Microwave Microfluidic Sensor Based on a Microstrip Splitter/Combiner Configuration and Split Ring Resonators (SRRs) for Dielectric Characterization of Liquids

Abstract

A microwave microfluidic sensor for dielectric characterization of liquids in real time is presented in this paper. The sensor is implemented in microstrip technology and consists of a symmetric splitter/combiner configuration loaded with a pair of identical split ring resonators (SRRs) and microfluidic channels placed on top of them (gap region). The sensor works in differential mode and sensing is based on frequency splitting. Thus, if the structure is unloaded or if it is symmetrically loaded with regard to the axial plane, only one transmission zero (notch) in the frequency response appears. However, if the axial symmetry is disrupted (e.g., by the presence of different liquids in the channels), two transmission zeros arise, and the difference in magnitude (notch depth) and frequency between such transmission zeros is indicative of the difference in the dielectric properties (complex dielectric constant). A circuit schematic, including transmission line sections to describe the distributed components, lumped elements to account for the SRRs and their coupling to the lines and lumped elements to model the liquid properties, is presented and validated. After proper calibration, the functionality of the proposed sensor is demonstrated by measuring the complex permittivity in solutions of deionized (DI) water and ethanol as a function of the ethanol content.

Real‐time monitoring of Escherichia coli concentration with planar microwave resonator sensor

“…The integration of microfluidic structures with microwave resonators brings very attractive characteristics to microwave sensors and enables high‐throughput analysis in constant volumes in the range of nanoliters to milliliters . This aspect allows inspection of live cells in a closed microfluidic chip, while reducing the risk of environmental contamination .…”

“…10 The integration of microfluidic structures with microwave resonators brings very attractive characteristics to microwave sensors and enables high-throughput analysis in constant volumes in the range of nanoliters to milliliters. 11,12 This aspect allows inspection of live cells in a closed microfluidic chip, while reducing the risk of environmental contamination. 13,14 The conjunction of microwave electromagnetics and microfluidic technology is starting to be utilized in cancer-cell identification, 15 and bacterial and viral contamination monitoring.…”

Multi-Negative Index Band Metamaterial-Inspired Microfluidic Sensors

“…MTM-inspired microfluid microwave sensors generally consist of a microstrip transmission line loaded with MTM structures and a fluidic channel of various shapes. Most MTM-inspired microfluidic sensor prototypes [1,6,18,20] presented a similar testing setup preforming the transmission response measurements, where the complex permittivity extracted from the transmission (S 21 ) and reflection (S 11 ) coefficients is used to classify the testing material. The MTMs designed for specific electromagnetic properties are integrated into the sensor system in order to effectively manipulate the shift of the resonant frequency and the change in Q-factors, resulting in a more pronounced dielectric perturbation phenomenon.…”

“…The sensing capability and efficiency are directly influenced by the correlation between the MTM structures and the fluidic channel. To differentiate the sensing performance, symmetry properties in transmission line loaded with one or more MTM structures need to be considered to avoid the lack of coupling due to the complete cancellation between electric and magnetic fields [6,[37][38][39][40]. All the MTM structures implemented for microfluidic microwave sensors are either a split ring resonator (SRR), a complementary SRR (CSRR), an open SRR (OSRR), or a splitring-cross resonator (SRCR) [1, 2-4, 7, 8, 10-13, 15-20, 22], which are typical magnetic resonators first introduced as negative permeability MTMs [41].…”

“…These SRRs with various orientation, e.g., single-ring, double-ring, square, circular, or rectangular shape, can be easily designed to generate a magnetic resonance, where the negative real part of the permeability is located. A single magnetic resonance from the SRRs is normally narrow, resulting in a narrow sensing frequency band; for example, a 0.05 GHz band for a sensor with a 1GHz operating frequency [6]. 4-circular double SRRs with different dimensions have been designed for multi-bands in order to optimize the permittivity characterization [1].…”