A Low-Cost Microwave Microfluidic Sensor Based on Planar Ring Resonator

In this article, a low-cost and very high-sensitive microwave microfluidic sensor implemented by two ring resonators is proposed for liquid characterization. Applying the liquid samples to the sensor was done using a polyvinyl chloride (PVC) microfluidic channel and dropping-based method which causes a shift in the resonance frequency and quality factor related to the value of the liquid’s complex permittivity. Several solvents were measured and a good agreement was achieved for simulations and experimental measurements. Also, ethanol-water mixtures were used for calibration and validation of a predictive model for the complex permittivity, which shows a maximum measurement error of 5.8%. A very high sensitivity of 0.7% using liquid volume of about
$30 ~\mu \text{L}$
was obtained which demonstrates the accuracy of characterization of the complex permittivity by using the proposed sensor. In comparison with previous designs, the proposed sensor has a compact size, simple design, and easy fabrication process and it is cost-effective to fabricate. Also, the proposed sensor can be used either with or without a microfluidic channel.