Vesicles uncorked

The hole formation in the liposomal membrane after treating the vesicles with reducing agents can be demonstrated by the fast and complete release of eosin, a fluorescent marker. Final proof that the polymeric backbone of the uncorked vesicles does not collapse comes from scanning electron microscopy. Fig. 56 shows the spherical structure of the liposomes with holes. 

There are several methods to selectively open up closed polymeric membrane compartments in order to release entrapped substances (Fig. 37). For uncorking a polymerized vesicle, its membrane has to contain destabilizable areas which could possibly be opened up by variation of pH 70), temperature increase71), photochemical destabilization 72), or enzymatic processes. Such an enzymatic process is the hydrolysis of a natural phospholipid by phospholipase A2 (Fig. 38). This enzyme cleaves the ester bond in position two of a natural phosphoglyceride producing a lysophospholipid and a fatty acid which are both water soluble. This leads to complete destruction of the membrane. 

In addition to enzymatic hydrolysis of natural lipids in polymeric membranes as discussed in chapter 4.2.2., other methods have been applied to trigger the release of vesicle-entrapped compounds as depicted in Fig. 37. Based on the investigations of phase-separated and only partially polymerized mixed liposomes 101, methods to uncork polymeric vesicles have been developed. One specific approach makes use of cleavable lipids such as the cystine derivative (63). From this fluorocarbon lipid mixed liposomes with the polymerizable dienoic acid-containing sulfolipid (58) were prepared in a molar ratio of 1 9 101115>. After polymerization of the matrix forming sulfolipids, stable spherically shaped vesicles are obtained as demonstrated in Fig. 54 by scanning electron microscopy 114>. 

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