Controllable Bending of Bi-hydrogel Strips with Differential Swelling

Abstract

Bi-hydrogel strips consisting of a highly swelling layer and a non-swelling layer are investigated in this study, in which the differential swelling capabilities and mismatch strain in the composite structure drive the strips into a bending configuration when being exposed to solvents. A modified analytical model originated from Timoshenko theory on bi-material beam is proposed to predict the swelling-induced bending shape of the bi-hydrogel strips, with explicit relations of bending curvature and mid-span deflection versus mismatch strain. Different sets of experiments were performed to verify the analytical model and its solutions, in which the swelling-induced mismatch strain was systematically tuned by the selection of saline concentration. The broad agreements between the theoretical and experimental results indicate that the modified analytical model can be used to guide the design of morphing elements by modulating layer-by-layer mismatches in gel composites to produce precise and complex shape transformations for practical applications.