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.