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.