The drug release pattern is defined by the drug incorporation models of SLNs, which have been categorized as follows (Fig. 12.1) [51]:

Solid solution model: When the cold homogenization technique is used to generate nanoparticles, the drug is molecularly dispersed in the lipid matrix and has strong interactions with the lipid matrix. No surfactant is required in this model.

Drug incorporation models of solid lipid nanoparticles

Figure 12.1 Drug incorporation models of solid lipid nanoparticles: (A) drug-enriched shell model, (B) drug-enriched core model, (C) solid solution model. (Modified from Shah R, Eldridge D, Palombo E, Harding I. Lipid nanoparticles: production, characterization and stability. Springer; 2015.)

Drug-enriched shell model: When the recrystallization temperature of lipid is attained, a solid lipid core is formed with drug molecules concentrating on the surface of SLNs.

Drug-enriched core model: Upon cooling, drug that is dissolved in a lipid melt at or near its saturation solubility precipitates before lipid recrystallization due to supersaturation. A membrane of recrystallized lipid is formed as the nanoemulsion is further cooled.

These models give an insight of modulating drug release by controlling the SLN structure during formulation. This in turn requires the necessary technical know-how for SLN production and fine tuning to achieve the desired drug release profile.

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