Lipid Bilayer and its Fluidic Nature

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects ofbiological cell membranes [5-7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, two-dimensional fluidity, bio-compatibility, elasticity, and rich chemical composition. 

For an artificial lipid bilayer of any size scale, it is a general feature that the bilayer acts as a two-dimensional fluid due to the presence of the water cushionlayer between the bilayer and the substrate. Due to this fluidic nature, molecules incorporated in the lipid bilayer show two-dimensional free diffusion. By applying any bias for controlling the diffusion dynamics, we can manipulate only the desired molecule within the artificial lipid bilayer, which leads to the development of a molecular separation system. 

Scale bars 5 pm. Adapted from Ref [29] with permission. 

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