Ultrahigh-Sensitivity Microwave Microfluidic Sensors Based on Modified Complementary Electric-LC and Split-Ring Resonator Structures

The paper proposes an ultrahigh sensitivity microwave sensor for retrieving the complex permittivity of the liquid sample. The proposed sensor originally evolves from the complementary split-ring resonator-based sensor, which confines the electric field in the slot of the resonator. A complementary electric-LC resonator structure is applied to substitute the CSRR due to its excellent ability of concentrating the electric field. To further improve the field confinement, a meander slot is embedded in the central arm of the resonator. Then, two etched complementary split-ring resonators are added on both sides of the central meander arm of the modified complementary electric-LC resonator. A polydimethylsiloxane microfluidic channel substrate is placed on the meander arm, and a mixture of water-ethanol is injected. The variations in the resonant frequency and peak attenuation are used to retrieve the complex permittivity of the injected liquid sample. The developed sensors are fabricated and tested, and a good agreement is observed between measured data and reference values. The peak sensitivities in retrieving the relative permittivity achieve 2.5% and 2.71% with the proposed sensors, which are much higher than previously reported designs.