Embedded Capacitive Proximity and Touch Sensing Flexible Circuit System for Electronic Textile and Wearable Systems

Electronic textiles (e-textiles) are an emerging technology comprised of electronic circuit systems embedded into the fabric using traditional textile techniques such as knitting and weaving. Such systems can be formed using off-the-shelf passive components and integrated circuits (ICs) available commercially. An e-textile switching mechanism can be provided by proximity and touch sensing input behavior from a human hand to switch circuits, for example, turning on an LED for visual verification of successful switching. Challenges to be overcome to achieve reliable systems include optimized circuit sampling rate, detection sensitivity, and sensing electrode dimensions when operating among textile fibers. This paper presents the development of a proximity and touch sensing e-textile flexible circuit system using a commercial capacitive detection chip PCF8883US by an NXP Semiconductor. Experiments determine the necessary sensitivity and sampling rate values for this chip to detect through fabric for its optimum capacitance performance value of 20 pF. This paper further details the fabrication of this $150~\mu \text{m}$ track width, $3.0~\text{mm}\times35.0~\text{mm}$ dimension flexible, copper-polyimide capacitive sensing circuit with a 0.0403 mm thick, 20:1 PDMS aqueous-resistant layer and its integration into two e-textile systems–a knitted textile yarn and woven textile fabric. Experiments revealed that when an electrode is strip-shaped for e-textile applications, its width is more influential than electrode length in enabling capacitive dual-functionality proximity and touch sensing. A square-shaped electrode offers a 58.3% increase in nominal proximity detection distance compared with multiple electrodes arranged in the same area.