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.