Abstract:

Objective Conventional flexible wearable sensing devices had typically operated on a single sensing mode, providing only one type of signal output. Their sensitivity to changes in multiple environmental factors had been insufficient, and their application scope had remained limited, failing to meet the diverse and evolving demands of the market. To overcome these limitations, this study had aimed to develop a yarn strain sensor with a dual-mode response mechanism of optics and electricity that would improve its sensing performance in the field of flexible wearable monitoring.

Method The photoelectric double response yarn strain sensor (L-E yarn strain sensor) had been fabricated through wet-spinning processing, heterogeneous structure construction, twisting into yarn, and functional material loading techniques.Scanning electron microscopy (SEM) was utilized to characterize the micromorphology of the L-E yarn strain sensor. The nano measurer tool was employed to measure the particle size of ZnS:Cu. X-ray energy spectrometry (EDS) was used to analyze the positional relationships between different components and the distribution of elements. A tensile testing machine and a digital source meter were used to test and record the changes in electrical signals of the sensor under different stretching conditions, to assess the sensor's electromechanical performance. An optical fiber spectrometer was used to measure the ML behavior of the L-E yarn strain sensor under various ZnS:Cu contents, different stretching strains, and different joint movements in a darkroom.

Results SEM characterization exhibited the non-uniform phase polydimethylsiloxane (PDMS) liquid droplets loaded on the L-E yarn strain sensor, the sensor's twisted structure, the successful loading of polydopamine/polypyrrole (PDA/PPy), and the tight wrapping of the PDMS/ZnS:Cu layer. By measuring the exposed ZnS:Cu particles on the cross-section of the sensor, the average particle size of ZnS:Cu was determined to be 16.9 μm. EDS measurements of characteristic elements such as Zn and S on the sensor further confirmed the presence of PDMS/ZnS:Cu. Analysis of the sensor's electromechanical properties indicated that the PDMS/ZnS:Cu soft matrix provided excellent protection for the sensor, preventing oxidation and enhancing sensor stability. Due to the presence of the PDMS non-uniform phase structure and the twisted yarn structure, the sensitivity value of the L-E yarn strain sensor gradually increased to 5.45, 19.18 and 48.93 when the tensile strain was in the ranges of 0%-62%, 62%-146%, and 146%-160% respectively. The response time was 100 ms, and the lowest detectable strain limit was 0.1%, exhibiting high sensitivity and a wide strain range. The L-E yarn strain sensor showed a broad applicability, maintaining stable curve changes under different strain magnitudes or different activity frequencies, meeting the needs of various application scenarios. Furthermore, in terms of durability and stability, the L-E yarn strain sensor passed a 2 000-cycle tensile test. ML testing of the sensor showed varying ML effects under different ZnS:Cu contents and different tensile strains, and considering both mechanical applicability and cost-effectiveness, a ZnS:Cu content of 30% was determined. To further demonstrate the wearable effect of the sensor, it was fixed on the test subject's fingers and wrists, and as the degree of joint bending increased, the corresponding ML optical fiber spectral curve peaks gradually increased. In terms of electrical signal sensing, the L-E yarn strain sensor was able to accurately recognize strain signals from fingers, wrists, elbows, knees, pulses, and facial expressions.

Conclusion The L-E yarn strain sensor has shown high sensitivity, broad strain range (0%-160%), and excellent cyclic stability. When the sensor was stretched, it could simultaneously generate both resistive and fluorescent signals, which intensifies as the strain increased. The sensor was applied to the field of wearable health monitoring, and through its optical-electric dual-mode response mechanism, not only was the visual sensing effect enhanced, but it could also accurately identify human physiological activities and joint movements, providing a reliable basis for doctors' health diagnoses.

Key words: yarn, strain sensor, wearable, double response, visualization, polyurethane elastomer, wet-spinning process