Studies of protein release mechanisms from polymer blends.
Liu, Kerh Lin.
Date of Issue2012
School of Materials Science and Engineering
The main objective of this work is to understand protein release mechanisms from polymer blends through microenvironment evaluation. Such evaluation would provide deeper insights on the importance of microenvironment properties especially polymer miscibility, protein partitioning and protein distribution, in affecting protein release mechanisms. Quantification of protein partitioning in a phase-separated polymer blend gives important insights into the protein release mechanism. Here, the author reports on the first visualization of protein-PEG colocalization in PCL/PEG blends using a combined application of confocal Raman mapping and confocal Laser Scanning Microscopy (CLSM) imaging. The degree of protein-PEG colocalization was further quantified via a novel image processing technique. This technique also allowed the characterization of the 3-D protein distribution within the films. The results showed homogeneous protein distribution within the film matrix, independent of PEG content and film thickness. However, the degree of protein-PEG colocalization was inversely proportional to PEG content, ranging from 65 to 94%. This quantitative data on protein-PEG colocalization was used along with in vitro PEG leaching profile to construct a predictive model for overall protein release. The predictive model matched well with the experimental protein release profile, which is characterized by an initial burst release and a subsequent slower diffusional release. More importantly, the success of this predictive model has highlighted the influence of protein-PEG colocalization and PEG leaching on protein release mechanism. This study suggests that a formulation strategy of enhancing polymer-polymer interaction would be desired for the successful protein sustained formulation using polymer blend. This has led to the use of PCL-PEG diblock copolymer in optimizing such interaction. The microenvironment evaluation for PCL-PEG copolymer blends validated its correlation with protein mechanisms. Protein release profiles of PCL/copolymer blends can be characterized by an initial burst release followed by a diffusional release. Interestingly, the magnitude of protein burst release is highly influenced by PCL-PEG block fractions. In spite of a homogeneous protein distribution found in both copolymer types, the massive burst release from copolymer blends of lower PCL-block fraction indicated a non-surface-segregation-dependent burst release. To sort out the cause of burst release, polymer miscibility of the copolymer blends was investigated. The results showed that the degree of polymer miscibility was proportional with the increasing PCL-block fraction, as an outcome of PCL-block partitioning into the homo PCL matrix. The better miscibility between PCL and copolymer of higher PCL-block fraction provides a non-leaching behaviour, which resulted in a low-burst-high-subsequent protein release profiles. The importance of microenvironment properties in governing protein release mechanisms has once again been highlighted.