Abstract:

Objective In recent years, there has been a surge in the research of flexible sensor devices, and owing to their lightweight, flexibility, and biocompatibility, they became a popular topic of scholarly investigation. These devices have found widespread application in flexible sensing, actuators, and wearable devices. However, in the realm of stress and strain sensing, fiber sensors often display poor resilience and low sensing sensitivity, which significantly limit their use and growth in this field. Therefore, thermoplastic polyurethane(TPU) is used as the fiber substrate and Ti₃C₂T_x MXene is used as the conductive filler to prepare a porous stress and strain sensing fiber with good resilience and high sensitivity.

Method The honeycomb-like structure of porous stress-strain sensing fibers was prepared by wet spinning, using the different solubilities of N,N-dimethylformamide (DMF) in water and isopropanol (IPA) to delay the solvent exchange process between the spinning solution and the coagulation bath. Flexible sensing fibers with high sensitivity and good resilience were achieved by adjusting the loading amount of MXene on the fibers. The influences of MXene addition on the microstructure, thermal stability, mechanical properties and electrical properties of the fibers were studied, and this fiber was applied to the detection of human motion.

Results The porous thermoplastic polyurethane-based conductive fibers were successfully obtained through wet spinning. In the cross section, TPU fiber presents a honeycomb-like structure, and the pores of MXene/TPU fiber are expanded by virtue of the addition of MXene, showing a non-uniform pore structure. The MXene nanosheets are successfully attached to these pores and build a good conductive network. At the same time, MXene is also embedded on the fiber surface to cause the fiber surface be flat tened. The Ti elements in MXene are evenly dispersed on the fiber. In the thermal stability analysis, the addition of MXene led to the advance of the thermal cracking peak from 430 ℃ to 310 ℃ and 400 ℃, and the increase of the MXene load increased the fiber residual mass percentage from 9.16% to 17.24%, 21.09%, 23.57% and 26.21%. The breaking strength of TPU fiber is 11.16 MPa, and the elongation at break is 1 257%, with the increase of MXene load, the mechanical properties of the fiber decrease. The breaking strength of MXene/TPU fiber is 2.12 MPa, and the elongation at break is 622%. It still maintains a certain breaking strength and a long elongation at break. The conductivity of MXene/TPU fiber is 0.86 S/m, which is significantly improved compared with other fibers. When the strain of MXene/TPU fiber is 10% and 50%, the sensing sensitivity (G_F) is 20.45 and 151.12, respectively. At the same time, it still maintains a relatively stable resistance recovery after 500 s cyclic stretching at 50% strain. Compared with the stress-strain sensing fibers reported, MXene/TPU fiber has higher sensing sensitivity at the same strain. MXene/TPU fiber is applied to wrist and finger joint motion detection, the fiber was tested for 10 cycles, exhibiting a stable electrical signal transmission effect.

Conclusion The thermoplastic polyurethane-based stress-strain sensing fiber with high sensing sensitivity under low strain conditions is prepared by modulating coagulation bath to strengthen the solvent exchange process in wet spinning. It solves the problem that the stress-strain sensing fiber is low in sensitivity and hard to detect under small strain applications. It has a broad application prospect in human motion detection.

Key words: MXene, thermoplastic polyurethane, conductive fiber, wet spinning, stress-strain sensing