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Compliant and Large-Strain Twisted String Actuators using Supercoiled Polymers

Twisted string actuators (TSAs) can produce linear motions by converting the electric motor’s rotary motion. TSAs exhibit high power density, energy efficiency, and translational force. However, a common challenge in utilizing TSAs as soft and compliant actuators is to simultaneously increase their compliance and maximum strain. Previous studies predominantly utilized strings with high stiffness. Recent studies attempted to use strings with low stiffness with a sacrifice of maximum strain, especially under large loading conditions. In this paper, a novel strategy is proposed to simultaneously increase the compliance and maximum strain of TSAs. By replacing stiff strings with stretchable and coiled nylon strings, called supercoiled polymer (SCP) strings, the compliance of the TSA is increased. Furthermore, by heating the TSA with SCP strings through Joule heating, additional linear contraction is obtained. The fabrication procedure of the proposed TSA is provided. Experimental characterizations of stiffness and actuation properties are conducted with different actuator configurations and loading conditions. Experiments confirm the enhanced performance of the proposed TSA using SCPs.