provide homogenous shear stress levels for the formation and regulation of the
spheroid sizes. Such bioreactor development or design studies can be supported by
numerical models that allow for optimization of the fluid flow regarding these issues
(i.e., homogenous hydrodynamic stress distribution).
2.2.2 Growth on Microcarriers
In order to overcome the limitations of the 2D culture systems, in 1967 van Wezel
[51] developed the concept of MC-based cultivation systems. In these systems, the
cells are expanded on the surface of small solid particles suspended in the cell culture
medium by slow agitation. The MC-based expansion represents a unit operation in
which both monolayer and suspension cultures are brought together. The MC
surface is available for cell growth, while the mobility of MCs in the medium
generates a homogeneity that is similar to the suspension environment used in
traditional mammalian submerged cultures [52]. Thus, MC-based expansion systems offer the following advantages:
1. A high surface to volume ratio, which can be further increased by increasing the
MC concentration
2. A homogenous environment that allows various process parameters (e.g., pH,
pO 2 , substrates and metabolites) to be both monitored and controlled
3. A possible scale-up of the MC-based expansion process within a suitable bioreactor series
4. Functionalization of the MC surface to improve cell attachment and in terms of
hMSCs to retain a high “stemness”
Different MCs, which are usually spherical, have been tested or even developed
over the years for the expansion of hMSCs (see Table 4). The MC types differ
greatly in size (90–380 μm), core material (e.g., polystyrene, cellulose, dextran,
gelatin), and surface coating (e.g., collagen, fibronectin, laminin, vitronectin). An
overview of commercially available MCs, including their material properties, can be
found in different reviews [15, 52, 53]. The core material and surface coating affect
not only the MC settlement and cell growth but also the impeller speed which is
required to hold the MCs in suspension and to guarantee sufficient mass transfer.
Rafiq et al. [54] and Leber et al. [55] screened different MC types in small-scale
bioreactors for hMSCs under predefined impeller speeds (N js ¼ N s1 ). Both found
significant differences in cell attachment, cell growth, glucose consumption, and
metabolite production depending on the MC type. They found that hBM-MSC grow
best on collagen-coated MCs from Solohill and Synthemax II and ProNectin F MCs
from Corning, something which comes as no surprise since these MCs are coated
with collagen and fibronectin, respectively. Both coatings are components of the
extracellular matrix, including the arginyl-glycyl-aspartic acid sequence which is
well-known to promote cell attachment and cell growth of fastidious cells [56]. Different studies have shown that the planar structure, including the material stiffness,
nanotopography, and local curvature, can impact cell proliferation, maintenance of
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V. Jossen et al.
spheroid sizes. Such bioreactor development or design studies can be supported by
numerical models that allow for optimization of the fluid flow regarding these issues
(i.e., homogenous hydrodynamic stress distribution).
2.2.2 Growth on Microcarriers
In order to overcome the limitations of the 2D culture systems, in 1967 van Wezel
[51] developed the concept of MC-based cultivation systems. In these systems, the
cells are expanded on the surface of small solid particles suspended in the cell culture
medium by slow agitation. The MC-based expansion represents a unit operation in
which both monolayer and suspension cultures are brought together. The MC
surface is available for cell growth, while the mobility of MCs in the medium
generates a homogeneity that is similar to the suspension environment used in
traditional mammalian submerged cultures [52]. Thus, MC-based expansion systems offer the following advantages:
1. A high surface to volume ratio, which can be further increased by increasing the
MC concentration
2. A homogenous environment that allows various process parameters (e.g., pH,
pO 2 , substrates and metabolites) to be both monitored and controlled
3. A possible scale-up of the MC-based expansion process within a suitable bioreactor series
4. Functionalization of the MC surface to improve cell attachment and in terms of
hMSCs to retain a high “stemness”
Different MCs, which are usually spherical, have been tested or even developed
over the years for the expansion of hMSCs (see Table 4). The MC types differ
greatly in size (90–380 μm), core material (e.g., polystyrene, cellulose, dextran,
gelatin), and surface coating (e.g., collagen, fibronectin, laminin, vitronectin). An
overview of commercially available MCs, including their material properties, can be
found in different reviews [15, 52, 53]. The core material and surface coating affect
not only the MC settlement and cell growth but also the impeller speed which is
required to hold the MCs in suspension and to guarantee sufficient mass transfer.
Rafiq et al. [54] and Leber et al. [55] screened different MC types in small-scale
bioreactors for hMSCs under predefined impeller speeds (N js ¼ N s1 ). Both found
significant differences in cell attachment, cell growth, glucose consumption, and
metabolite production depending on the MC type. They found that hBM-MSC grow
best on collagen-coated MCs from Solohill and Synthemax II and ProNectin F MCs
from Corning, something which comes as no surprise since these MCs are coated
with collagen and fibronectin, respectively. Both coatings are components of the
extracellular matrix, including the arginyl-glycyl-aspartic acid sequence which is
well-known to promote cell attachment and cell growth of fastidious cells [56]. Different studies have shown that the planar structure, including the material stiffness,
nanotopography, and local curvature, can impact cell proliferation, maintenance of
196
V. Jossen et al.