Bio-inspired hydrogel actuator with rapid self-strengthening behavior

Inspired by intelligent biomaterials which often have flexible responsiveness to multiple environmental cues and strengthen their mechanical properties by trainings, emerging soft actuators require programmable manipulation. Currently, the integration of such characteristics as rapid self-strengthening, strain-adaptive stiffening and smart actuation, into a single hydrogel actuator is urgently needed. Here, we report a self-strengthened hydrogel actuator based on the semi-interpenetrating polymer network consisting of polyvinyl alcohol (PVA) and poly(N-isopropylacrylamide) (PNIPAm), which can display diverse programmable actuations by responding to temperature/salt stimuli. In the design, the layer of freeze-thawed PNIPAm/PVA (PPGel-F) is assembled with another original PNIPAm/PVA hydrogel layer (PPGel) into one device. By taking advantage of the PVA crystalline nanofibrils in the PPGel-F matrix, the differentiated swelling degree across the bilayer structure gives rise to asymmetric deformations and the resultant shape transformation. Moreover, upon the mechanical training with less than 100 cycles, the anisotropic arrangement of PVA nanofibrils through strong hydrogen bonding interactions can swiftly immobilize the amorphous polymer chains orientation along the tensile direction. This enhances the strain-induced crystallization, thereby generating the rapid self- strengthening behavior. The proposed work provides potential solution for constructing dynamically adaptive hydrogel systems that can mimic biological tissues for more intelligent soft robotics and bionic research.