Abstract:

Objective Wearable flexible sensors have become an important part of smart textiles, and flexible capacitive wearable sensors continue to develop in the direction of flexibility, miniaturization, portability and flexibility. In order to develop new flexible capacitive sensors and promote the continuous production of wearable flexible sensors, low-cost and high-efficiency preparation processes have received attention of researchers.

Method Silver coated nylon (SCN) as core yarn and nylon DTY (PA) as outer wrapper were selected to produce SCN/PA wrapped yarn through the automated wrapping yarn production process. The SCN/PA wrapped yarns were obtained by double-layer reverse twisting method, the inner yarns were twisted with S-twist and the outer winding yarns were twisted with Z-twist, with a twist rate of 1 000 t/m under the process parameters. The SCN/PA wrapped yarn was impregnated with waterborne polyurethane (WPU) to obtain the SCN/PA/WPU composite yarn, and then the composite yarn was adopted to design and prepare a one-dimensional structured flexible capacitive sensor. The SCN core yarn, SCN/PA wrapped yarn, and SCN/PA/WPU composite yarn were subjected to morphological observation, mechanical property tests, and the strain sensing properties of the sensors were investigated and practically applied to the monitoring of human activities.

Results The mechanical properties of both SCN/PA/WPU composite yarn and SCN/PA wrapped yarn were significantly improved compared with SCN core yarn. The design of one-dimensional flexible capacitive sensors using SCN/PA/WPU composite yarns by spiral winding method showed good tensile strain-capacitance sensing performance. The sensor has an operating response range of 0-140%. As the tensile strain was increased sequentially, the capacitance value of the sensor got decreased, and when the strain was small, the sensor exhibited good linearity and sensitivity factor. When the strain continued to increase, the sensor linearity and gauge factor began to weaken. At 10% of the tensile strain, its gauge factor was 0.66. The sensor can withstand 1 200 times of tension-unloading cycle, and its capacitance value is relatively stable, demonstrating good repeatability. The change in capacitance of the sensor remained stable with the constant change in tensile velocity, both at small strains of 40% and at large strains of 100%, demonstrating that the change in tensile speed was hardly a negative influence on the sensing performance of the sensor. Applying the sensor to real-time monitoring of human movement, it can accurately monitor the continuous bending and intermittent movement states of the joint parts of the human body at the fingers, wrists, and elbows. In addition, the sensors can accurately record small amplitude differences between changes in continuous bending movements of the human body. The sensor was also attached to near the volunteer's mouth, the sensor also captured the signal of small tensile strainswhen the mouth made.

Conclusion A low-cost, high-efficiency, continuous processing automated production process of covering yarn was adopted to prepare SCN/PA wrapping yarn, then dip-coating WPU treatment was carried out to prepare SCN/PA/WPU composite yarn. The design of one-dimensional structure flexible capacitive sensors based on SCN/PA/WPU composite yarns demonstrated excellent gauge factor, stability, and repeatability. The application of the sensor to real-time monitoring of human movement can accurately monitor the continuous bending and intermittent motion state of the joint parts of the human body in the fingers, wrists and elbows, and can also monitor small changes in the activity of the human mouth. The sensor capacitance changes can also be utilized to distinguish the motion action as well as calculate the angle of human joint motion. In view of the superior sensing properties of this one-dimensional structured flexible capacitive sensor, it has potential applications in the fields of sports health monitoring, medical health promotion and smart wearable devices.

Key words: wrapped yarn, one-dimensional structure, capacitive sensor, human motion monitoring, waterborne polyurethane