Analysis of microneedles used for delivery in the scientific literature. (i) Classification based on a) type of microneedle used for delivery, b) type of material or energy delivered, and c) testing environment. (ii) The different types of molecules delivered are identified for each type of microneedle: a) dissolvable, b) coated, c) hollow, and d) solid. (iii) The different types of microneedles are identified for each type of molecule delivered: a) vaccine, b) biomolecule, c) small, d) other and e) energy.