CO<sub>2</sub> Control Strategy for Large-Scale Cell Culture Bioreactor Operation

In most mammalian cell culture operations, the pH is targeted to be close to neutral and the dissolved carbon dioxide [dCO₂] concentration is desired to remain between 5 and 15% to avoid any inhibitory effects on cell growth. Typical cell culture scale-up approaches include maintaining constant power by volume (P/V) or a constant tip speed to set the impeller agitation rate or constant vvm to set the gas flow rate. However, these approaches are only focused on keeping the shear in the bioreactor system to a minimum and do not account for controlling the [dCO₂] concentration within the desired range. Process engineers across industries have remediated the elevated [dCO₂] concentration problem in large scale bioreactors by increasing gas flow rates; however, this approach is often trial and error. Therefore, in this article we review the current understanding of various factors that impact the dCO₂ concentration during the scale up of the cell culture process to large-scale bioreactors. This article also describes an easy and practical approach to predict and control the dCO₂ concentration in large-scale cell culture bioreactors using a mathematical predictive model developed based on mass-transfer first principles. We demonstrate the effective application and verification of the model by running a CHO cell culture process with a peak cell density of up to 20 x106 Cells/mL in a 15,000 L bioreactor working volume.