Introduction
Biomedical engineers exploited as the first the possibilities of the chip technology to develop silicon based sensors, to be incorporated in the tip of a catheter (since about 1970). This technology should provide the clinicians with cheap sensors in analogy with the electronic chips, which would become continuously cheaper and cheaper, even with improved characteristics. Moreover the reproducibility of sensor characteristics should be highly improved compared to the usually piecewiseassembled sensors existing up to that time, due to the replication procedure on which the silicon technology relies. Therefore many of the first papers on silicon sensors appeared in the biomedical engineering literature, for instance with respect to the development of ion sensors. The ISFET sensor is one of the most well-known examples.
In order to be able to judge the value of ISFETs and to understand the operational mechanism, it makes sense to first elucidate the basics of potentiometric sensors in general, of which the pH glass-membrane electrode is the most well-known example.