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Role of Temperature Gradients and Convective-Diffusion Processes in Mechanisms of Biological Effects of Millimeter Waves


O.V. Betsky, A.S. Kozmin, E.E. Khizhnyak, E.P.Khizhnyak, M.A. Tsyganov, Yu.G. Yaremenko

The heating of biological tissue due to radiation absorption is one of the primary mechanisms of biological effects of electromagnetic irradiation (EMI). Biological effects of millimeter waves (EHF-radiation) are considerably different from the effects caused by the longer wavelength irradiation because: - The energy of millimeter wavelength radiation is absorbed within fractions of a millimeter, which leads to significant temperature gradients in the superficial layers of objects exposed to radiation; - The wavelength in biological tissues becomes comparable to the dimensions of biological structures; - Electromagnetic heterogeneity and the shape of biological objects become important factors; - Geometrical resonance occurs when objects are exposed in the near field area of radiating antenna, which leads to the formation of sharply non-uniform and highly frequency-dependent field distribution on the irradiated surface; - Temperature gradients that appear in the superficial layers of liquids become sufficient to modify convective flow patterns, and to create self-sustainable convective structures at very low levels of power density. Obviously each of the above mentioned factors has some contribution into the overall mechanism of biological action of millimeter waves. This paper takes an attempt to analyze the results of detailed studies of each of the above mentioned factors and to estimate the lowest threshold of millimeter-wave intensity up to which its biological effects can be explained by thermal mechanisms. Results of experimental studies of spatial heating patterns in biological tissue models exposed to millimeter wave radiation in the near field area of horn antennas are presented. Method of high resolution infrared thermography was used for real-time non-contact temperature measurements during millimeter-wave exposure. Hot spots formation due to heterogeneous properties and non-uniform shape of irradiated objects has been demonstrated. It was shown, that the evaporation process plays a significant role in the heating dynamics of liquids. Abnormal heating dynamics similar to a phase transition was observed 6-8 seconds after initiating irradiation. It has been demonstrated that millimeter waves have a significant influence on the convective and diffusion processes in the interfacial layers of multi-component water-containing solutions, which can lead to dissipative structures formation and temperature oscillations. Such effects can occur at extremely low levels of millimeter wave radiation and still nevertheless be explained by thermal mechanisms. It has been experimentally demonstrated, that insignificant in amplitude disturbances caused by millimeter waves can transform a stochastic system into a deterministic one.

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