Smaller unilamellar vesicles

The size and smface properties of liposomes vary with types of lipids, their compositions, their modification, and methods of preparation. For example, multilamellar vesicles (MLVs) several hundred nanometers in size can be produced by a reverse phase evaporation and extrusion, but smaller unilamellar vesicles (SUVs), whose size is less than 100 mn, can be produced by a sonication process [15]. Further, the membrane state of a bilayer is of primary interest not only for surface treatment but also for recognition of a cell surface and delivery of active ingredients. We will briefly review the microfluidity of bilayers and the interaction of liposomes with a cell surface. 

In order to prepare liposomes, the lipid preparation is dried at low temperature under an inert gas atmosphere (protect the lipid from oxidation). The lipid film is swollen with water or buffered aqueous solution and several freeze-thaw cycles are carried out to get optimal rehydration of the lipid. The rehydrated lipid preparation is filtered using membrane filters with defined pore size. After repeated filtration steps (extrusion) an unilamellar liposome preparation with a defined size distribution is obtained. Large unilamellar vesicles (LUV) are produced in this way. LUV s are about 100 nm in size the thickness of the lipid bilayer is about 4 nm. Even smaller liposomes can be derived from sonication (sonication probe or ultra-sonication bath). Separation of the prepared liposomes... 

Unilamellar vesicles are usually formed from lipid dispersions with sonication. Thus the suspension is metastablc, and vesicles aggregate to form a multilamellar structure. The decay time of sonicated phosphatidylcholine vesicles, as measured by DLS, showed a bimodal distribution 50. The hydrodynamic radius estimated from the smaller decay time was consistent with the values obtained by other experimental techniques such as ultracentrifugation. 

Intermediate-sized unilamellarvesicles (lUVs) have diameters of the order of magnitude of 100 nm, and are called large unilamellarvesicles (LUVs) if the size is more than 100 nm and they consist of a single bilayer. For unilamellarvesicles, the phospholipid content is related to the surface area of the vesicles, which is proportional to the square of the radius, while the entrapped volume varies with the cube of the radius. In addition, because of the Lnite thickness of the membrane (ca. 4 nm), as thf vesicles become smaller, their aqueous volume is further reduced since the phospholipids occupy more of the internal space. Consequently, for a given quantity of lipid, large unilamellar liposomes... 

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