An Enhanced Microwave Differential Microfluidic Sensing System for Detecting Small Variation of Dielectric Properties

This article proposes a microwave sensor based on differential structure to detect small changes in the dielectric properties of biological microfluids. By examining the underlying operating principles of the differential sensing system, the key factors influencing sensor performance are identified. The proposed design enhances sensitivity by optimizing both the sensing branches and the overall configuration. Two coupled eighth-mode (EM) substrate-integrated waveguide (SIW) resonators with concentrated electric field tips are employed in the sensing branches, which exhibit rapid phase variation as the sample under test (SUT) changes, thereby improving the sensitivity of the original differential zero point. Furthermore, the introduction of an additional zero point caused by self-resonance achieves double increase in sensor sensitivity within a narrow frequency band. The sensor operates in the frequency range of 4.7–5.7 GHz, and its performance has been validated using ethanol-water solution with varying concentrations to simulate the subtle changes in biofluids under physiological conditions. The proposed structure offers a potential solution to the sensitivity challenges posed by subtle changes of high dielectric permittivity samples in biosensing.