Infection diagnosis and antibiotic susceptibility testing (AST) are time-consuming and often laborious clinical practices. This paper presents a microwave-microfluidic biosensor for rapid, contactless and non-invasive device for testing the concentration and growth of Escherichia Coli (E. Coli) in medium solutions of different pH to increase the efficacy of clinical microbiology practices. The thin layer interface between the microfluidic channel and the microwave resonator significantly enhanced the detection sensitivity. The microfluidic chip, fabricated using standard soft lithography, was injected with bacterial samples and incorporated with a microwave microstrip ring resonator sensor with an operation frequency of 2.5 GHz and initial quality factor of 83 for detecting the concentration and growth of bacteria. The resonator had a coupling gap area on of 1.5 × 1.5 mm² as of its sensitive region. The presence of different concentrations of bacteria in different pH solutions were detected via screening the changes in resonant amplitude and frequency responses of the microwave system. The sensor device demonstrated near immediate response to changes in the concentration of bacteria and maximum sensitivity of 3.4 MHz compared to a logarithm value of bacteria concentration. The minimum prepared optical transparency of bacteria was tested at an OD₆₀₀ value of 0.003. The sensor’s resonant frequency and amplitude parameters were utilized to monitor bacteria growth during a 500-minute time frame, which demonstrated a stable response with respect to detecting the bacterial proliferation. A highly linear response was demonstrated for detecting bacteria concentration at various pH values. The growth of bacteria analyzed over the resonator showed an exponential growth curve with respect to time and concurred with the lag-log-stationary-death model of cell growth. This biosensor is one step forward to automate the complex AST workflow of clinical microbiology laboratories for rapid and automated detection of bacteria as well as screening the bacteria proliferation in response to antibiotics.

A planar microwave resonator sensor is designed, customized, and fabricated to detect coating breaches in industrial steel pipelines. The sensor, which utilizes a ring-shaped resonator to maximize the sensitivity at its core, is tuned to 2.5 GHz with a quality factor of 280. In the setup, the sensor is grounded to a piece of steel pipeline with an Epoxy-100 coating, which provides the substrate beneath the microstrip structure. It is demonstrated that any change in the gap height between the substrate layer and the pipeline, from 0 to 3.5 mm, produces a significant resonant frequency variation and bandwidth change in the sensor's response. The sensor structure demonstrates sensitivity and selectivity to air and water penetration to the breach. The sensor structure described in this work is a compact, low-cost solution and has potential for further miniaturization in mobile applications which may serve as a method for pipeline breach detection.

Sensitivity and resolution are important parameters that pose a significant challenge in using microwave microfluidic sensors to monitor low-concentration binary liquid mixtures. This work presents a microwave split-ring resonator (SRR) sensor equipped with a substrate-embedded fluidic channel and an active feedback loop to enhance sensitivity and resolution, respectively. The substrate-embedded channel eliminated the impact of the liquid-carrying tubes’ wall and maintained a significant interaction between the liquid in the channel and the electromagnetic (EM) field confined between the SRR and ground plane. Therefore, embedding the channel inside the substrate of the SRR sensor, operating at 2.63 GHz, enhanced the sensitivity by 60%. The constructive energy from the active feedback loop boosted the quality factor of the passive SRR by over $100\times $ (from 116 to 13 000), which significantly enhanced the resolution of the sensor. As a proof of concept, two prototypes of the sensor having different substrate-embedded channels with cross-sectional diameters of 1 and 0.762 mm were fabricated and used to detect low concentrations of ethanol (0–0.08 vol%) and salt (0–200 mM) in water. Embedding the channel inside the substrate of the active SRR sensor achieved a linear correlation ( $R^{2} = 0.99$ ) between the resonant frequency and the concentration of ethanol and salt in water with high sensitivity (19.95 MHz/vol% for ethanol and 4.35 kHz/mM for salt) and resolution (0.001 vol% for ethanol and 2.298 mM for salt). The achieved results demonstrate the feasibility of using the active SRR microwave sensor with a substrate-embedded channel for monitoring ethanol in fermentation broths and electrolytes in human sweat.

Ice accretion causes problems in vital industries and has been addressed over the past decades with either passive or active de-icing systems. This work presents a smart, hybrid (passive and active) de-icing system through the combination of a low interfacial toughness coating, printed circuit board heaters, and an ice-detecting microwave sensor. The coating’s interfacial toughness with ice is found to be temperature dependent and can be modulated using the embedded heaters. Accordingly, de-icing is realized without melting the interface. The synergistic combination of the low interfacial toughness coating and periodic heaters results in a greater de-icing power density than a full-coverage heater system. The hybrid de-icing system also shows durability towards repeated icing/de-icing, mechanical abrasion, outdoor exposure, and chemical contamination. A non-contact planar microwave resonator sensor is additionally designed and implemented to precisely detect the presence or absence of water or ice on the surface while operating beneath the coating, further enhancing the system’s energy efficiency. Scalability of the smart coating is demonstrated using large (up to 1 m) iced interfaces. Overall, the smart hybrid system designed here offers a paradigm shift in de-icing that can efficiently render a surface ice-free without the need for energetically expensive interface melting.

Microwave resonator sensors are attractive for their contactless and label-free capability of monitoring bacterial growth in liquid media. This paper outlines a new label-free microwave biosensor based on a pair of planar split ring resonators for non-invasive monitoring of bacterial growth on a solid agar media. The sensor is comprised of two split ring resonators with slightly different resonant frequencies for differential operation. The transmission coefficient (S21) of the sensor is considered as the sensor's response with a designed and measured quality factor above 200 to ensure a high-resolution operation of the biosensor. Two resonant frequencies of 1.95 and 2.11 GHz represent the sensing signal and the reference signal, respectively. The developed sensor demonstrates high performance in monitoring the growth dynamics of Escherichia coli (E. coli) on Luria-Bertani (LB) agar with 4 mm thickness. The sensor's resonant amplitude response demonstrated 0.5 dB variation corresponding to the bacterial growth over 48 hours when bacteria were spread on LB agar starting with initial OD600 = 1.5. Moreover, 0.6 dB change in the sensor's response was observed over 96 hours of bacterial growth starting with an initial OD600 = 1.17 spotted on LB agar. The measured results fit well to the curves created using Richards' bacterial growth model, showing the strength of the sensor as a potential candidate for use in predictive food microbiology systems.

In this letter, we present a novel sensor based on a planar resonator embedded within a 3-D printed fluidic channel for liquid sensing applications with a maximum detectable resolution of 1000 ppm (0.1 V%) ethanol in water. The fabricated sensor operates between 5.3 and 5.8 GHz with a quality factor of 116 and a resonant amplitude of −15 dB. The resonator sensor embedded within the fluidic channel enables 50- $\mu \text{m}$ proximity between the resonator and liquid material inside the channel. The encapsulating polymer material also protects the resonator’s ring from positional drift, allowing consistent measurements to be made easily without the need for complex fabrication processes. The sensor measured ethanol and water mixtures with a sensitivity of 2.5 MHz per ethanol volume percentage with a resolution of 0.1% ethanol. In addition to improved accuracy and sensitivity, the sensor device is affordable, easy to design and fabricate for a range of frequencies, and environmentally friendly with the continuous reusability.