Low-frequency dielectric properties of biological tissues : A review with some new insights
adio-frequency and microwave dielectric properties of biological materials have been of interest in connection with high-frequency and microwave dielectric heating, absorption of electromagnetic energy by living systems, and for nondestructive measurements of nonelectrical properties such as moisture content of agricultural products. These applications, including health hazards of energy absorption, medical diagnostics, and electromagnetic heating of tissues for medical purposes have spurred research on determining dielectric properties of such materials. There has been much less concentration on studies of low-frequency properties of biological materials. However, the behavior of materials exposed to low-frequency electromagnetic fields, and a better understanding of these phenomena, may open up new applications and potentially useful measurements of important characteristics in the future. Recent studies, related to material impedance and ion conduction through artificial membranes (Kuang, 1996), have opened an opportunity to present this review of low-frequency properties. With increasing interest in biological engineering opportunities, this review is written to provide some basic background for those interested in pursuing such studies. Readers are referred to the review by Foster and Schwan (1989) and other references cited for greater depth in the subject. Dielectric properties (electrical properties) of biological tissues have been of interest for many years. Information about tissue structure and composition, for example, water content or presence of a tumor, might be obtained by measuring the dielectric properties of the tissues. The understanding of interactions between electromagnetic energy and biological tissues must be based upon the knowledge of electrical properties of the tissues. This has many practical applications in agriculture, food engineering, and biomedical engineering. Dielectric properties of biological tissues are frequencydependent or dispersive. A significant change in dielectric properties over a frequency range, by convention, is called a dielectric dispersion. Although the dielectric properties of the tissues vary greatly from tissue to tissue, their typical behavior is characterized by three distinctly large dielectric dispersions, often referred to as α -, β-, and γ-dispersions (Schwan, 1957), as shown in figure 1. The α -dispersion usually occurs below a few kHz, the β-dispersion in the frequency region from tens of kHz to tens of MHz, and the γ-dispersion in the microwave frequency region. The γ-dispersion, arising mainly from polarization due to reorientation of water molecules, has been well studied and has found many applications in different areas (Thuery, 1991; Nelson, 1991; Metaxas and Meredith, 1983). Heterogeneous structure and composition and ionic activities inside the tissues dominate the low-frequency dielectric behavior of the tissues. The β-dispersion is well known to arise principally from interfacial polarization (Maxwell-Wagner effect) of biological membrane systems. This was observed mainly by studying blood tissues