MTL Annual Research Report 2011

Mammalian cell cultures dominate the biopharmaceutical industry because their products, valuable biotherapeutics including monoclonal antibodies, vaccines and growth factors, account for 60% of the market. However, mammalian cells are also the most sensitive to changes in the culture environment, e. g., mechanical agitation, nutrient depletion, and waste byproduct accumulation. Hence, maintaining cell viability in mammalian cell cultures for an extended period of time is an important limiting factor for mammalian cell cultures, in particular Chinese Hamster Ovary (CHO) cell cultures ^[1] . Currently, state-of-the-art micro-bioreactors estimate cell density by measuring the turbidity of the culture using an Optical Density (OD) sensor ^[2] . Unfortunately, an OD sensor measures biomass, which includes live and dead cells. An online sensor that explicitly measures viable cells in a micro-bioreactor is necessary.

Dielectric spectroscopy is a promising online sensor for cell viability in micro-bioreactors ^[3] . The difference between the low frequency and high frequency capacitance measurements (ΔC = C_LF – C_HF) in the radio frequency regime gives the capacitance contributed by the cells to the total capacitance of the suspension. Due to the fact that most dead cells no longer have an intact membrane, defined as a membrane that is selectively impermeable to ions in the solution, they do not contribute to the capacitance reading (ΔC). By performing an initial calibration with live cell suspensions of different density, the ΔC per cell per mL can be determined for the particular cell line. The modeled calibration curve is shown in Figure 1. The measured capacitance of a cell culture of unknown live cell density can then be determined by comparing with the calibrated capacitance values. By combining dielectric spectroscopy with OD measurements, the percent cell viability can be utilized to optimize the yield of a mammalian cell culture in a micro-bioreactor. The first part of our project involves designing and calibrating a dielectric spectroscopy sensor for a micro-scale bioreactor, shown in Figure 2.