Biological tissue, dielectric properties

Dielectric Properties of Biological Tissue and Biophysical Mechanisms of Electromagnetic-Field Interaction... 

A. Water and Tissue Water. The dielectric properties of pure water have been well established from dc up to microwave frequencies, approaching the infrared (3). For all practical purposes they are characterized by a single relaxation process centered near 20 GHz at room temperature. Static and infinite frequency permittivity values are, at room temperature, close to 78 and 5, respectively. Hence, the microwave conductivity increase predicted by Eq. (1) is close to 0.8 mho/cm above 20 GHz, much larger than typical low-frequency conductivities of biological fluids which are about 0.01 mho/cm. The dielectric properties of water are independent of field strength up to fields of the order 100 kV/cm. 

Characteristic frequencies may be found from dielectric permittivity data or, even better, from conductivity data. The earlier data by Herrick et al. (6) suggest that there is no apparent difference between the relaxation frequency of tissue water and that of the pure liquid (7). However, these data extend only to 8.5 GHz, one-third the relaxation frequency of pure water at 37°C (25 GHz), so small discrepancies might not have been uncovered. We have recently completed measurements on muscle at 37°C and 1°C (where the pure water relaxation frequency is 9 GHz), up to 17 GHz. The dielectric properties of the tissue above 1 GHz show a Debye relaxation at the expected frequency of 9 GHz (8 ) (Figure 3). The static dielectric constant of tissue water as determined at 100 MHz compares with that of free water if allowance is made for the fraction occupied by biological macromolecules and their small amount of bound water (1, 9). 

The impedance of the skin has been generally modeled by using a parallel resistance/capacitor equivalent circuit (Fig. 4a). The skin s capacitance is mainly attributed to the dielectric properties of the lipid-protein components of the human epidermis [5,8,9,12]. The resistance is associated primarily with the skin s stratum comeum layer [5,8,9,12]. Several extensions to the basic parallel resistor/capacitor circuit model have appeared in the literature [5,8,9,13]. Most involve two modified parallel resistor/capacitor combinations connected in series [5,8,9]. The interpretation of this series combination is that the first parallel resistor/capacitor circuit represents the stratum comeum and the second resistor/capacitor parallel combination represents the deeper tissues [5,8,9]. The modification generally employed is to add another resistance, either in series and/or in parallel with the original parallel resistor/capacitor combination [8,9]. Realize that because all of these circuits contain a capacitance, they will all exhibit a decrease in impedance as the frequency is increased. This is actually what is observed in all impedance measurements of the skin [5,6,8-15]. In addition, note that the capacitance associated with the skin is 10 times less than that calculated for a biological membrane [12]. This... 

K. R. Foster and H. P Schwan, Dielectric properties of tissues and biological materials a critical review. Grit Rev. Biomed. Eng., 17, 25-104 (1989). 

Gabriel, C., Gabriel, S., and Corthout, E. 1996. The dielectric properties of biological tissues I. Literature... 

Gabriel S, Lau RW, Gabriel C. 1996. The dielectric properties of biological tissue 11. Measurements in the frequency range 10 Hz to 20 GHz. Phys Med Biol 41,2251-2269. 

The electrical properties of any material, including biological tissue, can be broadly separated into two categories conducting and insulating. In a conductor, the electric charges move freely in response to the application of an electric field, whereas in an insulator (dielectric), the charges are fixed and not free to move. 

The dielectric properties of biological cells and tissues have been of interest for nearly a hundred years. The early work of Maxwell [1] and Wagner [2], Fricke [3], Cole [4], and... 

This review highlights current tissue engineering and novel therapeutic approaches to axonal regeneration following spinal cord injury. An important role in creation of functional, biological scaffolds for tissue engineering play theirs stiffness, thermostability, porosity and dielectric properties. 

The dielectric properties of human tissue have been extensively studied. Whereas some groups refer to one particular tissue, e.g. skin [2], Gabriel et al. (1996) created a database of a wide variety of biological materials [3-6], including an electronic database [7]. Although this database is useful and used by many research groups, it is not complete and has several limitations, especially at frequencies below 1 MHz, as mentioned by the authors themselves [6]. 

Nyboer, J., 1970b. Electrorheometric properties of tissues and fluids. Ann. N.Y. Acad. Sci. 170, 410—420. Pethig, R., 1979. Dielectric and Electronic Properties of Biological Materials. John Wiley. 

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