Scale-up analysis of geometrically dissimilar single-use bioreactors.

Cell culture scale-up is a challenging task due to the simultaneous change of multiple hydrodynamic process characteristics and their different dependencies on the bioreactor size as well as variation in the requirements of individual cell lines. Conventionally, the volumetric power input is the most common parameter to select the impeller speed for scale-up, however, it is well reported that this approach fails when there are huge differences in bioreactor scales. In this study, different scale-up criteria are evaluated. At first, different hydrodynamic characteristics are assessed using computational fluid dynamics data for four single-use bioreactors, the Mobius® CellReady 3 L, the Xcellerex™ XDR-10, the Xcellerex™ XDR-200, and the Xcellerex™ XDR-2000. On the basis of this numerical data, several potential scale-up criteria such as volumetric power input, impeller tip speed, mixing time, maximum hydrodynamic stress, and average strain rate in the impeller zone are evaluated. Out of all these criteria, the latter is found to be most appropriate, and the successful scale-up from 3 to 10 L bioreactor and to 200 L bioreactor is confirmed with cell culture experiments using Chinese Hamster Ovary cell cultivation.

Quantifying the hydrodynamic stress for bioprocesses.

Hydrodynamic stress is an influential physical parameter for various bioprocesses, affecting the performance and viability of the living organisms. However, different approaches are in use in various computational and experimental studies to calculate this parameter (including its normal and shear subcomponents) from velocity fields without a consensus on which one is the most representative of its effect on living cells. In this letter, we investigate these different methods with clear definitions and provide our suggested approach which relies on the principal stress values providing a maximal distinction between the shear and normal components. Furthermore, a numerical comparison is presented using the computational fluid dynamics simulation of a stirred and sparged bioreactor. It is demonstrated that for this specific bioreactor, some of these methods exhibit quite similar patterns throughout the bioreactor-therefore can be considered equivalent-whereas some of them differ significantly.

Numerical and Experimental Investigation of the Hydrodynamics in the Single-Use Bioreactor Mobius® CellReady 3 L.

Two-way Euler-Lagrange simulations are performed to characterize the hydrodynamics in the single-use bioreactor Mobius® CellReady 3 L. The hydrodynamics in stirred tank bioreactors are frequently modeled with the Euler-Euler approach, which cannot capture the trajectories of single bubbles. The present study employs the two-way coupled Euler-Lagrange approach, which accounts for the individual bubble trajectories through Langrangian equations and considers their impact on the Eulerian liquid phase equations. Hydrodynamic process characteristics that are relevant for cell cultivation including the oxygen mass transfer coefficient, the mixing time, and the hydrodynamic stress are evaluated for different working volumes, sparger types, impeller speeds, and sparging rates. A microporous sparger and an open pipe sparger are considered where bubbles of different sizes are generated, which has a pronounced impact on the bubble dispersion and the volumetric oxygen mass transfer coefficient. It is found that only the microporous sparger provides sufficiently high oxygen transfer to support typical suspended mammalian cell lines. The simulated mixing time and the volumetric oxygen mass transfer coefficient are successfully validated with experimental results. Due to the small reactor size, mixing times are below 25 s across all tested conditions. For the highest sparging rate of 100 mL min-1, the mixing time is found to be two seconds shorter than for a sparging rate of 50 mL min-1, which again, is 0.1 s longer than for a sparging rate of 10 mL min-1 at the same impeller speed of 100 rpm and the working volume of 1.7 L. The hydrodynamic stress in this bioreactor is found to be below critical levels for all investigated impeller speeds of up to 150 rpm, where the maximum levels are found in the region where the bubbles pass behind the impeller blades.

CFD-Based and Experimental Hydrodynamic Characterization of the Single-Use Bioreactor XcellerexTM XDR-10.

Understanding the hydrodynamic conditions in bioreactors is of utmost importance for the selection of operating conditions during cell culture process development. In the present study, the two-phase flow in the lab-scale single-use bioreactor XcellerexTM XDR-10 is characterized for working volumes from 4.5 L to 10 L, impeller speeds from 40 rpm to 360 rpm, and sparging with two different microporous spargers at rates from 0.02 L min-1 to 0.5 L min-1. The numerical simulations are performed with the one-way coupled Euler-Lagrange and the Euler-Euler models. The results of the agitated liquid height, the mixing time, and the volumetric oxygen mass transfer coefficient are compared to experiments. For the unbaffled XDR-10, strong surface vortex formation is found for the maximum impeller speed. To support the selection of suitable impeller speeds for cell cultivation, the surface vortex formation, the average turbulence energy dissipation rate, the hydrodynamic stress, and the mixing time are analyzed and discussed. Surface vortex formation is observed for the maximum impeller speed. Mixing times are below 30 s across all conditions, and volumetric oxygen mass transfer coefficients of up to 22.1 h-1 are found. The XDR-10 provides hydrodynamic conditions which are well suited for the cultivation of animal cells, despite the unusual design of a single bottom-mounted impeller and an unbaffled cultivation bioreactor.

Simulation of patient-specific bi-directional pulsating nasal aerosol dispersion and deposition with clockwise 45° and 90° nosepieces.

Numerical simulations of the dispersion and deposition of poly-disperse particles in a patient-specific human nasal configuration are performed. Computed tomography (CT) images are used to create a realistic configuration of the nasal cavity and paranasal sinuses. The OpenFOAM software is used to perform unsteady Large Eddy Simulations (LES) with the dynamic sub-grid scale Smagorinsky model. For the numerical analysis of the particle motion, a Lagrangian particle tracking method is implemented. Two different nosepieces with clockwise inclinations of 45° and 90° with respect to the horizontal axis are connected to the nostrils. A sinusoidal pulsating airflow profile with a frequency of 45 Hz is imposed on the airflow which carries the particles. Flow partition analysis inside the sinuses show that ventilation of the sinuses is improved slightly when the 45° nosepiece is used instead of the 90° nosepiece. The flow partition into the right maxillary is improved from 0.22% to 0.25%. It is observed that a closed soft palate increases the aerosol deposition efficiency (DE) in the nasal cavity as compared to an open soft palate condition. The utilization of pulsating inflow leads to more uniform deposition pattern in the nasal airway and enhances the DE by 160% and 44.6%, respectively, for the cases with clockwise 45° and 90° nosepieces, respectively. The bi-directional pulsating drug delivery with the same particle size distribution and inflow rates as the PARI SINUS device results in higher total DEs with 45° nosepiece than with the 90°. Thus, the numerical simulation suggests that the 45° nosepiece is favorable in terms of the delivered dose.

Large eddy simulation of the flow pattern in an idealized mouth-throat under unsteady inspiration flow conditions.

An excellent understanding of the airflow properties is critical to improve the drug delivery efficiency via the extrathoracic airway. The present numerical study focuses on the investigation the characteristics of important airflow structures such as the secondary vortices, the impinging jet and the recirculation zone under unsteady inspiration flow conditions in a circular idealized mouth-throat model using large eddy simulation (LES). Five inhalation cycles are simulated, the last one of which is analyzed in detail at five different times. Two times are chosen during the accelerating branch, one at the peak and two within the decelerating inhalation wave. The flow exhibits an extinct process of the flow transiting from laminar to turbulent during the accelerating phase and transiting back from turbulent to laminar in the decelerating phase. It is found that the flow is much more turbulent during the decelerating phase compared to the accelerating phase of the inspiration wave, which is associated with more smaller secondary vortices, a shorter and more unsteady laryngeal jet, a smaller and more unsteady recirculation zone, as well as an enlarged mixing zone. These differences during the unsteady inspiration require more attention in particular if particle transport and deposition in the upper airway are to be investigated.

Numerical study of the airflow structures in an idealized mouth-throat under light and heavy breathing intensities using large eddy simulation.

An excellent understanding of the airflow structures is critical to enhance the efficiency of drug delivery via the human oral airway. The present paper investigates the characteristics of both steady and unsteady airflow structures in an idealized mouth-throat using large eddy simulation (LES). Representative inhalation flow rates of 15L/min at rest and 60L/min in exercise are considered. The study shows that there are more secondary vortices in the pharynx and the laryngeal jet is much longer and more concave in the steady flow field at 15L/min compared to the higher inspiration rate, which decreases the possibility of drug impinging on the wall. In contrast, the laryngeal jet is much more unsteady at heavy breathing and its strong interaction with the recirculation zone in the trachea leads to a enlarged mixing zone, increasing the possibility for carrying the particles from the laryngeal jet into the recirculation zone, which will lead to a longer residence time of the particles in the trachea and this increases the possibility of drug deposition in this area. In addition, the recirculation zone size is larger, the separation region is far away from glottis, and the reversed flow is slower at light compared to heavy breathing. In conclusion, these airflow structures show distinct properties at light and heavy breathing conditions, particularly in the unsteady flow field. The study provides evidence about the physical processes leading to both enlarged mixing zones and recirculation zones. It is known that stronger secondary vortices, a stronger laryngeal jet and enlarged recirculation zones definitely increase the particle deposition in the upper airway. The present paper aims to uncover the physical properties of the airflow for different breathing conditions, and their detailed effect on particle deposition will be studied in future.