Sensitivity of bulk electrical impedance spectroscopy (bio-capacitance) probes to cell and culture properties: Study on CHO cell cultures

Bulk electrical impedance spectroscopy (bio-capacitance) probes, hold significant promise for real-time cell monitoring in bioprocesses. Focusing on Chinese hamster ovary (CHO) cells, we present a sensitivity analysis framework to assess the impact of cell and culture properties on the complex permittivity spectrum, epsilon mix, and its associated parameters, permittivity increment, Delta epsilon, critical frequency, fc, and Cole-Cole parameter, alpha, measured by bio-capacitance probes. Our sensitivity analysis showed that Delta epsilon is highly sensitive to cell size and concentration, making it suitable for estimating biovolume during the exponential growth phase, whereas fc provides information about cumulative changes in cell size, membrane permittivity, and cytoplasm conductivity during the transition to death phase. The analysis indicated that specific information about cell membrane permittivity or internal conductivity cannot be extracted from epsilon mix spectrum. Based on the sensitivity analysis, we proposed two alternative parameters for monitoring cells in bioprocesses: Delta epsilon 1 MHz and Delta epsilon 1 MHz/Delta epsilon 0.3 MHz, using measurements at 300 kHz, 1 MHz, and 10 MHz. Delta epsilon 1 MHz is suitable for estimating viable cell density during the exponential growth phase due to its lower sensitivity to cell size. Delta epsilon 1 MHz/Delta epsilon 0.3 MHz can replace fc due to similar sensitivities to cell size and dielectric properties. These frequencies are within most bio-capacitance probes' optimal operation range, eliminating the need for low-frequency electrode polarization and high-frequency stray capacitances corrections. Experimental measurements on CHO cells confirmed the results of sensitivity analysis.