Monitoring of yeast cell concentration using a micromachined impedance sensor

This paper describes the design, modeling and experimental characterization of a micromachined impedance sensor for on-line monitoring of the viable yeast cell concentration (biomass) in a miniaturized cell assay. Measurements in Saccharomyces cerevisiae cell culture show that the characteristic frequency describing the β-dispersion of S. cerevisiae cells is around 2.8 MHz. The permittivity change of the cell suspension was measured for the concentration range 0–9 g/l and depends linearly on the biomass concentration. In order to compensate the measurements for the electric properties of the background electrolyte, which increases the sensitivity and allows measurements in different media, the use of a three-electrode configuration in combination with a semi-permeable poly(2-hydroxyethyl methacrylate) (pHEMA) membrane was explored. Measurements show that the impedance of hydrated pHEMA varies with the background electrolyte conductivity only, and not with the concentration of cells, indicating that pHEMA is suitable for this purpose. The optimal pHEMA membrane thickness was determined using finite-element modelling and was found to be 1 μm for the electrode configuration under study.