Abstract:

Objective To investigate the influence of ply number on the strain sensing properties of conductive yarns, evaluate their effectiveness for human motion monitoring, and explore the feasibility of their integration into textile structures through weaving, this work aims to develop textile-based flexible strain sensors with a balance of high sensitivity and reliable mechanical performance for wearable applications.

Method Conductive yarns were fabricated using polyurethane (PU) film as the flexible substrate and carbon black (CB) as the conductive material. A spraying method was employed to coat the PU film with CB. The coated films were cut into strips, twisted with a Z-twist direction at a density of 30 twists per meter, and then plied to create yarns with different ply counts (designated as PU/CB-1C for single-ply, PU/CB-2C for two-ply, PU/CB-3C for three-ply, and PU/CB-4C for four-ply). The mass fraction of CB dispersion was kept constant at 50%. The surface morphology and chemical structure of the samples were characterized using scanning electron microscopy (SEM), optical microscopy, and Fourier transform infrared spectroscopy (FT-IR). The mechanical properties and electromechanical sensing performance were tested using an electronic fabric strength tester coupled with a digital source meter. Key sensing metrics, including the gauge factor (SGF, sensitivity) and resistance change rate, were measured. Furthermore, the optimal yarn (PU/CB-3C) was woven as the warp into a plain weave fabric using an elastic spandex yarn as the weft to create a textile-integrated sensor. The practical application was demonstrated by attaching the PU/CB-3C sensor to a human elbow and fingers to monitor bending motions.

Results The three-ply yarn (PU/CB-3C) exhibited the optimal overall sensing performance among the plied yarns. It achieved a maximum resistance change rate of 1 704% and a gauge factor of 8.9 within a 200% strain range, while also demonstrating good mechanical stability. In contrast, the four-ply yarn (PU/CB-4C) showed a decline in sensing performance due to reduced geometric deformation and potential interfacial slip within the thicker structure. The PU/CB-3C sensor maintained stable responsiveness across different stretching frequencies and stepwise strains, and showed only minimal signal drift over 200 stretching cycles at 15% strain, indicating good durability. When woven into a plain weave fabric as the warp yarn, the textile sensor retained a resistance change rate of 285% and a gauge factor of 6.2 within a 120% strain range, confirming successful textile integration. In practical tests, the PU/CB-3C sensor reliably detected and differentiated large-strain motions (elbow bending to 90°) and small-strain, fine motions (finger bending at 0°, 45°, 60°, and 90°) with clear and repeatable resistance change signals.

Conclusion The ply number significantly affects the performance of PU/CB conductive yarns. A three-ply structure offers an optimal balance, enhancing the stability of the conductive network and mechanical robustness compared to single-ply yarn, while avoiding the performance degradation observed in over-plied structures. The selected PU/CB-3C yarn is effective for monitoring diverse human joint movements. More importantly, it can be successfully integrated into a woven fabric structure, resulting in a functional textile strain sensor. This work demonstrates a viable pathway from material preparation to textile integration for developing wearable sensors, providing an experimental basis for applications in smart garments and health monitoring textiles.

Key words: carbon black, polyurethane, conductive plied yarn, tensile performance, strain sensing performance, flexible sensor, wearable electronic technology