by Oliver Yu
Microbes used in cell culture thrive in a narrow band of temperature between 30 to 40 degrees Celsius.
Humans have a deep body temperature of 37 degC so it stands to reason that anyone exploring the optimum temperature of mammalian cells in a bioreactor ought to start here.
Temperature is a measure of the "internal kinetic energy" of a system. It can be said that in a system with hotter temperature, the molecules are moving faster and "bumping into each other more frequently." The opposite is true in colder systems.
The cells in a bioreactor set to 37 degC "bump" into glucose molecule (and other molecules) at a higher frequency than cells in a bioreactor set to 33 degC. Should the cell culture be set to a higher temperature, key biological processes can fall apart - hence an upper and lower bound to the temperature.
Bioreactor temperature is controlled with a resistance temperature device (RTD) that sends the temperature reading to a controller. If the temperature drifts too low, the controller sends more cool water through the jacket; the temperature drifts too high and the controller sends more hot water through the jacket.
AgitationFor this method to work, the heat transferred through the bioreactor wall must be evenly dispersed throughout the cell culture, so agitation mixes the cell culture ensuring that the no volume of fluid stays local to the bioreactor wall for too long.
The agitation, itself, increases the temperature as the work into the system is dissipated as heat. But this temperature increase is negligible.
Gas SolubilitiesTemperature is crucial in determining the solubility of gas in the cell culture, which is mostly media...itself mostly water. The lower the temperature, the more carbon dioxide and oxygen the media can hold. So pH control and dO2 control are impacted by temperature control. In general, if you stick with temperatures found in nature, you're going to grow cells like they grow in nature.
How it works in the real worldTemperature control is a well-understood chemical engineering phenomenon long before the advent of biologics manufacturing. For seed/inoculum cultures, temperature is typically set at at fixed number (e.g. 37 degC). For production cultures that need to last, some temperature control strategies involve a temperature reduction based on biomass or time in order to cool the cell culture and stave off late culture viability.
If a temperature reduction happens, the dO2 controller will call for less oxygen because more oxygen is retained in the culture. The solubility effect for pH control is usually not observable in the control instruments because at this point in the culture the cells are evolving the carbon dioxide.