High-Range Dual-Coil NFC Sensor Tag for Battery-Less Physiological Monitoring

This paper presents a battery-less NFC sensor tag designed for contactless physiological monitoring, where extended read range and robust energy harvesting are critical to ensure reliable sensing. The sensor tag integrates three functional blocks: an electrochemical enzymatic sensor for analyte detection, the SIC4341 interface chip for data acquisition and wireless communication, and a dual-coil NFC antenna for efficient power transfer and signal exchange. The antenna is implemented on a thin, flexible polyimide substrate, with two in-phase coils printed on opposite sides and connected in series through a via hole. This configuration increases the overall inductance and magnetic flux, enhancing the tag communication range while maintaining a compact footprint. A full RLC circuit model is analyzed and tuned to operate at the standard NFC frequency of 13.56 MHz, with calibration to maintain resonance. The proposed device successfully demonstrates battery-less sensing of glucose levels under real operating conditions. Additionally, electromagnetic simulations and experimental validation confirm that the dual-coil design enables an almost 50% improvement in communication distance over a conventional single-coil layout, and is capable of extracting enough energy through inductive coupling over the large distance to drive the IC and sensing operation.