Liposomes are closed spherical structures formed by self-assembly of lipids into bilayers (monolayers) and/or concentric multiple bilayers (multilayers) enclosing a central aqueous cavity. The particle size of liposomes ranges from 30 nm to micrometers, and the membrane thickness of phospholipid bilayer is about 4~5 nm. Liposomes originated from the accidental discovery of British scientist Alec Bangham and his colleagues in 1961, who found that when phospholipids were dispersed in an aqueous medium, closed vesicles were spontaneously formed, and published the structure of liposomes for the first time in 1964, and started to use the term "liposomes" in 1968. In 1968, the name "liposomes" was used and is still used today. They were first discovered in 1965 by 

A.D. Bangham . Haematologist and inventor of liposomes. Born in Manchester, UK, on Nov 10, 1921, he died in Great Shelford, UK, on March 9, 2010. (From “Banghasomes” to liposomes). He found that amphipathic phospholipid molecules, when dispersed in water, became organized in vesicles consisting of concentric phospholipid bilayers, separated by aqueous compartments. The hydrophobic fatty acid chains of each bilayer were opposed to each other in the inner part of the bilayer, whereas the hydrophilic head groups of the fatty acids were exposed to the water compartments on the outer part of the bilayer. 

Structure of conventional liposome encapsulating hydrophilic and hydrophobic drugs Copyright （Pharmaceutics 2020, 12(3), 264）

Hydrophilic molecules solved in the aqueous solution used for preparation of the liposomes could be encapsulated in the water compartments between the phospholipid bilayers, whereas hydrophobic molecules could be associated with the phospholipid bilayers themselves, and it was GREGORY GREGORIADIS (Greece / UK), who proposed and initiated the application of liposomes as carriers of drugs in biology and medicine (drug carrier concept).

Because of its biodegradable, biocompatible, non-toxic, and non-immunogenic components, liposomes account for a large proportion of nanotherapy in clinical stage. The first liposomal formulation of the cytotoxic anticancer drug doxorubicin (DOX) was launched in the early 1990s. Currently, liposomes have been successfully used in all imaginable drug delivery methods, and their applications in solving various biomedical problems are also steadily increasing.

The amphiphilic phospholipid bilayer of liposome is very similar to mammalian cell membrane, which realizes the effective interaction between liposome and cell membrane and subsequent effective cell uptake. In addition, ligands can be added to liposomes to improve the efficiency and specifically target damaged cells, so as to improve the pharmacokinetics of liposomes and their ability to pass through the target membrane, achieve high concentration in cells, reduce toxicity and improve the therapeutic effect.

Liposomes can range in size from tens to hundreds of nanometers and are primarily composed of phospholipids, often combined with cholesterol to enhance stability. Liposomes are synthesized through processes such as hydration of dry lipid films followed by mechanical agitation. They are classified based on their size and the number of bilayers, including Small Unilamellar Vesicles (SUVs), Large Unilamellar Vesicles (LUVs), and Multilamellar Vesicles (MLVs). The versatility of liposomes allows them to encapsulate both hydrophilic and hydrophobic drugs, making them invaluable in targeted drug delivery.