Ply-hierarchical coiled yarns for two extreme applications: Strain sensors and elastic supercapacitor electrodes

Abstract

Stretchable electronics with sensory and conductive features are highly desirable for their diverse wearable applications. Herein, we report ply structure-based hierarchical-coiled yarn with tunable piezoresistive effect by pre-plying and subsequent over-coiling of a bunch of precursor fibers. The surface-buckled precursor fiber is prepared by introducing the strain mismatch between elastic spandex-core and conductive carbon nanotube sheath. The resulting ply-hierarchical coiled yarns can be stretched by 1000% without significant electrical fracture due to the increased complexity and linear packing density during successive yarn structure trans-formations. Notably, the two extreme applications of the ply-hierarchical coiled yarn as strain sensors and supercapacitor electrodes are demonstrated by inducing the difference of the coil index (the ratio of outer coil diameter to the fiber diameter)-dependent piezoresistivity based on the number of plies. The wearable single-ply sensor (with a coil index of 1.92) exhibits a sensitive resistance response against different deformations, such as a finger bending, load pressure, and tensile elongation. The stable 9-ply electrode (with a coil index of 1.28) can be used in super-elastic supercapacitors with excellent stretch tolerance (800%) and capacitance retention (95%).