# Recent Lipid Membrane-Based Biosensing Platforms
## 3 Lipid Membrane-Based Biosensing  
Methods and techniques for bilayer reconstitution have evolved considerably over time. Initial efforts to suspend the bilayer between two electrolyte solutions using the painting of the lipid solution over an aperture (Figure 5a) gave solvent-containing membranes at very low success rates [10]. Solvent-less forms could be produced with the expertise-demanding monolayer folding technique (Figure 5b), or the tip-dip method for short-lived bilayers (Figure 5c). These techniques produce bilayers that are accessible from both sides to permit the monitoring of ionic and molecular transport across the membrane. Supported lipid bilayers (sBLMs) can be constructed with a variety of methods. Self-assembly on a metal tip (Figure 5d) yields extremely rugged sensors, even for practical use [36]. Vesicle fusion, on the other hand, is very popular in membrane studies (Figure 5e). This method offers many options for bioelement immobilization: Incorporation of bioelements into the liposomes prior to their spreading, their addition during vesicle spreading, or after the formation of the bilayer [37]; the process is strongly driven by liposome-substrate ionic interactions and it is not easy to control. However, the close proximity of the bilayer to the support lessens options for both, biochemical strategy and transducer coupling. Transmembrane proteins cannot be used as bioelements, as the metal substrate does not permit ion translocation across the lipid bilayer [10]. In effect, only electrochemical and surface interrogation techniques may be utilized for signal transduction [38; 39]. Figure 5: Methods for bilayer formation. (**a**) Lipid painting technique: (i) A hydrophobic septum that separates two electrolyte chambers is punched to produce an aperture which is painted with lipid in n-decane. (ii) The lipids are arranged so as to protect the hydrophobic tails and the membrane is compressed laterally by the hydrostatic pressure difference across the film. (iii) Bilayer Lipid Membrane (BLM) thinning pushes the solvent towards the annulus at the edge of the aperture (Plateau–Gibbs border). (**b**) Monolayer folding: (i) A lipid monolayer is spread onto the surface of the electrolyte. (ii) The electrolyte in one chamber is gradually removed, driving the formation of a monolayer around the aperture. (iii) The chamber is gradually refilled with the electrolyte in order to allow the zipping of the second monolayer. (**c**) Tip-dip: (i) The tip of a glass pipette is immersed in electrolyte solution, and lipids are spread on the surface of the electrolyte. (ii) Raising the pipette slowly, allows a monolayer to attach on its tip. (iii) When re-immersed, the second monolayer is attached. (**d**) Self-assembly on metal support: (i) A freshly tipped electrode is dipped in lipid solution. (ii) When it is immersed in electrolyte, a lipid monolayer is pushed onto the tip. (iii) Gradually, lipids are attracted and form the second monolayer on top (s-BLM). (**e**) Vesicle fusion: when lipid vesicles come in contact with an oppositely-charged solid support, they rupture a form spontaneously a bilayer. (**f**) Tethering: A bilayer may be tethered by thiolipids (i), proteins that span the membrane (ii), or proteins that support the membrane (iii). (**g**) Supporting on porous materials. (**h**) Gel-cushioning that also serves as a reservoir for ions. (Reprinted from [10] with permission by John Wiley and Sons). Supported but floating membranes can be produced through tethering (Figure 5f). In that way, an ionic reservoir beneath the bilayer is made available and the embedded proteins can be kept away from the support [40]. The architecture; however, of the platform, is complex in order to isolate the tethering layer from water, but not the bilayer [11]; ion-channel recordings require high resistance membranes [41] or the use of protein supports [42]. Other approaches involve the formation of membranes on porous substrates, like alumina [43], glass microfiber [44], polycarbonate [45], or Teflon [46].