Tunable Actuation of Humidity-Driven Artificial Muscles via Graphene Nanofillers

Humidity-responsive soft actuators can be driven by external stimuli and provide biomimetic environmental adaptations. Here, we report a humidity-responsive axial soft actuator of sulfonated polyether ether ketone which shows greatly tailorable actuation performance upon embedding graphene nanoplatelets (GNPs). Analysis of the experimental data shows that adding only 0.5 wt % GNP increases the actuation by 50% and provides a maximum actuation stroke of 24% and work capacity of 230 J/kg. In addition, 0.5 wt % GNP facilitates faster actuation, with significantly enhanced rates of both contraction and expansion. However, the addition of 1 wt % GNP slightly decreases the actuation magnitude. The nonmonotonic actuation performance was correlated with ion exchange capacity, water uptake, and GNP dispersion. By utilizing actuation magnitude tunability via GNP, 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 tunable humidity actuator that demonstrates linear and bending actuation.