Adaptive Hybrid Architectures

Dynamic structures that respond reversibly to changes in their environment are central to self-regulating thermal and lighting systems, targeted drug delivery, sensors, and self-propelled locomotion.

Since an adaptive change requires energy input, an ideal strategy would be to design materials that harvest energy directly from the changing condition itself and use it to drive an appropriate response. 

Hu Y, Kim P, Aizenberg J. Harnessing structural instability and material instability in the hydrogel-actuated integrated responsive structures (HAIRS). Extreme Mechanics Letters. 2017;13 :84-90.Abstract

We describe the behavior of a temperature-responsive hydrogel actuated integrated responsive structure (HAIRS). The structure is constructed by embedding a rigid high-aspect-ratio post in a layer of poly(Nisopropylacrylamide) (PNIPAM) hydrogel which is bonded to a rigid substrate. As the hydrogel contracts, the post abruptly tilts. The HAIRS has demonstrated its broad applications in generating reversible micropattern formation, active optics, tunable wettability, and artificial homeostasis. To quantitatively describe and predict the system behavior, we construct an analytical model combining the structural instability, i.e. buckling of the post, and the material instability, i.e. the volume phase transition of PNIPAM hydrogel. The two instabilities of the system result in a large hysteresis in response to heating and cooling processes. Experimental results validate the predicted phenomenon of the abrupt tilting as temperature and large hysteresis in a heating-and-cooling cycle in the PNIPAM actuated HAIRS. Based on this model, we further discuss the influence of the material properties on the actuation of the structure.

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Based on this concept, we have developed a class of adaptive materials that, similar to skeletomuscular systems, use a hybrid architecture to interconvert energy between different forms and scales. To specify the materials’ functions, we use surfaces bearing arrays of nanostructures.

Their unique topography can be designed to confer a wide range of optical, wetting, adhesive, anti-bacterial, motion-generating, and other behaviors, similar to their natural counterparts used by lotus leaves to shed water, geckos to stick to surfaces, echinoderms to keep their skin clean, and fish to sense flow. An additional beauty of these nanostructured surfaces is that any of these properties can be switched or fine-tuned just by bending or tilting the structures to alter the patterns.

To harvest and channel energy to drive such reconfigurations, we embed the nanostructures in hydrogels that can be chemically tailored to sense a wide selection of chemical, mechanical, humidity, temperature, light, biochemical, and other environmental conditions.

Changes in these conditions cause the gel to swell or contract, generating not only the mechanical energy of bulk size change but an entire multiscale, 3D cooperative network of mechanical forces within the gel that provide the work for reconfiguring the nanostructures.

Using both experimental and modeling approaches as well as new fabrication methods, we are developing our ability to take full advantage of the immense potential for energy coupling within these hybrids to create a generation of sustainable, self-reporting, self-adapting materials.

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