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.

Solid lipid nanoparticles (SLNs) originally were introduced as an improvement over liposome delivery systems.¹ This suggests that liposomes, reviewed previously in 2005,² have some disadvantages. Advantages include the fact that they consist of biocompatible ingredients and are capable of including both water- and lipid-soluble actives; in addition, their membrane permeability and consequential release of actives can be regulated via the creation of single or multiple lamellar vesicles. However, disadvantages include their limited capability to enhance the stability of the incorporated active, as well as limited physical stability in real-life formulations.³

SLNs were therefore introduced in the early 1990s—and they are little more than o/w emulsions in which the oil droplet has been replaced by a solid fat at room and body temperature. Obviously, this overcomes the two disadvantages of liposomes described above in that they provide better chemical protection of the active and even greater control over its release, and better physical stability in real-life formulations, in addition to the other benefits of liposomes. The fats used to create SLNs are often biocompatible. In fact, most frequently, they are excipients already used in pharmaceutical and cosmetic products. Another added benefit is their relatively easy production, which frequently employs high pressure homogenization and microemulsion dilution.

Production methods and ingredient selection will be discussed in this first of a two-part series on SLNs and nanostructured lipid carriers (NLCs), whereas part two will discuss their characterization and formulation issues. Despite their benefits, however, SLNs have not yet been introduced to the cosmetic or pharmaceutical markets, although NLCs were introduced to the cosmetic market in 2005 in a repair cream and lotion^a; currently, more than 30 NLC-containing products are available worldwide.

SLN and NLC Structures

As noted, SLNs are essentially “solidified” o/w emulsions in which the oil droplets have been replaced by fat droplets. They typically contain 0.1–30% w/w solid lipid dispersed in an aqueous medium that, if necessary, is stabilized with 0.5–5.0% surfactant. Their mean particle size is 40–1,000 nm and as such, can be considered nano-sized. The active ingredients in SLNs are distributed throughout the lipid phase at a relatively low loading capacity.⁴

In NLCs, on the other hand, the oil phase is a blend of solid and liquid lipids in which the solid to liquid ratio is typically between 70/30 and 99.9/0.1, and where the fat content may be as high as 95%. As a consequence, NLC suspensions contain much less water than SLN suspensions. They also have a significantly higher loading capacity for active ingredients although they may suffer from active ingredient expulsion during storage.⁴ Figure 1 illustrates the differences between SLNs and NLCs.