Stanislav I. Alekseev

Characterization of the Interactions Between a 60-GHz Antenna and the Human Body in an Off-Body Scenario

Interactions between a 60-GHz microstrip patch antenna array designed for off-body communications and the human body are investigated numerically and experimentally. First, the array is characterized in free space and on a homogeneous skin-equivalent phantom in terms of reflection coefficient, radiation pattern, and antenna efficiency. Second, a multiphysics dosimetry technique is proposed and implemented to determine the specific absorption rate (SAR) and incident power density (IPD) from the heating dynamics measured on an experimental phantom using a high-resolution infrared (IR) camera. The SAR and IPD are found by fitting the analytical solution of the bio-heat transfer equation to the measured heating dynamics. The experimental and numerical results are in a very good agreement. They demonstrate that for the considered scenario the impact of the body on the antenna characteristics is almost negligible, and even relatively high radiated powers (up to 550 mW) result in exposure levels that are below international exposure limits.

Reflection and absorption of millimeter waves by thin absorbing films

Reflection, transmission, and absorption of mm-waves by thin absorbing films were determined at two therapeutic frequencies: 42. 25 and 53.57 GHz. Thin filter strips saturated with distilled water or an alcohol-water solution were used as absorbing samples of different thicknesses. The dependence of the power reflection coefficient R(d) on film thickness (d) was not monotonic. R(d) passed through a pronounced maximum before reaching its steady-state level [R(infinity)]. Similarly, absorption, A(d), passed two maximums with one minimum between them, before reaching its steady-state level [A(infinity)]. At 42.25 GHz, A(d) was compared with absorption in a semi-infinite water medium at a depth d. When d < 0.3 mm, absorption by the film increased: at d = 0.1 mm the absorption ratio for the thin layer sample and the semi-infinite medium was 3.2, while at d = 0.05 mm it increased up to 5.8. Calculations based on Fresnel equations for flat thin layers adequately described the dependence of the reflection, transmission, and absorption on d and allowed the determination of the refractive index (n), dielectric constant (epsilon), and penetration depth (delta) of the absorbing medium for various frequencies. For water samples, epsilon was found to be 12.4-19.3j, delta = 0.49 mm at 42.25 GHz, and epsilon = 9.0-19.5j, delta = 0.36 mm at 53.57 GHz. The calculated power density distribution within the film was strongly dependent on d. The measurements and calculations have shown that the reflection and absorption of mm-waves by thin absorbing layers can significantly differ from the reflection and absorption in similar semi-infinite media. The difference in reflection, absorption, and power density distribution in films, as compared to semi-infinite media, are caused by multiple internal reflections from the film boundaries. That is why, when using thin phantoms and thin biological samples, the specifics of the interaction of mm-waves with thin films should be taken into account.

Effects of millimeter waves on ionic currents of Lymnaea neurons

The effects of mm-waves 60.22-62.22 GHz and 75 GHz on A-type K+ currents and the effects of 61.22 GHz on Ca2+ currents of Lymnaea neurons were investigated using a whole-cell voltage-clamp technique. The open end of a rectangular waveguide covered with a thin Teflon film served as a radiator. Specific absorption rates at the waveguide outlet, inserted into physiological solution, were in the range of 0-2400 W/kg. Millimeter wave irradiation increased the peak amplitudes, activation rates, and inactivation rates of both ion currents. The changes in A-type K+ current were not dependent on the irradiation frequency. It was shown that the changes in the amplitudes and kinetics of both currents resulted from the temperature rise produced by irradiation. No additional effects of irradiation on A-type K+ current other than thermal were found when tested at the phase transition temperature or in the presence of ethanol. Ethanol reduced the thermal effect of irradiation. Millimeter waves had no effect on the steady-state activation and inactivation curves, suggesting that the membrane surface charge and binding of calcium ions to the membrane in the area of channel locations did not change.

Millimeter waves thermally alter the firing rate of the Lymnaea pacemaker neuron

The effects of millimeter waves (mm-waves, 75 GHz) and temperature elevation on the firing rate of the BP-4 pacemaker neuron of the pond snail Lymnaea stagnalis were studied by using microelectrode techniques. The open end to a rectangular waveguide covered with a thin Teflon film served as a radiator. Specific absorption rates (SARs), measured in physiological solution at the radiator outlet, ranged from 600 to 4,200 W/kg, causing temperatures rises from 0.3 to 2.2 degrees C, respectively. Irradiation at an SAR of 4200 W/kg caused a biphasic change in the firing rate, i.e., a transient decrease in the firing rate (69 +/- 22% below control) followed by a gradual increase to a new level that was 68 +/- 21% above control. The biphasic changes in the firing rate were reproduced by heating under the condition that the magnitude (2 degrees C) and the rate of temperature rise (0.96 degrees C/s) were equal to those produced by the irradiation (for an SAR of 4,030 W/kg). The addition of 0.05 mM of ouabain caused the disappearance of transient responses of the neuron to the irradiation. It was shown that the rate of temperature rise played an important role in the development of a transient neuronal response. The threshold stimulus for a transient response of the BP-4 neuron found in warming experiments was a temperature rise of 0.0025 degrees C/s.

New Method for Determining Dielectric Properties of Skin and Phantoms at Millimeter Waves Based on Heating Kinetics

Recent progress in millimeter-wave (MMW) wireless body-centric applications triggered an increasing interest to characterize the interactions between the millimeter waves and the human body. The determination of the dielectric properties of skin and phantoms (artificial models with tissue-equivalent dielectric properties) at MMW is crucial for the accurate evaluation of the power absorption and distribution in the skin. In this study, we show that the heating kinetics resulting from the MMW exposure can be used for the accurate determination of the penetration depth (δ) and power density (I) in different samples (1% and 4% agar phantoms, 20% and 25% polyethylene powder (PEP) phantoms, and human skin). The samples have been exposed at 60.4 GHz using an open-ended waveguide. The temperature distribution and dynamics are recorded using an infrared camera. The values of δ and I are defined by fitting the analytical solution of the bio-heat transfer equation to the experimental heating kinetics. The values of δ are further used to retrieve the permittivity spectra of materials described by Debye equation. Simultaneously, δ is calculated using the permittivity directly measured using a slim coaxial probe. Both results are in good agreement. Finally, our results demonstrate that the permittivity of a 20% PEP phantom is close to that of skin. Hence, this phantom can be used to model the MMW interactions with skin and to characterize on-body wearable MMW antennas.

Numerical and Experimental Millimeter-Wave Dosimetry for In Vitro Experiments

This paper provides extensive dosimetry data for in vitro experiments regarding the biological effects of millimeter waves. Two particular frequency ranges have been considered, which are: (1) the 57-64-GHz frequency range dedicated to near-future applications in high-speed wireless communication systems and (2) the discrete frequencies used in millimeter-wave therapy, namely, 42.25, 53.57, and 61.22 GHz. The dielectric properties of keratinocyte cells and culture media were determined using permittivity data of free water and Maxwells mixture equation. The local specific absorption rate (SAR) distribution within the cell monolayer located in a standard tissue culture plate was computed using the finite-element method and the finite-integration technique. The averaged near-surface SAR for the cell monolayer was determined using both numerical electric-field-based and experimental temperature-based approaches. The SAR was computed taking into account physiological variations of the water content in the keratinocyte cells, as well as variations in the cell monolayer thickness. Experimental and computational results are shown to be in very good agreement.

Complex permittivity of representative biological solutions in the 2–67 GHz range

The main purpose of this study is to provide experimental data on the complex permittivity of some biological solutions in the 2-67 GHz range at room and human body temperatures. The permittivity measurements are performed using an open-ended coaxial probe. Permittivity spectra of several representative monomolecular solutions of proteins, amino acids, nucleic acids, and carbohydrates are analyzed and compared. Furthermore, measurements have also been performed for complex biomolecular solutions, including bovine serum albumin (BSA)-DNA-glucose mixture, culture medium, and yeast extract solution. The results demonstrate that for concentrations below 1%, the permittivity spectra of the solutions do not substantially differ from that of distilled water. Measurements carried out for 4% and 20% BSA solutions show that the presence of proteins results in a decrease in permittivity. For highly concentrated RNA solutions (3%), a slight increase in the imaginary part of the permittivity is observed below 10 GHz. Experimental data show that free water permittivity can be used for modeling of the culture medium above 10 GHz. However, at lower frequencies a substantial increase in the imaginary part of the permittivity due to ionic conductivity should be carefully taken into account. A similar increase has also been observed for the yeast extract solution in the lower frequency region of the considered spectrum. Above 10 GHz, the high concentration of proteins and other low-permittivity components of the yeast extract solution results in a decrease in the complex permittivity compared to that of water. Obtained data are of utmost importance for millimeter-wave dosimetry studies.

Distortion of millimeter-wave absorption in biological media due to presence of thermocouples and other objects

Specific absorption rate (SAR) distributions in the vicinity of a thermocouple or air bubble in water and in the presence of hair or sweat duct in skin were calculated using analytical and two-dimensional impedance methods. The objects were exposed to uniform 42.25 GHz plane electromagnetic fields. Insertion of a 0.1-mm thermocouple or similarly sized air bubble into water produced a strong localized disturbance of the otherwise uniform SAR distribution. However, the average of SAR values immediately surrounding the thermocouple was close to the undisturbed uniform average SAR. This allows measuring the average SAR during exposure of both unbounded and bounded media using calibrated small thermocouples (up to 0.1 mm). The SAR distribution in the vicinity of a hair was qualitatively similar to that produced by an air bubble. The maximal value of SAR was more than 3 times higher than the overall average SAR value in the skin. Sweat ducts produced a smaller disturbance of the millimeter-wave (mm-wave) field.

Millimeter-wave-induced hypoalgesia in mice: dependence on type of experimental pain

Millimeter-wave therapy (MWT) is based on the systemic biological effects resulting from local exposure of skin to low-power electromagnetic waves of millimeter wavelength. The aims of the present study are to quantitatively evaluate hypoalgesic effects of MWT in murine experimental models of acute and chronic neuropathic pain, and to compare them with the previously determined MWT-induced hypoalgesia in an experimental model of chronic nonneuropathic pain, and also to assess the ability of local heating with a Holmium YAG laser to produce hypoalgesia in mice. The cold and hot water tail-flick tests and the unilateral chronic constriction injury (CCI) to the sciatic nerve were used as pain models. The MWT characteristics were: frequency =61.22 GHz; average power density =13.3 mW/cm/sup 2/; duration of exposure =15 min; and area of exposure-nose. This study demonstrated that a single MWT most effectively suppressed chronic nonneuropathic pain. Less effectively, a single MWT reduced pain sensitivity in the murine model of acute pain, and was ineffective in the model of chronic neuropathic pain. However, multiple MWT reduced the symptoms that developed following CCI. The local heating of the exposed area did not produce hypoalgesia. The findings support the use of MWT in chronic pain states.

Millimeter waves as a source of selective heating of skin

This study demonstrates that 20-100 GHz range can be used for spatially-accurate focusing of heating inside the skin achieved by varying frequency and exposure beam size, as well as by enforcing air convection. The latter is also used to reduce overheating of skin surface. Heating at different skin depths depending on these parameters is investigated in detail using the hybrid bio-heat equation. In particular, it is shown that decreasing frequency and/or increasing exposure beam size at forced airflow result in elevation of heating of deeper layers of tissue and decrease of skin surface temperature. Changes of water content within 15%, which exceed those due to aging and presence of tumors, only slightly affect heating. Exposure intensity necessary to reach a target temperature significantly increases in different areas of body with elevated blood flow. Dependence on exposure intensity and hyperthermia treatment duration is also investigated and discussed. Results of this study suggest that the lower part of the millimeter-wave range is an attractive alternative for non-invasive thermal treatment of skin cancer with a high spatial resolution.