Fast Large‐Stroke Sheath‐Driven Electrothermal Artificial Muscles with High Power Densities

Abstract Electrothermal carbon nanotube (CNT) yarn muscles can provide large strokes during thermal cycling. However, the slow cooling rate of thermal muscles limits their applications, since large diameter prior‐art thermal muscles cannot be rapidly cycled. Herein, a fast thermally powered sheath‐driven yarn muscle that uses a hybrid CNT sheath and an inexpensive polymer core, is reported. The stroke recovery rate for the hybrid muscle is much lower at all frequencies than for about the same diameter sheath‐driven muscle, which means that the full cycle contractile mechanical power is much higher than for comparable prior‐art hybrid muscle. More specifically, the coiled sheath‐driven muscle contracts 14.3% at 1 Hz and 7.3% at 8 Hz in air when powered by a square‐wave electrical voltage input, which is 2.9‐ and 11.4‐times the stroke of the coiled hybrid muscle at these respective frequencies. An average power density of 12 kW kg −1 is obtained for a sheath‐driven muscle, which is 42‐times that for human skeletal muscle. These high‐performance results since the heating that drives fast actuation cycles are largely restricted to the muscle sheath, and this sheath is in direct contact with ambient temperature air.