Thermoresponsive bilayer hydrogel with switchable bending directions as soft actuator

Shaping-deforming thermoresponsive hydrogel actuators have showed great potential toward comprehensive application in artificial intelligence including soft robots and advanced electronics. Despite research progress in designing hydrogels with complexed molecular and hierarchy structures, integrating feasible manufacturing process and switchable shape transformation remained a challenge. Herein, we reported a straightforward strategy to develop a novel actuator by asymmetric composed bilayer hydrogel. Poly(N-isopropylacrylamide) (PNIPAM) and poly(vinyl alcohol) PVA have been applied to fabricate PNIPAM/(PNIPAM/PVA) bilayer hydrogel in which water shrinkage speed could be tuned by PVA crystallinity. Interpenetration of PVA into PNIPAM network promised a faster water losing speed than PNIPAM while a slowed down water diffusion was observed when PVA crystalized during freezing/thawing cycle. Consequently, the hydrogel exhibits two opposite bending behavior under the same temperature-driven process. In addition, the hydrogen bond interaction between PVA and PNIPAM endows the bilayer hydrogel with excellent interfacial adhesion, which prevents delamination after several freezing/thawing cycles and shrinkage/swelling cycles. Approaches in this study points to a future direction in designing and fabricating intelligent materials for several scenarios including soft robotics and biomedical devices.