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 ).