(498f) Design and Manufacture of Novel Airlift Milibioreactors: A Low-Cost Lab Scale Approach for Proof-of-Concept Experiments

The need for platforms to scale-up bioprocesses is of the utmost importance for proof-of-concept experiments to collect information regarding operating conditions in order to maximize yield and minimize energy consumption. Commercially available systems are expensive, require large volumes and rely on tedious assembly protocols. This is particularly important for the case of bioprocesses where efficiency abruptly changes with small changes in the operating conditions. As an alternative, we propose to engineer lab-scale bioreactors with reduced volumes and manufacturing investments. Due to their ample applicability in a number of bioprocesses, we selected external loop airlift bioreactors as models to develop novel systems. Generally, in airlift bioreactors, the medium inside the vessel is pneumatically stirred by a gas stream (4), allowing oxygen transfer for the maintenance of the cultured cells (1). Additionally, this configuration significantly reduces the shear stress on the cells, which make them ideal for delicate cell cultures. Accordingly, we designed 200 mL airlift bioreactors, which we termed Milibioreactors. A schematic of the newly developed system is shown in Figure 1. As a conventional external loop airlift bioreactor, our system is equipped with a riser and a downcomer but at a much smaller scale. Additionally, the system can be easily instrumented with low-cost sensors for measuring key parameters such as biomass content, dissolved oxygen and pH. Moreover, the system can be customized to incorporate supports with encapsulated cells. For manufacturing, we made use of simple and inexpensive molding and 3D printing approaches.

For design and prototyping, we will explore two different gas flow configurations. In the first case, we considered injecting the gas directly into the reactor while in the second one, gas will be pre-injected into a liquid stream that goes to the reactor. For the assembly, the system was divided into three main components. The first one is the base where the required gas flow or liquid will be injected. The second one is the body of the Milibioreactor, which has the external loop and an outlet in case the rector is injected with liquid. In the event that gas is injected, the outlet would require a stopper. The last section is the lid, which is equipped with an outlet for gas in the upper section. To understand momentum transfer within the Milibioreactor, we conducted CFD simulations in Comsol Multiphysics® (3). We implemented a 2D computational domain as shown in Figure 2. Meshing was such that convergence was achieved with a maximum of 2% of change for the fluid velocity in randomly selected locations within the computational domain.

Figure 1. Milibioreactor design

A simulation for air injected at the bottom domain led to the streamlines for the fluid flow shown in Figure 2. Also, examples of the time evolution of velocity profiles for the gas and liquid are plotted in Figure 3.

Figure 2. Streamlines for the liquid phase into the  Milibioreactor

For the manufacture of the base and the lid, a negative 3D mold was prepared in silicone rubber. This mold was then filled up with polyester resin to obtain the part. The body was prepared in polypropylene. If required, all the reactor sections are autoclavable and tolerate operation at high temperatures.    

Figure 3. Fluid and gas velocity profiles

Proper assembly of the 3 sections of the milibioreactors was achieved by checking for leaks at the joints. We are certain that the newly developed systems provide a low-cost alternative to conduct proof-of-concept bioreactions in a simple and inexpensive manner. Further steps will be dedicated to test the performance of the system with both free and encapsulated cells such that yields and energy consumption levels can be collected and compared with larger testing systems at the lab-scale.    

References

1.Chisti, M., & Moo-young, M. (1987). Airlift reactors: characteristics, applications and design considerations. Chemical Engineering Communications, 60(1-6), 195-242. doi: 10.1080/00986448708912017

2. Chisti, M. (1989). Airlift bioreactors. London: Elsevier Applied Science.

3.Mohanty, K., Das, D., & Biswas, M. (2007). Mass transfer characteristics of a novel multi-stage external loop airlift reactor. Chemical Engineering Journal, 133(1-3), 257-264. doi: 10.1016/j.cej.2007.02.007

4.Zimmerman, W., Hewakandamby, B., Tesař, V., Bandulasena, H., & Omotowa, O. (2009). On the design and simulation of an airlift loop bioreactor with microbubble generation by fluidic oscillation. Food And Bioproducts Processing, 87(3), 215-227. doi: 10.1016/j.fbp.2009.03.006