The mechanical and electrical properties of flexible strain sensors based on carbonized cotton knitted fabric

In recent years, developing flexible strain sensors with excellent and practical mechanical–electrical properties has remained a challenge. Weft knitted fabric made of loops with excellent extensibility is widely used in the textile and garment fields. In this work, a flexible strain sensor based on carbonized plain cotton fabric and thermoplastic polyurethane film for encapsulation was prepared by a simple, relatively environment-friendly, low-cost and scalable method. The plain cotton fabric was translated to a conductive fabric with impregnation (10% diamine hydrogen phosphate), oxidation (pre-oxidation at 240°C for 60 min in air), high-temperature carbonization treatment. The tensile and sensing properties of the encapsulated sensor based on this carbonized plain cotton fabric were tested in the course and wale extensions. The experimental results showed that the strain sensor exhibited high sensitivity, stable repeatability and low creep properties overall. The resistance change rates of the strain sensor stretched in the two directions had no apparent dependence on the tensile speed and frequency. The sensor in course stretching revealed a broad strain of more than 250% with the maximum gauge factor of 10.1, while in contrast, these two data were 140% and 9.2 in wale stretching, respectively. These excellent sensing performances were given credited to the unique modified structure of carbonized cotton knitted fabric. The carbonized cotton knitted fabric strain sensor has great potential in wearable devices for physiological signal monitoring and human motion detection due to its scalable manufacturing process, low material costs and superior mechanical–electrical properties.