In order to avoid a thermal instability and a reaction of drug with solvents, a method was devised to load a drug in a polymer matrix by swelling. Using p-HEMA hydrogel as polymer matrix, the release mechanism of a model drug, citidine diphosphate coline(CNS stimulant),was studied. Also analyzed was the difference in the pH-related release mechanism of p-HEMA which has a non-ionic polymer chain and polyacrylate which has negative ions on the polymer side chain. For p-HEMA, ist break-point on the drug release curve occured due to swelling and degree of swelling did not change pH. On the other hand, swelling polyacrylate differed pH:the higher the pH, the higher the degree of swelling and faster the release rate. A formulation with a bioerodible coating membrane of polylactide was in-troduced to the co-polymer of HEMA and methylmethacylate. The difference in the drug release mechanism and changes relative to the polymer thickness in this system was observed. The formulation allows the bioerodible mem-brane to suppress the initial rate of drug release mechanism, which led to the sustained release pattern.