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In this paper, we demonstrate the optimal multi-modes for glucose droplet (GD) sensing depending on sample (glucose droplet) positions in a half-wavelength ( $\lambda $ /2) microstrip resonator. Using two different GD positions, i.e., an edge and a center position, in the resonator, the resonant behavior of microwave multi-modes for GD sensing was investigated. GDs with three different concentrations, i.e., 0.1, 0.2, and 0.3 g/ml, were tested in the present experiment. With increasing GD concentration, the ${S}_{21}$ -level of the edge-positional GD gradually was declined at all resonant modes. For a center-positional GD, the frequency not only was deviated into the high-frequency region, but its ${S}_{21}$ -level also gradually was declined for the second and fourth modes. Based on the experimental results, the resonant behavior indicated the edge-(center-) positional GD had the resolution (~0.01/gmL−1 or ~0.05 dB/gmL−1) at the second (second) and third (fourth) modes, respectively. Furthermore, we showed that the edge-positional GD sensing is more suitable when the resonator is associated with an active circuit system, and simultaneous sensing is also possible for different positions in multi-modes.

In this study, the microwave, i.e. 0.5–10 GHz, transmission characteristics of carbon nanofiber (CNF) with three different micrometer-scale thicknesses were experimentally investigated using a coplanar waveguide transmission line. In the experimental results, when the film of CNF was thick, the signal transmission level (S21-magnitude) was significantly lower and its phase (S21-phase) was shifted toward the low-frequency region. Based on the obtained S21-parameter (S21-magnitude and S21-phase), the electric permittivity (e) of CNF was extracted and showed clear differences depending on the thickness, i.e., ethin = 1.20 (0.28), emiddle = 1.84 (0.60), ethick = 6.30 (1.15) at 0.5 GHz (7.8 GHz). From the analysis of electromagnetic fields, the microwave conductivity (σmw) of CNF linearly increased with the increasing thickness, i.e., σmw/thin = 0.35 (S/m), σmw/middle = 0.58 (S/m), σmw/thick = 0.75 (S/m), due to enhanced electromagnetic field coupling between the film of CNF and the CPW line. As a result, we demonstrated that the film of CNF has a significant attenuation effect on signal transmission in the microwave regime, depending on micrometer-scale changes in film thickness.

In this paper, a microwave fluidic glucose sensor based on dual complementary split ring resonators (CSRRs) with a switching circuit is proposed for non-invasive and continuous glucose concentration detection. A CSRR has been designed to detect subtle changes in the dielectric constant of glucose solutions, and the electric field distribution is utilized to determine the effective sensing region. In a well-controlled environment, the glucose concentration can be detected by tracking the transmission coefficient of the designed CSRR. However, because the variations in dielectric constant with the glucose concentration are very small, the environmental factors (i.e., ambient temperature and relative humidity) can be dominant compared to the glucose concentration. To qualify the effect of the environmental factors, the variation in the transmission coefficient of the CSRR has been measured when the ambient temperature condition is varied from 293 K to 313 K and the relative humidity is varied from 30% to 90% inside temperature- and humidity-controlled chamber. Moreover, the environmental effects are calibrated out using two identical CSRRs with the switching circuit: one detects changes in the glucose concentration, while the other operates as a reference for environmental factors. After the elimination of the environmental effects, glucose concentrations in the range of 0 mg/dL to 400 mg/dL have been measured, and the sensitivity is 0.008dB/(100mg/dL). In addition, the reproducibility of the proposed sensor is verified by repeated measurements. It is demonstrated that the proposed sensor can detect glucose concentrations under practical environmental conditions.

In this paper, a microwave fluidic glucose sensor based on a microwave resonator coupled with an interferometric system is proposed for sensitivity enhancement. The proposed glucose sensor consists of two parts: a sensing part and a sensitivity enhancement part. The former is composed of a rectangular complementary split ring resonator (CSRR), and the latter is composed of a variable attenuator, a variable phase shifter, two hybrid couplers, and an RF power detector. Because the variation in the electrical properties, which is utilized in the microwave detection scheme, with glucose concentration over the possible concentration range in the human body is very small, improvement of the sensitivity is critical for practical use. Thus, the effective sensing area of the rectangular CSRR is determined by considering the electric field distribution. In addition, magnitude and phase conditions for the effective sensitivity enhancement are derived from a mathematical analysis of the proposed interferometric system. In the present experiment, aimed at demonstrating the detection performance as a function of the glucose concentration in the range of 0 mg/dL to 400 mg/dL, the sensitivity is significantly improved by 48 times by applying the derived conditions for effective sensitivity enhancement. Furthermore, the accuracy of the proposed glucose sensor for glucose concentrations at a step of 100 mg/dL is verified by the Clarke error grid. Based on the measurement results, the proposed glucose sensor is demonstrated to be applicable to noninvasive and continuous monitoring in practical environments.

In this paper, a relative humidity monitoring sensor based on an active resonator with PEDOT:PSS is studied in the microwave regime. The proposed active resonator is patterned on a printed circuit board and excited by electromagnetic field coupling. The active resonator consists of two parts: a passive core resonator and an active circuit. The passive core resonator senses the humidity via the PEDOT:PSS conducting polymer film, which is located in the area of the passive core resonator with the strongest electric field. The active circuit compensates for the loss of the passive core resonator to enhance the quality factor and thus improve the sensitivity of the relative humidity sensor. By combining the passive core resonator with the active circuit, the active resonator exhibits a high quality factor (2887), which is 90 times that of the passive core resonator alone. Moreover, the sensitivity of the proposed humidity sensor is improved when using the active resonator. To verify the performance of the proposed humidity sensor, a commercial sensor is placed alongside the proposed sensor in a well-controlled environment chamber. According to the experimental results, as the relative humidity increases, the transmission coefficient (S21) increases, while the resonance frequency decreases. As a result, the sensor exhibits an increase in S21 of 3.58 dB and a decrease in the resonance frequency of 8.85 MHz when the relative humidity changes from 30% to 85%.

In this paper, we characterize the thin conducting polymer film, PEDOT: PSS, using a microstrip line in a humid environment at microwave frequencies. To characterize the thin film alone over the microstrip line, a de-embedding technique of thru-reflect-line (TRL) calibration is utilized. After the de-embedding procedure, the thin film is modeled with a transmission line, and the film’s R, L, G, and C parameters are extracted from measured S-parameters. In addition, the characteristics of the thin film has been tested in terms of electrical conductivity with increasing relative humidity (RH). In the future, it is expected that this work can contribute to the improvement of stability as well as performance of conducting polymer-based electronic sensing devices.

This review paper focuses on radio-frequency (RF) biosensors for real-time and continuous glucose sensing reported in the literature, including our recent research. Diverse versions of glucose biosensors based on RF devices and circuits are briefly introduced, and their performances are compared. In addition, the limitations of the developed RF glucose biosensors are discussed. Finally, we present perspectives on state-of-art RF biosensing chips for point-of-care diagnosis and describe their future challenges.

This study aims to investigate the microwave properties of coplanar waveguide (CPW)-based poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) conducting polymer line in an ethanol gas atmosphere, with the frequency range of 0.5&ndash, 2 GHz. For an ethanol-exposed PEDOT:PSS line (test sample), the transmission coefficient (S21) decreased immediately, moreover, the microwave effective conductivity (&sigma, m/w) decreased simultaneously, compared with the ethanol-free PEDOT:PSS line (reference sample). The immediate variations in &Delta, S21 ( = S21,ethanol &minus, S21,free) and &Delta, &sigma, m/w ( = &sigma, m/w,ethanol &minus, m/w,free) were approximately 10.2 dB and 2.7 &times, 104 S/m, respectively. Furthermore, in the analysis of the circuit model of the PEDOT:PSS line, the characteristic impedance and distributed elements, i.e., resistance (R) and inductance (L) per length, of the test sample increased, compared with the reference sample. However, upon stopping the exposure to ethanol gas, the microwave properties of the test sample instantaneously recovered to those of the reference sample. According to these critical observations, we could confirm that the coplanar waveguide with a PEDOT:PSS line shows a significant difference in the diverse microwave properties, through rapid response to the ethanol gas at room temperature.

This Chapter describes the methods of a proximity coupling RF vital sensor. The methods are mainly divided into two categories: the chest wall sensor and the wrist pulse sensor. For each part, the theoretical considerations, simulation results, and experimental results on methods for efficiently enhancing the sensitivity of the sensors are described in detail. In addition, we introduce an autoregressive (AR) model which is one of the parametric spectral analyses suitable for vital sign detection.

In this paper, the advances in radio-frequency (RF)/microwave biosensors based on graphene nanomaterials including graphene, graphene oxide (GO), and reduced graphene oxide (rGO) are reviewed. From a few frontier studies, recently developed graphene nanomaterials-based RF/microwave biosensors are examined in-depth and discussed. Finally, the prospects and challenges of the next-generation RF/microwave biosensors for wireless biomedical applications are proposed.

In this review, the advances in radio-frequency (RF) /microwave chemical gas sensors using conducting polymers are discussed. First, the introduction of various conducting polymers is described. Only polyaniline (PANi), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), which are mainly used for gas sensors in RF/microwave region, are focused in this review. Sensing mechanism of the three conducting polymers are presented. And the RF/microwave characteristics and RF/microwave applications of the three conducting polymers are discussed. Moreover, the gas sensors using conducting polymers in RF/microwave frequencies are described. Finally, the the challenges and the prospects of the next generation of the RF/microwave based chemical sensors for wireless applications are proposed.

In this paper, a gas sensor using a modified hybrid coupler structure with conducting polymer which operates at 2.4 GHz is represented. Conducting polymers are used to the gas sensing material in proposed sensors. The conducting polymer varies its electrical property, such as work function and conductivity corresponding to the certain gas. To verify this variation of electrical property of conducting polymer at microwave frequencies, the conducting polymer is incorporated with the hybrid coupler structure, and this proposed sensor operates as reflection type variable attenuator and variable phase shifter. The conducting polymer is employed as impedence-variable transmission lines that cause a impedance mismatching between the general transmission line and conducting polymer. The experiment was conducted with 100 ppm ethanol gas at temperature of and relative humidity of 85 %. As a result, the amplitude deviation of is 0.13 dB and the frequency satisfying ${\angle}S_{21}

We present improved RF circuit modeling of a biosensing element based on a single-walled carbon nanotube combined interdigital capacitors at microwave frequencies. From the resultant circuit, the lumped element values for biomolecular binding are accurately obtained. It is thereby found that the completed RF circuit model shows excellent agreement with measured results. This implies that the electrical properties of a specific biomolecular binding system can be quantitatively analyzed if an optimal RF circuit model is constructed. Finally, we suggest that the suggested methodology can be used to analyze other biomolecular sensing methods.

We report the radio-frequency transmission properties and distributed elements of chemical vapour deposition (CVD)-grown graphene monolayers with width variation using a coplanar waveguide electrode in the microwave region. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This article investigates the high frequency characteristics of coplanar waveguide (CPW) lines fabricated with inkjet-printing technology on BT substrates. The optimal metallic thickness of a printed CPW line is approximately estimated to be 3 μm. When printed lines with higher electrical conductivity are realized through additional surface preparation steps, the findings of this study suggest that the RF performance level of these printed lines will be comparable to that of the copper clad laminate lines in the microwave frequency region. Finally, aspects of such improvements as well as applications of these types of printed lines are discussed. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 780–782, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24987

In this paper, we demonstrate the sensing feasibility of the proposed high-Q resonator using a phosphate-buffered saline droplet at microwave frequencies. In the experimental results, the resonant frequency, signal level, and Q-factor of the S21-parameter with and without a 1-μl droplet were changed to about 230 MHz, 32 dB, and 1500, respectively. The resonator system was found to be suitable for droplet sensing with a small volume due to its small and compact scheme. This resonator system is expected to play an important role in droplet sensing with different dielectric constants.

We present the transmission properties of graphene monolayer for three different coplanar waveguide electrode configurations at microwave frequencies. It is confirmed that the transmission level can be dependent on the ground shape due to electromagnetic coupling effect between graphene and ground pad in the CPW electrode. Among the three CPW electrodes with and without graphene, the CPW electrode having a deep ground pad shows the most superior transmission characteristics. From the resultant, we know that it is very important to consider the graphene as well as RF electrode design for the desired transmission level.

In this paper, a feasibility of using double split ring resonator (DSRR) having negative permeability as a biosensor has been demonstrated. The compact DSRR has been excited by a microstrip transmission line, and the DNA hybridization in which single-stranded deoxyribonucleic acid (ss-DNA) is coupled with a complementary deoxyribonucleic acid (c-DNA), it is recognized with change in S 21 resonant frequency. When ss-DNA is immobilized onto gold (Au) surface and then is coupled with c-DNA, the resonant frequency has been changed by Δƒ ss-DNA =20 MHz and Δƒ hybridization =60 MHz for two cases. It is clear that DSRR can be utilized as a DNA sensing element at microwave regime.

While inkjet printing technology shows a great potential for fabricating RF microwave circuitry and interconnects, the additional work is needed. Here we have studied the copper metal nanoparticle ink as a metal for printing coplanar waveguides transmission lines on BT substrates. Although the RF efficiency of the printed CPW lines exhibits an overall some lower characteristics than that of the CCL CPW lines, the improvements in printing techniques and the surface preparation will provide for lower conductor losses and then the printed line will be almost a match for CCL in RF performance. We confirm that the inkjet printing shows a very promising future for ultra-low-cost printing of passive microwave circuitry and interconnects. Low cost RF MEMS and CNT sensor device are also envisioned in the future of inkjet printing.

In this paper, we present a high quality (Q)-factor resonator utilizing biocompatible materials, thin glass and polydimethylsiloxane, at microwave regime. The proposed resonator is completely matched with a 50-ohm characteristic impedance via the materials. From the resulting matching, we find that it shows a half-wavelength resonance having an extremely high quality-value. Actually, the loaded quality-factor is about 1 670 at the resonant frequency of 12.52 GHz. From this work, we suggest that our device can be a good candidate as a simple, cost-efficient, and non-contact sensor device based on frequency and quality-factor variation.

In this paper, we have quantitatively evaluated the effective surface conductivity of chemical vapor deposition-grown graphene through a full-wave electromagnetic method and also investigated the intrinsic characteristics of the transmission line (TL) of the graphene at frequency ranging from 0.5 to 40 GHz. According to the simulated data based on the measured S-parameters, the effective conductivity of single- and multi-layer graphene (MLG) was about 4.3 × 106 S/m and 1.2 × 106 S/m, respectively. Furthermore, we confirm that multi-layer graphene is more suitable for use in transmission lines compared to single-layer graphene in the observed frequency region.

In this paper, the feasibility of utilization of a single element double split-ring resonator as a biosensing device has been demonstrated. The compact resonator has been excited by time-varying magnetic fields generated from the 50 Ω microstrip transmission line. In this work, DNA hybridization is recognized with shift in S21 resonant frequency. When thiol-linked single stranded-DNA is immobilized onto a gold (Au) surface and is then coupled with complementary-DNA, the frequency changes by Δfss-DNA=20 MHz and Δfhybridization=60 MHz, respectively. Thus, it is clear that the resonator can be utilized as a DNA sensing element in the microwave regime.

A split-ring resonator (SRR) array is experimentally demonstrated for a biosensing device at microwave frequencies. Each SRR in the array is excited by a time-varying H-field component through a microstrip transmission line in which microwaves propagate in the quasitransverse electromagnetic mode. It is found that the resonant frequency changes due to the binding of biotin and streptavidin onto the surface of the SRRs. The observed change values represent around ΔfB=120MHz and ΔfB-S=40MHz, respectively. Finally, the SRR-based biosensing device suggests a few improvements for increasing sensitivity and describes its possible application.