(418ae) Structure and Stability of Layersomes Formed with Dextran Sulfate and Poly-L-Arginine

The self-assembly of polyelectrolyte layers on lipid bilayer surfaces is of great interest in the areas of biological device fabrication, drug delivery, and biomaterial design.The structure, function, and stability of these systems ultimately depend upon the interactions between the polyelectrolytes and lipid bilayers, and how these interactions change as a function of time, temperature, and solvent composition. For example, polyelectrolyte layering on spherical lipid bilayer vesicles or liposomes (i.e. layersomes) can yield stable nanocapsules for drug delivery where the rate of release is dependent upon the permeability of the polyelectrolyte–lipid bilayer shell. The objective of this work was to determine how polyelectrolyte–lipid interactions lead to changes in shell structure and how these changes influence layersome morphology and stability.

Layersomes were prepared in deionized water (DIW) with cationic liposomes (130 nm hydrodynamic diameter) composed of dioleoylphosphatidylcholine (DOPC, zwitterioninc) and dioleoyltrimethylammonium-propane (DOTAP, cationic) at a 1:1 mole ratio. Negatively charged dextran sulfate (6500-10000 g/mol) and positively charged poly-L-arginine (5000-15000 g/mol) were used as FDA approved polyelectrolyte layers. Single (dextran sulfate) and multiple (up to 4) alternating layersomes were prepared using a ‘washless’ method. Layersome formation was confirmed by zeta potential, dynamic light scattering, and cryogenic transmission electron microscopy (Cryo-TEM) analysis. The layersomes ranged from 170 to 200 nm with zeta potentials alternating between +/-50 mV. There was evidence of polyelectrolyte complexes bound to the layersomes, but these complexes did not affect layersome stability. Long-term studies (30 days) showed that the layersomes were stable in DIW due to electrosteric repulsion. In phosphate buffered saline (PBS), layersomes with outer dextran sulfate layers were stable, but those with outer poly-L-arginine layers were not. Differences in layersome stability can be attributed to differences in the pKa and charge density of the polyelectrolytes, as well as restructuring of the polyelectrolyte shell. Fluidity measurements based on diphenylhexatriene (DPH) anisotropy were used to determine how polyelectrolyte layering affected lipid ordering within the bilayer. Dextran sulfate led to significant lipid ordering (layers 1 and 3), while the addition of poly-L-arginine (layers 2 and 4) partially neutralized dextran sulfate and reduced ordering. Collectively, this study shows that stable layersomes can be prepared using the ‘washless’ method and that layersome stability is most influenced by the outermost layer.