Understanding the effect of high gas entrance velocity on Chinese hamster ovary (CHO) cell culture performance and its implications on bioreactor scale-up and sparger design.

There are three main potential sources for cell shear damage existing in stirred tank bioreactors. One is the potential high energy dissipation in the immediate impeller zones; another from small gas bubble burst; and third is from high gas entrance velocity (GEV) emitting from the sparger. While the first two have been thoroughly addressed for the scale-up of Chinese hamster ovary (CHO) cell culture knowing that a wide tolerable agitation range with non-damaging energy dissipation exists and the use of shear protectants like Pluronic F68 guard against cell damage caused by bubble burst, GEV remains a potential scale-up problem across scales for the drilled hole or open pipe sparger designs. GEV as high as 170 m/s due to high gas flow rates and relatively small sparger hole diameters was observed to be significantly detrimental to cell culture performance in a 12,000 L bioreactor when compared to a satellite 2 L bioreactor run with GEV of <1 m/s. Small scale study of GEV as high as 265 m/s confirmed this. Based on the results of this study, a critical GEV of >60 m/s for CHO cells is proposed, whereas previously 30 m/s has been reported for NS0 cells by Zhu, Cuenca, Zhou, and Varma (2008. Biotechnol. Bioeng., 101, 751-760). Implementation of new large scale spargers with larger diameter and more holes lowered GEV and helped improve the cell culture performance, closing the scale-up gap. Design of such new spargers was even more critical when hole plugging was discovered during large scale cultivation hence exacerbating the GEV impact. Furthermore, development of a scale down model based on mimicry of the large scale GEV profile as a function of time was proven to be beneficial for reproducing large scale results.