Robust and Highly Sensitive Cellulose Nanofiber-Based Humidity Actuators

The design of humidity actuators with high response sensitivity (especially actuation time) while maintaining favorable mechanical properties is important for advanced intelligent manufacturing, like soft robotics and smart devices, but still remains a challenge. Here, we fabricate a robust and conductive composite film-based humidity actuator with synergetic benefits from one-dimensional cellulose nanofibers (CNFs) and carbon nanotubes (CNTs) as well as two-dimensional graphene oxide (GO) via an efficient vacuum-assisted self-assembly method. Owing to the excellent moisture sensitivity of CNF and GO, the hydrophobic CNT favoring rapid desorption of water molecules, and the unique porous structure with numerous nanochannels for accelerating the water exchange rate, this CNF/GO/CNT composite film delivers excellent actuation including an ultrafast response/recovery (0.8/2 s), large deformation, and sufficient cycle stability (no detectable degradation after 1000 cycles) in response to ambient gradient humidity. Intriguingly, the actuator could also achieve a superior flexibility, a good mechanical strength (201 MPa), a desirable toughness (6.6 MJ/m³), and stable electrical conductivity. Taking advantage of these benefits, the actuator is conceptually fabricated into various smart devices including mechanical grippers, crawling robotics, and humidity control switches, which is expected to hold great promise toward practical applications.