Three tools for mixing characterization were used: mixing time experiments, correlation software, and the particle image velocimetry
(PIV). Typical results of the PIV technique are shown in Figure 2.
Figure 2. Results generated by the particle image velocimetry technique
Mixing characterization was performed on the four selected technologies; their mixing configurations are shown in Figure 3.
Mixing performances were evaluated based on several criteria such as mixing time, maximum shear levels, etc. These performances
were then compared to the application's specific requirements: minimizing shear stress while assuring good homogeneity levels
and maintaining all the microcarriers in complete suspension. Some results of this study are depicted in Figure 4.
Figure 3. Mixing configuration of the four single-use bioreactors (SUB) evaluated
This figure shows the combination of maximum shear stress (represented by the tip speed) and mixing time in the minimum operating
conditions necessary to maintain microcarriers in complete suspension. In this graph, two technologies seem to show better
results in terms of acceptable mixing time combined with a low maximum shear stress (tip speed).
Figure 4. Mixing performances comparison of the four disposable bioreactors
Aeration performances also were compared based on gas transfer capacity measurements (kLa) in our end-of-cell-growth conditions for each bioreactor.
Gas transfer capacities in our operating conditions (minimum agitation speed required to maintain microcarriers in complete
suspension) were evaluated for the four bioreactor technologies. A high gas transfer capacity is useful to decrease the amount
of oxygen needed to maintain the dissolved-oxygen concentration to its set point. With low sparging flowrates, shear induced
by bubble break-up at the liquid surface will be decreased. These low flow rates also have a positive effect on foaming.
Cell Growth and Viral Production Results
Figure 5 shows a typical example of four cell growths obtained in one of the selected disposable bioreactors. These data demonstrate
that the cell growths obtained in this system are consistent and also equivalent to our control bioreactor (the control bioreactor
is a small-scale 10 L bioreactor that was validated as a representative scale-down model of the larger stainless steel vessels).
Figure 5. Cell growth profiles in one disposable bioreactor
A critical feature of microcarrier-based cell culture is the homogeneity of cell adhesion to the beads. This point was monitored
in all experiments performed in the different disposable systems selected and compared to the control bioreactor. Microcarrier
pictures by microscopy at different time points (days 0, 2, and 5) were analyzed for each culture. These pictures show that
a homogenous cell adhesion to the bead can be achieved using the right disposable bioreactor system, and the level of homogeneity
is similar to that obtained in a stainless steel bioreactor.
The most important process criterion for evaluating the performance of these systems was their ability to support the same
level of viral production as in a conventional bioreactor. To evaluate this point, several serotypes were produced in the
different disposable bioreactors selected. Figure 6 shows an example of the results obtained for one viral serotype with the
three disposable bioreactor systems. Viral production obtained with two systems are equivalent to the one obtained with the
Figure 6. Viral production in the three disposable bioreactors evaluated
By the end of this evaluation, we demonstrated that with the right disposable systems, it is possible to achieve process performances
equivalent to stainless steel bioreactors.