A simple and extremely-sensitive non-invasive coupled-line microwave sensor for biomedical monitoring applications

This paper presents a novel and extremely sensitive microwave sensor based on a coupled-line microstrip configuration. The proposed design offers a simple yet effective method for measuring and analyzing the dielectric properties of liquids, specifically tailored for biomedical monitoring applications. A glass capillary tube, serving as a sample holder, is strategically positioned above the coupled-line to exploit the region of strong electromagnetic field concentration. When liquid samples with varying relative permittivity flow through the capillary tube, their interaction with the electromagnetic fields induces noticeable shifts in the transmission zero. The sensor, measuring 80 × 21 × 0.787 mm³, is fabricated on a low-loss moderate-cost Rogers RO4003 substrate. Its performance was experimentally validated using water-ethanol mixtures and real blood samples, demonstrating a sensitivity of approximately 5.4 $\frac{\mathrm{MHz}}{∆{\epsilon }_r^{\prime }}$ at a frequency of 6.75 GHz. Experiments were also conducted using real human blood samples under fasting and non-fasting conditions. A strong correlation was observed between the simulated and measured results. The sensor offers several key advantages, including a simple structure, ease of fabrication, tunable operating frequency, seamless integration with other circuit systems, non-invasive operation, cost-effectiveness, and reusability of the capillary tube. These features make it a highly promising solution for diverse biomedical sensing applications.