Bioreactors provide a stable, controlled environment for biological cultivation of cells by maintaining biochemical processes that yield desired substances for biopharmaceutical applications. Bioreactors must be operated aseptically; be able to control pH, dissolved oxygen (DO), and temperature, to optimize cell growth. They provide a medium that provides the nutrients for cells to grow in; and deliver sufficient mixing and aeration to the culture to encourage growth. On the surface bioreactors can appear to be complex pieces of kit, so it is good to break down a bioreactor into its component parts, to gain a clearer understanding of how a bioreactor functions.
The vessel
Stirred-tank bioreactors may be composed of glass or stainless steel and allow cells to proliferate within the tank in an agitated suspension. A variation of this bioreactor is the single-use stirred tank that allows cells to proliferate in a pre-sterilized, single-use bag within the vessel. The most recent vessel introduced is the fixed-bed bioreactor that immobilizes cells on a fixed substrate, reducing the shear exposure common in the traditional stirred tanks and supporting cell lines that are not well adapted to the conditions of suspension culture.
Temperature control
Temperature regulation within the bioreactor is extremely important. The different methods of temperature control will depend on size and scale of the bioreactor. These options include heated blankets that can be latched around the vessel; a heating and cooling jacket incorporated under, and/or around the walls of the biocontainer; or internal heat coils. The smallest bioreactors may be placed in a separate temperature-controlled cabinet.
Baffles
Baffles are located on the inside walls of the bioreactor vessel and assist the impeller in providing uniform mixing by preventing vortexing of the media. Impeller
Impeller
It is necessary for any bioreactor to have homogeneous, low shear, continuous mixing capabilities to transport nutrients to, and waste products away, from the cells. Impellers provide agitation, heat transfer, and assist the spargers in aeration by breaking down and mixing bubbles throughout the system. Special consideration is given to the size, height, and shape of the impeller and will determine the allowable power inputs of the system.
Spargers
Dissolved oxygen (DO) concentration is an important parameter for cell growth in bioreactors and may be a critical limiting factor for very high cell concentrations such as microbial culture. Oxygen is transferred via oxygen-rich bubbles generated by spargers. Each bioreactor has its own aeration capacity which will determine the need for microspargers, macrospargers, or a combination of both. Microspargers are typically made of stainless steel and generate small bubbles. Macrospargers usually contain a pipe and ring and supply larger bubbles.
Overlay gas in-line
Where oxygen demand is low, aeration may not be required, and oxygen can be provided via a simple gas overlay. This overlay provides oxygen to the cell culture through diffusion, via the culture medium interface. Typically, the overlay also facilitates the removal of CO2 from the bioreactor via the same diffusive process.
Sterile filters
‘Gas in’ and ‘gas out’ lines go through a 0.2 µm sterile filter such that both the outer environment and the cell culture within is protected, confirming that the biochemical process is contained within the bioreactor.
Ports
Seed cultures, feed media, supplements, acid and base solutions, can all be supplied into bioreactors and fermenters through a number of different ports dedicated to each, without running the risk of being contaminated by injection syringes or inoculation needles. Ports are also available to take samples to confirm a bioreactor is functioning as expected and to harvest the cell culture fluid as required.
Foam control
It is necessary to decrease the amount of foam within the vessel to prevent exhaust gas filter blockage. Foam sensing and control are two elements that may be used to regulate this. The probe is inserted through the top of the bioreactor and is adjusted to a certain level that is above the broth's surface. A current will flow through the circuit if the foam level increases and reaches the tip of the probe. When the pump is turned on by the current, antifoaming agent is instantly discharged to combat the problem.
Valves and clamps
Valves and clamps allow shutting and closing lines and tubes when they are not in use. These aid containment and control.
Safety interlock
These are measures put in place that activate and stop an actuator to maintain safety and containment of the culture. For example, in the event of pressure build up, a safety interlock is activated when the pressure exceeds a defined threshold and stops aeration.
Sensors and control systems
To control and optimize the system, sensors and probes for monitoring and controlling parameters such as temperature, DO and pH are a critical part of the bioreactor. Pressure sensors are also implemented to monitor the pressure in the vessel.
Gassing system
Mass flow controllers for each gas, typically nitrogen, oxygen, carbon dioxide and compressed air, as well for ‘overlay’, and ‘sparger’, are implemented for a controlled and precise gassing regime.
Understanding the different component functions that make up a bioreactor is a first step in optimizing a cell culture process. However, much of the complexity in use of bioreactors is actually the science behind each individual cell line, and it is here where the bioprocessing specialist can optimize parameters to maximize yield.