Electromechanical Performance of Strain Sensors Based on Viscoelastic Conductive Composite Polymer Fibers

Abstract

Flexible conductive polymer composite (CPC) fibers that show large changes in resistance with deformation have recently gained much attention as strain-sensing components for future wearable electronics. However, the electrical resistance of these materials decays with time during dynamic cyclic loading, a deformation performed to simulate their real application as strain sensors. Despite the extensive research on CPC fibers, the mechanism leading to this decay in the electromechanical response under repetitive cycles remains unreported. Herein, this behavior is investigated using fiber-based strain sensors wet spun from thermoplastic polyurethane (TPU) consisting of a carbonaceous hybrid conductive filler system of carbon black (CB) and carbon nanotubes (CNTs). We found electrical viscosity to predict the observed electromechanical resistance decay. This implies that cycling these materials enables the relaxation of both the polymer chains and the conductive network. In addition, the resulting piezoresistive fibers are sensitive to deformation in the region of low strain (gauge factor of 6.0 within 3.0% strain), remain conductive under 280.5% deformation, and are stable for more than 2000 cycles. Finally, we demonstrate the potential of TPU/CB-CNT fibers as strain sensors for monitoring human motion.

Piezoresistive Fibers with Large Working Factors for Strain Sensing Applications

“…For instance, some conductive nanocomposite systems consisting of various conductive fillers have been reported to show values of t ranging between 1 and 6. ,,,, In other cases, t can either be very large ∼12 or very small ∼0.1 . Values of t < 1 like the case observed in this study are very synonymous with one-dimensional fiber-based conductive nanocomposites ,, and have been linked to explosive percolation where such materials show large values of ϕ c . After gaining insight into their electrical properties, we also examined the electromechanical properties of SEPP fibers.…”

“…The resultant neat fibers as depicted in Figure a-i show a porous and grooved morphology (Figure b-ii). This fiber structure is consistent with our previous report and linked to diffusional rate differences between NMP and water. After surface modification with PDA, the grooved and porous structure remained visible.…”

“…This cyclic performance is comparable to soaked bodily sensors and better than most piezoresistive nanocomposite systems whose signal stabilizes after more than 1000 cycles. In terms of signal quality, most bulk piezoresistive nanocomposites show nonmonotonic responses, that is, primary and secondary peaks in a single cycle. ,,, These peaks compromise the accuracy of applying the fatigue theory as only the primary peak is considered. However, the resistance signal for SEPP fibers remains monotonically in phase with strain after saturation as shown in Figure a,b and Figure S4.…”

“…DG decays (Figure c) because of the relaxation in the conductive network as softening occurs in the polymer chains of SEBS. This allows a fit to the relaxation function , for describing the observed phenomena to be applied in this regime (Figure c). In the conditioned regimes, DG becomes constant.…”

Preparation of Thermoplastic Polyurethane/Multi-Walled Carbon Nanotubes Composite Foam with High Resilience Performance via Fused Filament Fabrication and CO2 Foaming Technique

“…Typical stress-strain curves have three regions when subjected to strain, namely the initial elastic region, platform region and densification region. Viscoelastic materials exhibit a behavior called damping, which is obtained by the difference between the loading and unloading curves of the sample [ 32 ].…”