Controlling 3D deformations of bio-inspired active skins through designed geometrical imperfections

Certain biological organisms are born with shape, texture, and color morphing skin with the purpose of adapting to their surroundings or morphing their skin for camouflage, signaling, and hunting, among others. The recent demonstrations on artificial surfaces for mimicking biological capabilities, such as dry adhesives on geckos' feet or the low drag coefficient of sharks' skin, were achieved by controlling its surface topographies (i.e., shape, size, and distribution of asperities). Similarly, there have been tremendous interests in optimizing artificial surfaces that can continuously morph their surface texture for various applications. While several innovative artificial skins based on mechanical metamaterials have been developed, achieving controllable surface morphing remains challenging. In this study, a Bio-Inspired Active Skin (BIAS) that could selectively change its surface topography was designed and controlled by manipulating its local stress concentrations when subjected to strains. The 3D-printed and thin-film-like BIAS is based on a preconceived auxetic pattern designed to yield a Poisson's ratio of zero. When strained, these mechanical metamaterials can release stress concentrations in the form of bending and twisting, thereby enabling surface morphing. The main focus of this work was to investigate the geometrical dependence (i.e., width and rib angles) on surface morphing performance, as well as the effects of various designed geometrical imperfections (i.e., notch dimensions and locations) to prevent an uncontrollable and unpredictable morphing response. A slight adjustment in the notch design was enough to change the stress concentration, resulting in various deformed states. The nonlinear response of 3D-printed BIAS was characterized using both experiments and finite element simulations to design the unit cell geometries and to optimize the configurations and locations of the designed imperfections.