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Delivering Actives via Solid Lipid Nanoparticles and Nanostructured Lipid Carriers: Part I
By: Johann W. Wiechers, PhD, JW Solutions, and Eliana B. Souto
Posted: September 29, 2010, from the October 2010 issue of Cosmetics & Toiletries.
- Figure 1. The differences between SLNs and NLCs
- Figure 2. A schematic overview of both the hot and cold homogenization
- Figure 3. The lipid/active ratio will determine the SLN/NLC produced
- Figure 4. The effect of adding chemically different lipids to a pure lipid
- Figure 5. Selection criteria of lipid materials for SLNs and NLCs
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The purity of the lipids used to create SLNs is obviously much higher than that of NLCs since high purity lipids have a greater tendency to crystallize and, in turn, a much higher degree of crystallinity within the solid lipid phase when they solidify. Three different forms of lipid crystallinity have been described in the literature: an amorphous α form, an orthorhombic perpendicular β´ form, and a triclinic parallel β polymorphic form;5 the degree of crystallinity increases from the α, to the β´, to the β conformation.
Lipids normally take from minutes to sometimes days before they settle into their final, most stable, high crystallinity phase, which is why active ingredients may be expelled from SLNs during storage. This process can be observed by differential scanning calorimetry (DSC). Studies by Westesen et al. showed sharp peaks at high temperatures, i.e. perfect crystals, for the bulk lipids used to create the SLNs but wider peaks at lower temperatures, i.e. imperfect crystals, after mixing these bulk lipids with other ingredients.6 Such varying degrees of crystallinity can be used to influence the release of actives, as will be discussed in the next section.
Manufacturing SLNs and NLCs
Various methods to produce SLNs and NLCs exist, including high shear homogenization; ultrasound; high pressure homogenization, which can be subdivided into hot and cold homogenization; solvent emulsification/evaporation; and the microemulsion technique. Of these techniques, hot high pressure homogenization is generally the most frequently used.
High shear homogenization and ultrasound: The high shear homogenization and ultrasound methods are relatively easy but unpredictable processes. Basically, large-sized lipid particles are crushed into smaller particles via a high-speed, rotating metal blade. Although the typical particle size produced is between 100 and 200 nm, some microparticles may still be present. Metal contaminations, especially with the use of ultrasound, are also possible.
With these methods, the variables that must be considered include emulsification time, stirring rate and cooling speed. Mehnert and Mäder, for instance, describe a combination of lipids requiring 8 min of emulsification at 20,000 rpm and 10 min of cooling at room temperature as optimal processing conditions. Another combination of lipids required 10 min of emulsification at 25,000 rpm and 5 min of cooling at 5,000 rpm and 16°C. Higher stirring rates slightly improved polydispersity but did not reduce particle size.7 From these variations it can be concluded that high shear homogenization must be optimized on a case-by-case basis.