Breathable and Large Curved Area Perceptible Flexible Piezoresistive Sensors Fabricated with Conductive Nanofiber Assemblies

The rapid development of wearable electronics, humanoid robots, and artificial intelligence requires sensors to sensitively and stably detect external stress variations in large areas or on three-dimensional (3D) irregularly shaped surfaces while possessing the comfort. Most importantly, the flexibility and 3D compliance of sensors, and the fitting state of the interface between the sensor and the object are of great significance to the sensing accuracy and reliability. The ordered or random stacking and entangling of flexible and electrically conductive fiber materials can form a highly porous and mechanically stable fiber assembly. The changes in external stress can lead to the air trapped in the fiber assembly to flow in and out rapidly and repeatedly, as well as the reversible mechanical deformation of fiber materials. Correspondingly, the contact areas between electrically conductive fibers in the fiber assembly are reversibly changed, which makes the conductive and flexible fiber assembly be an ideal candidate for piezoresistive sensing material. It can be further expected that the statistical stability of contact points between conductive fibers under the stress may significantly increase with the decrease in fiber diameters. Herein, a new method to make a flexible piezoresistive sensor with conductive and porous fiber assembly was proposed. An ultrasensitive piezoresistive material was facilely prepared by fabricating conductive poly(vinyl alcohol-co-ethylene) (EVOH) nanofiber assemblies. The sensing performance of the piezoresistive sensor was optimized by regulating the nanofiber morphology, electrical conductivity, and mechanical properties. The flexible piezoresistive sensor exhibited a sensitivity of 2.79 kPa-1, a response time of 3 ms, and a recovery time of 10 ms. The sensing performance at different working frequencies was stable and durable within 4500 cycling tests. The flexible sensor showed good pressure-sensing accuracy and reliability when used on irregular surfaces and therefore was further applied in the static monitoring of large-area spatial pressure distribution and the wearable intelligent interactive device, demonstrating great application potential.