Hydrodynamics considerations in cells systems from ocean flow to perfusion cultivation process

Sammanfattning: Microorganisms and animal cells are grown surrounded by fluid, which is providing them with nutrients and removing their waste products. In nature and industry processes, cells/microbes can be subject to aggressive environments, such as turbulent flow or shear flow. Hydrodynamics force generated in these flows can affect the distribution of cells/microbes and even lead to cell damage. Understanding the mechanism and exploring the effect of hydrodynamic force in these environments could make the prediction of cells’ hydrodynamic response more systematic. In pharmaceutical industry, perfusion process is recognized as an attractive option for biologics production due to its high productivity. However, there are still some challenges and limitations for further process improvement due to lack of information of cell response to hydrodynamic force and nutrients. In both cases, hydrodynamics plays an important role and similar tool can be used to achieve a deeper understanding of these processes. This thesis is mainly aiming to elucidate the influence of hydrodynamic forces on microorganisms or cells in nature and during bioprocesses. In particular, shear stress in a natural environment and in a bioreactor operated in perfusion mode is studied.This work mainly investigates hydrodynamics in nature and bioprocess including three flow cases. The first study investigates the effect of turbulence on marine life by performing direct numerical simulations (DNS) of motile micro-organisms in isotropic homogeneous turbulence. The clustering level of micro-organisms with one preferential swimming direction (e.g. gyrotaxis) is examined. The second study uses Computation Fluid Dynamics (CFD) to simulate the fluid flow inside a Wave bioreactor bag. The phenomenon of mixing, oxygen transfer rate and shear stress in nine different operating conditions of rocking speeds and angles are discussed. In the third study, the cellular response to shear force including growth and metabolism in a cell retention device such as hollow fiber filters during a perfusion process is analyzed. Theoretical calculations and experiment validation is performed to compare two filtration modes, tangential flow filtration (TFF) or alternating tangential flow filtration (ATF). Further optimizations regarding mixing and feeding are performed in a screening scale of in a perfusion system of stirred tank bioreactor with cell separation device.The main findings can be summarized as that spherical gyrotaxis swimmers show significant clustering, whereas prolate swimmers remain more uniformly distributed due to their large sensitivity to the local shear. These results could explain how pure hydrodynamic effects can alter the ecology of micro-organisms for instance by varying shape and their preferential orientation (paper I). The simulations of Wave bioreactors show that the mixing and shear stress increase with the rocking angle but that increasing rocking speeds are not systematically associated with increasing mixing and shear stress. A resonance phenomenon is responsible for the fact that the lowest studied rocking speed generates the highest fluid velocity, mixing and shear stress (paper II). Theoretical velocity profile-based calculations suggested a lower shear stress for ATF by a factor 0.637 compared to TFF. This is experimentally validated by cultures of HEK (human embryonic kidney) 293 cells subjected to shear stress by a perfusion system that affects growth and metabolism using these cell separation devices (paper III). Thanks to optimization of mixing and oxygen transfer in a screening system for perfusion process, very high cell densities above 100 x 106 cells/mL of mammalian cells were achieved (paper IV).