Abstract:

Objective Wearable technology is currently one of the most promising fields and as a key component for wearable devices, flexible sensors are crucial to the development of intelligent wearable products. Sensing performance is the only criterion for the quality of a sensor. It is of great significance to propose a new one-dimensional flexible strain-capacitive sensor with good sensing performance.

Method Adopting the method of water bath electrospinning, experimental parameters were set with spinning solution concentration of 12%, electrostatic voltage of 20 kV, spinneret rate of 0.2 mL/h, core yarn winding speed of 0.16 m/min, and receiving distance of 5 cm. Silver coated polyamide 6(SCN) was chosen as the core yarn (capacitor electrode plate), and polyamide 6(PA6) nanofiber was used as the coating layer (dielectric layer) to prepare the silver coated polyamide 6/polyamide 6 (SCN/PA6) nanofiber core-spun yarn, which was wound on the rubber band to prepare the strain-capacitive flexible sensor. The human knee was selected as the experimental test site, and the test subjects performed intermittent knee bending, continuous knee bending, and walking on the treadmill at different speeds, and the capacitance changes of the sensor during the movement were recorded by the capacitance tester in real time.

Results By observing the structure morphology of SCN, analyzing the mechanical properties, and testing the sensing performance and practical application of the sensor, the following conclusions were obtained. PA6 nanofiber coating with complete structure was formed by electrospinning on the surface of SCN fiber (Fig. 4), with a thickness of 15-20 μm. The diameter distribution of nanofibers (Fig. 5) was uniform, mainly in the range of 80-100 nm, with an average diameter of 95.53 nm. Compared with the core yarn, the breaking strength and elongation at break of the nanofiber core-spun yarn were slightly increased and slightly decreased (Fig. 6), but the changes were small. The relative capacitance of the prepared sensor showed a decrease with the increase of elongation during stretching, while the recovery followed the opposite pattern, and the decreasing trend of the minimum capacitance gradually slows down (Fig. 7). When the elongation was small (6.67%), the linear fitting equation obtained with the elongation as the independent variable and C_p/C₀ as the dependent variable had a correlation coefficient as high as 0.988 6 (Tab. 1), showing good linearity. The sensitivity of the sensor gradually decreased with the increase of the elongation. When the elongation was 6.67%, the gauge factor value reached 3.93, while when the elongation was 66.67%, the gauge factor value was only 0.90 (Tab. 2). The maximum C_p/C₀ value of the sensor was about 1 for each stretch cycle of 450 s at different elongations, and the minimum C_p/C₀ value was close to the minimum C_p/C₀ value of the single stretch (Fig. 9), showing good repeatability stability. The variation of stretching speed has almost no effect on the relative capacitance of the sensor.

Conclusion By placing the sensor on the knee for intermittent, continuous bending and walking movements, regular and stable capacitance signal changes were obtained (Fig. 11), and C_p/C₀ values fluctuated stably between 0.6 and 1.0. According to the calculation and analysis of the signal changes at different speeds, the number and frequency of steps of the experimenter can be obtained (at speed of 3 km/h, the frequency is 86.4 steps/min; at speed of 4 km/h, the frequency is 107.2 steps/min; at speed of 6 km/h, the frequency is 151.8 steps/min). The strain sensor has potential applications in the field of real-time monitoring of flexible wearable human motion. It is suggested to select materials with better performance and improve the design of the structure to better play its original value.

Key words: water bath electrospinning, polyamide 6, nanofiber core-spun yarn, linear spiral structure, capacitive sensor, movement monitoring