A gradient-distributed liquid-metal hydrogel capable of tunable actuation

Although thermoresponsive hydrogels have numerous applications that range from soft robots, biomedical engineering, and actuators to sensors for artificial muscles, the existing hydrogel actuators undergo only unidirectional deformation under a single thermal stimulus and suffer from slow actuation and unstable interfacial adhesion in multiple layers. Herein, hydrogels containing gradient-distributed polydopamine-coated eutectic gallium-indium (PDA-EGaIn) nanodroplets in a poly(N-isopropylacrylamide) (PNIPAM) matrix and thus featuring a gradient distribution of thermal conductivity and an increased barrier towards water loss are shown to be capable of a rapid and tuneable thermoresponse. Notably, whereas hydrogels with a low content of PDA-EGaIn undergo rapid one-way bending under a single thermal (45 °C) stimulus, those with a high content of PDA-EGaIn undergo sequential bidirectional (bending) actuation. The ability of these hydrogels to undergo fast and tuneable actuation under a single thermal stimulus makes them suitable for use in grab-release instruments and soft robots.