Dielectric properties of porcine brain tissue in the transition from life to death at frequencies from 800 to 1900 MHz

Ten experiments on pigs were performed to investigate possible postmortem changes of the dielectric properties of brain gray matter in the frequency range of 800-1900 MHz. After keeping the animals in stable anaesthesia for at least 45 min, they were euthanatised by an intravenous injection of hypertonic potassium chloride (KCl), causing cardiac arrest within 3 min. Measurements of the dielectric properties were performed repeatedly from at least 45 min prior to death to 18 h after euthanasia. The anaesthesia regimen was chosen to minimize influence on brain tissue characteristics such as brain water content, intracranial blood volume, and cerebral blood flow. The data showed a decline of mean gray matter equivalent conductivity of about 15% at 900 MHz and about 11% at 1800 MHz within the first hour after death. The decline in permittivity was less pronounced (about 3-4%) and almost frequency independent. The results indicate that in vitro measurements of dielectric properties of brain tissue underestimate equivalent conductivity as well as permittivity of living tissue. These changes may affect the generally accepted data of dielectric properties of brain tissue widely used in RF dosimetry.     

Design,development, and performance of an electromagnetic illumination system for thawing cryopreserved kidneys of rabbits and dogs

Kidneys from rabbits and dogs were perfused with one of several DMSO concentrations (0.0, 0.7, 1.4, 2.1 m) in a K⁺-Mg²⁺-rich perfusate, frozen, and then thawed with equipment providing electromagnetic (EM) illumination. Electrical properties (dielectric constant and loss tangent) of kidneys were measured both before and after EM thawing. The kidneys thawed were evaluated by simple anatomical (macroscopic and microscopic) and physiological observations rather than by transplantation.Rabbit kidneys which are no thicker than 2 cm could be optimally (uniformly and rapidly) thawed by use of illumination at 2450 MHz, a frequency which has a penetration depth of 2.1 cm at 0 °C, Optimal thawing of canine kidneys, which are twice as thick as rabbit kidneys, required the insertion of steel spheres (electroseeds) into the renal pelvis prior to freezing and illumination at 7 MHz in addition to that at 2450 MHz. Increasing the DMSO concentration (0.0 to 2.1 m) in renal tissue illuminated with 2450 MHz increased the conductivity and the permittivity regardless of whether the renal tissue was frozen or thawed. The use of DMSO decreased the time for thawing with EM illumination and yielded kidneys with improved post-thaw morphology.     

SAR measurement due to mobile phone exposure in a simulated biological media

The specific absorption rate (SAR) measurements are carried out for compliance testing of personal 3G Mobile phone. The accuracy of this experimental setup has been checked by comparing the SAR in 10?gm of simulated tissue and an arbitrary shaped box. This has been carried out using a 3G mobile Phone at 1718.5?MHz, in a medium simulating brain and muscle phantom. The SAR measurement system consists of a stepper motor to move a monopole E-field probe in two dimensions inside an arbitrary shaped box. The phantom is filled with appropriate frequency-specific fluids with measured electrical properties (dielectric constant and conductivity). That is close to the average for gray and white matters of the brain at the frequencies of interest (1718.5?MHz). Induced fields are measured using a specially designed monopole probe in its close vicinity. The probe is immersed in the phantom material. The measured data for induced fields are used to compute SAR values at various locations with respect to the mobile phone location. It is concluded that these SAR values are position dependent and well below the safety criteria prescribed for human exposure.     

SAR measurement due to mobile phone exposure in a simulated biological media

The specific absorption rate (SAR) measurements are carried out for compliance testing of personal 3G Mobile phone. The accuracy of this experimental setup has been checked by comparing the SAR in 10 gm of simulated tissue and an arbitrary shaped box. This has been carried out using a 3G mobile Phone at 1718.5 MHz, in a medium simulating brain and muscle phantom. The SAR measurement system consists of a stepper motor to move a monopole E-field probe in two dimensions inside an arbitrary shaped box. The phantom is filled with appropriate frequency-specific fluids with measured electrical properties (dielectric constant and conductivity). That is close to the average for gray and white matters of the brain at the frequencies of interest (1718.5 MHz). Induced fields are measured using a specially designed monopole probe in its close vicinity. The probe is immersed in the phantom material. The measured data for induced fields are used to compute SAR values at various locations with respect to the mobile phone location. It is concluded that these SAR values are position dependent and well below the safety criteria prescribed for human exposure.     

The radial transmission line as a broad-band shielded exposure system for microwave irradiation of large numbers of culture flasks

The problem of simultaneously exposing large numbers of culture flasks at nominally equivalent incident power densities and with good thermal control is considered, and the radial transmission line (RTL) is proposed as a solution. The electromagnetic design of this structure is discussed, and an extensively bench-tested realization is described. Referred to 1 W of net forward power, the following specific absorption rate (SAR) data were obtained: at 835.62 MHz, 16.0+/-2.5 mW/kg (mean+/-SD) with range (11-22); at 2450 MHz, 245+/-50 mW/kg with range (130-323). Radio-frequency interference from an RTL driven at roughly 100 W is so low as to be compatible with a cellular base station only 500 m distant. To avoid potential confounding by temperature differences among as many as 144 T-75 flasks distributed over 9 RTLs (six irradiates and three shams), temperature within all flasks was controlled to 37.0+/-0.3 degrees C. Experience with over two years of trouble-free operation suggests that the RTL offers a robust, logistically friendly, and environmentally satisfactory solution to the problem of large-scale in vitro experiments in bioelectromagnetics.     

Formulas for preparing phantom muscle tissue at various radiofrequencies

The dielectric properties of various test samples of phantom tissue were measured using an automated and temperature-controlled slotted line. The ingredients for phantom materials were determined for simulating high-water content tissue at 13.56, 27.12, 40.68, 70, 100, 200, 300, 433, 750, 915, and 2,450 MHz. The ingredients consisted of water, TX-150 (a gelling agent), sodium chloride, and polyethylene powder (200-2,450 MHz) or aluminum powder (13.56-100 MHz). The dielectric constant and conductivity of these materials at different temperatures (15, 22, 30 degrees C) were characterized.     

Dielectric spectroscopy of single human erythrocytes at physiological ionic strength: dispersion of the cytoplasm.

Usually dielectrophoretic and electrorotation measurements are carried out at low ionic strength to reduce electrolysis and heat production. Such problems are minimized in microelectrode chambers. In a planar ultramicroelectrode chamber fabricated by semiconductor technology, we were able to measure the dielectric properties of human red blood cells in the frequency range from 2 kHz to 200 MHz up to physiological ion concentrations. At low ionic strength, red cells exhibit a typical electrorotation spectrum with an antifield rotation peak at low frequencies and a cofield rotation peak at higher ones. With increasing medium conductivity, both electrorotational peaks shift toward higher frequencies. The cofield peak becomes antifield for conductivities higher than 0.5 S/m. Because the polarizability of the external medium at these ionic strengths becomes similar to that of the cytoplasm, properties can be measured more sensitively. The critical dielectrophoretic frequencies were also determined. From our measurements, in the wide conductivity range from 2 mS/m to 1.5 S/m we propose a single-shell erythrocyte model. This pictures the cell as an oblate spheroid with a long semiaxis of 3.3 microns and an axial ratio of 1:2. Its membrane exhibits a capacitance of 0.997 x 10(-2) F/m2 and a specific conductance of 480 S/m2. The cytoplasmic parameters, a conductivity of 0.4 S/m at a dielectric constant of 212, disperse around 15 MHz to become 0.535 S/m and 50, respectively. We attribute this cytoplasmic dispersion to hemoglobin and cytoplasmic ion properties. In electrorotation measurements at about 60 MHz, an unexpectedly low rotation speed was observed. Around 180 MHz, the speed increased dramatically. By analysis of the electric chamber circuit properties, we were able to show that these effects are not due to cell polarization but are instead caused by a dramatic increase in the chamber field strength around 180 MHz. Although the chamber exhibits a resonance around 180 MHz, the harmonic content of the square-topped driving signals generates distortions of electrorotational spectra at far lower frequencies. Possible technological applications of chamber resonances are mentioned.     

Reliable Method for Fabricating Tissue‐Mimicking Materials With Designated Relative Permittivity and Conductivity at 128 MHz

Artificial materials that can simultaneously mimic the relative permittivity and conductivity of various human tissues are usually used in medical applications. However, the method of precisely designing these materials with designated values of both relative permittivity and conductivity at 3 T MRI resonance frequency is lacking. In this study, a reliable method is established to determine the compositions of artificial dielectric materials with designated relative permittivity and conductivity at 128 MHz. Sixty dielectric materials were produced using oil, sodium chloride, gelatin, and deionized water as the main raw materials. The dielectric properties of these dielectric materials were measured using the open‐ended coaxial line method at 128 MHz. Nonlinear least‐squares Marquardt–Levenberg algorithm was used to obtain the formula, establishing the relationship between the compositions of the dielectric materials and their dielectric properties at 128 MHz. The dielectric properties of the blood, gall bladder, muscle, skin, lung, and bone at 128 MHz were selected to verify the reliability of the obtained formula. For the obtained formula, the coefficient of determination and the expanded uncertainties with a coverage factor of k = 2 were 0.991% and 4.9% for relative permittivity and 0.992% and 6.4% for conductivity. For the obtained artificial materials measured using the open‐ended coaxial line method, the maximal difference of relative permittivity and conductivity were 1.0 and 0.02 S/m, respectively, with respect to the designated values. In conclusion, the compositions of tissue‐mimicking material can be quickly determined after the establishment of the formulas with the expanded uncertainties of less than 10%. Bioelectromagnetics. 2021;42:86–94. © 2020 Bioelectromagnetics Society.     

Rotational and translational water diffusion in the hemoglobin hydration shell: dielectric and proton nuclear relaxation measurements.

The dynamic properties of water in the hydration shell of hemoglobin have been studied by means of dielectric permittivity measurements and nuclear magnetic resonance spectroscopy. The temperature behavior of the complex permittivity of hemoglobin solutions has been measured at 3.02, 3.98, 8.59, and 10.80 GHz. At a temperature of 298 K the average rotational correlation time tau of water within a hydration shell of 0.5-nm thickness is determined from the activation parameters to be 68 +/- 10 ps, which is 8-fold the corresponding value of bulk water. Solvent proton magnetic relaxation induced by electron-nuclear dipole interaction between hemoglobin bound nitroxide spin labels and water protons is used to determine the translational diffusion coefficient D(T) of the hydration water. The temperature dependent relaxation behavior for Lamor frequencies between 3 and 90 MHz yields an average value D(298K) = (5 +/- 2) x 10(-10)m2 s-1, which is about one-fifth of the corresponding value of bulk water. The decrease of the water mobility in the hydration shell compared to the bulk is mainly due to an enhanced activation enthalpy.     

Development of a dielectric equivalent gel for better impedance matching for human skin

It would be useful to develop a tissue equivalent gel to improve the uniformity of the electromagnetic field in the human body, and for making a tissue equivalent dielectric human phantom. In this study, solid type, water based gelatin-honey gels were developed which have the electrical characteristics of skin tissue. It was demonstrated that a stable and homogeneous gel, with a relative dielectric constant epsilon ' chosen from desired ranges found in skin, can be made for 200-400 MHz.     

Dielectric dispersion in aqueous solutions of oxyhaemoglobin and carboxyhaemoglobin

The relative permittivity and conductivity of aqueous solutions of oxyhaemoglobin and carboxyhaemoglobin were measured over the frequency range 150kHz-100MHz. To minimize errors of measurement the investigations were carried out with three different samples of each type of haemoglobin, independent apparatus being used in two different laboratories. The dielectric increment and relaxation time were calculated at each of several temperatures from the results. These lead to a dipole moment of 400 Debyes and an activation enthalpy of 17.6+/-1.4kJ.mol(-1), both of which were found to be independent of temperature to within experimental error over the range 3-35 degrees C. The value of the dipole moment shows that the distribution of charge throughout the haemoglobin molecule is nearly symmetrical with respect to the centre of charge. The magnitude of the activation enthalpy is similar to that of the viscosity of water, in accord with the common observation that dielectric relaxation and viscosity are related phenomena. No significant differences are found between the dielectric parameters of oxyhaemoglobin and carboxyhaemoglobin. Combining the results with those obtained from X-ray diffraction of the solid a hydration value of 0.45g of water/g of protein is suggested, subject to the limitations of the model used. Finally, the results indicate the presence of a subsidiary dispersion, which could be attributed to the above quantity of bound water having a static permittivity of about 100 and a relaxation frequency in the region 100-200MHz.     

Dielectric spectroscopy on aqueous electrolytic solutions

In this work, detailed dielectric measurements are presented on aqueous electrolytic solutions of NaCl and KCl in a broad frequency range, typical for modern telecommunication techniques. The complex dielectric permittivity or equivalently the complex conductivity are systematically studied as function of frequency (100 MHz-40 GHz), temperature (10-60 degrees C) and molar concentration (0.001-1 mol/l). By a detailed analysis of the dielectric results using an asymmetrically broadened Cole-Davidson distribution of relaxation times, in addition to dc conductivity, the dielectric response as function of frequency, temperature, and molar concentration was fully parameterized by a total of 13 parameters. This model ansatz and the 13 parameters include an enormous predictive power, allowing a reasonable estimation of the dielectric constant, loss, and the conductivity for any set of external variables frequency, temperature and concentration. The proposed method is not only useful for rather simple electrolytic solutions, but also for cell suspensions and biological matter, if additional processes, especially at low frequencies, are adequately taken into account.     

Dielectric Properties of Normal and Metastatic Lymph Nodes Ex Vivo From Lung Cancer Surgeries

The dielectric properties of normal and tumor human tissues have been widely reported in recent years. However, the dielectric properties of intrathoracic lymph nodes (LNs) have not been reported. In this communication, we measured the dielectric properties (i.e., permittivity and conductivity) of ex vivo intrathoracic LNs obtained from lung cancer surgeries. Results show that the permittivity and conductivity of metastatic LNs are higher than those of normal LNs over the frequency range of 1 MHz–4 GHz. Statistically significant differences are observed at single specific frequencies (64, 128, 298, 433, and 915 MHz and 2.45 GHz). Our study provides the basic data to support future-related research and fills the research gap on the dielectric properties of LNs in the lungs. Bioelectromagnetics. 2020;41:148–155. © 2020 Bioelectromagnetics Society.     

Dielectric properties of mouse MCA1 fibrosarcoma at different stages of development

The in vitro bulk electrical properties of MCA1 fibrosarcoma induced in C57B1/6 male mice were measured at frequencies of 10 kHz to 100 MHz, with some tissues measured to 2 GHz. The properties of normal surrounding tissue also were measured. A comparison of the dielectric properties between three different stages of tumor development as well as that between various locations within the tumor is reported. Statistical analysis of the experimental results revealed statistically significant differences in the dielectric constant and conductivity of the tumor tissues at various stages of development as measured at frequencies below 10 MHz. Conductivity values at different stages also differ at a frequency of 100 MHz. At other frequencies these differences were found to be statistically insignificant.     

Metabolic effects of microwave radiation and convection heating on human mononuclear leukocytes

The effects of microwave radiation (2450 MHz, continuous wave, mean specific absorption rate of 103.5 +/- 4.2 W/kg) and convection heating on the nonphosphorylating oxidative metabolism of human peripheral mononuclear leukocytes (96% lymphocytes, 4% monocytes) at 37 degrees C were investigated. Metabolic activity, determined by chemiluminescence (CL) of cells challenged with luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) linked to bovine serum albumin, was detected with a brightness photometer. A significant stimulation after microwave exposure (p less than 0.005) over total CL of matched 37 degrees C incubator controls was observed. A similar degree of stimulation compared to incubator controls was also detected after sham treatment. There was no significant difference between changes in total CL or stimulation indices of the microwave and sham exposed groups. It appears that exposure to microwave radiation, under normothermic (37 +/- 0.03 degrees C) conditions, has no effect on the oxidative metabolic activity of human peripheral mononuclear leukocytes. However, the significant differences between microwave or sham exposed cells and their respective incubator controls occurred because the temperature of the incubator controls did not exceed 35.9 degrees C and this temperature required 39 minutes to reach from 22 degrees C. Slow heating of incubator controls must be accounted for in thermal and radiofrequency radiation studies in vitro.     

Topographic determination of zinc in human brain by atomic absorption spectrophotometry

Atomic absorption spectroscopy was used for measuring the zinc content, in ppb (μg/1), of brain tissue. A new method for determining the correction factor of atomic absorption interference is described. Measurements of the zinc content of twenty-four regions of adult human brains showed the maximum zinc content in resistent sector and endplate of the Amnion's horn, corroborating the histochemical data. The distribution of zinc in other regions was relatively uniform, but white matter showed lower values than gray matter. The zinc content of seventeen regions of human newborn brains was below that in adult brains, for all regions. The blood content of brain tissue contributed only insignificantly to its zinc content.     

Determination of safety distance limits for a human near a cellular base station antenna,adopting the IEEE standard or ICNIRP guidelines

This paper investigates the minimum distance for a human body in the near field of a cellular telephone base station antenna for which there is compliance with the IEEE or ICNIRP threshold values for radio frequency electromagnetic energy absorption in the human body. First, local maximum specific absorption rates (SARs), measured and averaged over volumes equivalent to 1 and to 10 g tissue within the trunk region of a physical, liquid filled shell phantom facing and irradiated by a typical GSM 900 base station antenna, were compared to corresponding calculated SAR values. The calculation used a homogeneous Visible Human body model in front of a simulated base station antenna of the same type. Both real and simulated base station antennas operated at 935 MHz. Antenna-body distances were between 1 and 65 cm. The agreement between measurements and calculations was excellent. This gave confidence in the subsequent calculated SAR values for the heterogeneous Visible Human model, for which each tissue was assigned the currently accepted values for permittivity and conductivity at 935 MHz. Calculated SAR values within the trunk of the body were found to be about double those for the homogeneous case. When the IEEE standard and the ICNIRP guidelines are both to be complied with, the local SAR averaged over 1 g tissue was found to be the determining parameter. Emitted power values from the antenna that produced the maximum SAR value over 1 g specified in the IEEE standard at the base station are less than those needed to reach the ICNIRP threshold specified for the local SAR averaged over 10 g. For the GSM base station antenna investigated here operating at 935 MHz with 40 W emitted power, the model indicates that the human body should not be closer to the antenna than 18 cm for controlled environment exposure, or about 95 cm for uncontrolled environment exposure. These safe distance limits are for SARs averaged over 1 g tissue. The corresponding safety distance limits under the ICNIRP guidelines for SAR taken over 10 g tissue are 5 cm for occupational exposure and about 75 cm for general-public exposure.     

Characterization of central nervous system structures by magnetic resonance diffusion anisotropy

Diffusion-weighted magnetic resonance imaging (MRI) provides information about tissue water diffusion. Diffusion anisotropy, which can be measured with diffusion tensor MRI, is a quantitative measure of the directional dependence of the diffusion restriction that is introduced by biological structures such as nerve fibers. Diffusion tensor MRI data was obtained in the brain, brain stem, and cervical spinal cord. For each region, scans were performed in four normal volunteers. Fractional anisotropy (FA), an index of diffusion anisotropy, was measured within regions of interest located in the corpus callosum, capsula interna, thalamus, caudate nucleus, putamen, brain cortex, pyramidal tract of the medulla, accessory olivary nucleus, dorsal olivary nucleus, inferior olivary nucleus, spinal white and gray matter. The highest FA value was measured in the corpus callosum (81 +/- 3%). The values of the other areas decreased in the following order: pyramidal tract in the medulla (72 +/- 1%), spinal white matter (65 +/- 4%), capsula interna (62 +/- 3%), accessory olivary nucleus (36 +/- 2%), spinal gray matter (35 +/- 5%), dorsal olivary nucleus in the medulla (29 +/- 2%), thalamus (28 +/- 2%), inferior olivary nucleus (15 +/- 2%), putamen (13 +/- 2%), caudate nucleus (13 +/- 2%), and brain cortex (9 +/- 1%). Our results indicate that the underlying fiber architecture, fiber density, and uniformity of nerve fiber direction affect anisotropy values of the various structures. Characterization of various central nervous system structures with diffusion anisotropy is possible and may be useful to monitor degenerative diseases in the central nervous system.     

Microwave dielectric relaxation in muscle. A second look.

The dielectric permittivity and conductivity of muscle fibers from the giant barnacle, Balanus nubilus, have been measured at 1, 25, and 37 degrees C, between 10 MHz and 17 GHz. The dominant microwave dielectric relaxation process in these fibers is due to dipolar relaxation of the tissue water, which shows a characteristic relaxation frequency equal to that of pure water, ranging from 9 GHz (1 degree C) to 25 GHz (37 degree C). The total permittivity decrease, epsilon 0 -- epsilon infinity, due to this process accounts for approximately 95% of the water content of the tissue; thus, the major fraction of tissue water is dielectrically identical to the pure fluid on a picosecond time scale. A second dielectric process contributes significantly to the tissue dielectric properties between 0.1 and 1--5 GHz, and arises in part form Maxwell-Wagner effects due to the electrolyte content of the tissue, and in part from dielectric relaxation of the tissue proteins themselves.     

Effects of a 2450 MHz high-frequency electromagnetic field with a wide range of SARs on the induction of heat-shock proteins in A172 cells

In this study, we investigated whether exposure to 2450 MHz high-frequency electromagnetic fields (HFEMFs) could act as an environmental insult to evoke a stress response in A172 cells, using HSP70 and HSP27 as stress markers. The cells were exposed to a 2450 MHz HFEMF with a wide range of specific absorption rates (SARs: 5-200 W/kg) or sham conditions. Because exposure to 2450 MHz HFEMF at 50-200 W/kg SAR causes temperature increases in culture medium, appropriate heat control groups (38-44 degrees C) were also included. The expression of HSP 70 and HSP 27, as well as the level of phosphorylated HSP 27 ((78)Ser) (p-HSP27), was determined by Western blotting. Our results showed that the expression of HSP 70 increased in a time and dose-dependent manner at >50 W/kg SAR for 1-3 h. A similar effect was also observed in corresponding heat controls. There was no significant change in HSP 27 expression caused by HFEMF at 5-200 W/kg or by comparable heating for 1-3 h. However, HSP 27 phosphorylation increased transiently at 100 and 200 W/kg to a greater extent than at 40-44 degrees C. Phosphorylation of HSP 27 reached a maximum after 1 h exposure at 100 W/kg HFEMF. Our results suggest that exposure to a 2450 MHz HFEMF has little or no apparent effect on HSP70 and HSP27 expression, but it may induce a transient increase in HSP27 Phosphorylation in A172 cells at very high SAR (>100 W/kg).