Structures closed bilayer

Upon the spontaneous rearrangement of anhydrous phospholipids in the presence of water into a hydrated bilayer structure, a portion of the aqueous phase is entrapped within a continuous, closed bilayer structure. By this process water-soluble compounds are passively entrapped in liposomes. The efficiency of encapsulation varies and depends, for example, on the method of preparation of liposomes and the phospholipid concentration during preparation. Different parameters can be used to describe the encapsulation efficiency ... 

The most versatile method to prepare such hollow capsules is self-assembly [203-205, 214, 215]. Owing to their amphiphilic nature and molecular geometry, lipid-based amphiphiles can aggregate into spherical closed bilayer structures in water so-called liposomes. It is quite reasonable that the hollow sphere structure of liposomes makes them suitable as precursors for the preparation of more functional capsules via modification of the surfaces with polymers and ligand molecules [205, 216, 217]. Indeed, numerous studies based on liposomes in this context have been performed [205, 209, 213]. 

Consequently exposed hydrophobic sides are relieved on collisions by Irreversibly forming a closed bilayer structure - a veslcle(2i). 

Supercritical fluid CO2 has been proposed for the encapsulation of hydrophilic and hydrophobic therapeutic agents into liposomes (144-149). Liposomes are closed bilayer structures that enclose an aqueous volume. These vesicles, comprising single or multiple bilayers, are typically formed from phospholipids. On the basis of their biological components and structure, liposomes are model cell membranes and artificial biomembranes. Their similarities to cellular membranes can be exploited for the parental administration of pharmaceuticals, an area of intense research since the 1970s (146). The presence of both hydrophilic regions of the vesicle (in the aqueous core)... 

Bangham published in 1967 that phospholipids in aqueous systems can form closed bilayered structures and since then numerous attempts have been made to use liposomes for pharmaceutical drug delivery. There are four commonly used methods to produce drug-loaded liposomes ... 

Non-ionic surfactant-based vesicles (niosomes) are formed from the self-assembly of non-ionic amphi-philes in aqueous media resulting in closed bilayer structures (Figure 51.23). The assembly into closed bilayers is rarely spontaneous and usually involves some input of energy such as physical agitation or heat. The result is an assembly in which the hydrophobic parts of the molecule are shielded from the aqueous solvent and the hydrophilic head groups enjoy maximum contact with the same. 

Many naturally occurring and synthetic surfactants and phospholipids that cannot undergo simple aggregation to form micelles will, when dispersed in water, spontaneously form closed bilayer structures referred to as liposomes or vesicles. They are constructed of alternating layers of lipid or surfactant bilayers spaced by aqueous layers or compartments arranged in approximately concentric circles (Fig. 15.13a). If the spontaneously formed multilayer vesicles are subjected to ultrasound or other vigorous agitation, the complex multilayer... 

Over 40 years since it what found that phospholipids can form closed bilayered structures in aqueous systems, liposomes have made a long way to become a popular pharmaceutical carrier for numerous practical applications. Liposomes are phospholipid vesicles, produced by various methods from lipid dispersions in water. Liposome preparation, their physicochemical properties and possible biomedical application have already been discussed in several monographs. Many different methods exist to prepare liposomes of different sizes, structure and size distribution. The most frequently used methods include ultrasonication, reverse phase evaporation and detergent removal from mixed lipid-detergent micelles by dialysis or gel-filtration. To increase liposome stability towards the physiological environment, cholesterol is incorporated into the liposomal membrane (up to 50% mol). The size of liposomes depends on their composition and preparation method and can vary from... 

It has been known for some time that the addition of some cationic surfactant to an anionic tends to increase the laundry performance [163-166], although the explanation is rather recent. In effect, the anionic and cationic surfactants react with each other to make a catanionic equimolar species that is much less hydrophilic but more surface active, particularly in relation to the wettability change. Such catanionic surfactants have been recently shown to produce microemuisions and vesicles (i.e., closed bilayer structures), so-called nanocapsules that may be considered as a water in water microemulsion. 

According to the depth profile of lithium passivated in LiAsF6 / dimethoxyethane (DME), the SEI has a bilayer structure containing lithium methoxide, LiOH, Li20, and LiF [21]. The oxide-hydroxide layer is close to the lithium surface and there are solvent-reduction species in the outer part of the film. The thickness of the surface film formed on lithium freshly immersed in LiAsF /DME solutions is of the order of 100 A. 

Liposomes (Fig. 8) are completely closed vesicular bilayer structures formed by exposing phosphoglycerides in water suspension to sonic oscil-... 

Phospholipids are a major component of living cell membranes. Physical and chemical properties of bilayer structure composed of phospholipids have been well studied, (ij.2) One of the intriguing properties of phospholipids is that they form a closed structure hereafter referred to as vesicles. Vesicles have attracted much attention since they are considered to mimic biocelIs. 

There is a close resemblance between fatty-acid salts and phospholipids (p. 790) in that both possess long hydrocarbon tails and a polar head. Phospholipids also aggregate in a polar medium to form micelles and continuous bilayer structures such as shown in Figure 18-5. The bilayer lipid structure is very important to the self-sealing function of membranes and their impermeability to very polar molecules. 

From these requirements we may infer a structure where the bulk of the intercellular lipids exist in the crystalline, close-packed state in stacked bilayer structures (Figure 2.5) due to the large amounts of long-chain saturated species. However, circumstantial evidence, for example, TEWL, indicates that a fraction of the lipid compartment should be in the liquid crystalline state, but as yet we do not know the composition of this fraction. Again the role of cholesterol may be crucial, as mentioned earlier. 

A bilayered structure in the form of a closed, hollow sphere is also possible [Fig. 6-3(a)]. This type of structure is called a vesicle. The primary concept of a vesicle is two sheets of lipid with their hydrocarbon chains opposed [a bilayer. Fig. 6-3(b)]. An isolated bilayer cannot exist as such in water, because exposed hydrocarbon tails would exist at the edges of the sheet however, this situation is obviated by the sheet s curving to form a self-sealed, hollow sphere. 

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