Stimuli responsive materials
Patrick Theatoa, Brent S. Sumerlinb, Rachel K. O'Reillyc and Thomas H. Epps, IIId
aInstitute for Technical and Macromolecular Chemistry, University of Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany. E-mail: theato@chemie.uni-hamburg.de
bGeorge & Josephine Butler Polymer Research Laboratory, Department of Chemistry, University of Florida, Gainesville, FL 32611, USA. E-mail: sumerlin@chem.ufl.edu
cDepartment of Chemistry, University of Warwick, Coventry CV4 7AL, UK. E-mail: Rachel.OReilly@warwick.ac.uk
dDepartment of Chemical & Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716, USA. E-mail: thepps@udel.edu
 Thomas Epps, III, Brent Sumerlin, Patrick Theato and Rachel O'Reilly |
The present themed issue focuses on recent developments in the design of polymers that change properties in response to a single stimulus or multiple stimuli. These so-called ‘smart’ or stimuli-responsive polymers represent a growing cadre of materials that support various applications (e.g., controlled release agents, responsive coatings, and adaptive shape memory materials). Stimuli-responsive materials have benefited from significant advances in polymer science in recent years, and this themed issue highlights several of the fascinating developments that could have a major impact on the implementation of new smart materials.
To fully address the field of stimuli responsive polymers, first we must understand the breadth of available stimuli that can induce a desired response, then we must design the polymer functionalities and systems that enable such a response; finally, we must develop methods to characterize the macromolecular changes as a result of that response.
As demonstrated in this issue, many of the interesting properties of responsive materials arise from a transition in solubility or conformation of a macromolecule in the presence of a solvent. In this manner, transitions at the molecular level can be amplified to result in a change in nanoscale structure and/or materials properties. Gibson and O'Reilly (DOI: 10.1039/C3CS60035A) overview these transitions in the specific area of thermoresponsive polymers with particular attention to the effect of nanoscale geometry on the resulting change in chain conformation following a temperature change. Sumerlin and co-workers (DOI: 10.1039/C3CS35499G) also highlight temperature-responsive polymers with particular emphasis on design parameters that facilitate tuning of the specific transition temperatures. Light-responsive materials have received significant attention, as discussed by Gohy and Zhao (DOI: 10.1039/C3CS35469E) in a review focused on reversible and irreversible transitions of photoresponsive copolymer micelles. Further, many systems can be designed to respond to multiple stimuli. Two examples are provided by Jochum and Theato (DOI: 10.1039/C2CS35191A), who describe recent advances in the area of synthetic polymers that respond to both light and temperature, and Huang and Heise (DOI: 10.1039/C3CS60063G), who overview synthetic polypeptides, many of which respond to changes in pH and temperature.
While the majority of work in the area of responsive polymers has involved stimuli of temperature, light, and pH, materials that respond to other less widely considered stimuli also have been reviewed in this issue, such as polymers that respond to magnetic fields (Lecommandoux, Sandre, and co-workers (DOI: 10.1039/C3CS60058K)), mechanical forces (Bielawski and co-workers (DOI: 10.1039/C3CS35493H)), and specific chemical triggers (Lowe and co-workers (DOI: 10.1039/C3CS60065C)). Additionally, Grinthal and Aizenberg (DOI: 10.1039/C3CS60045A) discuss materials with a chemomechanical feedback. These stimuli help diversify the range of targets and responses to aid in the development of a new generation of smart materials. As a further illustration of this trend, many chemical triggers are present in vivo, which provide particular promise for the use of responsive materials for biological applications, as discussed by Kloxin, Kiick, et al. (DOI: 10.1039/C3CS60040H), Kuckling and co-workers (DOI: 10.1039/C3CS60031A), and also Ge and Liu (DOI: 10.1039/C3CS60048C).
There is increasing interest in building on these fundamental concepts to induce or disrupt solution assemblies, as illustrated by Thayumanavan et al. (DOI: 10.1039/C3CS60094G), and Grubbs and Sun (DOI: 10.1039/C3CS60079C) who explore how stimuli can be employed to induce transitions in polymer assemblies in solution. Additionally, Feng and co-workers (DOI: 10.1039/C3CS35490C) highlight the specific potential and practical applications of stimuli-responsive wormlike micelles, while Zhai (DOI: 10.1039/C3CS60023H) discusses several unique aspects of responsive film systems. Finally, Epps, Sullivan, and co-workers (DOI: 10.1039/C3CS35512H) discuss a variety of stimuli that are especially relevant to solution-assembled and surface-assembled polymers for biomedical applications.
While there are numerous applications of responsive gels, several reviews in this issue focus specifically on their utility in biology and medicine. For example, Miravet, Escuder, and co-workers (DOI: 10.1039/C2CS35436E) discuss recent efforts to tune the reversibility of stimuli-induced gelation. As a further demonstration of designer gel systems, Kloxin and Bowman (DOI: 10.1039/C3CS60046G) describe methods for generating reconfigurable and responsive covalent polymer networks, while Balazs and co-workers (DOI: 10.1039/C3CS35497K) highlight self-oscillating gels that undergo biomimetic communication.
Though the majority of stimuli-responsive research efforts have focused on materials interactions in solvent-rich environments, other responsive systems are growing in prominence, including self-healing and shape memory materials. For instance, Zhao and co-workers (DOI: 10.1039/C3CS35489J) discuss links between the fundamental concepts in self-healing and shape memory materials with respect to light-induced responses, while Fiore, Rowan, and Weder (DOI: 10.1039/C3CS35471G) explore structurally dynamic polymers that allow healing by light irradiation. Yang and Urban (DOI: 10.1039/C3CS60109A) provide a broad overview of the various reactions and functional groups commonly employed in healable polymeric materials and offer insight regarding the physical principles governing self-healing behavior.
In addition to providing an overview of the underlying molecular interactions that give rise to responsive behaviour in polymers, many of the reviews in this issue highlight the specific fields and applications in which smart materials are finding utility. It is perhaps not surprising that many of the envisioned and realized applications are in the broadly defined area of biotechnology, as many biomacromolecules are known to dramatically alter their conformation and self-assembly in response to specific chemical cues in vivo. For example, the development of synthetic materials that respond to internal and external stimuli to effect therapeutic agent delivery is one attempt at biomimicry. However, the potential of responsive materials is not limited to biological applications. Indeed, many of the reviews in this issue describe the promise of smart polymers in the broad areas of materials science, catalysis, and coatings, to name a few.
In summary, we are grateful to the authors whose contributions comprise this themed issue of Chemical Society Reviews. We sincerely hope you enjoy reading these reviews as much as it was our pleasure collecting them. This compilation provides a window into the present state-of-the-art in responsive systems, and we expect the works described herein will be useful in informing and directing future research and teaching activities. Enjoy reading, and get inspired!
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