Ph.D Thesis

Ph.D StudentGefen-Azran Adi
SubjectBiocompatible Polymersomes for Controlled Drug Release
DepartmentDepartment of Chemical Engineering
Supervisor PROF. Havazelet Bianco-Peled
Full Thesis textFull thesis text - English Version


Polymersomes have emerged in the past decade as promising delivery vehicles for drug delivery due to their unique structure and versatility. Biocompatible and biodegradable polymersomes are the most promising candidates for biomedical applications, however so far only a few examples of biocompatible block copolymers have been researched as building blocks for their construction. In order to allow a rational design of polymersomes, insights into the formation and properties of biocompatible polymersome systems are needed.  Therefore, the overall goal of this research was to establish a better understanding of biocompatible block copolymer systems with an emphasis on systems that lead to formation of polymersomes. Film rehydration, a common technique for polymersome preparation, has been applied. Four biocompatible copolymers with poly(ethylene oxide) (PEO) as the hydrophilic block and a hydrolysable ester, either poly(lactic acid) (PLA) or poly(caprolactone) (PCL) as the hydrophobic block, have been evaluated. From these four, two PEO-PCL block copolymers designated as OCL2 and OCL3 were found to form polymersomes.  The nanostructure of the polymersomes was investigated  using DLS, cryo-TEM and SAXS. Further, the influence of preparation temperature and different stages of preparation procedure on the nanostructure of the formed aggregates was evaluated. Heating is introduced in all stages of 'Film Rehydration' technique rehydration, sonication, freezing/thawing and extrusion. Our hypothesis was that temperature with respect to the thermal transitions of the polymers, might have a large effect on the aggregation process. For OCL2, near the melting temperature of the comprising blocks, at 60 ˚C, sonication was identified as critical in size determination, as both the average size and the size distribution are set after the sonication step. In the course of preparation at either 30 ˚C or 80 ˚C the aggregate size gradually decreases yet a single population of aggregates is not obtained. Small angle x-ray scattering (SAXS) and cryogenic temperature electron microscopy (cryo-TEM ) illustrate that although OCL2 is expected to form polymersomes according to the thermodynamic theory,  polymersomes were not formed as preferred morphology. OCL3 displays behavior similar to that reported in literature: sonication and extrusion are required to obtain a narrow distribution, with decreasing distribution following extrusion, close to the melting temperature of the blocks. A Hollow Layered sphere model was best fitted to the SAXS data, in agreement with the cryo-TEM that revealed polymersomes as the predominant morphology.  The encapsulation efficiency of both copolymers was determined. Encapsulation was possible, supporting the suggestion that polymersomes were formed. The role of the extrusion process as a size determining stage in the preparation was evaluated by using a different filtration process with a lower shear forces,  syringe filtration. It was found that despite the similarity in average size obtained from DLS, the aggregates formed using the two methods are not equivalent. While the syringe filtration leads to size decrease to the nanometric regime, it is not suffice for forming the membrane structure of polymersomes.