A physicochemical study of the morphology of progesterone-loaded microspheres fabricated from...

Progesterone-loaded microspheres are fabricated by a solvent evaporation process from a poly(D,L-lactide-co-glycolide) (85/15 PLG) and from alpha-progesterone. Methylene chloride is used as solvent and polyvinyl alcohol and methylcellulose are used as surfactants. The microspheres are characterized by scanning electron microscopy, differential scanning calorimetry, and x-ray powder diagrams. Our study shows that the morphology and the thermal behavior of PLG microspheres can vary significantly with progesterone loading and sample thermal history. Below and at 16.5% loading the microspheres exhibit a smooth outer surface. Above 23% loading, the surface becomes rough, embedded by copolymer particles or well-defined crystals. Pores and cracks can also be observed. Below 35% the progesterone is molecularly dispersed. At 35% and above crystal domains of the steroid appear and two crystalline forms are found: alpha- and beta-progesterone. The physical state of progesterone and the nature of its crystal domains dispersed in the PLG matrix can change during storage. Also a progressive development of an endothermic peak at the Tg event of the copolymer is observed during storage. No well defined relationship of peak size to progesterone loading can be shown.