Effect of temperature softening on the actuation performance of twisted and coiled polymer muscles

Abstract

Twisted and coiled polymer (TCP) artificial muscles represent a promising class of actuators capable of emulating natural skeletal muscle, offering substantial linear actuation and energy density. The actuation characteristics of the thermally-activated TCP muscle and twisted fiber are characterized experimentally, emphasizing the influence of the temperature-softening effect. We find that the recovered torque of twisted fiber increases monotonically with temperature while showing a non-monotonical variation with the fiber bias angle. The variations of actuation strain and end torque with three structural parameters of TCP muscles, i.e., fiber bias angle, helical angle, and spring index, are comprehensively investigated. An optimal bias angle and spring index are present. A significant temperature-softening effect of precursor fiber is observed; a polymer becomes softer and more pliable as the temperature increases. A thermo-mechanical model for predicting the effect of temperature-softening on TCP muscle is established and validated with experimental data. This work facilitates precise control and guides the structural optimization design of the thermally-activated TCP muscle.