Abstract:

Objective To meet the requirements for functions, intelligence, and wearability of wearable devices for smart outdoor apparel applications, a self-powering knitted sensing fabric with bionic scales(BSK-TENG) is designed from the inspiration of natural selection, which combines the scales structure with protection and flexibility. The flexibility of scale-structured fabric not only satisfies the common wearing, but generates electrical outputs to supply energy for outdoors sensors. It is envisaged that this type of fabric with full fiber structure will provide novel ideas for multifunctional wearable electronics while maintaining the intrinsic performance of textiles.
Method The complex three-dimensional bionic scale knitting fabric was fabricated by a double-bed computerized flat knitting machine, representing the industrial production. The triboelectric nanogenerators were used as a convertor based on a coupled effect of contact electrification and electrostatic induction, generate periodic electrical outputs during mechanical movements. The single-sided scales were consistent with the single electrode working mode, which proved to be facile to construct the self-powering scale-structured knitted sensing fabric. For the triboelectric series of materials, polyamide (PA) yarns and a polytetrafluoroethylene(PTFE) yarn were selected as a pair of contact materials, and Ag-plated polyamide yarns were employed as the electrodes for electronic signal transfer.
Results The influence of structural features of scale-structured knitted sensing fabric on electrical and mechanical properties were comprehensively investigated for novel applications. The results show that the BSK-TENG as a novel wearable device can be manufactured in mass scale and formed in a single process (Fig. 2). Through the linear motor (Fig. 3), the PA yarn establish contact with the scales working as the single electrode (Fig. 4), generating the electrical outputs. In order to analysis the effect of fabric structural parameters on the electrical output performance, fabrics with different vertical spaces and scaly layouts were designed and fabricated to regulate the output performance, and the electrical outputs were measured. The electrical performance is enhanced as the vertical spaces increase, which caused the increase of the contact area (Fig. 5). For different layouts of the scaly fabrics, the electrical outputs show no difference between the parallel type and the imbricate type (Fig. 6). In addition, BSK-TENG exhibits satisfactory the stability and force sensitivity for monitoring the force change to obtain a high gauge factor (Fig. 7 and Fig. 8). Considering the asymmetry of fabric surface, the bending performance of scale knitting fabric demonstrates obvious differentiation, indicating lower stiffness on the intrados side than that on the extrados side (Fig.9). It turns out that the scale knitting fabric has special anisotropic mechanical property. With small interval spaces, the overlapping scale distribution has an obvious strain-stiffening response, which offers strong support for joint protection. BSK-TENG is utilized as the wearable device, which requires suitable level of air-permeability for wearing comfort. Due to the scaly structure, fabrics with different surface designs demonstrated distinctive testing results, but not decreasing the fabric breathability (Fig. 10).
Conclusion Industrial production of self-powering knitted sensing fabrics was achieved using knitting technology, achieving the one-piece complex three-dimensional fabric structure. The effect of interval spaces between scales on the electrical outputs were discussed and analyzed. It is found that fabrics with cover factor 0.7 generates higher electrical outputs, with the contact area equal to the scale area. This indicates that the design of interval space plays an important role in regulates the electrical output performance of BSK-TENG. Furthermore, a good linear relationship between electrical outputs and external force is established and it can be utilized for fabricate the self-powered sensor. The scaly layouts have little influence on the output performance, however there is a significant difference in stiffness performance. The scale knitted fabric has an apparent strain hardening effect, especially for the scaly section of the fabric, which could lead to a potential joint protection application. The smart textile with both intelligence and functions can satisfy the conflicting requirements of protection and flexibility while maintaining textile intrinsic good performances. It is envisaged that the high-speed production of soft bionic scale-structured fabric with both intelligence and functions will bring opportunities for the future development of wearables.

Key words: polyamide, polytetrafluoroethylene, bionic scale structure, fully-forming knitting technology, self-powered, sensing fabric, smart textile, anisotropic material