Performance evaluation of CNT-COOH coated dual-intertwined yarn for wearable resistive strain sensing

• This study presents a high-performance strain sensor developed using a dual-intertwined yarn (DIY) structure composed of nylon monofilaments wrapped around a spandex core, offering enhanced flexibility, strength, and mechanical responsiveness. • Carboxyl-functionalized multi-walled carbon nanotubes (MWCNT-COOH) were uniformly coated onto the DIY yarn using a layer-by-layer (LBL) assembly without surfactants, ensuring strong interfacial bonding and consistent conductivity. • Ecoflex encapsulation was applied to protect the conductive layer, significantly improving mechanical durability and environmental resistance, with minimal signal degradation after 2000 stretching cycles at 30% strain. • The sensor exhibited a high gauge factor of 11.40 (10%–40% strain), a rapid response time of 263 ms, and a conductivity of 6.03 × 10−3 S/m, making it suitable for real-time human motion monitoring. • The sensor demonstrated exceptional conformability, sensitivity, and repeatability when evaluated on the finger, wrist, elbow, and knee, validating its practical applicability for wearable electronics and smart textile applications. Stretchable resistive strain sensors are generating significant interest in wearable electronics due to their exceptional stretchability, rapid response, and ability to detect complex human movements. While yarn-based strain sensors show enormous potential for textile integration, they typically suffer from low sensitivity, poor interfacial adhesion between conductive materials and yarn surfaces, and limited mechanical durability under repeated strain, undermining their long-term applicability and fruitful use. This study presents a novel high-performance yarn strain sensor created by depositing multi-walled carboxyl-functionalized carbon nanotubes (MWCNT-COOH) onto a dual-intertwined yarn (DIY) substrate, consisting of a spandex core wrapped with nylon monofilaments using a scalable, cost-effective layer-by-layer (LBL) assembly without surfactants, followed by encapsulation with Ecoflex to enhance mechanical resilience and environmental endurance. The CNT-COOH dispersion exhibited excellent coverage and adsorption on the yarn surface, resulting in a reliable and conductive sensing layer. The CNT-COOH/DIY sensor demonstrated exceptional strain responsiveness, achieving a gauge factor (GF) of 11.40 for 10%–40% strain and 8.75 for 40%–60% strain. It also exhibited a response time of 263 ms and an electrical conductivity of 6.03 × 10 −3 S/m. The Ecoflex/CNT-COH/DIY sensor retained stable sensing performance across 2000 stretching cycles at 30% strain. Real-time assessments on the finger, wrist, elbow, and knee confirmed its practical viability for wearable motion sensors and smart textiles.