Dispersed lipid particles

Dispersed lipid particles 4,3,1 Dispersed liquid crystalline phases... 

Emulsifiers are surface active substances that modify interfacial eneigy at the interface of immiscible systems. Emulsifiers are often required in the film formation to disperse lipid particles in composite emulsion films or to achieve sufficient surface wettability to ensure proper surface coverage and adhesion to the coated surface (Krochta 2002). Some common emulsifiers are acetylated monoglyceride, lecithin, glycerol monopal-mitate, glycerol monostearate, polysoibate 60, polysoibate 65, polysoibate 80, sodium lauryl sulfate, sodium stearoyl lactylate, soibitan monooleate, and soibitan monostearate (Janjarasskul and Krochta, 2010). 

Lipid microparticles and nanopellets for oral use were first described by Speiser [11]. Nanopellets are prepared by dispersing melted lipids with high-speed mixers or via ultrasound techniques. Lipospheres developed by Domb are also prepared from dispersed lipids by stirring and sonication [12]. These preparations may contain a high degree of microparticles, which thus excludes an intravenous injection. For other routes of application (e.g., peroral administration), these microparticles might not be a serious problem. Furthermore, the dispersions may be contaminated by metal shed. With optimized conditions, however, mean particles sizes of 1(X) to 200 nm are possible [13]. 

Lipid particles can also be prepared by dispersing a hot microemulsion in cold water (2 to 3°C) under stirring. Drawbacks of this process are the frequent need for organic solvents and the relative low particle concentration as a result of the dilution with water [14]. 

Following the evaporation of water from the lipid nanodispersion applied to the skin surface, lipid particles form an adhesive layer, applying occlusion to the surface [17,40]. Therefore, the hydration of the stratum comeum may increase, which can facilitate drug penetration into deeper skin strata and even systemic availability of the drug. Occlusive effects are strongly related to particle size. Nanoparticles have turned out 15-fold more occlusive than microparticles [17], and particles smaller than 400 nm in a dispersion containing at least 35% high-crystallinity lipid proved to be most potent [41]. 

Solidification of the particles may not be the final step in the formation process of solid lipid particles. Lipidic materials exhibit rich polymorphism [69,70], which may also occur in the dispersed state. In nanoparticles, the polymorphic behavior of the matrix lipids may, however, differ distinctly from that in the bulk material. Polymorphic transitions are usually accelerated in the nanoparticles compared with the bulk lipids [2,62]. In some cases, polymorphic forms not observable in the corresponding bulk materials were detected in lipid nanoparticles [1,65]. Because polymorphism can affect pharmaceutically relevant properties of the particles, such as the drug incorporation capacity [65], corresponding investigations should also be included in the characterization process. As long as polymorphic or other crystalaging phenomena have not terminated, the particle matrix cannot be regarded as static, and alterations of the particle properties may still occur. 

Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) are the techniques most widely used for the characterization of crystallinity and polymorphism of solid lipid particles. Although DSC is usually more sensitive in detecting crystalline material, XRD is much more reliable in determining the type of polymorph present in the dispersions because it provides structural data. In contrast, DSC can detect the type of polymorph only indirectly via the transition temperatures and enthalpies. Because these parameters may be different from those observed in the bulk material, particularly for small colloidal particles [1,62], assigmnent of polymorphic forms in DSC curves should be supported by x-ray data. 

In DSC the sample is subjected to a controlled temperature program, usually a temperature scan, and the heat flow to or from the sample is monitored in comparison to an inert reference [75,76], The resulting curves — which show the phase transitions in the monitored temperature range, such as crystallization, melting, or polymorphic transitions — can be evaluated with regard to phase transition temperatures and transition enthalpy. DSC is thus a convenient method to confirm the presence of solid lipid particles via the detection of a melting transition. DSC recrystaUization studies give indications of whether the dispersed material of interest is likely to pose recrystallization problems and what kind of thermal procedure may be used to ensure solidification [62-65,68,77]. 

In milk plasma, fat may be present as extremely small globules, water-soluble fatty acids and other lipids, water-dispersible lipids, or lipoprotein particles. The amount is small, 0.02-0.03%. Obviously, most of the lipid is TG in the core of the globules. 

A colloidal dispersion in which the dispersed phase is of biological origin. Example a dispersion of lipid particles. 

Centrifuge the sonicated preparation at 105,000, for 1 h at 5°C to effect the removal of titanium particles as well as poorly dispersed lipids. Use only the top two-thirds of the supernatant (see Note 10). 

The protective effect of the surfactant can be compromised by lyophilization [48], It has been found that, to prevent an increase in particle size, the lipid content of the SLN dispersion should not exceed 5%. Direct contact of lipid particles is decreased in diluted samples. Furthermore, diluted SLN dispersions will also have higher sublimation velocities and a higher specific surface area [49], The addition of cryoprotectors will be necessary to decrease SLN aggregation and to obtain a better redispersion of the dry product. Typical cryoprotective agents are sorbitol, mannose, trehalose, glucose, and polyvinylpyrrolidone. 

Ingold, K.U., Bowry, V.W., Stocker, R., and Walling, C., Autoxidation of lipids and antioxidation by alpha-tocopherol and ubiquinol in homogeneous solution and in aqueous dispersions of lipids unrecognized consequences of lipid particle size as exemplified by oxidation of human low density lipoprotein, Proc. Natl. Acad. Sci. U.S.A. 90 (1), 45-49, 1993. 

Similar to emulsions, solid lipid particles consist of emulsifier-coated lipid droplets dispersed within an aqueous phase, with the main difference that the lipid phase is at the solid or semi-soUd state. 

Nanostructure lipid carriers indicate lipid particles with a disperse phase made of a mixture of solid and liquid lipids. Owing to the decreased melting point of the lipid phase, such systems can be produced at lower temperatures, reducing the extent of degradation of the thermolabile compounds. "... 

C.C. Trujillo, A.J. Wright, Properties and stability of solid lipid particle dispersions based on canola stearin and Poloxamer 188, J. Am. Oil Chem. Soc. 87 (2010) 715-730. 

An alternative approach is the use of pH-sensitive fluorophores (Lichtenberg and Barenholz, lOSS). These probes are located at the lipid-water interface and their fluorescence behavior reflects the local surface pH, which is a function of the surface potential at the interface. This indirect approach allows the use of vesicles independent of their particle size. Recently, techniques to measure the C potential of Liposome dispersions on the basis of dynamic light scattering became commercially available (Muller et al., 1986). 

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