This paper describes the development of a technique for protecting non-hermetically packaged integrated circuits for use inside the human body. A two layer combination consisting of a thin adhesion-promoting primer of plasma polymerized methane covered with a thicker layer of Parylene®-C has been found to provide excellent protection during long term exposure to simulated body fluids under continuous DC bias. The process for synthesizing the two layer combination in continuous vacuum is described. Results from adhesion and electrical testing of passive test structures are given. A new technique for the formation of small openings in thick polymer coatings for neural stimulation and sensing sites of implantable sensors is described. The technique uses biased thin film resistors formed on the passivation layer to provide localized heated areas which prevent polymer deposition during synthesis.