Free-standing single-helical woolen yarn artificial muscles with robust and trainable humidity-sensing actuation by eco-friendly treatment strategies

Twisted yarn artificial muscles have attracted great interests for diverse applications, such as soft robotics, miniaturization controllers and smart textiles. A challenging issue in fabricating the twisted yarn artificial muscles is to retain the inserted twist. Different from the exiting strategies of forming double-helical structures or harnessing complex chemical technologies, we herein propose a simple combination of plasma and UV-light treatments to train natural wools into twist-stable single-helical yarn artificial muscles without external torsional tethering, which realizes easy fabrication of twisted actuators, and achieves better moisture-actuating performance (nearly five times higher in maximum rotation) compared to equivalent double-helical actuators. The stable morphology of woolen yarn muscles affected by the opening and closing of disulfide bonds is explained from microstructure characterization and theoretical analysis. The charming properties of single-helical yarn muscles will provide new inspiration for the development of fiber-based actuators in industrial routines, which is expected to promote the practical application of yarn muscles in smart textiles and wider fields.