microparticles and nanoparticles prepared from oppositely charged polymers are provided in which a drug is incorporated into the core and is conjugated to one polymer by a schiff - base crosslink . the particles are suitable for use in injectable formulations in which the rate of release of the drug through the particle shell is slowed as compared to noncrosslinked drugs . enzymatically degradable polymers can be incorporated in otherwise hydrolytically stable particles to provide drug release at particular sites within the body where the enzyme of interest is present .