Microparticles are prepared by a method that includes: (a) forming a layer comprising a first polymer on a solid surface by depositing a first composition one or more times on the solid surface, wherein the first composition comprises the first polymer and a first solvent, and evaporating the first solvent in the first composition； (b) forming one or more layers comprising a second polymer and a therapeutic agent by depositing a second composition on all or part of the layer formed in step (a), wherein the second composition comprises the second polymer, the therapeutic agent, and a second solvent； and evaporating the second solvent in the second composition； and (c) forming an additional layer comprising a third polymer by depositing a third composition one or more times on a previously formed layer, wherein the third composition comprises the third polymer and a third solvent； and evaporating the third solvent in the third composition.
