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Tissue dehydration is known to have adverse effects on health and is highly challenging to characterize in vitro due to the complexity of the cell/tissue organisation. There have been a large number of studies reporting the dielectric properties of animal and human tissue explants or bodily fluids such as urine or blood over a wide frequency range from 10 Hz to 100 GHz. However, variations in blood composition involving the cell type, haematocrit (HCT) protein levels, ionic conductivity, electrolyte or glucose concentration will affect the dielectric measurements. The present study examined the effects of physiological osmotic solutions with NaCl (280 mOsmol/kg to 300 mOsmol/kg), HCT (35% to 50%) and albumin (3.5 g/dl to 5.5 g/dl) isolated from bovine blood on the dielectric properties measured at frequencies spanning from 0.5 GHz to 20 GHz using an open-ended co-axial probe. Measurements demonstrated linear correlations between permittivity and loss factor of blood solutions with varying HCT levels, albumin or osmolality such that a high protein concentration reduced the dielectric response in a dose-dependent manner. Whilst the spectral trends for the permittivity response to HCT and albumin were similar in a concentration-dependent manner, the loss factor profile was influenced by osmolality of the solution. In summary, we characterized the dependence of the microwave dielectric properties of blood on HCT, albumin and osmolality. The dielectric measurement technique has the potential to determine hydration levels for future diagnostic and therapeutic devices.

This paper features an effective and novel method to characterize dielectric materials using the reflected group delay of an over-coupled partially-filled coaxial resonator. The method employs a simple and cost-effective approach by utilizing one-port measurements from a Vector Network Analyzer to determine the dielectric constant and loss tangent of a dielectric material filled inside a coaxial resonator. Simulations using Advanced Design System by PathWave and ANSYS Electronics Desktop are done to test the mathematical model under different scenarios. Based on the results obtained from simulations, the proposed method can determine the dielectric constant and loss tangent with an error of less than 2% and 30% respectively, when operated to have a coupling coefficient in the range of 2.5 to 10. A region of confidence is derived by taking into account the effect of systematic errors to determine the maximum expected error in the calculated values of dielectric constant and loss tangent. The region of confidence describes that the resonator when filled to more than 50% of its capacity to have coupling coefficient greater than 2.5 can determine the dielectric constant and loss tangent of a material with an error of less than 5% and 80% respectively. However, by limiting the value of coupling coefficient the error in the loss tangent can be restricted under 30%. A coaxial resonator is designed and fabricated, and Teflon is tested in order to validate the proposed method. A hollow Teflon rod is tested twice for different values of coupling capacitance to confirm the repeatability of the method. Soil samples with different moisture contents are tested to confirm the capacity of the method in testing different types of materials. Each of the soil sample is tested three times and standard deviation in the calculated values of the dielectric properties re-confirmed the repeatability of the method. The proposed method is considered appropriate to test the dielectric properties of solid materials which can be machined to fit inside the hollow cavity of the resonator and other materials like soils, fine particle solids, grains and liquids.

Machine learning models have found their applications in solving problems that can traditionally handle ordinary or partial differential equations. The full-wave simulations solve such equations for functional materials in the microwave regime. These simulations heavily rely on the electrical properties of the materials in predicting microwave characteristics. Currently, most researchers are searching for novel materials and their electrical properties using trial and error methods, for example, microwave absorbing materials, which are used in various civil and military applications. To replace this trial and error method, there is a crucial need for an intelligent system to assist material scientists in fabricating and testing functional materials. This paper proposes a modified Physics-Informed Neural Network (PINN) learning model using an autoencoder with Generative Adversarial Networks (AGAN) and SHapley Additive exPlanations (SHAP) model to assist the researchers in modeling and characterizing any electromagnetic material depending on the user-defined application in a scientifically learned manner with minimum trial and error in selecting the electrical properties of the material. The proposed eXaplainable autoencoder PINN (XA-PINN) algorithm provides a deeper understanding of the decision-making process of microwave absorption frequency bandwidth classification. A proof of concept is demonstrated for dielectric materials by training the model over frequencies from 0.5 to 18 GHz with different permittivity and permeability values. A custom loss is introduced in the proposed XA-PINN model based on the solution of the Riccati equation and mean squared error (MSE).

Conductive polymer composites (CPCs) with nanocarbon fillers are at the high end of modern materials science, advancing current electronic applications. Herein, we establish the interplay between the chemistry and electrophysical properties of reduced graphene oxide (rGO), separately and as a filler for CPCs with the segregated structure conferred by the chemical composition of the initial graphene oxide (GO). A set of experimental methods, namely X-ray photoelectron spectroscopy (XPS), ultraviolet-visible spectroscopy, van der Paw and temperature-dependent sheet resistance measurements, along with dielectric spectroscopy, are employed to thoroughly examine the derived materials. The alterations in the composition of oxygen groups along with their beneficial effect on nitrogen doping upon GO reduction by hydrazine are tracked with the help of XPS. The slight defectiveness of the graphene network is found to boost the conductivity of the material due to facilitating the impact of the nitrogen lone-pair electrons in charge transport. In turn, a sharp drop in material conductivity is indicated upon further disruption of the π-conjugated network, predominantly governing the charge transport. Particularly, the transition from the Mott variable hopping transport mechanism to the Efros–Shklovsky one is signified. Finally, the impact of rGO chemistry and physics on the electrophysical properties of CPCs with the segregated structure is evaluated. Taken together, our results give a hint at how GO chemistry manifests the properties of rGO and the CPC derived from it, offering compelling opportunities for their practical applications.

A technique for solving the one-port closed coaxial transmission line sample holder scattering equation for complex permittivity inversion for lossy materials is presented. A non-linear least-squares procedure is used for the determination of parameters for the specification of the spectral functional form of the complex permittivity. The method allows for accurate retrieval of many low- and high-permittivity dielectric materials in the frequency range of 1 GHz to 3 GHz inserted into the coaxial cell. Using this method, the complex permittivity of a number of liquids and a Maltese soil known as Bajjad soil have been extracted by measurements using a short terminated coaxial transmission line sample holder. The proposed novel inversion method is mainly based on the reflection coefficient of the test material. The measured results of the complex permittivity of liquid dielectrics such as ethanol, methanol, and TX100 are validated and compared with previously published data obtained from measurements made by the National Physical Laboratory (NPL) using a two-port measurement setup made with the same commercial coaxial transmission line sample holder used in the one-port setup. Since the technique allows broadband measurements, it has been used to characterise the soil dielectric spectrum in the frequency range of 1–3 GHz, which is also compared with results from a two-port setup of the same coaxial line. The experimental results are a validation of the proposed approach for different types of materials.

Microwaves can safely and non-destructively illuminate and penetrate dielectric materials, making them an attractive solution for various medical tasks, including detection, diagnosis, classification, and monitoring. Their inherent electromagnetic properties, portability, cost-effectiveness, and the growth in computing capabilities have encouraged the development of numerous microwave sensing and imaging systems in the medical field, with the potential to complement or even replace current gold-standard methods. This review aims to provide a comprehensive update on the latest advances in medical applications of microwaves, particularly focusing on the near-field ones working within the 1–15 GHz frequency range. It specifically examines significant strides in the development of clinical devices for brain stroke diagnosis and classification, breast cancer screening, and continuous blood glucose monitoring. The technical implementation and algorithmic aspects of prototypes and devices are discussed in detail, including the transceiver systems, radiating elements (such as antennas and sensors), and the imaging algorithms. Additionally, it provides an overview of other promising cutting-edge microwave medical applications, such as knee injuries and colon polyps detection, torso scanning and image-based monitoring of thermal therapy intervention. Finally, the review discusses the challenges of achieving clinical engagement with microwave-based technologies and explores future perspectives.

This study describes preparing and characterizing chitosan/polyethylene oxide (Cs/PEO) nanocomposite films incorporated with different zirconia nanoparticles (ZrO2 NPs) weight percentages. The study involved the preparation of nanocomposite films using the casting technique, followed by a detailed analysis of their structural, optical, and dielectric properties. The X-ray diffraction (XRD) and infrared (FT-IR) spectroscopic measurements confirmed the interaction between Cs/PEO and ZrO2 NPs through hydrogen bonding between the polymeric chain and nanoparticles. The XRD analysis also indicated that the amorphosity of Cs/PEO matrices increased with the addition of ZrO2 NPs. The optical characteristics of the nanocomposite films, including absorption edge, band gap, refractive index, optical dielectric modulus (ε′ and ε′′), and optical conductivity (σ), were studied in detail. The results showed that adding ZrO2 NPs led to the creation of localized states in Cs/PEO and ZrO2 NPs, and the energy band gap reduced from 5.5 to 4.8 eV and from 4.4 to 2.7 eV, respectively. The dielectric measurements were performed, and it was found that both ε′ and ε′′ values were increased as the ZrO2 NPs content decreased and the frequency increased for the nanocomposite films. Additionally, the electrical conductivity was increased by adding ZrO2 NPs due to the presence of mobile charge carriers. Measurements of the real and imaginary dielectric modulus revealed that adding ZrO2 NPs improved both parameters. These findings indicate that the Cs/PEO- ZrO2 NPs nanocomposite films exhibit strong potential for use in optoelectronic devices, owing to their enhanced optical and electrical properties.

Dielectric-based measurement techniques have been shown to be very effective in determining the properties of various materials. These techniques have been widely used in a variety of fields and applications. Time Domain Transmission (TDT) techniques have grown in popularity because they are practical, non-destructive, provide measurements in real time and produce accurate measurements that are independent of multiple reflections. TDT techniques, on the other hand, are mostly performed with specialized bulky laboratory equipment, such as a Vector Network Analyzer (VNA) which makes TDT measurements prohibitively costly and unpractical. In fact, few works in the literature have reported portable on-site TDT systems. The aim of this paper is to design and implement a dedicated, compact, and low-cost microwave Time Domain Transmission (TDT) sensor for measuring superficial soil dielectric properties on-site. Our sensor uses a time-delay measurement technique over a microstrip transmission line to estimate the dielectric properties of the soil under test. Measurement results show that the computed mean absolute error (MAE) is less than 1.2 when compared to a calibrated dielectric assessment kit (DAK) with soils containing less than 20 % of water (ε′r

This work deals with the electromagnetic characterization of Carasau bread doughs, in the microwave range region. The microstructure, texture, product quality, and dielectric properties were investigated, jointly with a thorough thermogravimetric analysis. The goal is to link the physical properties with the product quality and to process variables. The variation of the dielectric properties during the dough leavening is studied. The dielectric properties of raw materials (water, yeast, salt, and semolina) were measured from 500 MHz to 8.5 GHz. Given the heterogeneous character of bread microstructure, the possibility of predicting by using mixing formula the final characteristic of a standard Carasau bread dough, manufactured with three different semolina wheat batches, is investigated. A power-law model can predict the average dough properties for frequencies higher than 2.45 GHz. This work proposes a dielectric spectroscopy model for this food material. A third-order Cole-Cole model can describe the dielectric spectra of the Carasau doughs over the entire frequency range. The decrease of the complex permittivity of the dough during leavening can be modeled with a reduction of the static permittivity and a decrement of the electrical conductivity. This study can foster the development of a microwave inline system for leavening monitoring.

This paper proposes a concept of dielectric characterization of low-volume liquid samples using the coupling coefficient of filters. The concept is validated through a two-pole substrate integrated waveguide filter in which the liquid under test is mounted on the coupling section between the two resonators. Unlike the conventional resonator perturbation method reported many times in the literature, this technique uses the coupling coefficient for sensing. The liquid sample is collected in a capillary tube and carefully positioned on the coupling section of the filter; the coupling coefficient of the two resonators varies compared to the relative permittivity of the sample; thus, an empirical model is established. The proposed sensor has been tested to compute the permittivity of different alcohols. Binary solutions of ethanol and water have also been characterized to calculate the volume ratio and relative permittivity as a proof-of-concept. The obtained results show that the proposed sensing technique is capable of characterizing a low quantity of liquids (≈44 µL) with good accuracy, and a worst case measured error of only 6.8% is noted. The ease of integration with other circuitry, low cost, reusability with no deterioration, and adaptability of the proposed sensor makes it a suitable choice for the chemical as well as for the pharmaceutical industry.

Due to their high surface coverage, good adhesion to metal surfaces, and their excellent corrosion resistance, epoxy thermosets are widely used as protective coatings. However, anticorrosion protection of these coatings can be improved against water uptake and can be tuned by changing the chemical nature of the curing agents. In this work, a comparative study has been performed on the water uptake of an epoxy–amine based on bisphenol A diglycidyl ether (DGEBA) cured with an aliphatic amine and the same epoxy initiated with a phosphonium ionic liquid (IL). Thus, the epoxy networks were immersed in saline water solution in a controlled temperature environment. Gravimetric and electric impedance measurements were carried out for a maximum of 3 months. Results were analyzed in order to assess the water diffusion coefficients and water saturation limits. Two models, the Brasher–Kingsbury and a novel mixing rule, were applied on permittivity values. Results highlighted that epoxy–ionic liquid systems are less sensitive to water uptake than conventional epoxy–amine networks. Due to their higher hydrophobic properties the water diffusion coefficient of epoxy–ionic liquid systems are two times less compared to epoxy–amine samples and the water saturation limit is more than four times less. The analysis also shows that the novel mixing rule model proposed here is prone to better estimate the water uptake with accuracy from electrical impedance measurements.

Anemia is a condition that affects over 1.6 billion people worldwide untreated, the disease could lead to increased morbidity and mortality during pregnancy, affecting both the mother and the unborn child. This paper presents the measured dielectric properties of whole blood samples from 176 patients obtained from a hematology clinic; with 80 samples from male patients and 96 samples from female patients. Measurements were performed using a Keysight slim form probe and Keysight network analyzer to obtain the dielectric properties over a wide frequency range (500 MHz-8.5 GHz). A multiple linear regression analysis is performed to identify which components of the blood show the highest correlation with changes in the dielectric properties. Hemoglobin (Hgb) is shown to be the biggest predictor of changes in complex permittivity, demonstrating that permittivity measurements at a single frequency can potentially be used to detect anemia. A support vector machines algorithm is trained and tested to classify between blood samples from healthy patients and blood samples from patients with anemia. The classifier is optimized using a Bayesian-optimization approach during 10-fold cross-validation and then the average performance of the final trained classifier is evaluated through 10-fold testing on unseen data sets. Using a clinical definition of anemia defined as patients having a concentration of Hgb

In the present work, series of Zn ion-doped Ni1−xZnxFe2O4 (0.0≤x≤0.5) ceramics were prepared by the double sintered solid-state reaction route to find out the influence of Zn2+ ions on the crystal structure, lattice structure and dielectric behavior of parent NiFe2O4. X-ray diffraction (XRD) study favors that all the prepared compounds belong to the cubic spinel structure. Lattice parameters found to have increasing value with increased Zn2+ ion substitution. The Raman scattering measurement discerns optical-active modes with blue shift as the doping increases. The dielectric constant (ε′) and dielectric tangent loss (tan δ) decrease with an increase in frequency and at higher frequency, both become constant. Dielectric parameters observe nonlinear behavior with increasing Zn2+ ion substitution. Room temperature dielectric constant of 10% Zn ion-doped NiFe2O4 [Ni0.9Zn0.1Fe2O4] is much higher as compared to other prepared ceramics. The room temperature ac conductivity is found to increase with increase in frequency and temperature-dependent ac conductivity increases with increase in temperature.

A new microwave sensor is proposed to characterize the complex relative permittivity of building non-magnetic materials and used in the characterization of three concrete samples. The proposed sensor structure consists of a log-periodic planar antenna with microstrip elements tilted forward by an angle β and printed, alternately, on the top and bottom sides of a dielectric layer. The operation principle is based on the measurement of the scattering parameters S11 and S21 in a free space propagation transmitter-receiver setup, for both cases with the material under test (MUT) sample (non-line-of-sight, NLOS) and without it (line-of-sight, LOS). A prototype is fabricated and measured to determine the scattering parameters of concrete samples. After measurements, the obtained results are used in the efficient and accurate Nicolson–Ross–Weir (NRW) method, making it possible to estimate the values of the complex relative permittivity of the concrete blocks. The sensor design is demonstrated from initial simulations to measurements for validation of the developed prototype. The obtained results for the complex relative permittivity of concrete are in agreement with those available in the literature and the difference between the simulated and measurement results for the sensor antenna resonant frequency is 4.71%. The used measurement setup can be applied to characterize different types of solid or liquid dielectric materials.

The dielectric properties of biological tissues are typically measured using the open-ended coaxial probe technique, which is based on the assumption that the tissue sample is homogeneous. Therefore, for heterogeneous tissue samples, additional post-measurement sample processing is conducted. Specifically, post-measurement histological analysis may be performed in order to associate the measured dielectric properties with the tissue types present in a heterogeneous sample. Accurate post-measurement histological analysis enables identification of the constituent tissue types that contributed to the measured dielectric properties, and their relative distributions. There is no standard protocol for conducting post-measurement histological analysis, which leads to high numbers of excluded tissue samples and inconsistencies in the resulting reported data for heterogeneous tissues. To this extent, this study examines the post-measurement histological process and the challenges in associating the acquired dielectric properties with the different tissue types present in heterogeneous samples. The results demonstrate that the histological process inevitably alters the morphology of samples, thus introducing errors in the interpretation of the dielectric properties acquired from heterogeneous biological samples. Notably, sample size was seen to shrink by up to 90% through the histological process, meaning that sensing volume determined from fresh tissues is not directly applicable to histology images.

Polymer/metal composites (PMC) comprising of polyvinylidene fluoride/nanocrystalline nickel with varying volume fractions of nickel (fcon) prepared under cold press show an insulator to metal transition (IMT) at percolation threshold (fc=fcon=0.27). The two kinds of generalized Johnscher’s universal dielectric response (UDR) laws on both sides of IMT hold good, while for the percolative sample, none of the two laws hold good. Neither the concept of dipolar relaxation nor anomalous low frequency dispersion stands valid for fc=0.27, while a completely different, neutral and competing electrical behavior is observed over the entire range of frequencies. The emerged third kind of Johnscher’s like UDR for fc is observed and the relaxation law has been formulated as the ratio of imaginary and real parts of dielectric constant remains constant over the entire range of frequency starting from dc to any higher frequency. The value of the constant is attributed to depend on the PMC, the dielectric constant of the polymer, the differences of conductivity and fractions of the components of the PMC and also on their connectivity arising due to the difference of their process conditions. The emerged unique dielectric relaxation consists of multiple relaxations arising due to the combination of other relaxations (arising due to the two different types of species) present in the sample, fcon=0.27. This novel material may be suitable for certain specific applications in electrical and electronics engineering.

To scan a vehicle’s environment frequency modulated continuous wave (FMCW) radar sensors are essential. The implementable driver assistance systems based on these sensors increase the comfort of an automobile. When integrating them into the car, the radar sensor’s cover must be taken into account. These parts serve as protection against external chemical and mechanical influences, but they should also support the vehicle design and appearance. Usually, painted polymer components are used as a radome. Depending on their material parameters (i.e. relative permittivity and loss-tangent) polymer covers lead to reflections and absorption, which may impair the radar performance, if they are not properly designed from a microwave point of view. For an appropriate design procedure, the polymer properties, have to be known precisely beforehand or need to be analyzed in realistic experimental configurations. Within this paper it is shown by measurements around the 80 GHz automotive radar bands and calculations based on the polymers repeating unit that the relative permittivity can be estimated from knowledge of the molecule structure. This allows the calculation of the relative permittivity of polymer molecules that have not yet been measured in the W-band at 80 GHz at a very early stage of the design process. Additionally it is shown that it has to be taken into account if the polymer is semi-crystalline or amorphous. Furthermore, the density or crystallinity has to be known.

Zn _1−x Sm _x O nanoparticles, with 0.00 ≤ x ≤ 0.10, were prepared using chemical co-precipitation method. The structure and morphology of the obtained samples were characterized using x-ray powder diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. However, the mechanical properties were investigated via digital Vickers microhardness tester. Vickers microhardness measurements were carried out at different applied loads, varying between 0.5 and 10 N at dwell time 60 s on pressed discs of average thickness 3 mm. H _v decreased as the Sm-content increased up to 0.02 and then it increased for higher concentrations. Whereas, it increased as the applied load increased, revealing that the samples exhibited a reverse indentation size effect (ISE). The microhardness measurements were interpreted using various models such as Meyer’s law, Hays and Kendall (HK) approach, elastic/plastic deformation (EPD), proportional specimen resistance (PSR) and the indentation-induced cracking (IIC). Mechanical parameters such as Young’s modulus (E), yield strength (Y), fracture toughness (K) and brittleness index (B) were calculated as a function of x. The most adequate model for the true microhardness of these samples is IIC. It was found that the addition of Sm content enhanced the mechanical properties of the prepared samples after x = 0.02. Dielectric measurements were used to compute different parameters such as real and imaginary parts of the complex permittivity, dielectric loss (tan δ ) and ac conductivity ( σ _ac ).

The viscosity variation of sputum is a common symptom of the progression of Chronic Obstructive Pulmonary Disease (COPD). Since the hydration of the sputum defines its viscosity level, dielectric sensors could be used for the characterization of sputum samples collected from patients for early diagnosis of COPD. In this work, a CMOS-based dielectric sensor for the real-time monitoring of sputum viscosity was designed and fabricated. A proper packaging for the ESD-protection and short-circuit prevention of the sensor was developed. The performance evaluation results show that the radio frequency sensor is capable of measuring dielectric constant of biofluids with an accuracy of 4.17%. Integration of this sensor into a portable system will result in a hand-held device capable of measuring viscosity of sputum samples of COPD-patients for diagnostic purposes.

A broadband soil dielectric spectra retrieval approach ( 1 MHz– 2 GHz) has been implemented for a layered half space. The inversion kernel consists of a two-port transmission line forward model in the frequency domain and a constitutive material equation based on a power law soil mixture rule (Complex Refractive Index Model - CRIM). The spatially-distributed retrieval of broadband dielectric spectra was achieved with a global optimization approach based on a Shuffled Complex Evolution (SCE) algorithm using the full set of the scattering parameters. For each layer, the broadband dielectric spectra were retrieved with the corresponding parameters thickness, porosity, water saturation and electrical conductivity of the aqueous pore solution. For the validation of the approach, a coaxial transmission line cell measured with a network analyzer was used. The possibilities and limitations of the inverse parameter estimation were numerically analyzed in four scenarios. Expected and retrieved layer thicknesses, soil properties and broadband dielectric spectra in each scenario were in reasonable agreement. Hence, the model is suitable for an estimation of in-homogeneous material parameter distributions. Moreover, the proposed frequency domain approach allows an automatic adaptation of layer number and thickness or regular grids in time and/or space.

Nowadays, the use of several methods for the measurement of space charges in dielectrics has become more relevant. The availability of different space charge measurement methods brings the necessity of calibration and characterization of measurement equipment to reach standardized measurements. In this paper, a method is presented to apply known charges at built samples which can be used for acoustic and thermal methods to compare measured values with calibration purposes. Experimental tests for validation purposes were performed in flat samples by comparison of results between a single layer and multilayer samples. Moreover, this method can be used to measure the resolution and accuracy of space charge measurement systems.

Electromagnetic (EM) medical technologies are rapidly expanding worldwide for both diagnostics and therapeutics. As these technologies are low-cost and minimally invasive, they have been the focus of significant research efforts in recent years. Such technologies are often based on the assumption that there is a contrast in the dielectric properties of different tissue types or that the properties of particular tissues fall within a defined range. Thus, accurate knowledge of the dielectric properties of biological tissues is fundamental to EM medical technologies. Over the past decades, numerous studies were conducted to expand the dielectric repository of biological tissues. However, dielectric data is not yet available for every tissue type and at every temperature and frequency. For this reason, dielectric measurements may be performed by researchers who are not specialists in the acquisition of tissue dielectric properties. To this end, this paper reviews the tissue dielectric measurement process performed with an open-ended coaxial probe. Given the high number of factors, including equipment- and tissue-related confounders, that can increase the measurement uncertainty or introduce errors into the tissue dielectric data, this work discusses each step of the coaxial probe measurement procedure, highlighting common practices, challenges, and techniques for controlling and compensating for confounders.