Emerging themes in soft matter: responsive and active soft materials

Anna C. Balazs

Julia M. Yeomans

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The general theme of this issue is the design and fabrication of “smart” materials that are capable of producing a global behavior in response to a local signal. A primary motivation for creating such materials arises from a technological need to produce energy efficient devices that can amplify or convert a small-scale input into a large-scale action. There are a number of design challenges that must be met in order to create such efficient systems. First, the material must be capable of not only sensing a local signal, but also reacting in a specified manner; hence our focus on responsive materials. Second, the material should ideally be capable of transducing one form of energy (e.g., electromagnetic, optical, chemical) into a mechanical action; hence the word active in the name of this issue. Meeting these design criteria poses significant and intriguing scientific challenges. For example, we must develop new strategies for optimizing the use of active materials in the fabrication of larger components or devices. This, in turn, demands that we develop robust models for the behavior of active materials.

Another inspiration for focusing on responsive and active materials comes from biology. One of the exquisitely evolved properties of biological systems is the ability to convert local information into a global action. Think of the way in which a bacterium changes direction in response to a tiny chemical gradient or how quickly you pull your hand away when your finger tip senses heat from a flame. This kind of rapid, large-scale motion, which results from sensing local change, is critical to survival and as yet, has few equivalents in the synthetic materials realm.

Within this issue, a number of authors have addressed the challenge of creating responsive and active materials by designing systems that undergo directed propulsion in response to an external cue. For example, Masoud and Alexeev (DOI: 10.1039/b916835d) harness a magnetic field to drive microcapsules, which encase superparamagnetic particles, to creep along a microchannel. Dayal et al. (DOI: 10.1039/b918434a) use light to guide the autonomous movement of responsive gel “worms” along circuitous paths. Energy from a chemical reaction is used to power nanodimers against a flow in an article by Tao and Kapral. (DOI: 10.1039/b918906h) These papers are complemented by a review by Ebbens and Howse (DOI: 10.1039/b918598d) that describes recent advances in creating nano- and micro-scopic swimming devices.

Polymer gels constitute optimal active materials since they can undergo large-scale changes in volume or shape, and thereby perform sustained mechanical work. A range of stimuli can be utilized to drive such structural changes in the gels; a number of authors in this issue (Kharlampieva et al., Swann et al.) (DOI: 10.1039/b917845g) (DOI: 10.1039/b918249g) investigate how to exploit and measure the properties of stimuli-responsive gels. Light serves as a useful stimulus for controlling not only the behavior of polymer networks, but also liquid crystalline polymers, as demonstrated in the paper by Serak et al. (DOI: 10.1039/b916831a), who use lasers and sunlight to photo-actuate a polymeric cantilever. Patterning also constitutes another means of controlling the behavior of active polymeric systems and Marenduzzo and Orlandini (DOI: 10.1039/b919113e) describe simulations that provide guidelines for regulating the hydrodynamic behavior of active gels.

In many applications, these soft, active materials will be sandwiched between two hard surfaces or serve as a coating on a substrate, and thus it is vital to understand how confinement affects the activity and responsiveness of the layers. Marcombe et al. (DOI: 10.1039/b917211d) address this challenge by developing a theory to capture the swelling of pH-sensitive gels that are bound by hard walls. Kim et al. (DOI: 10.1039/b920392c) also incorporate a substrate into their design of soft, active materials; they embed microscopic posts into a hydrogel layer. Changes in the thickness of the hydrogel drive the reconfiguration of the posts, thereby creating smart, reconfigurable, micropatterned surfaces.

In formulating this issue, we were also interested in the behavior of smart surfaces that transmit information to an overlying fluid layer or adsorbed species. Wischerhoff et al. (DOI: 10.1039/b913594d), provide a review of recent work in the area of smart bioactive surfaces and highlight examples of synthetic surfaces communicating with biological entities. McHale et al. (DOI: 10.1039/b917861a) consider the interactions between immersed superhydrophobic surfaces and the surrounding fluid, and thereby describe new, technologically important uses for these interfaces. Using a direct-write assembly, Wu et al. (DOI: 10.1039/b918436h) also consider the interactions between fluids and immersed surfaces; in their study, the surfaces are immersed in an epoxy matrix and form a network for fluid transport that resembles the microvasculature in biological systems.

Surfaces are commonly vulnerable to mechanical damage and wear; in their review Shchukin et al. (DOI: 10.1039/b918437f) describe recent advances in using responsive nano- and micro-containers that release healing agents or other encapsulated species when stimulated by the appropriate external cues. Such carriers can be used in coating applications, as well as for the controlled release of drugs. Along similar lines, Durbin and Buxton (DOI: 10.1039/b918476g) examine the release of drugs from polymer-core–shell nanoparticles and isolate optimal conditions for controlling the rate of release.

We thank the authors for their time and effort in contributing to this issue. We are particularly grateful to the staff of Soft Matter for all their help in this endeavor.

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