Coil order-transformational actuation for high tensile stroke up to 65% with coiled CNT/PDMS fiber

Abstract

Coiled fiber actuators have emerged as promising functional materials for soft robotics and intelligent systems. However, their inability to dynamically switch between primary and secondary coiled structures during operation severely limits their actuation performance. In this work, we propose a coil order transformational actuation mechanism based on twisted PDMS/CNT composite fibers that enables reversible switching between primary and secondary coils under dynamic loading and thermal stimulation. By exploiting temperature-induced variations in fiber diameter and mechanical properties, we modulate the critical twist density for secondary coiling and thereby trigger the spontaneous formation of higher-order coils. Under a load of 55 kPa at 200 °C, the actuator achieves a tensile stroke of 65% of its maximum value and can be fully reset under a load of 80 kPa. We further demonstrate a gas overheat protection valve based on this actuator, in which the structure autonomously regulates flow and shuts off at a critical temperature. This study introduces a new actuation paradigm based on reversible structural reconfiguration, thereby significantly enhancing the performance of coiled fiber actuators.