Three-directional knitted fabric sensor made of elastic carbon-based nanofiber yarn with excellent tensile and pressure sensing performance

Flexible strain sensors are in high demand for the detection of polymorphic changes in smart wearable devices, particularly in response to the tensile and pressure stimuli of the human body. Herein, conductive and elastic nanofiber yarns were combined by electrospun carbon nanohybrids/thermoplastic polyurethane composite nanofiber bundles using a lab-made self-bonding device and surface micro-dissolution of nanofibers. Subsequently, machine-knitted pure nanofiber yarn fabrics were directedly utilized as three-directional strain sensors for transverse tensile, longitudinal tensile, and vertical pressure. The sensing performance of these sensors was regulated by designing three different textures with varying loop structures. The resulting nanofiber fabric sensors exhibited significant tensile sensing anisotropy. The relative resistance change of a double-sided fabric with fully overlapped loops under 50% longitudinal strain was found to be 88 times higher than that under transverse strain (only -1%). Additionally, the sensor demonstrated outstanding pressure sensing capabilities, with a wide range of 0.05–100 kPa, a high sensitivity of 7.49 kPa-1 (compared to 0.028 kPa-1 for single-sided fabric), good linearity up to 0.99, and reliable durability exceeding 2000 cycles. A finite element model of stress distribution revealed the sensing mechanism, showing that fabrics with greater intersection points and overlap areas of yarn experienced lower tension levels under transverse stretching, increased transverse contraction under longitudinal stretching, and a higher degree of stress concentration under pressure. To complement the fabric sensors, stretchable wires with a resistance change of less than 0.5 Ω/cm under 50% elongation and tiny electrodes unaffected by external forces were designed. Finally, the fabric sensors were perfectly integrated into smart sports bandages and football socks to monitor vital signals. This facile and scalable approach offers a promising strategy for the development of tensile/pressure sensing fabrics applied in washable smart textiles.