Solar water evaporation-induced long-term locomotion of self-propelled soft robots

Self-propelled soft robots have attracted extensive attention because of their unique application in exploring dangerous and complex environments that are unsuitable for human beings. However, these soft robots require cyclical chemical stimulation or external power and have short locomotion times, which limits their practical applications. It remains challenging to create self-propelled soft robots exhibiting long-term locomotion. Here, we couple an active hydrogel with a solar absorbing coating to realize self-propelled soft robots with long-term locomotion. The active hydrogel can move freely on the water surface by continuously establishing asymmetric surface tension through dynamic wetting. The sunlight absorbers promote water evaporation inside the self-propelled soft robot to delay or even disrupt the swelling equilibrium of the hydrogel, thus establishing dynamic balance between water absorption and evaporation. In this way, the locomotion time of this self-propelled soft robot under constant light irradiation equivalent to 1 sun (1 kW/m 2 ) is 6.5 times higher than that of active hydrogel reported previously. Owing to the enhanced locomotion time through solar water evaporation water, this self-propelled soft robot is expected to be applied to oil pollution exploration, cargo transportation, and debris cleaning in small water areas.