Liposomes dispersed systems

With some molecules, a high concentration results in a lamellar phase, but no additional mesophases are formed if the concentration is reduced. The lamellar phase is dispersed in the form of concentric-layered particles in an excess of solvent (water or aqueous solution). This results in a vesicular dispersion. If the mesogenic material consists of phospholipids, the vesicular dispersion is called a liposomal dispersion. In principle, liposomes may be dispersed in oily continuous media too. However, the latter systems are of minor interest in drug formulation. 

Riaz, M. Weiner, N. Martin, F. Liposomes in theory of 47. emulsion. In Pharmaceutical Dosage Forms Dispersion Systems Lieberman, H.A., Reiger, M.M., Banker, G.S.,... 

When a bicontinuous cubic lipid-water phase is mechanically fragmented in the presence of a liposomal dispersion or of certain micellar solutions e.g. bile salt solution), a dispersion can be formed with high kinetic stability. In the polarising microscope it is sometimes possible to see an outer birefringent layer with radial symmetry (showing an extinction cross like that exhibited by a liposome). However, the core of these structures is isotropic. Such dispersions are formed in ternary systems, in a region where the cubic phase coexists in equilibrium with water and the L(x phase. The dispersion is due to a localisation of the La phase outside cubic particles. The structure has been confirmed by electron microscopy by Landh and Buchheim [15], and is shown in Fig. 5.4. It is natural to term these novel structures "cubosomes". They are an example of supra self-assembly. 

Emulsions and suspensions are disperse systems that is, a liquid or solid phase is dispersed in an external liquid phase. While emulsions are sometimes formulated from oily drugs or nutrient oils their main function is to provide vehicles for drug delivery in which the drug is dissolved in the oil or water phase. Suspensions, on the other hand, are usually prepared from water-insoluble drugs for delivery orally or by injection, usually intramuscular injection. An increasing number of modern delivery systems are suspensions - of liposomes or of polymer or protein microspheres, nanospheres or dendrimers, hence the need to understand the formulation and stabilization of these systems. Pharmaceutical emulsions and suspensions are in the colloidal state, that is where the particles range from the nanometre size to visible (or coarse) dispersions of several micrometres. 

As a consequence of this unacceptable situation, the interest in model systems suitable for the construction and study of complex lipid/protein membrane architectures increased steadily over the years [7], The classical portfolio of model membranes used for biophysical and interfacial studies of lipid (bi)layers and lipid/protein composites includes Langmuir monolayers assembled at the water/air interface, (uni- and multi-lamellar) vesicles in a bulk (liposomal) dispersion, the bi-molecular lipid membrane (BLMs), and various types of solid supported membranes [8], All these have their specific advantages but also suffer from serious drawbacks. 

The most common way to formulate injections is to dissolve the drug in an aqueous vehicle. Excipients are added to control solubility, osmotic pressure, pH, stability, specific gravity, and preservation. Injections may also be formulated as oily solutions, disperse systems like suspensions and emulsions (aqueous and oily), and liposomal dispersions. Drug formulation is an essential factor in photochemical stability of the drug substance. Excipients or impurities in the formulation can also participate in photochemical reactions, leading to decomposition of the drug substance or the formulation. 

The encapsulation of drug molecules also leads to delivery systems, especially in the case of hydrophilic compounds where a permeation barrier with depot effect is provided. The permeation velocity is controlled by the properties of the membranes, as well as by the lipophilicity and size of the incorporated drug. Even large molecules are released slowly in the body, but unfortunately this also occurs under storage conditions. As liposomes do not have a solid surface, an equilibrium is built up between incorporated or adhered drug and free drug molecules, and this can lead to bursf effects when liposome dispersions are diluted. 

Structure of the system disperse systems mcrocapsules liposomes polymersomes micro- and nanospheres... 

Several dispersed systems have been developed, on different size scales, which can be listed as follows nanoparticles, microemulsions, nanosuspensions, liposomes, dendrimers, niosomes, cyclo-dextrins, and so on, which can enhance the rate of ophthalmic drug delivery to a significant degree. 

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. 

General examples of this system are polymer micelles, microsphere, nanoparticles, liposomes, etc. The microparticle dispersion system has the following advantages ... 

The carrier materials of many microparticle dispersion systems are usually synthesized polymers whose accumulation in vivo could cause possible toxicity. In contrast, stearic acid is a better material for microparticle dispersion system due to its biocompatibility and nontoxicity. Generally, as solid lipid with the above advantages, stearic acid can be used for preparation of solid lipid nanopartical (SLN), which is supposed to be more stable than emulsion or liposome due to solid form of stearic acid at room temperature. 

All the above disperse systems contain self-assembly structures (i) micelles (spherical, rod-shaped, lamellar) (ii) liquid crystalline phases (hexagonal, cubic or lamellar) (iii) liposomes (multilamellar bilayers) or vesicles (single bilayers). They also contain thickeners (polymers or particulate dispersions) to control their rheology. All these self-assembly systems involve an interface whose property determines the structures produced and their properties. 

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