Hygrobot: A bio-inspired walking device driven by environmental humidity energy

Micro-robots that are light and agile yet requiring no artificial power input can be widely used in medical, military, and industrial applications. Here we report an actuation system that propels itself by harnessing the environmental humidity energy with its motion rectified by a ratchet. The bilayer-type actuator employs a hygroscopically responsive layer consisting of aligned nanofibers as inspired by the awns of self-burrowing seeds of Erodium and Pelargonium species. The ratchets based on asymmetric friction coefficients rectify the oscillatory bending motion into a directional locomotion like those found in inchworms, snails and snakes. We mathematically analyze the mechanical response of the actuator-ratchet system, the hygrobot, by solving the vapor diffusion and elastic bending of the active layer simultaneously, which allows us to optimize the robot design for maximum locomotion speed. We further demonstrate the utility of the humidity driven system for some biomedical applications.