Preparation of polymeric microcapsules: Formulation studies

Air-filled microcapsules were prepared by freeze-drying different oil-in-wa ter emulsions containing biodegradable polyester as the wall-forming materi al. The aim of this work was to find an acceptable formulation with respect to the microcapsule suspension and the stability of the emulsion during th e production process. The influence of various formulation parameters (conc entrations of mannitol, polymer, and surfactant; pH; oil-in-water phase rat io) was investigated in a factorial design. The results were treated by ord inary least-square (OLS) regression and partial least-square regression (PL SR). In a previous work, air-filled microcapsules were successfully made us ing human serum albumin as the surfactant in the emulsion (1). In the prese nt work, a new block copolymer based on poly(ethylene glycol) (PEG) was imp lemented as the surfactant to replace human serum albumin. It was found tha t the new block copolymer is a suitable replacement for human serum albumin . The concentration of the polymer in water and the concentration of the su rfactant ain the oil phase and the interaction between these variables had a significant influence on the stability of the emulsion at 60 degrees C. A surfactant concentration of approximately 2% (w/w) in water was necessary when the concentration of the wall-forming polymer was below 5% (w/v) in (- )-camphene. The concentration of the polymer in the oil phase influenced th e yield, measured as the volume concentration of particles in suspension pe r milligram of polymer added and as acoustic effect per milligram of polyme r. Low levels of polymer concentration in (-)-camphene (<5% w/v) gave the h ighest yield. Excess polymer in the oil phase did nor form microcapsules, b ut precipitated in the suspension or was included in the wall of the microc apsules. Addition of mannitol protected the microcapsules from being destro yed during freeze-drying and resulted in freeze-dried products with few cra cks, little shrinkage, and higher suspension yield.