Bioelectromagnetics
Bioelectromagnetics. Author manuscript; available in PMC 2012 Sep 1. Published in final edited form as: Bioelectromagnetics. 2011 Sep; 32(6): 423–433.
Published online 2011 Feb 22. doi: 10.1002/bem.20658
PMCID: PMC3118398
NIHMSID: NIHMS274632
ENHANCED ABSORPTION OF MILLIMETER WAVE ENERGY IN MURINE SUBCUTANEOUS BLOOD VESSELS
Stanislav I. Alekseev and Marvin C. Ziskin
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Abstract
The aim of the present study was to determine millimeter wave (MMW) absorption by blood vessels traversing the subcutaneous fat layer of murine skin. Most calculations were performed using the finite-difference time-domain (FDTD) technique. We used two types of models: (1) a rectangular block of multilayer tissue with blood vessels traversing the fat layer and (2) cylindrical models with circular and elliptical cross sections simulating the real geometry of murine limbs. We found that the specific absorption rate (SAR) in blood vessels normally traversing the fat layer achieved its maximal value at the parallel orientation of the E-field to the vessel axis. At 42 GHz exposure, the maximal SAR in small blood vessels could be more than 30 times greater than that in the skin. The SAR increased with decreasing the blood vessel diameter and increasing the fat thickness. The SAR decreased with increasing the exposure frequency. When the cylindrical or elliptical models of murine limbs were exposed to plane MMW, the greatest absorption of MMW energy occurred in blood vessels located on the lateral areas of the limb model. At these areas the maximal SAR values were comparable with or were greater than the maximal SAR on the front surface of the skin. Enhanced absorption of MMW energy by blood vessels traversing the fat layer may play a primary role in initiating MMW effects on blood cells and vasodilatation of cutaneous blood vessels.
Keywords: murine skin, FDTD technique, specific absorption rate
INTRODUCTION
Millimeter waves (MMW) have been used for the therapeutic treatment of different medical conditions including cardiovascular diseases, wound healing, pain relief, etc. [Rojavin and Ziskin, 1998]. Significant results were achieved with the application of three “therapeutic” frequencies: 42.25, 53.57 and 61.22 GHz at the incident power densities (IPD) of 10–30 mW/cm2. This has stimulated great interest in better understanding the biological mechanisms of MMW action and accurate dosimetry of MMW exposures. In recent publications we have described MMW dosimetry for human and murine skin [Alekseev et al., 2008a, b]. Since most experiments with MMW were performed on mice, the determination of the power density distribution in murine skin was important in order to make adequate extrapolation of the MMW effects found in mice to humans. Due to the small size of murine skin, MMW penetrate deep enough into tissue to reach the muscle layer. Up to 40% of MMW energy entering the skin is absorbed by muscle. Hence, the blood vessels located in the dermis and traversing the fat layer are subjected to MMW exposure with relatively high intensity. Some papers showed that MMW could affect blood cells causing significant biological effects [Gapeev et al., 1996; Roshchupkin et al., 1996]. Therefore, the accurate determination of MMW absorption by cutaneous blood vessels is important in understanding the primary mechanisms of the biological action of MMW.
The skin structures such as blood vessels and appendages (hair and sweat ducts) with electrical properties different from the average electrical properties of skin tissues cause selective absorption of MMW energy and local distortion of the MMW field in their vicinity [Alekseev and Ziskin, 2001, 2009a]. In our models the skin was irradiated with a plane wave normally incident on the skin surface. We found that in blood vessels located in the human and murine dermis and oriented parallel to the E-field, the specific absorption rate (SAR) could exceed the average SAR in the surrounding dermis by ~40% [Alekseev and Ziskin, 2009a].
Some papers showed that small holes in thin hydrophobic films with low permittivity (Teflon, acryl glass) used for the formation of bilayer lipid membranes could significantly absorb microwave energy [Eibert et al., 1999; Alekseev et al., 2009b, 2010]. As the hydrophobic film was placed in an electrolyte, the hole was also filled with the same bulk electrolyte. The enhanced absorption of microwave energy occurred at the orientation of the hydrophobic film perpendicular to the E-field, with the cylindrical axis of the hole being parallel to the E-field. Similar conditions for the enhanced absorption of MMW energy by blood vessels in murine skin may occur when the blood vessels traverse the fat layer. The fat layer, having lower permittivity than the surrounding dermis and muscle, may simulate a low permittivity hydrophobic film, while a blood vessel traversing the fat layer may simulate the membrane- forming hole.
The blood vessels feeding the epidermis and dermis enter and exit the dermis through the fat layer. At the boundary between the dermis and hypodermis they form a network located parallel to the boundary surface [Bloom and Fawcett, 1968]. From one side of this network small vessels enter the hypodermis and nourish fat cells. From another side of the network vessels enter the dermis. The sizes of blood vessels of murine skin do not differ significantly from those of human skin [Zweifach and Kossman, 1937; Algire, 1954; Braverman, 1989]. The internal diameters of the capillaries in mice are in the range of 3–10 μm. The diameters of the deepest dermal vessels are generally 40 to 50 μm.
When flat areas of skin are irradiated with plane MMW at normal incidence the E- and H-field vectors are directed parallel to the skin surface, and hence, perpendicular to the blood vessels traversing the fat layer. However, during whole-body exposure in the far field [Jauchem et al., 1999, 2004; Gapeyev et al., 2006, 2008] the orientation of the E-field with respect to the skin surface changes with the curvatures of the body parts. For example, the orientation of the E-field with the skin surface of limbs, fingers, and tail may vary from being perpendicular to being nearly parallel to the blood vessels traversing the fat layer. In some biological experiments, mice were exposed at the nose and limb areas in the near field of a horn antenna [Radzievsky et al., 2000, 2008]. In this case, the E-field around the nose and limbs may take any direction relative to the skin surface including the perpendicular direction. Therefore, it is important to evaluate the MMW energy absorption by blood vessels traversing the fat layer at different orientations of the E-field.
The aim of the present study was to determine the SAR and E-field distributions in blood vessels traversing the subcutaneous fat layer of murine skin exposed to a plane MMW.
MATERIALS AND METHODS
To calculate the E-field and SAR in blood vessels traversing the fat layer we used two types of models: a rectangular block of skin tissue (Model I) and a cylindrical model with circular or elliptical cross sections simulating a murine limb (Model II). Model I was purely theoretical. This model did not represent the whole of or a separate organ of animals; it reproduced just a small part of the body (limb, finger, etc.) being exposed to MMW. The orientations of the field vectors relative to the blood vessels in the fat layer used in the block model could occur in the limbs and other organs of animals as shown in simulations of MMW exposure of limbs. Because of its simple geometry, Model 1 is very useful in understanding the physics of MMW interactions with blood vessels. It allows us to evaluate the maximal SAR and E-field values achievable with the given geometries of skin tissues and blood vessels, especially when the diameter of blood vessels is extremely small (capillaries). Model II simulated separate organs (limbs) of animals.
Most calculations of the E-field and SAR distributions were performed using the 3D finite- difference time-domain (FDTD) technique [Kunz and Luebbers, 1993]. For estimation of the maximal SAR in the narrow blood vessels (capillaries), we used a quasistatic approximation [Olver, 1992].
A. Models used for FDTD calculations
In Model I, we used a rectangular block of murine tissue containing the epidermis plus dermis (skin), fat and muscle layers (Fig. 1). In most calculations the thicknesses of the epidermis plus dermis and fat layers were set equal to 0.25 and 0.2 mm, respectively. These values were within the range found for murine skin [Dang et al., 2005; Grover et al., 2007;Alekseev et al., 2008b]. To determine the dependence of the SAR on the length of a blood vessel equal to the thickness of a fat layer, the latter was varied in the range of 0.05–0.5 mm. All tissue layers were located parallel to the skin surface along the direction of a plane wave as shown in Figure 1. A blood vessel oriented parallel to the E-field was placed traversing the fat layer, and located 0.4–0.8 mm from the side of the block closest to the MMW source. This location corresponded with the penetration depth of the 30–80 GHz MMW used in our calculations. A blood vessel was modeled by circular cylinders of different diameters (0.01– 0.4 mm) and lengths (0.05–0.5 mm). Because the arteries feeding the skin are often paired with veins [Bloom and Fawcett, 1968], we examined the influence of the distance between two blood vessels on the SAR inside the blood vessels.
2011-Alexeev-Ziskin-Bioelectromagnetics