Liposomes Entrapped in Biopolymer Hydrogels Can Spontaneously Release into the External Solution.

Hydrogels of biopolymers such as agar and gelatin are widely used in many applications, and in many cases, the gels are loaded with nanoparticles. The polymer chains in these gels are cross-linked by physical bonds into three-dimensional networks, with the mesh size of these networks typically being 10-100 nm. One class of "soft" nanoparticles are liposomes, which have an aqueous core surrounded by a lipid bilayer. Solutes encapsulated in the liposomal core can be delivered externally over time. In this paper, we create liposomes with diameters ∼150 nm from an unsaturated phospholipid (lecithin) and embed them in agar gels (the aqueous phase also contains 0-50% of glycerol, which is an active ingredient in cosmetic products). Upon placing this gel in quiescent water, we find that the liposomes release out of the gel into the water over a period of 1-3 days, even though the gel remains intact. This is a surprising result that runs contrary to our expectation that the liposomes would simply remain immobilized in the gel. We show that the release rate of liposomes can be tuned by several variables: for example, the release rate increases as the agar concentration is lowered and the rate increases steadily with temperature. In addition to agar, release of liposomes also occurs out of other physical gels including those of agarose and gelatin. However, liposomes made from a saturated phospholipid do not release out of any gels. We discuss a possible mechanism for liposomal release, which involves intact liposomes deforming and squeezing through transient large pores that arise in physical networks such as agar. Our findings have relevance to transdermal delivery: they suggest the possibility of systematically delivering liposomes loaded with actives out of an intact matrix.

Nanoglycan complex formulation extends VEGF retention time in the lung.

To extend the retention time of aerosol-delivered growth factors in the lung for stem cell homing/activation purposes, we examined a formulation of vascular endothelial growth factor (VEGF) complexed to dextran sulfate (DS) and chitosan (CS) polyelectrolytes. Optimal incorporation of VEGF was found at a VEGF/DS/CS ratio of 0.12:1:0.33, which resulted in nanoparticle complexes with diameters of 612+/-79 nm and zeta potentials of -31+/-1 mV. The complexes collapsed in physiological solution, and released VEGF in a biphasic time course in vitro. In rat lungs, however, VEGF delivered in the complex was cleared at a constant exponential decay rate, 8-fold slower than that delivered in free form. The extended VEGF retention was likely due to equilibrium binding of VEGF to DS and to endogenous glycosaminoglycans. A similar retention effect is expected with other glycosaminoglycans-binding proteins (including many growth factors) when complexed with these glycans. Owing to its unique application, this type of complex is, perhaps, better described as a nanoglycan complex.