Medical devices having a plurality of nanoparticles disposed over a surface of the medical device. The nanoparticles have a core comprising a therapeutic agent and a shell surrounding the core, wherein the shell comprises a metal. A barrier layer is disposed over the nanoparticles. The barrier layer is water-permeable and comprises a metal that may be different from the metal used in the nanoparticle shells. In certain embodiments, the metal in the barrier layer undergoes galvanic corrosion. Also disclosed are medical device having a reservoir containing a therapeutic agent, with nanoparticles and a barrier layer being disposed over the reservoir; and medical devices having multiple barrier layers and multiple reservoirs containing therapeutic agents.