Figure 6 Characterization of the long-term stability of the hair artificial muscles. (A) Photographs of the homochiral and heterochiral artificial muscles before and after actuation in water and ethanol after 100 cycles. Tensile stroke of (B) the homochiral and (C) the heterochiral artificial muscles before and after actuation in water at 10 testing points (1, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100). Tensile stroke of (D) a homochiral and(E) a heterochiral artificial muscle in response to water and ethanol after 5 months. The response rate of (F) the homochiral and (G) the heterochiral hair muscle to water and ethanol after 5 months.

Applications of the hair artificial muscles

The extremely large tensile stroke upon water stimulation and its fast recovery in ethanol makes the hair artificial muscle suitable for various applications. Figure 7 displays several different application scenarios. First of all, a robotic “sea cucumber” that could climb long distances based on the heterochiral hair muscle with the spring index of 15 and the twist density of 2500 turns m-1 was made. The sea cucumber could crawl on a barbed plastic cord as long 200 mm by exchanging the water and ethanol stimulation two times (Figure 7A ). The barbs on the surface of the cord all tilted in one direction, thus guiding the sea cucumber to move in the fixed direction.
Next, the energy generated from the water actuation of the homochiral hair muscle was used to pull a wheel model (~2.8 g). The wheel model could move 40 mm within 143 s as the homochiral hair muscle contracted in response to water (Figure 7B ). This result revealed that the homochiral hair muscle could work as an engine to actuate the movement of a wheel model which was approximately 500 times heavier than itself.
As shown in Figure 7C (Video S5 ), a single 97-mm-long homochiral hair artificial muscle was able to lift a weight 10 times heavier than itself by 57 mm (59% strain) in response to water in 60 s. The contractile work generated by the hair muscle during weight lifting normalized to the total weight of the muscle was considered the work capacity of the hair muscle. It was 5.38 J kg-1for the homochiral hair muscle with the spring index of 8 and the twist density of 2500 turns m-1.
The extremely large tensile stroke of the homochiral hair muscle upon water actuation indicates high sensitivity of the hair muscle to water, which could be used for smart switches. As illustrated inFigure 7D , two homochiral hair muscles were bound to the switch of a circuit. When the environment is dry, the circuit was connected and the LED light was on. However, when water appeared in the ambient environment and contacted the hair muscles, they would contract and disconnect the circuit, thus turning off the LED light. Photos inFigure 7E (Video S6 ) showed the smart switch turned off the LED light when the homochiral hair muscle contacted with water.