**The Age-Dependence of Microwave Dielectric Parameters of Biological Tissues**

Mimoza Ibrani, Luan Ahma and Enver Hamiti

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51400

## **1. Introduction**

138 Microwave Materials Characterization

IEEE Trans Biomed Eng. 2000 Feb;47(2):202-8

frequency of 900 MHz Bioelectromagnetics, 21(4): 255-63.

[34] Pickard WF, Straube WL, Moros EG (2000) Experimental and numerical determination of SAR distributions within culture flasks in a dielectric loaded radial transmission line.

[35] Laval L, Leveque P, Jecko B (2000) A new in vitro exposure device for the mobile

Today's children are being exposed to electromagnetic fields even before they come to this world, while they are growing up in an environment polluted, amongst others, by the dense microwave electromagnetic flux.

Daily exposure, both indoor and outdoor, of younger generations to microwave electromagnetic fields is being followed with the raised concerns regarding possible biological and health effects induced as a result of exposure. Therefore there are many published scientific papers, ongoing research projects and awarness raising campaigns aiming to inform general public and other stakeholders about safety of microwave exposure.

In order to ensure the public safety, based on research evidence, the relevant authorities have developed and announced guidelines and limits for exposure to electromagnetic fields.

Even though there are recommendations and exposure standards and limits given at international level as ICNIRP [1] and IEEE [2], few countries have set even more rigorous country specific exposure limits in comparison with international ones, in terms of setting precautionary measures.

International safety standards and guidelines on exposure limits to electromagnetic fields have been developed based on research evidence for adults, and even though each of them include a specific safety margin it should be confirmed they remain valid for children as well.

Children have longer life time exposure to microwaves in comparison with adults since they are exposed to microwave electromagnetic fields at earlier age in comparison with adults, so the cumulative exposure effect is not to be neglected.

© 2012 Ibrani et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Ibrani et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

At ICNIRP standards [1] two classes of guidance are presented: 1) Basic restrictions: Restrictions on exposure to time-varying electric, magnetic, and electromagnetic fields that are based directly on established health effects. Depending upon the frequency of the field, the physical quantities used to specify these restrictions are current density (J), specific energy absorption rate (SAR), and power density (S) .Only power density in air, outside the body, can be readily measured in exposed individuals.; 2) Reference levels: These levels are provided for practical exposure assessment purposes to determine whether the basic restrictions are likely to be exceeded. Some reference levels are derived from relevant basic restrictions using measurement and/or computational techniques, address perception and adverse indirect effects of exposure to electromagnetic fields. The derived quantities are electric field strength (E), magnetic field strength (H), magnetic flux density (B), power density (S), and currents flowing through the limbs.

At microwave frequencies the basic restrictions are given in terms of SAR- parameter used to assess absorption of electromagnetic energy by biological tissue.

The SAR, time rate of RF energy absorbed per unit mass of body biological tissue, is given as:

$$SAR = \frac{(\sigma + w \,\varepsilon\_0 \,\varepsilon\_r^{\circ}) E\_i^2}{\rho} \quad \text{(W/kg)}\tag{1}$$

Where


*<sup>i</sup> E* - induced value of electrical field as a result of exposure to external field.

As noticed from relation (1) the SAR depends directly from: induced intensity of electrical field, density of biological tissue and electromagnetic properties of biological tissues at given point. Therefore, a first step in any analysis related with SAR determination is the derivation of electromagnetic properties of biological tissues.

Looking in the other aspect the electromagnetic parameters of human biological tissues are considered very important in the wide range of biomedical applications as reported on [3] such as functional electrical stimulation, diagnosis and treatment of various physiological conditions with weak electric currents, radio-frequency hyperthermia, electrocardiography etc. On a more fundamental level, knowledge of these electrical properties can lead to an understanding of the underlying basic biological processes, for example: one of the first demonstrations of the existence of the cell membrane was based on dielectric studies on cell suspensions.

As for all mediums, the constitutive electromagnetic parameters of biological tissues are the electric permittivity , the magnetic permeability and the electric conductivity σ. These parameters describe the interaction of external electromagnetic field with medium and determine the pathways of current flow through the human body.

140 Microwave Materials Characterization

as:

Where

*r* 

0 

density (S), and currents flowing through the limbs.



*w* Angular frequency, ''


suspensions.


to assess absorption of electromagnetic energy by biological tissue.

*<sup>i</sup> E* - induced value of electrical field as a result of exposure to external field.

derivation of electromagnetic properties of biological tissues.

 

As noticed from relation (1) the SAR depends directly from: induced intensity of electrical field, density of biological tissue and electromagnetic properties of biological tissues at given point. Therefore, a first step in any analysis related with SAR determination is the

Looking in the other aspect the electromagnetic parameters of human biological tissues are considered very important in the wide range of biomedical applications as reported on [3] such as functional electrical stimulation, diagnosis and treatment of various physiological conditions with weak electric currents, radio-frequency hyperthermia, electrocardiography etc. On a more fundamental level, knowledge of these electrical properties can lead to an understanding of the underlying basic biological processes, for example: one of the first demonstrations of the existence of the cell membrane was based on dielectric studies on cell

At ICNIRP standards [1] two classes of guidance are presented: 1) Basic restrictions: Restrictions on exposure to time-varying electric, magnetic, and electromagnetic fields that are based directly on established health effects. Depending upon the frequency of the field, the physical quantities used to specify these restrictions are current density (J), specific energy absorption rate (SAR), and power density (S) .Only power density in air, outside the body, can be readily measured in exposed individuals.; 2) Reference levels: These levels are provided for practical exposure assessment purposes to determine whether the basic restrictions are likely to be exceeded. Some reference levels are derived from relevant basic restrictions using measurement and/or computational techniques, address perception and adverse indirect effects of exposure to electromagnetic fields. The derived quantities are electric field strength (E), magnetic field strength (H), magnetic flux density (B), power

At microwave frequencies the basic restrictions are given in terms of SAR- parameter used

The SAR, time rate of RF energy absorbed per unit mass of body biological tissue, is given

'' 2 <sup>0</sup> ( ) (/) *w E r i SAR W kg*

(1)

The conductivity of a medium may be considered as a measure of the ability of its charge to be transported throughout its volume by an applied electric field. Similarly, permittivity is a measure of the ability of its dipoles to rotate or its charge to be stored by an applied external field [3].

For almost all mediums the electromagnetic parameters vary with the frequency of the applied signal. Such a variation is called dispersion. Biological tissues exhibit several different dispersions over a wide range of frequencies, as shown in Figure 1, and described in detail in [3]. Dispersion is characterized with the relaxation time or equivalently with relaxation frequency and can be described in terms of the orientation of dipoles and the motion of the charge carriers.

**Figure 1.** Frequency dependence of the dielectric parameters of biological tissues [3]

Biological tissues may be considered as materials with 0 thus it is of interest derivation of two others parameters: permittivity and conductivity known as dielectric parameters of tissues.

Dielectric parameters of tissues are functions of frequency, but they also depend on temperature and tissue organic composition.

A few studies have presented the variation of permittivity and conductivity as a function of tissue age, triggering scientific community to explore about possible difference of microwave energy absorption between ages and finding more sensitive age- groups to electromagnetic fields exposure.

The age –dependence of dielectric properties of biological tissues mostly relies on the fact that permittivity and conductivity may be expressed as a function of tissue water content (TBW- Total Body Water Content) that is function of age, respectively decreases with age.

The TBW also differs at people of the same age while it is a function of gender as well.

The newborns have higher water content in comparison with fat while biggest part of their mass is composed by visceral organs. Beside water content also development of different organs with age is to be considered.

Therefore there are arguments to elaborate the age-dependence of dielectric properties of biological tissue, and assign different values for different age-groups, especially pointing out the difference between the children, adults and elderly people dielectric characteristics.

*Up to date, based on our knowledge, there is no database with permittivity and conductivity of children biological tissues.* 

As a consequence, in many electromagnetic dosimetry research studies the dielectric properties of biological tissues for adults are used due to the lack of information related to children.

The age-dependence of dielectrical parameters of human biological tissues is still debatable at scientific forums and studies are mainly focused on two points:


For determination of children biological tissues dielectric parameters, and comparison of them with adult dielectric parameters afterward, based on ongoing research, there are two major possibilities:

 *In vivo* measurement of dielectric characteristics of animal biological tissues at different animal age aiming to confirm the age dependence of dielectric properties of animal tissues, where the main raised issue is the extrapolation of obtained results from animal to human biological tissues, respectively finding a correlation of dielectric characteristics of animal and human at different ages.

Up to date, based on our knowledge, there is no systematic scientific confirmed extrapolation methodology.


In the chapter, the above mentioned methods will be considered, and the main studies on the field will be discussed, by outlining also their advantages and disadvantages from our point of view.

It has to be mentioned that for few external biological tissues, as for the skin as an example, there is a possibility of *in vivo* measurement for determination of dielectric properties. In a report derived by a research project [4] the study with volunteers has been conducted presenting the results for permittivity and conductivity as obtained with in vivo measurements including a detailed analysis of possible measurement uncertainities.

142 Microwave Materials Characterization

*children biological tissues.* 

children.

and

major possibilities:

expressions.

point of view.

extrapolation methodology.

tissues as a function of age.

organs with age is to be considered.

The TBW also differs at people of the same age while it is a function of gender as well.

The newborns have higher water content in comparison with fat while biggest part of their mass is composed by visceral organs. Beside water content also development of different

Therefore there are arguments to elaborate the age-dependence of dielectric properties of biological tissue, and assign different values for different age-groups, especially pointing out the difference between the children, adults and elderly people dielectric characteristics. *Up to date, based on our knowledge, there is no database with permittivity and conductivity of* 

As a consequence, in many electromagnetic dosimetry research studies the dielectric properties of biological tissues for adults are used due to the lack of information related to

The age-dependence of dielectrical parameters of human biological tissues is still debatable

Finding a relation that will confirm possible age-dependence of dielectric properties of

 What impact will have the age-dependent dielectric properties on induced SAR values as a result of exposure to incident electromagnetic fields, respectively in dosimetric point of view what will be the difference of microwave energy absorption by different

For determination of children biological tissues dielectric parameters, and comparison of them with adult dielectric parameters afterward, based on ongoing research, there are two

 *In vivo* measurement of dielectric characteristics of animal biological tissues at different animal age aiming to confirm the age dependence of dielectric properties of animal tissues, where the main raised issue is the extrapolation of obtained results from animal to human biological tissues, respectively finding a correlation of dielectric

Up to date, based on our knowledge, there is no systematic scientific confirmed

Derivation of empiric formulas for estimating dielectric parameters of human biological


In the chapter, the above mentioned methods will be considered, and the main studies on the field will be discussed, by outlining also their advantages and disadvantages from our

Are the permittivity and the conductivity of human biological tissues a function of age?

at scientific forums and studies are mainly focused on two points:

age biological tissue for the same exposure conditions?

characteristics of animal and human at different ages.

human biological tissues, respectively answering the question:
