Figure 2 (A) Illustration of measuring the helical angles of the twisted hair fiber. (B) The morphology of the twisted hair fibers with the twist density of 1000, 1500, 2000, 2500, 2650 turns m-1. (C) The calculated and measured helical angles of the hair fibers with twist density of 1000, 1500, 2000, 2500, 2650 turns m-1.

Fabrication and characterization of thehair artificial muscles

After twisting, the torsional stress generated during the twist insertion tends to cause twist release of the hair fiber when the torsional tethering is removed. To balance the strong untwisting torque, the hair fiber was folded at its middle point and plied together to achieve a self-balanced structure (Figure 3A ). When the twist density was less than 1000 turns m-1, the self-plied fiber was too loose at the end. Whereas inserting more than 2650 turns m-1 of twist into the hair fiber would cause the fiber to break during twisting. Therefore, twist densities of 1000, 1500, 2000, 2500, and 2650 turns m-1 were used for the following experiment. The torque-balanced two-plied hair fibers were then wrapped tightly around cylindrical mandrels clockwise or counterclockwise and steamed for 30 min. When the direction of the fiber twist matches the coil’s wrapping direction, the obtained coiled muscles are referred to as homochiral artificial muscles. On the other hand, when the direction of the fiber twist and the coil’s wrapping direction are opposite, the obtained coiled muscles are referred to as heterochiral artificial muscles. The diameter of the mandrel was 1.6, 3.0, 5.0, 7.0, and 8.0 mm, respectively. The resulted spring index, which is the ratio of the mandrel diameter to the two-plied hair fiber diameter, was 8, 15, 25, 35, and 40.
After hydrothermal setting, coiled hair artificial muscles were untied from the mandrels and relaxed in the ambient air. The relaxed heterochiral hair artificial muscles and the homochiral muscles have distinct morphology. The coils of the heterochiral hair artificial muscles remained in contact with each other regardless of their diameters (Figure 3B ), while the coils of the homochiral muscles gradually loosened up (Figure 3C-i ) and extended to a long thin squiggle shape. For convenience, the diameter of the homochiral hair muscle before relaxation is considered as the homochiral muscle’s diameter, and the length before relaxation is considered as the homochiral muscle’s initial length, which can be achieved after water actuation (Figure 3C-ii ). It can be seen from Figure 3D that no significant correlation was found between the extension of the homochiral hair muscles and their twist densities. As for the influence of the diameter on the elongation of the homochiral hair muscles, however, it was found that the larger the initial diameter of the muscle, the longer the muscle extended (Figure 3E, 3F ). Coil pitch, which is the distance between the adjacent muscle coils and denoted as δ, was used to quantitatively analyze the extension of the homochiral artificial muscles (Figure 3G ). ImageJ was used to measure the coil pitch of the homochiral hair muscles with the diameter of 1.6, 3.0, 5.0, 7.0, and 8.0 mm. The result is shown in Figure3H . It can be seen that the coil pitch increases with the diameter of the homochiral muscle. The coil pitch for the fully relaxed homochiral muscles could be as large as 8 to 9 mm, about 3 times larger than the disulfide crosslinked hair muscle20. The elongation and coil pitch of the homochiral hair artificial muscles may provide a structural basis for their lateral actuation performance.