Showing papers by "Bart Nauwelaers published in 2019"

Modeling of Coplanar Interdigital Capacitor for Microwave Microfluidic Application

Abstract: Due to its noninvasive property, the interdigital capacitor (IDC) has been applied in dielectric liquid detection and characterization. In order to integrate the IDC sensor on a lab-on-chip, it is often required to minimize and optimize the sensor for sensitive and efficient performance. However, the conventional numerical simulation approach is quite time-consuming. Therefore, an efficient analytical method is proposed herein, leading to accurate capacitance and conductance expressions of an arbitrary multilayer-structured IDC. The model is validated with practical measurements of a series of coplanar waveguide (CPW) structure-based IDCs. In addition, an accurate characterization function, which relates the IDC capacitance and conductance to the complex permittivity of a material loaded on the top of the IDC sensing area, is obtained. The characterization function shows good agreement with the finite-element method (FEM) simulation results, which demonstrates the capability of the IDC sensor in dielectric spectroscopy measurements of $\mu \text{L}$ and even nL liquids.

An Interdigital Capacitor for Microwave Heating at 25 GHz and Wideband Dielectric Sensing of nL Volumes in Continuous Microfluidics

Abstract: 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.

A 20-GHz Microwave Miniaturized Ring Resonator for nL Microfluidic Sensing Applications

Abstract: In this article, a miniaturized microwave resonator is designed and fabricated. A loop-mode resonance is created when the permittivity difference between the two materials loaded in the two fluidic channels is 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 detect changes in the permittivity of the loaded liquid between the values of 3.2 and 42.4, making it a promising low-cost and label-free approach in applications requiring novel sensing techniques such as mixing monitoring and flow cytometry.

A Simplified Dielectric Material Characterization Algorithm for Both Liquids and Solids

Abstract: A simplified dielectric material characterization technique that combines an equivalent impedance algorithm and a general “line–line” trace method is introduced. The technique is applicable to every type of transmission lines. Due to the convenient fabrication and the allowance of integration with various polymers, coplanar waveguide transmission lines integrated with an SU-8 microfluidic channel is primarily used in this study. The conformal mapping method, an efficient and straightforward way, is introduced to build the foundation of dielectric material characterization and to optimize the sensor design. A validation measurement of the proposed technique is performed on deionized water, and show valuable consistency with previous reliable data presented in the literature. Next, the technique is applied on solid SU-8 characterization with four groups of on-wafer measurements. SU-8 measurement results and the related uncertainty analysis are demonstrated subsequently.

Microfluidic Biosensor for Bioengineering: High-frequency Equivalent-Circuit Modeling of Interdigital Capacitor

Abstract: Microwave biosensing is a rapidly growing field of bioengineering. The progress in micro- and nanotechnologies and the recent advances in microwave dielectric spectroscopy have allowed a rapid development in the miniaturization of high-frequency biosensors. There has therefore been intensive research during the last few years on investigating miniature microwave biosensors for liquid characterization. To contribute to the advancement of this challenging and stimulating field of research, the present contribution is devoted to the analysis of a microfluidic sensor based on a one-port coplanar interdigital capacitor (IDC). The high-frequency performance of the studied sensor is achieved by using the 3D finite-element method (FEM) by Ansoft’s high frequency structure simulator (HFSS). The simulations are used for extracting and validating an equivalent-circuit model that can be further exploited for complex permittivity extraction of the material under test.

Microwave Characterization of Liquid Mixtures with a Miniaturized Interdigital Sensor

Abstract: A miniaturized microstrip interdigital capacitor based sensor is proposed herein to characterize liquid mixtures. For low RF/microwave measurements, the sensing area is modeled as a capacitance in parallel with a conductance. According to the conformal mapping technique, a linear relationship between the capacitance and the permittivity of the liquid under test is obtained, which is further validated through a three-dimensional finite element simulation method. The sensor is then successfully used to characterize deionized water, pure isopropanol, and water-isopropanol mixtures.

Multisegment Multiconductor Transmission Line Model of High-Current Card-Edge Connectors With Analytically Calculated Model Parameters

Abstract: Modeling of two high-current card-edge connectors is presented. The modeling methodology consists of dividing the connectors into uniform segments and modeling each segment by a multiconductor transmission line. The calculation of the per-unit-length parameters of each segment is based on the analytic expression for a generalized capacitance matrix of widely separated conductors. The proposed modeling methodology generates the circuit model solely from the geometrical parameters of the connectors without the need to perform measurements and/or electromagnetic simulations. The modeling methodology is applied to two 62-pin high-current card-edge connectors that are used in the measurement setup for characterization of radiated electromagnetic emission of switched-mode power converters. Two printed circuit boards are designed to characterize the connectors and to validate the modeling methodology. The model of the complete characterization setup shows good agreement with the $S$ -parameter measurements up to 3 GHz.

Reliable, Fast and Reusable Interfacing of High-Frequency Signals to Disposable Lab-on-a-Chip Devices

Abstract: This work presents a novel manner of interconnecting high-frequency signals from laboratory instruments to a disposable lab-on-a-chip chip with an interconnect box (ICB). The ICB can connect with a standard coaxial connector to microwave lab equipment and uses a spring levered interface with elastomer conductive pins to connect to the disposable chip. With the spring-system, a new microwave-microfluidic chip 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 gold-on-quartz technology and casting of polydimethylsiloxane was used to develop an interdigital capacitor (IDC) for microwave sensing and heating for continuous flow microfluidics. Different isopropyl alcohol-water mixtures were flushed through the channel over the IDC and sensed based on the measured reflection coefficient measurements of the interconnected IDC on quartz. Using calibration techniques, the dielectric properties were extracted and good agreement with values in literature was obtained. At last, a deionized water sample of $2\mu \mathbf{L}$ was heated with heating rates of 50° C/s.

Electrically Controlled Tunable Broadband Interferometric Dielectric Spectroscopy: Groundwork for Single Cell Analysis

Abstract: An electrically controlled tunable interference based microwave sensor is demonstrated to perform time domain detection of single particles and measure complex permittivity of cell culture mediums within the broadband range of 12–18 GHz. Following set up is required to construct deep destructive interference nulls. Quadrature hybrid (QH), voltage controlled phase shifter and attenuator are utilized for each of the two branches. These comprise a reference branch and a branch containing the material-under-test (MUT). Coplanar waveguide (CPW) design was used as the microwave sensing structure and microfluidic wells were manufactured with polydimethylsiloxane (PDMS) to contain MUT, reference and calibration materials. The capability of detecting a single particle (diameter of 6-6.4um) is reported compared to simulations. Cell culture mediums measurements of Luria Agar (LA) and Luria Broth (LB) medium over the above-mentioned bandwidth were performed to demonstrate the operation processes and lay the groundwork for future research.

A Transmission Line Based Microwave Heater at 25 GHz for Continuous Flow Microfluidics Fabricated on Silicon

Abstract: In this work, a co-planar waveguide (CPW) microwave transmission line on high-resistivity silicon is employed as a microwave heater at 25 GHz to heat a sub-microliter volume of 0.23 μL. The CPW heater and fluidic layout are co-designed using COMSOL Multiphysics to achieve rapid, contact-less and uniform heating. The manufactured device is bond-wired and experimentally tested using the simultaneous microwave excitation and optical investigation. Heating performance is evaluated using a fluorescent mixture of Rhodamine B and deionised water loaded in the fluidic channel. A heating rate of 12°C/s is achieved with 1.58 W of power at 25 GHz and the heating uniformity of ±2°C is maintained during 10 seconds of heating.

Broadband Determination of Liquid Mixing Ratio through One-Port Microwave Measurements

Abstract: This paper presents a method for determining binary mixing ratio of two liquids for microfluidic applications. The liquid mixtures are measured through sensor constituted by interdigital capacitor. Dielectric permittivity of the fluid is derived from the high-frequency capacitance measurements. Finally, the mixing ratio is calculated using frequency-dependent modified Bruggeman formula. The method is purely measurement-based and does not require electromagnetic simulations. The error for determining the mixing ratio is below 5% at frequencies above 1 GHz for water and isopropanol mixtures.

Broadband Electrical Determination of Liquid Mixing Ratios for Microfluidics

Abstract: The paper shows a microwave method to determine mixing ratio of fluids in a microfluidic channel. The method uses modified Bruggeman formula that is fitted over a broad range of frequencies. The extraction of mixing ratio is based solely on the measurement results without any reference to EM simulation, which alleviates the requirements for chip fabrication accuracy. The procedure was tested with three lengths of microfluidic channels over CPW lines, and the mixing error was determined to be less than 5% error for at least 1.5 GHz frequency range. For some frequencies the maximum error drops to below 3%.

Microwave Dielectric Sensing for Sample Preparation in Digital Microfluidics

Abstract: This paper reports on uniplanar interdigital capacitors employed for dielectric sensing in digital microfluidics. Two interdigital capacitors on a microwave chip, a quadrature coupler and a phase shifter were used to build a tuneable 2-port network for glucose concentration extraction in glucose-DI water mixture using the interference principle. One capacitor in the setup was used as a sensor, while another capacitor was used as a reference. Microwave measurements were carried out at 4.5 GHz using the tuneable 2-port network for reference liquids being deionised water and a glucose-deionised water mixture. Measurement results demonstrated an accurate extraction up to 0.1 M glucose concentration.

Robust One-Tier Calibration for Microwave Microfluidics using Unknown Liquids

Abstract: Microwave measurements in microfluidic systems require accurate calibration methods. Unfortunately, this is inherently ill-conditioned problem prone to any error sources. This work presents a robust method of one-tier calibration, which minimizes the uncertainty by requiring only one connection. The calibration standards are realized by flushing different liquids through the microfluidic channel. The electrical parameters of these liquids do not have to be known, as they are determined during the calibration procedure. This alleviates the requirements for the manufacturing process, liquid purity, and measurement conditions, which is shown to significantly decrease the calibration error when comparing to corresponding method assuming perfect knowledge of standards.