Chemical-Free Engineering of Natural Bamboo into Highly Sensitive Humidity-Driven Actuators

Actuators driven by ambient humidity are of practical interest for various applications, such as soft robots and artificial muscles. However, the present methods involve high costs, complex chemical reactions, and associated environmental issues. Here, we demonstrated that by rationally controlling the thickness of bamboo via simple chemical-free processes, we could program the response sensitivity of bamboo to moisture. In particular, when the thickness of bamboo reached 25 μm, it exhibited rapid actuation and continuous locomotion under moisture stimulation. This ultrathin bamboo film achieved an impressive bending curvature of up to 300° within 9 s, and the bending curvature and speed surpassed the performance of the majority of previously reported materials, including graphene-based materials and synthetic polymers. Notably, this precise control of bending behavior via thickness was universal among different bamboo species but was not evident in other biomaterials such as black locust wood, reed stems, corn husks, and balsa wood. Based on the sensitive humidity-response property of bamboo film actuators, their tremendous potential in wide applications, including smart claws, walking robots, and electronic devices, has been demonstrated. Furthermore, bamboo film actuators demonstrated excellent recyclability and sustainability throughout their lifecycle, which was verified by conducting a life cycle assessment. This study provides new insights into designing all-natural actuators in a simple, mechanical way by utilizing the intrinsic characteristics of the biomaterial.