Agitation and aeration of stirred-bioreactors for the microcarrier culture of human mesenchymal stem cells and potential implications for large-scale bioprocess development

The impact of agitation rate and sparged aeration on BM-hMSC expansion in conventional stirred tank bioreactors was assessed. It was found that a decrease in impeller speed to below NJS caused sampling difficulties, clumping and an increase to ∼2 NJS decreased the growth rate though an intermediate value of ∼1.3 NJS did not. Additionally, over this range of agitation intensities, cell quality remained unchanged post-harvest suggesting that poor growth performance at the highest speed was due to a failure of the cells to attach efficiently to microcarriers rather than damage to the cells due to fluid dynamic stress. Further it was shown that direct aeration of the culture medium both with and without Pluronic F68 via a sparger at NJS was detrimental to BM-hMSC growth. Again, this reduction in growth seems to be associated with poor attachment rather than cell damage, which due to the mechanism of PluronicTM F68 reducing the cell hydrophobicity and thus the affinity of the BM-hMSCs to attach to the microcarrier, leads to a poorer performance in the presence of the surfactant. Certain post-harvest quality characteristics are also detrimentally impacted compared to headspace aeration. This problem is discussed in terms of the need to facilitate future large-scale process development where headspace aeration at NJS may not be sufficient to meet culture needs at higher cell densities.