This article mainly addresses the issues associated with the engineering of large-scale free suspension culture in agitated bioreactors >10,000 L because they have become the system of choice industrially. It is particularly concerned with problems that become increasingly important as the scale increases. However, very few papers have been written that are actually based on such large-scale studies and the few that do rarely address any of the issues quantitatively. Hence, it is necessary very often to extrapolate from small-scale work and this review tries to pull the two types of study together. It is shown that ‘shear sensitivity’ due to agitation and bursting bubbles is no longer considered a major problem. Homogeneity becomes increasingly important with respect to pH and nutrients at the largest scale and sub-surface feeding is recommended despite ‘cleaning in place’ concerns. There are still major questions with cell retention/recycle systems at these scales, either because of fouling, of capacity or of potential and different ‘shear sensitivity’ questions. Fed-batch operation gives rise to cell densities that have led to the use of oxygen and enriched air to meet oxygen demands. This strategy, in turn, gives rise to a CO2 evolution rate that impacts on pH control, pCO2 and osmolality. These interactions are difficult to resolve but if higher sparge and agitation intensities could be used to achieve the necessary oxygen transfer, the problem would largely disappear. Thus, the perception of ‘shear sensitivity’ is still impacting on the development of animal cell culture at the commercial scale. Microcarrier culture is also briefly addressed. Finally, some recommendations for bioreactor configuration and operating strategy are given.
Keywords: Large-scale bioreactors, ‘Shear sensitivity’, Homogeneity, pH control, Dissolved CO2, Osmolality