Many new bioreactors list operate with high productivity rates. The productivity rates and cell density that can be successfully maintained in some of the bioreactors are compare. It can be seen that the productivity increases atleast 50 fold by using immobilized cells which allow increase of cell density by an order of atleast 100 fold over batch reactor. Some of the criteria considered while designing a bioreactor system include the following: (i) microbiological and biochemical characteristics of the cell system, which may be a microbe, a mammaline cell line or a plant cell culture; (ii) hydrodynamic characteristics of bioreactor; (iii) mass and heat transfer characteristics of the bioreactor; (iv) kinetics of cell growth and product formation in the bioreactor; (v) genetic stability and characteristics of the cell system used in the bioreactor; (vi) aseptic equipment design to avoid contamination; (vii) control of bioreactor design on downstream processing; (ix) capital and operating cost of the bioreactor; (x) potential for bioreactor scale-up (scale up means, how to reduce the operational costs leading to lower cost of production).

As shown in the above criteria, in cell cultures (mammalian are needed for growth and synthesis of the product. Bioprocess engineering should also keep in mind that these requirements differ for different cell lines or different plant species used. Conditions also need to differ for suspension cultures or anchorage dependent (e.g. immobilized) cells, which should be kept in mind while designing bioreactors. In future, one of the priority areas will be the development of bioreactors for continuous fermentation both with free and immobilized cells. Development of computer software packages for simulation studies involving industrial production of substances like antibodies, etc. is another area of considerable research activity.

Downstream processing equipment for maximum recovery should also match the production system. Because most of the high valued bioproducts are synthesized in minute quantities, their recovery from fermented broth needs special attention. This necessitates studies on flocculation, selective adsorption and rapid precipitation of fermented broths. Novel separation/purification techniques like organic phase extraction, membrane separation and chromatographic separation should become integral components of downstream processing system.