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Responsive Materials and Methods
Scrivener Publishing100 Cummings Center, Suite 541J
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Advance Materials Series
The Advance Materials Series is intended to provide recent advancements of the fascinating fi eld of advanced materials science and technology, particularly in the area of structure, synthesis and processing, characterization, advanced-state properties, and applications. The volumes will cover theoretical and experimental approaches of molecular device materials, biomimetic materials, hybrid-type composite materials, functionalized polymers, super-amolecular systems, information- and energy-transfer materials, biobased and biodegradable or environmental friendly materials. Each volume will be devoted to one broad subject and the multi-disciplinary aspects will be drawn out in full.
Series Editor: Dr. Ashutosh TiwariBiosensors and Bioelectronics Centre
Linköping University SE-581 83 Linköping
SwedenE-mail: [email protected]
Managing Editors: Swapneel Despande and Sudheesh K. Shukla
Publishers at ScrivenerMartin Scrivener ([email protected])
Phillip Carmical ([email protected])
Responsive Materials and Methods
Edited by
Ashutosh Tiwari and Hisatoshi Kobayashi
State-of-the-Art Stimuli-Responsive Materials and
Their Applications
Copyright © 2014 by Scrivener Publishing LLC. All rights reserved.
Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts.Published simultaneously in Canada.
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Cover design by Russell Richardson
Library of Congr ess Cataloging-in-Publication Data:
ISBN 978-1-118-68622-5
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
v
Contents
Preface xiii
PART 1 Stimuli-Responsive Polymeric Materials 1
1 Smart Thermoresponsive Biomaterials 3Mohammed Yaseen and Jian R. Lu1.1 Introduction 31.2 Temperature-Responsive Polymers 5
1.2.1 Thermoresponsive Polymers Based on LCST 51.2.2 Biopolymers and Artifi cial Polypeptides 81.2.3 Temperature Sensitivity of Polymers 8
1.3 Development of Thermoresponsive Surfaces 101.3.1 Surface Modifi cations Using
Energetic Oxidation 101.3.2 Surface Grafting of Polymers 131.3.3 Graft Polymerization 14
1.4 Surface Characterization 151.5 Cell Culture and Tissue Engineering
Applications 161.6 Chromatography 201.7 Conclusion 22References 22
2 Light-Triggered Azobenzenes: From Molecular Architecture to Functional Materials 27Jaume Garcia-Amorós and Dolores Velasco2.1 Why Light-Triggered Materials? 282.2 Azobenzene-Based Light-Activatable
Materials 29
vi Contents
2.3 Photoswitchable Azobenzene-Based Materials 312.3.1 Photochromic Switches Based on
Azobenzene-Doped Liquid Crystals 312.3.2 Photochromic Oscillators Based on
Fast Thermal Isomerizing Azo Dyes 372.3.3 Fast Isomerizing Azobenzenes and Their
Potential Use for Biological Applications 392.3.4 Photoelectronic Switches Based on Azo Dyes 43
2.4 Photodeformable Azobenzene-Based Materials: Artifi cial Muscle-like Actuation 47
2.5 Conclusion and Perspectives 53Acknowledgements 54References 54
3 Functionalization with Interpenetrating Smart Polymer Networks by Gamma Irradiation for Loading and Delivery of Drugs 59Franklin Muñoz-Muñoz and Emilio BucioAbbreviations 603.1 Introduction 613.2 General Concepts 63
3.2.1 Graft Copolymers and Ionizing Radiation 633.2.2 Methods of Radiation for Preparing Grafts 65
3.3 Radiation Synthesis and Modifi cation of Polymers (Approaches) 743.3.1 Thermosensitive Networks 753.3.2 pH-Sensitive Networks 763.3.3 IPNs 773.3.4 Graft Copolymers 80
Acknowledgements 88References 88
4 Biomedical Devices Based on Smart Polymers 105Angel Contreras-García and Emilio Bucio4.1 Introduction 1064.2 Stimuli Responsive Polymers 1074.3 Sensitive Hydrogels 1084.4 Responsive Materials for Drug Delivery Systems 109
Contents vii
4.5 Intelligent Polymers for Tissue Engineering 1124.6 Types of Medical Devices 113Acknowledgements 117References 117
5 Stimuli-Responsive Polymers as Adjuvants and Carriers for Antigen Delivery 123Akhilesh Kumar Shakya and Kutty Selva NandakumarAbbreviations 1245.1 Introduction 1245.2 Responsive Polymers as Antigen Carriers 129
5.2.1 Charge Responsive Carrier 1295.2.2 Oxidation Responsive Carrier 1295.2.3 pH-Responsive Carrier 1305.2.4 Temperature-Responsive Carrier 131
5.3 Factors Affecting Adjuvant Potential of Stimuli-Responsive Polymeric Adjuvant 135
Acknowledgements 136References 136
6 Cyclodextrins as Advanced Materials for Pharmaceutical Applications 141Vesna D. Nikolic, Ljubisa B. Nikolic, Ivan M. Savic, and Ivana M. Savic6.1 Inclusion Complexes 1426.2 Preparation of Inclusion Complexes 1436.3 Historical Development of Cyclodextrins 1456.4 Equilibrium 1496.5 Confi rmation of Formed Inclusion Complexes 1526.6 Application of Cyclodextrins in the Pharmacy 1536.7 Cyclodextrins as a Drug Delivery System 1546.8 Cyclodextrin as Solubilizers 1576.9 Pharmaceutical Formulation
Containing Cyclodextrin 1586.10 Conclusion 160References 161
viii Contents
PART 2 Smart Nano-Engineered Materials 167
7 Advances in Smart Wearable Systems 169Rajesh Kumar Saini, Jaya Bajpai, and A. K. Bajpai7.1 Introduction 1707.2 Classifi cation of Smart Polymers 172
7.2.1 Shape-Memory Polymers 1737.2.2 Conducting Polymers 1757.2.3 Stimuli-Responsive Hydogels 1777.2.4 Nanomaterials 179
7.3 Applications 1817.3.1 Smart Fabrics 1827.3.2 Smart Skin 1857.3.3 Biosensors 189
7.4 Current Features of Wearable Systems 1927.5 Conclusions 1947.6 Challenges and Future Prospects 194References 195
8 Functionalization of Smart Nanomaterials 201Sharda Sundaram Sanjay and Avinash C. Pandey8.1 Introduction 202
8.1.1 Importance of Functionalization 2038.1.2 Advantages of Surface Functionalization 204
8.2 Functionalizing Agents 2058.2.1 Mode/Ways to Surface Functionalization 2068.2.2 Strategy for the Conjugation 2068.2.3 Classifi cation of Surface Functionalization
of Nanomaterials 2078.2.4 Methodology 2108.2.5 Conditions Favorable for
Biofunctionalization 2138.3 Carbon Nanomaterials 217
8.3.1 Functionalization of Carbon Nanotubes 2188.4 Silica Nanoparticles 2248.5 Confi rmation of Functionalization 225
8.5.1 Confi rmation through Infrared Spectral Analysis 225
8.5.2 Confi rmation through Optical/Colorimetric Assay 227
Contents ix
8.5.3 Confi rmation through Contact Angle Measurement 228
8.5.4 Confi rmation with the Help of Metathesis Reactions 228
Acknowledgements 229References 229
9 Role of Smart Nanostructured Materials in Cancers 237Rizwan Wahab, Farheen Khan, Javed Musarrat, and Abdulaziz A.Al-Khedhairy9.1 Introduction 238
9.1.1 What is cancer? 2389.1.2 Types of Cancers 2399.1.3 Importance of Nanostructures 244
9.2 Experimental 2469.2.1 Nanomaterials Synthesis 2469.2.2 Characterizations of Synthesized
Nanomaterials 2479.2.3 Biological Characterizations for the
Identifi cation of Cancers 2519.3 Results Related to Use of Smart Nanostructured
Materials to Control Cancers Cells 2589.4 Summary and Future Direction 265Acknowledgement 266References 266
10 Quantum Cutter and Sensitizer-Based Advanced Materials for their Application in Displays, Fluorescent Lamps and Solar Cells 273Raghvendra Singh Yadav, Jaromir Havlica, and Avinash Chandra Pandey10.1 Introduction 27410.2 Quantum Cutter and Sensitizer-Based
Advanced Materials 275 10.2.1 Visible Quantum Cutting 277 10.2.2 Near-Infra Red Quantum Cutting 284
10.3 Conclusion 297Acknowledgement 297References 298
x Contents
11 Nanofi bers of Conducting Polymer Nanocomposites 303Subhash B. Kondawar and Shikha P. Agrawal11.1 Conducting Polymers 30411.2 Nanostructure Conducting Polymers 311
11.2.1 Conducting Polymer Nanocomposites 315 11.2.2 Nanofi bers of Conducting Polymer
Nanocomposites 319 11.2.3 Electrospinning 326 11.2.4 Theoretical Modeling of Electrospun
Nanofi bers 328 11.2.5 Electrospun Nanofi bers of Conducting
Polymer Nanocomposites 33311.3 Electrical Conductive Properties of Nanofi bers
of Conducting Polymer Nanocomposites 33711.4 Applications of Nanofi bers of Conducting
Polymers Nanocomposites 341 11.4.1 Supercapacitors 341 11.4.2 Rechargeable Batteries 343 11.4.3 Sensors 344
11.5 Concluding Remarks 347References 348
PART 3 Smart Biosystems Engineering 357
12 Stimuli-Responsive Redox Biopolymers 359Sudheesh K. Shukla and Ashutosh Tiwari12.1 Introduction 35912.2 Method of Synthesis, Characterization
and Mechanism 36312.3 Stimuli-Responsive Redox and Electrical
Conductive Behavior 36712.4 Biosensor Applications 37212.5 Conclusion 373References 374
Contents xi
13 Commodity Thermoplastics with Bespoken Properties using Metallocene Catalyst Systems 377Nikhil Prakash13.1 Introduction 37813.2 Metallocene Catalyst Systems 379
13.2.1 Evolution of the Metallocenes 381 13.2.2 Categories of Metallocene Catalysts 382 13.2.3 Cocatalysts 383
13.3 Metallocene Thermoplastics 385 13.3.1 Polyethylene: Manufacture, Structure
and Properties 385 13.3.2 Polypropylene: Manufacture, Structure
and Properties 387 13.3.3 Polystyrene 391
13.4 Conclusions and Future Prospects 393References 393
PART 4 Theory and Modeling 397
14 Elastic Constants, Structural Parameters and Elastic Perspectives of Thorium Mono-Chalcogenides in Temperature Sensitive Region 399Krishna Murti RajuNomenclature 40014.1 Introduction 400
14.1.1 Primer of the Field 401 14.1.2 Overview 402
14.2 Formulation 40414.3 Evaluation 41014.4 Results and Discussions 414
14.4.1 Higher Order Elastic Constants 415 14.4.2 Pressure Derivatives 419
14.5 Conclusions 424Acknowledgment 424References 424
Index 429
xiii
Preface
The development of tuned materials by environmental require-ments is the recent arena of materials research. It is a newly emerg-ing, supra-disciplinary fi eld with great commercial potential. Stimuli-responsive materials answer by a considerable change in their properties to small changes in their environment. They are becoming increasingly more prevalent as scientists learn about the chemistry and triggers that induce conformational changes in mate-rial structures and devise ways to take advantage of and control them. New responsive materials are being chemically formulated that sense specifi c environmental changes and adjust in a predict-able manner, making them useful tools.
Stimuli-responsive materials are in widespread demand among researchers because they can be customized via chemistry to trigger induced conformational changes in structures or be taken advan-tage of in the form of structural or molecular regime via minute external environmental changes. Their effectors are both i) physical, i.e., temperature, electric or magnetic fi elds, mechanical stress; and ii) chemical, i.e., pH, ionic factors, chemical agents, biological agents. Thermoresponsive polymers represent an important class of “smart” materials as they are capable of responding dramatically to small temperature changes. The chapter on “Smart Thermoresponsive Biomaterials” describes a range of thermoresponsive polymers and the criteria that infl uence their thermoresponsive character for sur-face modifi cations and applications, in particular for cell culture and chromatography. In the chapter “Light-Triggered Azobenzenes: From Molecular Architecture to Functional Materials,” the prin-ciple of light-triggered materials is covered, for example, azoben-zene-based materials, their photochromic switching and oscillation ability, and potential biological and artifi cial muscle-like actua-tion applications. The chapter entitled “Functionalization with Interpenetrating Smart Polymer Networks by Gamma Irradiation for Loading and Delivery of Drugs,” discusses the γ-irradiation
xiv Preface
assisted graft copolymerization containing interpenetrating poly-mer networks and other architectures, mainly focusing on the performance of materials modifi ed with stimuli-responsive com-ponents capable of high loading therapeutic substances and their control release properties. The recently investigated applications of smart or intelligent polymeric materials for tissue engineering, regenerative medicine, implants, stents, and medical devices are overviewed in “Biomedical Devices Based on Smart Polymers.” The chapter “Stimuli Responsive Polymers as Adjuvants and Carriers for Antigen Delivery,” illustrates the promising advantages of respon-sive materials in immunology as carriers for an antigen and adju-vant for enhancing immunogenicity of an antigen. “Cyclodextrins and Advanced Materials for Pharmaceutical Applications” highli-ghts the combination of cyclodextrins and pharmaceutical exci-pients or carriers such as nanoparticles, liposomes, etc., and fosters the progress of the advanced dosage forms with the improved phy-sicochemical and biopharmaceutical properties.
“Recent Advances in Smart Wearable Systems,” presents an over-view of the smart nanoengineering that yields state-of-the-art wea-rable systems and sensor technologies, and underlying challenges are overviewed. The high surface functionalities available in such materials provide an opportunity to modify their outer surfaces and achieve multivalent effects. The chapter on “Functionalization of Smart Nanomaterials” describes the surface nanoengineering aimed at coupling advanced features for a range of optoelectronic applications. A thrust towards the development of novel nanoparti-cles has paved the way for sucessful cancer diagnosis and treatment. The chapter “Role of Smart Nanostructured Materials in Cancers,” summarizes different types of nanoparticles currently available for cancer therapy. Smart nanomaterials including visible quantum cut-ting and near-infrared quantum cutting phosphors such as fl uoride phosphors, oxide phosphors, phosphate phosphors and silicate phosphors, and their potential application for PDPs and Hg-free fl uorescent lamps, are the focus of “Quantum Cutter and Sensitizer-Based Advanced Materials for Their Application in Displays, Fluorescent Lamps and Solar Cells.” The chapter on “Nanofi bers of Conducting Polymer Nanocomposites” focuses on the preparative strategies of nanofi bers of conducting polymers and nanocomposi-tes and their electrical conductive properties and applications.
The biocompatible smart polymeric architect has signifi cantly in creased attention in biodevice and system managements.
Preface xv
“Stimuli-Responsive Redox Biopolymers” investigates arabic-co-polyaniline as pH-responsive redox copolymers and their properties for biosensor applications. The development of the metallocene catalysts, from their discovery to their present state-of-the-art, is portrayed in “Commodity Thermoplastics with Bespoke Properties Using Metallocene Catalyst Systems,” with an emphasis on weighing up discrete catalysts for stereo-specifi c polymerization and technologically important processes.
The study of elastic properties provides information about the magnitude of the forces and nature of bonding between the atoms. The impact of solids on the world of science and technology has been enormous, covering such diverse applications as solar energy, image processing, energy storage, computer and telecommunication technology, thermoelectric energy conversion, and new materials for numerous applications. The chapter “Elastic Constants, Structural Parameters and Elastic Perspectives of Thorium Monochalcogenides in Temperature Sensitive Region” predicts the anharmonic elastic properties of thorium chalcogenides having NaCl-type structure under high temperature using Born-Mayer repulsive potentials and the long- and short-range interaction approach.
This book is written for a large readership including university students and researchers from diverse backgrounds such as chem-istry, materials science, physics, pharmacy, medical science, and biomedical engineering. It can be used not only as a textbook for both undergraduate and graduate students, but also as a review and reference book for researchers in the materials science, bioengi-neering, medical, pharmaceutical, biotechnology, and nanotechnol-ogy fi elds. We hope the chapters of this book will provide valuable insight in the important area of responsive materials and cutting-edge technologies.
Editors
Ashutosh TiwariLinköping, Sweden
Hisatoshi KobayashiTsukuba, Japan
August 15, 2013
PART 1
STIMULI-RESPONSIVE POLYMERIC MATERIALS
3
Ashutosh Tiwari and Hisatoshi Kobayashi (eds.) Responsive Materials and Methods, (3–26) 2014 © Scrivener Publishing LLC
1
Smart Thermoresponsive Biomaterials
Mohammed Yaseen* and Jian R. Lu
Biological Physics Group, School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom
AbstractThermoresponsive materials represent an important class of advanced materials that have evolved over the past few decades. These materials are also designated as “smart” materials as they are capable of responding dramatically to small temperature changes. In this chapter we will present a select range of polymers that exhibit thermoresponsive behavior, with a particular focus on polyacrylamide-based polymers. We also review the criteria that infl uence their thermoresponsive character. Inherently, many materials are not thermoresponsive, thus approaches for surface modifi ca-tion of such materials resulting in unique thinly-coated thermoresponsive surface layers or fi lms are also shown. Finally, select biological applica-tions of thermoresponsive biomaterials are presented, in particular for cell culture and chromatography applications.
Keywords: Temperature responsive, functional polymers, nanofi lms, cell culture, chromatography
1.1 Introduction
Synthetic polymers that can respond to external stimuli in a con-trolled manner are increasingly of interest to science and industry. Such polymers have been designed to mimic natural biopolymers, such as proteins, polysaccharides and nucleic acids in living organ-isms within which responses to stimuli are common processes. Such “smart” or “intelligent” stimuli-responsive polymers are capable of undergoing relatively large and abrupt changes in response to
*Corresponding author: [email protected]
4 Responsive Materials and Methods
small external environmental changes. The exemplar stimuli are often classifi ed as either physical (temperature, electric or mag-netic fi elds, and mechanical stress) or chemical effectors (pH, ionic factors, chemical agents, biological agents), resulting in changes of the interactions between polymer chains or between chains and solvents at the molecular level (Figure 1.1). Such changes in the physio chemical properties of the polymers can subsequently affect their interactions with other systems, for example, adherent cells. These stimuli-responsive polymer systems are attractive to bio-related applications such as cell expansion, tissue engineering, con-trolled drug delivery, non-viral gene transfection, enzymatic activity control, biotechnology and chromatography for bio molecular sepa-ration and purifi cation [1, 2].
Signifi cant scientifi c research towards the understanding and development of dynamically responsive materials has resulted in a number of excellent reviews by other authors on the general topic of thermoresponsive polymer materials and related areas. The ref-erences in this chapter are hence primarily provided as starting points for further reading [3–7]. In this chapter we will describe the development of a select range of temperature-responsive polymers that exhibit thermoresponsive behavior. In particular we will review the use of polyacrylamide-based polymers and also the criteria that infl uence their temperature-responsive character. Inherently, many materials are not thermoresponsive, thus approaches for surface
Figure 1.1 A schematic representation of stimuli-responsive polymer change for (a) free polymer in aqueous bulk environment, and (b) surface immobilized polymer. The temperature-dependent soluble (hydrated below the LCST) to insoluble (dehydrated above the LCST) change of polymer in aqueous media is shown.
(a) In bulk solution
Stimulus
Stimulus
(b) At surface
WaterResponsive polymer
Soluble hydrated polymer Insoluble dehydrated polymer
Increasing temperature
Decreasing temperature
Smart Thermoresponsive Biomaterials 5
modifi cation of such materials need to be taken to produce unique thinly-coated thermoresponsive surface layers or fi lms. Finally, we will present cell culture and chromatographic purifi cation as select biological applications of thermoresponsive biomaterials.
1.2 Temperature-Responsive Polymers
1.2.1 Thermoresponsive Polymers Based on LCST
The change of temperature is a relatively easy and widely used stimulus for causing responsive behavior of polymers. A com-mon phenomenon is the change in solubility when the tempera-ture is shifted across the critical solution temperature at which the phase of a polymer solution or composite changes discontinuously. In general, solutions that appear as monophasic (isotropic state) below a specifi c temperature and turn biphasic above it, exhibit a lower critical solution temperature (LCST). LCST is hence the critical temperature beyond which immiscibility or insolubility occurs. Acquisition and control of LCST within the physiological temperature range is essential for applications such as cell culture and drug delivery. LCST is dependent on factors such as the ratios of monomers, their hydrophobic and hydrophilic nature, polydis-persity, branching and the degree of polymerization [5]. Thus the LCST of polymers in water can be altered by incorporating hydro-philic or hydrophobic moieties. For example, the copolymerization of N-isopropylacrylamide (NIPAAm) with hydrophilic monomers results in the increase of the LCST [7, 8]. In contrast, the LCST decreases when copolymerized with hydrophobic monomers, but this process may also affect the temperature sensitivity of NIPAAm-based copolymers. The copolymerization of ionizable groups such as acrylic acid (AAc) or N,N’-dimethylacrylamide (DMAAm) with NIPAAm can result in the discontinuous alternation or even disap-pearance of LCST at the pKa of the ionizable group [9].
For polymers such as poly(N-isopropylacrylamide) (PNIPAAm), an important characteristic is its intermolecular interaction with water molecules. Depending on its physical states, e.g., macro-molecular solution, micellar aggregation or gel, changes in tem-perature across LCST have a huge impact on hydrogen bonding and hydrophobic interactions resulting in big differences in their amphiphilic properties. The extent of hydrophobic interaction can be manipulated by tuning the balance of monomer ratios
6 Responsive Materials and Methods
and common examples are often seen from different diblock poly(ethylene oxide)–poly(propylene oxide) (PEOm-co-PPOn) and their triblock copolymers (PEOm-co-PPOn-co-PEOm), where changes in the ratio of m to n can lead to very different physio-chemical properties including thermoresponsive behavior.
Homopolymers: A number of other polymers can also display thermoresponsive behavior across their LCST with examples given in Figure 1.2. With some adjustments, the range of their thermo-responsive switches can be made useful for cell thermoresponsive detachment upon confl uence. For example, poly(vinyl methyl ether) (PVME) has the LCST of around 36°C and is usually synthesized via solution polymerization (Figure 1.2ii) [10]. Another exemplar thermoresponsive polymer is poly(N-vinyl caprolactam) which can be easily prepared by free radical polymerization of N-vinyl capro-lactam in solution, and has LCST of 32–34°C (Figure 2iii) [11].
Poly (N-substituted acrylamide) polymers are by far the most pop-ular and well-researched thermoresponsive polymers. PNIPAAm is the most well known of the thermo responsive polymers having a sharp phase transition in water (LCST) within the physiological range of about 32°C (Figure 1.2i). These polymers are also prevalent because of the fact that poly(N-substituted acrylamide) polymers are easy to prepare by radical polymerization [12, 13].
Other poly (N-substituted acrylamide) polymers shown in Figure 1.2 include poly(N, N’-diethylacrylamide) (PDEAAm) with LCST in the range of 25–35°C [14], poly(2-carboxyisopropylacryl-amide) (PCIPAAm) composed of a isopropylacrylamide group and carboxyl group, thus having the advantage of temperature response and additional functionality in its pendant groups [15]. Interestingly, the polymer poly(N-(L)-(1-hydroxymethyl) propyl-methacrylamide) (P(L-HMPMAAm)) (Figure 1.2iv) has optical activity associated with it and shows a different thermosensitive phase transition from that of optically inactive P(DL-HMPMAAm) [16]. The polymer poly(N-acryloyl-N’-propylpiperazine) with the LCST of 37°C is both temperature and pH responsive [17]. However, the Poly(N-acryloyl-N’-propylpiperazine) homopolymers based on methylpiperazine and ethylpiperazine were found not to exhibit LCST due to their weak hydrophobicity [18].
Copolymers: The co-polymerization of different N-substituted acrylamide monomers can provide further copolymer functionality and LCST tuning potential arising from the hydrophilic-hydrophobic balance of monomer units. The copolymer PNIPAAm-co-PCIPAAm has similar sensitivity and LCST to the homopolymer PNIPAAm
Smart Thermoresponsive Biomaterials 7
[15, 19]. It has structural similarity to PNIPAAm-co-poly(acrylic acid), but the two have very different temperature-responsive behavior. Triblock copolymers such as PEOm-co-PPOn-co-PEOm also exhibit temperature-responsive micellization and gelation
Figure 1.2 Chemical structures of polymers showing LCST; (i) poly(N-isopropyla crylamide) (PNIPAAm); (ii) poly(vinyl methyl ether) (PVME); (iii) poly(N-vinyl caprolactam); (iv) poly(N-(L)-(1-hydroxymethyl) propylmethacrylamide (P(L-HMPMAAm)); (v) poly(N, N’-diethylacrylamide) (PDEAAm); (vi) poly(2-carboxyisopropylacrylamide) (PCIPAAm); (vii) poly(N-acryloyl-N’-alkylpiperazine).
OR
(i) (ii) (iii)
(iv) (v) (vi)
(vii)
CH2 CH3
CH2
CH2
CH3
CH3
C
N H
O
N
R
CH2
H
CH3
H
CH3
R =
H2C
CH2
CH3 CH2
C
CH3 CH3
O
CH2
CH2
CH
CH2 CH2 CH2
CH2
CH
C
CH2
O
O
C
C C C
C
NN
N
N
N
HH
O
OO
O
O
O
H2C
CH2
CH
CH
CHCHCH
CH
CH
n n n
n n n
n
8 Responsive Materials and Methods
arising from their amphiphilic balance [20, 21]. The replacement of the PPO block with other hydrophobic groups such as the poly(1,2-butylene oxide) (PBO) results in a shift of the thermoresponsive LST behavior [22]. Likewise, its substitution by poly(L-lactic acid) (PLLA) and (DL-lactic acid-co-glycolic acid) (PLGA) can also result in the shift of the thermoresponsive performance with the added benefi t of biodegradable ester group incorporation [23, 24].
1.2.2 Biopolymers and Artifi cial Polypeptides
Temperature-responsive behavior is common in some biopoly-mers such as gelatin, agarose and gellan benzyl ester [25–27]. These polypeptides can form helix conformations, leading to physical crosslinking. Gelatin is obtained from collagen by breaking its tri-ple-helix structure into single-stranded molecules. It is a thermally reversible hydrogel and its fi lm stability at 37°C is poor. Thus, it is not ideal for direct use as thermoresponsive cell culture substrate under the normal cell culture conditions. Stable hydrogels of gela-tin have, however, been obtained by chemical crosslinking or con-jugation with chitosan by tyrosinases [28]. A substrate based on a triblock copolymer, poly (N-isopropylacrylamide)-co-poly[(R)-3-hydroxybutyrate]-co-poly (N-isopropylacrylamide) (PNIPAAm-co-PHB-co-PNIPAAm), co-coated with gelatin, has been developed for thermoresponsive cell culture. It was found to be superior to the PNIPAAm homopolymer coating in terms of fi lm stability, surface coating and cell growth [29]. Surface deposition of collagen in low density on PNIPAAm was also found to enhance cell adhesion but did not affect cell detachment compared to uncoated PNIPAAm [30].
Biomimetic polypeptides such as elastin-like polypeptides (ELPs), composed of Val-Pro-Gly-Xaa- Gly amino acid repeat units (where Xaa is a guest residue, not proline), have shown thermally reversible phase transition behavior. ELPs are water-soluble below their transition temperature. But above the transition temperature they precipitate, driven by hydrophobic aggregation. For example, a block co-polypeptide composed of ELPs segment and silk-like segment has been reported to undergo sol-gel transition [31, 32].
1.2.3 Temperature Sensitivity of Polymers
For the versatility of applications, temperature-responsive poly-mers require high sensitivity or fast response over a narrow tem-perature. The incorporation of phase-separated structures can
Smart Thermoresponsive Biomaterials 9
result in rapid swelling/deswelling within hydrogels, resulting in the change of physical form associated with a large shift in surface area and amphiphilicity [33]. The inclusion of hydrophilic moi-eties can also increase the deswelling rate of PNIPAAm hydrogel network. For example, the random copolymerization of NIPAAm with acrylic acid (AAc) or methacrylic acid (MAAc) provides the hydrogels with faster deswelling kinetics than PNIPAAm hydro-gel by itself [34]. However, for AAc-content above 1.3 wt%, the deswelling rate decreased when more AAc segments were added. An increase in the AAc content divided the long linear NIPAAm segments into short ones, causing the decrease of the driving force for hydrophobic aggregation and the subsequent disappearance of the LCST. In contrast, hydrophilic PEO grafts similarly introduced onto the PNIPAAm backbone were found not to interfere with long PNIPAAm sequences.
The copolymerization of PNIPAAm with poly(ethylene glycol) (PEG) onto porous culture membranes was carried out by elec-tron beam irradiation to provide better detachment of the cells. In this case, the NIPAAm monomers and PEG macromonomers (PEG methacrylate, MW = 4000) were dissolved in propanol con-taining 0.05% distilled water at a total concentration of 60 wt/wt%. This monomer-containing solution mixture was spread uniformly over the surface of a porous membrane (Cell Culture Insert) and irradiated using an electron beam resulting in the cova-lently bound polymer. In cell sheet detachment experiments, only 19 min were required to detach the cell sheets from PNIPAAm co-grafted with 0.5wt% of PEG, compared to approximately 35 min incubation at 20°C to completely detach the cell sheets from PNIPAAm coated on the same porous culture membranes. When the porous membranes were used, water molecules could access PNIPAAm molecules grafted on the surfaces from both underneath and peripheral to the attached cell sheets, resulting in more rapid hydration of grafted PNIPAAm molecules and faster detachment of cell sheet than nonporous tissue culture polysty-rene (TCPS) dishes [35].
Alternatively, rapid deswelling (faster acceleration of the poly-mer shrinking rate) was shown by PNIPAAm hydrogels having a comb-type molecular architecture rather than a linear-type struc-ture [36]. However, in the case of surface-immobilized PNIPAAm fi lms, the free mobile linear PNIPAAm showed a more rapid phase transition than PNIPAAm randomly crosslinked onto the surface, due to their different chain mobilities [37].
10 Responsive Materials and Methods
1.3 Development of Thermoresponsive Surfaces
Many applications utilize thermoresponsive polymers in solution or in the bulk state, however, temperature-responsive surface or inter-face has important biomedical applications such as temperature-modulated membranes, chromatography and cell culture dishes [38–40]. PNIPAAm polymers have been extensively investigated for the development of various temperature-responsive surfaces because of their specifi c advantages in biomedical applications. An important advantage includes a reversible temperature-dependent phase transition (LCST) in aqueous solution within the physiological range. Thus copolymers based on NIPAAm have been investigated and further developed for chromatography, tissue engineering and cell culture applications through controlled surface modifi cation, but not all surfaces are directly amenable to modifi cation.
1.3.1 Surface Modifi cations Using Energetic Oxidation
Surface properties of materials with no functional oxygen or nitro-gen groups are often required to be altered prior to monomer or polymer grafting. The modifi cation of surfaces is also required to facilitate cell attachment and growth. Surfaces for cell culture or tissue culture applications, other than polystyrene, include thermo-plastic polymers such as polyethylene terephthalate (PET) which are also easy to mold and manufacture. However, they are hydro-phobic in nature, so will exhibit a very different surface topology and chemical nature from the extracellular matrix (ECM). Other surfaces that require some form of surface modifi cation to further facilitate cell attachment and growth include polycarbonate and glass. An important route for surface modifi cation of materials includes the use of high-energy irradiation.
A number of methods that use high-energy irradiation are avail-able for modifying the surface of polymers. The resulting surface oxidation can make it more hydrophilic by introduction of hydroxyl and carboxyl functional groups. There are a variety of treatments that can be used to do this, such as UV, corona discharge, gamma irradiation, plasma treatments and electron beam irradiation. X-rays and electron beams are more penetrating than heat, light and microwave. Electrons, X-rays and gamma rays ionize the mate-rial they strike by stripping electrons from the atoms of the exposed material. This ionized environment is very damaging to bacteria or
Smart Thermoresponsive Biomaterials 11
viruses and can also change the chemical composition and surface structure of a material. Generally, these techniques can all effec-tively bombard the surface with ions, electrons or photons, result-ing in a subtle difference between the chemical groups formed on the surface. The bombarding radiation breaks some of the bonds in the polymer chains as well as the gaseous material surrounding them, to produce readily available free radicals on the culture sur-face, and these have the ability to quench nearby molecules. For a particular material, each technique can result in different density of surface oxidized groups, such as hydroxyl or carboxyl groups. For the polystyrene surface, research has shown that plasma treatment produces a wide variety of functional groups such as alkylperoxide, aldehyde, and carboxylic, and also up to 15% of hydroxyl groups, as shown in Figure 1.3. The percentage of hydroxyl groups formed by UV was 12%, and 8% by corona, but gamma radiation was found to produce very little of the hydroxyl groups. In summary, by using an energetic source for irradiation the surface of polymers like polystyrene can be oxidized. The resulting interactions with
Figure 1.3 Schematic drawing showing that the surface of polystyrene is hydrophobic but the oxidized surface has both hydroxyl and carboxyl functional groups making it more hydrophilic and susceptible to polymer attachment.
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12 Responsive Materials and Methods
polystyrene lead to carbon-carbon scissions and generation of a variety of oxygen-containing functionalities at the polymer surface.
Modifi cation of polymer surfaces can be performed cleanly and rapidly by plasma treatment, also leading to the formation of vari-ous active species on the surface of polymers such as polyethyl-ene (PE), polypropylene (PP), and polytetrafl uoroethylene (PTFE) [41]. However, this method also results in polymerization, func-tionalization, etching, roughening and crosslinking. Plasma can be described as a partially ionized gas consisting of free radicals, ions, photons and electrons. It can be created by gases such as oxygen and argon excited by an external energy such as heat or electric discharge. The effectiveness of plasma treatment depends on the chemical monomers or gases used to generate the reactive species. The plasma treatment of polyethylene terephthalate (PET) using air as the gas results in a large increase in hydrophilicity [42]. The treatment of poly(hydroxymethylsiloxane) surfaces by either O2 plasma or 6 keV Ar+ beams also resulted in different adhesion, pro-liferation and spreading of normal human dermal fi broblast cells. Low cell adhesion and scarce viability was found from O2 plasma treated surfaces, but complete cell confl uence, optimal spreading and proliferation were observed in the case of 6 keV Ar+ beams. The observed differences in cell responses were attributed to the relative surface free energy as a result of the two different plasma beams applied [43].
The high energy modifi cation treatments such as gamma radia-tion and lasers for surface modifi cation can lead to the modifi cation of polymers as well as providing improvement to surface biocom-patibility [44]. To achieve a specifi c gamma radiation effect it is nec-essary to apply a specifi c dose to the material. The radiation dose is a measure of the radiation energy deposited in unit mass of the material, measured in Gray (Gy) (1 Gy means 1 joule of radiation energy deposited in each kilogram of material). For example, to ster-ilize medical devices low doses of the order of 25 kGy are required, but the control of pathogens can be achieved with doses of 1.5 to 3.0 kGy, to allow preservation of the material and to render it biolog-ically inert. The crosslinking of plastics and polymers requires much higher doses of up to 200 kGy, and certain polymers are known to undergo chain scission whilst others predominantly crosslink. The susceptibility of different polymeric materials to radiation cross-linking depends mainly on their chemical structure and some can be crosslinked at low doses, while others containing non-reactive
