Microstrip Gap Sensor for Dielectric Spectroscopy in Microfluidic Applications

This paper presents a microstrip gap sensor designed for dielectric spectroscopy in microfluidic applications. The sensor enables the characterization of fluid properties based on the electrical permittivity using microwave measurements. Unlike conventional techniques, the sensor operates in a broadband manner and relies solely on the transmission coefficient, eliminating the need for de-embedding procedures. The experimental results demonstrate the sensor's performance and highlight the trade-off between frequency range (from 0.2GHz up to 6GHz) and accuracy.

I. Introduction

The analysis of fluid properties holds significant importance in various domains, particularly in the fields of biology and medicine. Among the diverse analytical techniques reported in the literature, this study focuses on dielectric spectroscopy (DS), which enables the determination of the electrical complex permittivity of fluids through the utilization of microwave sensors. Electrical permittivity is associated with numerous biological properties, and, compared to conventional analysis techniques, it offers the advantage of being non-invasive and not requiring the use of markers for the fluid being tested. These type of sensors, at a first approximation, can be categorized into two families: broadband sensors and resonator sensors [1]. Resonator sensors enable the determination of the complex electrical permittivity of the tested fluid primarily in the vicinity of the sensor's resonant frequency. Typically, the electrical permittivity of the fluid under examination is assessed by establishing the relationship between the change in resonant frequency and the real part of the complex permittivity, as well as the change in quality factor with respect to the imaginary part of the permittivity [2]–[4]. On the other hand, broadband sensors allow for an investigation of the permittivity of the fluid across a wide range of frequencies, spanning from low frequencies (MHz or less) to microwave frequencies (GHz). The utilization of broadband sensors proves valuable in obtaining insights into the dispersion phenomena exhibited by the fluid. However, the extensive frequency range necessitates the use of de-embedding techniques for each measurement, which can introduce inaccuracies in the results. Broadband approaches often involve the use of a perturbed two-port network transmission line [5], where the liquid induces a change in the characteristic impedance and, consequently, the S-parameters. Alternatively, a one-port network terminated with a capacitive gap perturbed by the liquid can also be employed [6], [7].