Vesicles and liposomes

It was mentioned that ordinary surfactants (soaps, etc.), when dissolved in water, form self-assembly micellar structures. Phospholipids are molecules like surfactants they also have a hydrophilic head and generally have two hydrophobic alkyl chains. These molecules are the main components of cell membranes. Lung fluid also consists mainly of lipids of this kind. In fact, usually, cell membrane are made up of two layers of phospholipids, with the tails turned inward, in an attempt to avoid water. The external membrane of a cell contains all the organelles and the cytoplasm. 

Phospholipids, when dispersed in water, may exhibit self-assembly properties (either as micellar self-assembly aggregates or larger structures). This may lead to aggregates that are called liposomes or vesicles. Liposomes are structures that are empty cells and that are currently being used by some industries. They are microscopic vesicles or containers formed by the membrane alone, and are widely used in the pharmaceutical and cosmetic fields because it is possible to insert chemicals inside them. Liposomes may also be used solubilize (in its hydrophobic part) hydro-phobic chemicals (water-insoluble organic compounds) such as oily substances so that they can be dispersed in an aqueous medium by virtue of the hydrophilic properties of the liposomes (in the alkyl region). 

A liposome is a spherical vesicle with a membrane composed of a phospholipid and cholesterol (less than 50%) bilayer. Liposomes can be composed of naturally derived phospholipids with mixed lipid chains (such as egg phosphatidylethanolamine) or 

FIGURE 4.13 Different self-assembly (SA) structures (a) micelle (b) monolayer (c) LB film (d) vesicle (e) liposome. 

The word liposome comprises two terms (from Greek—lipid [fat] and soma [body]). It does not in itself denote any size characteristics. Furthermore, the term liposome does not necessarily mean that it must contain lipophobic contents, such as water, although it usually does. The vesicles may be conceived as microscopic (or nano-sized) containers of carrying molecules (drugs) from one place to another. The structures are suitable for both transporting water-soluble or water-insoluble drugs. Since the lipids used are biocompatible molecules, this may also enhance their adsorption and penetration into the cells. 

The inherent lipid self-assembly characteristic is the main driving force in these structures. Monolayer studies are the only source of data which provides direct estimation of the stabilizing forces. Hence, it is safe to conclude that many important natural systems are based upon this molecular characteristics of lipids. Vesicles are unilamellar phospholipid liposome. 

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This range yields more highly tmncated cones. The main mesophase stmcture obtained from these units is a flexible bilayer such as that fonned in vesicles and liposomes. These arrangements are often obtained from doublechain surfactants such as lecithin, double tailed cationic surfactants and AOT. 

That vesicles and liposomes form at all is a consequence of the amphi-pathic nature of the phospholipid molecule. Ionic interactions between the... 

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

Antonietti, M. and Eoerster, S. (2003) Vesicles and liposomes a self-assembly principle beyond bpids. Adv. Mater., 15, 1323-1533. 

Vesicles and liposomes are versatile supermolecular systems and possess numerous potential applications in targeting agents, microreactors, encapsulations, and drug delivery. The Rotello group developed novel recognition-mediated polymersomes... 

We have also learned that self-replication is not a prerogative only of nucleic acids, but it can be shared by different kinds of chemical families see the formose reaction, the self-replicating peptides, and the self-reproducing micelles and vesicles. The list should include the cellular automata and the corresponding devices of artificial life. Self-reproduction of vesicles and liposomes is important because it represents a model for cell reproduction. 

Let us now look at some properties of vesicles and liposomes (liposomes can be defined as vesicles made out of lipids, although often the two terms are used synonymously). This will be a preliminary to the next chapter, where the reactivity of vesicles as models for biological cells will be considered in more detail. 

At the most fundamental level, monolayers of surfactants at an air-liquid interface serve as model systems to examine condensed matter phenomena. As we see briefly in Section 7.4, a rich variety of phases and structures occurs in such films, and phenomena such as nucleation, dendritic growth, and crystallization can be studied by a number of methods. Moreover, monolayers and bilayers of lipids can be used to model biological membranes and to produce vesicles and liposomes for potential applications in artificial blood substitutes and drug delivery systems (see, for example, Vignette 1.3 on liposomes in Chapter 1). 

Vesicles and liposomes are versatile supramolecular systems with unusual stability and great potential as functional materials with applications in biosensors, targeting agents, microreactors, and encapsulation/drug delivery. 

This research has been carried out in our laboratory and is still actively pursued— this is part of our enterprise in the direction of the minimal living cell. It is not the aim of this review to dwell upon this part. The interested reader is referred to our work concerned with enzymes in liposomes, in particular to the work dealing with lecithin-producing enzymes in lecithin liposomes, or the following work on enzymatic and molecular biological reaction taking place in vesicles and liposomes. " ... 

The region in the phase diagram denoted crystals + water often contains other structures as well. For phospholipids, which are generally mixtures and are very poorly soluble in water, vesicles and liposomes , i.e., fragments of liquid crystalline phases (or possibly of a-gel), have been observed. 

Fig. 19. Rectangular phase diagram of TC-L systems in 0.15 M NaCl at 20°C. Region I corresponds to concentrations where simple and mixed micelles coexist. In region II only mixed micelles are present, and in region III metastable vesicles and liposomes are found. (From ref. 102 with permission.)...

Antonietti M, Forster S. Vesicles and liposomes A self-assembly principle beyond lipids. Adv Mater 2003 15 1323-1333. 

Alternative and more complex surfactants continue to be explored as a resourceful option for MEKC separations. In situ generated micelles, which are anionic complexes formed by alkyl-or steroidal-glycoside surfactants and borate ions, the use of sodium 10-undecylenate (SUA) and sodium 10-undecylsulfate (SUS) oligomers as well as surfactants composed of two ionic groups and two liphophilic chains, such as sodium 5,12-h (dodecyloxymethyl)-4,7,10,13-(tetraoxa)-1,16-hexadecanedisulfonate (DBTD), bilayered aggregates such as vesicles and liposomes, and bilayer micelles are a few examples. 

Microemulsions and most surfactants in dilute solutions and dispersions self-assemble into a variety of microstructures spherical or wormlike micelles, swollen micelles, vesicles, and liposomes. Such systems are of biological and technological importance, e.g., in detergency, drug delivery, catalysis, enhanced oil recovery, flammability control, and nanoscale particle production. The macroscopic properties—rheology, surface tension, and conductivity—of these systems depend on their microstructure. As these microstructures are small (1-1000 nm) and sometimes several microstructures can coexist in the same solution, it is difficult to determine their structure. Conventional techniques like radiation scattering, although useful, provide only indirect evidence of microstructures, and the structures deduced are model-dependent. 

Other examples of bilayer structures already mentioned are the sponge phase and bicontinuous cubic phases. The sponge phase has been most studied for nonionic surfactants and is related to common microemulsions. Bilayers may also easily close on themselves to form discrete entities including unilamellar vesicles and multilamellar liposomes. Vesicles are of interest because of the division into inner and outer aqueous domains separated by the bilayer. Vesicles and liposomes are normally not thermodynamically stable (although there are exceptions) and tend to phase separate into a lamellar phase and a dilute aqueous solution. Lipid bilayers are important constituents of living organisms and form membranes, which act as barriers between different compartments. Certain surfactants and lipids may form reversed vesicles, i. e. vesicles with inner and outer oleic domains separated by a (reversed) amphiphile bilayer the bilayer may or may not contain some water. 

Antonietti, M., Forster, S. (2003). Vesicles and Liposomes A Self-Assembly Principle Beyond Lipids. 15(16), 1323-1333. 

Polymeric microencapsulates and lipid microencapsulates have extensive potential applications in food, cosmetics and pharmaceutics [1-5]. Microencapsulates can protect and conserve an active component until its release is desired and stimulated. Polymeric microencapsulates consist of a (biocompatible) polymer matrix in which an active component is encapsulated. Most frequently poly(lactic add) (PLA) or poly (lactic-co-glycolic acid) (PLGA) is used as the polymer [6,7], but alternatives have been investigated [8, 9]. Lipid microencapsulates, lipid vesicles and liposomes are composed of a (phospho-)lipid bilayer membrane that encapsulates an aqueous volume, thus mimicking a cell structure. 

Various body fluids, such as blood, digestive juices, lachrymal fluid fruit juices wastewater Gelatin gel Sephadex and other matrices, for example, for gel permeation chromatography Detergents, microemulsions, vesicles and liposomes, and biological membranes... 

Barauskas. J., Johnsson, M. Tiberg, F. (2005). Self-assembled lipid superstructures beyond vesicles and liposomes. Nano Letter, 5(8), 1615-9. 

Polymerizable surfactant aggregates such as vesicles and liposomes have been prepared from amphiphiles with diyne (Chapt. 2.4.), methadyloyl vinyl... 

The main goal of this study is to prove the synthesis of bilayer structures with lipids from the two monolayers drop and target monolayers. The place where the lipids from the drop monolayer could come in contact with the lipids from the target monolayer is under the thin liquid sheet of the drop impact pattern. If the insoluble lipids are captured under the drop bulk liquid they are in an aqueous medium and they form spontaneously bilayer and/or multilayer phases, like vesicles and liposomes, as shown in the literature [7-12]. Due to the fact that the new bilayer... 

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