Tunable Actuation Response of Humidity-Driven Artificial Muscles by Means of Nanofillers

Soft actuators can be driven via environmental humidity and provide biomimetic adaptation to environments. Humidity may serve as a passive stimulus, i.e., the desired actuation may be caused by uncontrolled changes in the humidity of the environment, or active stimuli, i.e., promoting an actuation response by controlling the humidity. Here, we report a humidity-responsive axial actuator and walking robot of sulfonated polyether ether ketone (SPEEK), which shows greatly tailorable actuation performance upon embedding graphene nanoplatelets (GNP). Three cases of SPEEK with no GNP, SPEEK with 0.5 wt.% GNP and 1 wt.% GNP was tested. Adding only 0.5wt% GNP increases the actuation by 50% and provides a maximum actuation stroke of 24% and work capacity of 230 J/kg. In addition, 0.5wt% GNP promotes faster actuation, with significantly enhanced rates of both contraction and expansion. However, the addition of 1wt% GNP slightly decreases the actuation magnitude and rates. The non-monotonic actuation tunability by GNP was correlated to changes in ion exchange capacity (IEC), water uptake, and GNP dispersion. By utilizing actuation magnitude dissimilarity, the axial actuators were converted into a walking robot stacked of two active layers consisting of fibers of the same material system. The bilayer robot demonstrated self-crawling and locomotion ability in response to humidity changes. This study shows a uniquely tailorable humidity actuator that demonstrates both linear and bending actuation.