PROCESSING AND CHARACTERIZATION OF POLYMER MICROPARTICLES

FOR CONTROLLED DRUG DELIVERY SYSTEMS

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree of Doctor of

Philosophy in the Graduate School of The Ohio State University

By

Aravind Chakrapani, M.S.

The Ohio State University

2006

Doctor’s Examination Committee:

Professor Derek J. Hansford, Advisor

Professor William E. Carson Approved by

Professor James T. Dalton

Advisor Biomedical Engineering Graduate Program

ABSTRACT

We report a novel soft lithography based technique to fabricate non-spherical

biodegradable polymeric microparticles of different sizes and shapes as drug delivery systems. Geometrical control over the shape and size of these microparticles renders them different aerodynamic and fluidic dynamic properties when compared to conventional spherical microparticles and may prove to be beneficial in certain drug delivery strategies, such as pulmonary and intravenous routes. The surface morphology of the particles was studied using a scanning electron microscope and size distribution of the particles was determined using a Coulter counter. The process is reproducible and millions of uniform biodegradable particles of various sizes and shapes with dimensions ranging from 2-30 µm have been fabricated and were collected by a simple vacuum filtration apparatus. In addition, we demonstrated encapsulation of a model drug (FITC), and FITC distributions within the particles were studied by confocal microscopy. Particle size is a critical parameter for several aspects of controlled drug delivery including control of drug-release kinetics, passive targeting to specific cell or tissue types, biodistribution upon administration and available routes of administration. In conclusion, the uniform, biodegradable polymeric microparticles produced have potential to be used in a variety of drug delivery applications and polymer-based microfabrication technology holds promise to produce sophisticated, multi-functional drug delivery devices.

ii

Dedicated to my loving parents and my sister

iii ACKNOWLEDGMENTS

First and foremost, I would like to thank my advisor Dr. Derek Hansford. No words can do justice to the gratitude I feel for the constant support, encouragement and guidance he provided throughout the course of my graduate studies. He has not only been an advisor but has also been a great friend and mentor over the last few years. Although my life in graduate school is coming to an end and I am looking forward to the next stage of my career, I will forever cherish the memories of my life in graduate school and hope to be in touch with Dr. Hansford for a long time.

I would like to thank Dr. James Dalton for being part of my candidacy and graduate committee and for his valuable suggestion towards the project.

I would like to thank Dr. William Carson for being part of my graduation committee and for having provided the use of his facilities and expertise. He got me in touch with Dr. Lesinski, who helped me with encapsulation experiments and patiently explained a lot of simple questions that I asked him about immunoassays. Needless to say, I am thankful to Dr. Lesinski for his help.

I would like to thank all the members of my research group for their suggestions with my work and for being such great friends. In particular, I would like to thank Nick

Ferrell, Jingjiao Guan for taking the time to answer a lot of my questions pertaining to research. Without their help my research work would not have been possible. I wish them all the best of luck in their endeavors.

iv I would like to express my appreciation and gratitude to Melanie Senitko, Kirsten

Gibbons, Vlad Marukhlenko, Ed Herderick and David Morelli at the biomedical

engineering center. I would also extend my thanks to Dr. Ruegsegger and Dr. Moldovan.

They have provided me constant support and encouragement with my academic work and

helped create a great atmosphere at work.

I wish to thank Derek Ditmer at MicroMD lab for his patience in training me on a

lot of equipments at MicroMD and for being a good friend. I would also like to thank

Jeremy Gaumer at Material Science engineering and Brian Kemmenoe for their patience

and having me trained on the optical profilometer and confocal microscope, respectively.

I am extremely grateful to all of my friends, for without their support, I would not

have been able to work here 9000 miles away from home. It would not be possible for me

to list all of their names and the memories they have provided over the last few years in the acknowledgments but I would have to at least mention the following: Amresh, Balaji,

Guru, Hassan, Jingjiao, Manmohan, Nishat, Nick, Pradeep, Ramesh, Rangarajan, Senthil,

Vinod, and Vyas. I express my sincere thanks to them for being such true friends, whom I could always count on.

Finally, I would like to thank my parents and my sister for their unconditional love and encouragement throughout my life. Without their sacrifices and prayers, I would not have had the opportunity to pursue my dreams. Thank you so much!

v VITA

April 23, 1978…………………Born----Chennai, Tamilnadu, India

1999…………………………... B.Tech. Chemical Engineering, University of

Madras, Tamilnadu, India

2003…………………………..M.S. Biomedical Engineering, The Ohio State University

Columbus, Ohio, USA

2003-present………………….Graduate Associate, The Ohio State University,

Columbus, Ohio, USA

PUBLICATIONS

Research Publications

1. Guan, J., A. Chakrapani, and D.J. Hansford, Polymer Microparticles Fabricated by Soft Lithography. Chemistry of Materials, 2005. 17(25): p. 6227- 6229.

FIELDS OF STUDY

Major Field: Biomedical Engineering

vi TABLE OF CONTENTS

Page

Abstract……………………………………………………………………………………ii

Acknowledgments………………………………………………………………………..iv

Vita…………………………………………………………………………………….….vi

List of Tables………………………………………………………………………….…xii

List of Figures………………………………………………………………………...…xiii

Chapters:

1. Introduction…………………………………………………………………………….1

2. Microparticles as Drug Delivery Systems……………………………………………...4

2.1. Biodegradable polymers for drug delivery systems………………………………..6

2.1.1. Properties of PLGA……………………………………………………………7

2.2. Fabrication techniques of biodegradable

micro/nanoparticles for drug delivery…………………………………………………..9

2.2.1. Solvent evaporation and extraction based processes…………………...... 10

2.2.1.1. Single emulsion process…………………………………………………...10

vii 2.2.1.2. Double emulsion process……………………………………………...... 13

2.2.2. Phase separation……………………………………………………………….15

2.2.3. Spray drying…………………………………………………………………...17

2.2.4. Microfabrication based drug delivery systems………………………………..18

2.2.4.1. Transdermal drug delivery systems……………………………………….20

2.2.4.2. Oral delivery………………………………………………………………24

2.2.4.3. Intravenous delivery………………………………………………………25

2.2.4.4. Microfabricated implants………………………………………………….26

2.2.4.5. Soft lithography…………………………………………………………...27

2.3. Practical examples………………………………………………………………...28

2.4. Summary...………………………………………………………………………...31

3. Fabrication of Biodegradable Polymeric Microparticles Using a Heat Press………....32

3.1. Photolithography…………………………………………………………………..33

3.2. PDMS stamp preparation………………………………………………………….33

3.3. Experimental methods…………………………………………………………….37

3.3.1. Materials…………………………………………………………………….37

3.3.2. Process overview……………………………………………………………37

3.3.3. Experimental set-up…………………………………………………………40

3.3.4. Fabrication process………………………………………………………….41

viii 3.4. Results and Discussion………………..…………………………………………..43

3.4.1. Coulter method of counting and sizing………………………………………47

3.4.2. Statistical analysis…………………………………………………………....49

3.5. Summary…………………………………………………………………………..49

4. Key Parameters and Process Improvements in Microtransfer Molding………………51

4.1. Introduction………………………………………………………………………..51

4.1.1Molds………………………………………………………………………...51

4.1.2. Spin-coating…………………………..…………………………………….52

4.1.3. Polymer solution…………………………………………………………....54

4.1.4. First stamp……………………..……………………………………………55

4.1.5. Uniform temperature and pressure……………………………………….....56

4.2. Summary...…………………………………………………………………………56

5. Fabrication of Polymeric Microparticles Using

the Hot Embosser as a Stamping Device………………………………………………58

5.1. Experimental methods…………………………………………………………….60

5.1.1. Materials…………………………………………………………………….60

5.1.2. Photolithography…………………………………………………………….60

5.1.2. PDMS stamp preparation…………………………………………………....61

ix 5.1.4. Preparation of PLGA microparticles………………………………………..62

5.1.5. Characterization……………………………………………………………..62

5.2. Results and Discussion…………………...……………………………………….63

5.2.1. PDMS stamps……………………………………………………………….63

5.2.2. PLGA microparticles………………………………………………………..64

5.2.3. Statistical Analysis…………………………………………………………..69

5.3. Summary…………………………………………………………………………..71

6. Polymeric Microparticles as Drug Delivery Devices…………………………………73

6.1. Introduction………………………………………………………………………..73

6.2. Experimental methods………………………………………………………….…73

6.2.1. Encapsulation………………………………………………………………..73

6.2.2. Filtration……………………………………………………………………..74

6.2.3. Bi-layered particles………………………………………………………….74

6.2.4. Encapsulation of Taxol……………………………………………………...76

6.3. Results an Discussion….………………………………..………………………...76

6.4. Summary...………………………………………………………………………...82

7. Conclusions and Recommendations…………………………………………………..83

x 7.1. Conclusions……………………………………………………………………..…83

7.2. Recommendations……………………………………………………………...….84

References………………………………………………………………………………..86

xi LIST OF TABLES

Table Page

2.1. Examples of pharmaceutical products based on drug loaded, biodegradable particles available on market…………………………25

5.1 Statistical comparison of results for the particles fabricated using the hot press and hot embosser processes…………………………………....70

xii

LIST OF FIGURES

Figure Page

2.1 Concentration (c) vs. Time (t) profiles for conventional ………………………6 and controlled release drug delivery devices

2.2 Different categories of microparticles…………………………………………...10

2.3 Encapsulation using oil-in-water emulsion technique…………………………...11

2.4 Schematic of w/o/w in-liquid drying process for microparticle preparation…….15

2.5 Spray-drying apparatus for fabricating uniform microspheres…………………..18

2.6 Microneedle fabrication sequence…………………………………………….…21

2.7 Scanning electron micrographs of microneedles………………………………...22

2.8 Method to fabricate polymer microneedles that encapsulate drug for controlled release…………………………………………………………...……23

2.9 Fabrication sequence for bioadhesive microdevice……………………………...25

2.10 Diagram showing design of cell isolation capsule……………………………….27

2.11 Drug loaded microparticles used to optimize cell growth and differentiation in cell therapies………………………………………………………………..…31

3.1 Schematic drawing of the photolithography process…………………………….34

3.2 Schematic diagram illustrating preparation of PDMS mold from the patterned wafer……………………………………………………………...……35

3.3 Silicon wafer with patterned features on the surface…………………………….36

xiii 3.4 PDMS mold with 10µm circular (wells) features…………………………….….36

3.5 Schematic illustration of microtransfer molding………………………………...39

3.6 Polymeric microstructures fabricated using µTM……………………………….39

3.7 Illustration of traditional imprint lithography……………………………………40

3.8 Heat press for fabricating polymeric microparticles……………………………..41

3.9 Schematic illustration of the soft lithography process for the fabrication of polymeric microparticles…………………………………………………….…..42

3.10 Optical micrograph of a clean PDMS mold with 10µm features………………..44

3.11 Optical micrograph of a PDMS mold filled with a PLGA from a solution in chloroform (5% w/w)…………………………………………………………….44

3.12 PLGA microparticles stamped onto a glass substrate……………………………45

3.13 PLGA microparticles stamped on a layer of PVA……………………………….45

3.14 PPMA particles…………………………………………………………………..46

3.15 PLGA particles a) PLGA particles stamped on PVA b) A thin residual film of PLGA…………..46

3.16 Size distribution profile of particles from a 10µm PDMS mold…………………47

3.17 Coulter method for counting and sizing of particles……………………………..48

4.1 SEM illustrating the presence of a residual polymer film between the cylindrical features…...…………………………………………………………53

4.2 PLGA particles fabricated using anisole as a solvent…………………………....55

5.1 Process schematics of the hot embossing process……………………………….59

5.2 Schematic drawing of the hot embossing equipment……………………………60

xiv 5.3 Schematic drawing of the soft lithography process to fabricate polymeric microparticles…………………………………………………………………….63 5.4 Surface data of the PDMS mold obtained from the profilometer………………..64

5.5 Electron micrograph showing a PDMS stamp with “microwells”………………65

5.6 Electron micrograph of polymeric material removed in the first stamp………....66

5.7 Electron micrograph of PLGA particles obtained after successful “first stamp” procedure…………………….……………………………………..66

5.8 PLGA particles made from a solution of PLGA in chloroform………………….67

5.9 PLGA particles with varying aspect ratios………………………………………67

5.10 Electron micrograph showing ‘hexagonal’ PLGA particles……………………..68

5.11 PLGA particles with higher aspect ratio (~1.8)……………………………….…68

5.12 Size distribution of particles obtained from different batches…………………...69

5.13 Volume distribution of the particles from the heat press process………………..70

5.14 Volume distribution of the particles from the hot embosser process………….…71

6.1 Schematic drawing illustrating ‘bi-layered’ microparticle fabrication process….75

6.2 Bi-layered microparticle………………………………………………………....75

6.3 PLGA particles observed under a confocal microscope………………………....77

6.4 PLGA particles loaded with FITC……………………………………………….77

6.5 Filtered PLGA microparticles……………………………………………………79

6.6 Electron micrographs showing bi-layered polymeric microparticles……………80

6.7 HPLC plot of Taxol in chloroform ( 0.5mg/ml)…………………………………81

6.8 HPLC plot of PLGA and Taxol co-dissolved in chloroform…………………….81

xv

CHAPTER 1

INTRODUCTION

The term ‘microparticle’ refers to a particle with a diameter of 1-1000 microns, irrespective of the interior structure [1]. After it was realized molecules of any size could be delivered slowly from biodegradable polymers, the field of polymeric delivery systems for the release of proteins and peptides has grown immensely [2]. Unlike traditional small molecule drugs, proteins and peptides cannot be administered orally because of their fragility and short in vivo half-lives. Encapsulation of proteins in

biodegradable polymeric devices from which the drug can be delivered locally or

systemically for a prolonged period of time has been a promising solution to these

problems [3]. The release of the drug may be constant over a long period, it may be

cyclic over a long period or it could be triggered by the local environment or other

external events. Ultimately, the purpose behind controlled drug delivery systems is to

achieve more effective therapies with optimal dosage of the drug.

As a field, drug delivery technology is experiencing a second wind coinciding with

the increased development of proteins and peptides with therapeutic potential. These

products, which commanded an estimated worldwide market of over $10 billion in 1998,

1 are currently almost exclusively delivered by injection, due to the fragile nature of the molecules and their digestion in the gastrointestinal tract [4]. With rapid advances in genomic research and biotechnology, drug companies are developing new protein and

peptide-based compounds such as interleukins, cytokines and enzymes for a variety of diseases [5]. This has presented major challenges in designing an optimal drug delivery system to increase the short circulatory half-life of these drugs, to increase their bioavailability, and to target molecules to responsive cells and tissues so as to limit systemic drug levels. Currently, there are at least 75 protein or peptide-based products approved for marketing in the US alone that are used in the treatment of cancer, diabetes, multiple sclerosis, and growth deficiencies [6]. With more than 100 other such drugs in human clinical trials, the market for protein-based drugs will continue to grow. Many of the new drugs being developed are macromolecules requiring delivery by injection.

Research has shown that injection of drugs like interferon-α have not been optimally

effective because the drug is cleared rapidly from the body without having the desired

therapeutic effect [7]. The United States drug delivery market grew by 10.9% in 2004 to

reach a value of $38.9 billion [8]. Therefore, significant opportunities exist for

technology solutions that are non-invasive alternatives to injection, reduce dosing

frequency, improve safety and efficacy or improve the stability of the macromolecule.

Several micro and nanoparticle based therapy systems are being investigated to meet

the above challenges. Although several techniques to fabricate microparticles have been

described in the literature, spraying [9, 10], phase-separation [11] and emulsion [12, 13]

techniques are the most commonly employed in the industry to produce micro and

nanoparticulate drug delivery systems [14]. With these methods, sphere size and

2 distribution are reproducible but often poorly controllable [15]. Conventional

microfabrication techniques [16], the use of supercritical fluids [17], and various other techniques [15, 18-20] are also being investigated by researchers worldwide to fabricate microparticles for more effective drug delivery systems.

The purpose of the presented research in this dissertation is the development of a novel soft lithography based process to fabricate and characterize uniform biodegradable polymeric microparticles of various sizes and geometry as controlled drug delivery devices [19, 21].The technology developed in this work provides a means to fabricate uniform biodegradable microparticles of controlled shape and size, thus allowing them to be specific for different drug delivery applications. The incorporation of drugs (proteins or peptides) provides unique opportunities for effective delivery of peptide-based drugs.

Geometrical control over the shape and size of these microparticles and the ability to fabricate complex polymeric structures offers distinct advantages for drug delivery

strategies. By controlling the processing conditions, particles with precise geometry and

sizes in the range of few hundreds of nanometers with specific fluid dynamic and

aerodynamic properties can be produced. The fabrication process, characterization and

encapsulation of a model drug in biodegradable polymer microdevices are presented in

this dissertation.

3

CHAPTER 2

MICROPARTICLE AS DRUG DELIVERY SYSTEMS

Microparticles offer a method to deliver macromolecules by a variety of routes and

effectively control the release of such drugs. They may also be used in the delivery of vaccines and molecules such as DNA for use in gene therapy. Microparticles offer effective protection of encapsulated agent against degradation (e.g. enzymatic), the

possibility of controlled and local delivery of the drug over periods ranging from few hours to months, and easy administration (compared to alternative forms of controlled release parenteral dosages, such as macrosized implants) [22]. Controlled drug delivery

systems could be extremely useful in providing the optimal therapy for a given drug

molecule [23]. Each drug has a characteristic ‘minimum effective concentration’, below

which no therapeutic effect is observed and a characteristic ‘minimum toxic concentration’ above which undesired side effects occur (as shown in Figure 2.1). The range in between is called the ‘therapeutic range’ or ‘therapeutic window’. Depending upon the type of drug and physiological factors, this therapeutic window could be narrow. The optimum effect of many medical treatments is obtained by maintaining the drug concentration in the therapeutic range over a sustained period of time. This is

especially true for highly potent drugs, such as anti-cancer drugs. Administration of the

entire drug dose at once using conventional pharmaceutical dosage (e.g. tablets, bolus

4 injection), the whole amount is rapidly released into the stomach, absorbed into the blood stream and distributed throughout the human body. As a result, the rate at which the drug reaches its site of action is often high. Depending on the therapeutic range and administered dose, the risk of toxic side effects can be considerable. As no continuous drug supply is provided and as the human body eliminates the active agent, the concentration decreases again. This results in a fluctuating concentration of the drug levels in the plasma and the therapeutic range is attained during only very short time periods. The idea behind a controlled drug delivery system is to incorporate the drug within a polymeric carrier that controls the release rate of the drug. Various processes, such as diffusion, erosion, and/or swelling can be involved in the control of the overall drug release rate, resulting in a broad spectrum of possible release profiles. For example, a continuous drug supply can be provided, compensating for the clearance of the drug from the human body, thus resulting in constant drug concentration at the site of action over a prolonged period.

Figure 2.1: Concentration(c) vs. Time(t) profiles for conventional and controlled release drug delivery devices [24]

5 2.1. Biodegradable polymers for drug delivery systems: Advances in polymer science

have opened up possibilities for using a wide variety of polymeric materials as drug

delivery systems. Biodegradable polymers, by virtue of their ability to degrade in the

body naturally, offer enormous advantages over conventional drug delivery systems. It eliminated the need for surgery and also does not elicit any adverse reactions from the

body. Polymeric drug delivery systems are mainly intended to deliver the drug over a period of time. Some of the materials that are currently being used/studied for controlled drug delivery include poly (methyl methacrylate), poly (vinyl alcohol), polyacrylamide,

polyethylene glycol, polylactic acid, polyglycolic acid, polylacticglycolic acid, and

polyanhydrides [25]. Most biodegradable polymers are designed to degrade as a result of

hydrolysis of the polymer chains into biologically acceptable and progressively smaller

compounds. For example in the case of polylactic glycolic acid, the polymer would eventually break down into lactic and glycolic acid, enter the Krebs cycle and further broken down into carbon dioxide and water. Drugs formulated in polymeric devices are released either by diffusion through the polymeric barrier, or by erosion of the polymer material, or by a combination of both diffusion and erosion mechanisms [26]. A wide variety of natural and synthetic biodegradable polymers have been investigated for drug targeting or prolonged drug release. Amongst them, the thermoplastic aliphatic poly

(esters) like PLA, PGA, and especially PLGA have generated tremendous interest due to their excellent biocompatibility and biodegradability. The wide acceptance of the lactide/glycolide polymers as suture materials, made them an attractive candidate for biomedical applications like ligament reconstruction, tracheal replacement, surgical dressings, vascular grafts, nerve, dental, and fracture repair. The first work on parenteral

6 controlled release of drugs using PLA was reported by Wise and Beck [26, 27]. Since

then various polymeric devices like microspheres, microcapsules, nanoparticles, pellets,

implants and films have been fabricated using these polymers for the delivery of a variety

of drug classes. Also they are easy to formulate into drug carrying devices for variety of

applications, such as, orthopedic drug delivery; they have been approved by the FDA for

drug delivery use [28].

2.1.1. Properties of PLGA: The understanding of physical, chemical and biological

properties of the polymer is helpful before formulating a controlled drug delivery device.

Lactic acid is more hydrophobic than glycolic acid and hence lactide-rich PLGA

copolymers are less hydrophilic, absorb less water and subsequently degrade more

slowly. The commercially available PLGA polymers are usually characterized in terms of

intrinsic viscosity, which is directly related to the molecular weight.

The mechanical strength, swelling behavior, capacity to undergo hydrolysis, and

subsequently the biodegradation rate are directly influenced by the crystallinity of the

PLGA polymer. The crystallinity of the PLGA copolymer is directly dependent on the type and molar ratio of the individual monomer components (lactide and glycolide) in the copolymer chain. PLGA polymers containing 50:50 ratio of lactic and glycolic acids are hydrolyzed much faster than those containing higher proportion of either of the two

monomers. Gilding and Reed have pointed out that PLGA containing less than 70%

glycolide are amorphous in nature. The degree of crystallinity and the melting point of

the polymers are directly related to the molecular weight of the polymer.

7 The Tg (glass transition temperature) of the PLGA copolymers are above the

physiological temperature of 37º C and hence they are glassy in nature. Thus they have a

fairly rigid chain structure which gives them significant mechanical strength to be formulated as drug delivery devices. The PLGA copolymer undergoes degradation in an

aqueous environment through cleavage of its backbone ester linkages. Hydrolysis of the

polymer backbone is accompanied by gradual erosion of the device. The biodegradation

rate of the PLGA copolymers are dependent on the molar ratio of the lactic and glycolic

acids in the polymer chain, molecular weight of the polymer, the degree of crystallinity

and Tg of the polymer. A three-phase mechanism for the PLGA biodegradation has been proposed.

- Random chain scission process: The molecular weight of the polymer decreases

significantly, but no appreciable weight loss and no soluble monomer products

formed.

- In the middle phase a decrease in molecular weight accompanied by rapid loss of

mass and soluble oligomeric and monomeric products are formed.

- Soluble monomer products formed from soluble oligomeric fragments. This phase

is that of complete solubilization.

The role of enzymes in any PLGA biodegradation has not been well established. Most of the literature indicates that the biodegradation of PLGA does not involve any enzymatic activity and is purely through hydrolysis [26]. However, some investigators have suggested enzymatic role in PLGA breakdown based upon the difference in the in vitro and in vivo degradation rates [27].

8 The polymer microparticles are usually prepared by a solvent evaporation technique or slight modifications of it. The biodegradable microparticles have a great range of applications for delivering drugs through oral formulations, injectable formulations and also as bioadhesive systems. Some of the commonly employed materials and techniques to produce microparticles for drug delivery and microfabricated drug delivery devices are discussed in the following sections.

2.2. Fabrication techniques of biodegradable micro/nanoparticles for drug delivery

applications: Within the broad category of microparticles (Figure 2.2), ‘microspheres’

specifically refer to spherical microparticles and ‘microcapsules’ applies to

microparticles which have a core surrounded by a material which is distinctly different from that of the core. The core may be solid, liquid or even gas. A microparticle usually refers to a homogeneous mixture of the polymer and active agent, whereas microcapsules have at least one discrete domain of active agent.

Figure 2.2: Different categories of microparticles [1]

9

A number of techniques to fabricated drug-loaded biodegradable Poly (lactide-co- glycolide) microdevices have been reported [28]. The method employed to encapsulate the drug in the polymeric device must meet the following requirements [29]:

I. The stability and biological activity of the drug must not be affected by the processing parameters employed in the fabrication of drug-loaded microparticles

II. The yield of microparticles should have the desired size range and the drug encapsulation efficiency should be high

III. The particle quality and the drug release profile should be reproducible

2.2.1. Solvent evaporation and extraction based processes

2.2.1.1. Single emulsion process: This process involves oil-in-water (o/w) emulsification. The o/w emulsion system consists of an organic phase comprised of a volatile solvent with dissolved polymer and the drug to be encapsulated, emulsified in an aqueous phase containing a dissolved surfactant.

10

Figure 2.3: Encapsulation using oil-in-water emulsion technique [1]

A surfactant is included in the aqueous phase to prevent the organic droplets from coalescing once they are formed. The polymer-solvent-drug solution is emulsified (with appropriate stirring and temperature conditions) to yield an o/w emulsion. The emulsion is created by using a propeller or magnetic bar for mixing the organic and aqueous phases. As seen in the figure 2.3, surfactants are used to stabilize the dispersed phase droplets formed during emulsification and inhibit coalescence. Surfactants are amphipathic in nature and will align themselves at the droplet surface promoting stability by lowering the free energy at the interface between the two phases. The surfactant also confers resistance to coalescence and microsphere flocculation. PVA is one of the widely used surfactants for producing the PLGA microparticles.

Once the emulsion is formed, it is subjected to solvent removal by either evaporation or extraction process to solidify the polymer droplets. In the case of solvent removal by evaporation, the emulsion is maintained at a reduced pressure or at atmospheric pressure

11 and the stir rate is reduced to enable the volatile solvent to evaporate. The organic solvent

leaches out of the droplet into the external aqueous phase before evaporating at the water- air interface. In the case of extraction, the emulsion is transferred to a large quantity of water or other quench medium, into which the solvent associated with the oil droplets is diffused out. The rate of solvent removal by extraction depends on the temperature of quench medium, ratio of the emulsion volume to quench medium and the solubility characteristics of the polymer, the solvent and the dispersion medium. A high extraction

result will result in formation of particles with a high porosity that could lead to

undesirable drug-release profiles [30, 31]. The solvent removal method by extraction is

faster (generally <30 minutes) than the evaporation process and hence the microspheres

made by this method are often more porous in comparison to those made by solvent-

evaporation method.

One of the disadvantages of the o/w emulsification process is the poor encapsulation

efficiency with moderately water-soluble drugs. The drug diffuses out or partitions from

the dispersed oil phase into the aqueous continuous phase and microcrystalline fragments

of the hydrophilic drugs get deposited on to the microsphere surface [32] and dispersed in

the polymer matrix. This results in poor trapping of the hydrophilic drug and initial rapid

release of the drug (burst effect) [29]. The oil/water emulsification process is thus widely

used to encapsulate lipid-soluble drugs. In order to increase the encapsulation efficiency

of water soluble drugs, an oil-in-oil emulsion method was developed [33]. In this method,

the drug may be dissolved or suspended in the oil phase before being dispersed in another oil phase. A water-miscible organic solvent like acetonitrile is employed to solubilise the

12 drug in which PLGA or PLA are also soluble. This solution is then dispersed in oil such as light mineral oil in the presence of an oil soluble surfactant like sorbitan oleate (Span) to yield the o/o emulsion. Microparticles are finally obtained by evaporation or extraction of the organic solvent from the dispersed oil droplets and the oil is washed off by solvents like n-hexane. This process is also referred to as water-in-oil (w/o) emulsion method [29].

2.2.1.2. Double emulsion process: A double emulsion process is usually employed for drugs not soluble in an organic solvent. A solid-in-oil-in-water emulsion (s/o/w) process could be used to encapsulate a drug provided its form is of small size. The size of the drug crystal should be at least an order of magnitude smaller than the desired microparticle diameter in order to avoid large bursts associated with dissolution of larger crystals. Smaller crystals will be homogeneously distributed throughout the organic droplets created in emulsion. Hydrophilic drugs (cisplatin, doxorubicin) have been encapsulated using this method. The problem with encapsulating hydrophilic drugs is the loss of drug to the external aqueous phase during the formation of the microparticle.

Along with the loss of drug to the external phase, the remaining drug may migrate to the surface of the droplet before solidifying. To minimize these problems, the organic droplets should be solidified into microparticles as quickly as possible following their formation [34]. This is achieved by using a viscous organic solution of polymer and drug and a large secondary volume of water that attracts the organic solvent into the aqueous phase immediately, thus leaving the microparticle with the encapsulated drug. The viscous dispersed phase minimizes the volume of organic solvent, facilitating its quick removal from the droplet and also makes it more difficult for the solid drug

13 particles/crystal to migrate to its surface, resulting in a more homogeneous distribution of

the drug within the particle.

Another alternative to encapsulate hydrophilic drugs is to employ the water-in-oil-in-

water (w/o/w) emulsion process (Figure 2.4). An aqueous solution of the drug is added to

an organic phase consisting of the polymer and organic solvent with vigorous stirring to

form the first w/o emulsion. This emulsion is then dispersed in another aqueous phase

containing more surfactant to form the w/o/w emulsion. A number of hydrophilic drugs

like the peptide leuprolide acetate, a lutenizing hormone-releasing hormone-releasing

hormone agonist [35, 36], vaccines [37, 38], proteins/peptides [39-41] and conventional molecules [42, 43] have been successfully encapsulated by this method. The problem with this type of emulsion occurs when the inner emulsion is not sufficiently stabilized, resulting in loss of aqueous droplets containing drug to the external aqueous phase. The choice of surfactants that can be used to stabilize the inner emulsion is limited to materials that will dissolve in the organic solvent. Typically, the fatty acid esters of polyoxyethylene or sorbitan are used due to their high solubility in organic solvents and good biocompatibility.

14

Figure 2.4: Schematic of w/o/w in-liquid drying process for microparticle preparation [1]

2.2.2. Phase separation: This process consists of decreasing the solubility of the encapsulating polymer by addition of a third component to the polymer solution [27, 29].

This process yields two liquid phases: the polymer containing coacervate phase and the supernatant phase depleted in polymer. The drug which is dispersed/dissolved in the polymer solution is coated by the coacervate. Thus the coacervation process consists of the following three steps: i) phase separation of the coating polymer solution, ii) adsorption of the coacervate around the drug particles, and iii) solidification of the microspheres.

The polymer is first dissolved in an organic solution. The water-soluble drugs like peptides and proteins are dissolved in water and dispersed in the polymer solution (w/o emulsion). Hydrophobic drugs like steroids are either solubilized or dispersed in the 15 polymer solution. An organic non-solvent is then added to the polymer-drug-solvent

system with stirring, which gradually extracts the polymer solvent. As a result, the

polymer is subjected to phase separation and it forms soft coacervate droplets that entrap the drug. The system is then transferred to a large quantity of another organic non-solvent to harden the microdroplets and form the final microspheres which are collected by

washing, sieving, filtration, or centrifugation, and are finally dried [26].

This process is suitable to encapsulate bother water-soluble as well as water-insoluble

drugs. However, the coacervation process is mainly used to encapsulate water-soluble

drugs like peptides, proteins, and vaccines. The rate at which the first non-solvent is

added should be such that the polymer solvent is extracted slowly, allowing sufficient

time for the polymer to deposit and coat evenly on the drug particle surface during the

coacervation process. The concentration of the polymer used is important as well, since

too high a concentration would result in rapid phase separation and non-uniform coating

of the drug particles. The coacervate droplets are extremely sticky and adhere to each

other before the complete phase separation or hardening stages of this method. Adjusting

the stirring rate, temperature, or the incorporation of an additive is known to rectify this

problem [26]. Dichloromethane, acetonitrile, ethyl acetate, and toluene have been used as

non-solvents in this process. The non-solvent affects both phase separation and the

hardening stages of the coacervation process. The non-solvents should not dissolve the

polymer or the drug and should be miscible with the polymer solvent [44]. The second

non-solvent should be relatively volatile and should easily remove the first viscous non-

solvent by washing. Some of the oils used as the first non-solvent are silicone oil,

vegetable oils, light liquid paraffin, low molecular weight liquid polybutadiene, and low

16 molecular weight liquid methacrylic polymers. Examples of the second non-solvent

include aliphatic hydrocarbons like hexane, heptane, and petroleum ether [26].

2.2.3. Spray drying: Spray-drying is a widely used method in the pharmaceutical industry and has been investigated by several researchers as a method for formulating biodegradable microparticles [15, 45-47]. It is rapid, convenient, easy to scale-up, involves mild conditions, and is less dependent on the solubility parameters of the drug and the polymer. This method typically uses drug dissolved or suspended in a polymer solution (either organic or aqueous solvent, depending on the polymer used). This solution/suspension is then fed into the spray-drying apparatus, of which the most

important component is the nozzle. The polymer/drug solution is mixed rapidly with air

and forced through a small diameter orifice. Nebulization of the polymer/drug solution

occurs at the nozzle [45] and the resultant droplets are very quickly dried by evaporation

(under high-pressure air) before collection.

Significant advantages of using this technique include high encapsulation efficiencies

and no residual surfactant on the surface of the microparticles. There is no external

aqueous phase that can act as a sink for the drug and there is no surfactant present

anywhere in the formulation. Parameters that affect the microparticle size and

morphology are temperature, pressure (of the air used for drying), nozzle diameter,

air/solution volume mixture, and polymer/drug concentrations.

Recently Pack et al have reported a novel technique [15] based on spray drying to

generate PLGA microspheres with uniform size distribution (see Figure 2.5). The

microsphere fabrication protocol is based on passing a solution containing the polymeric

17 material (and the drug to be encapsulated) through a small nozzle to form a smooth cylindrical jet. A piezoelectric transducer, driven by a wave generator at a frequency tuned to match the flow rate and the desired drop size, vibrates the nozzle and breaks down the jet into a train of uniform droplets. An annular, non-solvent carrier-stream over the polymer solution provided further control over the droplet size and they have fabricated uniform spheres with average diameters from ~5 to > 500 µm.

Figure 2.5: Spray-drying apparatus A) Schematic drawing of apparatus for fabricating microspheres portraying acoustic excitation with carrier stream B) Schematic drawing indicating the variables used for acoustic excitation theory development [15]

2.2.4. Microfabrication based Drug Delivery systems: Microfabrication techniques that are utilized presently to form novel drug delivery devices evolved from silicon-based integrated circuit manufacturing technologies, such as photolithography, thin film growth/deposition, etching, and bonding. Historically, microfabrication techniques have been focused on the development of diagnostic techniques for the health care industry.

However, with the emergence of a variety of protein-based drugs and the challenges

18 posed by them, researchers have begun to focus on the development of microdevices for therapeutic applications. Recent advances in polymer microfabrication [18, 21, 48, 49] and the advantages that it confers (geometrical control, surface chemistry, etc) over conventional methods indicate that there is tremendous potential for the application of microfabrication based techniques to create more effective and novel drug delivery systems [50]. However, there are also certain limitations associated with the application of conventional microfabrication technologies for fabricating polymeric drug delivery systems. For example, the use of photolithography to generate features is limited by optical diffraction and the high-energy radiation required to fabricate small structures requires complex facilities and technologies. It is also expensive, cannot be applied to non-planar surfaces, provides very little control over the chemistry of patterned surfaces and is limited in the variety of materials that can be used. Also, processes like etching involve the use of corrosive chemicals and present a great obstacle in fabricating effective polymeric drug delivery devices. Soft lithography techniques developed by

Whitesides et al presents a way to avoid some of the limitations imposed by conventional microfabrication and complement it to fabricate polymeric microdevices for drug delivery [51]. Soft lithography is a general term for several non-photolithographic techniques based on self-assembly and replica molding for micro and nano-fabrication.

The use of a patterned elastomeric material is a common feature among all these technologies, hence the name ‘soft lithography’. In the following section, we discuss some of the drug delivery systems developed using microfabrication and soft lithography to fabricate polymeric microdevices.

19 2.2.4.1. Transdermal Drug Delivery Systems: Transdermal drug delivery refers to

administration of drug across the skin. This approach seeks to avoid degradation of the

drug molecule in the gastrointestinal tract and first-pass effects of the liver associated

with oral delivery. Also the release of drugs across the skin from a patch like system

offers the possibility of controlled release over time and increases patient compliance compared to injections using a painful hypodermic needle [52]. The success of transdermal delivery has been limited by the inability of drugs to enter the skin at therapeutically useful rates [52]. To overcome this, the use of microneedles in increasing skin permeability has been proposed and this approach is been pursued by several research groups. The microneedles have been fabricated with various configurations

(solid or hollow) using different materials like silicon and different types of polymer [53-

56]. Although the concept of using microneedles for delivery across the skin was proposed in the 1970’s[57] it was not realized until the 1990’s when the tools to make such structures were borrowed from the microfabrication industry and applied for formulating novel drug delivery systems. Microneedles fabricated for transdermal drug delivery need to be long enough to penetrate the stratum corneum of the skin, but short enough to avoid the stimulating nerves. Since the skin’s stratum corneum has no nerves, the insertion of microneedles through this layer without stimulating the underlying nerves offers the promise of painless drug delivery. However, in current practices there is no evidence of microneedles penetrating the stratum corneum without entering the viable epidermis where nerves are found. Studies carried out to determine the pain caused by microneedles have reported that the subjects considered the use of microneedles as painless [58-60]. This is possibly because of their small size, which reduces the

20 probability of the needles encountering a nerve or stimulating it enough to produce a

painful sensation [52].

Initially, arrays of solid silicon microneedles were fabricated (as shown in figure 2.6)

as a microneedle-based transdermal patch system and were shown to increase the

permeability of skin to a variety

Figure 2.6: Microneedle fabrication sequence: a) defining the photoresist by selective exposure to UV light through a photomask, b) developing the photoresist, c) etching the mask layer and removing the photoresist d) performing an anisotropic etch to undercut the masks, leaving solid microneedles. This process yields microneedles shown in figure 2.7b and c [55]

of different molecules by orders of magnitude including macromolecules [61]. Further

work on this led to the development of hollow metal microneedles (see figure 2.7) that increased the permeability still further [62]. Several other groups have used a variation of the microfabrication approach borrowed from the microelectronics industry to fabricate silicon-based microneedles for drug delivery [63, 64]

21

Figure 2.7: Scanning electron micrographs of a) a 26-gauge hypodermic needle, b) a silicon microneedle array shown at the same magnification c) the array in b) at a higher magnification d) a hollow metal microtube array e) a hollow metal microneedle array and f) a tip of a hollow metal microneedle penetrating up through the underside of human epidermis [55]

Although silicon is attractive as a material for the microelectronics industry, it is relatively expensive, fragile, and not ideal for applications that require biocompatibility

[54]. To address this, polymer based microneedles were developed. The fabrication procedure for polymeric microneedles (figure 2.8) consists of making SU-8 structures using photolithography and fabricating PDMS micromolds from the SU-8 master

22 structure. The array of microneedles were coated with PDMS and peeled to make an inverse mold which were used for fabricating microneedles encapsulated with drug.

Figure 2.8: Method to fabricate polymer microneedles that encapsulate drug for controlled release. First, a suspension of drug particles is filled into a microneedle mold. Evaporation of the solvent leaves solid drug particles partially filling the mold. Pellets of biodegradable polymer are then melted into the mold under vacuum. Cooling and solidification of the polymer yields biodegradable polymer microneedles with encapsulated drug particles [53].

23 Microneedles have been successfully fabricated by different groups using

microfabrication techniques and demonstrated the delivery of genes and drugs into cells,

local regions of tissue, and across the skin. Hollow microneedles made out of materials

other than the industry standard silicon, such as metal (palladium), polysilicon, and

biodegradable polymers from micromolding techniques seem to be better designs as they

facilitate better control over drug delivery, are more biocompatible and are less expensive

for production [55].

2.2.4.2. Oral delivery: Oral delivery is one of the most preferred routes of administration

of drugs. However, oral delivery of peptide-based drugs presents a major challenge

because the human gastrointestinal tract resists absorption of peptides, proteins and other

large molecules unless they are broken down into smaller molecules. To enhance oral

delivery of peptides, several approaches have been pursued. They include protective

coatings [65] to protect the peptide during transport through the acidic environment of the

stomach, targeted delivery, permeation enhancers, protease inhibitors [48], and

bioadhesive microparticles [66, 67]. The microfabricated particles present certain

advantages in that they can combine several desired features like enteric coating, attachment of targeting moieties/bioadhesive agents, control over the size and shape of particles that could potentially enhance the uptake of the particles across the intestinal villi, and control over the release rate of the drug based on the size/shape of particles.

Desai et al reported a process for fabrication of bioadhesive microdevices for controlled drug delivery [68, 69]. This is illustrated in figure 2.9. The process demonstrated successful surface modification of PMMA microdevices and the ability to

24 design certain features on the device that could potentially increase the contact area between the particle and the intestinal lining. However, the methods and chemicals

involved in silicon-based microfabrication are not very desirable in making particles for

drug delivery as the drugs would be exposed to corrosive chemicals, UV radiation, and

the process is relatively expensive. To address this, soft lithography based methods for

fabricating polymer particulate devices have been developed [19, 49, 70].

Figure 2.9: Fabrication sequence for bioadhesive microdevice a) Clean silicon wafer b) PMMA spin coated on the wafer c) Photoresist on polymer device d) Polymer device defined e) Develop photoresist f) Carve device bodies g) define device reservoirs h) develop photoresist and carve device reservoirs i) remove photoresist [68]

2.2.4.3. Intravenous Delivery: Conventional particulate drug delivery systems described

earlier, such as polymer microspheres fabricated by emulsion and spray-drying

25 techniques, have been researched more extensively for use as intravenous drug delivery.

However, microfabrication techniques render the ability to produce more functionalized

structures for drug delivery. Imedd worked on fabrication of microparticles for

intravenous delivery for treatment of solid tumors [23]. Recently, soft lithography based

techniques to fabricate microparticles for drug delivery has been investigated by several

researchers [19, 21, 70]. The protocols developed so far have been focused on fabricating

the microparticles, and drug-encapsulation studies have not been conducted with these polymer microparticles.

2.2.4.4. Microfabricated Implants: The advances in the field of microfabrication have

fueled visions of controlled release/smart drug delivery systems that could deliver drugs

on demand. For example, a solid-state microchip that could provide controlled release of single or multiple chemical substances on demand was fabricated and demonstrated [24].

The release of drug is achieved via electrochemical dissolution of the thin anode membranes covering the microreservoirs filled with chemicals in solid, liquid, or gel-like form. Multipulse drug delivery from a resorbable microchip polymeric device has also been demonstrated [24].

Microfabrication techniques have also been applied to create a biocapsule (see figure

2.10) for effective immunoisolation of transplanted islet cells for treatment of diabetes

[48]. This process involved surface micromachining of nanochannels in a thin film on top of a silicon wafer and releasing the membrane by etching away the bulk of the silicon wafer underneath the membrane. The nanopore membranes were designed to allow the permeability of glucose, insulin and other metabolically active products, while at the

26 same time preventing the passage of cytotoxic cells, macrophages, antibodies, and complement. The membranes are bonded to a capsule that housed the pancreatic islet cells. Since the differences in the size of insulin, which must pass through the pores, and the size of immunoglobulins, which must be prevented from entering the pores is only a matter of few nanometers, the highly uniform pore size distribution provided by micromachining was essential for isolation of the capsule from the immune system and to achieve the desired therapeutic effect.

Figure 2.10: Diagram showing design of cell isolation capsule. The membrane pore size allows small molecules such as salts O2, glucose and insulin to freely diffuse through the membrane. Molecular components of the immune system including IgG and cellular elements (such as cytotoxic T lymphocytes) are excluded-protecting the cells from immune system –mediated rejection [48]

2.2.4.5. Soft Lithography: Soft lithography is an emerging technology that consists of

several methods for patterning two and three-dimensional structures with minimum

feature sizes in the nanometer regime using a patterned elastomer as the mask, stamp, or

mold [51]. Successful fabrication techniques that have emerged from development efforts

in microelectronics – photolithography, electron-beam lithography, etc, – are well-suited

to the tasks for which they were principally designed: forming structures of radiation-

sensitive materials on semiconductor surfaces [71]. However, significant challenges exist

27 in adapting these lithographic methods for emerging applications and areas of research that require unusual systems and materials (e.g. drug delivery, polymer electronics etc),

structures with nanometer dimensions, large patterned areas (larger than a few square

centimeters), or non-planar surfaces. The established microfabrication techniques also

involve high capital and operational costs. As a result, some of the oldest and

conceptually simplest forms of lithography are now being re-visited for their potential to

serve as the basis for micro and nanofabrication techniques [72]. Microcontact Hot

Printing technique was developed by Guan et al to prepare thin film microparticles with

well-defined lateral shapes out of common thermoplastic polymers [21]. The technique is

based on selectively transferring polymer features from a continuous film on a stamp to a

substrate. PDMS stamps with both isolated protruding and recessed features can be used

to make microparticles with this method.

Another method based on soft lithography techniques has been reported for fabrication

of particles which affords precise control over particle size, shape and composition.

Monodisperse particles of poly (ethylene glycol diacrylate), triacrylate resin, poly (lactic

acid), and poly (pyrrole) have been fabricated by this method. This method uses a low

surface energy, chemically resistant fluoropolymer as opposed to PDMS employed

commonly in soft lithography methods and eliminates the formation of a residual

interconnecting film between molded objects [70].

2.3 Practical Examples: Drug loaded microparticles have been applied to improve the

efficacy for various therapies. Some of the products that are commercially available in

the market are listed in table 2.1[22].

28

Drug Trade name Company Application Leuprolein acetate Lupron Depot Takeda Prostate cancer Recombinant growth hormone Nutropin depot Takeda Prostate cancer Goserelin acetate Zoladex I.C.I Prostate cancer Octrotide acetate Sandostatin LAR Novartis GH suppression depot anti-cancer Triptorelin Decapeptyl Debiopharm Cancer Recombinant bovine somatropin Posilac Monsanto Milk production in cattle Risperidone Risperdal consta Janssen Schizophrenia

Table 2.1: Examples of pharmaceutical products based on drug loaded, biodegradable microparticles available on the market [22]

Lupron Depot containing the anti-cancer drug leuprolein acetate (embedded within a

PLGA matrix) has been used for the treatment of prostate cancer since 1989. In addition to the possibility of a controlled release profile for the incorporated drug, microparticles offer the advantage of being directly injectable into the target tissue. Microparticles also offer the potential of targeted drug delivery wherein the microparticles can be coated with a targeting moiety that would attach itself to a specific tissue/cancer cells and release drug at a specific site. Potential natural barriers, which might normally hinder the drug to reach its site of action, can be overcome. For example, the blood-brain barrier protects the central nervous system against potential toxins and this renders treatment of brain diseases extremely difficult. Only low molecular weight lipid-soluble molecules and a few peptides and nutrients can cross this barrier to a significant extent, either by passive diffusion or using specific transport mechanisms. Thus, for most drugs it is difficult to achieve therapeutic levels within the brain tissue. The stereotaxic injection of drug-

29 loaded, biodegradable microparticles offers a promising possibility to overcome the challenges posed by the blood-brain barrier. Optimized drug concentrations at the site of action can be provided over prolonged periods of time, improving the efficiency of the pharmacotherapy. Recently, a phase II clinical trial with 5-fluorouracil-loaded PLGA- based microparticles has shown promising results [73].

Another example for the use of controlled release microparticles is the optimization of the growth and differentiation of cells used for cell therapy, where living cells are implanted into human tissue. Main difficulties with this type of therapy include limited cell survival, differentiation, and integration into the host tissue. The time-controlled release of drugs that can stimulate the growth and differentiation of the implanted cells can help to overcome these restrictions. For instance, nerve growth factor was incorporated in PLGA-based microparticles [74]. Cells adhered to microparticles, which released the growth factor in a time-controlled manner (as shown in figure 2.11). This led to improved cell survival and differentiation. These systems are intended to be implanted into human brains. The differentiated cells can produce dopamine, which is required to treat Parkinson’s disease.

30

Figure 2.11: Drug loaded microparticles used to optimize cell growth and differentiation in cell therapies. A. schematic illustration of the concept B. optical microscopy picture and C. scanning electron microscopy picture of cells adhering to surfaces of the microparticles [74]

2.4. Summary: Polymeric formulations could be particularly useful in cancer therapies

and in protein-based therapies. Many disease states, that are currently incurable, can

potentially be cured using these new therapies if the barriers to their delivery are

overcome. Research over the past three decades has generated an enormous amount of

design, formulation and performance data of the micro and nanoparticle systems. This

platform ensures that particulate-based systems will continue to flourish and be ranked amongst the primary options for delivery of biopharmaceuticals which are anticipated to flow from genomic and proteomic research and will require sophisticated delivery strategies to fully realize their potential. In addition, reformulation of existing compounds using novel delivery devices may lead to extension of patent life of these compounds.

31

CHAPTER 3

FABRICATION OF BIODEGRADABLE POLYMERIC MICROPARTICLES USING A HEAT PRESS

This chapter details the experimental methods employed to fabricate biodegradable polymeric microparticles using a heat press. A soft lithography based method was used to fabricate the polymeric microparticles. Soft lithography relies on replication of a patterned elastomeric stamp from a master. Standard photolithography procedures were employed to produce a silicon master with microfeatures. A PDMS mold (elastomeric stamp) was obtained from the silicon master to produce biodegradable polymeric microparticles of different sizes and shapes. The elastomeric stamp was coated with a polymer which was then transferred to a substrate to fabricate polymeric microparticles.

The substrate was coated with a sacrificial layer (release layer) to release the microparticles from the substrate. The methods employed for fabricating the silicon master, the PDMS stamp, conventional soft lithography processes, and the initial process employed using a heat-press to produce free-standing particles are explained in the following sections.

3.1. Photolithography: Photolithography is the most widely used form of lithography

[75]. The master used for producing the elastomeric stamp is a patterned photoresist (SU-

8 2005) surface with a vertical inverse of the desired pattern. Masters with structures

32 larger than 3µm were fabricated successfully using photolithography on silicon wafers. A schematic diagram of the photolithography procedure employed to fabricate the silicon master with patterned resist surface is shown in Figure 3.1. Figure 3.3. shows a wafer with patterned photoresist structures on it.

3.2. Poly (dimethyl siloxane) stamp preparation: The first step in the soft lithography technique is to obtain a patterned elastomeric poly (dimethyl siloxane) PDMS stamp. The schematic diagram of this process is illustrated in Figure 3.2. A 10:1 w/w mixture of

PDMS prepolymer (Silastic T2) and T2 curing agent (Dow Corning, Midland, MI) was prepared. The silicon wafer with the patterned photoresist surface was placed in a polystyrene petri dish and the prepolymer mixture was poured over the wafer. After pouring the prepolymer over the wafer, the dish was placed under vacuum to remove gas introduced into the mixture during stirring. The prepolymer was allowed to cure against the master for at least 24 hours at room temperature. The PDMS was then carefully peeled off the wafer and a 1-inch diameter mold was cut from it. These patterned PDMS mold had the negative features used as stamps to produce polymeric microparticles. The

PDMS molds were visually inspected under an optical microscope (Figure 3.4).

33

Silicon substrate

SU-8 Photoresist

Exposure to UV

Patterned photoresist on substrate

Figure 3.1: Schematic drawing of the photolithography process

34

Figure 3.2: Schematic diagram illustrating preparation of PDMS mold from the patterned wafer

35

Figure 3.3: Silicon wafer with patterned features on the surface

Figure 3.4: PDMS mold with 10µm circular (wells) features

36 3.3. Experimental Methods:

3.3.1. Materials: Poly lactic glycolic acid (PLGA, lactic to glycolic acid ratio = 50:50 and 65:35, Tg= 48.5ºC, according to the manufacturer) were obtained from Alkermes

(now Lakeshore Biomaterials) (Wilmington, OH). Cold-water soluble poly (vinyl alcohol) (PVA) was purchased from Sigma-Aldrich. PLGA 65:35 and PLGA 50:50 were extensively used for making the polymeric microparticles.

3.3.2. Process Overview: The process developed here to fabricate microparticles was based on a soft lithography method known as Microtransfer molding (µTM). In µTM (as shown in figure 3.5), a drop of liquid prepolymer is applied on the patterned surface of a

PDMS mold and then excess liquid is removed by scraping with a flat PDMS block or by

blowing off with a stream of nitrogen. The filled mold is then placed in contact with a

substrate and irradiated or heated. After the liquid precursor has cured to a solid, the mold

is peeled away carefully to leave a patterned microstructure on the surface of the

substrate. Figure 3.6 shows SEM images of some typical polymers fabricated by µTM

[76].

The most significant advantage of µTM is the ease with which it can fabricate

microstructures on non-planar surfaces, a characteristic that is essential for building

three-dimensional microstructures layer by layer. µTM can generate microstructures over

relatively large areas (ca. 3cm2) within a short period of time [76]. It has been used to

fabricate optical waveguides, couplers and interferometers from organic polymers. It also

has the capability of forming patterned microstructures with a wide variety of other

materials. However, one significant disadvantage of the µTM process is that

37 microfabricated structures on a flat surface usually have a thin film (~100nm) between the raised features [70]. The presence of this layer of thin film is undesirable in applications such as drug delivery, where we would ideally want to fabrication isolated microparticles. A schematic illustration of this thin film between micropatterned surfaces is also illustrated in Figure 3.7 and is a common feature of soft lithography based molding techniques.

The initial goals for the project were to develop protocols for production of fabricated microparticles of non-spherical geometries. Guan et al work demonstrated the fabrication of polymeric microparticles of various configurations using novel soft lithography techniques as a “proof of concept”. This work developed some of those methods to fabricate non-spherical particles, with higher drug-loading capacity, and to work on the processing conditions to stamp particles over a larger area and collect the particles for further applications.

38

Figure 3.5: Schematic illustration of microtransfer molding [76]

Figure 3.6: Polymeric microstructures fabricated using µTM [76]

39

Figure 3.7: Illustration of traditional imprint lithography in which a residual film is left between the features [70]

3.3.3. Experimental set-up: The initial set-up for fabricating the polymeric microparticles involved the use of a heat press (as shown in Figure 3.8) for stamping the microparticles. PDMS molds were cut into circular pieces (1-inch diameter). A spin- coater was used to apply polymer solutions of varying concentrations at different speeds to determine the effect of film thickness and the concentration of the polymer solution on the morphology of particles. The heat press was equipped with a digital microprocessor for heating of substrates. The anvils were 1-inch in diameter. Manual force was used to apply pressure for both presses and the analog gauges report the force. The presses were designed with a locking feature to allow sustained pressures. A 2-channel thermocouple attached to surfaces of the heated stages ensured measurement of surface temperatures. A light microscope was used for inspection of the mold and particles stamped on the substrate. 40

Figure 3.8: Heat press for fabricating polymeric microparticles [77]

3.3.4. Fabrication process: The fabrication procedure is illustrated in Figure 3.9. A 4-

5% PLGA solution in chloroform was spin-coated over a PDMS stamps fabricated from the silicon master at around 3000 rpm. The features on the PDMS stamp were ‘negatives’ of the features on the silicon master. The spin-coating process resulted in the evaporation of the solvent leaving behind a polymeric film on the mold. To make isolated particles, the polymer on the raised features had to be removed. This was accomplished by doing a

‘first stamp’ on a heated glass substrate. The stamp was bought into conformal contact with a glass slide heated to around 95°C, transferring the PLGA on the raised surfaces of 41 the stamp. A second stamp was made at a higher pressure with the same mold over a

cover slip coated with PVA (sacrificial layer) at around 80°C. The stamp and the slide

were in contact for around 15 seconds to transfer the polymer from the recessed features

onto the PVA layer.

PDMS mold with recessed features

Apply polymer

Second stamp

First stamp Microparticles

Release layer

Figure 3.9: Schematic illustration of the soft lithography process for the fabrication of polymeric microparticles

42 A Coulter Z2 multisizer (Beckman Coulter) equipped with a 100 micron aperture was

used to determine the size distribution. A PVA film spin-coated over a glass substrate

was used as the release layer and the particles (stamped over the PVA) were released in

approximately 20 ml of water. The solution with PLGA particles was mixed with an

equal volume of isoton electrolyte and 2-3 drops of type 1A dispersant was added to

prevent aggregation of particles.

3.4. Results and Discussion: The processes and experimental set-up facilitated

fabrication of polymeric microparticles by varying several parameters such as

concentration of polymer, spin speed, stamping temperature and stamping pressure. 5, 10

and 15µm diameter circular particles were fabricated. The results were observed to be dependent on the wetting behavior of polymer solutions with PDMS interface. PLGA particles with varying ratios of lactic and glycolic acid were fabricated.

PDMS molds were initially observed under microscope. Figure 3.10 displays a clean

PDMS mold, and Figure 3.11 shows a PDMS mold filled with 5% solution of PLGA in chloroform after evaporation of the chloroform. Figures 3.12 and 3.13 display particles fabricated using the same mold on a glass substrate and a PVA film, respectively. Figure

3.14 shows poly propyl methacrylate (PPMA) particles fabricated using anisole as a solvent. Figures 3.15a and b display defects caused in particles due to high stamping pressure employed in the process.

PLGA particles were stamped onto a glass cover slip coated with PVA to release them. The cover slip was placed in a small Petri dish and water was added along the sides of the container to release the particles. The solution was mixed with Isoton and the

43 analysis from Coulter counter indicated that approximately 200,000 particles were

fabricated from one step.

Figure 3.10: Optical micrograph of a clean PDMS mold with 10µm features

Figure 3.11: Optical micrograph of a PDMS mold filled with a PLGA from a solution in chloroform (5% w/w)

44

Figure 3.12: PLGA microparticles stamped onto a glass substrate

Figure 3.13: PLGA microparticles stamped on a layer of PVA

45

Figure 3.14: PPMA particles

Figure 3.15 a) PLGA particles stamped on PVA b) A thin residual film of PLGA due to improper stamping can be observed between the features

46

Figure 3.16: Size distribution profile of particles from a 10µm PDMS mold

The particles fabricated were analyzed under the SEM and the morphology of the particles indicated some of the limitations of the heat press. The pressure applied was not uniform throughout the substrate and also the diameter of the anvil (1-inch) limited the size of the stamp that was used for particle fabrication. The design of the equipment also limited the maximum force that could be used to apply pressure for stamping out the particles.

3.4.1. Coulter Method of Counting and Sizing: The Coulter method of counting and sizing is based on the detection and measurement of changes in electrical resistance produced by a particle or a cell suspended in a conductive liquid (diluent) traversing through a small aperture. When particles or cells are suspended in a conductive liquid, they function as discrete insulators. When a dilute suspension of particles is drawn 47 through a small cylindrical aperture, the passage of each individual particle momentarily

modulates the impedance of the electrical path between two submerged electrodes

located on each side of the aperture. Figure 3.17 illustrates the passage of particle through

an aperture. An electrical pulse, suitable for counting and sizing, results from the passage

of each particle through the aperture.

Figure 3.17: Coulter method for counting and sizing of particles [78]

While the number of pulses indicates particle count, the amplitude of the electrical pulse produced depends on the particle’s volume. The volume is assumed to be that of a spherical particle and the diameter of the particle is calculated based on the volume of a 48 sphere. For cylindrical particles, the thickness (height) could be calculated if the diameter of the particles is known.

3.4.2. Statistical Analysis: Data from the Coulter counter was imported and statistical

analysis was performed using statistical software R. The “kurtosis” of the distributions of the area and volume variable were calculated. The kurtosis value is a measure of the peakedness of the distribution. Higher kurtosis means more of the variance is due to infrequent extreme deviations, as opposed to frequently modest sized deviations. For example, when comparing two data sets with identical means and standard deviations, the data set with the higher kurtosis is a tighter peak with a few outlining data. The kurtosis of the distributions was also calculated using a truncated variance where all data points with less than 20 particles were removed as insignificant. The kurtosis values obtained using the volume variable was 14.9 and 2.0 respectively for non truncated and truncated data. These values were compared with the size distribution of particles obtained from a hot embosser in chapter 5.

3.5. Summary: We were able to demonstrate the fabrication of polymeric microparticles using a soft lithography technique. The heat-press was used as the stamping device and the process resulted in fabrication of polymeric microparticles with different aspect ratios. The particles fabricated were around 3µm tall and the particles had dents because of the variation in temperature and pressure parameters. However, it would be desirable to have a process that could be improved to produce more uniform particles with a higher yield and higher aspect ratio. Higher aspect ratio results in higher active surface area per

49 unit area of the substrate. This could prove to be useful in drug delivery application as it would result in higher loading capacity for the drug. The solution to these problems lies in improvements that could be made to this soft lithography process which has been discussed in the following chapters.

50

CHAPTER 4

KEY PARAMETERS AND PROCESS IMPROVEMENTS IN MICROTRANSFER MOLDING

4.1. Introduction: Ideally, the process for fabricating microparticles should result in

uniform microparticles with precise control over the shape and size of the microparticles.

The geometry of the particles should facilitate higher attainable drug loading than conventional formulations and simpler encapsulation methodology. Some of the important factors that affect the morphology of the particles fabricated using the heat press were the temperature and pressure applied during the stamping process. The wetting behavior of the polymer solution on the mold and the geometry of the mold are also important parameters affecting the morphology of the particle. The initial technique employed for fabricating particles using the heat press was instrumental in identifying several of these key parameters that had to be addressed to improve the processing conditions to increase the yield and uniformity of particles that were fabricated. They are discussed in the following sections.

4.1.1 Molds: Most of the initial experiments were done using PDMS molds that had shallow features on them (less than 3µm). This resulted in very thin disc-like particles that were around 1-2 microns thick. Also, some polymer was lost during the first stamp due to application of high pressure in the stamping process, resulting in even thinner

51 particles. To address this, the photolithography process was modified to produce silicon masters with a thicker patterned resist structure on them. The resulting PDMS molds made from the patterned resist structures resulted in PDMS molds with deep recessed features. The molds were characterized using an optical profilometer to assess the surface topography.

The molds were cut into small pieces (or circular pieces with 1-inch diameter) using a razor blade before spin-coating with polymer for fabricating the microparticles. This caused non-uniformity in the edges of the mold which again translated to a decrease in yield of particles during the stamping process. This was addressed by using an arch punch (McMaster-Carr, Chicago, IL) to cut the mold uniformly.

Initially, different trials were performed using the same mold. In between trials, molds were cleaned by applying a duct tape on the surface of the mold and peeling off any dirt/polymer left on the surface. However, this process did not take into consideration any residual polymer left in the ‘microwells’. Cleaning PDMS molds by applying pure solvent on the surface (spin-coating/immersing) was found to be more effective than just using the duct tape. This was determined by visually inspecting the mold under an optical microscope.

4.1.2 Spin-coating: The PDMS molds were filled by a spin-coating process. The spin coating process involves depositing the solution onto the substrate and spinning the substrate at high speed (around 3000-3500 rpm). It was observed that spin speed determined the thickness of the polymer film on the mold. Spin speed determines the thickness of the polymer film, which can cause a partially wetting liquid to break aprt if it

52 gets thin enough to be mechanically unstable. A low spin speed (less than 2000rpm)

resulted in a thick polymer film on the surface of the mold. Higher spin speeds (~3500) resulted in a thinner film on the surface of the mold. A thin film with discontinuities

facilitates successful removal of the polymer film between the features whereas a thicker

film results in removal of a continuous film (Figure 4.1). Optimization of the spin speed led to a good first stamp with minimal loss of polymer from the features. Also, increasing

the aspect ratio of the molds (deeper features) facilitated filling of the wells, leaving a

thin film on the surface of the mold that could be easily removed.

Figure 4.1: SEM illustrating the presence of a residual polymer film between the cylindrical features

53 4.1.3 Polymer Solution: Initial experiments indicated that the polymer concentration had

an important role in the ‘filling’ of the recessed features in the mold occurred. A higher

concentration (>4-5%) filled the 5µm deep features completely whereas a lower

concentration solution only partially filled the features. The polymer concentration had to

be optimized to fill the wells completely so that tall, cylindrical particles could be

fabricated from the PDMS mold.

The nature of the films deposited by spin-coating or dip coating is strongly influenced by the wetting and dewetting behavior of polymer solution on the substrate[79]. Initial experiments revealed that different solvents lead to differences in wetting of the substrate

that affected the morphology of particles. For instance, a solution of poly (propyl

methacrylate) in anisole exhibited dewetting on the surface of the PDMS. This resulted in

the polymer left only in wells. However, since the wells were not completely filled with

polymer particles with hemispherical morphology (as shown in Figure 4.2), were

fabricated from the soft lithography process. Part of this could also be attributed to

application of excess pressure during the stamping process but it was clear that the nature of the solution and how it ‘wets’ the substrate had a major effect on the morphology of the particles fabricated. Residual anisole in the polymer could also act as a plasticizer causing PLGA to melt on the surface resulting in hemispherical shape for the particles. In general, the wetting behavior could be attributed to the difference in surface energy between PDMS and the solvent used. Acetone and chloroform had a surface energy similar to that of PDMS and resulted in wetting of the substrate whereas the difference in surface energy of PDMS and solvents like anisole, were much greater and resulted in

‘dewetting’ of the substrate.

54

Figure 4.2: PLGA particles fabricated using anisole as a solvent. Change in solvent affects the wetting of the substrate and the morphology of the particle.

4.1.4 First stamp: Once the PDMS molds were coated with the desired polymer, a two

step stamping procedure was used to fabricate polymeric microparticles. To increase the

yield, the first stamping step to remove the polymer between the features had to be done at the optimum temperature and pressure. Too high a pressure could lead to removal of polymer present in the features as well. If the pressure applied is too low and the temperature parameters are not set correctly, then the polymer between the first features is not removed in the first step. Eventually, the second stamping results in a thin residual film between the particles. The pressure during the first stamping step had to be optimized to produce polymeric particles with uniform shape and size.

55 4.1.5 Uniform pressure and temperature to produce particles: The second stamp

requires application of uniform pressure and temperature across the substrate to produce

uniform microparticles. Too high a pressure causes deformation in the particles and too

high a temperature again distorts the particles due to melting or softening. The use of heat

press increased the uniformity of the pressure but it did not provide perfectly uniform application of pressure across the mold.

Temperature also plays an important role in this process. As the temperature is increased over the glass transition temperature of the polymer, the polymer becomes more mobile/rubbery and capable of viscous deformation. This causes the polymer to

‘flow’ from the wells and transfer the polymer from the PDMS stamp to the substrate.

Initial experiments were performed at different temperatures and it was determined that a surface temperature of around 70-80° C was sufficient to transfer the polymer from features. The temperature for stamping the particle is based on the glass transition temperature (Tg) of the polymer. The Tg of PLGA was 48.5ºC. So the working

temperature to stamp the particles had to be high enough to cause the polymer to flow out

of the features. The temperature for stamping polymeric particles would depend on the

glass transition temperature of the polymer. Ideally, a high temperature during processing

will not be desirable as it could affect the stability of the drug/protein.

4.2. Summary: The protocols developed so far demonstrated a proof-of-concept for

fabricating microparticles of various sizes and geometries. However, several factors were

identified that could further improve upon the existing processes to produce particles of

various morphologies with precise control over their size and geometry reproducibly. The

56 current method to fabricate microparticle was largely dependent on using a heat press as stamping equipment. The factors that were identified in this chapter for improvement were considered and the use of hot embosser (EV 620, EV Group, AZ) as a stamping device was incorporated with the experimental methods to further improve the process, explained in the following chapter.

57

CHAPTER 5

FABRICATION OF POLYMERIC MICROPARTICLES USING THE HOT EMBOSSER AS A STAMPING DEVICE

Commercialization of microsystems technology has led to the emergence of low-cost microfabrication methods suitable for high-volume production. In contrast to the substrates silicon, quartz, or glass, which are still used in some microfluidic systems,

polymers offer a wide variety of advantages [80]. Basically, the µTM process could be

likened to a printing process. All printing process can be logically divided into two main

steps, namely, defining an accurate pattern and bringing it close enough to the substrate

so that the desired process (ink transfer, chemical reaction, sealing or optical exposure)

can be executed. The main function of the printing tool is to establish the contact and

facilitate the release of a stamp from a substrate with precise control over the printing

time, the mechanical forces acting, and the placement of stamp on the substrate. To improve the processes developed on the heat-press as a stamping device, we investigated the use of hot embosser as a printing device to achieve conformal contact between the stamp and the substrate to achieve the desired pattern transfer.

Recent research has led to establishment of hot embossing as an alternative process for fabrication of polymer microcomponents [80]. Based on the work with the heat press as described in chapter 3, it was decided to use the hot embosser as a stamping device to produce particles. A schematic illustration of the process is shown in Figure 5.1, and

58 Figure 5.2 displays a schematic diagram of the hot embossing equipment. The soft lithography technique has already been described earlier. PDMS molds with deeper features (>5µm) were fabricated. Also hexagonal features were fabricated to demonstrate that particles with different geometries could be fabricated successfully using this technique.

Figure 5.1: Process schematics of the hot embossing process [80]

59

Figure 5.2: Schematic drawing of the hot embossing equipment [80]

5.1. Experimental Methods

5.1.1 Materials: Poly (dimethyl siloxane) (PDMS, Silastic T2) was purchased from

Dow-Corning. Poly (lactic-co-glycolic acid) (PLGA, lactic to glycolic acid ratio = 65:35,

Tg = 48.5 ºC according to the manufacturer) was purchased from Alkermes (Cincinnati,

OH). Poly vinyl alcohol (cold-water soluble) was purchased from Sigma-Aldrich

(St.Louis, MO). Silicon wafers (4-inch, coin roll/backside etch) were used as substrates

for stamping the polymeric microparticles and were obtained from Wafernet, Inc (San

Jose, CA).

5.1.2. Photolithography: A layer of SU-8(2005), a negative photoresist (Microchem,

CA), was spin-coated on the silicon wafers (Wafernet, San Jose, CA) to an approximate

thickness of 5 microns. Exposure was done through patterned chrome-plated masks. The

wafer was characterized using the scanning electron microscope (Hitachi S-3000H). The

60 following procedure was employed to obtain the patterned photoresist structure on silicon wafer:

• Spin-coat a layer of SU-8 2005 onto a silicon wafer (spin speed = 1000 rpm)

• Soft bake the wafer on hot plate to remove the remaining solvent from the

photoresist

- 1 minute bake on the first hot plate at 65ºC followed by another 1 minute bake on

the second hot plate at 95 ºC

• Align the coated wafer with the chosen mask in the aligner

• Expose the photoresist coated wafer to ultraviolet radiation

• Do a post-exposure bake to complete the cross-linking reaction in the exposed

area of the photoresist

- 1 minute bake at 65 ºC followed by another 1 minute bake at 95 ºC

• Develop the photoresist on the wafer with SU-8 developer

• Rinse the developed wafer with isopropyl alcohol and dry the wafer in a stream of

nitrogen

5.1.3. PDMS stamp preparation: PDMS stamps were prepared using soft- lithography technique. Silicon masters with designed microfeatures were produced by standard photolithography. Negative photoresist (SU8-2005) was used for pattern production. PDMS stamps were prepared by casting PDMS resin and curing agent at a

10:1 weight ratio against the master for 36 hours at room temperature.

61 5.1.4. Preparation of PLGA microparticles: 5% w/w solutions of PLGA in acetone

were prepared and spin-coated at a spin speed of 3000rpm over a 2 inch PDMS mold that

had the desired microfeatures on it. The PDMS stamp filled with polymer was applied

over a silicon wafer at a temperature of 90 ºC and the stamp was tapped against the wafer

to achieve conformal contact. This process removed the polymer from the surface of the

PDMS mold and was followed by a second stamping at a higher pressure (493 KPa) over

the substrate (clean silicon wafer) to transfer the polymer from the recessed features onto

the substrate. This process is outlined in Figure 5.3. In order to facilitate release of

particles and determine their size distribution, the particles were stamped onto a poly

(vinyl alcohol) (PVA) layer spin-coated over the silicon wafer at 2000 RPM. The PVA film served as a sacrificial layer and was chosen for its solubility in water and high glass transition temperature.

5.1.5. Characterization: An optical profilometer (WYKO NT3300, Veeco

Instruments, Woodbury, NY) was used to characterize the PDMS stamps. Scanning electron microscopy (Hitachi S-3000H) was used to image the surface morphology of the

PLGA microparticles. Samples were sputter coated (Technics Hummer VI) with gold before imaging. A Coulter Z2 multisizer (Beckman Coulter) equipped with a 100 micron aperture was used to determine the size distribution. A PVA film spin-coated over the silicon wafer was used as the release layer and the particles were released in approximately 20 ml of water. The solution with PLGA particles was mixed with an equal volume of isoton electrolyte and 2-3 drops of type 1A dispersant was added to prevent aggregation of particles.

62

Figure 5.3: Schematic drawing of the soft lithography process to fabricate polymeric microparticles a) Silicon wafer with desired microfeatures b) PDMS cast over the silicon master c) PDMS mold with recessed features d) PLGA solution spin-coated over the PDMS mold e) PDMS mld after removal of polymer from the surface of the mold by the first-stamping f) Individual polymer microparticles

5.2. Results and Discussion

5.2.1. PDMS stamps: Figure 5.4 displays the PDMS stamp with recessed features.

Molds with varying diameters (5, 10 and 15µm) were characterized with the profilometer. The depth of the features was determined to be 6.6µm using the optical profilometer. The center-to-center distance was 10, 20 and 30µm for molds with 5, 10 and 15µm diameter features. Fig 5.5 shows a scanning electron micrograph of a PDMS mold with ‘microwells’. PDMS molds with circular features of varying diameters (5, 10 and 15µm) were fabricated in this study. PDMS molds with hexagonal features of varying dimensions were also fabricated to demonstrate that particles of different geometries could be fabricated by applying this soft lithography technique. 63

Figure 5.4: Surface data of the PDMS mold obtained from the profilometer; 2-point profile shows the distance between the features and the depth of the wells

5.2.2 PLGA Microparticles: PLGA microparticles with different dimensions and aspect ratios were fabricated using this technique. Figure 5.6 shows the continuous film removed after the first stamping step. This basically comprises the polymer film between

64 the recessed features. Figure 5.7 depicts PLGA particles obtained after successful removal of the residual polymer film between the features. Figure 5.8 shows PLGA particles made from a solution of PLGA in chloroform. Figures 5.9a and b depict PLGA particles with varying aspect ratios. The same mask was used to make the silicon masters.

This illustrates how particles of varying aspect ratios could be made using the same mask by manipulating the spin speed in the photolithography process. Figure 5.10 shows hexagonal particles to demonstrate that particles with varying geometries could be fabricated by applying this method. Figure 5.11 shows particles of high aspect ratio that were fabricated using a solution that had a higher concentration of PLGA (~12%).

The results from the Coulter counter (Figure 5.12) and the statistical analysis performed indicate that the particles fabricated from the hot embosser have a much narrower size distribution. Approximately seven million particles were fabricated from a

PDMS mold with a diameter of 2 inches.

Figure 5.5. Electron micrograph showing a PDMS stamp with “microwells”

65

Figure 5.6: Electron micrograph of polymeric material removed in the first stamp – “the residual polymer film between the recessed features”

Figure 5.7: Electron micrograph of PLGA particles obtained after successful “first stamp” procedure. Solvent used was acetone.

66

Figure 5.8: PLGA particles made from a solution of PLGA in chloroform (4% w/w)

Figure 5.9: PLGA particles with varying aspect ratios. The particles have the same diameter and the silicon master is made from the same photomask

67

Figure 5.10: Electron micrograph showing ‘hexagonal’ PLGA particles

Figure 5.11: PLGA particles with higher aspect ratio (~1.8)

68

Figure 5.12: Size distribution of particles obtained from different batches using a mold that had circular features with a diameter of 5µm

5.2.3. Statistical Analysis: Data from the Coulter counter was imported and statistical

analysis was performed using statistical software R. The “kurtosis” of the distributions of the volume variable was calculated. The kurtosis value is a measure of the “peakedness”

of the distribution. Higher kurtosis means more of the variance is due to infrequent

extreme deviations, as opposed to frequently modest sized deviations. The kurtosis of the

distributions was also calculated using a truncated variance where all data points with less than 20 particles were removed as insignificant. The kurtosis values were compared for both the processes, heat press and the hot embosser, and are tabulated in Table 5.1.

Figures 5.13 and 5.14 show a plot of the number of particles against volume of the particles for the heat press and hot embosser processes, respectively.

69 Results/Process Heat Press Hot Embosser Number of particles fabricated 200,000 Approximately 7 million in one processing cycle Kurtosis Value 14.9 137.0 Variance 94204.5 1659.7 Kurtosis value using truncated 57121.9 290.0 data Variance value for truncated 2.0 983.1 data

Table 5.1: Statistical comparison of results for the particles fabricated using the hot press and hot embosser processes

Figure 5.13: Volume distribution: number of particles (y-axis) against the volume of particles (x-axis) for the “heat press” process

70

Figure 5.14: Volume distribution: number of particles (y-axis) against the volume of particles (x-axis) for the “hot embosser” process

5.3. Summary: We have observed that for a specific polymer solution, a continuous thin film forms within a certain concentration range. Solutions with low polymer concentration (<3%) lead to thin particles or particles with discontinuous features and inconsistent filling of wells, while too high a polymer concentration (>10%) leads to films that are too thick to maintain the contour of the stamp, preventing the selective removal of the polymer from the surface of the PDMS mold. A 5% solution provided a consistent filling of the wells and maintained sufficient contour in the film to allow selective removal during the first stamping process.

71 In conclusion, we have fabricated independent, uniform PLGA microparticles with different shapes and dimensions using a micro transfer molding technique. The polymeric particles were released in an aqueous solution by dissolving the release layer in water and were determined to have narrow size distributions, as indicated by their high kurtosis value. This technique can be applied to encapsulate proteins/peptides in the polymeric structure and their suitability as controlled drug delivery devices can be studied.

72

CHAPTER 6

POLYMERIC MICROPARTICLES AS DRUG DELIVERY DEVICES

6.1. Introduction: The previous chapters introduced the concept of fabricating free-

standing polymeric microparticles using a soft lithography based technique. To further

demonstrate the suitability of this technique towards making functional drug delivery

devices, encapsulation of Fluorescein iso thiocyanate(FITC) as a model drug was

performed. The particles obtained from the soft lithography technique using the hot embosser were isolated by a simple filtration set-up to demonstrate the ability to collect and isolate the polymeric microparticles for potential drug delivery studies. Also, a protocol was developed to fabricate bi-layered devices with the idea of controlling the release of the encapsulated agent. Preliminary experiments were conducted by dissolving

Paclitaxel (Taxol) to evaluate the feasibility of using the polymeric microdevices as a controlled release system for taxol delivery.

6.2. Experimental Methods

6.2.1. Encapsulation of FITC: The particles were fabricated using the hot embosser as described in the earlier chapter. Microparticles containing FITC (Sigma Aldrich,

St.Louis, MO) were fabricated by co-dissolving FITC with PLGA in acetone. The

polymer solution was spin-coated on the PDMS mold at around 3500 rpm. This was

followed by the stamping processes to fabricate free-standing polymeric particles

73 containing FITC. The particles were stamped on a glass substrate coated with PVA and

then released by adding water to it.

6.2.2. Filtration: A simple vacuum filtration set-up was done and the particles were collected on nanoporous polycarbonate filtration membrane. The particles were released from the sacrificial layer (PVA) by dissolving the release layer in water. The particles were collected on the filter membrane and imaged with the electron microscope.

6.2.3. Bi-layered polymeric microparticles: Polymeric microparticles that could release the drug in one direction could be useful in reducing the ‘burst effect’ and could be of use in several applications. The microtransfer molding process could be slightly modified to achieve this. This was accomplished by using two polymers with different solubility properties. Initially, a 1-1.5% solution of poly (caprolactone) polymer dissolved in chloroform was deposited in the microwells by spin coating. Since, the concentration of the polymer was very low, the features were not completely filled with polymer. A second spin-coating step was done over the PDMS mold with PLGA dissolved in acetone to fill the wells completely. The schematic of this process is illustrated in Figure 6.1.

Since PCL is only slightly soluble in acetone, the second spin-coating step does not affect the PCL layer. This process results in PLGA particles coated with PCL on all sides except one. It is illustrated in Figure 6.2

74 .

Figure 6.1: Schematic drawing illustrating ‘bi-layered’ microparticle fabrication process

Figure 6.2: Bi-layered microparticle (Blue represents PCL; Red represents PLGA)

75 6.2.4. Encapsulation of Taxol: The poor aqueous solubility of hydrophobic drugs is a

challenging formulation problem, particularly for intravenous delivery. It is estimated

that 40% of new chemical entities have poor aqueous solubility [81]. Thus creating

alternative formulations for such compounds is of significant value. Paclitaxel (Taxol®) is one of the best anti-neoplastic drugs to be found in the past decade. It has been clinically used in the treatment of various cancers especially breast and ovarian cancers. The clinical success of Taxol has been limited due to its low solubility in water and most of the other pharmaceutical solvents compatible for intravenous (i.v.) administration [82].

Paclitaxel is currently available as a solution in a vehicle comprised of Cremaphor EL

(polyoxyethylated castor oil) and ethanol. Side effects observed with intravenous administration of Taxol, such as hypersensitivity reactions, nephrotoxicity and neurotoxicity have been attributed to Cremophor [83].

Taxol (Sigma Aldrich, St. Louis, MO) was obtained from Sigma-Aldrich. Taxol was co-dissolved with PLGA in chloroform and microparticles were stamped out from the solution. The particles were analyzed under HPLC to detect presence of any paclitaxel.

Chromatographic conditions used for the detection of paclitaxel used UV detection at

254nm and a Vydac C-18 column (5µM, 2.1mm(i.d.) x 150mm). The eluent used was

60% Acetonitrile, 40% sodium acetate (50mM in H2O, pH = 6.7). The flow rate was set

0.2ml/min.

6.3. Results and Discussion: Fig 6.3a and 6.3b show polymeric particles loaded with

FITC. This demonstrates the potential of this technique to encapsulate biomolecules for controlled release systems. Optical cross sections, obtained using a Zeiss Meta 510 laser

76 confocal microscope, indicated the presence of FITC inside the polymer matrix. Three

dimensional projections of the particles (Figure 6.4) revealed that FITC was distributed throughout the particle.

Figure 6.3 PLGA particles observed under a confocal microscope: (left) 5µm PLGA particles loaded with FITC; (right) 15µm particles loaded with FITC

Figure 6.4: PLGA particles loaded with FITC. This picture shows complete distribution of FITC in the particle.

77 With 5µm particles, FITC was distributed uniformly throughout the polymer matrix. In

the case of 10 and 15-µm particles FITC tended to be present only on the edges and was

not present in the interior of the particle. This might have been due to the diffusion of

FITC to the edges along with acetone. However, it has not been determined conclusively.

Figures 6.5a and b show PLGA particles collected on a nanoporous polycarbonate

filter membrane, imaged under a scanning electron microscope (Hitachi S-3000H). There

was some non-uniformity in the particles attributed to a difference in the thickness of the

particles. The particles were collected from 5 different batches and imaged under the

SEM. The differences in thickness of the particles could also be a result of the lot-to-lot

variation in the morphology of the particles.

Figure 6.6 show bi-layered particles made from a PDMS mold with 5µm circular features. Since PCL is a much slower diffusing polymer compared to PLGA, PCL acts as a diffusion barrier. This ensures that the drug is released from the direction where PLGA

degrades. The lactic-glycolic acid ratios in the PLGA could be altered to make

microparticles with tailored degradation rates.

HPLC assays performed for detection of paclitaxel proved to be inconclusive as the results were not reproducible. The HPLC assay was performed on a solution of taxol in chloroform (0.5 mg/ml) (Figure 6.7), polymer and taxol co-dissolved in chloroform

(obtained from the microparticles) (Figure 6.8). Preliminary results indicated a peak for

both the positive control and the sample around the same time. However, the results were

not reproducible and the experimental parameters need to be optimized for conclusive

evidence to detect and determine the concentration of taxol encapsulated in the

microparticles.

78

Figure 6.5 Filtered PLGA microparticles a) Scanning electron microscopy of filtered PLGA particles on a polycarbonate nanoporous filter b) PLGA particles from a) collected on a polycarbonate nanoporous filter at different magnifications

79

Figure 6.6: Electron micrographs showing bi-layered polymeric microparticles 80

Taxol in Chloroform (1mg/2mL) (1)

0.1

0.08

0.06

0.04 mAU 0.02

0 0 400 800 1200 1600 2000 2400 2800 3200 3600 -0.02 Time (sec)

Figure 6.7: HPLC plot of Taxol in chloroform ( 0.5mg/ml)

Figure 6.8: HPLC plot of PLGA and Taxol co-dissolved in chloroform

81 6.4. Summary: Microparticles incorporated with FITC and Paclitaxel were fabricated by

co-dissolving them with the polymer in the organic solvent. The distribution of FITC in

the microparticles was determined by confocal microscopy. HPLC analysis of the

microparticles incorporated with paclitaxel were inconclusive and the experimental

parameters need to be optimized further to determine the concentration of paclitaxel in

the microparticles. Slight modifications to the technique resulted in fabrication of bi-

layered polymeric microparticles that could provide further control over release of the

drug from the particle. Microparticles released from the substrate and were collected on a filtration membrane and imaged with a SEM. The ability to collect isolated particles could prove to be useful in conducting degradation studies of the microparticles.

82

CHAPTER 7

CONCLUSIONS AND RECOMMENDATIONS

7.1. Conclusions: Conventional silicon-based microfabrication techniques suffers from

drawbacks such as the use of expensive clean-room based facilities, harsh processing

conditions and use of materials that are not ideally suited for drug delivery. In contrast,

the soft lithography technique developed is a benign, highly versatile method suitable for

production of isolated, monodisperse microparticles of various sizes. The microparticles

could be made from a variety of biocompatible/biodegradable polymers and the processing techniques cold be modified to produce more sophisticated devices.

The novel soft lithography technique was applied to produce polymeric microparticles of various sizes and geometries. Initial studies using a heat press to fabricate microparticles served as a proof-of-concept for fabricating non-spherical microparticles and were useful in identifying several parameters to further improve the processing to fabricate uniform microparticles. The use of hot embosser as a stamping tool provided precise control over temperature and pressure enabling improvement over the initial process to produce millions of microparticles in a single process. Using this technique, we were able to encapsulate FITC as a model drug and distribution of FITC in the particle was studied by observing fluorescence using confocal microscopy. This

83 method could be applied for encapsulating other compounds, including proteins for drug delivery applications. The particle morphology was analyzed by scanning electron microscopy and the size distributions were analyzed using a Coulter counter. These microdevices have potential for use in controlled drug delivery applications and the particles could be administered through different routes based on their size and geometry.

These studies demonstrate the promise that soft lithography based polymer microfabrication techniques hold for developing more sophisticated, multi-functional drug delivery devices. Further, this process can be employed for commercial viability by employing a roller based lithography system similar to gravure printing techniques [51,

70].

7.2. Recommendations: Although the fabrication of millions of polymeric microparticles for drug delivery applications has been successfully demonstrated in this dissertation, there is still a need to improve on the protocols further for the process to be commercially viable. The current protocol could be improved further to increase the yield. To make the process commercially viable, a roller based soft lithography system for continuous production of particles has been proposed in literature. Further characterization of the microstructures is needed to determine the degradation rates of the particles.

Controlled drug delivery applications being the target of these polymeric microdevices, more studies need to be conducted with loading of the biomolecules in the particles. Characterization of the microparticles loaded with drugs and their release kinetics need to be studied. Efforts should be made in order to further optimize this

84 process for encapsulating hydrophilic drugs. A survey of the literature suggests that proteins/peptides encapsulated could withstand exposure to organic solvents [84].

However, the structural integrity of the proteins could be affected under high temperatures and pressure involved in the process. Immunoassay and animal studies could be done to determine the efficacy of the drugs incorporated in the particles.

85

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