In a related post (see "What is a bioreactor"), we talked about the basics of what a bioreactor is and how the technology has evolved over time. Here we'll look at different types of bioreactors used in biopharmaceutical research, development, and production.
Types of bioreactors
There are many different types of bioreactors including: stirred-tank, rocker, air lift, and fixed-bed. Each type was developed to address certain challenges in cell culture or to suit specific applications.
Stirred-tank bioreactors
Stirred-tank bioreactors are equipped with an impeller for homogenizing culture media and a sparger for delivering oxygen to the cells. Stirred-tank reactors (STRs) are the most widely used bioreactors. These range in size from 15 mL to 2000 L for single-use and are available in sizes larger than 2000 L for stainless-steel. STRs are primarily used to scale up a process from research and development scale to manufacturing scale. The goal is to ensure that a process at a smaller volume can be representative of larger volumes. In addition to scale-up capacity, other key elements to consider are usability and process assurance of the single-use STR at the manufacturing scale.
Rocker bioreactors
Rocker bioreactors are offered as single-use systems. They involve a bag on a moving platform and rely on a rocking motion for the mixing. Oxygen diffuses through the headspace via liquid-gas interface, as these bioreactors are not equipped with a sparger. These bioreactors are typically smaller in scale than stirred-tank bioreactors, ranging from a few liters to ~100 L. They are usually used for small-scale production or for seeding into larger bioreactors.
Air lift bioreactors
Air lift bioreactors are less frequently used in the biopharmaceuticals industry due to them being lesser known with regulatory processes that are largely unexplored. They rely on air bubbles to aerate and carry the media around the reactor for mixing at the same time.
Fixed-bed bioreactors
Fixed-bed bioreactors are used for adherent cells, involving specialist cells that can only grow when attached to a surface. They usually have strips of fibers (carriers) resulting in a high surface area for the cells to adhere to. These carriers are packed together to make the fixed bed. Aerated culture medium circulates through it. Adherent cells can also be cultivated as suspended cells in stirred-tank, rocker, and air lift bioreactors. However, they need to be attached to 100-micrometer beads (i.e., microcarriers) and put in suspension. Scale up of this process is much more complex and time consuming than the equivalent suspension process or the adherent-cell process in the fixed-bed bioreactors technology.
Single-use vs multiple-use bioreactors
Traditional multiple-use bioreactors require cleaning after each cell culture run. Smaller-scale bioreactors are glass systems and can be sterilized in an autoclave. Larger bioreactors are stainless-steel systems that require additional plumbed-in systems for sterilization, such as cleaning in place (CIP) and steaming in place (SIP).
Single-use bioreactors were introduced into the biopharmaceuticals market in the late 1990s, with WAVE™ bioreactor systems among the earliest. They are made of single-use, closed, gamma-irradiated biocontainers, fitting on reusable hardware. Single-use bioreactors are increasingly used for small-scale, mid-scale, and large-scale production runs given their many benefits. As technology has improved, single-use bioreactors have become viable for manufacturing volumes up to and even beyond 2000 L.
What are the different modes of bioreactor operation?
There are four different modes to run a bioreactor; batch, fed-batch, perfusion and continuous.
Batch mode
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.
Fed-batch mode
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
Perfusion involves removing unwanted compounds, such as lactic acid, allowing the process to last longer than fed-batch. This allows for higher cell densities 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
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.
Choosing the right bioreactor
The best bioreactor for you depends on a wide range of factors. Universal considerations include cell type, scale, and application. Constraints such as culture feeding strategy, cost, and facility fit may also influence your choice. And, of course, it's important to identify factors specific to your goals, such as future scale-up plans, the level of support you need from a supplier, time constraints, and how you plan to use the bioreactor in future projects.
FAQs about bioreactor types
What are the main types of bioreactors?
Stirred-tank and rocking bioreactors are used frequently in biopharma research and production. Fixed-bed bioreactors are sometimes used for adherent cells.
How are bioreactors classified based on operation mode?
Bioreactors are sometimes referred to based on how the cells inside are cultured, as each type of culture has its own requirements. These modes are batch, fed batch, and continuous operation. In batch mode, the inoculum, media, and feed supplements are all added at the beginning of the culture period and grown until the desired cell density is obtained, usually over several days. In fed batch, fresh media or supplements are added periodically throughout the growth period. In continuous mode, fresh media is added and cells and waste are removed periodically. When only waste is removed, this is a type of continuous operation called perfusion.
How do different types of bioreactors affect product yield?
In mAb production, product yields typically are higher with fed-batch or continuous operation, as these enable cells to reach a higher density and thus produce more of the target molecule.
How do I choose the right type of bioreactor for my process?
Your choice of bioreactor is based largely on the cell type, target molecule, and amount of product you need.
Explore other articles in the series
Parts of a stirred-tank bioreactor and their function