Future perspectives and recent advances in stimuli-responsive materials
Introduction
At their most fundamental level, many of the most important substances in living systems are macromolecules with structures and behaviors that vary according to the conditions in their surrounding environment. A variety of biological processes rely on feedback-controlled communication involving nucleic acids, proteins, and polypeptides that have the ability to adopt conformations specific to their surroundings. Similar adaptive behavior can be imparted to synthetic (co)polymers such that their utility goes beyond providing structural support to instead allow active participation in a dynamic sense. Incorporating multiple copies of functional groups that are readily amenable to a change in character (e.g., charge, polarity, and solvency) along a polymer backbone causes relatively minor changes in chemical structure to be synergistically amplified to bring about dramatic transformations in macroscopic material properties.
The “response” of a polymer can be defined in various ways. Responsive polymers in solution are typically classified as those that change their individual chain dimensions/size, secondary structure, solubility, or the degree of intermolecular association. In most cases, the physical or chemical event that causes these responses is limited to formation or destruction of secondary forces (hydrogen bonding, hydrophobic effects, electrostatic interactions, etc.), simple reactions (e.g., acid–base reactions) of moieties pendant to the polymer backbone, and/or osmotic pressure differentials that result from such phenomena. In other systems, the definition of a response can be expanded to include more dramatic alterations in the polymeric structure. For example, degradation of hydrogels upon the application of a specific stimulus can occur by reversible or irreversible bond breakage of the polymeric backbone or pendant cross-linking groups. For the sake of this review, both concepts will be included, with particular attention being paid to those that hold promise in the areas of biomedical, sensing, and electronics applications.
Interest in stimuli-responsive polymers has persisted over many decades, and a great deal of work has been dedicated to devising examples of environmentally sensitive macromolecules that can be crafted into new smart materials. However, the overwhelming majority of reports in the literature describing stimuli-responsive polymers are dedicated to macromolecular systems that are sensitive to a few common stimuli, usually changes in pH, temperature, and electrolyte concentration. The purpose of this review is not to describe every stimulus being employed to induce a response in polymer systems. Rather, we aim to highlight recent results and future trends of a few particularly useful stimuli that have, in our opinion, not yet been exploited to a similar extent, despite their unique potential. In many cases, the topics represent clear opportunities for making advances in biomedical fields due to their specificity and the ability to respond to stimuli that are inherently present in biological systems. Indeed, designing synthetic polymers with the ability to adapt their properties in response to specific interactions with biomacromolecules and small molecules commonly associated with healthy or diseased states (e.g., glucose) may facilitate the application of smart polymers in drug delivery, diagnostics, sensing, separations, etc. Additionally, it is often advantageous to utilize a stimulus that is specifically applied from an external source so that the location and rate of response can be easily adjusted, as opposed to a stimulus that is encountered as an inherent feature of the system under consideration (e.g., change in pH occurring upon endocytosis). The ability to apply these sorts of stimuli in a non-invasive manner particularly lends itself to applications in vivo. The discussion that follows will focus on recent research in the area of smart materials by emphasizing these underutilized adaptive behaviors that have the ability to affect polymer conformation, solubility, degradability, and self-assembly behavior in aqueous media. The first areas covered will pertain to specific responses that may be encountered in biological systems. Polymers that alter their properties in response to glucose, enzymes, antigens/antibodies, and thiol/redox conditions are described. Secondly, we highlight recent results in the areas of field-responsive polymers, specifically macromolecules that exhibit adaptive behaviors when exposed to irradiation with light, electric, magnetic, or sonic energy.
Section snippets
Biologically responsive polymer systems
Smart polymers are becoming increasingly important in the context of biomedical applications. Whether for the purpose of controlled drug delivery, biosensing/diagnostics, smart films/matrices for tissue engineering, or for the in situ construction of structural networks, it is often advantageous to employ polymers that can respond to stimuli that are inherently present in natural systems. This approach is a form of biomimicry, since many biomacromolecules are known to dramatically alter their
Field-responsive polymers
Most methods of inducing a response in smart polymeric systems rely on kinetically restricted diffusion of the stimulus. For example, polymer gels that respond to changes in electrolyte concentration or pH require transport of externally introduced ions to or from the vicinity of the polymer backbone. Similarly, the response of temperature-sensitive polymers can be significantly limited by issues of heat transfer. As a result, the response of many traditional stimuli-responsive polymers is a
Conclusions
With inspirations arising from a variety of sources, novel polymers with sensitivities to other triggers are constantly being developed. A recent report by Rowan and co-workers capitalized on a phenomenon observed during the self-defense mechanism of the sea cucumber [275]. A nanocomposite of cellulose nanofibers embedded in a rubbery ethylene oxide-epichlorohydrin copolymer was prepared to mimic the sea cucumber dermis known to be composed of rigid collagen fibrils embedded within a
Acknowledgement
A portion of this material is based upon work supported by the National Science Foundation under Grant No. DMR-0846792.
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