Bioinspired Hygromorphic Actuator Exhibiting Controlled Locomotion

We report a bioinspired hygromorphic double-layered actuator (HDA), of which the movement is controlled by cyclical changes in relative humidity (RH). The basic principle of the HDA lies in the rapid swelling and deswelling of highly hygroscopic layer-by-layer (LbL) assembled films deposited on a moisture-resistant and flexible polytetrafluoroethylene (PTFE) ribbon. We engineer the geometry of the HDA to induce locomotion on a ratchet track. By controlling the exposure time and RH, the HDA is remotely controlled to move a precise number of steps on the ratchet track during one cycle of RH changes. We demonstrate that the step length of the HDA depends on the relative thickness change of the LbL film. We also provide theoretical considerations based on a plate theory and the Flory–Huggins theory to describe the actuation of the HDA. Our work provides fundamental insights into the fabrication and design of hygromorphic actuators driven by RH changes.