diffi cult to achieve a balance between effective levels and toxic levels when the concentration falls off so rapidly. Ideal delivery of drugs should follow zero- order kinetics, wherein the concentration of drugs in blood would remain constant throughout the delivery period. There are a number of technologies currently available that have been used to provide sustained release, but not necessarily zero-order release, for example, porous polymer micro- carriers that contain active pharmaceutical ingredients trapped within interstitial pore channels. The polymers themselves are not reactive, and drug delivery is accomplished utilizing diffusion (Fig. 9.1). This approach allows delivery for extended periods, but there is no evidence that zero-order kinetics is attained.51,52 In an attempt to approach zero-order

Microparticle containing drug

Polymeric matrix

9.1 Schematic diagram showing the diffusion of the drug from a drug loaded micro-particle embedded in a polymeric matrix.

kinetics, extensive work has been carried out in the attachment of biologically active peptides and proteins to poly(lactic acid), poly(lactic co-glycolic acid), and related polyesters.53 The ideal delivery is particularly important in certain classes of medicines intended, for example, for antibiotic delivery for healing of wounds which include diabetic ulcers, decubitus ulcer, venous static ulcer and other non-healing wounds. Controlled-release systems are most suitable for such wounds. These systems comprise a bioactive agent (a drug) incorporated in a carrier. The main objective of a controlled-release device is to maintain the concentration of the drug within therapeutic limits over the required duration. Compared with conventional drug formulations, such systems typically require smaller and less frequent drug dosage and minimize side effects. Here the release rate is a strong function of the physicochemical properties of the carrier as well as the bioactive agent and may also depend on environmental factors such as pH, ionic strength, temperature, enzymic concentration changes and degree of infection at the site of delivery. The drug release mechanism in some dressings is

inﬂ uenced by the inﬂ ammatory enzymes or microbial proteases (which are directly proportional to the rate of infection) where the polymer in the drug delivery dressings is degraded by these enzymes thereby releasing the drug from the polymer matrix (Fig. 9.2).

Collagen scaffold containing drug

Macrophage Neutrophil Fibroblast

(a)

MMPs

(b)

(c)

9.2 (a) Infected wound covered with drug-incorporated collagen scaffold; (b) release of MMPs from the inﬂ ammatory cells and ﬁ broblast; (c) degradation of scaffold and release of drug to the wound site.

The design and preparation of pH- and temperature-sensitive drug delivery dressings is another approach in which the dressing has a dual role to play: it must both retain the drug over prolonged periods and then release it relatively rapidly. Here, the drug is incorporated into the copolymer backbone and the solubility of the polymer is altered by temperature, protonation or deprotonation events. In the pH-sensitive linkages, they are designed to undergo hydrolysis under distinct physiological conditions to either directly release the drug, or alter the polymer structure to disrupt or break apart the dressing (see Fig. 9.3). In an ionic binding system, the release of the drug, which is already bound to the polymer backbone by ionic interaction, is regulated by the infected organisms present in the wound. Any alteration in the ionic behaviour of the wound caused by the infected organisms is controlled by the release of the drug from the polymer backbone due to anion–cation interaction (Fig. 9.4).54 In another method, responsive drug delivery can be achieved by externally triggering the drug administration from a delivery system with magnetic or electronic pulses. An alternative approach involves mixing drugs with an excipient that is slowly dissolved and relying on this

Hydrogel

Change in pH/ ionic strength/ increased temperature

9.3 Schematic diagram showing the drug delivery from an environmentally sensitive hydrogel matrix.

Succinylated collagen bilayer with drug

O – + (CH2)4 C O D

Ionic binding of drug (D) to succinylated collagen

Wound surface showing moderate infection Wound surface showing severe infection 9.4 Schematic diagram showing the release proﬁ le of an ionically bound drug from the ciproﬂ oxacin-incorporated collagen bilayer dressing based on the rate of wound infection.

prolonged dissolution to deliver effective amount of active ingredients over an extended period.55

9.5 Types of dressings for drug delivery There are occasions in surgery that require the use of a temporary cover for raw wounds. These include skin loss secondary to burns, trauma, amputation, chronic ulcers, leprosy and sites of skin transplant. The body needs its own regeneration time, while complications consequent to loss of skin cover wait for no one. The intact skin provides a productive layer over cutaneous nerves and the keratin layer of skin is a very effective antimicrobial barrier. Denuded areas of skin expose the nerves and cause pain and tenderness; they cannot prevent the loss of body heat as normal skin does by controlling vasodilation and sweating formation. Denuded areas continuously lose surface ﬂ uid and electrolytes, since the barrier of intact skin and keratin is not present to prevent the same. However, denuded areas are devoid of this protection, thus delaying wound healing by exposing vulnerable areas of subcutaneous tissues to infection. The orderly ingrowth of epithelium over denuded areas needs a layer of cover to act as the scaffold on which to grow and arrange itself but such areas are unable to provide this effectively leading to formation of extensive scars and even keloids. It is for this purpose that denuded areas need a temporary cover (dressing) until such time that the body is able to manufacture its own cover.

Wound dressings can be broadly categorized as natural and synthetic polymeric dressings. The natural polymers collagen, albumin and gelatine, as protein-based polymers, and agarose, alginate, carrageenan, hyaluronic acid, dextran, chitosan and cyclodextrins, as polysaccharides, are both widely used as wound-dressing materials. Both synthetic biodegradable and non-biodegradable polymers have been used successfully in designing wound-dressing materials. Poly(lactic acid), poly(glycolic acid), poly(hydroxybutyrate), poly(ε-caprolactone), poly(β-malic acid), poly- dioxanes, poly(sebacic acid), poly(adipic acid), poly(terphthalic acid), poly(imino carbonates), polyamino acids, polyphosphates, polyphospho- nates, polyphosphazenes, poly(cyanoacrylates), polyurethanes, polyorthoesters, polydihydropyrans and polyacetals are synthetic biodegradable polymers employed currently as wound-dressing materials. Carboxymethyl- cellulose, cellulose acetate, cellulose acetate propionate, hydroxypropyl- methylcellulose, polydimethylsiloxane, colloidal silica, polymethacrylates, poly(methyl methacrylate), poly(hydroxyethyl methacrylate), ethyl vinyl acetate, polyoxamers and polyoxamines are synthetic non-biodegradable polymers used for the same purpose. Cotton and synthetic gauzes are the most commonly used wound- dressing materials.56 They are preferred because of their low cost and high absorptive capacity. However, because of their porous structure they do not have a barrier to bacterial penetration and also, when in a wet condition, they promote migration of bacteria to the wound site. This can be prevented if an antimicrobial compound is present in the gauze.57 Gauze sticks to a wound surface and disrupts the wound bed when removed; these dressings are used only on minor wounds or as secondary dressings. Another dressing called tulle dressing is a cotton or viscose gauze dressing impregnated with paraffi n with or without antiseptic or antibiotic material. Paraffi n lowers the dressing adherence, but this property is lost if the dressing dries out. The hydrophobic nature of paraffi n prevents absorption of moisture from the wound, and frequent dressing changes are usually needed. Skin sensitization is also common in medicated types. Tulle dressings are mainly indicated for superfi cial clean wounds, and a secondary dressing is usually needed. Uses of gauze and tulle dressings as drug delivery systems are limited as they hold the drugs only for a limited period. Film dressings are highly comfortable and shower-proof, and their transparent surface facilitates the monitoring of wounds without dressing removal. These vapour-permeable fi lms allow diffusion of gases and water vapour throughout the surface when used on wounds. But these fi lms do not absorb wound exudates and are not preferred for use on heavily exudating wounds as fl uid tends to accumulate underneath the fi lm, leading to maceration of the wound and the surrounding skin. Film dressings are suitable for superfi cial, lightly exudating or epithelializing wounds. These

dressings vary in size and thickness, and may have an adhesive to hold the dressing on the skin. They conform easily to the patient’s body but do not hold well in high-friction areas, such as the sacrum or buttocks. Also, fi lms are semiocclusive and trap moisture, so they allow autolytic debridement of necrotic wounds and create a moist, healing environment for granulating wounds. They are impermeable to fl uids and bacteria, but they are permeable to air and water vapour, control of both is dependent on the moisture and vapour transmission rate of the fi lms and it varies depending on the polymer used in the dressings. Permeability of water and air in these dressings creates a moist wound environment.58 The shortcoming of fi lm dressings can be overcome by foam dressings. They are lightweight, superabsorbent, and easy to apply; they come as a highly compressed foam pad that wicks away moisture. It is a good choice for patients who have poorly vascularized, heavily draining leg wounds, because it only requires changing once a day. The foam can be attached with tape, gauze, or Ace bandages. In general, it is applied on dry to wet draining wounds as it wicks away the moisture during drainage of the wound. Foam dressings are designed to absorb large amount of exudates, they also maintain a moist wound environment.59 Hydrogel is another class of fi lm dressings; it consists of polymers with a very high intrinsic water content.60 They conform to wounds with unusual shapes owing to their gel-like nature. In contrast to hydrofi bres, hydrogels are used primarily to donate fl uid to dry necrotic and sloughing wounds, and their absorbency is limited. They are composed mainly of water in a complex network of fi bres that keep the polymer gel intact. Water is released to keep the wound moist. They are used for necrotic or sloughy wound beds to rehydrate and remove dead tissue.61 They are not used for moderate to heavily exudating wounds. There are other dressings called hydrocolloid dressings: they are indicated for minimal to moderately draining wounds. They are lightweight, with adhesive and absorbent characteristics. They can be tailor-made to fi t oddly shaped wounds and can be changed depending on the drainage, which may vary from a few days to as long as a week. They are composed of carboxymethylcellulose, gelatin, pectin, elastomers and adhesives that can be formed as a gel.60 Depending on the hydrocolloid dressing chosen, they can be used on wounds with light to heavy exudate, sloughing or granulating wounds and can be made available in many forms such as adhesive or non-adhesive pad, paste, powder. The most common form is self-adhesive pads. Hydrocolloid dressings, owing to their occlusive nature, do not allow water, oxygen, or bacteria into the wound. This may help to facilitate angiogenesis and granulation at the wound site.62 Hydrocolloids also cause the pH of the wound surface to drop and the acidic environment can inhibit

bacteria growth.63 Hydrocolloids absorb wound exudates and create a warm, moist environment that promotes debridement and healing. Like hydrogels, hydrocolloids helps a clean wound to granulate or epithelialize and encourages autolytic debridement in wounds with necrotic tissue. The occlusive nature of hydrocolloids makes them unsuitable for use if the wound or surrounding skin is infected, but hydrocolloid dressings containing antimicrobial agents may overcome this drawback.64 Hydrofi bre dressings are produced from similar materials to hydrocolloids and also form a gel on contact with the wound, but are softer and more fi brous in appearance, with a greater capacity to absorb exudate. Moisture from the gel assists in debridement and facilitates non-traumatic removal. They comprise soft non-woven pads or ribbon dressings made from sodium carboxymethylcellulose fi bres. They interact with wound drainage to form a soft gel and they absorb exudate to provide a moist environment in a deep wound that needs packing.65 Alginate dressings are called ‘seaweed’ dressing as it is derived from brown algae. Like polyurethane, it is applied dry so it can wick away moisture, making it another good choice for wet draining wounds. Alginate dressing contains calcium salt of alginic acid, it produces a highly absorbent dressing suitable for heavily exudating wounds. It is capable of absorbing up to 20 times its weight in fl uid and possesses haemostatic properties. It is available as fl at sheets or as rope, and can be used for packing cavities. Alginates change from a soft fi brous structure to gel when they absorb exudates. This facilitates easy removal of the dressing, thus preventing contamination of the wound and maintaining a moist wound environment. Alginate dressings can be used for both infected and non-infected wounds. On dry wounds or wounds with minimal drainage, these dressings are suitable as they dehydrate the wound and delay the healing process.66 Chitosan dressings are based on the natural biopolymer chitosan, which is derived from chitin, a major component of the outer skeletons of crustaceans. This material is known in the wound management fi eld for its haemostatic properties. Further, it also possesses other biological activities and affects macrophage function to achieve faster wound healing.67 It also has an aptitude to stimulate cell proliferation and histoarchitectural tissue organization.68 The bacteriostatic and fungistatic properties of this dressing are useful for wound management. Both in solution and gel forms, these dressings act as a bacteriostatic, fungistatic and coating agent. Gels and suspensions may act as carriers for the slow release or controlled action of drugs, as an immobilizing medium and as an encapsulation material. Film and membranes are used in dialysis, contact lenses, dressings and the encapsulation of mammallian cells, including cell cultures. Chitosan sponges are used in dressings to stop bleeding of mucous membranes.

Chitosan in the form of fi bres is used as resorbable sutures, non-wovens for dressings, and as drug carriers in the form of hollow fi bres. Collagen-based biomaterials are considered to be the most promising substitute for wound healing and skin regeneration. Collagen can be processed into a number of forms such as sheets, tubes, sponges, powders, fl eeces, injectable solutions and dispersions, all of which have found use in medical practice.69 Furthermore, attempts have been made to apply these systems in drug delivery in a variety of applications such as ophthalmology, wound and burn dressing, tumour treatment, and tissue engineering. Coll- agen inserts and shields can be used as a drug-delivery dressing over the corneal surface or to the cornea itself and to deliver the drug intra- ocularly.70 As a drug delivery dressing, collagen sponges are invaluable in the treatment of severe burns and have found use as a dressing for many other types of wounds, such as pressure sores, donor sites, leg ulcers and decubitus ulcers.71 The major benefi ts of collagen covers include their ability to easily absorb large quantities of tissue exudate, smooth adherence to the wet wound bed with preservation of this moist microclimate as well as its shielding against mechanical harm and prevention of secondary bacterial infection. Besides these physical effects, collagen promotes cellular mobility and growth and infl ammatory cells actively penetrate the porous scaffold.72 This allows a highly vascularized granulation bed to form and encourages the formation of new granulation tissue and epithelium on the wound. Based on the tissue repair and haemostyptic properties of collagen sponges, combinations with antibiotics were developed for local delivery in the treatment and prophylaxis of soft tissue infections. Collagen fi lms have also been used as drug carriers for antibiotics, having application in periodontal regeneration and infected dermal wounds.73 Composite dressings have multiple layers and can be used as primary or secondary dressings. They are appropriate for wounds with minimal to heavy exudates, healthy granulation tissue, necrotic tissue (slough or moist eschar), or a mixture of granulation and necrotic tissue. Composite dressings have two or more layers and each layer has a specifi c function. Two- layer dressings have a dense upper layer and a spongy lower layer. The outer membrane prevents body fl uid loss, controls water evaporation, and protects the wound surface from bacterial invasion, and the inner matrix encourages adherence by tissue growth into the matrix. These dressings may be considered to constitute an ideal structure that promotes wound healing. These dressings have excellent oxygen permeability, control the water vapour transmission rate, and promote water uptake capability.74 The spongy layer will usually be an antibiotic-impregnated polymer. Applications of both the collagen and chitosan bilayer dressings either alone or in combination with other polymers have been widely reported.75,76

Three-layer dressings consisting of chitosan, synthetic polymer, and a gauze layer have been developed. The second layer functions to transport exudates, protect the wound, and prevent the outer gauze layer from adhering to the wound. It is the second line of defence against bacteria that may try to invade after the chitosan layer breaks down. This polymer ﬁ lm layer, having the consistency of cellophane degrades and becomes part of the healed skin. The outermost layer, made of cotton or cotton–viscose, absorbs exudates and must be changed periodically. Because the wound is well protected underneath, removing the gauze layer does not hurt as much as removing traditional adhesive bandages. In some cases, a three-layer dressing consists of, ﬁ rst, a semi-adherent or non-adherent layer that touches the wound and protects the wound from adhering to other material. This layer allows the dressing to be removed without disturbing new tissue growth and exudates pass through it into the next layer, which is absorptive. If a topical agent is applied to the wound, such as an antibiotic ointment, this inner layer will not stick to the topical product. The second layer is absorptive and wicks drainage and debris away from the wound’s surface to prevent skin maceration and bacterial growth and maintain a moist healing environment. This absorptive layer is made of material other than an alginate, foam, hydrocolloid, or hydrogel. Besides protecting the intact skin from excessive moisture, the absorptive layer helps liquify eschar and necrotic debris, facilitating autolytic debridement. A bacterial barrier forms the third or outer layer that may have an adhesive border. This layer allows moisture vapour to pass from the wound to the air and keeps bacteria and particles out of the wound. It also helps maintain a moist healing environment. Unlike gauze, the bacterial barrier layer prevents moisture leakage to the outside of the dressing (strike-through), meaning that the dressing can be changed less frequently.

9.6 Applications of drug delivery dressings 9.6.1 Infection control Minor wounds usually are not serious, but even cuts and scrapes require care. A serious and infected wound requires infection control and attention. Goals of management of wounds are to avoid infection, minimize discomfort, facilitate healing and minimize scar formation. The use of topical antibiotics and antiseptics is a key approach for reducing the microbial load in wounds. There are several commercially available topical formulations (powders/creams) that release antimicrobial agents into the wound bed. Commonly used antibiotics include bacitracin, mupi- rocin and neosporin. However, owing to the increasing incidence of

bacterial resistance, antibiotics are being used less frequently and antiseptic agents such as slow-release iodine and silver ions have the advantage of rarely inducing any bacterial resistance. The combination of wound dressing with direct antibiotic release at the wound site provides obvious advantages over traditional wound dressings in preventing bacterial infection, especially in high-risk patients. The role of drug delivery dressing on an infected wound is to control the bacterial proliferation, absorb wound exudates and protect the wound from secondary bacterial contamination, thereby enhancing the healing process. A number of sophisticated dressings (dry and moist dressings) with antimicrobial properties have been introduced in the market for treating infected wounds.77 Some occlusive dressings, such as hydrogels and hydrocolloids, have bacterial and viral barrier properties. These dressings can be used to prevent contamination of the wound and reduce the spread of pathogens and cross-infection. Hydrogels allow them to be utilized to deliver topical wound medications like metronidazole and silver sulfadiazine by diffusion mechanism. The release of medications can be controlled by the degree of cross linkage in the gel. Both temperature- and pH-sensitive gels have been the subject of investigation with the objective of developing new products. The biological properties of chitosan including bacteriostatic and fungistatic properties are particularly useful for the treatment of infected wounds. These properties have been exploited in various forms: in gels and suspensions, Chitosan acts as an immobilising medium and an encapsulation material for slow release or controlled action of drugs; and as sponges and hollow ﬁ bres, it acts as a drug-carrier dressing.78 Collagen sponges impregnated with gentamicin and amikacin have been used for many years in human soft tissue and orthopaedic surgery.79

9.6.2 Healing improvement Although the main emphasis in the making of drug delivery dressings is on the prevention and control of infection, certain non-healing ulcers require additional care. Use of growth factors for the treatment of non- healing human wounds holds great therapeutic potential. Cytokines and growth factors, the major regulators, which are produced by various cell types and are recruited to or present at the wound site, are responsible for the successful repair of the injured tissues.80–83 Release of some substances from the dressings stimulates the cells to release cytokines, which enhance wound healing. For example, the release of acemannan from hydrogels has the ability to stimulate macrophages to release ﬁ brogenic and angiogenic cytokines (Interleukin-1 and TNF-α) which results in a positive effect on wound healing.

Various topical systems for delivery of cytokines and growth factors to the injured tissues have been examined. These include covering the wound with a gel, cream or ointment containing a cytokine or a growth factor,84 spraying the growth factor over the wounded site,85 application of the cytokine–soaked gauze86 to the wound, pre-incubation of the skin graft with a cytokine before grafting,87 and use of a genetically engineered biological bandage containing the culture of the cells producing growth hormone.88 Topical delivery of cytokines such as recombinant human granulocyte–macrophage colony stimulating factor (rhGM-CSF) and recombinant human granulocyte colony stimulating factor (rhG-CSF) by using collagen and polyurethane dressings may serve as effective tools for wound healing.89 Growth factors have also been incorporated into poly(lactic-co-glycolic acid) scaffolds successfully, leading to the formation of more viable tissue structures.90,91 Recombinant human-platelet derived growth factor-bb (rhPDGF-BB, Becaplermin) is the ﬁ rst approved growth factor available for clinical use. In clinical trials, it has been shown to increase the incidence of complete wound closure and decrease the time to achieve complete wound healing.

Collagen matrix with bovine transforming growth factor-β2 can be used as a potential dressing on closure of venous stasis ulcers. Controlled release of biologically active fi broblast growth factor (FGF-2) molecules from chitosan hydrogels caused induction of angiogenesis and collateral circulation occurred in healing-impaired diabetic patients. Some hydrocolloids have been shown to bridge the interactive and bioactive classifi cations by exhibiting fi brinolytic, chemotactic and angiogenic effects.92

9.6.3 Controlling transport of biological fl uid Wound exudates can pose problems in the treatment and care of a wound site and need to be handled. In the treatment of many wounds, it is benefi - cial to keep the wound moist while removing excess exudate. This provides an optimum wound healing environment, reduces pain, and helps autolytic debridement and re-epithelialization. Excess fl uid, however, can lead to problems such as maceration (skin breakdown) and microbial infection at the wound site. For this reason, many wound dressings are sometimes designed to have an absorbent pad with high moisture vapour transmission rates, i.e. the excess fl uid is allowed to transmit or evaporate through the wound dressing during application on the wound. Drug delivery dressings have been developed that contain a reservoir of a suitable medicament. The reservoir is directly placed in contact with the skin and the medicament is allowed or assisted to permeate the skin. Unfortunately, the amount of the drug contained within the dressing is

limited for a particular size of the dressing and these dressings do not have a capability to be recharged from a remote reservoir. Many of these dressings are not suitable for application over open wounds. For example, many transdermal drug delivery devices rely on the barrier provided by the dermis to regulate the rate of delivery of the drug. Hydrocolloids and hydroﬁ bres form a gel on contact with the wound and thus have a greater capacity to absorb exudates. Moisture from the gel assists in debridement and facilitates non-traumatic removal. This also enhances autolytic debridement of necrotic and sloughing tissues and promotes the formation of granulation tissue. Hydrogels are used primarily to donate ﬂ uid to dry necrotic and sloughing wounds, and their absorbency is limited. For burn dressings, it is a most important to control evaporative water loss in addition to the drug delivery. Therefore, materials impregnated with antimicrobial agent are required and wound-dressing materials developed in recent years meet these requirements.93,94

9.6.4 Haemostatic properties There exists a need for a wound dressing that can safely and effectively deliver a number of drugs to targeted tissue at a controlled rate, along with haemostatic function. A good example is oxidized cellulose dressing, which, owing to its biodegradable, bactericidal, and haemostatic properties, has been used as a topical haemostatic wound dressing in a variety of surgical procedures, which includes neurosurgery, abdominal surgery, car- diovascular surgery, thoracic surgery, head and neck surgery, pelvic surgery, and skin and subcutaneous tissue procedures. Collagen, in forms such as powder, fi bres or sponge, also has haemostatic properties when used as a wound dressing. Collagen in the form of fi ne fi bres demonstrates an unexpected and entirely unique self-adhesive property when wet with blood or fl uids in live warm-blooded animals: it adheres to severed tissues and requires no suturing. Similarly, chitosan dressing having chitosan fi bre with microporous polysaccharide microspheres containing a therapeutic agent provides both the functions of drug delivery and haemostatic property when applied on a wound. Gelatine sponge is another absorbent, haemostatic material used in surgical procedures characterized by venous or oozing bleeding. The sponge adheres to the bleeding site and absorbs approximately 45 times its own weight in fl uids. Owing to the uniform porosity of the gelatine sponge, blood platelets are caught within its pores, activating a coagulation cascade. Soluble fi brinogen transforms into a net of insoluble fi brin, which stops the bleeding.

9.7 Future trends 9.7.1 Tissue engineering Scaffolds (a term used for the latest innovations in wound-management and drug delivery dressings) are the central components of many tissue engineering strategies because they provide an architectural context in which extracellular matrix, cell and growth factor interactions combine to generate regenerative niches. Although three-dimensional porous structures have been recognized as the most appropriate design to sustain cell adhesion and proliferation, several speciﬁ c applications in tissue engineering may take advantage of other design formats or a combination of different material designs. In fact, as the demand for new and more sophisticated scaffolds develops, materials are being designed that have a more active role in guiding tissue development. Instead of merely holding the cells in place, these matrices are designed to accomplish other functions through the combination of different format features and materials. A good example of this is the use of drug delivery devices that can act simultaneously as scaffolds for cell growth. Drug delivery creates an appropriate chemical environment via soluble factors directly to cells. Con- trolled drug delivery and its applications for tissue engineering to support and stimulate tissue growth have attracted much attention over the last decade. Controlled release devices open the possibility of combining drugs and growth factors within scaffolds to promote tissue development and formation.95–99 In other approaches, microspheres or nanospheres with encapsulated cells, growth factors or other therapeutic agents in a polymeric matrix can enhance the ability of drug delivery dressings to resemble natural human tissues and therefore perform a better functioning in vivo condition.

9.7.2 Stem cell research Stem cells have the potential to reconstitute dermal, vascular and other elements required for optimum wound healing. Through tissue engineering and drug delivery, one may recapitulate developmental cues in a manner which is more physiologically relevant than the simple addition of growth factors to two-dimensional cultures. By combining microspheres containing different drugs with unique release proﬁ les, one can start to emulate the environment seen during development; this has profound implications for stem cell research. Thus, one may be able to gain greater control and insight into the capacity of stem cells and achieve the replacement and repair of complex tissues.

9.7.3 Micro and nanotechnology in drug delivery dressings The use of microspheres for sustained release in the drug delivery system has been of increasing interest. There are potential advantages for controlled release and absorbability. One of the most common methods in which growth factors have been incorporated into polymers is through the formation of microspheres.100–102 Microspheres that release growth factors have been used as scaffolds, providing another means to generate tissue- engineered structures with the necessary chemical environment to foster repair.103 Sustained release of bFGF from the gelatine microsphere incorporated in the bilayer dressing (inner gelatine sponge and outer polyurethane membrane) has shown improved healing on a York pig model.104 Cefazolin incorporated poly(lactide-co-glycolide) (PLAGA) nanoﬁ bres as antibiotic delivery system has shown potential healing in the treatment of wounds.2

9.7.4 Gene delivery Gene delivery is a versatile approach, capable of targeting any cellular process through localized expression of tissue inductive factors. Introduc- tion of the gene rather than a product such as a growth factor is thought to be cheaper and more effi cient for treating non-healing wounds. Poly- meric scaffolds, either natural, synthetic, or a combination of the two, and capable of controlled DNA delivery, can provide a fundamental tool for directing progenitor cell function; this has applications in the engineering of numerous types of tissue. Scaffolds are designed either to release the viral and non-viral vector into the local tissue environment or to maintain the vector at the polymer surface, which is regulated by the effective affi n- ity of the vector for the polymer. For example, DNA delivery by using a chitosan scaffold has been evaluated as wound-dressing materials105 and the in vivo delivery of a plasmid DNA encoding a platelet-derived growth factor gene using a polymer matrix, poly(lactide-co-glycolide), enhanced matrix deposition and blood vessel formation in the developing tissue.106,107 Effi ciency of hydrogel formed by PEG–PLGA–PEG was evaluated as non- viral delivery of pDNA for gene therapy in a skin wound model in CD-1 mice, which has shown promising wound healing.108

9.7.5 Environment-sensitive drug delivery dressings Some of the scaffolds under evaluation respond to several physiological stimuli such as pH, ionic strength, temperature or enzymic concentration changes and infection. Several studies have aimed to construct novel

triggered drug delivery systems that release antimicrobials at specifi c locations at required times. These new systems are usually triggered by certain endogenous host infection responses such as infl ammation- related enzymes, thrombin activity or microbial proteases. For example, S. aureus infection increases the local concentration of thrombin. A PVA– peptide–gentamicin conjugate was developed and investigated where the release of gentamicin depends on local thrombin concentration.109 In the same manner, ciprofl oxacin-conjugated polymers, synthesized from 1,6- hexane diisocyanate (HDI) and polycaprolactone diol (PCL), release ciprofl oxacin when the polymer degrades by the infl ammatory cell-derived enzyme cholesterol esterase.110 Thermosensitive micelles or thermoresponsive hydrogels are self- regulating carriers because the release can be induced by small temperature differences in the human body.111–112 Another way of achieving self-regulating drug delivery involves the application of hydrogels, which are swelling-controlled polymers that allow the release of incorporated drugs only at a certain pH.113 Responsive drug delivery can be achieved by externally triggering the drug administration from a delivery system with magnetic or electronic pulses.114 In a succinylated collagen bilayer system (outer membrane and inner sponge) with Ciprofl oxacin, collagen behaves as an anion after swelling and Ciprofl oxacin forms cations in the swelled network of sponge when applied on the wound. Owing to ionic binding, the drug diffuses slowly and its release rate is controlled. If wound exudates are greater, more drugs will be released to the wound site, which facilitates controlled delivery of the drug. When poly(vinyl-1-pyrrolidone) (PVP) is used along with the drug in the bilayer system, the drug is released as cations from the PVP and is then ionically bound to the succinylated collagen matrix under wet conditions. This further regulates the release of the drug. Infected wounds are polar in nature and therefore the release of the drug from the sponge is regulated by the ionic nature of the succinylated dressing. Once the dressing becomes wet, the role of PVP is negligible and the drug is released by overcoming the ionic binding between the drug and the sponge.115

9.8 Conclusions The design and development of drug delivery dressings involve various scientiﬁ c approaches. Drug delivery dressings require the drug delivery molecule to be designed to give it a enhanced chemical stability and phar- macokinetic properties. Thus, the drug can be tailored to give the required properties, in terms of its absorption, distribution, metabolism, excretion and transfer across biological barriers, to better target the site of action. Similarly, an understanding of the process by which the drugs pass through

the membranes, then choose their best route of administration followed by transportation to the target tissue, is essential for reliable drug delivery. Using various delivery devices the drug reaches the site of action and stays there to repair and heal the wound. While drug delivery dressings have their role in wound management, good nutrition is necessary for wound healing. During the healing process, the body needs an increased amount of calories, proteins, vitamin A and C and, sometimes, some minerals to promote wound healing and prevent infection and complications. If the patient has diabetes, it is necessary to monitor his blood sugar levels at regular intervals to ensure wound healing and control infection. Patients are advised that, if their appetite remains poor and the wound is not healing well and/or they are losing weight, they should make an appoint- ment to see a doctor. Wound healing needs an integrated approach and regular monitoring. Though drug delivery dressings may play a major role in the wound-healing process, other factors mentioned here are equally important and add to the successful management of a wound.

9.9 References

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J. F. K EN N EDY and K. BU N KO, Advanced Science and Technology Institute, UK

Abstract: Some recently developed ‘intelligent’ or advanced dressings, are reviewed and their mechanisms are detailed. Applications including the use of sensors, temperature control and non-contact dressings are described. The potential of such smart textiles in the treatment of particular wounds is explored.

Key words: wound care, smart textiles, wound dressings, sensors.

10.1 Introduction A ‘smart’ textile may be defi ned generally as a material designed to sense and react to differing stimuli or environmental conditions. The ‘intelligent’ textile industry is expanding very quickly and developing wearable composite materials for use in fi elds such as sport, defence, the military, aero- space or medicine. Garments equipped with electronic units, detector devices for ailments or various sensors designed to monitor changes in body temperature and moisture are becoming increasingly real, and not just the stock-in-trade of science fi ction fi lms. Smart textiles are being broadly developed in the fi eld of sports, e.g. outdoor garments with smart membranes to allow moisture to penetrate only in one direction, to keep the wearer warm and protect against the wind. Moreover, this development of ‘smart’ textiles is facilitating advances in the area of medical textiles. The system of wearable pads designed to monitor the vital signs of mother and foetus during pregnancy is one of many ‘smart’ material approaches in the health fi eld. In addition, special attention is being paid to the application of smart textiles in medical devices for the care of wounds. These innovative dressings have perhaps not yet been as fully investigated and developed as ‘smart’ garments, but this approach in medicine shows defi nite promise for improving and accelerating the healing process.

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10.2 Basic principles and types of smart textiles The term ‘smart textiles’ is used to describe materials that are advanced in their structure, composition and ‘behaviour’ in special conditions. Their ‘intelligence’ is classiﬁ ed into three subgroups:1–3

• Passive smart textiles, which are sensors and can only sense the environment; • Active smart textiles, which can sense stimuli from the environment and also react to them; simultaneously with the sensor function, they also play an actuator role; • Very smart textiles, which are able to adapt their behaviour to the circumstances.

To achieve an ideal wound care product, which may be classifi ed as ‘smart’, it is necessary to refi ne plain dressings. A deep understanding of the processes that occur on the injured surface (e.g. infl ammation, secretion of the exudate or epidermal regeneration) is fundamental when initiating trials to design intelligent textiles for wound care.4–6 It is very important for the healing process to provide a moist environment for wounds (a moist wound free from infection is an environment rich in white blood cells, enzymes, cytokines and growth factors benefi cial to wound healing), while still enabling adequate gaseous exchange. Therefore, the occlusive and adherent properties of covering dressings must be preserved. On the other hand, dressings should also be easily removable, without causing additional post-wound trauma, and they should support epithelialisation – the process of new epidermal cells moving across the wound surface and settling on it, which in turn causes wound closure and therefore healing. Epithelialisation occurs more effectively in moist environments. It is also very important to use the sort of dressing which will support healing but reduce scar size, e.g. pressure garments.7,8 Moreover, as a cover, the dressing should protect the wounded area from secondary contamination. It will ideally also provide the antiseptic and healing agents and accelerate the regeneration process. Hence, today, textiles with controlled drug-delivery systems are one of the most investigated categories of dressings. Finally, dressings should absorb excess wound exudate so as to provide relatively stable conditions for healing.4–6 In the light of these challenges, scientists are working to design innovative dressings which will fulfi l all these functions and provide the correct responses to particular wound conditions. These dressings, described as ‘intelligent’ or ‘smart’, will be able to react in different ways in various wound environments. The aim of this chapter is to gather information on the most sophisticated wound care textiles, which accelerate and

improve healing processes, and therefore provide more comfortable conditions for patients.

10.3 Characteristics of smart textiles 10.3.1 Sensors in smart textiles Living in the 21st century, we cannot imagine the world without electronic devices, computers and incredibly fast intercommunication. It follows that even wearable textiles might be controlled and manipulated by biosensors, and that all the components of their interactive electrome- chanical systems (such as sensors, actuators and power sources) can be incorporated or woven directly into garments (sensing and actuating micro-fi bers) or printed/applied onto fabrics (fl exible electronics).9 This means that science is heading towards the development of ‘intelligent’ communication between human beings and their inanimate creations. Electro-active fabrics and wearable biomonitoring devices are now being actively developed.10 Since electro-sensor-monitored garments are being extensively investigated, it seems probable that such electronic devices will soon be applicable as an element of wound dressings, tracking the changes in particular wounds, such as post-operative wounds which defi nitely need continual monitoring, and also providing cover-protection. Nowadays, dressings may contain different types of sensors, e.g. macromolecules such as environment-responsive polymers (ERP).10 These chemical sensors respond to the wound environment with a change of colour, release of fragrance, and swelling/de-swelling. On the other hand, the actuating mechanism for their action may be caused by different factors, e.g. changes in wound temperature, high secretion of exudate or pH changes in the local environment.

How are sensors arranged in dressings? Because different types of sensors monitor the wound environment, their location in the structure of the dressing depends on the particular situation. The structure of the dressing itself may allow the arrangement of the sensor within it. This occurs, for instance, in newly developed technology designed to diagnose infection in the wound using a silicon-based biosensor. This dressing detects lipid A, a component of the bacterial endotoxin lipopolysaccharide. Traditional detection of bacteria in the wound has been carried out using technology that involved several stages. Checking the presence of gram negative bacteria in the injured site involved making a smear slide of the wound sample, then performing staining, decolorisation, and ﬁ nally

examining the slide under a microscope. Using cutting-edge technology based on luminescence principles, it is possible to detect Gram-negative bacteria in situ, applying appropriate sensors within the dressing. This invention, developed by Miller, Fauchet and their colleagues from the Department of Chemistry, Rochester University (NY, USA),11 uses a porous silicon wafer on which millions of tiny holes are etched. This porous structure allows contact of the target molecules with a large surface area. In addition, this structure comprises nanocrystals, which are photolumi- nescent in the visible range of the spectrum at room temperature. More- over, for improved use of the luminescence method, it is possible to apply a proper bandwidth of light. The sensor material, therefore, is placed between further layers of porous silicon that only allow the escape of light for selected wavelengths. These devices are only a few micrometres thick and are known as porous silicon microcavity resonators.12

How does it work? In the detection process for lipid A, speciﬁ c binding occurs between the organic receptor molecule and the diphosphoryl lipid A in the water, via the precisely shaped molecular cavity. This organic molecular receptor, known as ter-tryptophan ter-cyclopentane (TWTCP), is blocked by added amine. This prevents all the TWTCP binding sites from binding to the silicon substrate. Finally, when the binding of lipid A with TWTCP takes place, causing a change in the refractive index of the silicon, an 8-nm red shift occurs in the wavelength of its photoluminescence peak. Unfortu- nately, this is not visible to the naked eye and an expensive machine reader must be used.11 However, when the process of reading the peak has been improved and simpliﬁ ed, this potential for easier and quicker detection of wound infection and the presence of hazardous bacteria will certainly be realised.

10.3.2 Temperature-controlled textiles Keeping wounds covered provides insulation from further injuries and helps to maintain normal body temperature. It has long been known that temperature may have a benefi cial infl uence on wound healing. All cellular functions, especially enzymic and biochemical reactions, are optimised at normal body temperature (normothermia), defi ned approximately as 37 °C.10,12,13–15 This is a very important point in the design of ‘smart’ textiles. But, on the other hand, body temperature may also be a factor that stimulates the ‘behaviour’ of the dressing and infl uences wound healing. The latest investigations in the fi eld of ‘smart’ wound dressings that are managed by temperature have revealed two directions:

• Textiles which change their properties (e.g. forming a gel) in response to different skin temperature; • Dressings that may be heated by speciﬁ c devices and thereby promote wound healing.

10.3.3 Temperature-sensitive dressings The ‘intelligence’ of hydrogels has long been recognised. These smart polymers absorb high amounts of water and have a soft consistency that renders them similar to natural tissue. Moreover, the hydrogel may take up water from the surroundings under special, strictly deﬁ ned conditions, e.g. the correct temperature. This means that this same polymer may exhibit hydrophobic or hydrophilic behaviour, according to the environment. When coating an area, or when embedded into membranes, hydrogels may therefore control the wettability of the surface. This feature has been exploited in the design of ‘smart’ dressings that combine hydrogel with textile.

How does it work? Hydrogels have been used in new cutting-edge textiles that protect the wound and support its healing. Being ‘smart’ chemical sensors, they respond to the wound environment in a special, predicted way. Poly(N-isopropylacrylamide) is a well-known thermosensitive polymer, which has both isopropyl (hydrophobic) and amide (hydrophilic) groups (Fig. 10.1).10,12,16 This polymer is able to assimilate water under strictly deﬁ ned con ditions. Its lower critical solution temperature is set at 32–33 °C, and below this point it absorbs water and extends its chain conformation. On the other hand, when immersed in aqueous solution above 32 °C, poly- N-isopropylacrylamide is extensively dehydrated and compact. This thermoresponsive nature of poly-N-isopropylacrylamide microgel polymer has, therefore, found an application in wound dressing.17,18 It may swell

H H2 2 C H3C C CH2 CH CH2 CO CO CO HN HN HN

CH CH CH H CCH H3CCH3 3 3 n H3CCH3 10.1 The chemical structure of poly(N-isopropylacrylamide) microgel.

and shrink according to the temperature of the aqueous solution (in this case the exudate). Below the lower critical solution temperature of poly- N-isopropylacrylamide, the swelling of the poly-N-isopropylacrylamide microgel beads will decrease the adhesive property of the polymer, resulting in lower peel strength and ﬁ nally in easier and more comfortable removal of the dressing from the wound. The studies have shown that poly- N-isopropylacrylamide microgel beads may be embedded into ﬁ lms, membranes or non-woven fabrics made from various materials (e.g. chitosan, polystyrene or polypropylene).10,19 The membranes have nanoporous structures and are very often transparent.

10.3.4 Applications A recently designed and studied dressing, which is sensitive to skin temperature, shows great and remarkable possibilities for the application of the poly-N-isopropylacrylamide polymer. It is a tri-layer membrane, which partially resembles artifi cial skin and may be applied to extensive burn injuries (Fig. 10.2).19 The fi rst layer is a three-dimensional tri-copolymeric sponge composed of gelatin, hyaluronan and chondroitin-6-sulphate. It has 70% porosity with a 20–100 μm range pore size. This layer fulfi ls the function of the extra-cellular matrix of the skin for cell retention. It is therefore a dermis-analogous layer, which stimulates capillary penetration, promotes dermal fi broblast migration and induces the secretion of an extra-cellular matrix. After migration of the dermal fi broblasts, which recognise the host dermis structure on the wound site, the biodegradable tri-copolymer matrix is gradually degraded by endogenous enzymes. The middle layer is composed of poly-N-isopropylacrylamide (PNIPAAm) and is called the auto-stripped layer. As explained earlier, poly-N- isopropylacrylamide is a polymer which exhibits hydrophilic properties below lower critical solution temperature (33 °C), and a hydrophobic nature above this temperature.

Non-woven polypropylene material

PNIPAAm hydrogel

Tri-copolymer sponge

10.2 Structure of the dressing with arranged poly(N-isopropylacrylamide) (PNIPAAm) microgel.

The third, external layer is a non-woven fabric made of polypropylene. Its function is to protect the wound from infection and facilitate exudate drainage. The most important property of this dressing is that the poly- N-isopropylacrylamide keeps the three layers together if the wound temperature is maintained in the inﬂ ammation state (above 37 °C). But when the healing process is advanced, new skin is being formed and the temperature decreases to normal skin level (31 °C), then the two exterior layers of dressing can be peeled off from the tri-copolymer layer. Wound dressings designed in this way can be used as biodegradable scaffolds for the induction of a ‘neodermis’ synthesis. ‘Neodermis’ is rebuilt new skin which covers the wounded areas. This dressing has huge potential application in large-surface burns, where skin grafts are required.19

10.3.5 Non-contact normothermic wound therapy (NNWT) It is obvious that open wounds endanger the organism as a result of the changing local environment in the body. Uncovered wounds are prone to internal contamination and are also being cooled, which is not beneﬁ cial to the healing process. Therefore, plain dressings such as bandages provide basic care of the injured surface, although they sometimes cause additional trauma when being applied or removed. It is very important, therefore, that the normothermic environment (at normal body temperature of about 37 °C) provides optimal conditions for enzymic and other biochemical reactions, and improves local circulation and tissue oxygen tension. Moreover, correct body temperature increases the availability of immune cells, e.g. macrophages, which may destroy and digest pathogens.13 Warming the wound, therefore, may stimulate the healing process by increasing blood ﬂ ow to the tissues, maintaining an optimal oxygen level and supporting the immune system.14,15,20 In the development of sophisticated dressings, designers have invented new dressings in the form of small wearable apparatus. The product called Warm Up ® Active Wound Therapy (Augustine Medical, Inc., Eden Prairie, MN) is one example of a dressing-device that supports heat input into the wound. This method of healing uses a non-contact radiant-heat bandage to treat chronic venous ulcers or acute injuries, when conventional treatment has failed.

How does it work? Warm Up ® Active Wound Therapy is a non-contact thermal dressing, which consists of a bandage, two plastic ﬁ lms, a heating element, a control unit and a power supply (Fig. 10.3).

ABC

38 °C

A1 A2 A3 A4

10.3 Diagram of a non-contact normothermic wound dressing. A: dressing with the heating element in the pocket (A1 foam ring pad, A2 pad window with two layers of the plastic ﬁ lms, which form a pocket for the heating element, A3 heating element, A4 fragment of bandage that surrounds the dressing pad); B: control unit that sets the temperature in the heating element; C: power supply, which is also a charger for the batteries located in the control unit.

The bandage surrounds two layers of plastic fi lm supported by and attached to an open-cell pad. The pad adheres to the skin surrounding the wound. The open window in the bandage is a two-layered pocket where the heating element is situated. The window is located straight over the wound and enables direct monitoring of the wound when the heating element is removed. The thin, fl at, resistant heating element, which is reus- able, fi ts into the pocket of the bandage and is linked to the control unit by a cable (Fig. 10.4). It is important that the heating element does not come into contact with the patient, but is held above the wound by the bandage and separated by one or more insulating layers. The temperature at the heating element is automatically maintained by the control unit at 38 °C. The power supply is a wall outlet pack that reduces the voltage to a safe level and recharges the batteries, which are located in the control unit.

The treatment proﬁ le There are different ways of using Warm Up® Active Wound Therapy. According to one method,21 this treatment was applied four times a day over a two-week period. The heating element was activated for one hour, followed by a period of one hour of non-heating. This regime was repeated four times a day. At the end of the last daily session, the radiant heat bandage was removed, the wounds were dressed with collagen–alginate

2 4 6

1 3 5 10.4 Diagram of the dressing section of Warm Up® Active Wound Therapy. 1: wound, 2: foam ring pad, which supports two plastic transparent ﬁ lms, 3: two layers of plastic ﬁ lms with a space between them, 4: heating element, 5: bandage around the pad, 6: connection to the heating control unit.

dressing and wrapped in an elastic compression bandage that was left in place overnight.

Results The method of use of Warm Up® Active Wound Therapy described above is one of several proposed. Generally, clinical treatment indicated that heating wounds defi nitely accelerated healing. The following results are signifi cant: • The use of local heat delivered by the non-contact normothermic wound therapy was shown to increase subcutaneous oxygen tension by 50% above the baseline measures in normal volunteers.15 • Healing with non-contact normothermic wound therapy is faster than the use of standard dressings and no adverse effects occur.15 • From the beginning to the end of the study, pain decreased in 92% of cases, while in the control group given standard treatment, the decrease was only in 27%, and the wound size decreased in 45% (for non-healing venous stasis ulcers).20 These results confi rm the benefi cial infl uence of non-contact normothermic wound therapy on the healing process.

10.4 Textiles in control of exudate from wounds It is very well known, and accepted by clinicians for more than 40 years, that a moist wound environment is beneﬁ cial during the healing process.21 Therefore, many modern textiles used in dressings are designed to provide moist and warm conditions for wounds. Body injuries are caused as a result of different stimuli, accidents or diseases (e.g. burns, acute wounds, post-operative wounds and ulcers). Some results suggests that

the secreting fl uid from wounds may have a benefi cial effect on healing, but, on the other hand, it may also inhibit this process. Studies have demonstrated that exudate from chronic wounds, e.g. ulcers, in fact causes deterioration of healing, whereas fl uid secreting from acute wounds may support the recovery of healthy skin.21,22 For this reason, it is common practice to manage the wound exudate and utilise its benefi cial effects for the healing process.23

10.4.1 Why does exudate play such an important role during the healing process? Within the injured area of the body, various infl ammatory reactions occur and fl uid derived from serum passes through the vessel walls into the wound bed in the process called extravasation. Finally, the exudate emerges at the wound surface and, according to the local environment, may contain micro-organisms or tissue debris.24 However, the wound exudate is derived from body serum and is therefore rich in components such as glucose, lactic acid, inorganic salts, proteolytic enzymes, growth factors, immune cells (e.g. macrophages and lymphocytes), plasma proteins, albumin, globulin, and fi brinogen. Some of these play remarkably signifi cant roles in the healing process. For example, macrophages play an immune-defence role releasing growth factors and proinfl ammatory cytokines. In addition, some components are a source of energy (glucose) while inorganic salts create typical pH values and cause the exudates to play the role of buffer solution.22 Moreover, the fl uid coming from the body through the wounded areas may be utilised as a medium for drugs that accelerate healing or for (other) antibacterial agents, which have bactericidal properties. When drugs have previously been administered into the digestion and circulation systems, they may subsequently appear in the exudate from the wound area.25 Thus it can be seen that the exudate is a heterogeneous liquid with dissolved contents, which undeniably has an infl uence on the healing process. Dressing selection should, therefore, be related to the type and size of wound, and – most importantly – it should be chosen according to the volume, viscosity and nature of the exudate.

10.4.2 Selecting a dressing After entering the wound area, the exudate may demonstrate either a beneﬁ cial or a noxious tendency in the healing process. Depending on the wound type, the exudate may appear as a clear, straw-coloured, thin ﬂ uid, which is very similar to serum and common in acute wounds.22 The seropurulent type of exudate, which has a characteristic yellow,

cream–coffee colour and creamy-thick consistency, indicates that infection is under way, as occurs in infected blisters. In chronic wounds on the other hand, the colour and consistency of the exudate may vary. In addition, an unpleasant odour from chronic wound exudate and excessive drainage on clothing or bedding are common causes of patients’ discomfort and distress.26 The features of the wound exudate also change over time after injury. Therefore, it is very important to select the dressing which is appropriate for current wound conditions. Ideally, the wound-care product would respond in a specifi c ‘smart’ way to the various fl uid environments, but, currently available dressings react in a predictable way to the defi ned circumstances and features of the exudate. Various mechanisms exist for handling the dressing of fl uid: gelling, absorption, retention and moisture vapour transmission. Some of these mechanisms sometimes occur simultaneously in one product.22 But all of these actions of dressing are clear objectives in the design of smart textiles, in order to manage the exudate and, as a result, provide the optimal environment for the quickest wound healing.27 Today, it is still necessary for us to assess which dressing will support the healing of a particular type of wound. We have to choose with care the product that can manage appropriately the predicted amount and type of exudate.

Absorbent textiles that form a gel with the exudate A wide range of dressings is available that uses gel formation as a response to interaction with the exudate. When incorporated in the dressing, the gel is the part of the system which not only protects the wound, but also accelerates the healing. The absorption and gelling functions of the dressing are very often combined and supplemented within one product. The gels applied in wound dressings might be set in sheets, pads or saturated gauzes. The most commonly used are alginates (and their combinations with other contents, e.g. with gelatine), poly(N-isopropylacrylamide) gels or hydrocolloid dressings. Hydrogels applied in wound dressings are well known for their hydroaffi nity properties. One very important feature of particular hydrogels is the amount of aqueous solution that they are able to donate or absorb. However, when excess exudate must be removed, a moist environment at the wound surface is still desirable.28 A further property that promotes the benefi cial use of hydrogels in dressings is the fact that gel contact with the exudate causes easier dressing removal, without additional pain and trauma for the patient.17 Other gelling dressings in use are hydrofi bres, which are soft, non-woven pads, useful for wounds with a heavy level of wound drainage, and for deep wounds. It is also important that hydrogels stay fi rmly in contact with the wound for a long period without removal, resulting in the reduction of scar formation.21,29

Fluid-retention dressings Another type of gelling dressing absorbs the exudate and causes retention of fl uid. The gel which is then formed can be directly removed from contact with the wound. However, this does not necessarily sustain the moist environment. One example of this kind of product is Hydrofi ber®, made from sodium carboxymethylcellulose fi bres. These are soft non-woven pads, which are useful for a heavy level of wound drainage, and for wounds that are deep and need packing.30 Hydrofi ber® may be used in dressings with a combination of other layers, e.g. with a polyurethane foam/fi lm layer (in the product VersivaTM).

Foams and absorbent pad dressings This type of dressing is appropriate for the ﬁ rst stage of healing when drainage from the wound is greatest because a large amount of exudate can be absorbed. The advantage is that foams can be applied in a variety of sizes, shapes and thicknesses and are gentle on the skin. While foam pads have good adhesive properties, some of them stick too ﬁ rmly to the wound or surrounding skin area, causing additional trauma upon removal. To prevent this, soft foam pads with an incorporated silicone layer have been designed.4 This allows for repositioning or gentle removing of the dressing.

A model of moisture vapour transmission in wound dressings Moisture vapour transmission through the dressing is a very important process. Depending on the dressing’s water absorption capacity, it determines the ﬂ uid balance at the wound site.31,32 The rate of moisture vapour transmission is a feature of all dressings which absorb and remove liquids. One type of dressing that absorbs particularly large amounts of liquid is hydrocolloid dressing, which usually has two layers. The ﬁ rst, which comes into contact with the wound site, is a hydrocolloid membrane with a large potential for swelling when taking up water. This layer is covered by another one, made of a polymeric material (e.g. polyurethane). Hydrocolloid dressings maintain a moist environment for healing, but also allow the liquid to permeate outside the dressing and facilitate autolytic debridement of the non-viable tissue.33

10.5 Examples of ‘smart’ textiles for wound care 10.5.1 Hydrogel dressings Some of the gel-containing dressings on the market are: Tegagel (from 3M Health Care), Curasol Hydrogel Saturated Dressing (from Healthpoint), DuoDerm Hydroactive Wound Gel (from Convatec), and Carrasyn

Hydrogel Wound Dressing (Carrington Laboratories). One of the examples of wound dressings that has gel formed in situ is based on oxidised alginate, gelatine and borax.34 Each of these components has a specifi c function in the dressing. The gelatine has a haemostatic effect, while the alginate is well known for promoting wound healing and is supported by the antiseptic quality of borax. The combination of these functions makes the dressings very effective for moist wound healing. The alginate is oxidised, rapidly cross-links the gelatine and in the presence of borax gives in situ formation of hydrogel. It does not wrinkle or fl ute in the wound bed and moreover is non-toxic and biodegradable. It protects the wound bed from the accumulation of excess exudate because it has a capacity of fl uid uptake that is 90% of its weight. It takes up the exudate, but still maintains the moist environment to facilitate and stimulate epithelial cell migration during the healing process.34

10.5.2 Fluid-retention dressings Dressings that are well known for fl uid retention very often contain hydrofi bres. Examples are Aquacel® Hydrofi ber® Wound Dressing and Versiva Composite Adhesive Exudate Management Dressing (both from ConvaTec).35 Versiva, for example, contains three components: a top polyurethane foam/fi lm layer, an absorptive non-woven fi brous blend layer (with Hydrofi ber®), and a thin perforated adhesive layer. All the components play specifi c roles in the dressing. The thin perforated adhesive layer has direct contact with the wound and accelerates exudate absorption. The non-woven fi brous-blend layer absorbs and retains the exudate by performing as a cohesive gel. The third layer, the outer-side polyurethane foam/ fi lm, protects the wound from contamination and manages the moisture vapour transmission of the exudate. This dressing is recommended for use in different types of ulcers (on legs, diabetic, pressure), and for surgical wounds, second-degree burns and traumatic wounds.

10.5.3 Foam dressings Some of the best known foam-dressing products are: Lyofoam (from ConvaTec), Polymem Non-Adhesive Dressings (from Ferris) and Allevyn Adhesive Dressings (from Smith and Nephew).

10.5.4 Silicone-foam pads Some brand-name dressings which contain soft silicone are: Tendra Mepitel (a silicone mesh contact layer), Tendra Mepilex Lite and Tendra Mepilex Border (all from Molnlycke Health Care).

10.5.5 Hydrocolloid dressings Some examples of hydrocolloid dressings available on the market are: Tegasorb (from 3M Health Care), Comfeel Ulcer Care Dressing (from Coloplast) and DuoDerm CGF Control Gel Formula Dressing (from ConvaTec).

10.6 Response of dressings to bacteria Wound infections caused by pathogens have always been a major problem that has slowed the healing process. Although the practice of dressing wounds dates from the start of civilisation, utilising dressings for infection control is relatively new. Dressings exist today that have speciﬁ c properties to inhibit infection in the wound site. There are different ways of achieving this. Sometimes the dressing applied may release drugs or other chemical compounds, which have bactericidal properties and kill the pathogen.36,37 In other cases, the dressings catch the whole bacteria population and close them within their own structure.38 All these treatments exploit advanced and ‘smart’ methods of accelerating wound healing, because while removing bacteria, they reduce the state of inﬂ ammation.

10.6.1 Dressings that remove bacteria from the wound site In the early 1990s, it was shown that bacteria may be removed from the wound site within the moisture-retentive hydrocolloid.38,39 Later, investigations demonstrated that alginate-fi bre dressings, which form a gel, may also partially immobilise bacteria in the fi brous matrix and, thus, effectively remove them from the wound site.39,40 Further investigations revealed other dressings that remove bacteria from the infected site with greater effi cacy than alginate.39,41,42

10.6.2 Application of textiles for removal of bacteria Wound dressings based on carboxymethylated spun cellulose, e.g. CMCH (carboxymethyl cellulose ), absorb the exudate and form a cohesive gel. Along with the exudate, whole populations of bacteria occupying the wound space are entrapped within the gel structure. In contact with the exudate, the CMCH dressing forms a coherent, continuous gel. The ﬁ bres of the dressing, therefore, become fully hydrated, swell and are indistinguishable from each other. The ﬂ uid is rapidly absorbed and the bacteria from the wound space are enclosed inside the gel. Most importantly, the great majority of the bacteria is taken into the cohesive gel structure and none are present on the non-hydrated

ﬁ bres surrounding the gelled area. These investigations have all been con- ﬁ rmed using a scanning electron microscope (SEM).38 This application has, therefore, the potential to inhibit wound infection without the release of drugs.

10.7 Future trends The wound dressing textile industry has a very fast rate of development. Smart devices or sensors, which may be incorporated into dressings, play a special role and are currently being investigated with particular urgency. In the past, people mostly considered the protective function of dressings, but today, they are loaded with drugs, different sensors and devices. On the basis of the achievements of current techniques and science, we can imagine that in the future, structures and applications of wound-care products will become more and more sophisticated, and patients’ recovery to health will be faster, painless and continually monitored.

10.7.1 Dressings and pathogens Dressings currently being investigated for detection of noxious bacteria in wound sites include, for example, dressings with a silicone-based biosensor that binds lipid A, which may be detected using the photoluminescence method.11 However, there is the potential development of new dressings, which can not only recognise the presence of microbiological organisms, but also distinguish between particular species and strains. However, detection methods will need to be simpliﬁ ed and avoid becoming too expensive. There is also the potential to develop new dressings containing antibacterial agents, which will only be active in the presence of the pathogen. Dressings already exist today which are loaded with drugs and antibiotics.43 Products containing silver as a bactericidal agent are especially popular, e.g. the nanocrystalline silver dressing ActicoatTM or SilvaSorb. Acticoat is a smart dressing which aims to accelerate the healing of burns. This product not only prevents wound adhesion, but is also an antibacterial agent. The Acticoat dressing contains a rayon/polyester nonwoven core, laminated between layers of silver-coated high-density polyethylene mesh. This dressing is effective against many bacterial strains, including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococci.37, 44–46 SilvaSorb is a synthetic, polyacrylate hydrophilic matrix which contains dispersed or suspended microscopic silver- containing particles. In contact with the moist exudate, the silver is released into the wound.3 These dressings are already smart because, apart from their protective function, they are also a source of antibacterial agents. The challenge for

future investigations is to design a dressing capable of releasing the antimicrobial agents in a controlled way, e.g. so that the amount of antimicrobial agent released to the wound site is just sufﬁ cient for the amount of pathogen, thereby preventing the adverse effects of any excess.

10.7.2 pH-sensitive textiles The pH value of the exudate is very important and signifi cantly infl uences patients’ health. A huge challenge remains for scientists to learn how the pH level may be used to accelerate the healing process, and how to monitor dangerous changes in the wound environment. As described above, wound liquids are particular mixtures of dissolved biological micro- and macromolecules. Some of them may be noxious and some may promote the healing process. Sometimes they have a negative or positive charge, which signifi cantly infl uences the pH of the exudate. There is, therefore, the possibility of designing dressings which can detect pH changes, or even react to them. Studies are currently under way to investigate how pH-sensitive chemical compounds may be applied in textiles and how they react in different conditions. Some colorimetric pH sensors react very rapidly and allow detection of changes with the naked eye, which makes them particularly attractive. On the other hand, fl uorescent sensors are even more sensitive than colorimetric ones.47 Fabrics which are chromic or chameleon materials, and can change their colour in different conditions (e.g. light, heat, electricity, pressure, liquid, electron beam), are representative of ‘intelligent’ textiles.48

How do colorimetric pH chemosensors work? It is well known that fl uorescent or colorimetric agents are good pH sensors.49 These compounds are highly sensitive to environmental changes, and when they possess high fl uorescence intensity with thermal and photo- stable properties, they may be used to dye natural or synthetic textile fi bres50 or as dyes for the structural coloration of polymer materials.51 One of the compounds investigated, which may be embedded on to a fabric matrix, is 1-[(7-oxo-7-benzo[de]anthracen-3-ylcarbamoyl)-methyl]-pyrid- inium chloride dye (BD).47,49 This novel water-soluble organic compound retains colorimetric pH-sensor properties even when it is immobilised on viscose textile. When BD is placed in acidic solution, it occurs in proto- nated form and produces a bright-yellow colour. At pH >10.4, deprotonation of BD occurs and the colour becomes reddish orange. Figure 10.5 shows two forms of electron movement as a result of deprotonation or protonation. BD may therefore occur in two canonical forms, as shown in Fig. 10.5. The absorption spectra of the dye are strongly dependent on the pH of the solution. It shows that the absorption maximum

OH– pH 3.3–7.3 OH– H H N N.. N .. N

O O

+ – H+ OH– H OH

H O+ H O+ 3 pH 10.4–12.6 3 .. .. N N N N

. – O .O..

. – O .O.. 10.5 Mechanism of protonation and deprotonation of BD dye. In acidic solution, the dye is bright yellow and in alkaline solution, a reddish orange.

occurs at two different wavelengths for the two ranges of pH. At pH 3.3– 7.3, the absorption maximum occurs at λ = 413 nm, whereas at pH 10.4– 12.6 the absorption maximum is at λ = 457 nm. Reaction is very fast and reversible.47 The example outlined above shows a dyed fabric which also plays a pH-sensor role. It would be a great challenge to investigate the textile matrix further, as an application for pH-sensitive dressings. Fabric dyeing with colorimetric agents gives a large sensory area, which might be convenient in the case of wound dressings, allowing the monitoring of the whole wounded surface. Furthermore, an immediate answer indicated by a colour change would give a warning in case of any dangerous pH variations. Although the ‘smart textile’ industry is expanding greatly, ‘smart dressings’ are still a relatively new branch. Therefore, some products already under development might not be included here because they have not yet been reported in the public domain. However, this chapter provides a

review of some ‘intelligent’ or advanced dressings, which have been invented and developed over the last few years. This detailed presentation of their mechanisms should encourage further investigation since it shows the potential of dressings in the treatment of particular wounds.

10.8 Sources of further information and advice Those interested in ‘smart’ wound dressings may fi nd other sources of further information, but the following reference list gives the most acces- sible and fundamental resources. Smart textiles for medicine and healthcare, edited by L. Van Langen- hove, is a signifi cant source of information not only for the wound care products area, but also for different smart textiles, such as intelligent garments for pre-hospital emergency care, smart textiles in rehabilitation or in pregnancy monitoring. The following volumes are also recommended: Medical textiles and biomaterials for healthcare, edited by S. C. Anand, J. F. Kennedy, M. Miraftab and S. Rajendran (Woodhead Publishing Limited, Cambridge, UK, 2006), which discusses issues relating to wound-care materials and intelligent textiles for medical applications. The previous editions of this book series are Medical textiles (2000) and Medical textiles 96 (1997), both edited by S. C. Anand. Conferences including discussion of topics relating to smart fabrics, textiles or dressings are an alternative source of information. During such events, changes are proposed in the functionality of textiles by applying new technologies and creating sensitive, interactive and intelligent materials. The most recent conferences and events are: • MEDTEX 07, Fourth International Conference and Exhibition on Healthcare and Medical Textiles, the latest conference of a series of MEDTEX conferences, The Holiday Inn Bolton Centre, Bolton, UK (16–18 July 2007), www.bolton.ac.uk/uni/research/medtex07 • Nanocomposites 2007 in Brussels, The Crown Plaza Europa in Brussels, Belgium (14–16 March 2007) • The Middle East’s fi rst symposium and exhibition, dedicated to non- wovens, Dubai, United Arab Emirates, (20, 21 February 2007) • 3rd International Congress on Innovations in Textiles and Fabrics, Berlin, Germany, (14, 15 February 2007) • Smart Textiles Europe, Paramount Carlton, Edinburgh, UK (13, 14 December 2006), www.paramount-carlton.co.uk • European Conference on Textiles and the Skin, Apolda, Germany, (11–13 April 2002), http://www.derma.uni-jena.de/04tagung/2002ects. pdf.

More information about conferences held over the last few years on textiles, fabrics and new trends in medical applications can be found on the website http://www.technical-textiles.net.

10.9 References

1. x zhang, x tao, ‘Smart textiles: passive smart’, June 2001, pp. 45–49; ‘Smart textiles: Active smart’, July 2001, pp 49–52; ‘Smart textiles: Very smart’, August 2001, pp. 35–37, Textile Asia. 2. l v langenhove, r puers and d matthys, ‘Intelligent textiles for medical applications: An overview’ In: S C Anand, J F Kennedy, M Miraftab & S Rajendran (eds.), Medical textiles and biomaterials for healthcare, Woodhead Publishing, Cambridge, 2006, 451–472. 3. l v langenhove, ‘Smart textiles for medicine and health care. Materials, systems and applications’, Woodhead Publishing, Cambridge, 2007. 4. t s stashak, e farstvedt and a othic, ‘Update on wound dressings: indications and best use’ Clinical Techniques in Equine Practice, 2004 3 149–163. 5. g m menaker, ‘Wound dressings in the new millennium’, Seminars in Cutan- eous Medicine and Surgery, 2002 21(2) 171–175. 6. s-y lin, k-s chen and l run-chu, ‘Design and evaluation of drug-loaded wound dressing having thermoresponsive, adhesive, absorptive and easy peeling pro perties’, Biomaterials 2001 22 2999–3004. 7. n yýldýz, ‘A novel technique to determine pressure in pressure garments for hypertrophic burn scars and comfort properties’, Burns, 2007 33 59–64. 8. l macintyre, m baird, ‘Pressure garments for use in treatment of hypertrophic scars – a review of the problems associated with their use’, Burns, 2006 33 10–15. 9. m engin, a demirel, e z engin and m fedakar, ‘Recent developments and trends in biomedical sensors’, Measurement, 2005 37 173–188. 10. b liu, j hu, ‘The application of temperature–sensitive hydrogels to textiles: a review of Chinese and Japanese investigations’, Fibers and Textiles in Eastern Europe, 2005 13(6) 54. 11. j whelan, ‘Smart bandages diagnose wound infection’, Drug Discovery Today, 2002 7(1) 9–10. 12. m-y zhou, l–y chu, w–m chen and x–j ju, ‘Flow aggregation characteristic of thermo-responsive poly(N-isopropylacrylamide) spheres during the phase transition’, Chemical Engineering Science, 2006 61 6337–6347. 13. j d whitney and m m wickline, ‘Treating chronic and acute wounds with warming: review of the science and practice implications’, J WOCN, 2003 30(4) 199–209. 14. j d whitney, g salvadalena, l higa and m mich, ‘Treatment of pressure ulcers with noncontact normothermic wound therapy: Healing and warming effects’, J WOCN, 2001 28(5) 244–252. 15. t ikeda, f tayefeh, d i sessler, a kurz, o plattner and b petschnigg, ‘Local radiant heating increases subcutaneous oxygen tension’, Am J Surg, 1998 175 33–37. 16. l m geever, d m devine, m j d nugent, j e kennedy, j g lyons, a hanley and c l higginbotham, ‘Lower critical solution temperature control and swelling

behaviour of physically crosslinked thermosensitive copolymers based on N-isopropylacrylamide’, European Polymer Journal, 2006 42 2540–2548. 17. l-s wang, p-y chow, t-t phan, i j lim and y-y yang, ‘Fabrication and characterisation of nanostructured and thermosensitive polymer membranes for wound healing and cell grafting’, Advanced Functional Materials, 2006 16 1171–1178. 18. m r guilherme, g m campese, e radovanovic, a f rubira, e b tambourgi and e c muniz, ‘Thermo-responsive sandwiched-like membranes of IPN- poly-N-isopropylacrylamide/PAAm hydrogels’, Journal of Membrane Science, 2006 275 187–194. 19. f-h lin, j-c tsai, t-m chen, k-s, j-m chen yang, p-l kang and t-h wu, ‘Fabrica- tion and evaluation of auto-stripped tri-layer wound dressing for extensive burn injury’, Materials Chemistry and Physics, 2007 102 152–158. 20. c robinson and s m santilli, ‘Warm-Up Active Wound Therapy: A novel approach to the management of chronic venous stasis ulcers’, Journal of Vascular Nursing, 1998 2 38–42. 21. a jones and d vaughan, ‘Hydrogel dressings in the management of a variety of wound types: A review’, Journal of Orthopaedic Nursing, 2005 9 51–511. 22. r white and k f cutting ‘Modern exudate management: a review of wound treatments’, World Wide Wound (Revision 1.0) 2006. 23. k lay-fl urrie, ‘The properties of hydrogel dressings and their impact on wound healing’, Prof Nurse, 2004 19 269–73. 24. k f cutting, ‘Exudate: composition and functions’, in White RJ (editor): Trends in wound care Volume III. London: Quay Books, 2004. 25. t yotsuyanagi, s urushidate, k yokoi, y sawada, m suno and t ohkubo ‘A study of the concentration of orally administered sparfl oxacin found in exudates from suture wounds beneath occlusive dressings’, Burns 1998 24 751–753. 26. s seaman, ‘Management of fungating wounds in advanced cancer’, Seminars in Oncology Nursing, 2006 22(3) 185–193. 27. t j coats, c edwards, r newton and e staun, ‘The effect of gel burns dressings on skin temperature’, Emerg Med J, 2002 19 224–225. 28. s thomas and p hay, ‘Fluid handling properties of hydrogel dressings’, Ostomy Wound Manage, 1995 41 54–56, 58–59. 29. d eisenbud, h hunter, l kessier and k zulkowski, ‘Hydrogel wound dressings: where do we stand in 2003?’, Ostomy Wound Manage, 2003 49 52–7. 30. g r newman, m walker, j a hobot and p g bowler, ‘Visualisation of bacterial sequestration and bacterial activity within hydrating Hydrofi ber® wound dressing’, Biomaterials, 2006 27 1129–1139. 31. c m mouës, g j c m van den bemd, f heule and s e r hovius, ‘Comparing conventional gauze therapy to vacuum assisted closure wound therapy: A prospective randomized trial’, Journal of Plastic, Reconstructive and Aesthetic Surgery, 2007 60 672–681. 32. p wu and j d s gaylor, ‘A model of water vapour transmission in hydrocolloid wound dressing’, Journal of Membrane Science, 1994 97 27–36. 33. m límová and j troyer-caudle, ‘Controlled, randomised clinical trail of 2 hydrocolloid dressings in the management of venous insuffi ciency ulcers’, Journal of Vascular Nursing, 2002 1 22–33.

34. b balakrishnan, m mohanty, p r umashankar and a jayakrishnan, ‘Evaluation of an in situ forming hydrogel wound dressing based on oxidized alginate and gelatin’, Biomaterials, 2005 26 6335–6342. 35. y qin, ‘Superabsorbent cellulosic ﬁ bres for wound management’, Textiles, 2005 1 12–14. 36. k moore, a thomas and k g harding, ‘Iodine released from the wound dressing modulates the secretion of cytokines by human macrophages responding to bacterial lipopolysaccharide’, International Journal of Biochemistry Cell Biology 1997 29 163–171. 37. f-l mi, y-b wu, s-s shyu, a-c chao, j-y lai and c-c su, ‘Asymmetric chitosan membranes prepared by dry/wet phase separation: a new type of wound dressing for controlled antibacterial release’, Journal of Membrane Science, 2003 212 237–254. 38. m walker, ja hobot, g r newman and p g bowler, ‘Scanning electron microscopic examination of bacterial immobilisation in carboxymethyl cellulose (AQUACEL®) and alginate dressings’, Biomaterials, 2003 24 883–890. 39. j lawrence, ‘Dressings and wound infection’, American Journal of Surgery, 1994 167 21S–4S. 40. f dehaut and m maingault, ‘Kinetic binding of bacteria on two types of dressing: algosteril (calcium alginate) and gauze’, Poster presentation, First European Workshop Surgery-Engineering: Synergy in Biomaterial Applica- tions. Montpelier, France, 1994. 41. p g bowler, s a jones, b j davies and e coyle, ‘Infection control properties of some wound dressings’, Journal of Wound Care, 1999 8 499–502. 42. m j waring and d parsons, ‘Physico-chemical characterisation of carboxymethylated spun cellulose ﬁ bers’, Biomaterials, 2001 22 903–12. 43. l martineau and p n shek, ‘Evaluation of bi-layer wound dressing for burn care. II In vitro and in vivo bactericidal properties’, Burns, 2006 32 172–179. 44. h n paddock, r fabia, s giles, j hayes, w lowell, d adams and g e besner, ‘A silver-impregnated antimicrobial dressing reduces hospital costs for pediatric burn patients’, Journal of Pediatric Surgery, 2007 42 211–213. 45. k kok, g a georgeu and w y wilson, ‘The acticoat glove – an effective dressing for the completely burnt hand. How we do it’, Burns, 2006 32 487–489. 46. r rustogi, j mill, j f fraser and r m kimble, ‘The use of ActicoatTM in neonatal burns’, Burns, 2005 31 878–882. 47. d staneva and r betcheva, ‘Synthesis and functional properties of new optical pH sensor based on benzo[de]anthracen-7-one immobilized on the viscose’, Dyes and Pigments, 2006 74 148–153. 48. c a norstebo, ‘Intelligent textiles, soft products’, Journal of Future Materials 2003 1–14. 49. d staneva, r betcheva and j-m chovelon, ‘Fluorescent benzo[de]anthracen- 7-one pH-sensor in aqueous solution and immobilized on viscose fabrics’, Journal of Photochemistry and Photobiology A: Chemistry, 2006 183 159–164. 50. n ayangar, r lahoti and r wagle, Indian Journal of Chemistry 1973 16B 106–108. 51. t konstantinova, p meallier, h konstantinov and d staneva, Polym. Degrad. Stab., 1995 48 161–166.

M. JOSH I and R. PURWA R, Indian Institute of Technology Delhi, India

Abstract: The structure of composite dressings is described as a combination of different textile structures such as woven, non-woven, knitted, net ﬁ lm and spacer fabrics. They are multi-layered, each layer having its distinct property to enhance the wound-healing process. Research and development in composite dressings is mainly focused on enhancing the functionality of these different layers so as to promote rapid wound healing and a reduction in the pain associated with wound treatment, and all this at a reduced cost. Recent developments towards improvements in non-adherent, absorbent and bacterial barrier layers are described that can reduce the frequent changes of the dressing and, thus, improve wound healing.

Key words: composite dressings, wound care, wound healing, multilayer dressings, wound dressing.

11.1 Introduction Wound management has recently become more complex because of new insights into wound healing and the increasing need to manage complex wounds outside the hospital. Modern wound dressings are designed to facilitate the function of wound healing rather than to just cover it. Healing of wounds is a biological cellular process linked to the process of repair. The physiological process of wound healing is a dynamic process and follows a complex pattern of four continuous phases, namely haemostasis, inﬂ ammation, proliferation and maturation or remodeling.1 The best dressing is the patient’s own skin, which is permeable to vapor and protects the deeper layer tissue against mechanical injuries and infection. The disadvantage of such dressings is their antigen properties, which limits the span of the application. A wound dressing creates a suitable microclimate for rapid and effective healing.2 A good wound dressing prevents dehydration and scab formation, is permeable to oxygen, is sterilizable, absorbs blood and exudates, protects against secondary infections, supplies mechanical protection to the wound, is non-adherent, non-toxic, non-allergic and non-sensitizing. A dressing should also be tear- and soil-resistant, stable over the range of

275

temperatures and humidity encountered during use, have a long shelf-life and be economical.3 Wounds require several kinds of dressing, depending on the type and dimension of the wound, its positioning on the body and, in particular, on the effusion intensity, the depth of tissue damage and the stage of healing. Appropriately used dressings may prevent complications such as infection, tissue maceration, excessive effusion, swelling, pain and smell. The dressing supports the body’s own cleaning mechanism and provides clean conditions in the wound.4 A traditional wound dressing comprises an absorbent pad of a fi brous material with gauze, which is in immediate contact with the wound. The diffusing wound fl uid will thus extend into the interstices and around the fi bre of the dressing so that the dressing is eventually adhesively and mechanically anchored into the wound surface and produces a protective covering to the wound. Commonly used wound dressings comprise cotton gauze, foams, sponges, cotton wads or other fi brous materials. Gauze and other fi brous materials absorb fl uid by capillary action with the disadvantage that when new tissue is formed as a part of healing process, it engulfs the fi bre and tears when the material is removed causing wound injury and disturbing new tissue growth. Consequently, removal or changing of dressings can cause disruptions of the healing process, delaying the healing process as well as being a painful procedure for the patient.5 Moreover, cotton gauze is generally used because of its good absorption properties and soft handle. However, a disadvantage of cotton gauze is that it allows moisture to evaporate from the wound which means that cotton gauze dressings do not maintain the moist environment that is said to facilitate faster wound healing. These dressings also need frequent changing. Thus, there is a need for a dressing which is non-adherent while being absorbent. The expectation from an ideal wound dressing at times may be very demanding and can not be fulfi lled by only one layer of material. A multi- layered composite dressing provides the ultimate wound protection, layer by layer. Such modern dressings have more than one layer of material that can be universally used as an initial form of treatment of the wound. They can be applied over a wound in one simple and effi cient step and prevent adherence of the dressing to the wound as well as help maintain a proper moisture level at the wound and prevent disturbance of wound caused by dressing changes.

11.2 Deﬁ nition of composite dressings Composite dressings are the products obtained by combining physically distinct components in to a single dressing that provides multiple functions. These functions must include (a) bacterial barrier (b) absorption (c) either

Table 11.1 Commercially available composite dressings

Serial Commercial name Manufacturing Features number company

1StratasorbTM Medline Island 2 3M Tegaderm 3M Sterile, thin ﬁ lm transparent 3AlldressMolnlycke Health Multi-layer Care 4 COVADERM PLUS DeRoyal Adhesive barrier 5 CD-1000 adult SDA Product Inc. Waterproof 6Versiva® ConvaTech Adhesive dressing with Hydroﬁ ber® 7 Coversite plus Smith & Nephew Waterproof Inc. 8Invacare SterileAracent HealthcarePolyurethane foam 9TELFA KendallIsland VENTEX 10 MPM MPM Multi-layer boderless 11 Viasorb Sherwood-Davis & Multi-layer Geck 12 Silon Dual-Dress 04P® Bio Med Sciences Multi-function Silon Dual-Dress 20F®

semi-adherence or non-adherence over the wound site and (d) an adhesive border. Composite dressings are extremely easy to use with a ‘band-aid’ type application. A composite dressing is deﬁ ned as a multi-layer product with an adhesive border which comprises:6 (a) a physical (not chemical) bacterial barrier that is present over the entire dressing pad and extends out into the adhesive border; (b) an absorptive layer other than an alginate or other ﬁ ber-gelling dressing, foam, hydrocolloid or hydrogel; (c) a semi-adherent or non-adherent layer over the wound site. Table 11.1 summarizes the commercially available composite dressings in the market. The shape and size of the composite dressing depend on the wound size and its place.

11.3 Structure of composite dressings Composite dressings have multiple layers and can be used as primary or secondary dressings. Most composite dressings have three layers, namely a semi-adherent or non-adherent layer, an absorptive layer and a bacterial barrier layer as shown in Fig. 11.1. A semi-adherent or non-adherent layer

Bacterial barrier layer

Adsorptive layer

Exudates

Moisture

Non-adherent or semi- adherent layer

Wound

11.1 Layered structure of a composite dressing.

touches the wound and protects the wound from adhering to other material. This layer allows the dressing to be removed without disturbing new tissue growth. Since exudates pass through it into the next layer, it has to be permeable to ﬂ uids. An absorptive layer wicks the drainage and debris away from the surface of the wound and this prevent skin maceration and bacterial growth and maintains a moist healing environment. In addition to protecting the intact skin from excessive moisture, the absorptive layer helps liquify eschar and necrotic debris, facilitating autolytic debridement. A bacterial barrier layer may have an adhesive border. This outer layer allows moisture vapor to pass from the wound to the air and keeps bacteria

and particles out of the wound. It prevents moisture leakage to the outside of the dressing (strike-through), meaning that the dressing can be changed less frequently.7

11.4 Materials and textile structures used in composite dressings 11.4.1 Non-adherent or semi-adherent layer In the composite dressing, the fi rst layer needs to be a fi ne mesh ‘non- adherent’ type material. ‘Non-adherent’ is actually a misnomer since the purpose of this layer is to allow the substances that come out of the wound to go through this layer and then be removed with the dressing change. The major function of the non-adherent layer is that it reduces shearing of the epithelium, permits passage of blood and fl uids, does not trap heat or moisture, is non-absorbent and non-adhesive. This layer does not promote or speed up the healing process. Instead, it just allows for simple dressing changes to occur without disturbing the healing process. The non-adherent material is generally only one layer thick.8 Various textile materials such as nylon, cotton, polyester, polypropylene, rayon, silk, and cellulose derivatives in the form of woven, non-woven, net, and perforated fi lm structure can be used as a non-adherent layer as shown in Fig. 11.2. Initially, paraffi n gauze was used as non-adherent material for dressings. Jelonet, Paranet, Paratulle and Unitulle are some of the commercial non-adherent dressings based on paraffi n gauze; these consist of a cotton or rayon cloth impregnated with white or yellow soft paraffi n. The soft paraffi n in the dressing reduces adherence to the wound bed, if applied in suffi cient layers. However, these dressings require frequent changing in order to prevent drying out and incorporation into the granulation tissue in the wound.9 The main drawback of the natural polymer-based textile structure is that although they are non-adherent relative to the absorbent layer, they are not completely non-adherent. Woven or net structures have interstices in which the exudates can dry. Moreover, in certain situations such as pressure dressing or a packing dressing, the dressing is pressed on to the treatment area causing non- adherent material to be pressed into the wound so that granulation tissue and epithelial cells are forced into the interstices of the non-adherent material. The non-adherent layer of composite dressing can be prepared by changing the texture of the surface. The US patent 3 006 33810 discloses the preparation of a non-adherent surface of the dressing, wherein the non-woven fabric gets impregnated with minute particles or globules of

Non-woven Woven net

Perforated film Woven (gauze) 11.2 Basic textile structures for non-adherent layer of composite dressing.

polythene and then passes through a hot calendar to provide a smooth surface covered with fi lm-like patches of polythene and free of projecting fi bres. In general, a non-adherent layer comprises a highly porous, self- sustaining, discontinuous fi lm of fused and coalesced non-woven inert thermoplastic synthetic polypropylene.11 N-TERFACE (high-density polyethylene sheeting) by Winfi eld laboratory is an example for non-adherent layer composite dressing.12 More recently, a polyamide fabric partially embedded into the biosynthetic silicon fi lms has been used. Collagen is incorporated in silicon and polyamide components. Mepitel® is a non-adherent silicon dressing made of a medical-grade silicon gel, bound to a soft and pliable polyamide net. Another example is a fi ne polyamide net, containing pores of 50 μm in diameter, under the trade name of Tegapore. These pores are big enough to permit the free passage of exudates into the secondary absorbent layer but are too small to allow the ingress of granulation tissue. The perforated net prevents the dressing from adhering to the surface of the wound while the holes allow the passage of exudates to the absorbent layer. The net dressings themselves do not absorb any body fl uids. A non-adherent dressing layer is also produced by coating them with a thin layer of aluminum by vacuum deposition. Such dressings are commercially available under

Spun bonded Air laid Woven fabric

Wet laid Spun lace Needle punch 11.3 Various textile structures used for absorptive layer in composite dressing.

the trade name Metallene®.13 In general, the non-adherent layer comprises a porous polyethylene ﬁ lm having a thickness of about 3.5–5.0 mil.

11.4.2 Absorbent layer As the name suggests, the second layer comprises an absorbent material for absorbing exudates, blood, moisture, and other fl uid materials from the inner layer and also maintain the wound moist environment. The absorbent layer may contain one or more layers of absorbent material in an adequate quantity. Many non-toxic conventionally known absorbent materials are readily available. The most commonly used absorbent materials are gauze, non-woven sheet materials such as needle-punched non-woven rayon, cellulosic pulp, synthetic pulp, cotton rayon, creped cellulose wadding, an airfelt or airlaid pulp fi bers and absorbent sponges, some of which are shown in Fig. 11.3. The absorptive layer needs to be fl exible, soft and approximately 0.1–0.5 cm thick. The absorptive layer size and shape will vary with the size and shape of the wound. It should be large enough to cover the wound and capable of absorbing at least 2 to 20 cm3 g −1 of exudate.12 Superabsorbents are water-insoluble materials, capable of absorbing and retaining large amounts of water or aqueous fl uid in comparison to their

own weight. A cellulose matrix containing a superabsorbent, e.g. carboxymethylcellulose, sodium polyacrylate in powder form is generally used for the absorbent layer in composite dressings. Other absorbents used are composed of alginic acid, dextran, carboxymethyldextran, starch, modifi ed starch, hydroxyethyl starch, hydrolyzed polyacrylonitrile, poly- acrylamide, carboxymethylcellulose and their derivatives in the form of powder or granules. The most preferred superabsorbent material is a crosslinked dextran derivative which absorbs 2 to 10 g g−1 of liquid. These are commercially available under the trade names Sephadex® (from Sigma Chemical Co. St. Louis, MO), Debrisan® (from Pharmacia of Sweden) and Separan®, AP30 (from Dow Chemical Co.). It is preferable that the absorbent is uniformly laid so as to wick the blood and body fl uids away from the wound surface and prevent the pooling of fl uids next to the wound to prevent maceration of the wound. The absorbent should breathe and not trap excessive heat. The absorbent can be designed to dispense microencapsulated medication or traditional medication to the wound. The absorbent can be moistened with various medications or solutions before application or can be impregnated with a desired medicament. Examples of medicaments that may be absorbed into the absorbent are antimicrobial drugs, analgesics, metal oxides and enzymes. The absorbent should readily wick away any fl uids before they dry, thereby helping to prevent any bonding or adherence of the contact component to the wound surface. Commercially available absorbent textiles such as Sunbeam Process absorbent materials (Gelman Technology), the Compos- ite Air Laid Superabsorbent Pad (Dry Forming Processes) and Polyester Superabsorbent Fiber Flock SAFF (Hanfspinnerei Steen & Co.) are the preferred materials for making composite dressings.12 US patents 516761314 and 546573515 disclose the combination of high- density and low-density absorbent pad structures to absorb the exudates. Such structures consist of two separate but contiguous elements, namely a lower high-density woven or non-woven fabric having optimum spreading or wicking characteristics and upper low density fabric having optimum absorption capacity. The high-density fabric will have a density of the order of 0.1 to 0.2 g cm−3 while the low-density fabric will have a density 0.05 g cm−3. The high-density and low-density fabric can be prepared by using rayon, rayon/polyester blend, polyester/cotton blend or cellulosic material. US patent 607752616 discloses the gradient density needle-punch felt of polyester and viscose fi ber, which absorbs a large quantity of exudate. Material with varying density across its depth is achieved by varying the degree of needle penetration and thus fi bre entanglement across the material depth. Thus, a graded density felt may have a relatively loose volu- minous central region and more dense surface layers. The increased density

of the surface layer is achieved by more entanglement, with the result that there is less spacing at its surface. This construction of graded density felt, acts as a regulator or gate. Another US patent17 claims a composite wound dressing having an outer layer having controlled permeability and an absorptive layer which is highly blood and exudate absorbent. It comprises a PTFE ﬁ bril matrix having hydrophilic absorptive particles enmeshed in a matrix and optionally a partially occlusive ﬁ lm coated on one surface of the matrix.

11.4.3 Bacterial barrier layer The third or outer layer comprises a bacteria-impermeable and air- permeable cover sheet. The cover sheet also consists of a layer of pressure- sensitive adhesive in its peripheral as shown in Fig. 11.4. The adhesive layer may be of the order of 1 mil thick. This layer helps to seal the cover sheet in a liquid and bacteria tight relationship around their common periphery so that exudate cannot escape through the edges of the dressing nor can any external contaminants, including bacteria, enter into the dressing and then pass through the slits to the underlying wound. The bacterial barrier layer comprises a fl exible, waterproof and breathable material which protects the injury from exposure to contaminants from the outer atmosphere, while preventing leakage of any moisture from the injury. In general, this layer comprises a waterproof, breathable polyurethane fi lm of an approximately 0.5–1.5 mil thickness. Low density, opaque polyethylene or polypropylene fi lm, or waterproof and breathable fi lms are other suitable materials for the bacterial barrier layer. The bacterial

Bacteria impermeable, waterproof Bacterial barrier layer

Adhesive layer Air permeable 11.4 Function of bacterial barrier layer along with adhesive layer.

barrier layer may be of the order of 1.0–3.0 mil thick.18 A bacterial barrier air ﬁ lter that can also be used as an outer layer is commercially available as Nucleopore®, Millipore® or Gellman®.

11.5 Types of composite dressings Various types of composite dressings of differing absorbances for wound exudates have been patented and are commercially available. The major classiﬁ cation includes (a) island composite dressings (b) multi-layer and functional composite dressings.

11.5.1 Island composite dressings Island dressings consists of an absorbent layer sandwiched between semipermeable backing sheets as shown in Fig. 11.5. The absorbent pad is sub- stantially centrally disposed on an adhesive layer of greater dimension so that the free adhesive surface surrounds the periphery of the absorbent pad for securing the dressing to the skin. The backing sheet thus extends outwardly from the edges of the absorbent layer for the attachment of the dressing over a wound by adhesion to the skin surrounding the wound. US patent 656657719 discloses a low-adherence island dressing for a bleeding or weakly exuding wound. The wound dressing consists of an absorbent layer cover with a thermoplastic fi lm. The fi lm has a textured perforated surface on the front and a smooth perforated surface on the back. The back is covered with a semi-permeable backing sheet. This sheet is extended to the sides as an adhesive material. US patent 616880020 discloses the antimicrobial multilayer island dressing which includes an inner absorbent assembly having a fi rst layer comprising of a wound contacting non-absorbent, non-adhering porous polymeric fi lm, which is impregnated with an antimicrobial agent, a second layer comprising a semi-permeable continuous polymeric fi lm joined to

Semi-permeable backing sheet Absorbent layer Sandwiched Adhesive layer absorbent material Bacterial barrier layer

Non-adherent layer

Island dressing Island composite dressing 11.5 Schematic representation of island dressing and island composite dressing.

the ﬁ rst layer to form a sealed interior reservoir compartment, an absorbent material positioned within the interior reservoir compartment to collect discharged exudate from a wound, and an outer layer extending beyond the peripheral edges of the inner absorbent assembly, the outer layer having at least a portion coated with an adhesive material for adhering the island dressing to the wound area. US patent 560738821 discloses a liquid and pathogen impermeable island wound dressing that can be adhesively bonded to the skin surrounding a wound and is provided with a liquid and gas permeable exudate- absorbing pad, and a plurality of sequentially removable liquid and micro-organism impermeable, but gas and moisture vapor permeable cover sheets disposed adjacent to and covering the pad.

11.5.2 Multi-layer and functional composite dressing US patent 634842322 discloses a multi-layer wound dressing which includes a fi brous absorbent layer for absorbing wound exudate, an odor layer for absorbing odor and a barrier layer interposed between the fi brous absorbent and barrier layers. Preferably, the barrier layer is vacuum perforated and the fi brous absorbent layer is of highly absorbent fi bers capable of absorbing 25 g g−1 of exudate. US patent 487547323 discloses a multi-layer wound dressing that facilitates wound healing by creating hypoxia followed, after 3 to 72 h, by an aerobic environment. The dressing is made of (a) an outer layer of low oxygen permeability; (b) an oxygen-permeable inner layer, affi xed on one side to the outer layer; and (c) an adhesive applied to the other side of the inner layer. The adhesive may be applied in a continuous or discontinuous manner, and may be applied only around the perimeter of the dressing, leaving an adhesive-free window. The entire dressing is applied to the wound and creates a hypoxic environment until the outer layer of low oxygen permeability is removed after 3 to 72 h. The oxygen-permeable layer is left on to provide protection during a subsequent aerobic healing phase. An absorbent dressing containing a slow-release agent comprises a wound-contacting layer covered with tapered apertures. The bottom surface of the said tapered aperture contacts a wound area to discharge exudate from the wound area and transmit exudate via a guiding layer to an absorbent layer. The absorbent layer formed of high-molecular polymeric fi bers is mixed with a certain concentration of water-soluble agents, such as antiseptic agents, enzymes and growth factor agents, in a suitable proportion. After the exudate passes into the absorbent layer, the polymeric fi bers expand becoming gel-like and forming into a shape that prevents exudates fl owing backward to the wound area. Also, a

translucent evaporating layer having numerous micro-pores for air venting is above the absorbent layer and the peripheral edges are joined together in the form of a sealed structure by heat-sealing to stop side escape of exudate, which is more effective in preventing secondary infection in the wound area.23 CarboFlexTM is a non-adhesive multi-layer dressing. The fi rst layer is an absorbent wound contact layer consisting of fi bers of AquacelTM and KaltostatTM. This layer absorbs and retains exudate, so the problem of maceration and excoriation is reduced as the retention of exudate controls the lateral wicking. As this layer forms a soft gel, a moist environment is maintained and so should allow pain-free removal of the dressing. The second layer is an ethylene methyl acrylate (EMA) fi lm that contains one-way water-resistant micro-valves, which delay strike-through to the charcoal layer. This mechanism prolongs the action of the odor-absorptive properties of the dressing. The third layer is a black activated charcoal cloth for the absorption of odor. The fourth layer consists of a soft non- woven absorbent pad that will absorb any excess exudate, thus delaying the strike-through and providing extra softness to the dressing for patient comfort. It is also thought that patients may dislike the appearance of a black dressing, so this layer makes it esthetically pleasing to patients. The fi nal fi fth layer is another layer of ethylene methyl acrylate (EMA) fi lm, which again delays strike-through of exudate and makes the dressing soft, smooth and water resistant.9 CarbonetTM is another multi-layered, low-adherent, absorbent deodorizing dressing. The fi rst layer is a TricotexTM wound contact layer that protects any granulation tissue in the wound bed and allows pain-free removal of the dressing. The middle layer is an absorbent layer of MelolinTM fl eece, that absorbs exudate, and the top layer is made of activated charcoal cloth.9 The Triosyn T40TM Antimicrobial Wound Dressing is a sterile, primary wound dressing. It is a multi-layer composite dressing consisting of an absorbent polyester non-woven pad, a permeable adhesive, a single layer of Triosyn iodinated resin beads, and a non-adherent high-density polyethylene (HDPE) mesh.

11.6 Trends in composite dressings: embroidery technology Embroidery technology is being widely used for medical textiles and tissue engineering. Non-healing wounds require intensive wound care for a very long time. In textile-based implant materials, tissue formation and vascularization depend on the size and distribution of pores and ﬁ bers.

11.6 Embroidery technology for wound dressing.23

An arrangement of pores of different orders of magnitude will favor tissue growth and the formation of new blood vessel and capillaries. St. Gallen, Switzerland have developed a wound dressing TISSUPOR® as shown in Fig. 11.6 based on embroidery for treatment of chronically non-healing wounds. Embroidery technology allows a 3-dimensionally structured textile architecture to be achieved that combines pores for directed angiogenesis and elements for local mechanical stimulation of wound ground. In TISSUPOR pad, the layers of dense fabric and spacer fabric are made of polyester and the superabsorbing material used is polyacrylate.24 Wound dressings based on using woven fabric have the disadvantage that it has a hard surface, which adapts poorly to the wound. For this reason many wound dressings are made up of knitted structures which are soft because of the movements of the threads within the interfacing. However, the disadvantage is that they harden because of exudates emerging from the wound and thus lose their fl exibility. Mono-fi laments, multi-fi laments or mixtures of these can be used in the embroidery process. The advantage of embroidery over knit structure is that the thread cannot move in the interfacings. The mechanical properties of the embroidery are defi ned by the arrangements of the interfacings and hardly affected by the incorporation of exudates or extracellular matrix into the thread, which leads to interfacings sticking together. In knitted structures, the mechanical properties are mainly defi ned by the moveabil- ity of the thread in the open interfacing. Thus, adhesive exudate leads to an increase in the rigidness of the textile in some circumstances far more than an order of size. Stiffness can cause local loading conditions, which can lead to local tissue necrosis.25

11.7 Conclusions A composite dressing is a multilayered product, which is a combination of different textile structures such as woven, non-woven, knitted, net ﬁ lm and spacer fabric. Each layer has its distinct property to enhance the wound healing process. Research and new developments in composite dressings are mainly focused in enhancing the functionality of its different layers so as to promote rapid wound healing, reduction in pain associated with wound treatment and all this at a reduced cost. The improvements in non- adherent, absorbent and bacterial barrier layers can thus reduce the frequent changes of the dressing, which helps improve wound healing.

11.8 References

1. w paul and c p sharma, Trends in Biomaterials and Artifi cial Organs, 18(1) (2004) 18. 2. r f diegelmann and m c evans, Bioscience, 9 (2004) 283. 3. v jones, j e grey and k g harding, British Medical Journal, 332 (2006) 777. 4. n f s waston and w hodgkin, Surgery, 23(2) (2005) 52. 5. b griffi ths, e jacques and s bishop, US Patent No. 6458460 (2002). 6. tri-centurian, Region A/B DME PSC Bulletin September 2006, www. tricenturian.com. 7. c thomas, Nursing 2000, 30(5) (2000) 26. 8. s thomas (1990) Primary wound contact material, In Wound Management and Dressing, London. The Pharmaceutical Press. 9. d morgan (2000), Formulary of Wound Management Products 8th Ed. Haslemere Euromed Communication Ltd. 10. p thomas davies, US Patent 3006388 (1961). 11. c l eldredge and k j petters, US Patent 3285245 (1966). 12. g w cummings and r cummings, U S Patent 5910125 (1999). 13. r punder, JCN online 15(8) (2001). 14. h karami and r f vitaris, US Patent 5167613 (1992) 15. h a patel, US Patent 5465735 (1994). 16. d c scully and c mccabe, US Patent 6077526 (2000). 17. l a errede, j d stoesz and g d winter, US Patent 4373519 (1981). 18. w andrews, l hammett and c robert, US Patent 5437621 (1993). 19. d addison, j s mellor, m w stow and m c biott, US Patent 6566577 (2003). 20. j a dobos and r d mabry, US Patent 6168800 (1998). 21. r ewall, US Patent 5607388 (1994). 22. b griffi ths, e jacques and s bishop, US Patent 6348423 (1999). 23. o m alvarez, US Patent 4875473 (1986). 24. e karamuk, m billia, b bischoff, r ferrario, b wagner, r mose, m wanner and j mayer, European Cells and Materials, 1 (2001) 3. 25. tissupore ag, US Patent 6737149 (2004).

M. KUN, C. CHAN and S. RAMAKRISHNA, National University of Singapore, Singapore

Abstract: The principles of tissue engineering are described and the properties required for ﬁ brous scaffolds are discussed. A review is presented of the various types of textile structure and common fabrication technologies, with a wide range of examples of tissue- engineered scaffolds for repair or replacement of various organs or tissues.

Key words: tissue engineering, textile scaffolds, nanoﬁ bers, stem cells.

12.1 Introduction: principles of tissue engineering Tissue engineering is the application of engineering and life sciences principles in the development of biological substitutes for restoration, maintenance, or improvement of tissue function or a whole organ (Lanza et al. 2000). There are three essential components as shown in Fig. 12.1. Firstly, a porous matrix or scaffold, preferably biocompatible, i.e. absorbable, is required to serve as an extracellular matrix (ECM) support structure for self-organization of cells. Secondly, various cell types including autologous, allogeneic, xenogeneic cells or cell lines can be cultured on scaffolds for proliferation, differentiation or other biomedical purposes. Lastly, key biomolecules such as growth factors, differentiation factors or other cytokines can also be incorporated into the scaffolds for the desired molecular cues or cellular signals imparted to the seeded cells. Besides the above-mentioned triad, various tissue engineering parameters, such as two-dimensional (2D) cell expansion, three-dimensional (3D) tissue formation, in vitro cell culture conditions, e.g. static, stirred or dynamic fl ow conditions with or without bioreactors, can be manipulated to enhance the performance of the biological functions of the engineered tissue in an artifi cial matrix. This chapter is divided into six sections. The fi rst section provides a brief overview of principles of tissue engineering. The second section is concerned with the major functions, specifi c requirements and current materials used for fi brous scaffolds as well as the relationship between

289

Cells

A scaffold Cell–matrix interaction: attachment, migration, proliferation, differentiation Regulators: growth factors, cytokines, etc.

A triad 12.1 Elements of tissue engineering (a triad): cells, a scaffold/matrix and regulators.

textile architecture and cell behaviors. The third section presents various types of textiles classiﬁ ed by fabrication methods and intended applications. The fourth section covers an ever-expanding range of applications of tissue engineering for replacement and repair of various organs or tissues. This section also introduces several novel smart scaffold devices. The ﬁ fth section offers insight into new trends and future directions for tissue engineering developments. The last section is a survey of some important sources of information and references.

12.2 Properties required for ﬁ brous scaffolds Of the three key components in tissue engineering, scaffolds is the one that can be manipulated to the greatest extent. A 3D scaffold similar to natural ECM topography is considered to be a critical component for a successful tissue engineering strategy. The desirable scaffold is a regeneration substrate with special properties such as: • architectures and functions that are similar to natural ECM; • a pore size within a critically deﬁ ned range; and • a degradation rate that matches the rate of tissue regeneration at the host bed.

12.2.1 Functions of ﬁ brous scaffolds Living cells in tissue engineering are generally seeded into an artiﬁ cial architecture capable of retaining cells and guiding their growth and tissue

regeneration in three dimensions. This artifi cial ECM plays an important role in mimicking the in vivo milieu to allow cells to interact in their own micro-environments. The fi brous scaffolds fabricated may have a branched confi guration extending outwardly from a central stem (Vancanti et al. 1998), and serves at least one of the several purposes. Firstly, fi brous scaffolds with hierarchical structures can provide a superior surface area for cell attachment, migration, proliferation and differentiation. Secondly, they can also serve as a carrier for biochemical factors to deliver the biochemical factors to the target organs for therapeutic purposes. In addition, these highly porous fi brous scaffolds allow free entrance of vital cell nutrients such as oxygen and cell growth factors, and allow easy diffusion of secreted biomolecules during cell growth. Recently, scaffolds fabricated from nanofi bers (NF), referred to here as fi bers with diameters below 1000 nm, are gaining increasing attention. It is being recognized that nanofi bers when used as scaffold materials in tissue engineering exhibit superior functions when compared with macrofi bers. Some of the attributed advantages are as follows (Li et al. 2002, Li et al. 2007): 1. The high surface area-to-volume ratio of nanofi brous scaffolds has enhanced absorption of adhesion molecules such as vitronectin and fi bronectin which are important for cell adhesion to the scaffold. 2. Nanofi bers being smaller by two-orders than a cell create a 3D environment that resembles extracellular matrix (ECM) favorable for cell interaction. 3. Nanofi brous scaffolds provide a favorable environment for proliferation and maintenance of certain cell phenotypes. 4. Stem cells maintained in nanofi brous scaffolds can be induced to differentiate into different cell lines, thus offering the possibility of engineering complex tissue consisting of different cell lines starting from a single stem cell line. 5. It appears that nanofi brous scaffolds can provide physical as well as spatial cues that are essential to mimic natural tissue growth.

12.2.2 Specifi c requirements for fi brous scaffolds To achieve successful tissue restoration and organ functions, a fi brous scaffold must meet certain fundamental criteria. First, the pore size must be adequate and the porosity suffi ciently high to facilitate cell seeding and allow for an effi cient exchange of cell nutrient and metabolic waste between the adherent cells and the host bed. Cells residing in a fi brous scaffold are capable of amoeboid movement pushing the surrounding fi bers aside and thus expanding pores within the scaffold. In this way, fi brous scaffold offers

cells the opportunity to optimally adjust the pore diameter and migrate into areas where some of the initial pores are relatively small (Li et al. 2002). The relationship between pore diameter and tissue in-growth will be discussed in the section 12.4. In addition, mechanical properties of a fi brous scaffold is another contributor to its success in tissue engineering because a scaffold not only provides a substrate for cell residence but also assists to maintain the mechanical stability at the defect site of the host (Li et al. 2002). An effective tissue-engineered scaffold has to meet at least two mechanical requirements. First, it must be suffi ciently stable for a physician to handle and implant the scaffold into the target site of the host. Second, after transplantation the architectures of the scaffold in the process of biodegradation need to provide suffi cient biomechanical supports during tissue regeneration (Li et al. 2002). The mechanical properties of an engineered scaffold are determined not only by intrinsic factors like chemical compositions of the material, but also by extrinsic factors, such as construction geometry or architectural arrangement of the building blocks (Li et al. 2002). For certain applications such as vascular grafts involving pulsatile stress, there will be special requirements in terms of compliance and elasticity of the materials (Wang et al. 2005). If the graft’s compliance and distensibility are not met, blood fl ow disturbance would occur and endothelial injuries would be caused by the increase in mechanical stress near the anastomotic sites (Doi et al. 1997). However, incorporation of elastic fi bers or polymers in scaffolds for vascular grafts will enhance the compliance of the scaffolds to hemodynamic pressure (Wang et al. 2005). Last but not least, biodegradability is an important consideration for selecting a scaffold material. Ideally, a biodegradable scaffold is absorbed by the surrounding tissues and metabolized in the body after fulfi lling its intended purposes. For example, poly-(dl-lactide-co-glycolide) (PLGA) is a widely used biomaterial that can hydrolyze into monomers of lactide and glycolide, which subsequently break down into water and carbon dioxide through the Krebs cycle (Bazile et al. 1992). The degradation rate should match the rate of tissue formation. This means the residing cells should proliferate, differentiate and build up ECM suffi cient for tissue reconstruction while the scaffold materials gradually degrade over time. An ideal skin substitute is expected to have certain characteristics:

1. Easy to manipulate for transplantation and readily adherent to wound sites. 2. Impervious to bacteria, but porous for nutrients and oxygen diffusion. 3. Sterile, non-toxic and non-antigenic.

4. Have physical and mechanical properties appropriate to wound coverage. 5. Have a controlled degradation rate. 6. Ability to regain lost skin functions, including sensitivity, elasticity, normal physiological function and pigmentation. 7. Assimilated into the host with minimal scarring and pain. 8. Readily vascularized. The ultimate goal of the skin tissue engineering is to satisfy most if not all of the criteria, and meeting these criteria can be better achieved with novel, smart skin substitutes (Metcalfe et al. 2007).

12.3 Materials used for scaffolds Currently, there are a variety of biomaterials suitable for the fabrication of scaffolds. Biomaterials can be defi ned as any biocompatible substances other than those used for food or drugs (Peppas et al. 2007). Owing to their functional properties and design fl exibility, polymers are the primary class of materials for fabrication of scaffolds (Murugan et al. 2006). The polymers used in scaffold fabrication can be classifi ed into two groups: naturally derived polymers and synthetic polymers. The naturally derived polymers are biodegradable, such as collagen, dextran, elastin, fi brin, hyaluronic acid, fi bronectin, polypeptides, hydroxy- apatites, chitosan, and glycosaminoglycans (GAGs) (Metcalfe et al. 2007, Pellegrini et al. 1999, Ronfard et al. 2000, Feng et al. 2006). Such materials have the advantage that they facilitate biological recognition and provide a better environment for tissue regeneration. However, rapid absorption and weak mechanical strength often compromise effi cient applications of natural biomaterials. They may also elicit infl ammatory and allergic responses. Amongst the naturally derived polymers, collagen elicits the least immunological response. Synthetic polymers can be classifi ed further as either biodegradable or non-biodegradable. Biodegradable polymers are of great interest in the development of tissue engineering. Synthetic polymers such as poly- l-lactide (PLA), polyglycolide (PGA), polycaprolactone (PCL) and polyglactin are biodegradable. The degradation rate of these synthetic polymers varies greatly, with PGA degrading in weeks whereas PCL may take up to several years. As the seeded cells begin to break down the scaffold, the surface of the polymer is continuously replaced by ECM secreted by the cells. This is a dynamic process of cell–substrate interactions (Lanza et al. 2000). Recently, copolymers such as PLGA and poly-(l-lactic acid-co-ε-caprolactone) (PLLA-CL) are increasingly used for fabricating tissue-engineered scaffolds or as implants because the

degradation rates can be readily controlled by adjusting the ratio of their components. Another advantage of synthetic polymers lies in their potential to be produced in large quantities with controlled properties in terms of strength, degradation rate, and microstructure. However, compared with natural polymers, synthetic polymers lack biological recognition signals for cellular interaction. Current strategies to address this problem include immobilization of naturally derived polymers on the surface of synthetic polymer matrix or simply physically blending a natural polymer with a synthetic polymer before fabrication. This is to improve biocompatibility of the hybrid scaffold while preserving their mechanical strength (Feng et al. 2006, He et al. 2005).

12.4 Relationship between textile architecture and cell behavior Cellular responses to ﬁ brous scaffolds as discussed previously are inﬂ u- enced by various characteristics of the scaffold including surface chemistry and texture. The following properties are of particular importance:

12.4.1 Scaffold topography Micro-architecture of a porous scaffold is known to affect cell behaviors. For example, from 5–500 nm to 7–10 μm, topographic alterations on 2D substratum can result in different cell responses (Curtis et al. 2007, Massia et al. 1991, Sun et al. 2006). In addition, it is well documented that cell responses to 2D and 3D environment are quite different. Compared with 2D substrates, cells loaded within a 3D matrix display enhanced biological activities and a lower requirement of integrin usage for cell adhesion (Cukierman et al. 2001). Other reports also observed that epithelial cells and fi broblasts are more likely to proliferate in 3D than 2D culture conditions (Sanders et al. 2003b, Sanders et al. 2000). This is attributed to the 3D environment being similar to natural ECM in terms of structural dimension. Fiber orientation within a fi brous scaffold is also found to affect cellular behavior. In our studies, smooth muscle cells (SMCs) tended to orientate and elongate themselves along the alignment of the fi bers and to express a spindle-like contractile phenotype, as shown in Fig. 12.2 (Xu et al. 2004a).

12.4.2 Fiber diameter Cells are able to organize around the ﬁ bers with diameters smaller than themselves (Pham et al. 2006). Sanders and co-workers showed that, for

200.00 μm

12.2 Laser scanning confocal microscopy image of immunostained α-actin ﬁ laments in SMCs after one day of culture on aligned PLLA-CL nanoﬁ brous scaffold (Xu et al. 2004a).

fi ve weeks after polypropylene fi bers were implanted in the subcutaneous tissue in the dorsum of rats, small fi bers with diameters less than 6.0 μm had signifi cantly less macrophage density than those fi bers between 6.0 and 27.0 μm in diameter, which had substantial fi brous encapsulation, a sign of a foreign-body reaction (Sanders et al. 2000). This may be because of the reduced cell–material contact surface area or due to a curvature threshold effect that triggers certain cell signaling (Sanders et al. 2000). Induction of a fi brous layer between a scaffold implant and host soft tissue can create unstable mechanical coupling at the material–tissue interface. This can trigger local stress responses and fi brous capsule formation. It has been shown that the threshold value of microfi bers to form fi brous capsules is 5.9 μm in diameter (Sanders et al. 2003a). One explanation why fi bers with diameters above this threshold value can elicit capsule formation is that the larger fi bers may separate from collagen fi bers in ECM, thus creating dead space regions adjacent to themselves, and recruit infl ammatory cells to stimulate fi brous capsule formation (Sanders et al. 2003b).

12.4.3 Scaffold porosity Porosity is a measure of void spaces in a material. The volume of void spaces and the distribution of pore size are two important parameters that characterize the scaffold porosity. Of particular importance in tissue engineering is the inter-connectivity of the pores. Most studies have focused on the effect of the pore size.

A scaffold with a high porosity tends to have a large surface area for cellular attachment and formation of multiple focal adhesion points on the interconnected fi bers (Pham et al. 2006). The relationship between pore diameter and tissue ingrowth has been extensively investigated. For percutaneous implants, it was reported that epithelial migration was signifi cantly improved when pore size is 0.025–3 μm (Squier et al. 1981). For corneal implants, stratifi cation was more often observed for materials with a pore size of 0.1–0.8 μm than for 0.8–3.0 μm (Dalton et al. 1999, Evans et al. 1999, Fitton et al. 1998). In addition, a pore size of 40–50 μm was shown to facilitate migration of endothelial cells cultured on vascular grafts (Hermens et al. 1995, Takahisa et al. 1996). Another study indicated that the optimal pore dimensions for encouraging neovascularization is 0.8–8.0 μm (Brauker et al. 1995), whereas for effective osteoblast ingrowth for bone graft, the optimal pore diameter is 75–150 μm (Desai, 2000). The porosity of fi brous scaffolds can also affect the degradation rates. Highly porous fi bers degrade more slowly because acidic by-products secreted by cells would be removed from the implanted sites at a faster rate (Wang et al. 2005). Porosity, fi ber diameter and mechanical strength are inter-related parameters for nanofi brous scaffolds. It has been shown that a decrease in the porosity of a nanofi brous scaffold is associated with a decrease in fi ber diameter and an increase in mechanical strength and density (Wang et al. 2005).

12.4.4 Surface property Surface properties of a fi brous scaffold, such as surface hydrophobicity, charge, and roughness, are largely dominated by free functional groups of the materials used in fabricating the scaffolds. Chemical composition and biological functions of these materials play an important role in determining cell–scaffold interactions. Hydrophobic surfaces are usually considered initiators of the foreign- body reaction and reduced biocompatibility (Sanders et al. 2003a). There- fore, a preferred current approach for scaffolds that are constructed from hydrophobic synthetic materials is to either incorporate hydrophilic natural materials or to modify the surface of the material to make it less hydrophobic. For instance, it was reported that grafting concentrated hydrochloric acid on a PGA fi brous scaffold enhanced the surface hydro- philicity of the scaffold, indicated by increased attachment and proliferation rates of rat cardiac fi broblasts cultured on it (Boland et al. 2004). Similarly, incorporation of chitosan in fi brous scaffolds was used in skin

tissue engineering because chitosan can improve the biocompatibility of the scaffold and have wound-healing effects and anti-microbial properties as well (Wang et al. 2005). Some in vitro studies have shown the effects of surface charge of scaffolds on cell adhesion and expansion. The proliferation rate of fi broblasts is greater on positively charged copolymers such as hydroxyethyl methacrylate and amine-containing N,N-dimethylaminoethyl methacrylate hydrochloride; hydroxyethyl methacrylate and trimethylaminoethyl methacrylate chloride than on negatively charged copolymers (such as methacrylic acid and hydroxyethyl methacrylate) or on a neutral homo- polymer such as hydroxyethyl methacrylate (Hattori et al. 1985). Another group demonstrated that the preferred scaffold surface charge for endothelial cells is in the following order: a positively charged hydroxyethyl methacrylate copolymer (with trimethylaminoethyl methacrylate HCl) > tissue culture polystyrene (TCP) (slightly negative charge) > a negatively charged hydroxyethylmethacrylate copolymer (with methacrylic acid) (Vanwachem et al. 1987). However, this is not always the case. The interaction between seeded cells and a scaffold surface can vary with different surface materials. For example, the proliferation rate of endothelial cells on either negatively charged or positively charged methylmethacrylate copolymers (with methacrylic acid and trimethylaminoethyl methacrylate HCl salt, respectively) was observed to be higher than that on the TCP surface. In contrast, positively charged hydroxyethyl methacrylate copolymer (with trimethylaminoethyl methacrylate HCl) was found to support attachment of endothelial cells, while the same copolymer, when being negatively charged (with methacrylic acid), would not facilitate endothelial cell adherence (Vanwachem et al. 1987). Subtle differences in the surface roughness of fi brous scaffolds can affect cellular responses. It has been observed by seeding human coronary artery endothelial cells (HCAECs) on a smooth solvent-cast fi lm and on an electrospun PLA nanofi ber mesh that there is an inverse relationship between surface roughness of substrates and adhesion and proliferation rates of HCAECs. Cells on the smooth fi lm exhibited round morphology and can organize into capillary-like microtubes, which is the expected functional phenotype. In contrast, cells on the nanofi ber mesh have an undesired spread-out morphology (Wang et al. 2005). However, different cell types have different preferences for surface roughness. Another group reported that the attachment and expansion of human umbilical vein endothelial cells were signifi cantly enhanced when the surface roughness was increased. This roughness was created by grafting polyethylene glycol (PEG, mol. wt. 2000) and a cell surface peptide (GRGD) onto the polyurethan (PU) surface (Chung et al. 2003). Similarly, vascular smooth

muscle cells have lower cell adhesion and proliferation on the smooth PLA surfaces (Xu et al. 2004b). However, each of the above-mentioned properties is only a component factor rather than an exclusive determinant for directing cell–matrix interactions. It should also be kept in mind that the overall effects of ﬁ brous scaffolds on behaviors of their resident cells can be tailored by adjusting these component factors.

12.5 Textiles used for tissue scaffolds and scaffold fabrication A multitude of research work on various types of optimum scaffolds for tissue engineering has been carried out in the last decade. According to processing methods, these scaffolds can be broadly categorized into three groups: (1) foams/sponges, (2) 3D printed substrates/templates, (3) textile structures (Ramakrishna 2001). Textile structures form an important class of porous scaffold in tissue engineering.

12.5.1 Textile structures for medical application Textile structures including non-woven, weave, braid and knit have been applied in the medical ﬁ eld for many years, from the initial uses, in sutures, wound gauze, plasters, vessel prosthesis and hernia nets, etc., to current applications, in vascular implants, artiﬁ cial liver, tendons, skin and other vital organs or tissues. Table 12.1 shows a wide range of applications of textile structures in tissue-engineered scaffolds.

Microstructural aspects of textile structures Textile structures can be customized to give the required porosity in terms of size, amount, and distribution pattern. Table 12.2 lists the microstructural aspects of different textiles and their wide uses in healthcare- related fi elds. The porosity of scaffolds is governed by the construction of textile fabrics. A typical textile scaffold shows three levels of porosity that can be selectively controlled. The fi rst level is the inter-fi ber gap that can be controlled by changing the number of fi bers in the yarn and the yarn- packing density. The gap between yarns forms the second level of porosity. For knitted scaffolds, variations in stitch density and stitch pattern can affect this level of porosity, whereas in the case of woven scaffolds, the porosity can be changed by controlling the inter-yarn gaps through a beating action. The third level of porosity is created by subjecting the textile structures to secondary operations including crimping, folding, rolling and stacking.

© 2009 Woodhead Publishing Limited Textile-based scaffolds for tissue engineering 299 -caprolactone), -caprolactone), ε -lactic acid-co- -lactic l Textile w) (n, Textile (n), foam (n), Textile k),Textile foam w, (n, Textile y) (n, Textile foam (n), (porous membrane) Foam,Textile 3D (n), printed Scaffold structures: yarn (y), weave weave Scaffold (y), structures: yarn braid(w), knit (b), (k), non-woven (n) Textile (n) Textile (n) Textile (n, w, b, k)Textile w, (n, foam Textile (n), Textile (y, b, n, k), foamTextile (y, -lactide), PLLA-CL: poly( PLLA-CL: -lactide), l uoroethylene, PDMS: poly(dimethyl siloxane), PNIPAAM: * + hydroxyapatite; polyethylene brin, polyester, copolymer of PDMS and PNIPAAM Foam chitosan, alginates viscose rayon PLGA (Vicryl), PLLA-CLPLGA PGA PGA, collagen-glycosaminoglycan; PLGA, nylon, chitin/ PGA; PETE; silk Collagen; PETE; polyethylene; PGA; PLGA; PLA; PLGA (Vicryl) Collagen, fi Collagen, PGA; PLLA; PLGA Examples of scaffold materials scaffold of Examples PGA Polyester(Dacron); PETE; polyurethane; PGA; PTFE; PLA; PGA; PLLA; PLGA PGA; PLA; PLGA; polyorthoesters; PLGA; Polyanhydride; Collagen-glycosaminoglycan; PGA -lactide-co-glycolide), PGA: poly(glycolide), PLA: poly( poly(glycolide), PGA: -lactide-co-glycolide), dl Various scaffolds used in tissue engineering 2001) (Tao, -isopropylacrylamide) N PLGA: PLGA: poly( Heart valve Ligament Liver Skin Tendon Nerve Dental Cornea Table 12.1 * Tissue engineered engineered Tissue substitutesbiological Bladder Blood vessel Cartilage PETE: polyethylene terephthalate, PTFE: polytetrafl Poly( Bone © 2009 Woodhead Publishing Limited 300 Advanced textiles for wound care cal tendons bers artifi and ligaments, stents, compression bandages bending properties current of biodegradable fi Knitted 50–1000 40–95 Uniform Good Good to excellent to Good Excellent Vascular implants, Limitations of low cal ligaments, ligaments, cal sutures, vascular vascular sutures, implants uniform cross- shapes sectional Uniform Excellent 0.5–1000 30–90 Excellent Excellent Artifi Only tubular or Braided vascular implants, dressing, plasters, tissue engineering hospital scaffolds, bedding and uniforms Excellent Uniform Woven Excellent 0.5–1000 30–90 Excellent Surgical gowns, Limited shape cal cal incontinence pads, nappies, sanitary artifi wear, ligment, tissue scaffoldengineering questionable control porosity over Good 10–1000 40–95 Poor High equipment cost, m) μ Microstructural aspects and medical applications of textile scaffolds 2001) (Tao, porosity Porosity (%) Porosity Pore distribution Random Reproducibility of applicationsMedical gowns, Surgical Pore connectivityPore Processability Good Others Table 12.2 Non-woven Pore size ( © 2009 Woodhead Publishing Limited Textile-based scaffolds for tissue engineering 301

Table 12.3 Mechanical properties of textile scaffolds (Tao, 2001)

Non-woven Woven Braided Knitted

Strength Low High High Low Stiffness Low High High Medium Structural Poor to Excellent Excellent Poor to good stability good Others Isotropic Anisotropic Anisotropic, with The behavior can behavior behavior good properties be tailored from in axial anisotropic to direction and isotropic poor properties in transverse direction

Mechanical properties of textile structures For certain applications, the scaffold is more than a simple vehicle for cell delivery. It must maintain its structural integrity and a certain amount of load-carrying capacity for the desired amount of time, for example in bone, cartilage and ligament reconstructure (Yang et al. 2001). Table 12.3 compares the mechanical properties of various textile structures. In general, non-woven fabrics are webs of non-aligned ﬁ laments allowing for the largest variation in pore characteristics, while woven fabrics possess a dimensionally stable structure, characterized by pores of regular size and shape. Warp-knitted fabrics, in which threads are laid along the direction of fabric production, have been proven to be suitable for the demand of high elastic deformation, for example, in vascular grafts applications, the grafts are required to resist continuous dynamic stress. Braided fabrics are formed by interlacing three or more threads, so that threads cross one another in a diagonal formation. They can be made either ﬂ at or tubular to meet for different purposes (Mather, 2006, Wollina et al. 2003). Com- pared with woven or knitted fabrics, the braided textile composites can better resist twisting, shearing and impact. However, they show their poor stability under an axial compression (Wollina et al. 2003, Tan et al. 1997).

12.5.2 Technologies for fabrication of textile scaffolds Embroidery technology Recently, embroidery technology in which thread direction can be arranged at almost any angle, has elicited great interests in the design and fabrication

of tissue-engineered scaffolds. The process involves a needlework pattern of sewing reinforcing ﬁ bers onto a ground material. The arrangement of these reinforcing ﬁ bers is computer controlled and, thus, almost any desired textile forms can be produced. Various materials including woven and non- woven fabrics or foils mentioned above can be used as the ground material. By stacking the embroidery, a multi-layered construct can be built to form 3D textile scaffolds (Ramakrishna et al. 2001). Custom-made embroidery technology has been widely employed to design and construct tissue-engineered scaffolds. For example, Ellis and co-workers have successfully developed hernia patches, stents for the repair of abdominal aortic aneurysms and implants for intervertebral disc (Ramakrishna et al. 2001). Another group has developed a 3D embroi- dered substrate for the improved regeneration of skin tissue in chronic wounds. Besides, Mai et al. (2006) observed that osteoblast-seeded poly- 3-hydroxybutyrate (PHB) embroidery can successfully induce ectopic bone formation. Embroidery technology is particularly attractive for making tissue-engineered scaffolds. The degree of construction of the embroidery can be controlled and, therefore, a broad spectrum of scaffolds with a wide range of properties is possible.

Technologies for fabrication of textile nanofi bers The processing of fi bers with diameters less than 1000 nm plays an increasingly important role in the construction of tissue-engineered scaffolds. Several fabrication techniques such as electrospinning, phase separation, melt-blown, template synthesis and self-assembly have been used to produce suitable polymer nanofi bers for various purposes (Zhang et al. 2005). Among the techniques mentioned above, electrospinning is the most commonly used method to fabricate nanofi bers because it is relatively easy to set up in the laboratory and the resultant scaffolds have a large surface area-to-volume ratio and interconnected pores. Synthetic and natural polymers, as well as ceramics have been electrospun into nanofi bers. The fabrication process involves an electrostatic fi eld of the order of 5–30 kV between a collector and the spinneret (usually in the form of a needle). The polymer melt or solution is pumped out of the spinneret at a controlled rate. When the suffi ciently high electric fi eld overcomes the polymer solution surface tension, a thin jet of liquid fl ies towards the collector plate. The thin jet of liquid solidifi es owing to evaporation of the solvent or cooling of the molten polymer and nanofi bers are collected on the collector. The fi ber morphology is controlled by adjusting the electrospinning conditions, such as applied voltage, feed rate, types of collector, diameters of needle (spinneret) and distance between the needle tip and the collector. It is observed that the shorter the distance between needle tip and

collector, the more beads are formed along the fi bers. It has been reported that increased voltage correlates with increased beads density and at an even higher voltage, the beads will join to form a thicker diameter fi ber (Demir et al. 2002, Zong et al. 2002, Megelski et al. 2002). For a given voltage and a given needle–collector distance, reducing the internal diameter of the needle orifi ce or reducing the feed rate often leads to a decrease in the fi ber diameter or size of beads. In addition, the type of collectors will infl uence the morphology of the collected nanofi bers. For instance, collectors made from conductive materials such as aluminum foil are observed to have a higher fi ber-packing density than non-conductive ones. Thus, nanofi bers collected on a non-conductive plate are more likely to form a 3D structure, such as honeycomb architecture, owing to the repulsive forces of the accumulated charges on the non-conductive collector (Deitzel et al. 2001). Additionally, experiments on collectors with smooth surfaces such as metal foils showed a high fi ber-packing density compared with collectors with porous surfaces such as metal mesh. Moreover, the texture of the fi bers formed can also be varied by using a patterned collector like a braided Tefl on sheet, on which the yield fi bers take a topography that follows the surface pattern of the Tefl on sheet (Ramakrishna et al. 2005). Additionally, whether or not the collector is static or moving may have an effect on the morphology of fi bers. A rotating cylinder collector can be used to obtain unidirectional nanofi bers with alignment along the rotating direction. Other ambient parameters that would infl uence the evaporation rate of the solvent, including concentration of the polymer solution, temperature and humidity, also infl uence fi ber morphology. By varying those parameters synergistically, several special morphologies can be achieved, for instance, porous nanofi bers (Bognitzki et al. 2001), fl attened (Koski et al. 2004), ribbon-like fi bers (Huang et al. 2000), helical fi bers (Kessick et al. 2004) and hollow fi bers (Kessick et al. 2004, Hou et al. 2002). Nanofi bers have an extensive application in tissue engineering.

12.6 Applications of textile scaffolds in tissue engineering The range of tissue engineering applications has expanded in recent years. Some typical examples, with special emphasis on the use of textile scaffolds, are described here.

12.6.1 Skin grafts Skin grafts are perhaps the most successful tissue-engineered constructs and several have been approved by the US Food and Drug Administration

(FDA) and commercially manufactured. For example, in 1998, Advanced Tissue Science, Inc. introduced Dermagraft®, a cryopreserved dermal substitute, in which human fi broblast cells derived from newborn foreskin tissue were seeded on a biodegradable polyglactin mesh scaffold. The fi broblasts were found to proliferate and fi ll interstices of the scaffold and secrete dermal collagen, matrix proteins, growth factors and cytokines suitable as dermal substitute. Some commercialized skin graft substitutes employ natural ECM molecules in their fi brous scaffolds, such as collagen (BiobraneTM, IntegraTM, AllodermTM), fi brin (BioseedTM), hyaluronic acid (LaserskinTM), fi bronectin (TransCyteTM), and glycosaminoglycans (GAGs) (TransCyteTM). Although there have been considerable improvements in tissue- engineered skin grafts, none of them could reproduce the normal architecture of natural skin, including hair follicles, Langerhans cells, sebaceous glands, and sweat glands. Therefore, intensive research is still ongoing in order to improve existing skin graft substitutes to regenerate the important properties of natural skin.

12.6.2 Vascular grafts An important requirement for tissue-engineered vascular grafts is that the tubular conduits are made of materials capable of incorporating into host tissues with a functioning self-renewing endothelial layer. Scaffold materials can be either synthetic or natural polymers. Synthetic materials for fabricating vascular grafts include biodegradable polymers such as PGA, PLA, PLGA, or non-biodegradable polymers such as polyurethanes (Kim et al. 1998, Tiwari et al. 2002, Niklason et al. 2001) and polyethylene terephthalate (DacronTM). Biological materials that have been used in vascular grafts can be generally classiﬁ ed into three groups: acellular xenogeneic grafts (Clarke et al. 2001, Lantz et al. 1992, Huynh et al. 1999), acellular allogeneic grafts (Conklin et al. 2002, Schaner et al. 2004, Teebken et al. 2000, Kaushal et al. 2001, Yow et al. 2006) and prefabricated grafts made with natural polymers (Yow et al. 2006, Weinberg et al. 1986). Major disadvantages of natural material-based scaffolds are their rapid absorption rate and poor mechanical strength. To improve both biocompatibility and mechanical strength, many novel grafts use a combination of synthetic and biological materials. For example, a collagen-coated PLLA- CL (70 : 30) nanoﬁ ber vascular conduit is shown in Fig. 12.3. It demonstrated in such a construct that there are enhanced attachment, proliferation and viability of human coronary artery endothelial cells (HCAECs) and, more importantly, the cellular phenotype is preserved (He et al. 2005). Traditional problems associated with vascular grafts, including clotting and scar tissue formation, is still a problem for current vascular grafts and

12.3 Electrospun PLLA-CL nanoﬁ brous vascular conduit (length 3.8 cm, diameter 3 mm) (He et al. 2005).

this has prevented new grafts from entering clinical trials (Sanders et al. 2000). Recently the US FDA has approved a vascular graft (Gore Pro- paten), made of expanded polytetraﬂ uoroethylene (ePTFE)-heparin for treatment of peripheral artery disease. The addition of heparin to the luminal surface of the graft via proprietary end-point covalent bonding was intended to reduce the occurrence of thrombosis in clinical performance between synthetic and vein grafts. Other strategies such as embed- ding antibiotics and antithrombotic agents in the graft materials have also been attempted in order to improve the performance of vascular substitutes (Blanchemain et al. 2005, Guimond et al. 2006, Peeters et al. 2006).

12.6.3 Tissue-engineered liver Several extracorporeal bioartifi cial liver (BAL) systems have been developed to support essential hepatic functions. In a typical BAL device, patient’s plasma or blood is circulated through a bioreactor with immobilized primary hepatocytes or hepatoma cell lines between artifi cial plates or capillaries (Strain et al. 2002, Cao et al. 1998). From 1990, nine BAL systems have been tested clinically and most of them performed well in preclinical tests (Park et al. 2005). The BAL devices may be based on hollow fi ber technologies such as ELAD (Sussman et al. 1994 and 1993, Ellis et al. 1996), HepatAssist (Demetriou et al. 1995), LSS (Gerlach 1996, Sauer et al. 2001), and BLSS (Mazariegos et al. 2001); porous matrix systems such as RFB-BAL (Morsiani et al. 2001), AMC-BAL (Flendrig et al. 1998, Sosef et al. 2003, Van De Kerkhove et al. 2002), encapsulation systems such as UCLA, (Strain et al. 2002), and AHS-BAL (Lee et al. 2004); or fl at membrane systems such as FMB-BAL (De Bartolo et al.

2000). For instance, the HepatAssist liver device, developed by Demetriou and co-workers (1995), consists of a microcarrier for attachment of primary porcine hepatocytes, two charcoal ﬁ lters, a membrane oxygenator, and a pump. The hollow ﬁ ber open membrane has a pore size of 0.2 μm, small enough to prevent the passage of hepatocytes, but large enough to allow an exchange of proteins and protein-bound toxins between plasma and hepatocytes. The hepatocytes are housed in the extracapillary space (Demetriou et al. 1995). As liver is a vital and complex organ, investigation into a BAL with the broader essential functions of liver is ongoing.

12.6.4 Nerve grafts Neural tissue engineering involves the use of biomaterials as scaffolds with tissue-specifi c architecture and a controlled pore structure that facilitates growth and organization of resident cells for nerve repair (Liu et al. 1997). Nerve guidance conduits to bridge the gap between the nerve stumps and to direct nerve regeneration have been developed recently (Widmer et al. 1998, Seckel 1990, Giardino et al. 1995, Foidart et al. 1997). For example, Bini et al. (2004) fabricated a peripheral nerve conduit made of microbraided PLGA biodegradable polymer fi bers. Fibrin matrix cable formation was observed one week after implantation into the right sciatic nerve of the rat and nerve generation was seen in nine out of ten rats tested three weeks later. Many biodegradable and non-biodegradable materials have been investigated to construct nerve conduits such as collagen (Archibald et al. 1991, Chamberlain et al. 1998, Yoshii et al. 2001, Yoshii et al. 2001), polyethylene (Seckel et al. 1995, Hekimian et al. 1995, Stevenson et al. 1994), PLGA (Hadlock et al. 1998, Hadlock et al. 2000), polyphosphoester (Wang et al. 2001, Wan et al. 2001), silicone (Valentini et al. 1989, Lundborg et al. 1997), PTFE (Valentini et al. 1989). Electrospun biodegradable PLA nanofi bers have been used to fabricate a nerve conduit. Neural stem cells (NSC) seeded on these conduits were observed to attach and interact favorably with the aligned nanofi ber ECM (Yang et al. 2004). For future research, the challenge is to develop more effi cient nerve conduits so that nerve generation can occur across extended gaps.

12.6.5 Bone grafts Tissue engineering in bone has also undergone major advances in recent years. Natural materials such as collagen (Komaki et al. 2006, von Arx et al. 2006, Tal et al. 2005), ﬁ brin (Ito et al. 2003, Ito et al. 2006, Huh et al. 2006), chitosan (Kong et al. 2005, Rhee et al. 2005), hyaluronic acid (HA) (Aslan et al. 2006) and the synthetic polymers such as PCL

(La et al. 2005, Rhee et al. 2004), poly(propylene fumarates) (Shung et al. 2002, Behravesh et al. 1999), poly(phosphazenes) (Laurencin et al. 1993, Ibim et al. 1997, Laurencin et al. 1996), and PLGA (OsteofoamTM) (Holy et al. 2000) have been applied to establish 3D bone scaffolds. Cell sources include osteoblasts (Heath 2000, Platt 1996), adult stem cells from bone marrow (Aslan H et al. 2006, Dong et al. 2002, Mendes et al. 2002, Dong et al. 2001) or periosteum (Perka et al. 2000). Meanwhile, a plethora of growth factors including bone morphogenetic proteins (BMPs), transforming growth factor beta (TGF β), ﬁ broblast growth factors (FGFs), insulin growth factor I and II (IGF I/II), platelet derived growth factor (PDGF), and chrysalin have been incorporated into scaffolds for ingrowth and differentiation of osteoblasts and bone tissue formation (Jadlowiec et al. 2003, Boden 1999, Lind et al. 2001, Yoon et al. 2002, Malafaya et al. 2002).

12.6.6 Other organs Besides the applications mentioned above, rapid developments in tissue engineering have extended its scope to encompass many other important human organs, such as cartilage, (Hoben et al. 2006, Kaﬁ enah et al. 2006, Moroni et al. 2006, Shangkai et al. 2006), ligament (Freeman et al. 2006, Heckmann et al. 2006, Doroski et al. 2007), heart (Wu et al. 2006, Batten et al. 2006, Mendelson et al. 2006), pancreas (Cui et al. 2001, Iwata et al. 2004, Papas et al. 1999), larynx (Blaimauer et al. 2006, Kingham et al. 2006, Ringel et al. 2006), and cornea (Lai et al. 2006, Alaminos et al. 2006, Zorlutuna et al. 2006). However, further research and development are still required before ‘off-the-shelf’ products are available.

12.6.7 Examples of smart textile scaffolds The convergence of information technologies and advances in textile materials has created smart textile fabrics. Smart textiles are intelligent textile structures or fabrics that can sense and react to environmental stimuli, which may be mechanical, thermal, chemical, biological, and magnetic amongst others (Tao 2001). The initial application was in military and defense systems and it is currently being introduced in biomedicine and tissue engineering; two examples are described here. The incorporation of miniature electronic devices into textiles promises to have a tremendous impact on tissue engineering and medical textiles. Calvert and his team created an electronic sensor-studded textile using ink-jet printers (Sawhney et al. 2006). In the process, ‘wires’ made of the conducting polymer, poly(3,4-ethylenedioxythiophene)–poly(4-styrene sulfonate) (PEDOT-PSS) were printed between the silver lines on the

fabric. This intelligent textile device has been used to sense body motions like twisting of a wrist and bending of a knee using its piezoresistive properties (Bethany et al. 2005). Another example is smart textile scaffolds made of shape memory materials. These shape memory scaffolds can be transferred into memorized, permanent shapes from their original temporary conﬁ guration upon an external stimulus, e.g. an increase in temperature. Therefore, such a smart textile scaffold made of biodegradable materials such as PCL and copoly- esters of diglycolide and dilactides may be introduced into the human body through a small incision and the implanted device then returns to its original shape to fulﬁ l the desired functions. Such a device would degrade after successfully completing the desired function (Tao 2001).

12.7 Future trends The burgeoning research in tissue engineering has impacted signifi cantly on how tissue replacement and restoration will be done in the future. Further developments in material sciences, molecular biology, and scaffold fabrication technologies, as well as further understanding of stem cells and controlled manipulation of cell differentiation, will further broaden and enhance successful applications of tissue engineering. Advances in intelligent materials will give textile scaffolds greater functions, and allow novel materials to be fabricated. For example, electroac- tive polymers and elastomers could be used in artifi cial muscles (Tao 2001). Auxetic fi bers made from polymers like PTFE, polypropylene and nylon can be constructed into knitted and woven textiles that are particularly suitable for skin grafts as these scaffolds will expand in response to wound swelling (Alderson 2002). Growth factors, drugs or monitoring devices may also be incorporated into textile scaffolds and implanted in the human body to perform certain functions that are not possible today. For example, a novel temperature- sensitive and biodegradable glycidyl methacrylated dextran (Dex-GMA)/ gelatin scaffold containing micro-spheres loaded with BMP has been developed. The phase transition temperature of the resulting scaffold could be tailored for controlled release with a half-life ranging from 18 days to more than 28 days by changing the ratio of Dex-GMA to gelatine. Such smart hybrid scaffolds have both self-regulated drug delivery and tissue scaffold functions (Chen et al. 2006). The electrospinning technique is versatile and relatively cost-effective and therefore is likely to continue to be used in the future for fabrication of nanofi bers. Applications of nanofi bers in the biomedical and biotechnological fi elds are increasingly important as these applications exploit

some of the unique properties attributable to the nanoscale effects. It is envisaged that more natural polymers will be incorporated into nanofi brous scaffold designs in order to improve biological compatibility and enhance functional performance. The challenge of scaling up and making better quality fi bers is being met with the introduction of novel and improved electrospinning techniques. Progenitor or stem cells with high capabilities of self-renewal and multilineage differentiation have a remarkable potential to develop into many different cell types in various tissues. For instance, embryonic stem cells could give rise to almost all cells deriving from three germ lines (Salgado et al. 2004). Adult stem cells, residing in the fully differentiated or adult tissues, e.g. bone marrow, periosteum, muscle, fat, brain, and skin, can differentiate into a specifi c cell lineage from which they derive (Salgado et al. 2004). Moreover, recent studies found higher plasticity of adult stem cells than what was previously expected. For example, it was indicated that adult stem cells derived from the dermis could differentiate into muscle, brain and fat cells (Toma et al. 2001), and another group found that adult stem cells isolated from bone marrow would differentiate into keratinocytes in vivo (Borue et al. 2004). Mesenchymal stem cells have been used for regeneration and repair of tendon, (Chong et al. 2007), bone (Rust et al. 2006) and heart (Zhou et al. 2006). Loading these multipotent cells onto textile scaffolds and directing them to differentiate into desired lin- eages broadens the potential applications of textile scaffolds. Gene modulation will make possible the seeding of recombinant proteins or genetically modifi ed cells on textile fabrics for specifi c therapeutic applications. For example, one recent group loaded the Bcl-2 transduced human umbilical vein endothelial cells (HUVECs) on a skin substitute. Enhanced vascularization and engraftment were observed after trans- planting to immunodefi cient (C.B-17SCID/Bg) mice. The improved angiogenesis was attributed to Bcl-2 resulting in secretion of an antiapop- totic protein which increases the capacity of HUVEC to form mature vessels. (Enis et al. 2005). Therefore, it is hoped that, by loading genetically modifi ed cells on textile scaffolds as skin graft substitutes, improved functions, including those provided by hair follicles, sebaceous glands, sweat glands and dendritic cells, will be achieved in the near future (Hermens et al. 1995). In addition, the transgenic approach may be employed to produce biomaterials with unique characteristics for fabricating fi brous textile scaffolds. For example, in 1999, Nexia Biotechnologies Inc. succeeded in extracting a recombinant spider silk proteins (BioSteel®) from the milk of transgenic goats. Although the strength of BioSteel® is far from satisfac- tory, its ductility is unique as it can undergo a high degree of elongation

before it breaks. Following this, a research group has successfully co- electrospun this spider silk protein with carbon nanotubes. Such a combination enabled tailoring nanofi bers with mechanical properties superior to that of natural spider silk. The feasibility of using such ‘super silk’ as tissue engineering scaffolds has been explored (Ko et al. 2004). However, there are a number of potential risks that are not fully addressed at this time including the toxic effects of textiles with nanoscale texture and the unknown risks of nanopolymer materials on human health. Other issues of contention are the ethics of using stem cells and genetically modifi ed cells. As there is no single ideal scaffold for all tissue types, rapid advances in fi brous textile scaffold research will allow more fl exibility in tailoring scaffolds for different requirements. The unique properties attributable to nanoscale effects are just beginning to be exploited in textile scaffold. We expect further developments will progress along this line.

12.8 Sources of further information and advice 12.8.1 Books Cohn D., Reis R. L. (2002), Polymer based systems on tissue engineering, replacement and regeneration, The Netherlands, Kluwer Academic Publishers. Ikada Y. (2006), Tissue engineering: fundamentals and applications, UK, Elsevier Ltd. Lewandrowski K. U. (2002), Tissue engineering and biodegradable equiva- lents: scientifi c and clinical applications, New York, Marcel Dekker, Inc. Ma P. X., Elisseeff J. H. (2005), Scaffolding in tissue engineering, Boca Raton, FL, US, CRC Press. Peppas N. A., Hilt J. Z., Thomas J. B. (2007), Nanotechnology in thera- peutics: current technology and applications, UK, Horizon Bioscience. Ramakrishna S. (2005), An introduction to electrospinning and nanofi - bers, Singapore, World Scientifi c Publishing Co. Pte. Ltd. Reis R. L., Román J. S. (2005), Biodegradable systems in tissue engineering and regenerative medicine, USA, CRC Press. Saltzman W. M. (2004), Tissue engineering: engineering principles for the design of replacement organs and tissues, New York, Oxford University Press, Inc. Selcuk I. Güceri, Vladimir. Kuznetsov (2004), Nanoengineered nanofi brous materials, Netherlands, Kluwer Academic Publishers. Yannas, I. V. (2001), Tissue and Organ Regeneration in Adults, New York, Springer.

12.8.2 Trade and professional bodies CellTran Limited, UK, http://www.celltran.co.uk/index.php. Degradable Solutions AG, http://www.degradable.ch/. Exploit Technologies Private Limited, Singapore, http://www.exploit-tech. com/Home.aspx/. Japan Tissue Engineering Co., Ltd., http://www.jpte.co.jp/english/index. html. Nanomatrix, http://www.nanomatrix.biz/. The Japanese Society for Tissue Engineering, http://www.jste.net/english. htm. Tissue Engineering Inc. (TEI), www.tissueengineering.com. Tissue Engineering International and Regenerative Medicine Society, http://www.termis.org/. Tissue Engineering Sciences (TES), www.tissueeng.com. Transtissue technologies, www.transtissue.com.

12.8.3 Research groups Centre for Biomaterials and Tissue Engineering, University of Shefﬁ eld, UK, http://www.cbte.group.shef.ac.uk/. Center for Biomedical Engineering, Massachusetts Institute of Technol- ogy, USA, http://web.mit.edu/afs/athena.mit.edu/org/c/cbe/www/. Charité Tissue Engineering Laboratory, Germany, http://ctel.tissueengineering.net/index.php?seite=Startseite. Healthcare and Energy Materials Laboratories, http://www.bioeng.nus. edu.sg/seeram_ramakrishna/ Polymeric Biomaterials and Tissue Engineering Laboratory, University of Michigan, USA, http://www.dent.umich.edu/depts/bms/personnel/ faculty.php?uname=mapx. Tissue Engineering and Organ Fabrication Laboratory, Boston, USA, http://www.mgh.harvard.edu/tissue/. Tissue Engineering Laboratory, National University of Singapore, http:// www.bioeng.nus.edu.sg/research/tissueengineering/.

12.8.4 Websites http://www.ameriburn.org. http://www.collagenesis.com/documents/products.htm. http://www.collagenmatrix.com/. http://www.lifecell.com. http://www.netdoctor.co.uk/diseases/facts/footandlegulcers.htm. http://www.organogenesis.com.

http://www.ortecinternational.com. http://www.synthecon.com/products/bioscaffold.htm. http://www.tissue-engineering.net/. http://www.worldwidewounds.com.

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