Lipid-based drug delivery systems — such as liposomes, micro-and nanoemulsions, self-emulsified drug delivery systems, and solid lipid micro-and nanoparticles — are becoming more popular because lipid materials are easily characterized, contain a high range of well-defined/tolerated surfactant molecules and can be developed for several administration routes.

These systems are particularly suitable for topical delivery of oily substances, and are already common in a variety of topical pharmaceutical and cosmetic applications. This is because many suitable compounds are soluble in these materials, they do not irritate the skin and they have extremely low acute and chronic toxicities. The majority of existing topical formulations are applied to achieve a protective effect. Although the skin is widely recognized as a barrier against permeability for topically applied compounds, these formulations cannot be considered safe — even when used solely for protective purposes. For such purposes, well-tolerated and biocompatible materials are also required.

Additionally, for several physiological conditions, the pharmacological activity of the applied drug is required to achieve the treatment, penetration and/or absorption of a particular active ingredient. To achieve topical/dermatological drug delivery, it is necessary to develop systems that enhance drug penetration and have a high encapsulation efficiency for the APIs.

The LPs have been developed by exchanging the liquid lipid (oil) of the oil-in-water emulsions (o/w) by a lipid that is solid at both room temperature and body temperature.¹ The presence of a solid core has many advantages compared with a liquid core, including enhanced protection for chemically labile drugs/actives, less chance of an emulsion forming and a relatively fast release rate from liposomes.

Depending on the matrix size, particles can be below (nano) or above (micro) the nanometre range. Both are composed of a matrix that is in a solid state; that is, the melting temperature should be above 40 °C to remain solid after topical administration. Thus, the melting point of the systems can be adjusted by the choice of the lipid materials used to produce them.

Several lipid compounds from animal and vegetable sources, such as mono-, di-and tri-acylglycerols and waxes, can be used to produce LPs. When very pure lipids (e.g., tristearin and tripalmitin) are used to produce these systems, a highly ordered matrix with a low encapsulation efficiency for incorporated drug molecules is obtained. Alternatively, less-defined mixtures of acylglycerols, such as glycerol behenate and glycerol palmitostearate, can create voids and vacancies within the lipid matrix of the particles, resulting in higher encapsulation efficiencies.

Incorporating liquid lipids (oils) within the solid matrix of LPs has many advantages. In fact, many drugs/APIs are more soluble in liquid lipids than solid lipids, which increases the loading of the systems. In addition, when mixing solid lipids with sufficient amounts of oils an imperfect crystal matrix can be obtained.² The physiochemical characterization of these oil-loaded solid lipid nanoparticles (SLNs) revealed that the oil is encapsulated and partially segregated within the nanoparticles,^3–5 although some controversial results have also been published.^6,7

To produce SLNs, the lipid phase is melted and then immediately emulsified in a hot surfactant aqueous solution to prepare an emulsion. The obtained emulsion is then cooled from the molten state to room temperature to crystallize and form solid particles.

Figure 1 Morphological difference between SLPs and oil-loaded LPs.