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A three-dimensional (3D) stack concept for the assembly of photolithographically fabricated microfluidic components is presented and discussed. The system uses silicon-based micropumps and simple planar structures which mimic standard elements of conventional flow systems. Detection is provided either by solid state electrochemical sensors or small volume optical detection. The general advantages of using micromachined flow manifolds for microchemical analysis are addressed. The particular benefits to be derived from this an approach compared with other assembly methods are also examined.

This paper reviews the results that have been reported on ISFET based enzyme sensors. The most important improvement that results from the application of ISFETs instead of glass membrane electrodes is in the method of fabrication. Problems with regard to the pH dependence of the response and the dynamic range as well as the influence of the sample buffer capacity have not been solved. As a possible solution we introduce a coulometric system that compensates for the analyte buffer capacity. tf the pH in the immobilized enzyme layer is thus controlled, the resulting pH-static enzyme sensor has an output that is independent of the sample pH and buffer capacity and has an expanded linear range.Key words: ISFET, enzyme, urea, K-technology, buffer dependence, coulometry, pH-static enzyme sensor. INlXODUCTIONPotentiometrically operating enzyme sensors suffer from a number of problems, partly due to the properties of immobilized enzymes and partly resulting from the use of the particular ion sensitive electrode. Focussing on enzyme electrodes in which the conversion of a substrate causes a change in the local pH, these problems are: pH-dependence of response, detection limit and dynamic range; response dependency on the analyte buffer capacity; dependence on immobilization technology; and individual membrane casting and control of membrane thickness. 161Biosensors 0265-928X/88/$03-50

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