ABSTRACT
Liposomes are supramolecular aggregates possessing one or more closed lipid bilayers and thus can vary in a number of attributes, including bilayer thickness, number of lamellae, and overall size and shape (usually spherical) depending upon their composition and how the formulation has been processed [1]. These lipid vesicles are typically composed of phospholipids and sterols in which both hydrophobic fatty acid tails of each phospholipid molecule are oriented toward the center of the bilayer, while the hydrophilic head group, for example, the phosphatidylcholine (PC) moiety, is oriented toward the internal or external aqueous phases [1,2]. The formation and stabilization of these bilayers is driven both by entropic factors related to the sequestration of the hydrophobic fatty acid tails from the aqueous medium as well as enthalpic elements due to hydrogen bonding between the hydrophilic head groups and van der Waals interactions between the aligned acyl groups of the fatty acid chains. Even for a specic liposomal formulation manufactured under controlled conditions, a range of liposome sizes and shapes will be present. The selection of the manufacturing processes, the controls over the various processing steps, and the ingredients that constitute the liposome as well as the solvents and buffers used during manufacturing will determine the extent of this heterogeneity and the reproducibility from batch to batch.
