Transdermal Drug Delivery Systems: Methods for Enhancing Skin Permeability and Their Evaluation

Transdermal drug delivery (TDD) is an increasingly important non-invasive method for administering active pharmaceutical ingredients (APIs) through the skin barrier, offering advantages such as improved therapeutic efficacy and reduced systemic side effects. As demand increases for patient-friendly and minimally invasive treatment options, TDD has attracted substantial attention in research and clinical practice. This review summarizes recent advances enhancing skin permeability through chemical enhancers (e.g., ethanol, fatty acids, terpenes), physical (e.g., iontophoresis, microneedles, sonophoresis), and nanotechnological methods (e.g., liposomes, ethosomes, solid lipid nanoparticles, and transferosomes). A comprehensive literature analysis, including scientific publications, regulatory guidelines, and patents, was conducted to identify innovative methods and materials used to overcome the barrier properties of the stratum corneum. Special emphasis was placed on in vitro, ex vivo, and in vivo evaluation techniques for such as Franz diffusion cells for assessing drug permeation and skin interactions. The findings highlight the importance of active physical methods, passive nanostructured systems, and chemical penetration enhancers. In conclusion, integrating multiple analytical techniques is essential for the rational design and optimization of effective transdermal drug delivery systems.