ABSTRACT
Model biological membranes on solid substrates provide a versatile platform for studying membrane physicochemical properties and functions. Micropatterning is an attractive feature of these membranes because it enables us to study an array of model membranes in a parallel fashion. Complex biological events can also be studied in vitro by using purposefully designed membranes. We have developed a methodology for generating micropatterned bilayers composed of polymerized and fluid lipid bilayers. The polymeric bilayer acts as a framework that supports embedded lipid membranes with defined boundaries. The embedded lipid membranes, on the other hand, retain some important characteristics of the biological membrane, such as fluidity, and are used as a model system. The fact that polymeric and fluid bilayers are integrated as a continuous bilayer membrane gives
various unique features such as enhanced incorporation of fluid bilayers by preformed polymeric bilayer scaffolds. Controlling the composition of polymeric and fluid bilayers enables us to modulate the mobility and distribution of membrane-associated molecules. Owing to these features, micropatterned composite membranes of polymeric and fluid lipid bilayers should provide a versatile platform for constructing complex model systems of the biological membrane used in basic biophysical studies as well as biomedical applications. 23.1 Introduction
Artificial model membranes have played important roles in the development of our understanding on the structure and function of the biological membrane [1-4]. Substrate-supported planar bilayers (SPBs) are a relatively new type of model membranes introduced in the 1980s [5-7]. They typically comprise a single lipid bilayer immobilized on the solid surface by physical interactions or chemical bonds. The lipid bilayer is trapped in the vicinity of the surface by colloidal interactions with an estimated separation of about 10 Å [8]. The presence of the water layer ensures lateral mobility of lipid molecules in the bilayer (fluidity), which is an important property of the biological membrane [7]. SPBs have some unique features compared with other formats of model membranes (lipid vesicles, black lipid bilayers [BLMs], etc.). First, they are mechanically stable due to the fact that the membrane is supported by a solid surface (in contrast to freestanding black lipid membranes). Second, there are a wide range of analytical techniques that can detect interfacial events with an extremely high sensitivity, such as surface plasmon resonance (SPR) and quartz crystal microbalance with dissipation monitoring (QCM-D) [9-12]. The third important feature of SPBs is the potential to generate micropatterned membranes on the substrate by utilizing various microfabrication techniques. These features render SPBs attractive for the development of model systems that utilize artificially mimicked cellular functions [7, 13]. Micropatterned lipid membranes have been applied to various basic biological studies such as immunological synapse formation [14] and signal transduction in cells [15]. They also provide an attractive platform for microarrays of membrane proteins [16]. Micropatterning approaches reported to date can be categorized into three classes (Fig. 23.1). The first type is deposition of lipid
membranes on prepatterned substrates. In this approach, materials on which lipid membranes do not adsorb or form SPBs (e.g., metals, polymeric materials) are prepatterned on the substrate and lipid membranes are subsequently introduced [17-19]. Materials such as self-assembled monolayers and proteins were reported to be an effective barrier to confine lipid bilayers [20-22]. The second type is spatially controlled deposition or removal of lipid membranes. This approach includes the use of mechanical scratching [23], microcontact printing [24, 25], microfluidics [26-28], micropipettes [29], scanning probe microscope tips [30-32], inkjet-printers [16], and air bubble collapse [33]. The third type is photolithographic modification of lipid bilayers. This approach includes decomposition of lipid membranes by deep ultraviolet (UV) light and UV polymerization of lipid bilayers [34, 35].
