Oxygen supply in aerobic bioprocesses is of crucial importance. For this reason, this paper presents the oxygen demand of different cells and summarizes experimental and numerical possibilities for the determination of oxygen transfer in bioreactors. The focus lies on the volumetric oxygen mass transfer coefficient (k_La) calculation using computational fluid dynamics and state‐of‐the‐art models for surface‐aerated and forced‐aerated bioreactors. In addition, experimental methods for the determination of thek_Lavalue and the gas bubble size distribution are presented.

No matter the scale, stirred tank bioreactors are the most commonly used systems in biotechnological production processes. Single-use and reusable systems are supplied by several manufacturers. The type, size, and number of impellers used in these systems have a significant influence on the characteristics and designs of bioreactors. Depending on the desired application, classic shaft-driven systems, bearing-mounted drives, or stirring elements that levitate freely in the vessel may be employed. In systems with drive shafts, process hygiene requirements also affect the type of seal used. For sensitive processes with high hygienic requirements, magnetic-driven stirring systems, which have been the focus of much research in recent years, are recommended. This review provides the reader with an overview of the most common agitation and seal types implemented in stirred bioreactor systems, highlights their advantages and disadvantages, and explains their possible fields of application. Special attention is paid to the development of magnetically driven agitators, which are widely used in reusable systems and are also becoming more and more important in their single-use counterparts.Key Points• Basic design of the most frequently used bioreactor type: the stirred tank bioreactor• Differences in most common seal types in stirred systems and fields of application• Comprehensive overview of commercially available bioreactor seal types• Increased use of magnetically driven agitation systems in single-use bioreactors

Optimizing bioprocesses requires an in-depth understanding, from a bioengineering perspective, of the cultivation systems used. A bioengineering characterization is typically performedviaexperimental or numerical methods, which are particularly well-established for stirred bioreactors. For unstirred, non-rigid systems such as wave-mixed bioreactors, numerical methods prove to be problematic, as often only simplified geometries and motions can be assumed. In this work, a general approach for the numerical characterization of non-stirred cultivation systems is demonstrated using the CELL-tainer bioreactor with two degree of freedom motion as an example. In a first step, the motion is recordedviamotion capturing, and a 3D model of the culture bag geometry is generatedvia3D-scanning. Subsequently, the bioreactor is characterized with respect to mixing time, and oxygen transfer rate, as well as specific power input and temporal Kolmogorov length scale distribution. The results demonstrate that the CELL-tainer with two degrees of freedom outperforms classic wave-mixed bioreactors in terms of oxygen transport. In addition, it was shown that in the cell culture version of the CELL-tainer, the critical Kolmogorov length is not surpassed in any simulation.

HEK293 is a widely used cell line in the fields of research and industry. It is assumed that these cells are sensitive to hydrodynamic stress. The aim of this research was to use particle image velocimetry validated computational fluid dynamics (CFD) to determine the hydrodynamic stress in both shake flasks, with and without baffles, and in stirred Minifors 2 bioreactors to evaluate its effect on the growth and aggregate size distribution of HEK293 suspension cells. The HEK FreeStyle^TM 293-F cell line was cultivated in batch mode at different specific power inputs (from 63 $\mathrm{W} {\mathrm{m}}^{-3}$ to 451 $\mathrm{W} {\mathrm{m}}^{-3}$), whereby $\approx 60 \mathrm{W} {\mathrm{m}}^{-3}$ corresponds to the upper limit, which is what has been typically described in published experiments. In addition to the specific growth rate and maximum viable cell density VCD${}_{\max }$, the cell size distribution over time and cluster size distribution were investigated. The VCD${}_{\max }$ of $(5.77\pm 0.02)·{10}^6$$\mathrm{cells}$$\mathrm{m}$${\mathrm{L}}^{-1}$ was reached at a specific power input of 233 $\mathrm{W} {\mathrm{m}}^{-3}$ and was $23.8\%$ higher than the value obtained at 63 $\mathrm{W} {\mathrm{m}}^{-3}$ and $7.2\%$ higher than the value obtained at 451 $\mathrm{W} {\mathrm{m}}^{-3}$. No significant change in the cell size distribution could be measured in the investigated range. It was shown that the cell cluster size distribution follows a strict geometric distribution whose free parameter p is linearly dependent on the mean Kolmogorov length scale. Based on the performed experiments, it has been shown that by using CFD-characterised bioreactors, the VCD${}_{\max }$ can be increased and the cell aggregate rate can be precisely controlled.