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 sheathdriven 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.

Enhanced energy harvester performance by a tension annealed carbon nanotube yarn at extreme temperatures

“…For comparison, both neat and ITAP CNT yarns were studied. As described in our previous paper, 11,13 a coiled neat CNT yarn was made by inserting an extreme twist into forest-drawn MWCNT sheets until it forms a fully-coiled structure. The ITAP CNT yarns were prepared by applying tension annealing on coiled or twisted CNT yarns, which were named ITAP 1 and ITAP 2 , respectively.…”

“…Recently, various types of mechanical energy harvesters have been developed, such as triboelectric nanogenerators (TENGs) [3][4][5] and piezoelectrics, [6][7][8][9] they are efficient at high frequencies (∼20-100 Hz) but suffer from their low output electrical power. Twisted and coiled carbon nanotube (CNT) yarns have been demonstrated for diverse applications such as artificial muscles, [10][11][12][13] supercapacitors, 14,15 batteries, 16,17 and intelligent textiles 18 due to their excellent mechanical and electrical properties. Recently described coiled CNT yarn harvesters (called twistron) generate electrical energy by a stretchinginduced electrochemical capacitance change.…”

Pretension-Free and Self-Recoverable Coiled Artificial Muscle Fibers with Powerful Cyclic Work Capability

“…As observed, when the fiber was driven by applying a square-wave voltage (0–11V) with a 5% duty cycle, the contractile stroke decreased from 51.3% at 0.17 Hz to 6.2% at 5 Hz. The contraction decreased at high frequency, which has been widely observed in previous thermal artificial muscles, ,, can be attributed to the limited heat dissipation. Figure S35 demonstrates that the contraction rates of the coiled fiber tested at varying frequencies remained almost the same.…”

Modeling dynamic swelling of polymer-based artificial muscles

“…This nearly order of magnitude reduction of coil strain emphasizes the importance of using an inner core that maintains approximately constant modulus and an outer shell that swells to increase radius, as has been done with sheath-run artificial muscles. 33,66,67 Composites might be an alternative artificial muscle design approach to maintain nearly constant moduli and anisotropic swelling.…”