One aspect of the present invention relates to a nucleic acid delivery system including a polymeric structure formed of a biocompatible polymer and a mixture comprising one or more nucleic acid molecules and a first co-dispersant, the mixture being contained within the polymeric structure, wherein the first co-dispersant is present in an amount effective to control diffusion of the one or more nucleic acid from the polymeric structure. Compositions including the nucleic acid delivery system and a pharmaceutically-acceptable carrier are disclosed. Methods of making the nucleic acid delivery system and their use in delivering nucleic acid into a patient and modifying gene expression in a target cell are also disclosed.