This paper describes a strategy to overcome the problems associated with the use of MEMFETs as selective chemical sensors. The problems arising from the thermodynamically ill-defined insulator-membrane interface have been eliminated by a new sensor design, viz., the introduction of an intermediate hydrogel layer. Experiments to assess the beneficial effect of such a hydrogel are described. Furthermore, new membrane materials have been designed, synthesized, and characterized. Hence, photopolymerizable polysiloxanes are used which allow both the covalent attachment of electroactive components and the covalent bonding of the sensing membrane to the hydrogel. It is shown that polysiloxanes are versatile materials for the sensing membrane of an ISFET-based chemical sensor. The described technology results in a robust and stable sensor.