Bioreactors are vessels designed to carry out reactions brought about by cells, or portions thereof (e.g., enzymes); their ultimate purpose is to generate extra biomass or specific metabolites for a number of practical applications—following harvest, separation, and purification of their contents. In either case, the biocatalyst must gain access to substrate(s)—which, in the case of living cells, also serve as nutrients for metabolism lato sensu; all such starting compounds are to be supplied in liquid form, usually dissolved in water—and even such gaseous substrates as O2 or CO2 can be taken up only if in solubilized form.