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.