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.