Liposome-hydrogel bead complexes prepared via biotin-avidin conjugation

Liposomes immobilized onto polymeric hydrogel microbeads have potential advantages both in tissue engineering applications and as drug delivery vehicles. Here we demonstrate, quantify, and optimize lipid vesicle binding to polymeric hydrogel microbeads via the avidin-biotin conjugation system and characterize the stability of the resulting microgel-bound liposomes. Microgels consisting of a copolymer of N-isopropylacrylamide (NIPAM) and acrylic acid (AA), cross-linked with bis-acrylamide, that is, p(NIPAM-co-AA), were biotinylated using aqueous carbodiimide chemistry. Extruded liposomes consisting of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) plus a small fraction of a biotin-derivatized phosphatidylethanolamine (B-PE) were saturated with avidin and allowed to bind to biotinylated hydrogel beads. Using a combination of fluorescence spectroscopy, quenching, and microscopy and 31P NMR static and magic angle spinning (MAS) spectroscopies, we demonstrate conditions for near-quantitative liposome binding to p(NIPAM-co-AA) microbeads and show that liposome fusion does not occur under such conditions, that the liposomes remain intact and impermeable when so bound, and that they can function as slow release vehicles for entrapped aqueous species.