Abstract:

Objective Conductive fabric can be easily fabricated into smart clothes comfortable to wear. However, a common problem is that a large mismatch in mechanical properties between the conductive layer and the fabric substrate affects the performance of the flexible sensors. In order to improve the interfacial adhesion between the conductive layer and fabric, and construct an effective contact conductive network to obtain excellent sensing response characteristics, a reduced graphene oxide (RGO) and polypyrrole (PPy) flexible sensor was prepared by surface modification of polyester-spandex knitted fabric with polydopamine (PDA).
Method A knitted fabric substrate was modified by PDA, a PDA-RGO fabric was prepared by impregnation-drying and chemical reduction, and PPy was self-assembled on the PDA-RGO fabric via in-situ polymerization. The PDA modified RGO/PPy conductive fabric sensor was characterized and analyzed by Fourier infrared spectrometry (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), a self-made KTC sensor box, a four-probe square resistance tester, a Martindale abrasion and pilling tester and a universal tensile testing machine.
Results The PDA fabric, PDA-RGO fabric, PDA-RGO/PPy fabric and RGO/PPy fabric were prepared. A comparative study of the influence of PDA modification on the electrical conductivity and sensing properties of knitted fabrics was then carried out. The results indicated that the PDA filled the gaps among the yarns of the knitted fabric and improved the continuity of the conductive layer. The square resistance of the conductive fabrics showed that PDA enhanced the conductivity of the conductive fabric. The square resistance of the PDA-RGO/PPy fabric was about 0.08 kΩ/□. The PDA-modified knitted fabric had a strong adsorption to the conductive layer. RGO and PPy had a synergistic effect on the electrical properties, and the conductive fabrics containing RGO/PPY had better conductivity than fabrics with a single conductive component. The conductive layer of the PDA-modified RGO/PPY fabric had increased interfacial bonding by virtue of the bonding of the PDA. The change in resistance after rubbing was smaller for the PDA-RGO/PPy fabric than for the RGO/PPy fabric (Fig.5). The study of fabric sensing characteristics showed that PDA-RGO/PPy fabric had better sensing properties than RGO/PPy fabric. The stretching range of the PDA-RGO/PPy fabric flexible sensor was 0%~130%, the sensitivity was increased to 39.1, and the response time was 0.06 s. Moreover, the peak value of the relative change of resistance of PDA-RGO/PPy fabric was essentially the same for different stretching rate (Fig.6), proving the accuracy of this flexible sensor. This phenomenon can be explained by the fact that PDA deformed the conductive layer synchronously with the fabric substrate. The PDA-RGO/PPy fabric flexible sensor can be worn on joints such as fingers, wrists and knees to monitor motions. The fabric flexible sensor captures the motions steadily and outputs the relative change of resistance (Fig.11).
Conclusion The results of above characterizations indicate that the interfacial adhesion between the PDA-modified fabric and RGO/PPy is significantly improved, and the conductive network is constructed more continuous. Compared with unmodified fabrics, the modified fabrics has improved durability and rubbing resistance. The experimental results show that the sensing mechanism of the fabric sensor is mainly the disconnection mechanism and crack propagation. Monitoring of different joint motions can be achieved according to the resistance change curve and the data can be used for building human joint motion sensing systems. In the future, the conductive properties of the PDA-RGO/PPy fabric flexible sensor can be optimized by controlling the combination options and shape of the conductive materials for further adjusting the surface morphology of the conductive layer.

Key words: flexible sensor, conductive fabric, polydopamine, reduced graphene oxide, polypyrrole, sensing response characteristic