A diffusion controlling bioreactor that selectively controls the molecular diffusion between fluids through a microchannel in fluid communication with a reaction reservoir. The length and cross-sectional area are selected to obtain a predetermined rate of molecular diffusion between fluids. When the fluids are liquids, flow through the microchannel is laminar and the capillary action of the microchannel and fluid is such that the fluid does not flow into the reaction reservoir unless the pressure of the fluid is increased by an external source, thereby minimizing contamination of the bioreactor. The instant invention may also utilize at least one microchannel and reagent reservoir to regulate, rather than prevent, the passage of various molecules into the bioreactor. A pressure equalizing vent operating on similar principles to the microchannel may have a structure configured to minimize the chances of fluid leakage from the bioreactor, even if the bioreactor is turned in various directions.