In this article, we introduce an array of uncoupled and cascaded microwave sensors with a novel structure for the dielectric characterization of liquid samples for biomedical applications. The developed array consists of four cascaded resonators and can measure the complex permittivity of multiple samples simultaneously and independently, contributing to microwell plate measurements. The fabricated sensors consist of couplers and defected grounded interdigitated capacitors (IDCs), yielding a high sensitivity. Each sensor in the array acts as a resonator, with the resonance frequency and quality factor changing according to the material under test (MUT). We have used water/ethanol mixtures in different concentrations to calibrate the sensor and later relate the changes in resonance behavior to the one in dielectric permittivity. The corresponding fitting functions based on IDC’s model have a high $r$ -squared value and result in a high precision permittivity extraction of the liquids with the volume of 20 $\mu$ L in the frequency range of 3.87 GHz (for the sample with the highest permittivity) to 8 GHz.

This paper presents a highly sensitive microwave dielectric sensor for determining glucose concentration using a novel metamaterial-inspired Interdigital Capacitor (IDC)–Complementary Split-Ring Resonator (CSRR) structure. The sensor’s sensitivity is improved by a factor of 80 when used as a one-port impedance-matched structure compared to a conventional two-port configuration. By utilizing a digitally controlled step capacitor, we were able to achieve impedance matching at the two distinct frequencies of 3.69 GHz and 4.38 GHz while conserving the resonant mode of the IDC-CSRR. Both matched frequencies were then used for glucose concentration measurements, resulting in a maximum sensitivity of 8.727 dB/mg/ml and 53.01 kHz/mg/ml. Principal component regression (PCR) was used to predict glucose concentration with R² of 0.9997 and mean average error (MAE) of only 0.045 mg/ml, achieving a resolution of 0.42 mg/ml, corresponding to the minimum detected concentration difference between the two measured samples. The proposed setup demonstrates substantial improvements over conventional two-port sensors, surpassing the performance of more complex interferometric systems.

This work presents the design and evaluation of a planar device for microwave heating of liquids in continuous microfluidics (CMF) made in polydimethylsiloxane (PDMS). It deals with volumes in the µL range, which are of high interest and relevance to biologists and chemists. The planar heater in this work is conceived around a complementary split-ring resonator (CSRR) topology that offers a desired electric field direction to—and interaction with—liquids in a microwell. The designed device on a 0.25 mm thick Rogers RO4350B substrate operates at around 2.5 GHz, while a CMF channel and a 2.45 µL microwell are manufactured in PDMS using the casting process. The evaluation of the performance of the designed heater is conducted using a fluorescent dye, Rhodamine B, dissolved in deionized water. Heating measurements are carried out using 1 W of power and the designed device achieves a temperature of 47 °C on a sample volume of 2.45 µL after 20 s of heating. Based on the achieved results, the CSRR topology has a large potential in microwave heating, in addition to the already demonstrated potential in microwave dielectric sensing, all proving the multifunctionality and reusability of single planar microwave-microfluidic devices.

In this study, complementary split-ring resonator (CSRR) metamaterial structures are proposed for label-free dielectric spectroscopy of liquids in microplates. This novel combination of an array of sensors and microplates is readily scalable and thus offers a great potential for non-invasive, rapid, and label-free dielectric spectroscopy of liquids in large microplate arrays. The proposed array of sensors on a printed circuit board consists of a microstrip line coupled to four CSRRs in cascade with resonant frequencies ranging from 7 to 10 GHz, spaced around 1 GHz. The microwells were manufactured and bonded to the CSRR using polydimethylsiloxane, whose resonant frequency is dependent on a complex relative permittivity of the liquid loaded in the microwell. The individual microstrip lines with CSRRs were interconnected to the measurement equipment using two electronically controllable microwave switches, which enables microwave measurements of the 4 × 4 CSRR array using only a two-port measurement system. The 4 × 4 microwell sensor arrays were calibrated and evaluated using water-ethanol mixtures with different ethanol concentrations. The proposed measurement setup offers comparable results to ones obtained using a dielectric probe, confirming the potential of the planar sensor array for large-scale microplate experiments.

With widespread applications in a variety of disciplines, mainly biology and medicine, Polymerase Chain Reaction (PCR) technology has established itself as one of the most significant discoveries of the last 100 years. However, the primary drawback of commercially available PCR instruments is their slow thermal cycling. On the other hand, rapid and efficient microwave (MW) heating offers a viable solution to drastically decrease the time needed for PCR experiments. In this study, we utilize a Complementary Split Ring Resonator (CSRR), operating as a microwave heater at around 3.75 GHz when combined with a microfluidic structure with a 5.4 $\mu$ l volume. The resulting device exhibits excellent temperature uniformity with high heating and cooling rates of 19 $^\circ$ C/s and 18.6 $^\circ$ C/s, respectively. Furthermore, model-based frequency-adaptive MW heating was investigated based on optimal heating frequency shift due to the temperature increase of the sample during MW heating, yielding 1.2 W lower applied power and an 8% higher heating efficiency when compared to fixed-frequency heating.