Cracking the Skin Barrier: Models and Methods Driving Dermal Drug Delivery

Dermal drug delivery is a promising alternate route of drug administration, offering localized therapeutic effects, reduced systemic effects, and improved patient compliance. However, the skin's intricate configuration, especially the stratum corneum (SC), presents formidable barriers, restricting drug permeation. This review summarizes biological, synthetic, and methodological models employed to study dermal absorption and permeability. Ex vivo human skin is a reference point, but limited availability and ethical constraints necessitate reliance on animal models, including porcine, rodent, rabbit, monkey, and even snake skin, each with unique advantages and drawbacks. Synthetic substitutes, e.g., reconstructed human epidermis and Strat-M^® membranes, provide reproducibility and economic practicality, though none fully mimic the barrier functions of human skin. Innovative analytical methods, including diffusion cells, skin-PAMPA, tape stripping, and advanced imaging techniques, enable quantitative, semi-quantitative, and qualitative insights into drug transport. Collectively, these tools support formulation optimization and aid regulatory bioequivalence assessments. However, challenges remain in correlating in vitro, ex vivo, and in vivo outcomes and in replicating the skin's dynamic physiology. This review highlights current opportunities and limitations, emphasizing the need for more physiologically relevant models to advance safe, effective, and innovative dermal drug delivery systems.