At first, a decrease in the DO level below the specified set point for all Dasgip vessels during the initial days of the production step was observed. It turned out that the choice of sparger entailed a different maximum gassing rate from manufacturing scale to maintain the DO to set point. The reason for this behavior is the difference between the size of the bubbles produced by the different spargers, which results in diverse bubble residence times.
Since the open steel pipe as well as the L-sparger produce bigger bubbles their gassing rates has to be increased by nearly a factor of six, leading to a final 0.083 vvm, instead of the 0.015 vvm used for the 12 kL bioreactor. The gassing of the vessels using the sintered microsparger, however, could be kept the same. This sparger generates small bubbles transferring oxygen more efficiently to the culture and providing better dissolved oxygen control at low cell density.
For both cultures run with the L-sparger and the open steel pipe (default), cells grew to a maximum viable cell density on day 9, and then entered a stationary growth phase followed by a moderate death phase. These trends are comparable to those observed at manufacturing scale (Figure 3).
The Dasgip system set up with a sintered microsparger exhibited comparable growth until day 5; a more dramatic decrease of viable cell density was subsequently observed. Those findings are caused by the sintered microsparger design: small bubbles provide a greater interfacial area than larger bubbles, leading to a higher oxygen transfer rate. However, smaller bubbles also carry more cells to the top gas-liquid interfacial area, where cell damage occurs due to bubble bursting.
At inoculation, all sparger configurations present lower viabilities than at manufacturing scale (Figure 3). This is due to medium-exchange operations, which are not performed at manufacturing scale.
From day 3 on, the viabilities are comparable between manufacturing scale and all sparger configurations in the Dasgip system, up until day 9 when the loss of viability is more pronounced in the Dasgip system. Here the set up with the L-sparger showed the best results, with viabilities closer to manufacturing scale.
Figure 3 demonstrates that in both Dasgip systems set up with the L-sparger and the open steel pipe, titers are comparable to those obtained at manufacturing scale with slightly higher titers using the L-sparger configuration. Titers are, however, significantly lower with the sintered microsparger configuration due to the lower cell growth and viability.
Finally the product quality attributes were measured only for the Dasgip systems run with the tube open steel pipe and the L-sparger. All product quality attributes were comparable to those obtained at manufacturing scale and within specifications.
A scale-down model is considered comparable to manufacturing scale when all critical quality attributes—growth, productivity, and product quality—are comparable between scales.
As such, the Dasgip system proved to be a viable alternative to the use of 2 L glass bioreactors as a reliable scale-down model. The L-sparger configuration provided more comparable growth, productivity, and product quality data at manufacturing scale than the default sparger and the sintered microsparger. Its volumetric gas flow rate was higher than at manufacturing scale due to a lower bubble residence time in the system. However, this difference had no impact on process performance.
From an economic standpoint, the Dasgip system proved to be a cost- and time-effective model. The preparation time for the Dasgip vessels is approximately one-third of the time needed to set up a 2 L bioreactor. Moreover, only half the amount of raw materials is needed for the Dasgip system due to its smaller working volume compared to the 2 L glass bioreactor.