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Here we identify the recruitment of solvent ions to lipid membranes as the dominant regulator of lipid phase behavior. Our data demonstrate that binding of counterions to charged lipids promotes the formation of lamellar membranes, whereas their absence can induce fusion. The mechanism applies to anionic and cationic liposomes, as well as the recently introduced amphoteric liposomes. In the latter, an additional pH-dependent lipid salt formation between anionic and cationic lipids must occur, as indicated by the depletion of membrane-bound ions in a zone around pH 5. Amphoteric liposomes fuse under these conditions but form lamellar structures at both lower and higher pH values. The integration of these observations into the classic lipid shape theory yielded a quantitative link between lipid and solvent composition and the physical state of the lipid assembly. The key parameter of the new model, κ(pH), describes the membrane phase behavior of charged membranes in response to their ion loading in a quantitative way.

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We here for the first time describe the encapsulation of single liposomes within ultrathin networks made from poly -electrolytes. Prevention of aggregate formation and encapsu -lation of single liposomes were achieved using a titration approach, where limited amounts of polyelectrolyte were added to the liposome suspension. The lipid bilayer was found to be stable during the coating procedure. True formation of cage-like structures was verified by detergent treatment, encapsulation of macromolecules, and electron microscopy. Solubilization of the inner liposomes allows gentle removal of the templating core, leaving unaffected hollow shell structures behind.Polymer encapsulated lipo-somes can be manufactured from synthetic and natural polymers, and even proteins, and constitute a novel bio-material with promising characteristics. The entire structure can be constructed from biological materials or other substances that are generally regarded as being safe, a parti-cular advantage for biomedical applications. These capsules are expected to be useful in the development of novel oral drug delivery systems that are well suited for the transport of biological macromolecules, i.e. DNA, proteins, and peptides. 238

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