The capacitance of suspensions of CHO and HeLa cells (0.5-3 x 10(6) cells/ml) has been measured between 0.2 and 10 MHz. As frequencies decrease, there is a continuous increase in capacitance of both the cell suspension and the spent growth medium free of cells, a phenomenon which is partially attributed to an increased polarisation of the electrodes. At a given frequency, subtraction of the capacitance of the spent medium from that of the cell suspension allows one to determine the capacitance of the cells only. The intensity of this signal varies linearly with the biomass and cell size. At low frequencies such as those used in this study (0.25 MHz), where sensitivity is the highest, concentrations as low as 0.5 x 10(6) cells/ml can be accurately measured. Suggestions are made how to make these measures on-line, non-invasive and in real time.
Low-frequency dielectric spectroscopy has been used in situ, i.e. while the cells are still attached to their microsupport, to monitor the changes of biomass accompanying the growth of anchorage-dependent cells. This method, when compared to Aperture Impedance Pulse Spectroscopy (also called electronic sizing), is characterized by a somewhat lower degree of resolution. Suggestions are made on how to determine the capacitance of the spent growth medium alone, still keeping the probe inserted in the bioreactor. This will make dielectric spectroscopy the first truly in situ, on-line, in real time, non-invasive measure of the biomass.
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