A Microfluidic-Integrated SIW Lab-on-Substrate Sensor for Microliter Liquid Characterization

Abstract

A novel microfluidic-integrated microwave sensor with potential application in microliter-volume biological/biomedical liquid sample characterization and quantification is presented in this paper. The sensor is designed based on the resonance method, providing the best sensing accuracy, and implemented by using a substrate-integrated-waveguide (SIW) structure combining with a rectangular slot antenna operating at 10 GHz. The device can perform accurate characterization of various liquid materials from very low to high loss, demonstrated by the measurement of deionized water and methanol liquid mixtures. The measured relative permittivity, which is the real part of complex permittivity, ranges from 8.58 to 66.12, which is simply limited by the choice of test materials available in our laboratory, not any other technical considerations of the sensor. The fabricated sensor prototype requires a very small liquid volume of less than 7 $\mu \text{L}$ , while still offering an overall accuracy of better than 3%, as compared with the commercial and other published works. The key advantages of the proposed sensor are that it combines: 1) a very low-profile planar and miniaturized structure sensing microliter liquid volume; 2) ease of design and fabrication, which makes it cost-effective to manufacture; and 3) noninvasive and contactless measurements. Moreover, since the microfluidic subsystem can potentially be detached from the SIW microwave sensor and, afterward, replaced by a new microfluidic component, the sensor can be reused with no life-cycle limitation and without degrading any figure of merit.