Long-life, integrated multiple-ISFETs with CMOS interface circuits have been developed. The interface circuits are designed for impedance conversion and rapid multiplexing of the signal from each ISFET. Three kinds of ion-selective membranes for sodium, potassium and chloride are formed on the gates and separated by a patterned thick polyimide layer. The poor adhesion of the polymeric membrane to the ISFET gate is improved by selecting the proper plasticizer for use in the membrane. As a result, a chemical response for sodium, potassium and chloride can be obtained for more than two months.
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