Mechanical dispersion, liposome preparation

For the preparation of large quantities of liposomes, mechanical dispersion using a commercially available emulsifier is probably the best route. For limited quantities, the use of simple shaking or ethanolic dispersion techniques works well. 

Mechanical dispersion methods are the most frequently used methods in the production of the laboratory scale liposomes. Usually two steps are included in these methods the Llm preparation anc... 

In mechanical dispersion, the lipids are dried down onto a solid support from organic solvents, followed by the dispersion of liposomes by adding the aqueous media through shaking. Other methods using mechanical dispersion techniques include freeze drying, pro-liposome preparation, hand shaking, and the non-shaken method (175). 

For the study of drug membrane interactions and of the influence of drug structure and membrane composition, artificial membranes simulating especially mammalian membranes can easily be prepared because of the readiness of phospholipids to form automatically lipid bilayers, i.e. their tendency for self-association in water. The macroscopic structure of dispersions of phospholipids depends on the type of lipids and on the water content. The structure and properties of self-assembled phospholipids in excess water have been described [32], and the mechanism of liposome formation has been reviewed [33]. While the individual components, i.e. the membrane proteins and lipids, are composted of atoms and covalent bonds, their association with each other to produce membrane structures is governed largely by the hydrophobic effect This hydrophobic effect is derived from the structure of water and the interaction of other components with the water structure. Because of their enormous hydrogen-bonding capacity, water molecules adopt a structure in both the liquid and the solid state. 

Perhaps the simplest solvent dispersion method is that developed by Batzri and Korn (1973). Phospholipids and other lipids to be a part of the liposomal membrane are first dissolved in ethanol. This ethanolic solution is then rapidly injected into an aqueous solution of 0.16 M KC1 using a Hamilton syringe, resulting in a maximum concentration of no more than 7.5% ethanol. Using this method, single bilayer liposomes of about 25-nm diameter can be created that are indistinguishable from those formed by mechanical sonication techniques. The main disadvantages of ethanolic injection are the limited solubility of some lipids in the solvent (about 40 mM for phosphatidyl choline) and the dilute nature of the resultant liposome suspension. However, for the preparation of small quantities of SUVs, this method may be one of the best available. 

A model system which is most useful is a dispersion of a pure phospholipid in water or in aqueous buffer, prepared with vigorous mechanical agitation. Under these conditions, phospholipids form "multilamellar suspensions," sometimes called "liposomes," which consist of concentric spherical lipid bilayers separated from one another by excess water. The assumption is that one can gain some understanding of the lipid bilayer component of natural biomembranes through the study of these simpler molecular systems. 

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