Encapsulation amphiphilic molecules

Concepts Protocells Amphiphilic molecules spontaneously form cell-like structures that, by a process of encapsulation, acquire a chemical inventory... 

We know very little about how this event might have occurred at the origin of cellular life, but recent advances have provided clues about possible sources of amphiphilic molecules, assembly of membrane structures, and encapsulation mechanisms by which large molecules can be captured in membrane-bounded microenvironments. Here we will describe the chemical and physical properties of such systems and several experimental models that incorporate certain properties related to the origin of cellular life. 

Lipid bilayers are formed by many amphiphilic molecules in the presence of water. Their interest derives not only from the fact that they are a major, if not the only, organizing principle of biological membranes (1), but also because they tend to form closed (usually) spherical structures (liposomes or lipid bilayer vesicles) in which inner and outer aqueous spaces are separated by the lipid bilayers (2) which thereby provides a means of encapsulation (3, 4). 

A liquid crystal is a general term used to describe a variety of anisotropic structures formed by amphiphilic molecules, typically but not exclusively at high concentrations. Hexagonal, lamellar, and cubic phases are all examples of liquid crystalline phases. These phases have been examined as drug delivery systems because of their stability, broad solubilization potential, ability to delay the release of encapsulated drug, and, in the case of lamellar phases, their ability to form closed, spherical bilayer structures known as vesicles, which can entrap both hydrophobic and hydrophilic drug. This section will review SANS studies performed on all liquid crystalline phases, except vesicles, which will be considered separately. Vesicles will be considered separately because, with a few exceptions, generally mixed systems, vesicles (unlike the other liquid crystalline phases mentioned) do not form spontaneously upon dispersal of the surfactant in water and because there have been many more SANS studies performed on these systems. 

The structural units of liposomes are amphiphile molecules, mainly phospholipids, Alec Bangham and co-workers observed self-closed lipid structures after they had been dissolved in water [357- This first observation took place after egg yolk lecithin had been dispersed in water. According to D.D.Lasic and D. Papahadjopoulos, liposomes are self-assembling colloidal particles in which a lipid bilayer encapsulates a fraction of the surrounding medium [36],... 

The ability to encapsulate large aqueous volumes and consequently whatever is dissolved in it is one prerogative of polymersomes and represents their most promising ability. Nevertheless, while for hydrophobic and amphiphilic molecules the encapsulation process is more or less straightforward, the complex kinetics of polymersomes formations hampers the encapsulation of hydrophilic molecules. Clearly the most important parameter to consider in encapsulating water-soluble molecules is the polymersome membrane permeability. Early work carried out by Discher and co-workers [6] showed that the water permeability of PEO-PEE polymersomes was established by measuring the reduction in polymersome swelling as a function... 

Polymersomes are, like any other vesicular structure, able to encapsulate hydrophilic, hydrophobic and amphiphilic molecules, but unlike other vesicular structure, their macromolecular nature makes them stronger and more stable with the intrinsic responsiveness of polymers. All these properties make polymersomes one of the most interesting supramolecular structures with potential applications in drug... 

Alternative approaches involving molecules that combine the properties of a monomer with those of a surfactant (so-called polymerizable surfactants) have also been reported. For example, quaternary alkyl salts of dimethyl aminoethyl methacrylate (CnBr) surfactants were used to promote polymer encapsulation of silica gels in aqueous suspension [43, 44]. The polymerizable surfactant formed a bilayer on the silica surface, the configuration of which enabled the formation of core-shell particles. The CnBr amphiphilic molecule was either homopolymer-ized or copolymerized with styrene adsolubilized in the reactive surfactant bilayer. This concept of admicellar polymerization is detailed in Sect. 3.1. In the recent... 

This versatile modifiable property allows liposomes to encapsulate functional molecules in the interior, inserted in the bilayer or attached on the bilayer membrane surface. Hence, they have been considered for application in multifunctional platforms, that is, therapy and imaging. They protect the amphiphilic, hydrophobic, and hydrophilic therapeutic agents against various threats that lead to their immediate dilution and degradation. 

Self-assembled membranes constructed from phospholipids and other surfactants have been extensively investigated to understand their formation, encapsulation and release, and templating properties (7-25). Lipids and surfactants are amphiphilic molecules with hydrophilic, polar headgroups and nonpolar tails. As a result of the hydrogen bonding and electrostatic interactions of the hydrophilic headgroups and the van der Waals interactions between the hydrophobic tails, amphiphiles form organized membrane microstructures when dispersed in water or oil. When... 

Figure 7.2 Encapsulation of small guest molecules and dispersion of nanoparticles by amphiphilic molecules with linear structure (top) and highly branched pol aners with micelle-like topology (bottom).

Apart from their morphological diversity, polymer vesicles have been at the focus of extensive research also because of their potential applications, especially in medical fields. These kinds of applications usually exploit the unique ability of polymersomes to encapsulate hydrophilic compounds within the core and, at the same time, hydrophobic and amphiphilic molecules within the membrane. This feature combined with the enhanced mechanical properties of polymeric membranes renders them ideal delivery devices. As a result, polymer vesicles have been extensively utilised in drug delivery, gene therapy, protein delivery, medical imaging, cancer diagnosis and therapy, etc. [48-54]. 

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