Tribology in Industry Friction and Adhesion in Porous Biomaterial
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Vol. 38, No. 3 (2016) 361-370 Tribology in Industry www.tribology.fink.rs RESEARCH Friction and Adhesion in Porous Biomaterial Structure a a a a b a a a F. Živić , N. Grujović , S. Mitrović , D. Adamović , V. Petrović , A. Radovanović , S. Đurić , N. Palić a Faculty of Engineering, University of Kragujevac, Sestre Janjić 6, 34000 Kragujevac, Serbia, bFundación Empresa Universidad Gallega - FEUGA, Spain. Keywords: A B S T R A C T Porosity The paper presents short review of different aspects of the introduction of Biomaterials porosity into the bulk biomaterial and effects on different material Scaffolds characteristics, especially related to friction and adhesion. Nowadays, there is a Tissue engineering great interest to investigate relations between porosity, different mechanical Friction responses due to controlled topography and cell responses generated Adhesion accordingly. Examples of current investigations of custom developed scaffolds for tissue engineering related to cell seeding and hip stem component are Corresponding author: shown. Friction, adhesion and adhesive forces are briefly defined as related to F. Živić porous material structures and the relevance of nano- and micro- level surface Faculty of Engineering layers in such structures. Patterning techniques and micro-fabrication Universty of Kragujevac, techniques for production of controlled and random porous surface layers are Sestre Janjić 6, given. Influence of porosity on adhesion and friction is presented through 34000 Kragujevac, Serbia. several existing experimental results. However, there is still general lack of data E-mail: [email protected] related to many aspects of these novel porous materials and structures. © 2016 Published by Faculty of Engineering 1. INTRODUCTION biomaterial structures for design of medical implants. Depending on the end application, Porosity and permeability are two com-monly different properties are important and series of used parameters to define porous structures. studies can be found accordingly, related to the Porosity is a measure of existing voids within a changes of mechanical properties such as dense material structure. Permeability strength, strain, modulus, permeability, as well determines the ease of fluid flow through the as damage mechanics, fatigue, creep, different porous material, in case of open cell porosity reinforcements, coatings, effects of design (e.g. and it is usually defined by Darcy’s law. stent struts thickness or shape), effects of pore Porous metallic materials have long been shape, etc. Some approaches include complex considered as good candidates in many areas of metal - polymer composites with remaining con- applications, such as vibration and sound trolled porosity, in order to improve certain absorption, light materials, heat transfer media, specific property. Generally, introduction of sandwich core for different panels, various porosity in otherwise dense material, inevitably membranes and during the last years as suitable leads to decrease of strength but this is not 361 F. Živić et al., Tribology in Industry Vol. 38, No. 3 (2016) 361-370 always the case, especially for some novel describing some cellular processes such as complex classes of materials. Based on the shape adhesion, migration and differentiation of of pores, size and orientation, a general various types of cells. Mechanical contact is classification of porous structures can be given certainly the important physical cue governing as: 1) Foams (open cell and closed cell) and 2) the features of cell structures. Different physical Periodic cellular metals (prismatic, shell, and signals very often transform into biochemical truss) [1]. signals, changing some of the cell’s functions. This phenomenon is known as It is accepted that metal foams have in-creased mechanotrunsduction and it is responsible for internal friction compared to the same dense changing the irritability of some sensory cells. material, as well as that the stress-strain state is Topography of ECM varies from nanometer to developed as a very complex non-homogenous micrometer scale, hence behavior of cells and under cyclic loads, but the governing sub-cells (cell morphology, organization, mechanisms are still un-der discussions [2]. migration etc.) depends on a range of length scales [5]. 2. POROSITY RELATED TO APPROPRIATE In order to provide cell support, adhesion, TISSUE RESPONSE proliferation and suitable mechanical properties of the scaffold, various fabrication processes has Initially, research in tissue engineering was been used, such as different chemical related only to cell seeding and biological approaches, textile technologies, particulate- responses and mechanisms. Nowadays, it is a leaching techniques, phase separation, broad area comprising different sub-categories electrospinning, freeze drying, and other, but all of research, where engineering of scaffolds on of these produce random cell sizes or which the cell cultures are investigated became interconnections [4]. Topography at micro level one promising area of future advancements. and with various geometries can be generated Ideally, a scaffold should closely resemble, or using different fabrication methods [28]. These duplicate, the natural architecture of methods mostly depend on the type of designed extracellular matrix (ECM). So far, the majority structure or end applications. Randomly of developed scaffolds have been made of arranged topographies can also be achieved polymer materials, but very recently metallic using processes such as polishing, grinding, foams have started to be studied extensively. abrasion, plasma spraying, sandblasting, grit- The largest number of human cells needs blasting, etc. something to adhere to, in order to survive and function, usually ECM and scaffolds which mimic For surfaces with ordered structures micro- ECM geometry and structure are of great fabrication is recognised as the most suitable importance. Generally, cell environment which is technology today [3]. Micro-fabrication important for proper functioning is within a technologies can provide better control over nano to micro scale. For example, collagen fiber cell’s functions on micro and nano-meter scale is of 0.5 m length and 50 nm thick and can produce controlled structures such as approximately (0.5 - 3 m), while cells pits, grooves, pillars, wires and many others. In a themselves can be of different sizes, ranging very recent time researchers have intensively from 1 m to 100 m or biological membranes studied methods for control of cell’s behavior. with pores 1 - 5 m [3]. Accordingly, 3D scaffold Ventre et al. [6] demonstrated possibility of structures with features within a range of these controlling adhesion and migration of osteoblast scales are needed. MC3T3 cells by adjusting micro-topographic features and chemical condition of the surface. In recent years some of investigations have been They also presented cell’s trajectories and focused on the influence of biophysical signals described migration of cells on untreated and on properties of cell-substrate interface. oxygen treated substrates in the case of 2 μm Biochemical or biophysical signals comes mostly and 5 μm size of patterns. from extracellular matrix and adjacent cells. On the other hand, topography patterns and Two examples of custom developed scaffolding for mechanical stiffness have major role in tissue engineering are given in Figs. 1 and 2, 362 F. Živić et al., Tribology in Industry Vol. 38, No. 3 (2016) 361-370 showing structures produced by two different new perform two-dimensional cellular additive manufacturing technologies which can be differentiation. In contrast, in foams with used for controlled porosity generation, but with a improved cell-cell contact, with sub-cellular recognised problem to obtain fine roughness and pores of 67 μm three-dimensional hepatocyte high level of precision of the pore sizes. reorganization has been achieved. Better cell- cell interactions promote higher degree of cell spreading throughout the scaffold (approximate 17 - 82 μm size of voids). Travis and Horst [30] presented three-dimensional artificial nano scale fibers by using electrospinning for fabrication. These structures which contain high porosity and interconnectivity resemble the nature collagen fibers, and can be used to improve interaction between scaffolds and cells. Park et al. [8] seeded hepatocytes on glass substrates with micro-channels between each substrate. They demonstrated that these cell’s cultures have better characteristics suited for liver replacements, due to the surface Fig. 1. SEM image of the surface of porous Ti6Al4V modification: 100 m high micro-channels on sample, fabricated by additive manufacturing glass substrate which provides protection from technology, Electron beam melting (EBM), aimed for shear stress. the artificial hip stem (AIMME Institute Spain). Different types of cells and their responses to various cues (mechanical, biological, chemical etc.) have been studied, but generally the majority of these investigations were related to their functions within the immune system. For instance, macrophages were studied through their immune defense capability and they have been used in various healing processes [9]. Recently, many researchers have started to investigate design of the scaffold material in order to interact with the living tissue, such as the research related to the characteristics of macrophage response to various topographies [10]. In this way, controlled design of the scaffold geometry