The invention is directed to eluting medical devices that enable consistent “on-demand” delivery of therapeutic agents to a vessel. The medical device of the current invention comprises an expandable member, a hydrophilic coating comprising at least one therapeutic agent about the expandable member or structural layer and an outer sheath with a variably permeable microstructure. The design and methods disclosed herein ensures that therapeutic agent delivery occurs essentially only during expansion of the expandable member, minimizing coating and/or therapeutic agent loss to the bloodstream and providing controlled delivery to the treatment site.