Bilayer vesicles solute encapsulation

Lipid vesicles (also known as liposomes) are spherical membrane shells of lipid bilayer in solution. You could think of them as a bubble with water inside and out. Vesicles are stable with aqueous solutions on either side of the membrane provided the osmotic difference between the two solutions is not too large. Vesicles can be large or small, multilamellar or unilamellar and provide us with a useful experimental tool. In biotechnology, liposomes are widely used to encapsulate specific molecules and can provide a mechanism for drug delivery with a targeted or slow release. 

The permeability of solutes across lipid bilayers is a product of the partition coefficient and the transverse diffusion coefficient [30]. Bilayer polymerization can alter solute diffusion by modifying either or both of these processes. In order to examine the effect of polymerization on bilayer permeability a nonionic solute of moderate permeability, [3H-glucose], was encapsulated in the vesicles prior to polymerization, removed from the exterior after polymerization, and its permeation across the bilayer was measured periodically [31]. Quantitative measurements of the 3H-glucose leakage revealed that the formation of linear polymer chains from methacryloyl lipids reduced the permeability coefficient to 0.3 to 0.5 of that of the unpolymerized lipid vesicles. A larger reduction (two orders of magnitude) was only found when crosslinked polymer networks were formed [31]. 

Release of liposome-encapsulated CF from HA/PLL films has been observed at temperatures above the lipid transition temperature (Fig. 4f). Below this temperature, the vesicles were stable at least for a few hours. The polyelectrolyte network destabilizes the embedded vesicles, which show higher lipidic bilayer permeability upon heating than do vesicles in solution [84], No change in film properties upon heating has been reported as proof of the polyelectrolyte destabilization effect. 

Earlier reports [50] showed that vesicles composed of oleic acid can grow and reproduce as oleoyl anhydride spontaneously hydrolyzed in the reaction mixture, thereby adding additional amphiphilic components (oleic acid) to the vesicle membranes. This approach has recently been extended by Hanczyc et al. [51], who prepared myristoleic acid membranes under defined conditions of pH, temperature, and ionic strength. The process by which the vesicles formed from micellar solutions required several hours, apparently with a rate-limiting step related to the assembly of nuclei of bilayer structures. However, if a mineral surface in the form of clay particles was present, the surface in some way catalyzed vesicle formation, reducing the time required from hours to a few minutes. The clay particles were spontaneously encapsulated in the vesicles. The authors further found that RNA bound to the clay was encapsulated as well. 

Block copolymer vesicles, or polymersomes, are of continued interest for their ability to encapsulate aqueous compartments within relatively robust polymer bilayer shells (Fig. 7) [66, 67]. Eisenberg and coworkers were the first to report the formation of block copolymer vesicles from the self-assembly of polystyrene-h-poly(acrylic acid) (PS-h-PAA) block copolymers. They also have described the formation of a wide range of vesicle architectures in solution from the self-assembly of five different block copolymers PS-h-PAA. PS-h-PMMA, PB-h-PAA, polystyrene-h-poly(4-vinyIpyridinium methyl iodide), and polystyrene-h-(4-vinylpyridinium decyl iodide) [68]. Small uniform vesicles, large polydisperse vesicles, entrapped vesicles, hollow concentric vesicles, onions, and vesicles with hollow tubes in the walls have been observed and the formation mechanism discussed. Since vesicles could be prepared with low glass transition polymers such as PB [69, 70] and PPO [71], it has been established than these structures are thermodynamically stable and not trapped by the glassy nature of the hydrophobic part. 

Unilamellar phosphatidylcholine vesicles can be readily prepared by sonicating dispersions of the lipid in aqueous solution at a temperature above the gel-liquid transition point. When formed in the presence of metal ions, the internal space contains encapsulated species that can subsequently undergo crystallization reactions with membrane-permeable species such as OH and H2S (Fig. 21). Alternatively, coreactants can be transported into the interior of the vesicles via ionophores sited in the lipid bilayer. The following materials have been invest -... 

When natural phospholipids are the surfactant, the formed vesicles are termed liposomes. They are made of fragmented phospholipid bilayers in aqueous solution, and closed liposome structures encapsulate some aqueous solution within. Lipids are natural surfactants having two hydrocarbon tails per molecule and they behave similarly to synthetic surfac-... 

Nanoscaled particles with a disk-shaped lipid bilayer stabilized by an apolipo-protein scaffold (curcumin nanodisks, NDs, with <50 nm size) have been used for encapsulating curcumin. NDs self-assemble in solution upon presentation of the scaffold protein to a phospholipid vesicle substrate to which an appropriate hydro-phobic bioactive agent has been introduced. The incorporation of curcumin into NDs enhanced growth inhibition of human hepatocellular carcinoma cells (HepG2) and induced apoptosis in mantle cell lymphoma (Jeko cells) [115,116]. Natthakittaet al. reported the development of mucoadhesive curcumin nanospheres. Ethyl cellulose... 

Sustained Release. Depending on permeation coefficient, vesicle radius, and bilayer thickness, encapsulated low molecular weight solutes will be released on timescales of minutes to days. This can be used for the controlled release of drugs, where the dose can be predicted from the encapsnlated volume and the initial drug concentration in the vesicle using eqnation 21. Large ionic solutes, in particular proteins, will have low release rates and are practically permanently encapsulated until the vesicle is ruptured. 

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