Locomotion Based On Hygromorphic Bilayer Actuator: Humidity-Induced Swelling/Deswelling of Layer-By-Layer Films

Several organisms in nature exhibit complex actuation in response to changes in environmental factors. Humidity-induced motions and actuations are utilized for seed dispersal by pine cones and wild wheat seeds. In general, humidity-induced actuations observed in these biological systems rely on the differential swelling and deswelling of bilayer structures under cyclically changing humidity conditions. Inspired by these natural actuators, we develop a hygromophic bilayer actuator (HBA) that shows locomotion on a ratchet track in response to changes in relative humidity (RH). The basic principle of the HBA lies in the rapid swelling and deswelling of highly hygroscopic layer-by-layer (LbL) films of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) deposited on a moisture-resistant Teflon ribbon. We demonstrate that the swelling of the PAH/PAA LbL films is a governing parameter that controls the step length of the HBA on the ratchet track. We also present the humidity-induced swelling/deswelling behavior of (PAH/PAA) films assembled under different conditions, which provides fundamental insights into the interactions between water and these LbL films.