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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. Schematic representation of SLNs (left) and NLCs (right); modified from Reference 4.
- Figure 2. The production process of lipid nanoparticles using cold (left, light gray) and hot (right, dark gray) high pressure homogenization; reproduced with permission from Reference 4.
- Figure 3. Models of actives incorporated in lipid nanoparticles, homogeneous matrix; a) type I SLNs, b) type II SLNs, and c) type III SLNs; modified from Reference 4.
- Figure 4. The effect of adding chemically different lipids to a pure lipid; at left, the melting and crystallization temperatures of a pure lipid are shown—both quite high and with a relatively small difference (i.e., supercooling).
- Figure 5. Summary of selection criteria of lipid materials to be used in SLNs and NLCs
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As noted, the first cosmetic products using the NLC technology entered the market in October 2005,4 only a few years after NLCs were first investigated. Since then, a whole series of products has followed incorporating cosmetic actives including, among others: avocado oil, black currant seed oil, co-enzyme Q10, kukuinut oil, Macadamia ternifolia seed oil, Manoi tiare tahiti, and ω-3 and ω-6 unsaturated fatty acids. And even very recently, new papers have emerged in the scientific literature describing the cosmetic benefits of NLC-containing products.11 NLCs therefore still have a long road ahead of them.
Selecting Lipids, Waxes and Actives for Use in SLNs, NLCs
This column thus far has discussed how lipid nanoparticle-containing preparations differ from o/w-emulsions, differences between SLNs and NLCs and how they are produced, and the typical constituents of each type; but for cosmetic formulators who have never made SLNs or NLCs, the selection criteria for the lipids, waxes and actives to be included in SLNs or NLCs are lacking. For instance, can one use any fatty material, or does the choice of excipient influence the skin delivery from SLNs and NLCs to the same extent as was described for o/w emulsions?12 The following section aims to provide some guidelines, but it will become clear that research in this field is only rudimentary and observational rather than based on systematic studies investigating the influence of the chemical nature of excipients on the various characteristics of SLNs and NLCs—such as particle size, release characteristics, polydispersity, etc.; these characteristics will be discussed in the second part of this review.
Lipids: The choice of the lipid(s) used to produce SLNs and NLCs is, of course, very important. When higher melting point lipids are used, larger nanoparticles generally are obtained. This can be explained by the fact that higher melting point lipids result in a higher viscosity of the dispersed phase and therefore, at the same level of energy input, in larger particles. The reversed argument is also true: because homogenization is less efficient at the same level of energy input, increased particle agglomeration is obtained; therefore, broader particle distribution is obtained when using higher melting point lipids.7 For instance, the average particle size of ingredients such as hydrogenated coco-glycerideb SLNs was found to be significantly smaller (117.0 ± 1.8 nm) than the size of tristearinc SLNs (175.1 ± 3.5 nm).13
Hydrogenated coco-glycerides contain shorter fatty acid chains and considerable amounts of mono- and diglycerides, which possess surface active properties. The melting points of hydrogenated coco-glyceridesb and tristearinc are 33.5–35.5°C and 68°C, respectively, therefore SLNs made with the former are smaller than those made with the latter. The reduced homogenization efficiency leading to larger particles is also the reason that increasing the lipid content above 5–10% in most cases results in larger particles (including microparticles) and broader particle size distributions.
Other, less clear variables that affect the characteristics of SLNs and NLCs include: the velocity of lipid crystallization; lipid hydrophilicity—i.e., self-emulsifying properties; the shape of the crystal, since this determines the surface area; and variations in the chemical composition of an ingredient from the same or different suppliers, and even between batches of the same excipient from the same supplier. Impurities, for instance, can have a considerable impact on the quality of SLN dispersions by affecting the zeta potential or retarding the crystallization process.7