A REVIEW ON SOLID LIPID NANOPARTICLES AND EVALUATION

Solid lipid nanoparticles (SLNs) are submicron colloidal carriers composed of biocompatible lipids that remain solid at room temperature and at body temperature, stabilized in an aqueous phase by surfactants. SLNs were introduced to address limitations of emulsions, liposomes, and some polymeric nanoparticles by offering improved physical stability, potential for controlled drug release, and the possibility of scale-up using high-pressure homogenization. SLNs can improve delivery of poorly water-soluble drugs, enhance local retention in topical products, and protect sensitive drug molecules; however, challenges such as limited drug loading, burst release, polymorphic transitions of lipids, and drug expulsion during storage remain important. This review elaborates SLN fundamentals (composition, structure, and incorporation models), major production methods, advantages and disadvantages, applications, and a detailed evaluation framework including particle size distribution, zeta potential, morphology, entrapment efficiency, crystallinity/polymorphism (DSC/XRD), in vitro release, permeation testing, stability, and route-specific quality requirements. Current trends emphasize moving from “classical SLN” to nanostructured lipid carriers (NLCs), applying Quality-by-Design (QbD), surface engineering, co-delivery strategies, and more predictive in vitro testing to improve robustness and translation.