Self-replenishing soft actuator driven by liquid–gas phase change of harvested atmospheric moisture

Liquid–gas phase-change soft actuators generate large deformation by exploiting the volumetric expansion of a working fluid upon vaporization. However, their operation is fundamentally limited by depletion of the fluid through gas permeation. Here, we present a self-replenishing phase-change soft actuator whose working fluid is harvested from atmospheric moisture in high-humidity environments. The actuator passively absorbs moisture from the surrounding air and is actuated by external near-infrared photothermal heating. The actuator consists of a porous silicone composite containing hygroscopic silica-gel particles. Water vapor absorbed from the ambient air serves as the working fluid, and its liquid–gas phase change under near-infrared photothermal heating generates bending deformation. Over repeated cycles of 30-min moisture absorption and 15-s photothermal heating at 91–93% relative humidity, the actuator maintained a nearly constant curvature change (~0.033 mm⁻¹ for the 1-mm-thick sample), with the absorbed water mass recovering in every cycle. In contrast, repeated heating without moisture replenishment caused the actuation to decay to a small residual level attributable to solid thermal expansion. These results show that the actuation capability is autonomously recovered from ambient humidity without external liquid reservoirs or manual refilling. Proof-of-concept demonstrations further illustrate the potential of the actuator for autonomous operation in humid environments.