microcarriers, which are suspended by agitation in stirred tank bioreactors (STRs)

or are used in packed or fluidized bed reactors. Microcarrier systems have been

scaled up to 6,000 L for Vero cells [31]. For suspension cells, the concentration of

metabolites is the restricting factor, thus, the volume of the vessel is of interest for

scaling up. STRs using suspension cells could be scaled up to 10,000 L, but 2,000 L

is the preferred size. Another drawback of adherent cell cultivation is the necessity

for cell detachment when scaling up or during subcultivation, which can be realized

via proteases such as trypsin. Moreover, cell counting for cell growth monitoring

requires cell detachment from the carriers, which is difficult during a production

run, while measurement for suspension cells is conducted offline by taking a sample

via a dip tube. For adherent as well as suspension cells today online measurement of

the cell volume/concentration via the permittivity of polarized cells is now possible,

overcoming this drawback.

5.4.4.1

Cell Attachment and Use of Microcarriers

The adhesion of adherent cells to a surface is divided into two main phases as

shown in Figure 5.2A [32]. After cell seeding, cells attach to the surface and are

characterized by a round cell shape. To provide a suitable growth surface, flasks as

well as microcarriers are often coated, as cell attachment is dependent on a

Ca2+

Mg2+

Mn2+

Fn

Fn

MHS

MHS

(a) adsorption

(b) contact

(c) attachment

(d) spreading &

flattening

SCM

SFM

(a)

(b)

FIGURE 5.2 Schematic representation of cell adhesion (a) without shear stress (T-flasks),

and (b) with shear stress on microcarriers either in serum containing medium (SCM) or in

serum-free medium (SFM). Cells that do not flatten will be easily detached from the carrier

surface due to the shear forces from stirring (indicated by arrows). Figure A adapted from

[ 33]. MHS: multivalent heparan sulfate; Fn: fibronectin.

94

Bioprocessing of Viral Vaccines