Smart Polymers and their Applications

Maria Rosa Aguilar, Julio San Román

• Publisher: Woodhead Publishing
• Year: 2014
• eBook ISBN: 9780857097026
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Smart polymers are polymers that respond to different stimuli or changes in the environment. Smart Polymers and their Applications reviews the types, synthesis, properties, and applications of smart polymers. Chapters in part one focus on types of polymers, including temperature-, pH-, photo-, and enzyme-responsive polymers. Shape memory polymers, smart polymer hydrogels, and self-healing polymer systems are also explored. Part two highlights applications of smart polymers, including smart instructive polymer substrates for tissue engineering; smart polymer nanocarriers for drug delivery; the use of smart polymers in medical devices for minimally invasive surgery, diagnosis, and other applications; and smart polymers for bioseparation and other biotechnology applications. Further chapters discuss the use of smart polymers for textile and packaging applications, and for optical data storage. Smart Polymers and their Applications is a technical resource for chemists, chemical engineers, mechanical engineers, and other professionals in the polymer industry; manufacturers in such sectors as medical, automotive, and aerospace engineering; and academic researchers in polymer science. - Reviews the different types of smart polymer, discussing their properties, structure, design, and characterization - Reviews applications of smart polymers in such areas as biomedical engineering, textiles, and food packaging

1

Introduction to smart polymers and their applications

M.R. Aguilar and J. San Román, Institute of Polymer Science and Technology (ICTP-CSIC), Spain and Networking Biomedical Research Center in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Spain

Abstract:

The scientific community tries to mimic nature in the way that living organisms adopt their behavior as a function of environmental conditions to improve survival. In this sense, smart polymers offer materials that respond to numerous stimuli (temperature, pH, electric and magnetic fields, light intensity, biological molecules, etc.), and scientists must devise the best way to apply them in all research areas. This chapter comprehensively summarizes the content of this book which tries to provide a wide overview of smart polymers and the most interesting applications developed recently.

Key words

smart polymers; stimuli-responsive polymers; sensitive polymers; applications

1.1 Introduction

Living systems respond to naturally varying environmental conditions, adapting their structure and functionality to suit these changes by using complex sensing mechanisms, actuating and regulating functions, and feedback control systems. Nature can be considered the best example a scientist can have when developing new materials and applications, and the overall challenge for scientists is to create materials with dynamic and tunable properties, mimicking the active microenvironment that occurs in nature.

Smart polymers or stimuli-responsive polymers undergo large reversible changes, either physical or chemical, in their properties as a consequence of small environmental variations. They can respond to a single stimulus or multiple stimuli such as temperature, pH, electric or magnetic field, light intensity, biological molecules, etc., that induce macroscopic responses in the material, such as swelling/collapse or solution-to-gel transitions, depending on the physical state of the chains (Aguilar et al., 2007).

Linear and solubilized smart macromolecules will pass from monophasic to biphasic near the transition conditions giving rise to reversible sol–gel hydrogels. Smart cross-linked networks undergo chain reorganization at the transition conditions where the network passes from a collapsed to an expanded estate. Smart surfaces change their hydrophilicity as a function of a stimulus-providing, responsive interface. All these changes can be used in the design of smart devices for multiple applications, for example, minimally invasive injectable systems (Nguyen and Lee, 2010); pulsatile drug delivery systems (Arora et al., 2011; Tran et al., 2013) or new substrates for cell culture or tissue engineering (Duarte et al., 2011).

Moreover, most polymers can easily be functionalized by pre-polymerization (Guillerm et al., 2012) or post-polymerization (Arnold et al., 2012) methods incorporating functional molecules to the structure, such as biological receptors (Shakya et al., 2010). Therefore, polymer scientists have a wide range of possibilities in terms of polymer chemical structures, polymer architectures and polymer modifications to develop an infinite number of applications for these smart materials (Stuart et al., 2010).

The aim of this book is to guide the reader through the amazing world of smart polymers in order to understand not only the state of the art in this area but also shed some light on future directions in this research field. The first part of the book gives the reader a broad overview of different stimuli-responsive polymers. Temperature, pH, light intensity, magnetic field and enzyme responsive polymers are described. Moreover, due to their actual and future applications, special attention is paid to smart hydrogels, shape memory materials and self-healing polymers.

1.2 Types of smart polymer

Polymers that respond to temperature changes and, more specifically, those that undergo a phase transition in aqueous solution are currently gaining special attention due to their potential applications in the biomaterials field (Bajpai, 2010), architecture (Yang et al., 2013) or water-recovery strategies (Yang et al., 2013) amongst others. Temperature-sensitive polymers present low critical solution temperature (LCST) or upper critical solution temperature (UCST) depending on their transition behavior from monophasic to biphasic when temperature is raised or, on the contrary, from biphasic to monophasic when temperature is raised, respectively. LCST polymers have been widely investigated whereas UCST polymers are quite rare. Most common LCST polymers are the poly(N-substituted acrylamide), poly(vinyl amide) and poly(oligoethylene glycol (meth)acrylate) families. However, many other polymers can present LCST behavior if the proper hydrophilic–hydrophobic balance is present in the macromolecules. Poly(vinyl ether)s (Aoshima and Kanaoka, 2008), poly(2-oxazoline)s (Guillerm et al., 2012) and poly(phosphoester)s (Turánek et al., 2009) also present temperature-responsive behavior and are reviewed in this book.

pH-sensitive polymers bear weak polyacidic (poly(acrylic acids) or poly(methacrylic acids)) or polybasic (poly(N-dimethylaminoethyl methacrylate), poly(N-diethylaminoethyl methacrylate), poly(ethyl pyrrolidine methacrylate)) moieties in their structure that protonate or deprotonate as a function of the surrounding pH. Drug delivery systems, gene carriers (Pezzoli and Candiani, 2013) or glucose sensors (Kost and Langer 2012) are three of the multiple applications described for this kind of smart polymer.

Photo-sensitive polymers undergo a reversible or irreversible change in conformation, polarity, amphiphilicity, charge, optical chirality, conjugation, etc., in response to a light stimulus. Reversible chromophores or reversible molecular switches (e.g., azobenzenes, spiropyran, diaryl ethane or coumarin) undergo a reversible isomerization upon light irradiation (Wang and Wang, 2013) while irreversible chromophores are cleaved from the polymer chain upon light exposure (e.g., o-nitrobenzylphotolabile protecting group) or induced reactivity resulting in the coupling of two species (e.g., 2-naphthoquinone-3-methides). Both molecular switches and irreversible chromophores have been applied in multiple applications such as drug delivery systems, functional micropatterns, responsive hydrogels, photodegradable materials or photoswitchable liquid crystalline elastomers for remote actuation (Ohm et al., 2010).

Polymer hydrogels play a key role in the development of new biomaterials as their high level of hydration and 3D structure resemble natural tissue. However, despite the superior performance of hydrogels, they present several limitations mainly due to their poor controllability, actuation and response polymers. Several advances have been made in this sense by the use of smart polymers in the preparation of hydrogels (Ravichandran et al., 2012). For example, magnetically responsive polymer gels and elastomers are composites based on magnetic nanoparticles dispersed in a high elastic polymeric matrix. Magnetic field quickly deforms the polymer matrix with no noise, heat evolution or exhaustion which makes these materials ideal for the preparation of sensors, micromachines, energy transducing devices, controlled delivery systems or even artificial muscles (Li et al., 2013). One of the limiting steps in the development of these materials has been the precise coupling of magnetic nanoparticles to the gel; however, this problem has been overcome when magnetic nanoparticles form the cross-linking nodes of the hydrogel (Ilg, 2013).

Macroscopic transitions can also be triggered by ‘biology-to-material’ interactions in the so-called biointeractive polymers. These materials incorporate receptors for biomolecules which, when stimulated, cause localized or bulk modifications in the material properties. Those polymers that respond to selective enzyme catalysis are known as enzyme responsive polymers. These materials represent an important advance in the integration of artificial materials with biological entities as they link together the polymer properties with specific biological processes controlled naturally by either regulating enzyme expression levels or availability of cofactors (Hu et al., 2012b). Enzyme responsive polymers can also display reversible and dynamic responses to a stimulus, thus being of great interest in the formulation of new biomaterials such as cell supports, injectable scaffolds or drug delivery systems (de la Rica et al., 2012).

Shape memory polymers represent one of the most active areas in material science due to their easier processability and lower cost when compared with shape memory metals or ceramics. This kind of smart polymers have the ability to recover their (permanent) predefined shape when stimulated by an external stimulus. A stable network and a reversible switching transition of the polymer are the two pre-requisites for shape memory effect. The stable network is responsible for the original shape, and reversible switching transition fixes the temporary shape, which can be crystallization/melting transition, liquid crystal anisotropic/isotropic transition, reversible molecule cross-linking (such as photodimerization, Diels – Alder reaction, oxidation/redox reaction of mercapto groups), and supramolecular association/disassociation (such as hydrogen bonding, self-assembly metal–ligand coordination and self-assembly of β-cyclodextrin). In addition to the reversible switches mentioned, other stimuli that change chain mobility can also trigger shape memory effect, such as light, pH, moisture, electric field, magnetic field, pressure, etc. (Pretsch, 2010). Shape memory polymers allow large, recoverable strains; however, they normally present poor mechanical properties and do not support great shape-recovery stresses. As a result, great efforts are being made in the development of shape memory composites with reinforced properties. Shape memory polymers present numerous actual and potential applications in medicine, aerospace, textiles, engineering, microfluidics, lithography and household products (Meng and Li, 2013).

Self-healing or restoration of lost functionalities without external help is a dream come true with self-healing polymers (Aїssa et al., 2012). Healing mechanisms can be extrinsic (the healing compound is isolated from the polymer matrix in capsules, fibers or nanocarriers) or intrinsic (the polymer chains temporarily increase mobility and flow to the damaged area) (Billiet et al., 2013) and are responsible for restoration of properties such as structural integrity (White et al., 2001), surface aesthetics (Yao et al., 2011), electrical conductivity (Tee et al., 2012), hydrophobicity and hydrophilicity (Ionov and Synytska, 2012), mechanical properties (Jones et al., 2013), etc.

1.3 Applications of smart polymers

The second part of the book comprises relevant applications of smart polymers and their future trends according to the opinion of well-known researchers in the field. Most of the important developments were registered in the biomedical field and use smart polymers in the development of new therapies for the treatment of several diseases or sophisticated medical devices that react to the environment of the surrounding tissues (pH, temperature, enzymes, analyte concentration, etc.) or external stimuli (light, magnetic radiation, etc.).

Responsive polymeric substrates or instructive substrates regulate cell behavior in response to external factors and are of significant importance in tissue engineering application...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• 1. Introduction to smart polymers and their applications
• Part I: Types of smart polymer
• Part II: Applications of smart polymers
• Index