Shortly the differences between liposome and micelle is : Liposomes are composed of a lipid bilayer separating an aqueous internal compartment from the bulk aqueous phase. Micelles are closed lipid monolayers with a fatty acid core and polar surface, or polar core with fatty acids on the surface (inverted micelle).

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MICELLE
Micelle formation is essential for the emulsification and subsequent absorption of fat-soluble nutrients such as vitamins E, D and K, the carotenoids and omega-3 EFAs. It is the bile salts formed in the liver and secreted by the gallbladder that allow micelles of fatty acids to form.


A typical micelle is a nanosized vesicular membrane made soluble in water by having a hydrophillic (water-loving) "head" facing the outside, while the hydrophobic (water-hating) "tails" surround the fat-soluble nutrient inside.3 Such micelle formation is known as emulsification, a process that allows a compound normally insoluble (in the solvent being used) to dissolve. Technically, this natural formation is referred to as a nanoemulsion.

Polymeric micelles have several advantages over conventional surfactant micelles in that they have better thermodynamic stability in physiological solution, as indicated by their low critical micellar concentration, which makes polymeric micelles stable and prevents their rapid dissociation in vivo. Micelles have a fairly narrow size distribution in the nanometer range.

Polymeric micelles provide a safer alternative for parenteral administration of poorly water-soluble drugs. Drugs can be partitioned in the hydrophobic core of micelles and the outer hydrophilic layer forms a stable dispersion in aqueous media, which can then be administered intravenously. The distribution of drug-loaded polymeric micelles in the body is determined mainly by size and surface properties. Their individual particle size is less than 50 nm in diameter, which provides obvious benefits over liposomes. It makes them ideal drug-delivery carriers because they avoid renal exclusion and the RES but also provides them with enhanced endothelial cell permeability in the vicinity of solid tumors by passive diffusion. Targeted drug delivery by polymeric micelles is, in most cases, hindered by either premature drug release from the micellar nanomedicine before the nanomedicine reaches the specific targets.

The drug-delivery potential of polymeric micelles may be enhanced by conjugating targeting ligands, including antibodies to the micelle surface.

LIPOSOME
Lipid spheres that contain an aqueous core are called liposomes, meaning literally "fat body." Liposomes are structurally different from micelles in that they have a bi-layer membrane. In the body, natural liposomes, like micelles, are composed of lecithin phospholipids. Though generally larger, they have the advantage of being able to carry both fat-soluble and water-soluble nutrients. Liposomes vary greatly in size, most are 400 nm or less. Because of their size, hydrophobic and hydrophilic character, biocompatibility, biodegradability, low toxicity and immunogenicity, liposomes are promising systems for drug delivery.

Liposomes also can be multi-laminar, like a ball within a ball within a ball. They carry a much bigger payload of both water-soluble and fat-soluble ingredients. Multi-laminar liposomes may reach up to perhaps 500 nm (0.5 micron) in size.
Once nanosized, fat nutrients are emulsified ("mycellized") or encapsulated in liposomes, they are absorbed by simple passive diffusion through the walls of the enterocytes of the small intestine. They can do this more easily than proteins or carbohydrates because the phospholipid membranes that make up the micelle and liposome are similar to the phospholipid membranes that make up the cell wall of the enterocytes.

Once inside the enterocytes, the micelles or liposomes are incorporated into chylomicrons. Together, these travel through the lymph system, bypassing the liver (portal circulation) into the subclavian vein. In the blood stream, these micelles and liposomes are digested by lipoprotein lipases derived from the capillary walls, releasing the nutrients. The liposome itself, usually made of principally lecithin, becomes a nutrient source of phospholipid choline (PC).

Liposome surfaces can be modified by attaching PEG units to the bilayer (producing what is known as stealth liposomes) to enhance their circulation time in the bloodstream.

Liposome surfaces can be attached with targeting ligand (antibodies for immunoliposomes) molecules for active targeted delivery to specific tissues/cells (i.e., cancer cells).

Parenteral (intravenous) injection of some liposomes can cause acute hypersensitivity reactions in a high percentage (up to 45%) of patients, with hemodynamic, respiratory and cutaneous manifestations. The phenomenon can be explained with activation of the complement system on the surface of lipid particles, leading to anaphylatoxin liberation and subsequent release reactions of mast cells, basophils and possibly other inflammatory cells in blood. In one study, idiosyncratic reactions occur after infusion of stealth systems, such as PEG-grafted liposomes. Based on in vitro and animal studies, it was proposed that the phenomenon might represent an unusual allergic reaction called complement activation-related pseudoallergy.

Read More :
Liposome : Definition, Stucture and Phospolipid

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