An unilamellar liposome is a spherical chamber/vesicle, bounded by a single bilayer of an amphiphilic lipid or a mixture of such lipids, containing aqueous solution inside the chamber. Small unilamellar liposomes/vesicles (SUVs) have sizes up to 100 nm; large unilamellar liposomes/vesicles (LUVs) may have sizes more than 100 nm up to few micrometers (µm). There are giant unilamellar liposomes/vesicles (GUVs), which have an average diameter of 100 µm. GUVs are mostly used as models for biological membranes in research work. Each lipid bilayer structure is comparable to lamellar phase lipid organization in biological membranes, in general. In contrast, multilamellar liposomes (MLVs), consist of many concentric amphiphilic lipid bilayers analogous to onion layers, and MLVs may be of variable sizes up to several micrometers.
Phospholipid liposomes are used as targeted drug delivery systems. Hydrophilic drugs can be carried as solution inside the SUVs or MLVs and hydrophobic drugs can be incorporated into lipid bilayer of these liposomes. If injected into circulation of human/animal body, MLVs are preferentially taken up phagocytic cells, and thus drugs can be targeted to these cells. For general or overall delivery, SUVs may be used. For topical applications on skin, specialized lipids like phospholipids and sphingolipids may be used to make drug-free liposomes as moisturizers, and with drugs such as for anti-ultraviolet radiation applications.
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