A microwave microfluidic sensor for detecting binary liquid mixtures with a dielectric method at RF/microwave frequencies is presented in this article. The sensor is based on a split-ring resonator (SRR) that is implemented in a microstrip transmission line, with interdigitated electrodes (IDEs) being integrated into the ring for liquid detection. Based on the equivalent circuit of the IDE-SRR device and with a series of finite element simulations, the detection theory is developed, and the device design optimization is investigated. The validation measurements on water-isopropanol liquid mixtures with various concentrations show that the proposed IDE-SRR sensor has higher sensitivity than the previous standard SRR sensor. The IDE-SRR sensor is then used to detect two binary liquids, i.e., water-methanol mixtures and water-tetrahydrofuran mixtures. The measured effective permittivity results of the binary mixtures at RF/microwave frequency range are compared with the existing mixing models for binary dielectric mixtures at zero frequency.

This paper describes the design, fabrication, and assessment of an on-wafer measurement platform for non-invasive quantitative characterization of small volumes of liquids over the frequency range from 500 kHz to 1 GHz. The platform, integrated with a micrometer scale SU-8 microfluidic channel, allows accurate placement of very small liquid volumes at a well-defined location. For the first time, a miniaturized one-port interdigital capacitor (IDC) sensor is used for dielectric spectroscopy (DS) measurements at RF/lower microwave frequency range. 3D simulation on the IDC sensor with the finite element method (FEM) verifies the complex permittivity extraction model. A validation of this platform is performed using water-isopropanol mixtures, and then it is utilized for liquid temperature measurement and baker's yeast cell concentration detection. An observation over 5 minutes is also carried out on a cell suspension, which has shown the capability of this platform in monitoring dynamic process in chemical or biological fields.

This paper proposes a miniature microwave-microfluidic chip based on continuous microfluidics and a miniature interdigital capacitor (IDC). The novel chip consists of three individually accessible heaters, three platinum temperature sensors and two liquid cooling and mixing zones. The IDC is designed to achieve localized, fast and uniform heating of nanoliter volumes flowing through the microfluidic channel. The heating performance of the IDC located on the novel chip was evaluated using a fluorescent dye (Rhodamine B) diluted in demineralized water on a novel microwave-optical-fluidic (MOF) measurement setup. The MOF setup allows simultaneous microwave excitation of the IDC by means of a custom-made printed circuit board (connected to microwave equipment) placed in a top stage of a microscope, manipulation of liquid flowing through the channel located over the IDC with a pump and optical inspection of the same liquid flowing over the IDC using a fast camera, a light source and the microscope. The designed IDC brings a liquid volume of around 1.2 nL from room temperature to 100 °C in 21 ms with 1.58 W at 25 GHz. Next to the heating capability, the designed IDC can dielectrically sense the flowing liquid. Liquid sensing was evaluated on different concentration of water-isopropanol mixtures, and a reflection coefficient magnitude change of 6 dB was recorded around 8.1 GHz, while the minimum of the reflection coefficient magnitude shifted in the same frequency range for 60 MHz.

In this paper, a miniaturized microwave resonator is designed and fabricated. A loop mode resonance is created when the permittivity difference of the two materials loaded in the two fluidic channels are sufficiently large. A highly concentrated electrical field is generated in a microfluidic channel loaded with deionized water to achieve high sensing spatial resolution within a sensing volume of 45 pL. On-wafer measurements are performed by keeping one channel unloaded as a reference while loading the other channel with different mixtures of isopropanol and deionized water. The acquired microwave-microfluidic measurements confirm that the proposed resonator can resolve changes in the permittivity of the loaded liquid between the values of 3.2 and 42.4, making it promising for applications such as mixing monitoring and flow cytometry.

This work presents a novel platform conceived as an interconnect box (ICB) that brings high-frequency signals from microwave instruments to consumable lab-on-a-chip devices. The ICB can be connected to instruments with a standard coaxial connector and to consumable chips by introducing a spring-levered interface with elastomer conductive pins. With the spring-system, microwave-microfluidic chips can be mounted reliably on the setup in a couple of seconds. The high-frequency interface within the ICB is protected from the environment by an enclosure having a single slit for mounting the chip. The stability and repeatability of the contact between the ICB and inserted consumable chips are investigated to prove the reliability of the proposed ICB. Given the rapid interconnecting of chips using the proposed ICB, five different interdigital capacitor (IDC) designs having the same sensing area were investigated for dielectric permittivity extraction of liquids. The designed IDCs, embedded in a polydimethylsiloxane (PDMS) channel, were fabricated with a lift-off gold patterning technology on a quartz substrate. Water–Isopropanol (IPA) mixtures with different volume fractions were flushed through the channel over IDCs and sensed based on the measured reflection coefficients. Dielectric permittivity was extracted using permittivity extraction techniques, and fitted permittivity data shows good agreement with literature from 100 to 25 GHz.

A fully automated electrically controlled tunable interference-based microwave sensor is demonstrated with broadband complex permittivity measurements of aqueous solutions within the frequency range of 12–18 GHz. To construct deep destructive interference over the above-mentioned bandwidth, two quadrature hybrids (QHs or 90° coupler) split and combine the signals of material-under-test (MUT) channel and reference liquid (REF) channel with 180° phase shift. Together with voltage variable phase shifters (PSs) and attenuators in both channels, the destructive interference nulls of the desired frequency can be easily tuned having a $Q$ -factor of $2.67\times {10}^{3}$ . A coplanar waveguide (CPW) sensor design was used as the microwave sensing structure and closed microfluidic channel was designed and manufactured with polydimethylsiloxane (PDMS) to facilitate the alteration of MUT, REF, and calibration liquids. Benefiting from electrically controlled PS and attenuator, the measurement system is fully automated for accurate and fast broadband microwave liquid characterization. Measurement of 2-propanol-water solutions (isopropyl alcohol (IPA) solutions) with volume concentration of 1% is performed to illustrate automation processes and its high sensitivity. A mathematical model improved from those in the literature by taking conductor loss and mismatch effects into consideration was applied to extract MUT permittivity, generalizing the calibration procedure with the air and deionized (DI) water as calibration materials, which results in improvements in accuracy, sensitivity, and effective MUT volume. Comparing to state-of-the-art sensors, the proposed setup has an average relative error (ARE) of 1.443% for the effective volume of 5.18 $\mu \text{L}$ . The complex permittivity of 2-propanol-water solutions with different mole fractions were obtained and confirmed with current literature data.