New twist on artificial muscles

Abstract

Lightweight artificial muscle fibers that can match the large tensile stroke of natural muscles have been elusive. In particular, low stroke, limited cycle life, and inefficient energy conversion have combined with high cost and hysteretic performance to restrict practical use. In recent years, a new class of artificial muscles, based on highly twisted fibers, has emerged that can deliver more than 2,000 J/kg of specific work during muscle contraction, compared with just 40 J/kg for natural muscle. Thermally actuated muscles made from ordinary polymer fibers can deliver long-life, hysteresis-free tensile strokes of more than 30% and torsional actuation capable of spinning a paddle at speeds of more than 100,000 rpm. In this perspective, we explore the mechanisms and potential applications of present twisted fiber muscles and the future opportunities and challenges for developing twisted muscles having improved cycle rates, efficiencies, and functionality. We also demonstrate artificial muscle sewing threads and textiles and coiled structures that exhibit nearly unlimited actuation strokes. In addition to robotics and prosthetics, future applications include smart textiles that change breathability in response to temperature and moisture and window shutters that automatically open and close to conserve energy.

Keywords: actuators; artificial muscles; carbon nanotubes; textiles; yarns.

Fig. 1.

Twisted and coiled artificial muscles. (A) Optical image of a nylon monofilament before (Left) and after (Right) twist insertion. The ink line represents the alignment direction of the fiber’s constituent molecular chains before and after twisting, which reorient to a bias angle α_f (∼45° in this example). Note that the bias angle is constant along the surface of fiber, but nearly horizontal lines appear as artifacts from the back side due to cylindrical lensing through the transparent fiber. (B) Schematic of a homochiral coil where the fiber twist and coil share the same Z chirality. When heated, the fiber untwists, drawing coils closer together to cause tensile contraction, thereby decreasing the coil bias angle (α_c).

Fig. 2.

Tensile stroke of a coiled nylon muscle. (A) Optical picture of a homochiral coil prepared by twisting a 300-µm-diameter nylon 6/6 monofilament sewing thread to just before the point of coiling and then annealing the fiber around a mandrel. The coil spring index, C, is depicted as the ratio of D, the average coil diameter (typically the average of the outer and inner coil diameter) and d, the filament diameter. (B) Plot of the tensile actuation measured for the coil in A, vs. temperature (red line), and the theoretical actuation predicted for this coil (black circles). The predicted actuation was calculated from Eq. 2 by considering the coil’s geometry, and data measured for the torsional stroke of the precursor, highly twisted fiber shown in the Inset. The theoretical and experimental results agree at low temperature, but diverge when adjacent coils come into contact above 140 °C.

Fig. 3.

Giant-stroke actuation of a spiral coil muscle in which coil-coil interference is eliminated. (A) The length of the coil vs. temperature and corresponding strain when normalized to the initial muscle length. The picture inset on the Bottom Left shows the spiral mold used to anneal the coil, and the picture inset on the Top Right shows the coil after fabrication. (B) Optical pictures of the spiral coil when heated electrically, showing the progression of actuation during heating. A brass rod is inserted through the muscle to guide actuation horizontally.

Fig. 4.

Coiled polymer muscle threads and textiles. (A) A spool of 320 denier, coiled muscle (235-μm outer coil diameter) made from 125-µm-diameter nylon 6/6 monofilament sewing thread by a continuous process, which has been prestretched to provide space between coils for actuation, and the same fiber with an insulated copper wire wrap for electrothermal actuation, without prestretch, pictured on the Left and Right, respectively. (B and C) Closeup photographs of the fibers in A. (D) Woven fabric made from coiled, 225-µm-diameter nylon sewing thread muscle. (E) Stitches made by sewing the coiled fiber in A into a polymer sheet using a conventional sewing machine. (F) Machine-knitted textile made from a coiled 225-µm-diameter nylon sewing thread muscle.