Actuation mechanism
To the best of our knowledge, the
tensile stroke achieved in this study is far larger than that of the
previously reported artificial muscles (Figure 5A ).
Furthermore, it is more than 3 times higher than the largest stroke
reported so far, which results from hair artificial muscles prepared
through disulfide cross-linking (3000%)20. Simply by
twist insertion, coiling and steaming, hair artificial muscles with
extremely large contraction or elongation stroke could be obtained.
It is known that the actuation mechanism of the twisted fiber based
artificial muscles is muscle untwisting driven by the volume change,
which arises from their response to external stimuli1.
Moreover, by transforming the twisted fibers into coiled muscles, the
untwisting torque derived torsional actuation upon external stimulus can
be translated into tensile actuation4, 6. Hair is one
of the most important natural keratinous fibers based on
α-keratin24. The stability of the keratin is
maintained by intramolecular hydrogen bonds, making it susceptible to
the ambient water content25.
The structural change of the hair after treatment was characterized by
DSC analysis. As shown in Figure 5B , in spite of the
peak at around 230 ℃ that generally depicts the melting of the α-form
crystallites, another peak at around 239 ℃ appeared on the DSC curve of
the steamed hair. This endothermic event indicated the irreversible
denaturation of α-keratin caused by steaming. Interestingly, no obvious
peak showed at around 239 ℃ on the DSC curve of the heat hair,
suggesting the important role of water vapor. It might be inferred that
the intramolecular hydrogen bonds of the keratin were interfered with
the ambient water vapor during the steaming process. Moreover, when the
hair artificial muscle was immersed into water, the hydrogen bonds
tended to rearrange, thus leading to the contraction or elongation of
the artificial muscle. When the wet hair muscle was immersed in ethanol,
it would dehydrate quickly and recover. Furthermore, torsional tethering
is not required here to preserve the twist in the hair fibers.