Solid supported lipid bilayers: From biophysical studies to sensor design

Abstract

The lipid bilayer is one of the most eloquent and important self-assembled structures in nature. It not only provides a protective container for cells and sub-cellular compartments, but also hosts much of the machinery for cellular communication and transport across the cell membrane. Solid supported lipid bilayers provide an excellent model system for studying the surface chemistry of the cell. Moreover, they are accessible to a wide variety of surface-specific analytical techniques. This makes it possible to investigate processes such as cell signaling, ligand-receptor interactions, enzymatic reactions occurring at the cell surface, as well as pathogen attack. In this review, the following membrane systems are discussed: black lipid membranes, solid supported lipid bilayers, hybrid lipid bilayers, and polymer cushioned lipid bilayers. Examples of how supported lipid membrane technology is interfaced with array based systems by photolithographic patterning, spatial addressing, microcontact printing, and microfluidic patterning are explored. Also, the use of supported lipid bilayers in microfluidic devices for the development of lab-on-a-chip based platforms is examined. Finally, the utility of lipid bilayers in nanotechnology and future directions in this area are discussed.

Fig. 1

Illustration of a black lipid membrane. The phospholipid membrane spans a 100 μm–1 mm pin hole in a hydrophobic support.

Fig. 2

The formation of a folded lipid bilayer. The solution on the side containing a lipid monolayer is slowly lowered and then raised. This deposits a monolayer with each pass producing the black lipid film.

Fig. 3

Stochastic sensing with black lipid membranes. A pore protein such as $\alpha$-Hemolysin can be used to sense single molecule binding within the protein’s ion channel. The binding process is observed by a decrease in the current flowing through the pore in the presence of the analyte.

Fig. 4

Schematic diagram of a solid supported phospholipid bilayer. The membrane is separated from the substrate by a 10–20 Å thick layer of water.

Fig. 5

Common techniques for the formation of supported lipid bilayers. (a) The Langmuir–Blodgett technique is carried out by pulling a hydrophilic substrate through a lipid monolayer and sequentially pushing it horizontally through another lipid monolayer. (b) Vesicles in solution adsorb and spontaneously fuse to the surface to form a solid supported lipid bilayer. (c) A combination of the Langmuir–Blodgett and vesicle fusion processes.

Fig. 6

Proposed method of vesicle fusion. Adsorbed vesicles deform and either rupture or fuse with one another to form larger vesicles which in turn rupture to form a continuous surface supported membrane. Reprinted with permission from the Biophysical Journal. (Biophys. J., 2006, 90: 1241–1248.)

Fig. 7

Illustration of the formation of an air-stable supported bilayer. PEG-PE lipids are incorporated into vesicles which can be fused to solid supports imparting air stability to the system. The PEG layer retains water and increases the bending elastic modulus of the membrane, thus protecting it as it is passed through an air–water interface. Reprinted with permission from Langmuir. (Langmuir, 2005, 21: 7476–7482.)

Fig. 8

Schematic illustration of a hybrid bilayer. A single phospholipid monolayer rides on an alkanethiol SAM.

Fig. 9

Peripheral domains of transmembrane proteins can become immobilized and denatured on a solid support. A polymer cushion helps shield the protein from the substrate.

Fig. 10

Illustration of the in situ dialysis process for the formation of supported bilayers anchored around transmembrane proteins. The transmembrane proteins are anchored to the surface via the formation of a nickel His-tag complex. Bio-Bead removal of the detergents used to solublize the proteins is carried our simultaneously as the bilayer is filled in by vesicle fusion. Reprinted with permission from the Journal of the American Chemical Society. (J. Am. Chem. Soc., 2004, 126: 16199–16206.)

Fig. 11

(a) Composition arrays generated by photopatterning. A mask is used to selectively bleach different sized areas of a membrane array. After diffusive mixing within each corral, a concentration array is observed. (b) Microcontact printing of different sized bilayer patches is used to fabricate a concentration array. After printing, the empty space in each corral is backfilled with SUVs. This forms a continuous bilayer in each corral. Shown here is an epifluorescence image of printed Texas Red labeled membranes backfilled with Cascade Blue labeled lipids. The red image is shown on the right and the blue on the left. Reprinted with permission from Accounts of Chemical Research. (Acc. Chem. Res. 2002, 35: 149–157.)

Fig. 12

Depiction of the method for patterning supported bilayers using high intensity deep-UV illumination. Illumination through a photomask in close proximity to a supported bilayer creates localized ozone and singlet oxygen. These highly reactive species decompose the lipids in the regions under illumination. The products of the reaction are soluble and transfer into the bulk solution. This leaves behind a patterned lipid bilayer as can be seen in the fluorescence image. Reprinted with permission from Advanced Materials. (Advanced Materials, 2004, 16(14): 1184–1189.)

Fig. 13

(a) The spatial addressing of solid supported phospholipid bilayers. A pulled microcapillary tip is used to address individual corrals on a pre-patterned substrate. (b) A bright field image of a pulled microcapillary tip addressing 50 μm corrals. (c) An EFTIR macroscope image of an individually addressed 7×9 membrane array. Darker squares have been addressed with Texas Red labeled lipids and the lighter squares with fluorescein labeled lipids. Reprinted with permission from the Journal of the American Chemical Society. (J. Am. Chem. Soc., 1999, 121: 8130–8131.)

Fig. 14

Arrays of solid supported bilayers stamped with a molded agarose gel. (a) Illustration of the stamping technique. (b) 1 micron supported lipid bilayer patches stamped with a high density array. (c) A low density array demonstrating the spatial addressing capabilities of this technique. Reprinted with permission from the Angewandte Chemie International Edition. (Angew. Chem. Int. Ed., 2005, 44: 6697–6700.)

Fig. 15

Addressing by laminar flow in a microfluidic channel. Diffusive mixing in a microchannel under laminar flow conditions provides a concentration gradient of different dye-labeled vesicles. The concentration of vesicles in the gradient is reflected in the surface concentration of each membrane in the resultant array. The array shown is a mixture of Texas Red labeled lipids (shown in red) and DiD labeled lipids (shown in green). Since the dyes have opposite charge, they can be separated in an electric field. Reprinted with permission from Accounts of Chemical Research. (Acc. Chem. Res. 2002, 35: 149–157.)

Fig. 16

(a) Schematic diagram showing phospholipid bilayers coating the interior of PDMS-glass microchannels. (b) Epifluorescence image of the system. Reprinted with permission from Analytical Chemical. (Anal. Chem., 2001, 73: 165–169.)

Fig. 17

(a) Schematic diagram of a microfluidic device used to measure the phase transition temperature of a DPPC bilayer. (b) Calibration curve showing the temperature at various positions within the device. Reprinted with permission from the Journal of the American Chemical Society. (J. Am. Chem. Soc., 2001, 124: 4432–4435.)

Fig. 18

Epifluorescence image of an NVN stained with a green dye. The red regions show where 30 nm latex beads have been injected into the system. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Reprinted with permission from the Proceedings of the National Academy of Sciences. (Proc. Natl Acad. Sci. USA, 2002, 99: 11573–11578.)