The invention is directed to delivery medical devices that enable consistent “on-demand” delivery of therapeutic agents to a vessel. The medical device of the current invention comprises retractable sheath comprising neckable elements. 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 a retractable outer sheath with a selectively permeable microstructure. The design and methods disclosed herein ensures that therapeutic agent delivery occurs essentially only during retraction of the outer sheath, minimizing coating and/or therapeutic agent loss to the bloodstream and providing controlled delivery to the treatment site.