Dual-gradient PNIPAM-based hydrogel capable of rapid response and tunable actuation

Thermoresponsive hydrogels have been received myriad attention and applied in numerous fields of soft robots, artificial muscles, and multifunctional sensors. However, developing a facial and low-cost method to obtain a thermoresponsive hydrogel with distinguished, multidirectional actuating performance and suitable mechanical properties remains a big challenge. Herein, by the introduction of the inorganic particle (IP), a thermoresponsive, dual-gradient (IP and crosslinking density) hydrogel with multidirectional actuating behaviors and improved mechanical properties were prepared by ultraviolet (UV)-induced copolymerization of the N-isopropylacrylamide (NIPAM) monomer and dispersed IP. The high thermal conductivity of IP endowed the prepared hydrogel with relatively rapid heat transmission and actuating deformation, and the deformation degree and direction could also be adjusted by changing the content of IP. Notably, although hydrogels containing a low content of IP undergo one-way deformation under a single thermal (45 ℃) stimulus, those containing a higher content of IP can exhibit bi-and even tri-directional stable actuations under the same and continuous thermal stimulus. Therefore, inspired by the adjustable and steady multidirectional actuations under a single thermal stimulus, the IP-containing hydrogels were suitable for use as the hydrogel-based gripper and jack to grab/release and lift/lower objects, respectively. Our work provides a facial and practical method for the design of anisotropic thermoresponsive hydrogels, paving the way for the further development of hydrogel-based soft robots and artificial muscles.