Soft actuators have received intensive attention in the fields of soft robots, sensors, intelligent control, artificial intelligence, and visual intelligence. By combination of tensile and torsional deformations, different types of motions can be realized, such as bending, rolling, and jumping. Soft robotics need soft actuators, such as artificial muscles to lift or move objects to perform some work. Additionally, actuation integrated with functions of sensing, signal transmission, and control is also needed in the development of advanced intelligent systems, which further stimulates the requirement of multifunctional actuators. Here different types of soft actuators that can perform tensile and torsional actuations are summarized, including twisted fiber artificial muscles, shape memory polymers, hydrogels, liquid crystal polymers, electrochemical actuators with conducting polymers, and some natural materials. Examples are also included regarding the bending or rolling deformations of the actuators for lifting objects. Then, recent interesting reports about multifunctional soft actuators combined with sensing and signal transmission performances, are summarized. Last, a summary of different ways to realize tensile and torsional actuations, different materials, and designs for lifting or moving objects, as well as construction of multifunctional actuators with actuation and sensing functions is provided.

Tensile and torsional artificial muscles from biocompatible and biodegradable materials are highly desired for soft robotics, sensors, and controllers in bio-related applications. Twisted fibers can be used to prepare tensile...

Developing moisture-sensitive artificial muscles from industrialized natural fibers with large abundance is highly desired for smart textiles that can respond to humidity or temperature change. However, currently most of fiber artificial muscles are based on non-common industrial textile materials or of a small portion of global textile fiber market. In this paper, we developed moisture-sensitive torsional artificial muscles and textiles based on cotton yarns. It was prepared by twisting the cotton yarn followed by folding in the middle point to form a self-balanced structure. The cotton yarn muscle showed a torsional stroke of 42.55 °/mm and a rotational speed of 720 rpm upon exposure to water moisture. Good reversibility and retention of stroke during cyclic exposure and removal of water moisture were obtained. A moisture-sensitive smart window that can close when it rains was demonstrated based on the torsional cotton yarn muscles. This twist-based technique combining natural textile fibers provides a new insight for construction of smart textile materials.

Conspectus Nature’s evolution over billions of years has led to the development of different kinds of twisted structures in a variety of biological species. Twisted fibers from nanoscale- to micrometer-scale diameter have been prepared by mimicking natural twisted structures. Mechanically inserting twist in a yarn is an efficient and important method, which generates internal stress, changes the macromolecular orientation, and increases compactness. Recently, twist insertion has been found to produce interesting fiber properties, including chemical, mechanical, electrical, and thermal properties. This Account summarizes recent progress in how twist insertion affects the chemical and physical properties of fibers and describes their applications in artificial spider silk, artificial muscles, refrigeration, and electricity generation. Twist and associated chirality widely arise in nature from molecules to nano- and microscale materials to macroscopic objects such as DNA, RNA, peptides, and chromosomes. Such twisted architectures play an important role in improving the mechanical properties and enabling biological functions. Inspired by the beauty and interesting properties of twisted structures, a wide range of artificial chiral materials with twisted or coiled structures have been prepared, from organic and inorganic nanorods, nanotubes, and nanobelts to macroscopic architectures and buildings. An efficient way to prepare twisted materials is by inserting twist in fibers or yarns, which is an ancient technique used to make yarns or ropes (Wang, R., et al. Science 2019, 366, 216–221. Mu, J., et al. Science 2019, 365, 150–155). During the twisting process, torque is generated in fibers or yarns, the structure of the polymer chains becomes helically oriented, and the fibers in a yarn become more compact. Therefore, the twisting of fibers and yarns can produce novel chemical, mechanical, electrical, and thermal properties (Dou, Y., et al. Nat. Commun. 2019, 10, 1–10. Kim, S. H., et al. Science 2017, 357, 773–778). This Account focuses on the novel properties generated by twist insertion. The mechanical stress and strain can be optimized in a yarn by twist insertion, and different types of fibers exhibit rather different mechanisms. In the first section, we will focus on recent progress in improving the mechanical properties of twisted fibers, including carbon nanotube yarns, single-filament fibers, and hydrogel fibers. Torque was generated by twist insertion in a fiber or a yarn, and the balance of internal torsional stress can be changed by causing a change in yarn volume. This will result in twist release and torsional and tensile actuations of the yarn, which will be described in the second section. Twisting a yarn generally makes it more compact, which will result in a mechanically induced change in capacitance, supercapacitance, and other useful electrochemical properties when a conducting yarn is in an electrolyte. Such processes were used to develop novel devices for twist-based electricity generation, called t...

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,

Smart textiles responding to the ambient environment like temperature, humidity, and light are highly desirable to improve the comfortability and realize multifunctions. The bamboo yarn has merits like air permeability, biodegradability, and excellent heat dissipation performance, but it has not been prepared for responsive materials and smart textiles. In this paper, the moisture-responsive twisted bamboo yarns were plied to form a self-balanced torsional actuator and wrapped around a mandrel to form a coil, followed by water immersion and evaporation to fix the shape and serve as a tensile actuator. A torsional actuation of 64.4°⋅ mm−1 was realized for the twisted actuator in 4.2 s; a maximum elongation of 133% or contraction of 50% was achieved for a coiled tensile actuator with good cyclability. The porous structure of bamboo yarns helped improve the water absorbance speed and decrease the response time of moisture. The self-balanced two-ply physical structure and reversible generation of chemical phase after soaking in aqueous solution fixed internal stress and provided good cyclability. With the unique properties including aqueous water-induced shape fixation and moisture-induced actuation, the application of tensile actuation of bamboo yarns was demonstrated, showing promising prospects on smart textiles.

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