Penetration enhancement of transdermal delivery--current permutations and limitations

In order to achieve enhanced topical drug delivery, it is necessary to make physical or biomolecular structural alterations to the stratum corneum by suitable techniques or by the use of specific chemical agents or drug carriers. The role of the chemical penetration enhancer is to reversibly alter the barrier properties of the stratum corneum by disruption of the membrane structures or by maximizing drug solubility within the skin. Alternatively, permeant delivery to the dermal vasculature using one of several physical methods to reduce diffusional resistance within the skin may be used to promote drug penetration. In the present article, we summarize the major facets of the diverse spectrum of penetration enhancement techniques that include modification of the stratum corneum, lipid-based delivery systems, drug/vehicle interactions, bypassing the stratum corneum, and electrical techniques of enhancement.