THE DIELECTRIC PROPERTIES OF FLUID SATURATED BONE

The dielectric permittivity and electrical conductivity of fluid saturated rat femoral bone were studied as a function of the matrix condition (i.e. freshly excised and formalin fixed specimens) and as a function of the conductivity of the immersion solution. The electrical properties were measured at 37(DEGREES)C in a direction normal to the long axis of bone at frequencies from 10 Hz to 100 MHz. The electrical measurements were made using an impedance analyzer and a vector impedance meter under microcomputer control. An impedance cell of new design was employed that permits in vitro measurements on the tissue while preserving its natural geometry. The results of these measurements indicate that the dielectric dispersion in bone can be summarized by a Cole-Cole function having a mean relaxation frequency that is proportional to the conductivity of this fluid; the mean relaxation frequency is about 6 kHz for bone immersed in a solution having a conductivity of 1.0 S/m. The distribution parameter of the Cole-Cole function is about 0.5, and limiting values of the permittivity are about 1000 at low frequencies and about 10 at high frequencies. The high frequency limit is consistent with the value determined from mixture theory for a tissue having a water content of 15-20% by volume and with a permittivity of the anhydrous matrix of 6. The dc conductivity of the tissue is proportional to about 1 percent of the conductivity of the immersion fluid. A model was developed to predict the dc conductivity from the number and orientation of the capillaries in the bone and the conductivity of the fluid within these canals. This simple model was demonstrated to adequately characterize the dc conductivity of the tissue. The dielectric dispersion in bone has been shown to be most likely due to a frequency dependent surface admittance, which has been attributed to either a counterion polarization mechanism occurring in the region adjacent to the collagen molecules, or to ion diffusion along various interfacial surfaces of the bone