Effect of Freeze-Thawing Process on the Size and Lamellarity of PEG-Lipid Liposomes

The influence of the freeze-thawing process on the size and lamellarity of multilamellar PEG-lipid liposomes prepared from a mixture of egg yolk phosphatidilcholine (EggPC) and distearoyl phosphatidylethanolamine polyethyleneglycol (DSPE-PEG) 2000 was investigated. Trapped volume measurement, quasielastic light scattering (QELS) and freeze-fracture electron microscopy were used to estimate the morphology and lamellarity of liposomes. During the freeze-thawing process, the lamellarity of multilamellar vesicles (MLVs) depended strongly on both PEG-lipid concentration and the number of freeze-thaw cycles. The decrease in the number of lamellae was a function of the number of freeze-thaw cycles. The increase in trapped volume coincided with the decrease in the number of lamellae observed in electron micrographs. After comparing the results obtained from EggPC/DSPE-PEG2000 MLV and from pure EggPC MLV, it was concluded that during the freeze-thawing the liposomes with DSPE-PEG2000 achieved a unilamellar structure more readily than the pure EggPC liposomes.