Hydrogel-actuated integrated responsive systems (HAIRS): Moving towards adaptive materials

The move toward sustainability and efficiency in nearly every field calls for dynamic materials that can harvest energy from and adapt to a changing environment. Here we review our recently developed, widely applicable strategy for adaptive surface design that integrates two rarely associated categories of materials-nanostructured surfaces and hydrogels-into a hybrid architecture. The nanostructure arrays provide unique topographic patterns that confer wetting, optical, and many other functions but on their own are generally static; by embedding them in a layer of responsive hydrogel, we channel the mechanical forces generated within the swelling/contracting gel to reversibly reconfigure the nanostructures in response to stimuli. Since the sensing and responding components are structurally distinct, they can each be programmed independently to match potentially almost any type of environmental change with almost any type of output. Several of our recent advances in nanofabrication make it possible to choose from an entire spectrum of nanostructured materials, stiffnesses, shapes, symmetries, orientations, and large-scale surface gradients, enabling a given stimulus to be translated into a vast assortment of complex multiscale patterns and adaptive responses. The gel chemistry and nanostructure flexibility can be further optimized for incorporating the surfaces into a variety of structures and environments. We envision using this platform to create a generation of sustainable, self-adapting, and self-reporting materials.