Solid Lipid Nanoparticles: A Carrier for Modern Therapeutics

Solid Lipid Nanoparticles (SLNs) are colloidal drug delivery systems consisting of a solid lipid matrix stabilized by surfactants. They are particularly advantageous for poorly soluble drugs (BCS Class II and IV) owing to their biocompatibility, reduced toxicity, and ability to enhance stability, absorption, and controlled release. The solid lipid matrix minimizes drug migration, lowers molecular mobility, and delays lipid digestion, thereby sustaining drug release and protecting chemically sensitive compounds. SLNs can be prepared by several techniques, including high-pressure homogenization, microemulsion, solvent emulsification–diffusion, double emulsion, membrane contactor, precipitation, solvent injection, and film–ultrasound dispersion. Post-processing methods such as lyophilization and spray drying are used to improve long-term stability. Characterization involves particle size and zeta potential determination using Dynamic Light Scattering (DLS), Static Light Scattering (SLS), electron microscopy, acoustic methods, and Nuclear Magnetic Resonance (NMR). Crystallinity is assessed by Differential Scanning Calorimetry (DSC) and Powder X-ray Diffraction (PXRD), while entrapment efficiency and drug loading are quantified by direct extraction or separation followed by analytical methods such as HPLC or UV–Visible spectrophotometry. In vitro release studies commonly employ dialysis tubing and reverse dialysis, whereas ex vivo intestinal models assess permeability and absorption. Despite limitations such as gel formation and reduced drug loading, SLNs remain a promising platform for improving bioavailability and controlled delivery of therapeutic agents.