Biochronomer (AP Pharma) is a fourth-generation poly(ortho ester) prepared by the condensation of diols and a diketene acetal. The polymer contains a copolymerised latent acid whose concentration controls erosion rate. The polymer has been shown to undergo a surface erosion process and a number of applications have been explored. Among these, the delivery of plasmid DNA for vaccines is currently of most interest. This application takes advantage of the acid-labile nature of the polymer, which leads to rapid polymer hydrolysis and hence rapid release of plasmid DNA once internalised in the acidic environment within the endosomes, and the non-acidic environment within the polymer that conserves plasmid DNA conformation. A low molecular semisolid polymer is now in Phase II clinical trials for the delivery of mepivacaine to control postoperative pain, and in Phase I clinical trials for the systemic delivery of granisetron to control nausea.