Dielectric properties of lens tissue at microwave frequencies

The relative permittivity and conductivity of rabbit eye lens were measured in the frequency domain between 2 and 18 GHz at temperatures of 37 and 20°C. An analysis of the data suggested that a significant proportion of the bulk water in nuclear and cortical lens tissue may behave differently to pure water. In addition, the Maxwell-Fricke mixture theory was used to estimate the amount of hydrated water that relaxes far below 1 GHz.     

Comparison of the dielectric properties of normal and wounded human skin material

Measurements have been made of the permittivity and conductivity of normal and wounded human skin material over the frequency range 10 MHz-10 GHz. The permittivity of the wounded tissue was found to be about 12% higher than that of the normal tissue. A similar percentage increase was observed for the conductivity. These differences are attributed to the presence of a higher proportion of bulk water in the wounded material.     

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.     

Modeling of proton spin relaxation in muscle tissue using nuclear magnetic resonance spin grouping and exchange analysis.

NMR spin relaxation experiments performed on healthy mouse muscle tissue at 40 MHz and 293 K are reported. The spin-lattice relaxation experiments were performed using different combinations of selective and nonselective radio frequency pulses. Relaxation experiments in the rotating frame at H1 = 10, 5 and 1 G are also reported. The experimental results were analyzed using the spin-grouping method, which yields the sizes of the resolved magnetization components as well as their T2's and T1's (or T1p's) for the nonexponential relaxation functions. These results were analyzed further for the exchange between different spin groups. It has been found that to explain all of these experimental data it was necessary to use a four-compartment model of the muscle tissue that consists of a lipid spin group, a "solid-like" spin group (mainly proteins), a "bulk water" spin group and a "bound water" spin group. The chemical exchange rate between "bulk" and "bound" water was found to be 29 +/- 9s-1 at room temperature. The exchange rate between the bound water and the solid moderator was estimated to be approximately 500 s-1.     

RF absorption involving biological macromolecules

The fundamental intramolecular frequency of a globular protein can be obtained from the measurements of acoustic velocities of bulk protein matter. This lowest frequency for common size molecules is shown to be above several hundred GHz. All modes below this frequency would then be intermolecular modes or bulk modes of the molecule and surrounding matter or tissue. The lowest frequency modes of an extended DNA double helix are also shown to be bulk modes because of interaction with water. Only DNA modes, whose frequency is well above 4 GHz, can be intrahelical modes, that is, confined to the helix rather than in the helix plus surroundings. Near 4 GHz, they are heavily damped and, therefore, not able to resonantly absorb. Modes that absorb radio frequency (RF) below this frequency are bulk modes of the supporting matter. Bulk modes rapidly thermalize all absorbed energy. The implication of these findings for the possibility of athermal RF effects is considered. The applicability of these findings for other biological molecules is discussed.     

Some observations on the dielectric properties of hemoglobin's suspending medium inside human erythrocytes.

Dielectric permittivity and conductivity data for human erythrocytes exhibit two consecutive dispersions with frequency in the range from 0.1 to 250 MHz. The dispersion observed above 50 MHz yields a circle in the complex impedance plane with its center on the real axis, suggesting a Debye relaxation, for the erythrocyte interior medium, centered in the microwave frequency range as for bulk water. Moreover, the Maxwell mixture equation, indicates that this "free" water forms most of the suspending medium of hemoglobin macromolecules. These results extend to lower frequencies (earlier results obtained by Schwan) with biological tissues such as muscles, skin, or liver at microwaves frequencies.     

A generalized activating function for predicting virtual electrodes in cardiac tissue.

To fully understand the mechanisms of defibrillation, it is critical to know how a given electrical stimulus causes membrane polarizations in cardiac tissue. We have extended the concept of the activating function, originally used to describe neuronal stimulation, to derive a new expression that identifies the sources that drive changes in transmembrane potential. Source terms, or virtual electrodes, consist of either second derivatives of extracellular potential weighted by intracellular conductivity or extracellular potential gradients weighted by derivatives of intracellular conductivity. The full response of passive tissue can be considered, in simple cases, to be a convolution of this "generalized activating function" with the impulse response of the tissue. Computer simulations of a two-dimensional sheet of passive myocardium under steady-state conditions demonstrate that this source term is useful for estimating the effects of applied electrical stimuli. The generalized activating function predicts oppositely polarized regions of tissue when unequally anisotropic tissue is point stimulated and a monopolar response when a point stimulus is applied to isotropic tissue. In the bulk of the myocardium, this new expression is helpful for understanding mechanisms by which virtual electrodes can be produced, such as the hypothetical "sawtooth" pattern of polarization, as well as polarization owing to regions of depressed conductivity, missing cells or clefts, changes in fiber diameter, or fiber curvature. In comparing solutions obtained with an assumed extracellular potential distribution to those with fully coupled intra- and extracellular domains, we find that the former provides a reliable estimate of the total solution. Thus the generalized activating function that we have derived provides a useful way of understanding virtual electrode effects in cardiac tissue.     

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.     

The high frequency properties of brain tissue

Computer modeling is becoming increasingly important in the realm of brain biomechanics and injury. New computer simulations range from modeling of brain surgery, a low frequency, high strain event, to predicting injury as a result of an impact to the head, a high frequency event with varying strain magnitudes. This range of modeling efforts requires characterization of the tissue over as wide a frequency and strain range as possible. Research done to date has concentrated on the low frequency properties of the tissue. Complex compression and complex shear moduli have been measured at frequencies up to 350 Hz. Impact modeling requires use of frequency data at significantly higher frequencies than these. The "wave-in-a-tube" ultrasonic method was applied to brain tissue to determine mechanical properties at frequencies between 100 kHz and 10 MHz. Of these properties, only complex bulk modulus |K*| is fairly invariant (2133 MPa) with respect to frequency. Complex shear and complex Young's moduli vary with frequency and approach an asymptotic upper limit. Some variation in complex Poisson's ratio was also observed.     

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.     

Auxin increases the hydraulic conductivity of auxin-sensitive hypocotyl tissue

The ability of water to enter the cells of growing hypocotyl tissue was determined in etiolated soybean (Glycine max (L.) Merr.) seedlings. Water uptake was restricted to that for cell enlargement, and the seedlings were kept intact insofar as possible. Tissue water potentials ( _w) were measured at thermodynamic equilibrium with an isopiestic thermocouple psychrometer. _wwas below the water potential of the environment by as much as 3.1 bars when the tissue was enlarging rapidly. However, _w was similar to the water potential of the environment when cell enlargement was not occurring. The low _w in enlarging tissue indicates that there was a low conductivity for water entering the cells.The ability of water to enter the enlarging cells was defined as the apparent hydraulic conductivity of the tissue (Lp). Despite the low Lp of growing cells, Lp decreased further as cell enlargement decreased when intact hypocotyl tissue was deprived of endogenous auxin (indole-3-acetic acid) by removal of the hypocotyl hook. Cell enlargement resumed and Lp increased when auxin was resupplied exogenously. The auxin-induced increase in Lp was correlated with the magnitude of the growth enhancement caused by auxin, and it was observed during the earliest phase of the growth response to auxin. The increase in Lp appeared to be caused by an increase in the hydraulic conductivity of the cell protoplasm, since other factors contributing to Lp remained constant. The rapidity of the response is consistent with a cellular site of action at the plasmalemma, although other sites are not precluded.Because the experiments involved only short times, auxin-induced changes in cell enlargement could not be attributed to changes in cell osmotic potentials. Neither could they be attributed to changes in turgor, which increased when the rate of enlargement decreased. Rather, auxin appeared to act by altering the extensibility of the cell walls and by simultaneously altering the ability of water to enter the growing cells under a given water potential gradient. The hydraulic conductivity and extensibility of the cell walls appeared to contribute about equally to the control of the growth rate of the hypocotyls.     

Microwave dielectric studies on proteins,tissues, and heterogeneous suspensions

We summarize the results of several of our recent studies on the dielectric properties of protein solutions, tissues, and nonionic microemulsions at microwave frequencies extending to 18 GHz. The data in all cases are analyzed using the Maxwell mixture theory to determine the dielectric properties of the suspending water and the amount and dielectric properties of the water of hydration associated with the suspended phase. The dielectric data from the protein solutions and tissues are broadly consistent with the results of previous studies at UHF frequencies; they indicate hydration values in the range of 0.4–0.6 g water/g protein. There is evidence of a dielectric relaxation process occurring at low-GHz frequencies that can be attributed in part to dielectric relaxation of the “bound” water in the system. The remaining solvent water appears to have dielectric properties close to, if not precisely the same as, those of pure water. The average relaxation frequency of the suspending water in the microemulsions is reduced from that of pure water, evidently reflecting an average of that of the water of hydration (～5–6 GHz) and that of pure water. This reduced average relaxation frequency implies an increased average viscosity of the water and (by Walden's rule) accounts for the unexpectedly low ionic conductivity of the preparations.     

Relaxation dynamics of a protein solution investigated by dielectric spectroscopy

In the present work, we provide a dielectric study on two differently concentrated aqueous lysozyme solutions in the frequency range from 1MHz to 40GHz and for temperatures from 275 to 330K. We analyze the three dispersion regions, commonly found in protein solutions, usually termed β-, γ-, and δ-relaxations. The β-relaxation, occurring in the frequency range around 10MHz and the γ-relaxation around 20GHz (at room temperature) can be attributed to the rotation of the polar protein molecules in their aqueous medium and the reorientational motion of the free water molecules, respectively. The nature of the δ-relaxation, which is often ascribed to the motion of bound water molecules, is not yet fully understood. Here we provide data on the temperature dependence of the relaxation times and relaxation strengths of all three detected processes and on the dc conductivity arising from ionic charge transport. The temperature dependences of the β- and γ-relaxations are closely correlated. We found a significant temperature dependence of the dipole moment of the protein, indicating conformational changes. Moreover we find a breakdown of the Debye-Stokes-Einstein relation in this protein solution, i.e., the dc conductivity is not completely governed by the mobility of the solvent molecules. Instead it seems that the dc conductivity is closely connected to the hydration shell dynamics.     

Patterns of water use and the tissue water relations in the dioecious shrub,Salix arctica: the physiological basis for habitat partitioning between the sexes

Summary Within the high arctic of Canada, Salix arctica, a dioecious, dwarf willow exhibits significant spatial segregation of the sexes. The overall sex ratio is female-biased and female plants are especially common in wet, higher nutrient, but lower soil temperature habitats. In contrast, male plants predominate in more xeric and lower nutrient habitats with higher soil temperatures that can be drought prone. Associated with the sex-specific habitat differences were differences in the seasonal and diurnal patterns of water use as measured by stomatal conductance to water vapor and the bulk tissue water relations of each gender. Within the wet habitats, female plants maintained higher rates of stomatal conductance (g) than males when soil and root temperatures were low (<4° C). In contrast, within the xeric habitats, male plants maintained higher g and had lower leaf water potentials _leaf at low soil water potentials and a high leaf-to-air vapor pressure gradient (w) when compared to females. Female plants had more positive carbon isotope ratios than males indicating a lower internal leaf carbon dioxide concentration and possibly higher water use efficiency relative to males. Tissue osmotic and elastic properties also differed between the sexes. Male plants demonstrated lower tissue osmotic potentials near full tissue hydration and at the turgor loss point and a lower bulk tissue elastic modulus (higher tissue elasticity) than female plants. Males also demonstrated a greater ability to osmotically adjust on a diurnal basis than females. These properties allowed male plants to maintain higher tissue turgor pressures at lower tissue water contents and _soil over the course of the day. The sex-specific distributional and ecophysiological characteristics were also correlated with greater total plant growth and higher fecundity of females in wet habitats, and males in xeric habitats respectively. The intersexual differences in physiology persisted in all habitats. These results and those obtained from growth chamber studies suggest that sex-specific differences have an underlying genetic basis. From these data we conjecture that selection maintaining the intersexual differences may be related to different costs associated with reproduction that can be most easily met through physiological specialization and spatial segregation of the sexes among habitats of differing conditions.     

Dielectric properties of hydrated collagen

Dielectric measurements have been carried out on partially hydrated collagen in the frequency ranges 100 kHz–5 MHz, 100 MHz–1 GHz, and 8–23 GHz. In the low-frequency range, a dispersion was observed around 100 kHz which results from inhomogeneous conductivity of the samples. A dielectric relaxation was observed aroud 0.3 GHz using time-domain-spectroscopy techniques. This relaxation can be considered to originate from mobile side chains. Microwave measurements indicate that the water relaxation may extend into the 10-GHz region. An apparent discrepancy between the main water relaxation time and the average rotational correlation time of water as measured by nmr line widths was resolved by the assumption that a fraction of the water molecules is bound to the collagen with residence times on the order of 10^?6 sec, whereas the remainder of the water is only weakly bound and exhibits rotational rates on the order of 10^?10 sec.     

Water relations and growth of tomato fruit pericarp tissue

The water relations of young tomato fruit pericarp tissue were examined and related to tissue expansion. The relationship between bulk turgor pressure and tissue expansion (as change in fresh mass or length of tissue) was determined in slices of pericarp cut from young, growing fruit by incubation in different osmotic concentrations of polyethylene glycol 6000 or mannitol. The bulk turgor of this tissue was low (about 0.2 MPa), even in fruit from plants that were otherwise fully turgid, whether measured psychrometrically or by length change in osmotic solutions. The rate of tissue growth at maximum turgor was less than that at moderate turgor unless calcium was added to the incubation medium. However, added calcium also decreased the rate of growth at lower turgor pressures. Yield turgor was < 0.1 MPa, but it was increased by the addition of calcium ions. Electrolyte leakage from tissue was greatest at maximum turgor pressure but was decreased by the addition of calcium ions or osmoticum. Tissue growth was unaffected by a range of plant growth regulators (IAA, abscisic acid, benzyladenine and GA₃) but was inhibited, particularly at high turgor, by low concentrations of malic or citric acid. The low turgor pressure of pericarp tissue could be due to the presence of apoplastic solutes within the pericarp, and evidence for this is discussed. Thus, fruit tissue may be able to maintain optimal expansion rates only at moderate turgor and low calcium concentration.     

A thermal method for the determination of tissue blood flow.

An isothermal flowmeter for the determination of local tissue blood flow is described. Flow is determined by the measurement of the thermal conductivity of the tissue in the vicinity of a heated thermistor maintained at a fixed temperature difference above a reference thermistor. Direct heating of the thermistor is utilized to eliminate the need for specially constructed indirectly heated thermistors. This design results in a device with a voltage output directly proportional to tissue thermal conductivity and to tissue blood flow. The device is shown to be adequate for the qualitative measurement of myocardial blood flow under various situations. Construction is simplified and the size of the circuit reduced by the use of readily available integrated circuits.     

Hydration properties of adenosine phosphate series as studied by microwave dielectric spectroscopy

Hydration properties of adenine nucleotides and orthophosphate (Pi) in aqueous solutions adjusted to pH = 8 with NaOH were studied by high-resolution microwave dielectric relaxation (DR) spectroscopy at 20 °C. The dielectric spectra were analyzed using a mixture theory combined with a least-squares Debye decomposition method. Solutions of Pi and adenine nucleotides showed qualitatively similar dielectric properties described by two Debye components. One component was characterized by a relaxation frequency (f_c = 18.8-19.7 GHz) significantly higher than that of bulk water (17 GHz) and the other by a much lower f_c (6.4-7.6 GHz), which are referred to here as hyper-mobile water and constrained water, respectively. By contrast, a hydration shell of only the latter type was found for adenosine (f_c ~ 6.7 GHz). The present results indicate that phosphoryl groups are mostly responsible for affecting the structure of the water surrounding the adenine nucleotides by forming one constrained water layer and an additional three or four layers of hyper-mobile water.     

The effects of electromagnetic radiation of extremely high frequency and low intensity on the growth rate of bacteria Escherichia coli and the role of medium pH

It has been shown that coherent electromagnetic irradiation (EMI) of extremely high frequency (45-53 GHz) or millimeter waves (wavelength 5.6-6.7 mm) of low intensity (flux capacity 0.06 mW/cm2) of Escherichia coli K12, grown under anaerobic conditions during the fermentation of sugar (glucose) for 30 min or 1 h, caused a decrease in their growth rate, the maximum inhibitory effect being achieved at a frequency of 51.8 or 53 GHz. This effect depended on medium pH when the maximal action was determined at pH 7.5. In addition, separate 30-min of 1-h irradiation (frequency 51.8 or 53 GHz) of doubly distilled water or some inorganic ions contained in Tris-phosphate buffer where the cells were transferred induced oppositely directed changes in further growth of these bacteria under anaerobic conditions; irradiation of water caused a decrease in the growth rate of bacteria. A significant change in pH of water (0.5-1.5 unit) was induced by a 30-irradiation at a frequency of 49, 50.3, 51.8, or 53 GHz, when the initial pH value was 6.0 or 8.0, but not 7.5. These results indicate the changes in the properties of water and its role in the effects of EMI of extremely high frequency. The marked effect of EMI on bacteria disappeared upon repeated irradiation for 1 h at a frequency of 51.8 or 53 GHz with an interval of 2 hours. This result indicates some compensatory mechanisms in bacteria.     

Water balance in the arborescent palm, Sabal palmetto. I. Stem structure, tissue water release properties and leaf epidermal conductance

The functional importance of water storage in the arborescent palm, Sabal palmetto, was investigated by observing aboveground water content, pressure-volume curve parameters of leaf and stem tissue and leaf epidermal conductance rates. The ratio of the amount of water stored within the stem to the leaf area (kg m^?2) increased linearly with plant height. Pressure-volume curves for leaf and stem parenchyma differed markedly; leaves lost turgor at 0.90 relative water content and –3.81 MPa, while the turgor loss point for stem parenchyma occurred at 0–64 relative water content and ?0.96 MPa. Specific capacitance (change in relative water content per change in tissue water potential) of stem parenchyma tissue was 84 times higher than that of leaves, while the bulk modulus of elasticity was 346 times lower. Leaf epidermal conductance rates were extremely low (0.32–0.56 mmol m^?2 s^?1) suggesting that S. palmetto are able to strongly restrict foliar water loss rates. Structurally, stems of S. palmetto appear to be well suited to act as a water storage reservoir, and coupled with the ability to restrict water loss from leaf surfaces, may play an important role in tree survival during periods of low water availability.