Solid lipid nanoparticles and nanostructured lipid carriers: A review emphasizing on particle structure and drug release.

Colloidal systems based on lipids have attracted attention during the last two decades as a delivery method of drugs that are poorly soluble in water. The investigations carried out have focused on the development of different formulations using a wide variety of excipients and active molecules. However, there is no consensus on the structure of the particles in these colloidal systems. This is partly due since there are few studies oriented to understand both the preferential location of the drug in the particle and the arrangement of the lipids and the stabilizing agents during the particle formation. As a contribution in this sense, in this review the most common materials and preparation methods to obtain lipid particles are presented. Also, the particle characteristics, including the shape, size and size distribution, zeta potential, drug load capacity and drug entrapment efficiency are synthesized and analyzed with the help of scientometrics tools. Emphasis has been placed on the latest advances regarding the particle structure especially as it relates to the drug release behavior. In addition to the available evidence, a model of particle structure based on the formation of the different polymorphic forms of the solid lipid because of the starting materials and the processing conditions is proposed. In general, the importance of obtaining a detailed knowledge of the structure of the lipid nanoparticles is highlighted, which is useful for the rational design of this type of nanocarriers and propose solutions to the challenges involved in their formulation.