Numerical Simulation on Mass Transport in a Microchannel Bioreactor for Co-culture Applications

Mass Transport in a Microchannel Bioreactor With a Porous Wall

A two-dimensional flow model has been developed to simulate mass transport in a microchannel bioreactor with a porous wall. A two-domain approach, based on the finite volume method, was implemented. For the fluid part, the governing equation used was the Navier–Stokes equation; for the porous medium region, the generalized Darcy–Brinkman–Forchheimer extended model was used. For the porous-fluid interface, a stress jump condition was enforced with a continuity of normal stress, and the mass interfacial conditions were continuities of mass and mass flux. Two parameters were defined to characterize the mass transports in the fluid and porous regions. The porous Damkohler number is the ratio of consumption to diffusion of the substrates in the porous medium. The fluid Damkohler number is the ratio of the substrate consumption in the porous medium to the substrate convection in the fluid region. The concentration results were found to be well correlated by the use of a reaction-convection distance parameter, which incorporated the effects of axial distance, substrate consumption, and convection. The reactor efficiency reduced with reaction-convection distance parameter because of reduced reaction (or flux), and smaller local effectiveness factor due to the lower concentration in Michaelis–Menten type reactions. The reactor was more effective, and hence, more efficient with the smaller porous Damkohler number. The generalized results could find applications for the design of bioreactors with a porous wall.

Design of well and groove microchannel bioreactors for cell culture

Microfluidic bioreactors have been shown valuable for various cellular applications. The use of micro-wells/grooves bioreactors, in which micro-topographical features are used to protect sensitive cells from the detrimental effects of fluidic shear stress, is a promising approach to culture sensitive cells in these perfusion microsystems. However, such devices exhibit substantially different fluid dynamics and mass transport characteristics compared to conventional planar microchannel reactors. In order to properly design and optimize these systems, fluid and mass transport issues playing a key role in microscale bioreactors should be adequately addressed. The present work is a parametric study of micro-groove/micro-well microchannel bioreactors. Operation conditions and design parameters were theoretically examined via a numerical model. The complex flow pattern obtained at grooves of various depths was studied and the shear protection factor compared to planar microchannels was evaluated. 3D flow simulations were preformed in order to examine the shear protection factor in micro-wells, which were found to have similar attributes as the grooves. The oxygen mass transport problem, which is coupled to the fluid mechanics problem, was solved for various groove geometries and for several cell types, assuming a defined shear stress limitation. It is shown that by optimizing the groove depth, the groove bioreactor may be used to effectively maximize the number of cells cultured within it or to minimize the oxygen gradient existing in such devices. Moreover, for sensitive cells having a high oxygen demand (e.g., hepatocytes) or low endurance to shear (e.g., human embryonic stem cells), results show that the use of grooves is an enabling technology, since under the same physical conditions the cells cannot be cultured for long periods of time in a planar microchannel. In addition to the theoretical model findings, the culture of human foreskin fibroblasts in groove (30 microm depth) and well bioreactors (35 microm depth) was experimentally examined at various flow rates of medium perfusion and compared to cell culture in regular flat microchannels. It was shown that the wells and the grooves enable a one order of magnitude increase in the maximum perfusion rate compared to planar microchannels. Altogether, the study demonstrates that the proper design and use of microgroove/well bioreactors may be highly beneficial for cell culture assays.

Numerical Simulation of Mass Transport in a Microchannel Bioreactor With Cell Micropatterning

Micropatterning of two different cell types based on surface modification allows spatial control over two distinct cell subpopulations. This study considers a micropatterned coculture system, which has release and absorption parts alternately arranged at the base, and each part has a single cell type. A micropattern unit was defined and within each unit, there are one release part and one absorption part. The cells in the absorption parts consume species, which are secreted by the cells in the release parts. The species concentrations at the micropatterned cell base were computed from a three-dimensional numerical flow model incorporating mass transport. Different combined parameters were developed for the release and absorption parts to make the data collapse in each part. Combination of the collapse data in the release and absorption parts can be used to predict the concentration distribution through the whole channel. The correlated results were applied to predict the critical length ratio of the release and absorption parts for an actual micropatterned system (Bhatia et al., 1999, “Effect of Cell-Cell Interactions in Preservation of Cellular Phenotype: Co-Cultivation of Hepatocytes and Nonparenchymal Cell,” FASEB J. 13, pp. 1883–1900) to avoid species insufficiency based on basic fibroblast growth factor (bFGF). The mass transfer effectiveness was found to be higher with more numbers of micropattern units. The optimal condition for micropatterned coculture bioreactors is achieved by having the product of the length ratio and the reaction ratio equal to 1. This condition was used to optimize the mass transfer in the micropatterned system (Bhatia et al., 1999, “Effect of Cell-Cell Interactions in Preservation of Cellular henotype: Co-Cultivation of Hepatocytes and Nonparenchymal Cell,” FASEB J. 13, pp. 1883–1900) based on bFGF.