An implantable medical device is provided for use with an external detector to detect an analyte in vivo. In one embodiment, the device consisting essentially of a substrate; a plurality of discrete reservoirs located in the substrate, each reservoir having at least one opening; a reacting component contained in each reservoir; and at least one non-degradable barrier layer covering each reservoir opening, the barrier layer being permeable to an analyte to be detected, wherein the reacting component remains inside the reservoirs and can react with the analyte to be detected, and wherein the device is adapted for implantation into the body of a patient.