Mixing theory for culture and harvest in bioreactors of human mesenchymal stem cells on microcarriers

The use of human mesenchymal stem cells (hMSCs) in regenerative medicine is a potential major advance for the treatment of many medical conditions, especially with the use of allogeneic therapies where the cells from a single donor can be used to treat ailments in many patients. Such cells must be grown attached to surfaces and for large scale production, it is shown that stirred bioreactors containing ~200 μm particles (microcarriers) can provide such a surface. It is also shown that the just suspended condition, agitator speed NJS, provides a satisfactory condition for cell growth by minimizing the specific energy dissipation rate, εT, in the bioreactor whilst still meeting the oxygen demand of the cells. For the cells to be used for therapeutic purposes, they must be detached from the microcarriers before being cryopreserved. A strategy based on a short period (~7 min) of very high εT, based on theories of secondary nucleation, is effective at removing >99% cells. Once removed, the cells are smaller than the Kolmogorov scale of turbulence and hence not damaged. This approach is shown to be successful for culture and detachment in 4 types of stirred bioreactors from 15 mL to 5 L

Power curves and flow patterns for a range of impellers in Newtonian fluids: 40<Re<5×10(5)

The mixing pattern and power number in a stirred vessel depend not only on the impeller design but also on the impeller/tank geometry and flow regime. The relationship between these parameters is reported for a Rushton turbine (6DT), six-bladed 45°-pitched turbine pumping down (6MFD) and (6MFU) up and three proprietary impellers. The impellers were used over a wide range of impeller diameter (D)/vessel diameter (T) and impeller clearance (C)/T values. Flow patterns were recorded and power numbers were measured as a function of Reynolds number from about 40 to about 50,000 in Newtonian fluids. The results obtained were used to aid in the study of the impellers in solid-liquid and three-phase systems

Comparing impeller performance for solid-suspension in the transitional flow regime with Newtonian fluids

Particle suspension in Newtonian fluids of viscosities from 0.01 to 1 Pa s have been studied using Rushton turbines, pitched blade turbines, Chemineer HE-3 and Lightnin' A310 hydrofoils (all pumping downwards), and Ekato Intermig agitators. By comparison to the turbulent system (Ibrahim and Nienow, 1996), at high viscosity, there was less random particle movement across the base prior to suspension. On the other hand, once the agitation speed, N, was high enough to achieve suspension, i.e., N = N(js), particles remained longer in suspension after a reduction to N 3000, the optimum overall configuration was a D/T of about 0.4 with the downward pumping HE-3 and pitched blade turbines, whilst at Re < 3000, the optimimum was the HE-3 at a D/T of about 0.5. The performance of dual Intermigs with D/T ratios of ~0.6 dramatically improved as the viscosity increased to 0.1 Pa s, and the single Intermig was the most efficient impeller at 1 Pa s. The Zwietering equation was found unsuitable for prediction of N(js) at low Re. Particle suspension in Newtonian fluids of viscosities from 0.01 to 1 Pa s have been studied using Rushton turbines, pitched blade turbines, Chemineer HE-3 and Lightnin' A310 hydrofoils (all pumping downwards), and Ekato Intermig agitators. By comparison to the turbulent system, at high viscosity, there was less random particle movement across the base prior to suspension. On the other hand, once the agitation speed, N, was high enough to achieve suspension, i.e., N = Njs, particles remained longer in suspension after a reduction to N&lt;Njs, though eventually with little or no hysterisis. Suspension at high viscosity was achieved with a lower mean specific energy dissipation rate, (εT)js, when using large D/T impellers whether of the radial or axial type. Thus, at Re&gt;3000, the optimum overall configuration was a D/T of about 0.4 with the downward pumping HE-3 and pitched blade turbines, whilst at Re&lt;3000, the optimimum was the HE-3 at a D/T of about 0.5. The performance of dual Intermigs with D/T ratios of approximately 0.6 dramatically improved as the viscosity increased to 0.1 Pa s, and the single Intermig was the most efficient impeller at 1 Pa s. The Zwietering equation was found unsuitable for prediction of Njs at low Re

The effect of viscosity on particle suspension in an aerated stirred vessel with different impellers and bases

The agitator speed required to suspend solids under gassed conditions, N JSg, has been studied in water and in corn syrup of 0.01 and 0.1 Pas giving Reynolds numbers from the full turbulent region down to ~10 3. Of the impellers tested, the downpumping, three-blade, axial flow hydrofoil impellers are generally unsuitable for this duty, and although six-blade, mixed flow down-pumping impellers require the lowest mean specific energy dissipation rates to suspend the solids, (T) JSg, at low gas flow rates, they are still prone to flow instabilities and torque fluctuations. The latter poor characteristics are made worse by reducing the size of the impeller relative to the vessel and by increasing viscosity and gas flow rate, QGV. Thus, they are of limited use for such systems. The Ekato InterMIG impeller has the highest (T) JSg and tends to cause vessel vibrations when dispersing the gas, and this weakness is also enhanced by increasing viscosity and gas flow rate. Again, they are generally not appropriate for three-phase systems. The radial flow Rushton turbine is quite stable and able to suspend the solids in all the fluids. However, it requires the second highest (T) JSg, and both (T) JSg and NJSg increase substantially with increasing QGV. The up-pumping six-blade, mixed flow impeller of approximately half the vessel diameter is able to suspend the solids and is very stable in all the fluids. In addition, both (T) JSg and NJSg are very insensitive toQGV, with (T) JSg generally being the lowest at the highest QGV. It is thus the preferred agitator among those tested. As in ungassed systems, modifying the base of the vessel can significantly lower (T) JSg and NJSg for a given impeller type in water compared to a flat base. The concept of keeping constant torque as a means of maintaining suspension has been tested and found not to be valid in this work. Another approach to generalizing the results is also suggested. © Taylor &amp; Francis Group, LLC

Particle suspension in the turbulent regime: the effect of impeller type and impeller/vessel configuration

Particle suspension in water has been studied using Rushton turbines, pitched blade turbines (pumping upwards and downwards), Chemineer HE3 and Lightnin A310 hydrofoils pumping downwards, and Ekato Intermig agitators. Flat and profiled bottoms have been used. The dimensionless parameter S, which is related to the minimum impeller speed for complete solids suspension based on the work of Zwietering, has been used to generalize the results for two different particles. The configuration requiring the lowest specific energy dissipation rate in the flat-bottomed tank was a downward- pumping impeller of 0.35 to 0.4 times the vessel diameter with a clearance off the base of 1/4 the vessel height. Under these conditions, the main flow pattern was such that the piles of solids associated with the regions of flow reversal at the periphery and of the centre of the base were both removed at about the same speed. By modifying the base to fill in these zones where solids collect, the minimum specific energy dissipation rate associated with this configuration using the HE3 hydrofoils could be further reduced by a factor of about 4 to 5. Compared to the HE3 hydrofoils, the Rushton turbines and the Intermigs required minimum specific energy dissipation rates for suspension 5 to 10 times higher at equivalent clearances and vessel base configurations

Mixing in the process industries

xii, 414 p. : il.; 24 cm