Apparatus and methods for transferring a principal substance to a substrate involve the use of a gating agent that blocks the substance from or attracts the substance to the substrate. The apparatus and methods may utilize ink jet technology to apply the gating agent directly to the substrate or to an intermediate surface. The substance may be an ink, an electrically conductive material, a magnetic material, a carrier for a therapeutic, diagnostic, or marking substance other than an ink, or a carrier for any other type of substance.