2.2. Preparation of PLA and PLLA Microspheres

Microspheres of PLA and PLLA were prepared using the conventional emulsion-solvent evaporation technique (O/W). At first, a 1% aqueous solution of PVA was prepared by stirring with a magnetic stirrer (Heidolph Instruments GmbH & Co. KG, Schwabach, Germany) and heating up to 70 °C. Then both PLA and PLLA were dissolved in methylene chloride (CH₂Cl₂) to obtain the 4 wt % transparent solution. The volume ratio of organic phase to aqueous phase was 1:3. Next, the organic solution was dropped into PVA solution by using a peristaltic pump (Zalimp, Warsaw, Poland) while the PVA solution was stirred with magnetic stirring at 800 rpm at room temperature. After emulsification, the methylene chloride was removed by evaporation at room temperature under stirring over 48 h. PLA and PLLA microspheres were separated by centrifuging and drying in a vacuum at 40 °C for 24 h. After preparation, the microspheres were additionally annealed at 40, 70, 90, and 120 °C for 5 h. The series of microspheres were abbreviated PLA_T and PLLA_T, where T stands for the conditioning temperature.