Wearable strain sensor based on highly conductive carbon nanotube/polyurethane composite fibers

Highly conductive and stretchable fibers have recently attracted increasing attention owing to their potential for application in flexible wearable electronics. Carboxylated carbon nanotubes (c-CNTs) are coated onto flexible fibers as a convenient way of fabricating wearable strain sensors. However, the conductivity of a c-CNT is reduced due to the destruction of the graphitized structure of the CNT during carboxylation. It still remains a significant challenge to endow c-CNT composite fibers with high conductivity. In this study, highly conductive fibers were prepared by coating metal ion-linked c-CNTs onto polyurethane (PU) fibers in order to improve the electron transport rate between the c-CNTs. 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^-1). The high conductivity is the result of coordination junctions with strong electronic state coupling facilitating electron transport, which was proved by density functional theory calculations. The resulting coordination effect enhanced the interaction between the c-CNTs, 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. The fibrous strain sensor was successfully used to monitor joint movement and facial expression.