A noninvasive contactless binary gas sensing system is enabled using coupled passive flexible tag integrated with porous adsorbent beads tower. The platform is based on two flexible passive tags coupled with a dual-reader resonator operating at 4.86 GHz, while the distance between passive reader and tags can be extended to 10 mm. The sensor demonstrates the capability of detecting and monitoring more than one gas simultaneously and independently from each other. Various concentrations of methyl ethyl ketone (MEK) and humidity have been detected distinctively by using a hyper cross-linked polymer as volatile organic compounds (VOCs) and silica gel as humidity adsorbent beads. Selective accumulation of the gas molecules in the beads' pores is monitored with less than 5% error and by separate coupled tags and their associated resonant frequencies. The proposed structure offers a low-cost conformal solution and has the potential for real-time contactless multiple gas sensing in environmental applications. The structure demonstrates the sensitivity of 0.18 MHz/ppm in VOC sensing and 0.19 MHz/ppm while performing selective binary sensing.

Here, the feasibility of using a non-contact high-resolution microwave sensor for the detection and mitigation of High pH Stress Corrosion Cracking (HpHSCC) through monitoring the pH range beneath the dsibonded coating in buried pipeline steel has been investigated. A modelling method and microwave dielectric sensing for aqueous solution has been studied in order to relate relative permittivity and loss to the pH level using the concentration as a bridge. The experimental results showed the potential of the high quality-factor sensor for monitoring the pH level variation. The resonant frequency of the microwave sensor was the main variable considered in the characterization of the sensor's response to pH level changes or concentration variations in the defect beneath the pipe coating. Additionally, the extracted experimental results for near-neutral and acidic pH environment detection demonstrated the significance of non-contact sensing performance and the sensor's potential to detect and study the pH variation. The results demonstrate significantly distinct frequency shift of 174 kHz as the pH increases from 7 to 11 which is the range of the pH created in HpHSCC. This method, as an example, could be employed in the third stage of Stress Corrosion Cracking Direct Assessment (SSCDA), Direct examination step, to determine whether breeding ground for HpHSCC is created or not.

In this article, a new coupled structure based on microwave planar resonators is proposed to demonstrate the capability of ultrahigh quality factor performance using a chipless passive resonator. The platform is based on two passive split-ring resonators (SRRs), one as a reader, and the other one as the tag. The reader resonator is designed to operate at 2.6-GHz resonance frequency and is coupled to an active feedback loop with a microwave amplifier to compensate for the resonator's loss and increase the loaded quality factor of the response. The bandwidth of the feedback amplifier is modified such that the amplifier's gain is higher at the resonance frequency of the tag than that of the reader by adding a capacitor between emitter and collector of the bipolar junction transistor (BJT) amplifier. The tag is designed at 1.6 GHz and is located at a 2.5-mm vertical distance from the reader. As a result, the overall quality factor of about 75 000 is achieved for the tag performing the sensing. The presented technique provides a great practical solution for highly sensitive, noninvasive, and real-time sensing applications. The proposed sensing tag is integrated with a microfluidic chip to highlight its capability for small volume sensing and lab-on-a-chip applications. The sensitivity potential of the design is verified by detecting the concentration of acetone in deionized water. The average sensitivity of the presented sensor is more than 35 kHz/(1% of acetone concentration variation), which is offering extremely high sensitivity of the structure considering the very small volume of the exposed material under the test and the distance between the sensor and the sample.

This research study reports on a new coupling structure to demonstrate a constant coupling strength for longer distances in planar microwave sensors. It is shown that there is a positive correlation between the gap size in a pair of gap-coupled transmission lines (GCTLs) and the effective distance of reader and sensing resonators in a planar microwave sensor. On the other hand, the coupling strength in a pair of GCTLs is negatively correlated to the gap size between the two lines, where the larger gap sizes result in weaker coupling strength or vice versa. A technique is introduced to make the coupling strength partially independent of the gap size in GCTLs. The technique enables a sensor designer to increase the gap size, and subsequently, the distance between the reader and sensing resonators of a sensor. Strong electromagnetic coupling is locally demonstrated between a pair of GCTLs using a new artificial perfect electromagnetic conductor (PEMC) boundary, which does not allow electromagnetic power reflect toward the source over larger gap sizes in GCTLs. The concept is experimentally verified with a gap size up to 10 mm between GCTLs with a constantly measured insertion loss of 0.9 dB. Moreover, the structure is employed to design distant microwave resonator for humidity sensing applications. The design has demonstrated the ability of sensing, while the sensing resonator is placed at 35-mm distance from the readout circuitry. The structure is reported to increase the sensing distance with a ratio of about 12 (36:3) compared to the conventional microwave sensors.

Glycerol concentration measurement is one of the most important biomarkers for many diagnostic applications, such as indication of hyperglyceridaemia and coronary heart disease. A new distant microwave sensing platform to monitor the concentration of glycerol in a water and serum solution inside a microfluidic channel is presented. The sensor is based on a microstrip line reader with a defected ground plane coupled to a chipless microwave resonator tag. Due to the strong coupling between the reader and the tag, the distance between them can be increased up to 45 mm. The high level of sensitivity of the structure makes it suitable for nanolitre volume scale chemical sensing inside the microfluidic channel. The sensor demonstrates a frequency shift of 6 MHz when the volumetric percentage (V%) of glycerol in DI water changes from 70% to 0%. The proposed solution provides an opportunity to integrate the chipless passive tag with the microfluidic channel while the reader is distant from the microfluidic system.

Microwave split-ring resonators are utilized as sensors in a wide variety of applications due to their remarkable features, such as extremely low cost, high sensitivity, and relatively high quality factor. In this article, another application is enabled according to a recently demonstrated chipless tag-reader structure providing the possibility of simplifying the sensor structure from a “multilayer structure” consisting of a dielectric substrate sandwiched between two metallic layers to a single-layer structure formed from a conductive material. This capability is specifically important for strain sensing applications as it brings the possibility of utilizing low stiff conductive materials instead of copper (which is the primary material used in microwave application) while keeping the reader structure with high-quality microwave application-specified substrates intact. With the explained approach in this work, a low tensile silver-aluminum silicone elastomer conductive material is considered for the tag providing a very high tensile dynamic range. According to the whole sensing system structure, as high elongation range of as 20% and the high sensitivity in the range of 25 MHz/1% of strain is achieved. Multiple simulations and experimental results support the idea of the novel microwave strain sensor proposed in this work.