**6. Biological effect of microwave power**

Research studies on the interactions between the EM field and biological systems have been the subject of high interest during the last decades. Here, we would like to give more details about such kind of research and obtained technical and biological results (i.e., basic description of implemented exposure systems). Two of our recent projects were oriented on the research of thermal effects of EM field (using either

waveguide or array applicators). And the third one then on the research of nonthermal effects. Whole-body exposure chamber, operating at 900 MHz, was developed for small animals in the frame of this research project. The setup was designed with respect to homogeneity of induced EM field, elimination of external radiation, and exact determination of absorbed power. Further sufficient space for mice movement was taken into account. The whole-body exposure chamber with an anatomical mouse model was simulated by two different numerical methods, e.g., finite-differencetime-domain method (FDTD) and finite integration technique (FIT), and compared computed SAR values and its dosimetry results.

#### **6.1 Exposure chamber**

The major advantage of the system we will describe here is the capability of direct measurement of the whole-body averaged SAR, which is performed by analysis of measured scattering parameters. As the basic idea and principle of the discussed exposure chamber, a circular waveguide was chosen. The advantage of the waveguide structure is a perfect shielding of EM field generated either inside (in order to protect the operators) or generated outside the system (in order to eliminate interference caused by external EM fields). The circularly polarized wave TE11 is excited inside the exposure chamber with the aid of two monopoles that have mutually orthogonal orientations, and the distance between them is equal to one-fourth of the wavelength. Such circularly polarized wave provides relatively constant field coupling to each mouse regardless of its position, posture, or movement. The discussed exposure chamber is displayed in **Figure 11**.

EM field distribution and impedance matching of the discussed exposure chamber were optimized and verified by 3D EM field simulators SEMCAD X resp. Sim4Life. Dimensions of the exposure chamber were calculated to use the desired frequency of operation and the volume needed to expose mice. The exposure chamber is made of a copper cylinder with dimensions of 1650 mm in length and 240 mm in diameter. It is terminated by matched loads at both ends (conical shape, 500 mm long, and made of RF absorbers). The reflection loss of the matched load is more than 20 dB at 900 MHz.

The exposed mice are kept in a cylindrical box that is made of Styrofoam. Styrofoam has a dielectric constant of 1.03, i.e., very close to that of air, and thus, the disturbance of exposure and measurements is negligible. The box provides space for two separated mice. Punctured slit-like holes are set on the cover and side of the box for air ventilation. In the study, the mice were held in the chamber only during RF exposures, and therefore, no food or drinking water was necessary.

For the survival of experimental animals inside the exposure chamber, it is important to create efficient ventilation, which will maintain a constant temperature and good air quality in the chamber. The air comes toward mice through the ventilation

**Figure 11.** *Waveguide-type exposure chamber for animal experiments.*

hole placed below the styrofoam box and flows toward the second opposite ventilation hole placed above the box.

To be able to evaluate the results of experiments with small animals (mice in our case), we need to specify appropriate dosimetry. It is the quantification of the magnitude and distribution of absorbed EM energy within biological objects that are exposed to EM fields. In the case of radiofrequency and microwave frequency bands, there is the dosimetric quantity, which is called SAR (i.e., specific absorption rate). It is defined as the rate at which energy is absorbed per unit mass. The SAR is determined and influenced not only by the incident EM waves but also by the electrical and geometric characteristics of the irradiated subject and objects nearby it. It is strongly related to the internal electric field strength E as well as to the electric conductivity σ and the density of tissues ρ as discussed above and as can be seen, reminded by the following equation.

$$\text{SAR} = \sigma \text{.E}^2 / 2\rho \text{ (W/kg)}\tag{10}$$

Therefore, SAR is a suitable dosimetric parameter, even when a studied mechanism is determined to be "athermal." SAR distributions are usually determined from measurements in animal tissues or from numerical calculations. It generally is difficult to measure the SAR directly in a living biological body, and therefore, dosimetry efforts are forced to rely on computer simulations mainly.

An anatomically based dielectric model of an experimental animal is essential for numerical dosimetry. It can be developed commonly from MRI or CT scans. In order to develop it, original gray-scale data must be interpreted into tissue types known as a process of segmentation. In our studies, the CT scans for mouse model development were obtained from the website: http://neuroimage.usc.edu/Digimouse\_download.h tml. The mouse model has the resolution 0.1 mm, meaning voxel size 0.1 x 0.1 x 0.1 mm. Each voxel was assigned to one of 14 different tissue types, such as bone, muscle, brain, etc.

For dosimetry with the numerical voxel models, proper permittivity and conductivity values must be assigned to each tissue. The data from 10 MHz to 6 GHz, derived from 4-Cole-Cole extrapolation based on measurements for small animals, constitute the most widely accepted database for this information. The data are recommended by various international standardization organizations and can be accessed, e.g., from the website http://www.fcc.gov/fcc-bin/dielec.sh.

#### **6.2 Result of biological experiment**

In order to verify and rely on numerical dosimetry results, the simulations of the exposure chamber were done in two different EM field simulators (based on two different numerical methods). Our choice was SEMCAD X, which uses the finite difference time domain (FDTD) method, and CST Microwave Studio, which uses the finite integration technique (FIT) method. We used these simulations to the determination of SAR distribution inside the mice during experiments.

Researchers from Medical Faculty in Pilsen, Charles University (Prof. František Vožeh, MD., Jan Barcal, MD.), did biological experiments with the aid of this exposure chamber. With the aim of whether EM exposure can increase the content of free radicals in the exposed tissue, a series of EM exposures to small animals (mice) was done. SAR level was at the level of 0.8 W/kg in the case of these experiments.

*Applications of Microwaves in Medicine and Biology DOI: http://dx.doi.org/10.5772/intechopen.105492*

#### **Figure 12.**

*Results of an animal experiment: in all four studied organ specimens (brain, heart, kidney, and liver), significantly increased content of free radicals was found.*

Evaluation of preliminary results is displayed in **Figure 12**. It can be interpreted as a significantly increased content of free radicals being found.
