The present invention provides an implantable substrate sensor comprising electronic circuitry (70) formed within, or on, a substrate (52). A protective coating (56) then covers the substrate, forming a hermetically sealed package having the circuitry under the coating. The circuitry has electrically conductive pads (72a and 72b) for communicating and/or providing power to the circuitry. Electrical pathways provide hermetic electrical connection to the conductive pads for external connection to the sealed circuitry. In a first embodiment, the pathway is a via that is made from biocompatible material that is made hermetic by either increasing its thickness or by ion beam deposition. Alternatively, the pathways are formed from metal traces (74), surrounded by a biocompatible insulation material, essentially parallel to the surface of the substrate that are connected to the conductive pads by first vias and have second ends externally accessible to the sealed package to provide external electrical connection to the hermetically sealed circuitry within.