by Oliver Yu
Pressure control strategy is the last and the least of the cell culture environmental parameters. Mostly, this is because cell culture media is mostly water and water is an incompressible fluid.
The other reason is that the only thing pressure can impact is the solubility of oxygen and carbon dioxide in the media--and not by much:
The main reason to control pressure is to maintain positive pressure in the bioreactor for contamination risk reduction. Positive pressure means that the pressure in the bioreactor is greater than the outside pressure so that if there is a breach in the sterile envelope (e.g. filter, probe port, rupture disk) that the flow is outwardly.
This outwardly-directed flow does not prevent contamination, it gives the operator an opportunity to identify the breach and address it before the situation gets worse.
How it happens in the real world
Pressurizing bioreactors is a well understood, well-engineered process. Pressure is controlled by shutting off all the valves and modulating the flow of sterile-filtered air in the headspace of the bioreactor. Filtered air comes in and moist air goes out and if more pressure is needed, the more the valve is commanded to open.
In large-scale cell culture, however, there is an opportunity for the vent filter to get clogged, in which case the pressure will slowly increase. Efforts to change the vent filter must be weighed against contamination risk; if this happens mid-culture, the risk is worth it. If this happens late culture, people often wait.
Also, the vessel pressure can drop to zero (same as atmospheric pressure) if there are breaches.
For commercial manufacturing, controlling product formation and product quality for biologics means controlling the cellular environment. Controlling the cellular environment means having pH, dO2, agitation, temperature and pressure control strategies defined by manufacturing instructions that pH, dO2, dCO2, shear forces, mixing, bubble size distribution, temperature and osmolality.
Big ups to: Bob Kiss, Dan Stark, Jesse Bergevin and Reddy for teaching me everything I need to know about large-scale cell culture.