Figure 5 (A) Comparison of the tensile strokes between different artificial muscles5, 8, 9, 12, 19-21, 23.(B) Differential scanning curves (DSCs) of the pristine hair, hair treated with only heat and with steam. Right panels magnified the events in the dashed boxes.

Reversibility and the long-term stability of the hair artificial muscles

To investigate the reversibility of the hair artificial muscles, 100 fully reversible water-ethanol stimulation cycles were demonstrated. The homochiral hair muscles contracted in water and re-elongated in ethanol, while the heterochiral hair muscles elongated in water and re-contracted in ethanol. This water and ethanol stimulation cycle is called a reversibility cycle and can be repeated many times. As shown inFigure 6A , after 100 water-ethanol stimulation cycles, the homochiral hair muscle could still contract to a short spring in 21 s in response to water and re-elongate to 100 times its initial length in ethanol, while the heterochiral hair muscle could elongate to 100 times its initial length in 40 s in response to water and re-contract to a short spring in only 25 s in ethanol. The change of the tensile stroke for both the homochiral and heterochiral hair muscles during the 100 reversible cycles is shown in Figure 6B andFigure 6C . It can be seen that there is no significant performance decrease, indicating the reversibility for both the homochiral and heterochiral hair artificial muscles.
Moreover, the long-term stability of the hair muscles was also studied.Figure 6D shows the function of the tensile stroke against time for a homochiral hair muscle with the diameter of 8 mm and twist density of 2650 turns m-1 after placing in ambient environment for 5 months. The hair muscle responded to both the water and ethanol stimulation and achieved a tensile stroke as large as 10000% within 30 s. The response rate shown in Figure6F exhibits that both the maximum contraction speed in water and the maximum elongation speed in ethanol could be over 1000% per second. Apart from the homochiral hair artificial muscles, the actuation of a heterochiral hair muscle with the diameter of 8 mm and twist density of 2650 turns m-1 after 5 months was also investigated. It can be seen from Figure 6E andFigure 6G that the tensile stroke in response to water was also 10000% and the maximum actuation rate could be over 800% per second. These results demonstrated the excellent actuation performance of both the homochiral and heterochiral hair artificial muscles after 5 months, indicating their good long-term stability.