Bioreactors and their Design Features

What are bioreactors?

Bioreactors are such mechanisms (vessels) that are manufactured to maintain or shift in a biological system (including enzymes, tissues, organism cells, and microorganisms) and carry out a bioprocess to result in outcomes with chemical substances/substances of organisms (such as antibodies and vaccines). Establishing a bioreactor demands strategic thinking, as one needs to study the biological system they’re working with and identify the suitable parts associated with the system’s cell growth, product expression, and more to apply to the bioreactor. The biological system within the bioreactor is maintained by the presence of oxygen, pH, and temperature suitable to such cells, tissue, enzymes, and organisms. Bioreactors can be both aerobic and anaerobic and are mostly cylindrical, however, range in their size measurement.

How to determine the design of a bioreactor?

The design of a bioreactor depends on the biological system and the bioprocess that needs to be carried out, where certain designs drive more successful results. After reading into the bioreactor design cumulation of articles on sciencedirect.com and a variety of other sources (listed at the bottom of this article), I cumulated a list of specific considerations in the design of bioreactors: shape, turbulence flow, agitation systems, impellers, shear stress, sparging, temperature, ports, etc.

Design Features and Considerations

[1]Design Feature/Consideration #1: Shape

1. Abioreactors’ format and shape depend on the culture capacity in production environments and the performance of the system.
2. Cylindrical bioreactors have been used for cell culture and fermentation for a number of years.
3. Single use bioreactors are commonly used for cell culture and the volumes are 500ml-2000l and fermentation systems are 25 to 500l.
4. The shape of the bioreactor comes hand in hand with the performance of the bioreactor, as fluids within the system can change the heat and cool transfer in the bioreactor.
5. One of the critical considerations in the shape of a bioreactor is the height to diameter ratio. For example, with a long height and small diameter, the bioreactor could perform differently, as liquid gas bubbles will lead into a gas to liquid transfer and the pressure forces will be limited in the bioreactor.
6. Wide bioreactors are used for high gas flow rates and high mixing rates, 3:1 dimension (H:D) and typically associated with microbial fermentations.
7. For cell culture, typically a bioreactor of dimensions 1:5:1 and 2:1:1 is used to control the lower gas flow and low mixing.
8. Sterilized bioreactors have top and bottom heads whereas single use have a bottom head; having one or the other (or both) can change the performance and features of the system.

[1] Design Features/Considerations #2: Turbulence flow, Agitation Systems, Impellers

1. Mass transferring is impacted by the interior design and agitation system of the bioreactor and flows differently with turbulent and laminar flow conditions. Turbulent flow has a higher chance than laminar, however, that is a good and bad thing (as it gives a better result but also affects fragile bioprocesses).
2. Turbulence flow can be introduced through baffles and agitation systems, which allow fluid resistance and deduct laminar flow. Agitation systems have a common position with a 15 degree angle and a center drive, and in some cell cultures, the system positioning makes a great turbulent flow, whereas in microbial fermentation, baffled tanks for higher turbulent flow are recommended.
3. The angle and positioning of the agitation system plays a significant role in the design of the bioreactor, as one could choose either a top/bottom system with integrants like motors, shafts, impellers, mechanical seal, gearbox, etc. The shaft size comes hand in hand with the H:D proportion of the bioreactor and the carrying out volume in the biosystems. Motors are not internally within the bioreactor, so the shaft plays a crucial role in driving forces to fluid motion.
4. Impellers are necessary for mixing fluids, and there are a variety of impellers that can sustain the energy transfer of the bioreactor designs.
5. All impellers are graded based on their flow, with either considered axial, radial, or tangential.
6. Impellers all contain a power number that can increase ( Np, P0)
7. Rushton impellers create an axial and radial flow and are used for microbial fermentations, whereas pitch blades and marine bioreactors are used for cell culture.
8. Mixing the rushton and axial flow impellers would lead to an improvement in mixing, a change in the systems main characteristics, and it would be useful for both fermentation and cell culture. The location of the impellers on the shaft would be varied depending on the use of the bioreactor.

[2] Design Features/Considerations #3: Shear Stress and Impellers in Bioreactors and Fermentation

1. The cell shear stress sensitivity affects the parameters in fermentation and affects agitation systems, impellers, and aeration rates .The two types of shear stress bioreactors are Mechanical and Hydrodynamic. Smaller cell organisms, such as bacteria and yeast, aren’t as shear stress sensitive as plant cells and insects.
2. Impeller designs are modified in consideration of the shear sensitivity of cell cultures and are produced to mix cells, gases, and nutrients within the culture vessel. Mixing evenly distributes oxygen and nutrients to the entire culture and helps maintain temperature.
3. Mixing will have either a radial flow, axial flow, or both.
4. Rushton impellers are most common in the fermentation industry. They produce an unidirectional radial flow for mass transfer and heat and generate a high shear stress in mixing. They are considered for lower shear sensitive cultures, such as yeast, fungi, and bacteria. They create a zone of high AND low turbulence.
5. The blades on a pitch blade impeller produce axial and turbulent flows and are set at an 45 degree angle. They provide a high mass transfer oxygen rate and better mixing in comparison to marine impellers. They are low-shear and are used for insects, high shear-sensitive cells and are suitable for very viscous cultures.
6. Marine blade impeller front can be flat and concave while the back is convex. They use gentle mixing, low shear to prevent damage to the cells.
7. Cell lift impellers are found in fed and batch-fed processes for shear-sensitive animal cells and are used in continuous perfusion within micro-carrier culture.
8. Packed Bed Basket Impellers protect shear sensitive cells, have low turbulence and can have high cell densities.

[3] Design Features/Considerations #4: Sparging

1. Spargers are an important aspect of bioreactors and are found within the inside vessels of the design. Some types of spargers are; ring spargers(macro), open pipe(macro sparger), sintered(micro), and etc. Sparging is the entry of air, most significantly oxygen, into the cell culture channel. When this air dissolves into the culture, the cells use it quickly and need to be fed with spargers. Oxygen is less soluble than culture media, for example oxygen is molar 16000x less than glucose in the culture. With that in consideration, the sparging of oxygen is critical. Low oxygen solubility means that the oxygen dissolve can minimize, limit cell growth. The area increases as the bubble size decreases. Small bubbles have a lower surface area to a higher volume ratio. A larger interfacial area allows oxygen to dissolve more easily into the culture media. Small bubbles created by spargers rise slowly through bioreactors which leads up to a longer residence time. Again, the position of the sparger and impellers of the bioreactor are crucial for a good distribution of bubbles. Sparger designs are divided into two main categories; micro and macro spargers. The larger the bubbles in macro spargers means the lower the residence time, easy cleaning, and scale production whereas in a microsparger with smaller bubbles, there is a higher residence time, difficult cleaning, and a lab production. Sintered spargers are typically used one time in bioreactors.
2. Smaller bubbles can produce excessive foam and halt gas exchange. During the later time of a cell culture process, larger bubbles will be required. In the use of larger bubbles, the culture respiration adds more CO2 to the liquid in the reactor, and some of the gas mixes with the liquid to create HCO3. However, this is avoidable with the sparging of CO2, where the bubbles are large and the CO2 is dissolved out of the culture and into the headspace. If the bubbles were smaller, they would dissolve back into the liquid rather than into the headspace. With all of this in consideration, the bubble size and spargers are crucial to the application and use of the bioreactor.

What are the most crucial design features?

[4]

Scientists have primary considerations and implement certain factors into their bioreactors. If you look at both the bioreactor sketches above, you’ll see certain similarities: pH monitor/probe, heat/temperature probe, foam/antifoam, dissolved oxygen/oxygen probe, conditioning system, and nutrient medium.

Sources/References:

[1] “Bioreactor Design.” Bioreactor Design — an Overview | ScienceDirect Topics, www.sciencedirect.com/topics/engineering/bioreactor-design.

[2] “Shear Stress & Impellers in Fermenters / Bioreactors.” YouTube, YouTube, 8 Dec. 2019, www.youtube.com/watch?v=W9kMeBTSBeY.

[3] “The Impact of Sparging on Cell Culture in Bioreactors — Two Minute Tuesday Video.” YouTube, YouTube, 14 Sept. 2016, www.youtube.com/watch?v=7Pnrnjt6pyo.

[4] https://www.researchgate.net/publication/301680545_Bioreactors_-_Technology_Design_Analysis

*This research was inspired by my task to help design a bioreactor prototype for the nonprofit Open Insulin Project in the summer of 2020.