Extending the Linear Range of Piezoresistive Yarn Sensors via Sequential Activation of Gradient Nanofiber Sheaths

The limited linear sensing range of piezoresistive yarn sensors, caused by premature saturation of conductive networks in homogeneous architectures, remains a critical barrier to their use in medium- and high-pressure applications. Here, we develop a gradient nanofiber sheath that enables sequential activation of conductive pathways to address this challenge. The yarn is constructed by assembling multilayer TPU/PAN nanofiber sheaths with progressively increased carbon nanotube loading through capillary-assisted confinement and ultrasonic adsorption. This design establishes a gradient in conductivity and stiffness, allowing deformation to initiate from the compliant inner layer and propagate outward under compression. As a result, conductive networks are activated in a stepwise manner across the sheath, effectively delaying resistance saturation and maintaining continuous signal evolution. The optimized yarn sensor exhibits an ultrawide multistage linear response over 0-300 N, exceeding conventional homogeneous designs by more than an order of magnitude, while delivering fast response, low hysteresis, and excellent cycling stability. Integration into woven textile arrays further enables reliable spatiotemporal pressure mapping in applications such as driver fatigue monitoring and plantar pressure analysis.