Microemulsions as Advanced Drug Delivery Systems: Formulation, Evaluation, Applications, and Future Perspectives

Microemulsions are thermodynamically stable, isotropic liquids of oil, water, surfactant, and co-surfactant with nanoscale droplets (10–100 nm) that form spontaneously and improve drug solubility and bioavailability. This review summarizes their structural classification (Winsor I–IV), key formulation components, and preparation methods—high-pressure homogenization, ultrasonication, phase inversion, and spontaneous emulsification—comparing scalability and energy needs. Choice of oils and surfactants dictates compatibility, stability, and efficacy. Applications span oral, parenteral, topical, ocular, nasal, and tumor-targeted delivery. Challenges include packaging, storage, and environmental sensitivity. Future trends emphasize green formulations, stimuli-responsive systems, and targeted platforms for clinical translation.