Although fluid lipid films have been used widely in biosensing devices, they lack the high stability desired for technological implementation because the noncovalent forces between the constituent lipids are relatively weak. In this work, polymerized, planar supported lipid bilayers ((poly)PSLBs) composed of diene-functionalized lipids have been prepared and characterized. Several parameters relating (poly)-PSLB structure and stability to observations made in studies of polymerized bilayer vesicles were examined, including a comparison of UV photopolymerization and redox-initiated radical polymerization, the number and location of the polymerizable moieties in the lipid monomer, and a comparison to PSLBs produced with diacetylene lipids. Redox-initiated polymerization of films composed of bis-substituted diene lipids with at least one polymerizable moiety located near the acyl terminus produced dried PSLBs that were highly uniform and stable. All other conditions yielded PSLBs that contained a high density of defects after drying, including those formed from diacetylene lipids. In most cases, defect formation is attributed to desorption of unreacted monomers or low molecular weight polymers when the film was passed through the air/water interface. Studies on highly stable (poly)PSLBs doped with nonpolymerizable lipids showed that 40-80% of the dopants are retained when the film is dried. Thus to ensure quantitative lipid retention upon PSLB removal from water, all of the lipid monomers must be covalently anchored to the polymer network.

Planar supported lipid bilayers (PSLBs) composed of phosphorylcholine (PC) lipids are known to be highly resistant to nonspecific adsorption of soluble proteins. However, these structures lack the stability desired for implementation in molecular devices (e.g., biosensors).

Substrate-supported lipid bilayers have been prepared from bis-diene functionalized phosphorylcholine (PC) lipids and polymerized by UV irradiation. The overall bilayer structure is largely preserved upon removal from water, although significant loss of material occurs from the upper leaflet of the bilayer, likely due to desorption at the air/water interface. The morphology and surface structure of the bilayer, as observed by AFM, indicate a substantially different arrangement of the lipids in the hydrated and dehydrated states, presumably due to the loss of water from the near surface region. These changes have been correlated with infrared spectral shifts sensitive to the conformation of the hydrocarbon chains. Protein adsorption studies show that rehydrated, polymerized bilayers retain a degree of resistance to BSA adsorption intermediate between model hydrophobic and fluid PC lipid bilayer surfaces. The degree of protein adsorption is correlated with desorption of material from the upper leaflet of the bilayer upon drying, which produces voids at which hydrophobically driven protein adsorption occurs.

The cross-linking polymerization of hydrated amphiphiles in monolayers, bilayers, and nonlamellar phases, i.e., bicontinuous cubic and the inverted hexagonal phases, is an effective method to modify their properties. Polymerization of monomeric amphiphiles in an assembly proceeds in a linear or cross-linking manner depending on the number and location of polymerizable groups per monomer. Polymerization of hydrated lipids with reactive groups in each hydrophobic tail yields cross-linked polymers. Sisson et al. (1996) examined the cross-linking of bilayers as a function of the mole fraction of bis-substituted lipids (bis-SorbPC), where the reactive groups were located at the end of the lipid tails. The onset of cross-linking was determined by changes in lipid lateral diffusion, bilayer vesicle stability, and polymer solubility. These data indicated that a substantial mole fraction (0.30 ± 0.05) of the bis-substituted lipid was necessary for bilayer cross-linking. Analysis of the cross-linking and competing reactions suggested that the location of the reactive group, i.e., reaction site, in the amphiphile and therefore within the bilayer assembly influences the cross-linking efficiency. To assess this possibility, the cross-linking of dienoyl-substituted phospholipids, (E,E)-DenPC, where the reactive diene is located near the glycerol backbone of the lipid, was compared with SorbPC. The cross-linking of (E,E)-DenPC was found to be substantially more efficient than that of SorbPC. It is proposed that this effect is partly a consequence of the relative probability of macrocyclization and cross-linking reactions.

The cross-linking of supramolecular assemblies of hydrated lipids is an effective method to stabilize these assemblies to disruption by surfactants or aqueous alcohol. The heterobifunctional lipids, Acryl/DenPC(16,18) and Sorb/DenPC(18,21), are examples of a new class of polymerizable lipid designed for the creation of cross-linked lipid structures. The robust nature of cross-linked liposomes was demonstrated by lyophilization of the liposomes followed by their essentially complete redispersion in water. The resulting liposomes were compared to the original sample by quasi-elastic light scattering and transmission electron microscopy. There was no major change in the size or structure of the cross-linked liposomes after rehydration of the freeze-dried powder of liposomes. Moreover, the rehydrated cross-linked liposomes continued to be resistant to surfactant solubilization. Neutral cross-linked liposomes were predominantly redispersed after freeze-drying with the aid of bath sonication. The small amount of residual liposome aggregation observed with neutral liposomes could be prevented by incorporating a surface charge into the liposome or attaching hydrophilic polymers, for example, poly(ethylene glycol), onto the liposome.