Introduction

Artificial muscles that can actively contract, expand or rotate when stimulated by electricity, light, heat, or solvent have drawn considerable attention owing to their potential applications in smart devices and soft robotics1-3. Among various types of developed artificial muscles, twisted fibers have drawn considerable attention due to their reversible tensile actuations upon volumetric expansion and similarity to the biological muscle in form4-6.
For the past decades, a wide range of actuating materials have been used for twisted fiber based artificial muscles, including shape memory materials7, 8, carbon nanotube (CNT) yarns9-12, graphene oxide fibers13, 14, polymers15, 16, silk17-19, and hair fibers20. CNT yarns can deliver large strokes when stimulated by heat, electricity, or solvent. For instance, a helical fiber actuator from CNT can respond to organic solvents with a tensile contraction of 60% while a self-sensing coaxial CNT based muscle fiber can maintain a ~11% tensile stroke between electrothermal stimulation and solvent adsorption21, 22. Despite the advantages, the high cost of CNT limits its scalable fabrication and practical applications. Inexpensive polymer fibers like polyethylene and nylon fishing lines have also been developed into artificial muscles23. The extreme twisting technology caused coiling of the polymer fiber and dramatically amplified the tensile stroke. Nevertheless, it is also demonstrated that the coiling of twisted fibers with a mandrel can obtain larger stroke than that caused by the extreme twist insertion.
In addition to the synthetic materials, there is also a growing need for artificial muscles from natural biocompatible and biodegradable materials. Silk is one of the most impressive natural materials due to its hierarchical structure and superior mechanical properties. A humidity-driven artificial muscle prepared by coiling and thermal setting of the twisted silk fibers could contract by 70% in about 1 min when the ambient humidity changed from 20% to 80%19. Another natural fiber material, human hair, has also been developed into tether-free tensile artificial muscles by disulfide cross-linking20. When the twist density of the hair fiber was 3000 turns m-1 and the spring index was 15.8, the hair muscles exhibited the best performance, with 94% contraction for the homochiral and 3000% extension for the heterochiral. However, either the acquisition of the dual-filament silk fibers or the chemical crosslinking adds the complexity of the muscle fabrication and artificial muscles with stroke up to 10000% have not been reported so far.
In this study, long human hair was successfully transformed into highly reversible, tether-free, solvent-driven artificial muscles with extremely large tensile stroke and fast recovery by a simple two-step method, namely, twist insertion followed by a coiling and steaming process. Leaving out the chemical reduction and oxidation of the disulfide bonds of the hair, the method adopted here is not only simple and cost-effective, but also environmentally friendly. By simply adjusting the twist density and the diameter of the coiled hair muscles, a tensile stroke of as large as 10000%, more than 3 times of the largest stroke reported so far, was achieved upon stimulation. Meanwhile, ethanol could significantly shorten the recovery time of the artificial muscle to merely 10 seconds. Moreover, the hair artificial muscles were also demonstrated for weight-lifting, climbing, and sensitively control the switch of a circuit.