Inhomogeneous instability patterns in polyacrylamide hydrogel bilayers

Abstract

Instability-induced patterns with spatially varying dimensions and geometric shapes, or inhomogeneous instability patterns, are ubiquitous in biological structures and have attracted much interest in both fundamental research and engineering applications. However, it remains a challenge to create such patterns using materials with stiffnesses and scales comparable to biological structures. Here, we demonstrate the creation of inhomogeneous instability patterns using film–substrate bilayers of polyacrylamide hydrogels. Swelling of thickness-gradient films on the substrates allows for the coexistence of different instability patterns, including creases and wrinkles, on the surface. The film thickness and the stiffness ratio between the film and substrate are the factors that control the characteristic length of the patterns on the millimeter scale, as well as the switching of the patterns between creases and wrinkles. Furthermore, these patterns can be generated at prescribed local areas on the surface. Our approach to creating inhomogeneous instability patterns on a single surface would help to gain insight into the morphological development of biological patterns and to fabricate bio-inspired functional surfaces.