A Split-Ring Resonator-Based Planar Microwave Sensor for Microfluidic Applications

This paper demonstrates a high-sensitivity microwave microfluidic sensor for retrieving the liquid permittivity. To increase the sensitivity and notch depth, the equivalent circuit model of split-ring resonator (SRR) structure is analyzed. Then, the interdigital capacitor structure (IDC) and defected ground structure (DGS) are introduced. A polydimethylsiloxane (PDMS) block is placed above sensing area to realize microfluidic channel, which completely covers interdigital gap and makes full use of strong electric field. Ethanol-water mixed solution is injected into the channel as the measured liquid. The effective permittivity of the channel is thereby changed to affect the resonance frequency, which is used to detect the liquid sample. Our proposed sensor achieved an average sensitivity of 1.461% using very small liquid sample volume about $0.68\ \mu \mathrm{L}$, and the values extracted from the prototype are in good agreement with the actual data. The average sensitivity is defined as the average of the frequency shift per unit permittivity at each ethanol fraction.