The Power of Fiber Twist

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 twistrons, which will be discussed in the third section. Twist insertion or release also changes the polymer chain orientation or crystal structure, resulting in changes in entropy. This is called the twistocaloric effect, which was used to develop a new cooling method, and will be discussed in the last section.