(a) Schematic illustration of in situ vaccination with an autonomous dissolvable active MN patch for the treatment of B16F10 melanoma by the release and delivery of plant virus nanoparticles (cowpea mosaic virus, CPMV). (b) Microneedle tip composition and corresponding scanning electron micrograph (SEM). Scale bar, 200 μm. (c) Schematics of fabrication steps for the dissolvable active MN array: infiltration of Magnesium (Mg) microparticles onto the negative MN features of PDMS mold, polymer and CPMV loading, drying, and demolding. (d) Digital photograph of a dissolvable active MN patch comprised of 225 MN tips, corresponding SEM image, and Energy Dispersive X-Ray (EDX) elemental analysis of Mg within the active MN tips. Scale bars, 5 mm, and 400 μm respectively. (e) Fluorescent microscopy time-frame images of the dissolution of an active MN tip, displaying the rapid polymer matrix dissolution and Mg microparticle hydrogen reaction (0, 30, 60 and 120 s intervals). Scale bars, 400 μm. (f) Release kinetics of the delivery of Cy5-conjugated CPMV (Cy3-CPMV) from active and passive MNs. (g) Mechanical strength analysis of a dissolvable active MN array.