shear stress turns this flow into a turbulent flow; the fluid moves randomly in all

directions but yields a flow in one direction with an average velocity. Turbulent

flow regimes are typically associated with higher shear stress.

Shear stress occurs in a bioreactor by stirring or when air bubbles disrupt.

Likewise, high shear stress conditions can occur, when cells or virus particles are

pumped through tubings. Peristaltic pumps are often used for such pumping actions

for example for inoculum or virus addition or harvesting. But peristaltic pumps can

cause very high shear stress that often results in cell damage or cell death. An

alternative option is, e.g., the use of magnetic levitating pumps from Levitronix.

Furthermore, for the transfer of all liquids containing cells or virus particles, the

flow rate and inner tube diameter should be well chosen. In particular, switching

between diameters can cause additional shear (equally the transfer through con-

nectors). For vessel-to-vessel transfers, tubes should end as close as possible to the

liquid level to avoid further cell damage.

5.6.2.4

Oxygen Supply

Lastly and of great relevance is the oxygen supply. Oxygen is an essential substrate

for the cell, due to its crucial role as last electron acceptor within the respiratory chain.

However, for their cultivation, cells are submerged in an aqueous liquid phase. Since

O2 is a gaseous non-polar molecule, its solubility is extremely low, around 7 mg/L at

37°C and 1 atm [56]. The biomass, salts, sugars, and other complex components in

conventional culture media lower O2 solubility in an additive way as a function of

their concentration, so it can be up to 30% lower in comparison to its solubility in

water at the same temperature and pressure [57,58]. To make O2 available to the cells

in culture, it has to be transferred from the gaseous to the liquid phase, which is the

limiting step in this process. The overall oxygen transfer rate (OTR) is described by

the following expression:

(

)

OTR

k a C

C

=

L

O

O

2

2

(5.2)

where k a

L

[h⁻¹] is the volumetric mass transfer coefficient, CO2 [mM] is the oxygen

saturation concentration and CO2 [mM] the actual oxygen concentration.

As long as the O2 demand increases during the cell growth phase or early virus

production phase (before virus-induced apoptosis and cell death), pO2 decreases

progressively [56]. Below a threshold value (about 5% pO2), animal cells reshape

their metabolism to cope with their energy and reducing power needs. Under this

scenario, it is said that the cell culture is O2 limited. In industrial scale bioreactors,

O2 limitation not only is a function of time, but also of space in case of the in-

efficient mixing. The latter results in a poor dispersion of air bubbles, so that regions

with different O2 concentrations and OTRs can appear [59]. The heterogeneities to

which the cells are exposed to and that are magnified with the increase in scale, are

often responsible of their low process performance.

Maximizing the OTR in bioreactor design and operation might prolong cells’

productive time, from the high-throughput screening systems to the tenths m³

production bioreactors. The fundamental guidelines in the strategies to increase

Upstream processing for viral vaccines

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