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.
Artificial muscle systems have the potential to impact many technologies ranging from advanced prosthesis to miniature robotics. Recently, it has been shown that twisting drawn polymer monofilaments, such as nylon fishing line or sewing thread, can result in a biomimetic thermally activated torsional actuator. The actuation phenomenon in these twisted polymer actuators (TPAs) is thought to be a result of an untwisting that occurs about the fiber’s axis due to an anisotropic thermal expansion. Before being twisted, the precursor fibers are comprised of polymer chains that are aligned axially. During fabrication of TPAs, the polymer chains reorient as the precursor fiber is twisted about the central axis of the monofilament. At the end of the fabrication process, the TPA is annealed in order to relieve internal stresses and to keep the fiber in the twisted configuration. The mechanism of untwisting actuation is generally thought to be a result of radial expansion and axial contraction. After being twisted, these radial and axial expansion relationships remain relatively unchanged, but the polymer chain direction is no longer axially aligned. Thus, upon heating the twisted fibers of the TPA, the fibers untwist and torsional actuation occurs. This actuation phenomenon has been used in the past to create linear actuators, but can also be use directly as a torsional actuator. Compared to other torsional actuators TPAs are low cost, lightweight, and can actuate reasonably high torques per unit volume. However, because TPAs are thermally activated, they may not be suitable for all applications. In this work, we present a novel TPA design for use as a torsional actuator for miniature actuation and artificial muscle applications. Our design bundles twisted monofilaments to increase the torque. Both fabrication and testing methods of the new design are presented. Results for temperature versus torsional displacement under various loads give insights as to how these actuators may be used and the reversibility of the actuation process under different fabrication loads. Additionally, comparisons are made between these bundled actuators and similarly loaded single TPA monofilament actuation.
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