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Silver-coated nylon actuators - a form of artificial muscle - are potential candidates in biomedical applications, , as they yield a large strain (5-20%+), high force (>20 MN/m2 ), are compact and low-cost. But the on skin or internal application of these thermal actuators is limited by the heat released and the high activation temperatures (typically >80°C), which could cause tissue damage. We present a hybrid coating that reduces the temperature at the interface of the nylon actuator and surrounding tissue/skin, while maintaining the inner nylon actuator activation temperature. By taking advantage of the high heat capacity of water-swollen polyacrylamide (PAAm) hydrogel and the low thermal conductivity of silicone elastomer, we develop a hybrid coating for nylon actuators that provides effective heat dissipation and encapsulation without impacting strain. Hydrogel is used to absorb and dissipate heat. Using it alone dissipates heat quickly, and in turn, excess power is needed to achieve full strain. Therefore, silicone is used as a thin, inner insulating layer to retain the heat, so full strain can be achieved without excess power. We examined the strain and temperature of uncoated nylon fibres (control), single-layered silicone-coated nylon fibres, single-layered hydrogel nylon fibres and hybrid-coated (inner layer of silicone, outer layer of hydrogel) nylon fibres. At a constant current of 0.55 A, the mean strains of hybrid coated nylon fibres (6.0 %), and silicone coated nylon fibres (5.5%) were comparable to uncoated nylon fibres (5.3%). The mean strain for the hydrogel-coated nylon fibres was considerably lower (1.4%). The hybrid coating effectively maintains the fibre temperature (80-87°C) while cooling the outer surface (hydrogel) of the hybrid-coated nylon fibre (30-35°C). This provides a possible solution for use of these actuators in temperature sensitive applications.
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