Microencapsulation of ovalbumin in poly(lactide-co-glycolide) by an oil-in-oil (o/o) solvent evaporation method

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Abstract
The objective of this study was to produce biodegradable poly(lactide-co-glycolide) (PLGA; 50/50) microspheres by an oil-in-oil (o/o) solvent evaporation method to prolong the in vitro release of ovalbumin (OVA) as a model protein. The effects, on loading efficiency, microsphere yield, morphology and drug release, of two dispersing agents, aluminium tristearate and Span 80, in mineral oil were examined. PLGA 50/50 microspheres containing OVA powder (sieved through a 53 microns mesh) were prepared using an o/o solvent evaporation method. When aluminum tristearate was employed as a dispersing agent, the loading efficiency and yield of OVA had maximum values of 89 and 72% at 0.15% (w/v) aluminum tristearate, respectively. Morphology studies suggested that the obtained microspheres were spherical, and had a smooth surface. The diameters of the microspheres ranged between 100 and 200 microns. The loading efficiency, or yield, for microspheres decreased significantly above or below 0.15% (w/v) aluminum tristearate, and microspheres with irregular shapes were observed. The minimum sedimentation volume ratio (F) was obtained at a dispersity of carbon black particles in ethanol containing 0.15% (w/v) aluminum tristearate by a sedimentation study, and the cloudy supernatant suggested a deflocculated suspension. However, on the contrary, when Span 80 was added into the mineral oil as a dispersing agent, the concentration of Span 80 had little or no effect on the characteristics of the prepared microspheres. Drug loadings (60-70%) were obtained within the Span 80 concentrations employed in the present study (0.05-1.0% (w/v)). The yields were also in the same levels. The microspheres prepared in mineral oil containing Span 80 had an average diameter less than 50 microns in all cases. Sustained-release characteristics were demonstrated for PLGA microspheres prepared in mineral oil containing aluminum tristearate as a dispersing agent, even though a burst release at the initial phase was observed. This initial burst release from PLGA microspheres was reduced to some extent by micronization of the OVA powder using a planetary-type ball mill. However, PLGA microspheres prepared in mineral oil containing Span 80 as a dispersing agent, exhibited a large initial burst release. This burst release seems to be due to the smaller size of microspheres and the OVA powder adhering to the surface of PLGA microspheres (confirmed by scanning electron microscope (SEM) study).