An implantable medical device is provided for controlled delivery or exposure of reservoir contents. The device may include a substrate; a plurality of discrete reservoirs in the substrate, wherein the reservoirs have interior walls and at least one opening in the substrate; reservoir contents disposed in the reservoirs; a reservoir cap closing the at least one opening of each of the reservoirs to seal the reservoir contents in each of the reservoirs; and control circuitry for selectively disintegrating the reservoir caps to release or expose the reservoir contents in vivo, wherein the interior walls of the reservoirs comprise a material to protect the substrate in vivo. For example, to enhance the resistance of the substrate to etching in vivo, the interior sidewalls of the reservoirs may include a protective coating (such as gold, platinum, carbon, silicon carbide, silicon dioxide, or platinum silicide), or the sidewalls may include a doped silicon.