Quantitative experimental study of shear stress and mixing in progressive flow regimes within annular flow bioreactors

Annular-flow bioreactors are normally operated under laminar Couette flow conditions in order to minimise shear-induced damage to cells. In this study, we computed the fluid shear stresses in model annular vessels over a range of laminar flow regimes, from Couette flow to Taylor-vortex flow, and at two geometric scales, using a shear rate model for freely suspended particles, together with experimental Laser Doppler Anemometry data for a 2-D velocity field. The shear stresses were greatest in the boundary layers adjacent to each wall in each case, with values typically 6 times higher than the mean stresses in the annular space; their respective magnitudes were significantly lower in the larger of the two vessels studied, however. Cell viability studies were also performed in which mammalian cells were cultured under dynamic conditions in a functional bioreactor having the same dimensions as the smaller vessel. The results of these studies demonstrated that a significantly greater number of cells remained in suspension in Taylor-vortex flows than in Couette flow, but at the expense of cell viability at higher Taylor numbers. Taken together, these findings suggest that the benefits of enhanced convective mass transport afforded by Taylor-Couette flows could be realised without risk of appreciable shear induced damage of cells and tissues in larger vessels operating under dynamically similar flow conditions