Design and optimization of miniaturized flexible NFC antennas for battery-free biomedical sensing

Abstract

Thesis (M.S.)-- Wichita State University, College of Engineering, Dept. of Biomedical EngineeringWireless data transfer capabilities enable real-time, user-centered monitoring for wearable and implantable biomedical devices. Among wireless technologies, near-field communication (NFC) holds great potential due to its ability to be ultra-thin, flexible, and miniaturized, while also supporting battery-free wireless communication. NFC technology operates through inductive coupling between two coils, which exchange electrical power and sensing data when in close proximity. Although NFC has been widely explored for wearable devices, few studies have focused on optimizing working distances while minimizing antenna size, which is crucial for the development of unobtrusive and long-term wearable devices that function without interruption. The goal of this study is to develop highly flexible antennas suitable for miniaturized biomedical devices and to construct a compact wearable sensing system. Since antenna sizes directly affect the working distance, smaller antennas typically suffer for shorter working distances, due to low inductive coupling. Therefore, our study primarily focused on the physical dimensions of antenna coils (e.g. coil shape, diameter, turns, spacing) and analyzed their impact on working distance. Based on our theoretical studies, we designed 12 tag antennas with diameters of 10, 15, 20, and 30 mm and corresponding inductance’s of 2, 4, and 6 μH, respectively. Thus antennas were then fabricated using 2 μm thick copper film through micro fabrication techniques. For experimental analysis, antenna calibration was performed to achieve the desired resonant frequency ( 13.56 MHz). Our results demonstrate that, antenna diameter significantly impacts the working distance, with the 30 mm diameter achieving the maximum range of 15 mm, followed by the 20 mm (12 mm), 15 mm (7.5 mm), and 10 mm (4 mm) antennas. To assess the stability, antennas’ electrical and mechanical properties were evaluated through bending tests over multiple cycles of bending to determine their flexibility and durability. Notably, all antennas exhibit stable coil resistance and resonant frequency under different bending conditions. To validate the sensing capabilities, we conducted pH testing using lab fabricated working electrodes (Platinum) and reference electrodes (Ag/AgCl). Their electrochemical performance was evaluated over the range of pH values, focusing on sensitivity, reproducibility and stability. In conclusion, our findings suggest that miniaturized antennas and fabricated electrodes have the potential to the development of battery-free wireless sensing for wearable health monitoring, and broader biomedical use. Further advancing this research by integrating fabricated electrodes with an NFC tag could lead to the development of self-powered sensing systems