Photopolymerization of biomembrane templates: Nanometer-scale hydrogels and the photoinduced release of vesicle contents

Long nanotubes of fluid-bilayers were used to create templates for photochemical polymerization into solid-phase conduits and networks. Micromechanical methods were developed which allowed each nanotube to be pulled from a micropipette-held feeder vesicle by mechanical retraction of the vesicle after molecular bonding to a rigid substrate. The caliber of the tube was controlled precisely in a range from 20 to 200 nanometers by setting the suction pressure in the micropipette. Branched conduits were formed by coalescing separate nanotubes drawn serially from the feeder vesicle surface. Single nanotubes and nanotube junctions could be linked together between bonding sites on a surface to create a functionalized network. After assembly, the templates were stabilized by photoinitiated radical cross-linking of hydrophilic monomer contained in the aqueous solution confined by the lipid bilayer boundary. Nanometer-sized vesicles that were prepared by extrusion were also used as templates for photopolymerization. Results from dynamic light scattering and electron microscopy experiments suggest that UV initiated, free-radical polymerization of vesicle-encapsulated monomer resulted in the formation of cross-linked polymer networks that were surrounded by a bilayer lipid membrane. Using fluorescence spectroscopy to monitor the release of an initially entrapped marker, it was determined that lumenal polymerization neither disrupted the semi-permeable membrane, nor did it effect the osmotic release of encapsulated solutes. The addition of detergent to a suspension of polymerized vesicles completely dissolved the bilayer membrane, leaving behind a rigid gel replica of the vesicular template. Two polymerizable amphiphiles with reactive headgroups were prepared and incorporated into the phospholipid bilayer, so as to provide a means to copolymerize the bilayer sheath with the liposome-encapsulated monomer. By monitoring the release of entrapped solutes, it was discovered that photopolymerization of vesicles constructed with either one of the amphiphilic monomers in combination with encapsulated monomer resulted in membrane destabilization, and the complete release of the entrapped solutes. This represented a new approach to the photoinduced release of vesicle contents. In fact, vesicles that contained the polymerizable lipid but not the entrapped monomer also exhibited contents release upon polymerization. During polymerization, the propagating membrane-bound polymer destabilized the membrane. Two simple models were put forth to explain the photoinduced release from polymerized vesicles. One takes into consideration the mechanical stresses that develop in the membrane asymmetric polymerization; the second suggests that release of contents was caused by formation of a membrane-bound, polymeric surfactant which forms “pores” in the membrane