Multifrequency permittivity measurements enable on‐line monitoring of changes in intracellular conductivity due to nutrient limitations during batch cultivations of CHO cells

Lab and pilot scale batch cultivations of a CHO K1/dhfr − host cell line were conducted to evaluate on‐line multifrequency permittivity measurements as a process monitoring tool. The β‐dispersion parameters such as the characteristic frequency ( f C ) and the permittivity increment (Δε max ) were calculated on‐line from the permittivity spectra. The dual‐frequency permittivity signal correlated well with the off‐line measured biovolume and the viable cell density. A significant drop in permittivity was monitored at the transition from exponential growth to a phase with reduced growth rate. Although not reflected in off‐line biovolume measurements, this decrease coincided with a drop in OUR and was probably caused by the depletion of glutamine and a metabolic shift occurring at the same time. Sudden changes in cell density, cell size, viability, capacitance per membrane area ( C M ), and effects caused by medium conductivity (σ m ) could be excluded as reasons for the decrease in permittivity. After analysis of the process data, a drop in f C as a result of a fall in intracellular conductivity (σ i ) was identified as responsible for the observed changes in the dual‐frequency permittivity signal. It is hypothesized that the β‐dispersion parameter f C is indicative of changes in nutrient availability that have an impact on intracellular conductivity σ i . On‐line permittivity measurements consequently not only reflect the biovolume but also the physiological state of mammalian cell cultures. These findings should pave the way for a better understanding of the intracellular state of cells and render permittivity measurements an important tool in process development and control. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010