In vitro degradation of hydroxyapatite nanoparticle-coated biodegradable microspheres

In vitro degradation behavior of hydroxyapatite (HAp) nanoparticle-coated poly(l-lactide-co-ε-caprolactone) (PLCL) microspheres, which were fabricated via 'Pickering emulsion' route, has been examined in terms of weight, molecular weight, thermal property, and morphological changes through incubation in a phosphate buffered saline up to 48 weeks. Gel permeation chromatography, scanning electron microscopy (SEM) and gravimetrical method were employed to characterize their degradation profiles. It was found that molecular weight of PLCL decreased rapidly after immersing the microspheres in the buffered saline and, on the other hand, the weight of the microspheres started to decrease after 16 weeks. These results support bulk degradation for the HAp-coated PLCL microspheres. During the degradation, the enthalpy of melting increased progressively, which should be due to the increase of crystallinity because of chain rearrangement of amorphous region induced by water uptake and the preferential degradation of amorphous regions. The microspheres retained spherical morphology at least for 48 weeks and the HAp nanoparticles detached from the microsphere surface, which was confirmed by SEM study. The microspheres were easy to be broken by external pressure when the molecular weight became below 35,000 g/mol after 24-week incubation.