Elastic carbon nanotube straight yarns embedded with helical loops

Introducing stretchability and elasticity into carbon nanotube (CNT) yarns could extend their applications to areas such as stretchable and deformable fiber-based devices and strain sensors. Here, we convert a straight and inelastic yarn into a highly elastic structure by spinning a predefined number of helical loops along the yarn, resulting in a short helical segment with smooth structural transition to the straight portions. The loop-forming process is well controlled, and the obtained straight-helical-straight hybrid yarn is freestanding, stable, and based entirely on CNTs. The elastic and conductive yarns can be stretched to moderate tensile strains (up to 25%) repeatedly for 1000 cycles without producing residual deformation, with a simultaneous and linear change of electrical resistance depending on the strain. Our results indicate that conventional straight CNT yarns could be used as fiber-shaped strain sensors by simple structural modification.