Fluid dynamics of suspension bioreactors

2017-03-01T03:35:52Z (GMT) by Ismadi, Mohd Zulhilmi Paiz
The successful engraftment of pluripotent stem cells has revolutionised regenerative medicine in the treatment of many degenerative diseases and injuries. For many years, bioreactors have been widely used to improve cell culturing efficiency. Although the importance of mechanical stresses on cell culture has been often reported in the literature, little attention has been given to understanding the fluid dynamics aspect of the bioreactor. This thesis demonstrates the use of experimental fluid dynamics in uncovering the underlying physics of the flow mechanics in bioreactors. The first section of the study comprised the flow visualisation in a rotating-top disk cylindrical bioreactor, with the height-radius ratio of 2. Qualitative analysis determined the effect on the flow when the density of the dye injection was varied. The injection of a relatively denser fluid created a buoyancy force downward, which counteracted the meridional recirculation in the cylinder and thus enhanced the formation of a vortex breakdown bubble. On the other hand, the injection of a lighter fluid did not destroy the vortex breakdown. However, for large enough density differences (larger than 0.03%), the lighter fluid was able to pierce through the bubble and led to a new structure of the vortex breakdown. Translating this knowledge to the use of microcarriers in a rotating lid bioreactor, it is expected the microcarriers will accumulate and thicken at the centre of the container. Then, the behaviour is followed by destabilisation of the flow with an oscillatory effect creating a phenomenon similar to dense dye experiments. The flow field within a spinner flask at varying speeds (10RPM to 80RPM) and impeller positions was characterised experimentally. Particle Image Velocimetry (PIV) was employed to visualise the fluid flow and calculate the stresses and vorticities associated with the flow within the flask. The highest shear stress region was observed at the base of the spinner flask due to fluid-wall interaction. The study provides an overview of the fluid structure within the spinner flask in the meridional and azimuthal planes. Furthermore, the quantitative results of this study gave an accurate numerical stress margin for the given impeller speeds. The optimum flow condition for culturing mouse induced pluripotent stem cells (iPSCs) was investigated. It was found that the mouse iPSCs achieved the optimum number of cells over 7 days in a 25RPM suspension culture. This condition translated to a 0.0984 Pa maximum shear stress caused by the interaction of the fluid flow with the bottom surface. However, negative cell growth was obtained in the 28RPM culture condition. Such a narrow margin demonstrated that mouse iPSCs cultured on microcarriers are very sensitive to mechanical forces. This study provides insight to biomechanical parameters, specifically the shear stress distribution, for a commonly-used spinner flask over a wide range of Reynolds number. Additionally, these results provide estimates of the biomechanical properties within the type of spinner flask used in many published cell studies. In an effort to measure three-dimensional (3D) velocity fields in biological flows, a correlation-based approach, termed Holographic Correlation Velocimetry (HCV), was developed. The direct flow reconstruction approach developed here allows for measurements at high seeding densities. Moreover, because the system is based on in-line holography, it is very efficient with regards to the use of light, as it does not rely on side scattering. This efficiency makes the system appropriate for high-speed flows and low exposure times, which is essential for imaging dynamic systems. The method was rigorously tested, validated and applied to measure flows in a cuvette and a spinner flask bioreactor. This thesis provides invaluable information and deep insight into flow behaviour within bioreactors. Furthermore, the wide range of variables covered in the study will allow the estimation of the magnitude of mechanical forces to be made, thus improving the understanding of the relations between mechanical forces and the culture outcome. The proposed imaging technique will allow thorough characterisation of the biomechanical parameters and define the design criteria for future bioreactor development.