Prepared by Max Darnell - AP225 Fall 2011
Lipids can be thought of as micelles with both inner and outer hydrophobic portions. These constructs are comprised of phospholipids which are ampipathic, meaning that they contain both hydrophilic and hydophobic regions. In aqueous solutions, these phospholipids self-assemble to shield their hydrophobic regions from the solvent. Lipososmes form when the "shell" consists of a double layer of lipids such that the interior is an aqueous solution as well. They were discovered in 1961.
Applications/Connections to Soft Matter
Liposomes are extremely attractive materials since the lipid bilayer is analogous to that of the cell membrane and is thus compatible. This provides a number of advantages. First, lipid bilayers are good at excluding certain types of molecules while passing others. For example, small uncharged molecules such as steroids pass freely, while large molecules such as DNA and charged ions do not. This effect is partially due to the difficulty of charged molecules to pass through the hydrophobic inner region of the bilayer.
Also, the bilayer can be modified and functionalized with various receptors or ligands, such as glycoproteins or glycolipids which can be used to identify the cargo and target the liposome to a specific site. Such an approach is extremely attractive in drug delivery, where the liposome can be targeted to a tissue of interest.
Along the lines of drug delivery, although many desired payloads like DNA cannot pass through the lipid bilayer, the nature of the liposome as a vesicle allows for fusion with existing bilayer membranes. Hence, if a liposome is tagged with a ligand that binds a cell surface receptor, it is possible that the two membranes will fuse, emptying the contents of the liposome into the cytoplasm of the cell. This approach represents a very efficient means for protecting a payload that might otherwise be subject to degradation. As many potential therapeutics are limited by an inability to actually get a payload inside of a cell, engineering liposomes and polymersomes, their polymer counterpart, is definitely a worthwhile area of interest.
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