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.
Low-cost, highly versatile thermally driven coiled nylon actuators have demonstrated great tensile stress (>10 MPa) and large stroke (>5%). The work density of this material is 100 times greater than mammalian muscle, which makes coiled nylon actuators good candidates for applications in soft robotics. Similar to other thermally driven actuators, heat transfer rate limits their frequency response and benefits from extensive cooling. The cooling time for these actuators is dependent on heat conduction and convection. For instance, an 860 µm multi-stranded coiled nylon actuator is limited to 0.2 Hz frequency of actuation, above which tensile stroke drops due to heat accumulation. We analyzed the thermal behavior of silver-coated nylon actuators and investigated the actuation under air flow, in hydrogel, and in water, to improve the frequency response. An improved frequency response was observed under air flow (compressed air) in relation to still air. The measured heat transfer coefficient under air flow reaches 137 W/m2 /K enabling 5% strain at 0.8 Hz. The fastest frequency responses were observed in water and within hydrogel, where the nylon actuators demonstrated ~10% strain at 1 Hz (add water and hydrogel heat transfer coefficient). The application of a hydrogel coated actuator is demonstrated through an actuated 3D printed finger, which makes use of antagonistic coiled nylon actuators.
KEYWORDS: Actuators, Silver, Transistors, Biomimetics, Modulation, Electroactive polymers, Signal processing, Digital signal processing, Computer engineering, Process engineering
In muscle variable impedance and ability to recruit fibers as needed helps enable actions such as walking and catching. A new biomimetic structure of nylon actuator is presented that imitates the human pennate muscle in structure, ability to vary stiffness and the ability to increase force by recruiting additional fibers. The actuator consists of 16 silver coated nylon coiled fibers attached to a central tendon at an angle of 20°. Each nylon coil produces 20 MPa of stress at constant length and nearly 20% strain at fixed load. Fibers are individually switched ON and OFF using transistors so that each element can be recruited, and the stiffness varied. The amount of input power is controlled with pulse width modulation (PWM) techniques. It is observed that the spring constant of the pennate structure varies from that of its passive state, 503 N/m and a resonance frequency of 1.4 Hz, to 1480 N/m with resonance frequency of 3.1 Hz in the active state where all the fibers are switched on under a 25 N load. Stiffness can be varied by a factor of 9.
Components in automotive and aerospace applications require a wide temperature range of operation. Newly discovered thermally active Baughman muscle potentially provides affordable and viable solutions for driving mechanical devices by heating them from room temperature, but little is known about their operation below room temperature. We study the mechanical behavior of nylon coil actuators by testing elastic modulus and by investigating tensile stroke as a function of temperature. Loads that range from 35 MPa to 155 MPa were applied. For the nylon used and the coiling conditions, active thermal contraction totals 19.5 % when the temperature is raised from -40 °C to 160 °C. The thermal contraction observed from -40 °C to 20°C is only ~2 %, whereas between 100 and 160 °C the contraction is 10 %. A marked increase in thermal contraction is occurs in the vicinity of the glass transition temperature (~ 45°C). The elastic modulus drops as temperature increases, from ~155 MPa at – 40 °C to 35 MPa at 200 °C. Interestingly the drop in active contraction with increasing load is small and much less than might be expected given the temperature dependence of modulus.
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