Liposomes were first described by British haematologist Dr Alec D Bangham in 1961 (published
1964), at the Babraham Institute, in Cambridge. They were discovered when Bangham and R.
W. Horne were testing the institute's newelectron microscope by adding negative stain to dry
phospholipids. The resemblance to the plasmalemma was obvious, and the microscope pictures
served as the first real evidence for the cell membrane being a bilayer lipid structure.
The word liposome derives from two Greek words: lipo ("fat") and soma ("body"); it is so named
because its composition is primarily of phospholipid.
A liposome encapsulates a region of aqueous solution inside a hydrophobic membrane;
dissolved hydrophilic solutes cannot readily pass through the lipids. Hydrophobic chemicals can
be dissolved into the membrane, and in this way liposome can carry both hydrophobic molecules
and hydrophilic molecules. To deliver the molecules to sites of action, the lipid bilayer can fuse
with other bilayers such as the cell membrane, thus delivering the liposome contents. By making
liposomes in a solution of DNA or drugs (which would normally be unable to diffuse through the
membrane) they can be (indiscriminately) delivered past the lipid bilayer. A liposome does not
necessarily have lipophobic contents, such as water, although it usually does.
Liposomes are used as models for artificial cells. Liposomes can also be designed to deliver
drugs in other ways. Liposomes that contain low (or high) pH can be constructed such that
dissolved aqueous drugs will be charged in solution (i.e., the pH is outside the drug's pI range).
As the pH naturally neutralizes within the liposome (protons can pass through some membranes),
the drug will also be neutralized, allowing it to freely pass through a membrane. These liposomes
work to deliver drug by diffusion rather than by direct cell fusion.
A similar approach can be exploited in the biodetoxification of drugs by injecting empty
liposomes with a transmembrane pH gradient. In this case the vesicles act as sinks to scavenge
the drug in the blood circulation and prevent its toxic effect. Another strategy for liposome drug
delivery is to target endocytosis events. Liposomes can be made in a particular size range that
makes them viable targets for natural macrophage phagocytosis. These liposomes may
be digested while in the macrophage's phagosome, thus releasing its drug. Liposomes can also be
decorated withopsonins and ligands to activate endocytosis in other cell types.
The use of liposomes for transformation or transfection of