Degradation and release profile of microcapsules made of poly[L-lactic acid-co-L-lysine(Z)]

Poly(L-lactic acid-co-L-lysine(Z)) with different Lys(Z) contents was synthesized by Sn(II) salt-catalyzed ring-opening copolymerization of 3(S)-benzyloxycarbonylaminobutyl-6(S)-methylmorpholine-2,5-dione with lactide. Microcapsules of the copolymers were prepared by solvent evaporation from w/o/w emulsion, and FITC-dextran release from the microcapsules was investigated. The FITC-dextran release was dependent on the composition and molecular weight of the copolymers. The release from the microcapsules containing Lys(Z) of 6.5 mol% was slowest among the present microcapsules, which is due to smooth surface and very small microcapsules included in a large microcapsule. On the other hand, the release from microcapsules containing Lys(Z) of 31 or 50 mol% became faster after several days of incubation. GPC measurement of the microcapsules revealed that the copolymers were degraded during the incubation. Cracks and pores were formed on the microcapsule wall. PLLA microcapsules having comparable molecular weight to the copolymers showed neither release acceleration nor degradation in short-time incubation. Therefore, the introduction of Lys(Z) units made PLLA susceptible to degradation to result in delayed acceleration of release.