Substitution and spreadsheet methods for analysing dielectric spectra of biological systems

Abstract

1. Two major problems are encountered when one wishes to fit audio- and radio-frequency dielectric spectra of biological cell suspensions (or other materials): (a) changes in the apparent frequency-dependent permittivity of the system due to the phenomena of electrode polarisation can dominate those due to the biological system, and (b) because of the overlap of different dispersions it may be very difficult to deconvolute the individual contributions of the underlying biophysical mechanisms. 2. The extent of electrode polarisation depends substantially upon the conductivity of the medium surrounding the cells, but only marginally on the nature of the ions of a given valency contribution to it. 3. This, and the fact that the apparent time constants of the phenomena contributing to electrode polarisation are much greater than those of biological dielectric dispersions, permits one to use a simple substitution method to extract the latter in the presence of the former. This is shown both by simulation and by experiments using suspensions of human erythrocytes. 4. A spreadsheet method is described for the display of dielectric data and their conformance to the double Cole-Cole equation. The method provides a rapid and convenient approach, based on interactive graphical outputs, for the fitting of dielectric data to this equation. 5. Estimates derived from the spreadsheet program may be used in a BASIC program to arrive at the optimal fit. 6. The method is applied to the strongly-overlapping α- and β-dispersions of erythrocytes, permitting their deconvolution and providing a high level of accuracy.