ABSTRACT
For a living cell, the main barrier between the inside and outside world is a 4-6 nm thick layer mainly composed of lipids and proteins, the plasma membrane. Th e lipids, arranged in a bilayer, form a barrier, impermeable for water-soluble ions and molecules. Membrane spanning proteins are embedded, which are in part responsible for the selective transport of otherwise impermeable ions and molecules. Th e fundamental understanding of these membrane proteins as well as their applications in biosensors and screening assays make the development of membrane structures attached to a surface, which allow for the functional insertion of transmembrane proteins, very attractive. Such bilayers provide, however, only an appropriate environment for membrane proteins if several requirements are fulfi lled. Each leafl et of the lipid bilayer should be in the fl uid state and the entire membrane should be surrounded by an aqueous phase. Several diff erent bilayer systems have been developed over the past 30 years to fulfi ll these requirements, two of which will be briefl y discussed here: (1) Solid supported membranes, fi rst described by Brian and McConnell in 1984 on glass surfaces [1], allow to functionalize conducting as well as nonconducting surfaces [2-8]. Th e resulting membranes have, however, some major drawbacks. Th e membrane is in direct contact to the surface, which hampers the insertion and functionality of transmembrane proteins. Due to the lack of a second aqueous compartment, the transport of ions and small molecules mediated by ion channels, transporters, and pumps cannot be followed easily. To monitor the transport of ions or molecules from one compartment to another, a lipid
bilayer needs to be prepared that spans a small aperture, thus separating two aqueous compartments. Such membranes have been developed in the 1970s by Müller and Rudin [9]. However, they are very fragile, and automation and parallization as required for the design of biosensors and automated screening systems, is not possible. Hence, attempts have been made to suspend lipid bilayers across small apertures, manufactured in glass or silicon [10-14]. In recent years, a hybrid system has been established, which is based on highly ordered mesoporous substrates, on which lipid bilayers are prepared in a way that they suspend the pores. Th ese membranes are to be expected to combine the merits of solid supported and freestanding lipid bilayers suspending a 1-100 µm sized single pore.
