Coated microneedles have been the most extensively studied technique for ID microneedle vaccination (Figs. 3a, b) . Using this approach, vaccine forms a solidstate coating on the surface of solid microneedles that dissolves off within the skin upon application. Typically, this method provides a bolus delivery of a sub-milligram dose of antigen within minutes of application, which is often suitable for delivery of vaccines. An effective microneedle coating process typically involves dip-coating metal microneedles in a coating solution containing the vaccine, a surfactant to promote wetting of the microneedle surface, and a viscosity enhancer to increase coating thickness (Gill and Prausnitz 2007b; Gill and Prausnitz 2007a) . Using this technique, compounds over a large range of sizes including small molecules, proteins, DNA, and virus particles have been coated onto microneedles. Novel coated microneedle designs for improved delivery have been demonstrated, such as the three-dimensional grooves-embedded microneedle (Han et al. 2009 ) and the pocketed microneedle (Gill and Prausnitz 2008) . The first ID vaccination using coated microneedles delivered ovalbumin as a model protein antigen to (Courtesy of University of Queensland). b Array of solid stainless steel microneedles coated with yellow dye. Each 12 mm by 12 mm device contains 50 microneedles measuring 700 lm tall. Inset shows magnified view of two coated microneedles (Courtesy of Georgia Institute of Technology). c Dissolving microneedles shown intact before insertion into skin, partially dissolved 1 min after insertion into skin and fully dissolved 5 min after insertion into skin (Reproduced from (Sullivan et al. 2010 ); Courtesy of Georgia Institute of Technology) hairless guinea pigs (Matriano et al. 2002; Widera et al. 2006) . In these studies, ID microneedle vaccination showed a better immune response than an equivalent SC or IM injection at low dose. The investigators also found that immune response by microneedle vaccination was dose-dependent.