Coupled Mode Theory-Inspired Design of Ultrasensitive Planar Biosensor Based on SIW Resonator

This article proposes a highly compact and ultrasensitive microfluidic sensor based on the cavity resonator for biofluids. Aiming at the problem that cavity sensors under planar channels have difficulty achieving high sensitivity, a simple and efficient design method based on coupled mode theory (CMT) is proposed, and a highly miniaturized device is optimized based on the proposed concept. Instead of the drilled channel of a typical substrate-integrated waveguide (SIW) cavity with a top and bottom metal layer, the proposed resonator is based on a planar integration of the channel. An additional patch resonator is introduced in the cavity center for establishing a coupled resonator system. The high sensitivity is obtained by mode splitting generated from the strong coupling of the SIW resonator and the introduced patch resonator. The miniaturization strategies of folded SIW and spiral complementary split ring resonator (CSRR) are applied to the two-coupled resonators, to put the channel position at maximum field levels. The highly miniaturized device reduces the amount of necessary fluid during testing while maintaining the performance. The sensor is operational in the frequency range 0.6–1.7 GHz and has a size 0.21 x 0.21 λ² at 1.7 GHz. The proposed sensor has been verified with sodium chloride (NaCl) solutions to emulate the concentration range of sweat under physiological conditions. The results demonstrated that the sensor provides excellent resolution, thereby confirming its suitability for biological fluid analysis.