The bioreactor

A bioreactor is an engineered vessel of glass or steel, usually cylindrical, in which microbial, plant or animal cells grow under sterile (aseptic), controlled conditions.

It must provide:

• a sterile environment;
• enough air (oxygen);
• uniform mixing, without the tearing forces (shear stress) that damage cells;
• the best temperature;
• constant monitoring of pH and dissolved oxygen.

Its main parts:

• Agitator shaft with an impeller near the bottom: stirs and mixes the contents.
• Sparger: bubbles sterile air (passed through microfilters) into the medium.
• Baffles: strips along the wall that break up the whirlpool (vortex) the stirrer would otherwise create, which wastes power and mixes poorly.
• Jacket: an outer layer through which water circulates at the set temperature.
• Probes that measure temperature and pH.

A digital controller runs the water bath and adds small amounts of 1M NaOH or 1N HCl to hold the pH steady, and displays the temperature, pH and stirring speed (rpm).

Three common types:

• Stirred tank: the conventional bioreactor, mixed by an agitator.
• Air-lift: air pumped into a central draft tube lifts the liquid, making it circulate round the vessel.
• Bubble column: air bubbles rising from a sparger do all the mixing. It has low shear and transfers a lot of oxygen for the power used.