What Are the Different Bioreactor Processes?
April 9, 2020
In this blog, we discuss bioreactor processes for cell cultures. A cell culture is limited by factors such as the living cells having a life cycle and using up nutrients and secreting harmful toxins. These different bioreactor processes are a result of these limitations or aim to solve them. There are four different modes to run a bioreactor; batch, fed-batch, perfusion and continuous.
- A batch process is the simplest process. For a batch process, the bioreactor is filled with a predetermined amount of media. The cells are then inoculated into the bioreactor. The cell culture grows until the nutrients are consumed, and then the run is harvested. Nothing is added or removed from the bioreactor during a batch process. The cells go through four main phases. These are known as: lag, growth, stationary (which is when the product is secreted) and then finally, death.
- A fed-batch process is similar to a batch process, with one extra part added. For a fed-batch process, the bioreactor is filled with media and inoculated with cells. As the cells grow and consume the initial media in the system, fresh ‘feeds’ of nutrients are introduced as required throughout the run. The addition of nutrients (feeds) throughout the process prevents the depletion of nutrients and provides additional cell growth. This can extend the process duration, resulting in a higher cell density capabilities and higher product yield. A fed-batch process is the most common process, as it is one of the simplest and can have better results than a batch process. There are different strategies for a fed-batch culture: high cell-density, constantly-fed-batch and exponential-fed-batch. High cell density is achieved by adding a high concentration feed in order to not dilute the cells. Constantly-fed-batch involves simply adding a constant stream of feed throughout the process. Finally, exponential-fed-batch involves matching the feed rate with the feed consumption rate, so as the cells grow exponentially the feed rate is increased exponentially.
- Perfusion involves removing unwanted compounds, such as lactic acid, allowing the process to last longer than fed-batch. This allows for higher cell density’s and a higher product yield. It involves recycling some of the media and utilizing tangential flow filters to remove unwanted compounds. The pump and filter used in perfusion is very important as to not destroy the cells or clog up the filter, and to help maintain sterility. Diaphragm or peristaltic pumps can be used in conjunction with a tangential flow filter (TFF). For single-use technologies, alternating tangential flow (ATF) is a common perfusion method paired with a single-use bioreactor. Because of this waste flow of media out of the bioreactor there is a constant fresh feed of media into the bioreactor to maintain volume. However, this process is not continuous as the bioreactor does not reach steady state as the cells grow and will reach a limit in cell density.
- Continuous processing is when the bioreactor reaches a steady state, where the growth rate of cells is equal to the death rate. There is a continuous stream of feed into the bioreactor and a continuous feed of product out of the bioreactor. Continuous bioprocessing is currently in the early stages of development; however, the process has the potential to go indefinitely, manufacturing the product at a constant rate. A continuous process is beneficial as there is no downtime between batches, there are no expensive and laborious cleaning tasks, and there is no batch-to-batch variability in your product.
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Alex Williams – Associate R&D Engineer, Research and Development
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