Tunable, Fast, Robust Hydrogel Actuators Based on Evaporation-Programmed Heterogeneous Structures

The ability to topographically structure and fast controllably actuate hydrogel in two and three dimensions is the key for their promising applications in soft robots, microfluidic valves, cell and drug delivery, and artificial muscles. Inspired by evaporation-induced concentration differentiation phenomenon in the production process of beancurd sheet, we introduce a facile one-step evaporation process to create laminated layer/porous layer heterogeneous structure within graphene oxide-clay-poly(N-isopropylacrylamide) hydrogel in vertical direction and pattern the heterogeneous structure in lateral direction to form tunable, fast, and robust hydrogel actuators. The laminated layer/porous layer architecture is highly stable and robust without possibility of delamination. The evaporation-programmed heterogeneous structures tune thermoresponsive actuations from global bending/unbending for global heterogeneous structure to local bending/unbending and site-specific folding/unfolding for segment-patterned heterogeneous structure, then to directional bending/unbending and chiral twisting/untwisting for stripe-patterned heterogeneous structure. These actuations are instant and reversible without detectable fatigue after many cycles.