The use of biosensors in control of fed-batch processes - Opportunities and limitations

The introduction of biosensors in an industrial environment is only possible if they present a well adapted solution to an important analytical problem. In the past few years, this has been demonstrated with enzyme sensors for glucose, lactate and glutamine measurement. These biosensors have been connected via flow-injection analysis (FIA) devices and applied to batch cultures of hybridoma cells and also perfusion systems using recombinant BHK cells for the production of IL-2. It has been shown that these biosensors allow direct feedback control of the concentrations of the main nutrients. Furthermore, the on-line determination of the biotechnological product (e.g. monoclonal antibodies, interleukin-2) is a prerequisite for the development of reliable process control strategies. Novel amperometric immunosensors and automated immunocartridge systems allow the analysis of protein products in minutes (compared to hours with ELISA techniques). It has been clearly demonstrated that glucose, lactate, glutamate and high molecular weight product determinations with associated process control is possible. Renneberg et al., (1991), for example, showed that on-line control was possible for the process control of perfused bioreactor systems containing mammalian cell lines producing interleukin - 2. Glucose, lactate and glutamate were determined using enzyme sensors. Interluekin - 2 was detected using immunosensors. Figure 1 illustrates the experimental set-up of the flow injection analysis (F.I.A.) biosensor-bioreactor coupling and compares on-line and off-line data. Figure 2 illustrates the principle of the immunosensor and the experimental results obtained using it (Warsinke, 1992; Renneberg et al., 1992). Such systems, however, have not been utilised extensively by the biotechnological industry. A possible reason for this is that early expectations that biosensors could be directly inserted( like pH and DO probes) into the bioreactor have clearly underestimated the problems of fouling and calibration of the biosensor and activity loss of the biocomponent. We feel, therefore, that in-situ biosensors are not yet a practical industrial solution but rather by-pass systems integrating sampling, sample transport, sample pretreatment, separation and data handling are more appropriate. These biosensor systems, used as "measuring fingers", are the appropriate strategy for control of fed-batch processes and the optimisation of productivity.