3D Porous Thermoplastic Polyurethane/Carbon Nanotube@Silver Nanoparticle Foam with Multidimensional Conductive Networks for Flexible Electronic Sensing

Abstract

Porous structures are a common design in the preparation of compressive, flexible strain sensors. It can endow the flexibility and permeability of flexible sensors while effectively increasing the specific surface area and reducing its mass. However, efficient preparation of porous strain sensors with accurate measurement results, high stability, wide operating range, and excellent durability remains challenging. Herein, the salt template method combined with vacuum casting and freeze-drying processes were used to prepare a pristine three-dimensional porous foam model, and a porous lightweight thermoplastic polyurethane (TPU)/carbon nanotube (CNT)@silver nanoparticles (AgNPs) (Vc-TPU/CNT@AgNPs) strain sensor with high compressibility was prepared by impregnating CNTs and growing AgNPs in situ. Thanks to the reduction of AgNPs inside the foam as an interlayer contact point, the resulting microstructure effectively changes the force on the sensor during compression. Meanwhile, the lap of AgNPs as a conductive filler between the layers effectively reduces the overall resistance during foam compression, resulting in a significant increase in sensor sensitivity (gauge factor = 1.40) and giving the sensor a superior linear fit (R² = 0.99875), a wide sensing range (5–70% strain, 88 pa ∼35 kPa pressure), and a rapid response and recovery time (20 ms). The in situ growth of AgNPs and π–π bonding interaction between TPU and CNT then provide excellent durability (500 cycles, 50% strain) for the Vc-TPU/CNT@AgNPs strain sensor. Furthermore, the strain sensors can be successfully used to monitor human motion, ranging from small vibrations in tendons and ears to large strain movements, such as finger flexion and foot stamping. This work provides a proven method for the preparation of porous flexible strain sensors with excellent linearity, good sensitivity, lightness and breathability, and durability, which have promising applications in the field of wearable electronics.