transepidermally

Transepidermally is an adverb describing movement or transfer that occurs through the epidermis, the outermost layer of the skin. It is used in dermatology and pharmacology to discuss how substances, water, or other molecules traverse the epidermal barrier. The term often appears alongside transepidermal water loss (TEWL), a standard measure of skin barrier function.

Etymology: from Latin trans- meaning through and epidermis, the outer skin layer. It is related but not identical to transdermal, which emphasizes entry into the systemic circulation. Transepidermal describes the path across the epidermal barrier, whether within the skin layers or into the dermis.

Mechanisms: movement transepidermally typically occurs by passive diffusion, primarily through intercellular lipids in the stratum corneum, though routes via hair follicles or sweat ducts can also contribute. The extent of transepidermal transfer depends on factors such as molecule size, polarity, hydration, skin integrity, and occlusion.

Applications and considerations: In cosmetics and pharmaceuticals, transepidermal transfer is a key factor in evaluating irritation, permeability, and efficacy. Measurements of TEWL or in vitro skin permeation assays are used to study transepidermal transport. Clinically, enhanced transepidermal absorption can occur in damaged skin, inflammatory conditions, or with penetration enhancers.