Figure 3 (A) The morphology of the self-balanced two-plied hair fiber. (B) The morphology of the heterochiral hair muscles with the spring index of 8 (i) and 15 (ii). (C)The morphology of the homochiral hair muscles after being untied from the mandrel (i), and fully contracted in water (ii). (D) The morphology of the homochiral hair artificial muscles with the diameter of 7 mm and the twist densities of 1000, 1500, 2000, 2500, and 2650 turns m-1. (E) The morphology of the homochiral hair muscles with the twist density of 2650 turns m-1 and the diameter of 1.6, 3.0, 5.0, 7.0, and 8.0 mm. (F) Digital photos of homochiral hair muscles with the twist density of 2650 turns m-1 and the diameter of 1.6, 3.0, 5.0, 7.0, and 8.0 mm. (G) Schematic illustration of a homochiral artificial muscle made from a self-balanced two-plied hair fibers and the measurement of the coil pitch δ. (H) Measured coil pitch for homochiral hair muscles with the diameter of 1.6, 3.0, 5.0, 7.0, and 8.0 mm. Scale bars for A, C, D, and E: 1 mm.

Actuation performance of the hair artificial muscles

The actuation performance of both the homochiral and heterochiral hair artificial muscles in response to water and ethanol were studied. To investigate the impact of both the twist densities and the spring index of the hair muscles on their actuation performance, homochiral and heterochiral hair muscles with the same twist density but different diameters or with the same diameter but different twist densities were prepared and used for the study.
Results are shown in Figure 4 . Homochiral hair artificial muscles contract in water and elongate in ethanol (Figure 4A, Video S1 and S2 ), while the heterochiral muscles elongate in water and contract in ethanol (Figure4B ). Figure 4C presents the photos of a homochiral hair muscle with the twist density of 2500 turns m-1and the diameter of 7.0 mm at different time points of the actuation process. The muscle was about 235 mm long in the beginning. When activated with water, the hair coils would rotate radially and contract along the axial direction, bringing the coils closer until they were adjacent to each other. After shrinking into a 2 mm tight spring structure, the hair muscle was put into the ethanol to extend. It could quickly rotate radially and extend along the axial direction to 220 mm long in 50 s.
Similarly, the length change of a heterochiral hair muscle with the twist density of 2650 turns m-1 and the diameter of 8.0 mm in response to water and ethanol is shown in Figure4D . The 1.5 mm long hair muscle extended to 150 mm long in only 32 s in water and shortened back to 1.5 mm long in ethanol within 10 s, exhibiting a very fast response rate to ethanol. As can be seen fromFigure 4E and Figure 4F(Video S3 and S4 ), the actuation was really fast during the first 10 to 20 s, and slowed down later on. The maximum actuation speed could reach 500% s-1. Tensile stroke of both homochiral and heterochiral hair artificial muscles increases with not only the spring index of the helical muscles (Figure4E, 4F ), but also the twist density of the hair fibers (Figure 4G, 4H ). Surprisingly, when the twist density reached 2500 turns m-1 and the diameter reached 7.0 mm, the tensile stroke was as large as 10000% for either the heterochiral or homochiral hair muscles. Since the homochiral hair muscle extended after untying from the mandrel, the length before extension is considered as its initial length. In addition to the heterochiral hair muscle with the highest twist density and largest diameter, nearly all heterochiral hair muscles could contract to its initial length within 20 s (Figure 4F, 4H ).