Wearable strain sensor based on highly conductive carbon nanotubes polyurethane composite fibers

Highly conductive and stretchable fibers have been increasingly investigated owing to their promising applications in flexible wearable electronics recently. Carboxylated carbon nanotubes (c-CNT) are coated on the flexible fibers, which is a convenient way to fabricate the wearable strain sensors. However, the conductivity of c-CNT is reduced due to the destruction of the graphitized structure of CNT during carboxylation. It still remains a significant challenge to endow the c-CNT composite fibers with high conductivity. In this study, highly conductive fibers based on polyurethane (PU) fibers coated with metal ions-linked c-CNT were prepared through improving the electron transport rate of c-CNT. The metal-coordination junctions formed by Fe2+ ions and carboxyl significantly enhanced the conductivity of the PU/CNT@Fe2+ fibers (up to 72 S/m). The high conductivity is the result of the coordination junctions with strong electronic state coupling to facilitate electron transport, which was proved by density functional theory (DFT) calculations. The resulting coordination effect enhanced the interaction between the c-CNT, which made the conductive network more flexible. The strain sensor based on PU/CNT@Fe2+ fibers exhibited high sensitivity (gauge factor =36 at 50% strain), a large strain range, inconspicuous drift and durability. Fibrous strain sensor was successfully used to monitor joint movement and facial expression.