Steric stabilization of phospholipid vesicles by block copolymers – Vesicle flocculation and osmotic swelling caused by monovalent and divalent cations

Steric stabilization of small unilamellar phospholipid vesicles (liposomes) following conjugation of (tri-)block copolymer molecules by physical means can be carried out either by allowing the copolymer molecules to participate in vesicle bilayer formation, or by adsorbing the copolymers onto preformed liposome surfaces. These systems and a bare liposome system have been compared. The degree of steric stability was determined relative to vesicle resistance against flocculation and osmotic swelling induced by cations. The pronounced affinity of monovalent (Na+) and divalent (Mn2+) cations for the phospholipid surface was exploited to cause flocculation of the vesicle dispersions. Moreover, the formation of an osmotic gradient by varying the electrolyte concentration in the outer and inner vesicle aqueous phases was used to produce swollen liposomes. Size determination by quasi-elastic light scattering (QUELS), surface characterization by laser Doppler electrophoresis (LDE), turbidity measurement by UV-VIS spectroscopy, NMR measurements and phase contrast video-microscopy were used to characterize and compare the vesicle systems. In both ways of copolymer addition the large hydrophilic polymer chains coat the phospholipid surface, as manifested by the considerable resistance to cation-induced flocculation. The presence of polymer at the vesicle surface offers increased protection, owing to the repelling forces due to polymeric steric barriers acting on vesicle close contact. Only the vesicles formed in the presence of copolymer molecules exhibited inert behaviour to osmotic swelling, confirming the alteration of the bilayer structure and its related properties. The advantages of constructing sterically stabilized vesicles by allowing the hydrophobic block of the copolymer to form an integral part of the bilayer are thus evident, since a robust surface coating is achieved by firmly anchored copolymer molecules rather than surface-adsorbed polymers, while avoiding the costly cumbersome chemical modification process of attaching polymer chains onto lipid headgroups.