A modified method to determine protein encapsulation efficiency in polymer matrices has been developed and applied to two proteins and two polymers to demonstrate its wide range of applicability. This study was pursued due to the wide variation in reported protein encapsulation efficiency of polymer-based microcapsules, even when the protein, the polymer, and the microcapsule manufacturing method were consistent. Hemoglobin (Hb) and bovine serum albumin (BSA) were chosen as model proteins and ethylcellulose and poly(lactic-co-glycolic acid) (PLGA) as model polymers. The polymer of the microcapsule was dissolved in dichloromethane/ethanol or dichloromethane/ethyl acetate for ethylcellulose or PLGA microcapsules, respectively. Liberated proteins were simultaneously precipitated, pelleted by centrifugation, isolated by decanting the polymer solution, redissolved in 10% w/v sodium dodecyl sulfate in 0.8 N sodium hydroxide, and quantified using a modified Lowry assay. Blank microcapsules and exogenously added proteins demonstrated ≥ 93.8% recovery of proteins. The mean encapsulation efficiency of ethylcellulose or PLGA microcapsules was 52.4 or 76.9% for Hb and 86.4 or 74.7% for BSA, respectively. This demonstrates the effective use of centrifugation and the importance of an appropriate cosolvent system in the measure of encapsulation efficiency where one solvent dissolves the polymer while the other solvent quantitatively precipitates the liberated protein. It is evident that an alkaline solution of sodium dodecyl sulfate is efficient at quantitatively dissolving precipitated proteins. Remediation of problems observed with current methods and high reproducibility suggest that this modified method is generally applicable to the measure of protein encapsulation efficiency of polymer microcapsules.
Keywords: encapsulation efficiency; microcapsule; polymer; protein precipitation; spray drying.