Human mesenchymal stem cell (hMSC)-based therapies are of increasing interest in the field of regenerative medicine. As economic considerations have shown, allogeneic therapy seems to be the most cost-effective method. Standardized procedures based on instrumented single-use bioreactors have been shown to provide billion of cells with consistent product quality and to be superior to traditional expansions in planar cultivation systems. Furthermore, under consideration of the complex nature and requirements of allogeneic hMSC-therapeutics, a new equipment for downstream processing (DSP) was successfully evaluated. This mini-review summarizes both the current state of the hMSC production process and the challenges which have to be taken into account when efficiently producing hMSCs for the clinical scale. Special emphasis is placed on the upstream processing (USP) and DSP operations which cover expansion, harvesting, detachment, separation, washing and concentration steps, and the regulatory demands.

Suspension cultures, in which human mesenchymal stem cells are cultivated on microcarriers in scalable single‐use stirred bioreactor types, have been shown to be a promising alternative to planar flask cultures. However, stirred single‐use bioreactors were originally developed for production processes with robust, permanent cell lines. Human mesenchymal stem cells are adherent primary cells and thus expanding them in such bioreactor systems imposes more stringent requirements on bioreactor systems. For low‐serum conditions (5%) and different types of stirred single‐use bioreactors, a suspension criteria‐based approach for expanding human adipose tissue‐derived mesenchymal stem cells (hASCs) from milliliter to pilot scale was successfully developed. For process scale‐up, experimental and numerical investigations were performed to (i) predict optimum impeller speeds, (ii) determine the main engineering parameters (local shear stress, turbulent dissipation rate, Kolmogorov microscale), and (iii) verify suspension criteria NS1 and NS1u for rapid process transfer from 100 mL to 2 L and 35 L cultures. Using optimized medium‐microcarrier combinations as well as NS1 and NS1u as scale‐up factors, total hASC quantities between 3 × 107 (100 mL scale) and 1 × 1010 (35 L scale) were obtained. The cell quantities obtained are the highest reported to date for scalable single‐use bioreactors under low‐serum conditions.

The fluid flow and suspension characteristics inside small‐scale, stirred, single‐use bioreactors were investigated experimentally and by means of computational fluid dynamics. The required impeller speeds for homogenous suspension were determined for two microcarrier types. The shear stress level and turbulence distribution were predicted using a numerical model, which was verified by particle image velocimetry measurements. In subsequent cultivations of primary mesencymal adipose‐derived stem cells, up to 31.4‐fold expansion in cell number was achieved for serum concentrations as low as 5 %.

The potential of human mesenchymal stem cells (hMSCs) for allogeneic cell therapies has created a large amount of interest. However, this presupposes the availability of efficient scale-up procedures. Promising results have been reported for stirred bioreactors that operate with microcarriers. Recent publications focusing on microcarrier-based stirred bioreactors have demonstrated the successful use of Computational Fluid Dynamics (CFD) and suspension criteria (N S1u, N S1) for rapidly scaling up hMSC expansions from mL- to pilot scale. Nevertheless, one obstacle may be the formation of large microcarrier-cell-aggregates, which may result in mass transfer limitations and inhomogeneous distributions of stem cells in the culture broth. The dependence of microcarrier-cell-aggregate formation on impeller speed and shear stress levels was investigated for human adipose derived stromal/stem cells (hASCs) at the spinner scale by recording the Sauter mean diameter (d 32) versus time. Cultivation at the suspension criteria provided d 32 values between 0.2 and 0.7 mm, the highest cell densities (1.25 × 106 cells mL−1 hASCs), and the highest expansion factors (117.0 ± 4.7 on day 7), while maintaining the expression of specific surface markers. Furthermore, suitability of the suspension criterion N S1u was investigated for scaling up microcarrier-based processes in wave-mixed bioreactors for the first time.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Dedicated to Prof. Dr.-Ing. Matthias Kraume on the occasion of his 65th birthday 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 L a) 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 the k L a value and the gas bubble size distribution are presented.