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[[Image:Liposome.jpg|thumb|right|250px|Liposomes are polymeric nanoparticles with various targeting ligands attached to their surface allowing for their surface-attachment and accumulation in pathological areas for treatment of disease<ref>  Torchilin VP. (2006)Adv Drug Deliv Rev. 2006 Dec 1;58(14):1532-55</ref>.]]
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==Overview==
A '''liposome''' is a spherical [[vesicle (biology)|vesicle]] composed of a bilayer membrane.  In biology, this specifically refers to a [[biological membrane|membrane]] composed of a [[lipid|phospholipid]] and [[cholesterol]] [[bilayer]] (see on the right). Liposomes can be composed of naturally-derived phospholipids with mixed lipid chains (like egg [[Phospholipid#Phosphatidyl ethanolamine|phosphatidylethanolamine]]), or of pure [[surfactant]] components like DOPE (dioleoylphosphatidylethanolamine). Liposomes, usually but not by definition, contain a core of aqueous solution; lipid spheres that contain no aqueous material are called [[micelles]], however, reverse micelles [http://en.wikipedia.org/wiki/Micelle#Reverse_Micelles] can be made to encompass an aqueous environment.

==Etymology==
The name liposome is derived from two Greek words 'Lipid' meaning fat and 'Soma' meaning body. The word liposome does not in itself denote any size characteristics as is frequently presumed and therefore is not an alternative to a Nanosome. Furthermore the term liposome does not necessarily mean that it must contain [[lipophobic]] contents, such as water, although it usually does.

==Discovery==
Liposomes were first described by British haematologist Sir. Alec D Bangham in 1961 (published 1964), at the Babraham institute, Cambridge. They were discovered when Bangham and R. W. Horne were testing the institute's new [[electron 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.

==Application==
Liposomes are used for [[drug delivery]] due to their unique properties. A liposome encapsulates a region on aqueous solution inside a [[hydrophobic]] membrane; dissolved [[hydrophilic]] [[solutes]] can not 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 [[medication|drugs]] (which would normally be unable to [[diffusion|diffuse]] through the membrane) they can be (indiscriminately) delivered past the lipid bilayer.

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. 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 magrophage's [[phagosome]], thus releasing its drug. Liposomes can also be decorated with [[opsonin]]s and [[ligand]]s to activate endocytosis in other cell types.

* The use of liposomes for transformation or [[transfection]] of DNA into a host cell is known as [[lipofection]].

==Targeting cancer==
Another interesting property of liposomes are their natural ability to target [[cancer]]. The [[endothelial]] wall of all healthy human [[blood vessel]]s are encapsulated by endothelial cells that are bound together by [[tight junction]]s. These tight junctions stop any large particle of in the [[blood]] from leaking out of the vessel. [[Tumour]] vessels do not contain the same level of seal between cells and are diagnostically ''leaky''. This ability is known as the [[Enhanced Permeability and Retention effect]]. Liposomes of certain sizes, typically less than 400nm, can rapidly enter tumour sites from the blood, but are kept in the bloodstream by the endothelial wall in healthy [[Biological tissue|tissue]] [[vasculature]]. Anti-cancer drugs such as Doxorubicin (Doxil) and Daunorubicin (Daunoxome) are currently being marketed in liposome delivery systems.         

==Manufacturing==
Liposomes can be created by [[sonication|sonicating]] phospholipids in water. Low shear rates create multilamellar liposomes, which have many layers like an onion. Continued high-shear sonication tends to form smaller unilamellar liposomes. In this technique, the liposome contents are the same as the contents of the aqueous phase. Sonication is generally considered a "gross" method of preparation, and newer methods such as extrusion are employed to produce materials for human use.  

==Prospect==
Further advances in liposome research have been able to allow liposomes to avoid detection by the body's immune system, specifically, the cells of [[reticuloendothelial system]] (RES). These liposomes are known as "stealth liposomes", and are constructed with PEG ([[Polyethylene glycol|Polyethylene Glycol]]) studding the outside of the membrane. The PEG coating, which is [[inert]] in the body, allows for longer circulatory life for the drug delivery mechanism.  However, research currently seeks to investigate at what amount of PEG coating the PEG actually hinders binding of the liposome to the delivery site.  In addition to a PEG coating, most stealth liposomes also have some sort of biological species attached as a ligand to the liposome in order to enable binding via a specific expression on the targeted drug delivery site. These targeting ligands could be [[monoclonal antibodies]] (making an immunoliposome), [[vitamin]]s, or specific [[antigen]]s. Targeted liposomes can target nearly any cell type in the body and deliver drugs that would naturally be systemically delivered. Naturally toxic drugs can be much less toxic if delivered only to diseased tissues.

==References==
{{Reflist}}

==External links==

* [http://www.encapsula.com Liposome Formulator and Manufacturer for Academic Research Labs and Pharmaceutical companies]

* [http://www.liposome.org International Liposome Society]

* [http://www.avantilipids.com  Lipid manufacturer for liposomes]

* [http://www.controlledreleasesociety.org/  Controlled Release Society]

[[Category:Membrane biology]]

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