by Oliver Yu
Because dissolved oxygen impacts cell growth and viability, a dissolved oxygen strategy is required. Overall, the dO2 control objective can be summarized as
Don't let the dO2 of the culture drop below 5% air saturation.To achieve a dissolved oxygen between 20 and 60% air saturation, we use a combination of sparging, agitation and media ingredients.
Sparging Air/OxygenIf you decided to oxygenate the cell culture by blowing air or oxygen into the bioreactor (at the bottom since gases tend to bubble to the top), this would be better but not enough as the cells still sit at the bottom of the bioreactor.
AgitationTo get the cells off the bottom, an agitator spins with impellers pushing the fluid downward. The mixing dissipates the cells from the bottom of the bioreactor and suspends the cells in the media. The agitation also ensures that air, oxygen (as well as CO2 acid, carbonate alkali and media components) are evenly distributed throughout the cell culture.
The downward pumping impellers are to help impede the speed of air/oxygen bubbles to increase their residence time in the culture.
Shear ForcesNow that you've introduced air/oxygen to oxygenate the cell culture (in addition to the CO2 for pH control), you've added shear forces to which mammalian cells are not accustomed. Between agitation and bubbles, the greater shear force is with the bubbles.
To help the cells cope with shear forces, surfactant is added to the cell culture media.
How dO2 works in the real world
dO2 probes are calibrated when the bioreactor is filled with media. The final step of this calibration is to saturate the media with oxygen and span the probes to 95 or 100%. As the bioreactor awaits inoculation, the saturated media will lose oxygen naturally.
Once inoculated, however, the cells begin using dissolved oxygen and the dO2 of the cell culture drops. When it drops below setpoint, the dO2 controller will begin sparging air. When the maximum flow rate for air is unable to meet the oxygen demand, the air is supplemented with pure oxygen. The culture will peak begin to slow; as the culture slows, less oxygen is demanded and oxygen-supplementation is withdrawn.