Humidity-induced actuation of silk yarn

Abstract

Silkworm silk is a promising material for smart textiles due to its excellent mechanical properties, biocompatibility with human body, and remarkable hygroscopicity. Silk yarn can provide reversible and repeatable actuation responses during humidification-and-drying cycles. In this study, we investigate the torsional and tensile actuation performances of silk yarn stimulated by humidity from an experimental and theoretical perspective. We find that increasing the twist level and number of silk fibers significantly improves the actuation performance. Silk yarn undergoes an irreversible relaxation upon first being exposed to moisture. It exhibits a repeatable humidity-contraction stress response when wetted further. The actuation strain of silk yarn varies with humidity and increases monotonically with twist level. A theoretical model based on the water absorption equation and the standard reinforcing model is developed to describe the humidity-induced torsional and tensile actuation of silk yarn. The theoretical predictions agree well with the experiments. This work sheds light on the physical mechanisms of humidity-induced actuation in silk yarn and inspires the design of miniature soft actuators.