Bioinspired Smart Moisture Actuators Based on Nanoscale Cellulose Materials and Porous, Hydrophilic EVOH Nanofibrous Membranes

Biomimetic actuators with rapid response speed, high sensitivity, and selectivity to external stimulus have found potential applications in smart switches, artificial muscles, and soft robots. The nanoscale structures of actuators enhance the exposed area to stimulus as well as enable versatile control of the actuation behaviors. Freestanding, flexible, and porous water-driven actuators with poly(vinyl alcohol- co-ethylene) (EVOH) nanofibers as the substrate and super hydrophilic nanoscale cellulose materials (cellulose nanofibers, cellulose nanocrystals, bacterial cellulose) as the active substance via uniform mixing or surface depositing were fabricated. The effects of the EVOH nanofiber substrate, the structures and concentrations of nanoscale cellulose materials, as well as the different environmental stimuli like humidity and temperature on the performance of actuators were studied. The water-driven actuation mechanism was proposed from the macroscopic and molecular aspects and the analysis of Gibbs free energy and mechanical energy. The actuator could bend to an angle of 180° and recovered less than 1 s for more than 100 circles without compromising properties when the environmental moisture changed. Furthermore, the multidimensional deformation behaviors of the water-stimulated actuators could also be well tuned by varying the orientations of the nanoscale materials. Additionally, the applications of the prepared actuator were demonstrated.