Full-body NFC: body-scale near-field sensor networks with machine-knittable meandered e-textiles

TL;DR

Full-body NFC tackles the challenge of scalable, battery-free on-body sensing by employing digitally knitted twin meander coils that confine the NFC magnetic field near the body while maintaining strong coupling to small sensor tags. The system uses a balanced bridge between two identical reader coils to achieve impedance balance and motion-robust readout at 13.56 MHz, enabling time-division NFC communication with multiple tags. The authors demonstrate body-scale coverage of roughly 70-80% and battery-free operation for hundreds of kbps data rates from multiple tags, with tag footprints under 0.3% of the coverage area, while remaining robust to daily activities and washing. This work lays the groundwork for Internet of Textiles applications by enabling distributed, battery-free sensing across garment-enabled bodies.

Abstract

Wireless body networks comprising battery-free on-body sensors and textile-based wireless readers can enable daily health monitoring and activity tracking by continuously monitoring physiological signals across the body. However, previous textile-based wireless networks made of coils or antennas have limited the data and power transmission area because covering the whole body results in undesirable levels of electromagnetic interactions with the body, degrading the scalability, power consumption, and data rate. Here, we report Full-body NFC, digitally-knitted electronic textiles based on a twin meander coil design that enables body-scale near-field communication (NFC) with battery-free sensor tags arbitrarily placed around the body. Full-body NFC features i) a meander coil that enhances the magnetic field intensity on the body's surface while suppressing undesired interactions with deep tissues, in addition to ii) paired identical coil structure that enables highly-sensitive and motion-robust NFC using a differential architecture. Additionally, industrial digital knitting machines loaded with conductive yarn allow the integration of the Full-body NFC system into daily garments supporting approximately $70-80\%$ large-scale NFC-enabled area of the body. We demonstrate Full-body NFC could achieve mW-class energy-efficient near-field sensor networks with hundreds of kbps-class NFC battery-free sensor tags occupying less than $0.3\%$ of the coverage area under severe body movements.

Figures (7)

• Figure 1: Design overview of Full-body NFC. (a) Photograph of prototype. (b) Schematic and simulated inductive field of helical coil, meander coil, and twin meander coil that divides a body-scale meander coil into two parts symmetrically (i.e., reader coil #1 and #2). (c) Circuit diagram and (d) frequency spectrum of Full-body NFC.
• Figure 2: Measured impedance characteristics of two types of body-scale reader coil: (a) body-scale meander coil and chip elements impedance-matched with the reader coil at 13.56 MHz and (b) twin meander coil consisting of the two identical meander coil structure. (c) Impedance difference ratio in (a) and (b). The impedance-balanced frequency band is here below 10% of the impedance difference ratio.
• Figure 3: Fabrication process of Full-body NFC. (a) Photograph and (b) schematic of knitting process of the twin meander coil. The knitting machine joins multiple yarns via a carrier to decrease the wire resistive characteristics. (c) Sheet resistivity of knitted conductive wire for a joining number of conductive yarns. (d) Photograph of the fabrication post-process of the twin meander coil.
• Figure 4: Experimental verification of body-scale wireless charging capability. (a) Photograph of wireless charging demonstration of lighting up six NFC sensor coils equipped withred LED. (b) Schematic of power supply scheme using two reader coils. (c) Result of output power for eight different user motions.. (d)(e) Result of output power and AC-to-AC power transfer efficiency for (d) sensor's placement and (e) distance of the sensor coil from the reader coil.
• Figure 5: Experimental verification of body-scale wireless communication capability. (a) Configuration of bit-error-rate (BER) measurement of the listening mode. (b) Results of BER for different bit rate with different keying. (c) Results of BER for mannequin or human. (d) Results of BER of five types of readout methods. (e) Results of BER of the tops and bottoms for various motions. (f)(g) Results of BER according to (f) distance of the sensor coil from the reader coil and (g) sensor's placement.