A 20-GHz Microwave Miniaturized Ring Resonator for nL Microfluidic Sensing Applications

In this article, a miniaturized microwave resonator is designed and fabricated. A loop-mode resonance is created when the permittivity difference between the two materials loaded in the two fluidic channels is sufficiently large. A highly concentrated electrical field is generated in a microfluidic channel loaded with deionized water to achieve high sensing spatial resolution within a sensing volume of 45 pL. On-wafer measurements are performed by keeping one channel unloaded as a reference while loading the other channel with different mixtures of isopropanol and deionized water. The acquired microwave-microfluidic measurements confirm that the proposed resonator can detect changes in the permittivity of the loaded liquid between the values of 3.2 and 42.4, making it a promising low-cost and label-free approach in applications requiring novel sensing techniques such as mixing monitoring and flow cytometry.