Spider-Silk-Like Single-Fiber Actuators with Two Actuation Modes Driven by Water

Soft actuators can contribute to the movement of soft robots by generating mechanical force. Among the various types of soft actuators, fiber actuators are particularly advantageous due to their untethered, flexible, shape-configurable, and lightweight properties. Herein, we propose single-fiber actuators with programmable water-driven actuation ability. This design is based on the intrinsic hierarchical structure of keratin, including rich reversible hydrogen bonds and reversible conformational changes in the secondary structure (α-helix and β-sheet). The fiber actuators exhibit two responsive modes similar to those of spider silk: a supercontraction response upon the first time of wetting and a cyclic contraction-extension response during hydration-dehydration cycles. This intrinsic water responsiveness enables the keratin fibers to generate significant energy actuation compared to natural muscles and outperforms liquid crystal elastomer and composite actuators. The discovery of this dual-mode responsiveness in keratin fibers will enhance the value of keratin and inspire the design of biobased soft actuators.