We are investigating the properties and dynamics of environmentally responsive composite surfaces, which we call HAIRS (Hydrogel-Actuated Integrated Responsive Structures). These hybrid surfaces consist of flexible, high-aspect-ratio polymer or silicon structures coupled with a responsive hydrogel, where the hydrogel, upon expansion and contraction, acts as a "muscle" to move or actuate the passive polymer "bones". We have investigated humidity, pH, and temperature responsive surfaces, and aim to expand to light and redox sensitive surfaces.
We are able to control actuation direction by a variety of methods, including topographical patterning of the hydrogel, "shadow curing" of the hydrogel, the use of anisotropic or asymmetric "microfin" actuating structures, and by inducing thickness gradients in the hydrogel using laminar flow in microfluidic channels. We are evaluating these surfaces with microfin structures for use in active optics, in which the transparency, color, polarization, etc. of the surface can change in response to an external stimulus.