New methods enable better performance in smart e-fabric devices

Recently, the latest research results of Ye Tao’s research group in the Department of Electronic and Electrical Engineering of Southern University of Science and Technology were published in Nature-Communication. The research team proposed a method to model and measure the parasitic capacitance of conductive yarns, and systematically analyzed and quantified the parasitic elements of conductive yarns, which is of great significance for evaluating the electronic performance of flexible wearable fabrics in high-frequency applications. The paper was selected as one of the articles in the Editorial Highlights section of Nature Communications.

Research diagram Courtesy of SUSTech

As an emerging field, flexible fabric electronics integrates textile and electronic technology and has great potential and advantages. Fabric electronics can be seamlessly and aesthetically integrated in soft, wearable fabric materials, realizing the combination of intelligence, immersion and comfort, and have a wide range of application prospects in medical monitoring, health management and motion tracking.

As an important part of fabric electronics, conductive yarns have been widely used in smart clothing applications such as antennas, inductors, and interconnects, becoming a viable alternative to traditional metal wires. However, parasitic capacitance caused by the microstructure of conductive yarns can greatly affect the performance of equipment in high-frequency applications.

In this regard, the research team proposed an overall and interturn model of a hollow helical inductor based on conductive yarn, and systematically analyzed and quantified the parasitic elements of conductive yarn. By comparing the frequency responses of copper-based and yarn-based inductors with the same structure, the parasitic capacitance values were quantified and extracted.

Experimental measurements show that the parasitic capacitance per unit length of commercial conductive yarn ranges from 1 farad to 3 farads per centimeter, depending on the microstructure of the yarn. These experiments and measurements provide important quantitative evaluation criteria for conductive yarn parasitic elements and provide valuable guidance for the design and characterization of fabric-based flexible wearable devices.

Conductive yarns can be divided into three types according to the properties of their constituent fibers: pure conductive metal fibers (such as stainless steel), intrinsically conductive polymer fibers, and conductive polymer composite fibers. By choosing different materials, conductive yarns with different mechanical and electrical properties can be designed to suit a wide range of applications. However, due to its special microstructure, conductive yarns bring many challenges to the design and manufacture of the inherent electrical and mechanical properties of conductive yarns.

For example, conductive yarns generally have higher resistance than metallic wires, and poor conductivity leads to increased energy losses and lower quality factors. In addition to their inherent resistance, conductive yarns have inherent parasitic capacitance, which affects the high-frequency characteristics of flexible fabric electronics.

According to the research team, although parasitic capacitance may have an important impact on the high-frequency electromagnetic characteristics of fabric electronic devices, it is difficult to accurately quantify the parasitic capacitance value using only a multimeter or network analyzer. In previous studies, no attempt has been made to create circuit models and specifically estimate the parasitic capacitance of conductive yarns.

In this regard, the research team proposed an overall model and an interturn model to estimate the parasitic capacitance of conductive yarns, and proposed a systematic method to extract and estimate the parasitic capacitance of these two forms. The research team built two spiral air-core inductors with the same geometric parameters, i.e. the same diameter, the same turn spacing and the same number of turns.

One of the spiral inductors is wound using conductive yarn, while the other spiral inductor is wound using copper wire of the same specification. By comparing the resonant frequencies of these two spiral inductors, the overall parasitic capacitance of the yarn can be measured.

In addition, by comparing the impedance curves of the two spiral inductors at different turns and fitting the impedance curves of the equivalent circuit, the interturn parasitic capacitance of the conductive yarn can be extracted. The overall parasitic capacitance and inter-turn parasitic capacitance are correlated, which further verifies the accuracy of the parasitic capacitance model and extraction method in this study. (Source: China Science News, Diao Wenhui)

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