OTR in bioreactors rely on improving mixing and gas dispersion as well as in

increasing O2 solubility.

To increase the oxygen transfer, the following strategies can be used:

a. Increase the partial pressure of O2 in the gas fed to the STR

According to Henry´s law, O2 concentration in liquid phase is directly

proportional to the O2 partial pressure in the gaseous phase. Therefore,

increasing the O2 content in the aeration gas stream will allow more O2 to

be dissolved. Thus, aeration can be carried out either with air (21% pO2)

and CO2 or with N2, pure O2 and pure CO2. The use of enriched O2

mixtures has allowed increasing cell density [60,61]. However, high pO2

concentration might trigger oxidative stress responses [62,63].

b. Pressurization

Increasing the pressure in the bioreactor increases pO2 and oxygen solu-

bility in the culture medium. This allows a more efficient use of the vo-

lumetric power input that is reflected at high OTR (Knoll et al., 2005). A

disadvantage is that dissolved CO2 can reach inhibitory concentrations,

since it is five times more soluble than O2 [58].

c. Decreasing culture temperature

Although mammalian cells are often cultivated at 37°C, decreasing tem-

perature to 30°C increases O2 availability in the culture medium up to 10%.

Increasing the kLa value can be achieved as follows:

The volumetric mass transfer coefficient kLa in equation 5.2 is a key factor that

determines the OTR. It is strongly influenced by aeration rate, sparger type,

properties of the culture medium, mixing and vessel geometry including baffles,

vessel pressure, and gas composition.

a. Gas phase dispersion

Most spargers are tubular structures, generally ring-shaped with drilled

holes through which the gas stream is dispersed. The greater the number

and the smaller the diameter of the holes, the smaller the size of the

bubbles and consequently the larger the gas-liquid contact surface area.

The latter applies, in particular, for the use of sintered steel or ceramic

microspargers that allow for very small bubble sizes and contribute to an

increased gas hold-up and bubble residence time [64]. However, this ex-

tent of gas dispersion can be critical for shear stress sensitive cells [65,66].

The geometry, size, and position of the impellers, as well as the sparging

rate, influence importantly gas dispersion in terms of hold-up [67]. The

composition and the flow rate of the gas input controls the amount of

oxygen supplied, but can also have the disadvantage that it pushes CO2 out

of the liquid and with that increases the pH value [68]. Gas sparging results

undesirably in foaming. Foam formed from medium compounds or pro-

teins released into the extracellular medium can trap cells and could

eventually clog vent filters resulting in an unwanted termination of a

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Bioprocessing of Viral Vaccines