Biomimetic Microstructured Hydrogels with Thermal-Triggered Switchable Underwater Adhesion and Stable Antiswelling Property.

The design of hydrogels with switchable adhesion and stable antiswelling property in a wet environment has remained a challenge. Here, we report a biomimetic hydrogel that can adhere and detach on-demand on various material surfaces, which is realized by thermal-triggered switchable shape transformation on hexagonal micropillar patterned hydrogels. The hydrogels are cross-linked by two cross-linkers of poly(ethylene glycol) dimethacrylate and 2-ureidoethyl methacrylate, which guarantee the strong mechanical property and stable antiswelling property in a wet environment. The hydrogels can maintain stable water content in solutions with variable pH, temperature, and salt concentration, and the change in volume does not exceed 2%. In addition, due to the dynamical hydrogen bonds and dipole-dipole interaction in the hydrogels, the hydrogels exhibit a thermal-triggered shape-memory effect. The hydrogel can recover shape more than 80% in 15 s. Furthermore, inspired by the surface structure of tree-frog footpads, the hexagonal micropillar patterned hydrogels exhibit improved underwater adhesion strength. The underwater adhesion strength of hexagonal micropillar patterned hydrogels is seven times more than that of flat hydrogels. Based on the shape-memory effect of hydrogels, the adhesion strength can be altered by a thermal stimulus. The adhesion strength of the microstructures recovered from the hydrogel surface decreased to 15.4% of the initial adhesion strength. The switchable underwater adhesion of hydrogels can be applied in the fields of transfer printing, medical adhesives, mobile robots, etc.