A method for controlled delivery of a substance into a body includes administering a plurality of containment vessels into the body, in which each of the plurality of containment vessels includes a quantity of the substance loaded therein prior to the administering; and providing a time-varying magnetic field such that the plurality of containment vessels are exposed thereto to cause a release of at least a portion of the substance from the plurality of containment vessels. Each of the plurality of containment vessels has an average outer diameter less than about 1 μm.