Steric stabilization of liposomes — a review

The performance of two principally different kinds of intended stabilizers is reviewed. Especially any influence of the stabilizers on the liposome properties, such as structure, permeability and surface potential, is discussed. Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers have not been shown to function satisfactorily as stabilizers in combination with phospholipids. The incorporation of triblock copolymers of different segment length leads to structural breakdown of the liposome structure even at low concentrations. In addition, incorporation of the copolymers results in extensive leakage of encapsulated material. Neither have there been any reports of unambiguous proof of efficient steric repulsion due to the coating by copolymers. In contrast poly(ethylene glycol) lipids [PEG(2000)-phosphatidyl ethanol-amine] efficiently provide a steric barrier to the liposomes. This is, however, only true if the surface concentration is kept below a rupture limit. An important additional effect is that the permeability of encapsulated hydrophilic cargo is reduced in the presence of PEG lipids. The most common PEG lipids contain a carbamate linkage that introduces a negative surface potential at the liposome surface. However, at medium ionic strength similar to physiological conditions the surface potential is small and does not contribute to any important extent to colloidal stabilization.