Thermoresponsive Gel-Yarn Composites Enable Low-Temperature Tensile Actuation in Twisted and Coiled Polymer...

Soft robotic systems require actuators that combine large deformation, mechanical robustness, and reliable operation under practical conditions. Twisted and coiled polymer (TCP) artificial muscles, typically fabricated from fibrous materials, offer high power density and large stroke; however, their reliance on elevated operating temperatures (>80°C) for high performance limits their use in temperature-sensitive environments. Here, we report a thermoresponsive gel-yarn composite actuator that enables actuation at reduced temperatures within TCP-based systems. Twisted polyester yarns were functionalized with a biocompatible thermosensitive hydrogel, poly(MAA-co-OEGMA), forming a fiber-gel composite architecture. Temperature-induced swelling and deswelling of the hydrogel in cold and hot water, respectively, generated internal stresses within the yarn, which drive untwisting of the coiled structure and result in lengthwise actuation. The effect of ionic crosslinking was systematically investigated by varying FeCl₃ concentration, with 1 M identified as the optimal condition balancing structural stiffness and actuation performance. The composite actuators exhibit isotonic free strains of up to 39% when cycled between room temperature and 50°C and work densities of up to 8.8 × 10^{- 3} J g^{- 1}, with reversible and repeatable actuation behavior. Compared to conventional TCP systems, which typically operate at substantially higher temperatures, the present system achieves similar deformation but within a lower and more practical thermal range. These findings suggest that fiber-gel composite architectures provide a viable route for tuning actuation behavior in TCP systems while maintaining compatibility with temperature-sensitive applications.