MXene-PS-Based Soft Actuator with Deformation Reversal Capability through Time-Sequence Difference of Asymmetric Water Absorption Kinetics

Abstract Many soft actuators have been developed to achieve bending deformation under external stimuli through asymmetric structures or nonuniform material distributions. However, most existing actuators can only produce bending in a single direction, and developing soft actuators with deformation reversal capability remains a challenge. To address this issue, this study proposes an MXene-PS/MXene bilayer soft actuator, which is fabricated by sequential vacuum-assisted filtration, resulting in a loose MXene-PS top layer and a dense MXene bottom layer. The MXene-PS layer features a high water absorption/desorption rate but low capacity, while the MXene layer exhibits a low rate but high capacity. This complementary kinetic-capacity coupling mechanism, termed the time-sequence difference of asymmetric water absorption kinetics, enables the actuator to achieve reversible deformation reversal under NIR light and humidity stimuli. Furthermore, simply by changing the placement orientation of the actuator, two distinct self-sustained oscillation behaviors can be obtained in a humidity gradient environment, namely, periodic motion toward or away from the water surface. Leveraging this deformation reversal capability, we demonstrate two proof-of-concept applications: a biomimetic flower and a self-sustained motion sailboat. This work introduces a deformation mode and actuation principle for MXene-based soft actuators and further broadens their application prospects in biomimetic devices and soft robotics.