Experimental characterization and model predictions for twisted polymer actuators in free torsion

Thermally driven artificial muscles, such as twisted polymer actuators (TPAs), are a promising new development in the field of smart materials. TPAs have potential applications in advanced prostheses, robotics, or any operation that produces excess heat and requires actuation. The theory explaining the actuation phenomenon of TPAs is based on the anisotropic thermal expansion of drawn polymers, which expand radially and contract axially under thermal loading. When the monofilaments are twisted, these thermal expansion properties remain relatively unchanged, but the internal fibers become helically aligned, thus causing the TPA to untwist when heated. TPAs can be used as torsional or linear actuators, depending on the configuration of the twist. In this work, we present experimental methods for acquiring untwisted monofilament thermal properties and thermal actuation data of straight twisted polymer actuators (STPAs). STPAs act as torsional actuators and can be thought of as elemental sections of the coiled linear actuators. The experimental data is then used to assess current, kinematic models for predicting STPA responses under free torsion. The results suggest that current models capture first order torsional and axial response due to thermal load and indicate areas for future refinement and research.