Figure 2. Principal of rocking mixing technology.
|
Energy input is affected by rocking the chamber back and forth, generating a fluid movement in the cell culture and medium
(Figure 2). In this way, the surface of the medium is continuously renewed so bubble-free aeration can take place. Therefore,
a maximum of half of the total bag volume can be filled with the cultivation media. The other half is needed for the overlay
aeration. Depending on the sensitivity of the cultivated cell line, the angle and amplitude of the rocking motion, as well
as velocity can be varied. Specially designed sterile plastic cultivation bags contain tubing for filling, harvesting and
sampling. This allows simple handling and optimal cell growth for different cell types and cell lines.
Figure 3. Aeration strategy with BIOSTAT RM Tower optical (Sartorius BBI Systems).
|
The measuring and control unit can facilitate regulation of pH, DO, temperature, rocking rate and gas flow using the gas mixing
system. An example for gas supply strategy is shown in Figure 3. The pO2 cascade control and pH control with CO2 additionally
to base and acid is provided.
The used cultivation bags can be sterilized by autoclaving and then destroyed by incineration. The PE foil from which the
cultivation bag is made, is environmentally compatible when incinerated. This is less critical for the environment than the
cleaning of conventional bioreactors with alcohol, acids and basic solutions. An additional safety factor is the fact that
no cell or protein residues are released into the wastewater cycle.
Potential and limits
The advantages of disposable bioreactors are mainly ease of use, prevention of cross-contaminations, and cost and capital
investment savings. The costs compared with conventional stainless steel bioreactors are lower because the capital investment
requirement is lower, and the necessity for qualification and validation processes, as well as maintenance, is reduced. A
cost reduction of up to 41% compared with conventional stirred bioreactor technology is possible.4 Additionally, the disposable bioreactors enhance process safety because the contamination risk falls under 1%.10 The disadvantage
of using disposable bioreactors is the limitation in scale-up (300 L) and aeration capabilities for cultivation of some microorganisms,
which require high oxygen uptake rates.
Oxygenation or aeration can be achieved by three methods, individually or in combination; that is, membrane aeration, surface
aeration or direct sparge aeration.
Microbial fermentations exhibit high metabolic oxygen demand together with high biomass concentration; this combination requires
an energy-intensive means of maintaining adequate culture aeration. This aeration is achieved by simultaneously sparging air
at high rates and vigorously agitating the culture to increase the bubble interfacial area and affect the mass transfer of
oxygen.
Table 1. KLa(h-1) and OTR(h-1) values of different cultivation systems (source: Enfors 2003 and Wave Biotech AG, Switzerland).
|
The biomass concentrations and cellular oxygen uptake rates observed in mammalian cell cultures are low compared with microbial
systems (Table 1). Even if the cellular oxygen uptake rate is low, it does not mean that oxygenation does not present a problem.
The reason is the shear sensitivity of mammalian cells, which prevents the use of vigorous aeration or agitation. As a result,
the volumetric oxygen mass-transfer coefficient is low and becomes a limiting factor in scale-up.
|
