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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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If the two lipids combined in NLCs are chemically too different, the amount of supercooling may increase to unacceptable levels (see Figure 4) and this should also be considered. This can be determined by testing the solidification of the lipid mix at room temperature (see Figure 5b). When it solidifies, no phase separation between the lipids or visible expulsion of the active ingredient should be observed. A separation of the two lipids indicates the choice of the lipids in the mixture is not optimal and another liquid lipid (oil) should be selected. Visible expulsion of the active also indicates another liquid lipid should be selected since the LC% will be too low. It may also indicate that too much active has been included, and therefore a reduction of the level of active in the lipid mixture may also be advisable. Remember that the lipid/active ratio also determines the release characteristics from the SLN/NLC, as illustrated in Figure 3.
This first of a two-part series on SLNs and NLCs describes the differences between the two types and their delivery capabilities. The terms solid lipid nanoparticles and nanostructured lipid carriers are not very useful to distinguish these two delivery systems since both are solid, both are lipids, both are nanoparticles, and both are carrier systems. The only real difference between the two is the purity of the single lipid used in SLNs or multiple lipids used, i.e. one solid and one liquid, in NLCs. This factor has an enormous impact on the crystallinity of the lipid phase, which subsequently influences the loading capacity of the system for encapsulated active ingredients or API.
Whereas most formulators would likely opt for NLCs due to their higher loading capability, care must be taken to select two lipids that are not chemically too different otherwise supercooling will prevent crystallization of the internal oil droplet, resulting in a normal emulsion. Typical ingredients such as lipids, waxes and emulsifiers have been discussed as well as production methods for SLNs and NLCs. Hot high pressure homogenization is by far the most frequently used method but the thermolability of the active ingredient or API should be considered. Finally, some general guidelines for the selection of the lipids in combination with the active ingredient and API have been provided.
Following these guidelines, Teeranachaideekul et al.15 designed a series of NLCs that were subsequently characterized. The second part of this review will detail the characterization methods of these SLNs and NLCs, their skin moisturizing properties, other formulation factors as well as their enhancement factors, i.e., the extent to which they enhance skin delivery.
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