PerspectiveMaterials Science

Adaptive Composites

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Science  25 Jan 2008:
Vol. 319, Issue 5862, pp. 420-421
DOI: 10.1126/science.1152931

Imagine a search-and-rescue robot that can change shape to squeeze through crevices with the suppleness of an octopus, or an aircraft skin with a circulatory system that enables temperature regulation, cooling, and self-healing similar to an animal. Such concepts are driving the development of adaptive composites that mimic biological responsive functionality while operating in extreme environments.

Traditionally, the high-performance, load-bearing substructures of aircraft, satellites and robots are designed for structural efficiency. Thus, they are rigid and passive; active functions such as sensing, energy harvesting, and propulsion are added by attaching components to the structure. This compartmentalization of functions into attached subsystems streamlines manufacturing and maintenance and facilitates upgrades (1). Advanced passive material technologies, such as continuous-fiber organic-matrix composites, have revolutionized applications from sporting equipment and prosthetics to satellites and aircrafts.

However, this approach is in stark contrast to natural organisms, in which flexible, jointed frameworks and complex materials impart active functionality at multiple length scales within the materials (see the figure). The realization of analogous synthetic structures depends on the combination of new materials that deliver active properties and autonomic response, as well as new computational tools that enable design, analysis, and optimization of the collective and hierarchical dynamic character.

CREDIT: CORNERSTONE RESEARCH GROUP, INC.

The transformation of a rigid substructure to a dynamic, articulated structure is particularly interesting for the aerospace industry. Large-scale actuated bending, extending, and folding structures would enable the deployment of large-scale antennae in space and the development of morphing wings on unmanned aerial vehicles. Other applications for new adaptive materials include energy-efficient locomotion and concealment.

Materials with reversible property changes—such as piezoelectric ceramics—have been studied for at least 100 years (2), but these materials have been monolithic. In contrast, new multicomponent, hierarchical material systems are inspired by biology. For example, a cell's ability to change shape and recover from large deformations arises in part from the arrangement of actin filaments in the cellular cytoplasm (3). Similar highly deformable networks can be created from cellular materials, bistable composite laminates, and bimorph strips. Alternatively, the elastic buckling of carbon nanotube mats (4) and arrays (5) may be used to store and recover substantial amounts of strain energy.

Combining these concepts with materials that exhibit a reversible stiffness enables trapping and subsequent release of the mechanical deformation. For example, the stiffness of shape-memory polymers can be reversibly changed between a thermoplastic glass and an elastomer when triggered (by temperature or other means) and will “remember” their original shape as they are softened. Studies on the International Space Station recently showed that hinges made from an elastic memory composite (consisting of predeformed carbon fibers in a shape-memory polymer matrix) are a viable replacement for current complex mechanical hinges used for unfurling structures such as solar arrays (6).

Reversibility needs to be triggered easily and controllably. The trigger for mechanical property changes is usually temperature, which can limit the speed of the transition and complicates control systems. Recent demonstrations of other triggering stimuli have broadened the suite of possible control concepts.

For example, Baughman and co-workers used a coating of catalytic nanoparticles on a shape-memory alloy wire to convert enough chemical energy to thermal energy to trigger a shape recovery of the wire (7). Advances in understanding natural systems are also highlighting chemical triggers. For example, addition of acetylcholine and calcium ionophores shifts the iridescence of the mantle iridophores of the cephalopod Lolliguncula brevis from red to blue, providing dynamic coloration. Recombinant derivatives of comparable proteins have been self-assembled in vitro to form fibers and diffraction gratings with dynamic coloration (8).

Radiation can also be used as a trigger for adaptive response. Recent advances in optically triggered, reversible colloid surface chemistry (9) may lead to rheological fluids that do not require a bias to maintain an ordered electrical or magnetic state. Photoisomerization on the molecular scale can alter liquid-crystal phase stability or control domain orientation, resulting in substantial changes in mechanical and optical characteristics (10). Koerner and co-workers used carbon nanotubes dispersed in shape-memory polymers to trigger shape recovery both electrically and optically (11). Finally, Buckley and co-workers have presented a scheme for shape-changing medical devices activated by radio-frequency radiation (12).

Materials for adaptive composites need not be restricted to solid constituents, but can incorporate fluid networks. For example, Sottos and co-workers have developed a self-healing system based on a three-dimensional microvascular network embedded in the substrate (13). The network autonomously delivers a healing agent to repair cracks in a damaged polymeric surface sheet.

Natural systems also provide inspiration for design and pumping concepts for circulation. For example, the heart of the zebra fish efficiently pumps fluids along a longitudinally graded channel via resonate coupling of an excitation. Also, the xylem and phloem vessels of vascular plants exemplify the benefits of axially graded channel walls.

Inspired by biology.

The communication and camouflage of squids (Loligo pealeii, top) arises from a precise arrangement of organelles in an elastomeric skin (middle). Chromatophores modulate adsorption by laterally swelling (bottom, left to right), while iridophores modulate reflections (iridescence) through swelling an effective Bragg grading. Scientists are now attempting to realize composite materials that reproduce the performance of such biological systems. [Adapted from (16)]

PHOTO CREDITS: (TOP) ROGER HANLON/MARINE BIOLOGY LABORATORY, WOODS HOLE, MA; (BOTTOM PAIR) LYDIA MATHGER//MARINE BIOLOGY LABORATORY, WOODS HOLE, MA PERSPECTIVES

By including fluids and associated mass transport within a composite, many active functions—such as thermal management, variable modulus, and dynamic optical transmission—can be envisioned. However, the use of fluids also poses substantial challenges, because the resulting materials are more complex and less durable. For example, pores and voids are known to drastically reduce strength in engineered materials. However, recent reports indicate that the strength and toughness of composites is less impacted by embedded channels than by voids that form during processing (14).

Even with this ever-expanding array of material concepts, adaptive composites will fall short of their perceived potential without the development of verified, validated, and streamlined computational design tools that capture the properties of the many possible configurations or states. Further complexity arises in considering control concepts for these multiple active materials.

Alternative concepts such as emergent, morphogenic, or evolutionary design may be best adapted to deal with this complexity. For example, Stoy and Nagpal have provided elegant computational examples of how a set of local control rules can be used to construct mobile robots or self-repairing structures from discrete reconfigurable pieces (15). The challenge is how to harness these concepts with the process and fabrication tools of materials science and nanotechnology.

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