Continuous Ag Nanolayer/Interior Network Formation on PU Fibers via an Alkali-Enhanced Process for Conductive Stretchable Strain Sensors

Flexible wearable electronics made with conductive fibers (CFs) show significant potential for applications in health monitoring, entertainment, and sports. This study proposes a fabrication process for conductive silver nanoparticle fibers that forms a continuous silver nanolayer on highly elastic polyurethane (PU) fibers while also creating an internal conductive network, resulting in a significant CF strain sensor. By integration of an alkaline pretreatment (AP) process, the surface structure of PU fibers is modified to achieve high polarity, accelerating the reduction of silver nanoparticles and increasing the deposition rate by 40%. The AP process enables the conductive PU fibers to achieve ultralow resistance (5.46 Ω) with a single cycle of in situ silver nanoparticle reduction. A three-factor response surface methodology experimental design was utilized to identify the optimal conditions for AP and in situ silver nanoparticle reduction while determining the key influencing factors. Surface and structural analyses using XPS, XRD, TGA, and SEM effectively confirm the formation of the surface silver nanolayer and the internal silver network. The developed fiber strain sensor demonstrates a 72.8% reduction in initial resistance, a 25% improvement in response time, and stable strain signal performance over 4000 stretch–release cycles. Moreover, incorporating a hydrophobic protective layer enhances the waterproof properties of AP-CFs, maintaining long-term electrical stability, even when immersed in water and acid–base solutions. Finally, the AP-CFs were integrated into a respiratory monitoring waistband equipped with Internet of Things cloud-based monitoring technology, enabling the real-time detection of vital signs in bedridden individuals and providing effective medical protection.