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Protein polarization in packed hollow fibre bioreactors Koska, Jurgen

Abstract

Osmotically active proteins and cells, retained in the extra capillary space (ECS) of hollow fibre bioreactors (HFBRs), can influence the hydrodynamics of such devices. A mathematical model was developed to describe the coupled hydrodynamics and high molecular weight protein transport in a cell filled HFBR. It was assumed that the multi-fibre reactor can be represented by a single, straight fibre surrounded by a symmetry envelope containing fluid and a homogeneous packed bed of cells. The low Reynolds number flow in this porous medium was described by Darcy's law. Because of difficulties associated with operating a reactor filled with mammalian cells, a suitable analogue was used to experimentally investigate protein polarization in packed HFBRs. The ECS side of the reactor was filled with an agarose/protein solution which, upon cooling, formed a porous medium with a uniform initial concentration of protein. A constant lumen flow was established for several days before the cartridge was sacrificed and the axial ECS protein distribution was measured. Since the protein transport and HFBR hydrodynamics were coupled, numerical methods were required to solve the governing equations of both the two-dimensional (axial plus radial variations) and the one-dimensional (axial variations only) models developed to predict axisymmetric transient ECS protein concentrations. Computer modelling results indicated that, because of the large length/radius ratio of the representative fibre unit, the two-dimensional ECS protein concentrations could be accurately duplicated by the simpler one-dimensional model. The latter model, required about two orders-of-magnitude less computational time than the former. The one-dimensional model results were compared to experimentally obtained ECS protein profiles and, subsequently, the model was used to predict protein polarization in ITFBRs for different conditions. The hydraulic conductivity of the agarose gel, required for the model, was experimentally determined using the falling head method. The measured conductivity values failed to adequately describe the observed protein polarization in the ECS of HFBRs. However, by using a gel conductivity which was about an order-of-magnitude higher than the measured value, it was found that the model agreed well, in general, with the measured ECS protein polarization profiles obtained for initial protein loadings of 5 - 30 g/L. The higher apparent conductivity, needed to fit the model to the experimental results, was attributed to the inability of the gel to completely fill the geometrically complex ECS. Since in HFBRs, packed anchorage-dependent mammalian cells are expected to achieve hydraulic conductivities similar to values encountered in tissues, several model simulations were carried out at low tissue conductivity values. The results indicated that, for these conditions, protein transport in the ECS is mainly governed by diffusion. Protein polarization, a dominant feature of empty ECS protein transport, is greatly reduced. Also, it was shown that the removal of product protein from a packed ECS space can be difficult, since ECS flows are reduced. Models, such as those developed here, can be used to further investigate HFBR operation and process optimization.

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