Abstract:

Objective Wearable flexible strain sensors are widely used in fields of healthcare, public health, and human-computer interaction, among which textile-based flexible strain sensors have attracted great attention due to the advantages of high flexibility, comfort, and easy integration with clothing. However, the preparation of strain sensors with both high sensitivity and wide workable strain range remains a challenge. In this work, a flexible strain sensor with high stretchability and sensitivity is prepared by choosing polyurethane filament (PUF) to form a net-like three-dimensional braided fabric, and constructing a synergistic conductive network with one-dimensional tubular carbon nanotubes (CNT) and two-dimensional graphene sheets (GNP).

Method The three-dimensional (3-D) braided fabrics were prepared by four-step braiding process using PUF as raw material. The conductive fillers, CNT and GNP, were loaded onto the fabric surface in different ratios with the aid of ultrasound-assisted impregnation-drying. To enhance the fastness of the conductive coatings, the fabric was then immersed in dopamine solution for 6 h to obtain the 3-D braided flexible strain sensors. The sensors were characterized in terms of morphology, mechanical-electrical property and sensing performance. The effects of braided structures and CNT/GNP mass ratio on the performance of the prepared strain sensors were investigated.

Results Flexible strain sensor was prepared by loading CNT/GNP on the surface and forming a synergistic conductive network. Results showed that the sensing range of 3-D braided strain sensor was much higher than that of the PUF sensor. Under the same treatment conditions, the workable strain range of 3-D braided strain sensor was up to 280%, while the PUF was up to 126% in comparison. Thus the introduction of braided structure increased the workable strain sensing range significantly. The decrease in mass ratio of CNT in the impregnating solution led to the increase in the mass ratio of GNP, decrease in the workable strain range and significant increase in sensitivity. Consequently, in the conductive network of the strain sensor, CNT mainly played the role of circuit bridging to provide a large strain monitoring range for the sensor, while GNP sheets provided higher sensing sensitivity for the sensor through the significant change of the contact surface under strain. In addition, the surface polymerization of polydopamine(PDA) made the conductive coating firmer, enhancing stability and repeatability of the prepared strain sensor. Considering both of the sensitivity and strain range, the 3-D braided flexible sensor with CNT/GNP mass ratio of 1∶1 demonstrated better sensing performance, with a sensitivity coefficient as high as 249.8 and a strain sensing range of not less than 154%. Meanwhile, the electrical signal output was stable after more than 6 000 cycles of 50% stretching, and the water washing resistance was also proved well. It exhibited stable transmission signal in the motion monitoring of human face, neck, elbow, wrist, knuckle, knee and other parts of the body. The consistency and reliability of the signal were maintained for repeated movements with different speeds and amplitudes, proving the great potential of the prepared braided strain sensor for application in the fields of health assessment and rehabilitation training.

Conclusion CNTs/GNP/PDA-PUF flexible strain sensors were prepared with a flexible 3-D braided fabric as the substrate, and the introduction of braided structure increased the workable strain sensing range significantly. As the mass ratio of CNTs decreases and the mass ratio of GNP increases, the sensitivity of the braided strain sensor increases and the workable strain range decreases. Surface PDA-PUF improves the coating fastness, resulting in a significant increase in sensor stability. The prepared CNTs/GNP/PDA-PUF flexible braided strain sensor has both high sensitivity and large workable strain range, proving the potential in fields of human motion and health monitoring applications.

Key words: wearable flexible strain sensor, 3-D braided structure, carbon nanotube, graphene, sensitivity, polyurethane filament, health monitoring, polydopamine