Abstract:

Objective The development of intelligent wearable products has put forward new requirements for conductive circuit, in which high flexibility and outstanding adaptation to the human body are essential. Flexible textile circuit made with metal wire guarantees reliable electric conduction, but faces challenges in processibility, wearing comfort, safety and conductive stability. The purpose of this paper is to improve the flexibility and stability of textile circuit made from cotton/metal wire core yarns to establish a flexible wearable sensing network.

Method This paper proposed a flexible circuit based on knitting structure that integrates sirospun technology with knitting technology. By exploring different spinning parameters including stainless steel wire diameter, twist factor and draft ratio, siro core-spun yarns were prepared from stainless steel wire and cotton. The non-conductive part of flexible knitted circuit was fabricated with polyamide/spandex yarn as plating yarn and polyester yarn as ground yarn. The conductive part was integrated by intarsia knit with polyamide/spandex yarn as plating yarn and conductive core-spun yarn as ground yarn. Then, mechanical properties, electrical stability under various mechanical conditions and conductive durability of flexible knitting circuit were characterized and analyzed.

Results The yarn used stainless steel wire as core and cotton fibers as the sheath (Fig. 3). Research on spinning parameters showed that increase in stainless steel wire diameter, reduction in draft ratio and proper twist factor have positive effects to conductive core-spun yarn appearance (Tab. 3). Considering the exposed core length and strength of the yarn, the twist factor of 400, the draft ratio of 17.28 and the stainless steel wire diameter of 0.05 mm were selected as the final spinning parameters. The break strength of the prepared conductive core-spun yarn is about 7.6 times that of bare stainless steel wire, but the elongation at break decreases about 2.8-3.6 times (Fig. 4). It was confirmed that the resistance of the yarn was affected by the strain and the structure of core-spun yarns caused reduction in the resistance change rate compared to bare stainless steel wire (Fig. 5). The prepared conductive core-spun yarn can be knitted on the knitting machine, exhibiting good processibility. In order to increase elasticity of the knitted circuit, polyamide/spandex yarn was introduced as plating yarn under conductive core-spun yarn and polyester yarn which increases circuit breaking force, breaking strain and maximum resistance change rate (Tab. 4). The trend of resistance change rate of knitted circuit in extended state is similar to that of conductive core-spun yarn, but the resistance change value is smaller than that of core-spun yarn, attributing to knitted structure (Fig. 7). The knitted structure also offered the knitted circuit good transverse mechanical property, with the transverse resistance change rate of the knitted circuit below 0.38%, which is smaller than the longitudinal resistance change rate (Fig. 8). The electrical durability test showed a resistance change rate of 0.61% after 5 000 cycles of 20% longitudinal strain, confirming circuit with knitted substrate has good durability after repeated deformation (Fig. 10). When bursting test was carried out to simulate complex three-dimensional strains on the knitted circuit, the resistance change rate was lower than 0.38% when the mean strain was lower than 150%. The load was then concentrated on the conductive core-spun yarn, the resistance change rate increases linearly with mean strain after elongation took place in the yarn (Fig. 12).

Conclusion Sirospun technology was adopted to prepare core-spun yarn with stainless steel wire as the core and cotton as the sheath. The spinning parameters were determined based on better covering degree and higher yarn strength by exploring the influences of twist factor, draft ratio and stainless steel wire diameter on the performance of core-spun yarn. The prepared conductive core-spun yarn was found to have good processibility. The research on the conductive properties of knitted circuit with different conductive core-spun yarns and fabric specifications shows that the conductivity is related to fabric elasticity, stainless steel wire diameter and tensile direction. The resistance change rate of the fabricated circuit can be less than 0.31% indicating good conductive stability of the knitted circuit. Similar conductive stability was also observed in three-dimensional bursting test under 150% mean strain. The knitted circuit has good electrical durability and can maintain stable resistance during 5 000 cycles of longitudinal tensile tests with an average strain of 20%.

Key words: cotton, stainless steel wire, sirospun technology, intarsia structure, core-spun yarn, flexible knitted circuit, conductive stability