Topic overview

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Drug movement across skin is governed by concentration gradient and skin permeability, as described by Fick’s law; passage through the stratum corneum is typically rate-limiting. Transdermal candidates are generally potent drugs with suitable molecular size, dose, lipophilicity, melting point, and skin tolerability. Drugs may cross by intercellular, transcellular, or appendageal routes. Patch designs include reservoir, matrix, adhesive-dispersion, and micro-reservoir systems, with components such as backing, drug-containing layers, adhesive, and release liner. Chemical enhancers and physical methods can increase permeation. TDDS can provide steady delivery and avoid first-pass metabolism, but use is constrained by drug suitability, skin reactions, adhesion, and variability. Quality assessment includes content uniformity, adhesion, permeation, pharmacokinetics, and irritation.