Higher efficiency, lower cost refrigeration is needed for both large and small scale cooling. Refrigerators using entropy changes during cycles of stretching or hydrostatically compression of a solid are possible alternatives to the vapor-compression fridges found in homes. We show that high cooling results from twist changes for twisted, coiled, or supercoiled fibers, including those of natural rubber, NiTi, and polyethylene fishing line. By using opposite chiralities of twist and coiling, supercoiled natural rubber fibers and coiled fishing line fibers result that cool when stretched. A demonstrated twistbased device for cooling flowing water provides a high cooling energy and device efficiency. Theory describes the axial and spring index dependencies of twist-enhanced cooling and its origin in a phase transformation for polyethylene fibers.Summary: Twist-exploiting mechanocaloric cooling is demonstrated for rubber fibers, fishing line fibers, and NiTi shape-memory wires.3

The development of artificial muscles is an interdisciplinary field of science involving materials science, mechanical engineering, chemical biology, and chemistry. The artificial muscle is a type of actuator composed of a single-component device that can generate external work by producing deformations (such as reversible expansion, rotation, and tensile actuation) under various external stimuli, including heat, humidity, electric current, pressure, light, etc. [1][2][3][4][5] Similar to human muscles, artificial muscle is expected to work as the muscle of soft robotics for lifting things, for walking or locomotion, and even for sensing and medical applications. Such a singlecomponent design shows a high volumespecific or weight-specific energy density and work output, when compared to the traditional electric motor. Moreover, it provides high flexibility and simplifies the design of soft robotics. Inspired by natural smart systems, numerous responsive materials have emerged that can transfer dynamic and reversible shape changes into mechanical motions under various external stimuli. Based on the basic motions of expansion, rotation, and contraction, other actions of artificial muscles, such as bending, can be realized using an anisotropic double-layer structure where the expansion of one side is greater than the other. [6][7][8] Table 1 summarizes the comparison of different types of artificial muscles. Among them, fiber-based artificial muscle can transform volume expansion into radial rotation and axial contraction of the fiber through its spiral structure and more complex movements can be achieved through weaving. In addition, the energy conversion efficiency, power density, and work of artificial muscle fiber are much higher than those of existing membrane actuators. It also has excellent mechanical properties, good flexibility, and is closer to natural biological muscle in form. Therefore, we focused on twisted-fiber artificial muscle, which is designed to show torsional rotation, tensile contraction, or extension.Twisted-fiber artificial muscles have been developed and have received widespread attention from scientists since the 1990s. A variety of materials have been used for twisted-fiber artificial muscles, including carbon nanotube (CNT) yarns, [6,9] graphene fiber, [10] fishing line and sewing thread, [11] shape memory polymer, [12] metal alloys, [13,14] elastomers, [15] and their composites. [16][17][18] These fibers can be fabricated to be artificial muscles simply by twist insertion and coil formation, which is an ancient technique used to make yarns or ropes. [19] During the twisting process, torque is generated in fibers or yarns, the morphology of the polymer chains becomes twisted and forms a spiral configuration, and the fibers in the yarn become more compact. Therefore, twisting of the fibers produce novel mechanical,