Figure 1 Schematic illustration of the fabrication process of (A) a torque-balanced two-plied hair fiber and(B) heterochiral and homochiral hair artificial muscles and the solvent-driven actuation.
First, a certain length of human hair was hanged vertically with a load torsionally tethered, and twisted counter-clockwise with a motor for some time (Figure 2A ). By adjusting the twisting time, hair fibers with different twist densities (1000, 1500, 2000, 2500, 2650 turns m-1) were obtained. The morphology of these hair fibers was examined with a micro lens and shown in Figure2B . It can be seen that the longitudinal elements changed from straight to helical after twisting and their angular displacement increased with the increase of the twist density. The helical angles can be approximated with the following equation:
\begin{equation} \alpha=\operatorname{}{(2\pi rT)}\nonumber \\ \end{equation}
where r is the radial distance from the center of the hair fiber (~100 μm), and T is the amount of inserted twist divided by the length of the hair fiber. In addition to the theoretical value, image J was also used to measure the actual helical angles of these fibers. It can be seen from the graph in Figure 2Cthat the measured values of these angles agree well with the calculated values.