Abstract:

Objective In order to solve the problems of poor flexibility and limited working range of conventional rigid sensors, smart sensoring devices are constantly shifting to miniaturization, flexibility and portability. Flexible wearable sensors can monitor human signals accurately and quickly, facilitating fitting into the human body or combine with clothing. The yarn-based flexible strain sensors have attracted much research attention to the engineering of flexible strain transducers taking advantages of their processability, high elasticity and wide adaptability.

Method With polyester-coated spandex yarn as the substrate, a layer of polydopamine (PDA) was deposited on the surface of the substrate by impregnation to improve the adhesion of polypyrrole (PPy) to the substrate. Then, PPy was synthesized by in-situ freezing interface polymerization to effectively avoid unfavorable cross-linking or branching in the polymer, improve the conductivity, and obtain a flexible yarn strain sensor aiming for excellent tensile strain sensing performance. The conductive yarn's microscopic morphology and chemical structure were characterized by scanning electron microscopy, X-ray spectroscopy and Fourier transform infrared spectroscopy.

Results The characterization results all proved the successful load of PDA and PPy. After PDA modification, a uniform and uneven PDA coating layer was formed on the surface of the yarn, and the hydrophilicity of the fiber surface was greatly improved. After in-situ polymerization of PPy, a granular PPy conductive layer was observed on the surface of the yarn, forming a conductive path. With the optimal ratio of n(pyrrole)/n(ferric chloride) of 1, the yarn resistance value was the lowest at 0.33 kΩ/cm. Weights of different mass were loaded on the same section of the yarn, and as the weight mass increased, the tensile deformation of the yarn gradually increased, resulting in a gradual increase in the yarn resistance value. Three stages of the main resistance were observed in the diagram of relative resistance and strain. In the first stage, the relative resistance change increased very rapidly at a strain of 0% to 6%, with a gage factor (G_F) value of 4.039. In the strain range of 6% to 18%, the relative resistance change increased gradually slowing down as the strain increased, and the G_F value was 1.006. At 18% to 30% strain, the relative resistance change increased slowly, and the G_F value was 0.318. The change in resistance was attributed to the deformation and movement of polyester fibers after tensile strain. The prepared strain sensor simultaneously achieved a broad working range of 60%, a fast response time of 166.67 ms, which is of almost frequency-independent reliability, and stable cycle durability over 1 000 cycles. In addition, human activity could be detected when the yarn sensor was connected directly to different body parts, such as the mouth, abdomen, fingers, and knees. In the example, the yarn sensor was fixed beside the mouth, and when the tester spoke different words such as "Jiangnan", "Zhongguo", "Shaxiang" and so on, the yarn sensor recorded a specific waveform signal for each vocalization because of the different amplitudes and patterns of the mouth opening and closing. The sensor recorded almost the same waveform when the same words were repeated. Connecting the yarn to the human abdomen the sensor detected the slight deformation caused by different shades of breathing state. All these verified that the PDA/PPy/polyester-coated spandex conductive yarn had good sensitivity.

Conclusion PDA/PPy/polyester-coated spandex conductive yarns have excellent stability, sensitivity, durability and repeatability to meet the requirements of wearable strain transducers. In addition, conductive yarns can be combined into woven, knitted and embroidery fabrics to monitor human joint activities in real time and have great potential in speech recognition, rehabilitation training, monitoring of respiratory. The outcome of the research demonstrate potentials in helping patients with joint injury and monitoring vital vegetative signs.

Key words: polydopamine, polypyrrole, polyester-coated spandex elastic yarn, conductive property, in-situ freezing interfacial polymerization, yarn strain sensor