MXene/Carboxymethyl Chitosan Moisture Responsive Soft Actuator with Diode-Like Actuation for Versatile Applications Driven by Human Metabolism.

Moisture responsive soft actuators are receiving increasing attention due to their unique potential in reducing external energy dependence and carbon footprint. For the conventional moisture responsive soft actuators, their bending deformation under moisture stimulation is usually bidirectional, and the orientation of the bending axis is random. Achieving a moisture responsive monolithic actuator with controllable unidirectional deformation remains a challenge. Here, a Ti₃C₂T_x MXene/carboxymethyl chitosan composite film actuator with thickness gradient along length direction is fabricated via a vacuum-assisted "tilt-filtration" approach. The actuator exhibits a "diode-like" controllable unidirectional deformation behavior under moisture gradient, and its deformation direction is strictly correlated to its thickness gradient direction and moisture source direction. Based on this highly correlated actuation behavior with internal structural asymmetry, a self-sustained oscillator under a constant moisture gradient is achieved. Besides, various multifunctional applications based on this actuator driven by human metabolism are also demonstrated, including non-contact switch with unidirectional conductivity, intelligent keyboard for non-contact character input, biomimetic crawling robot, wearable intelligent thermal management clothing, and a self-powered respiratory sensor. This work paves the way for the realization of moisture responsive soft actuators with unidirectional controllable deformation, and further promotes the development of sustainable intelligent materials in soft robotics and electronics.