{'fraction_non_alphanumeric': 0.016500217108119844, 'fraction_numerical': 0.0, 'mean_word_length': 5.295081967213115, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Percutaneous Absorption: In Vivo Methods in Humans and Animals

Is skin viability important in percutaneous absorption? The concept of skin as a passive membrane has led to the domination of the study of percutaneous absorption by laws of mass action and physical diffusion. This concept has also led investigators to use skin excised from cadavers (human and animal) and then to isolate skin sheets or sections physically (e.g., by freezing or heat separation) and chemically and determine chemical diffusion across these treated tissues. A consequence of this earlier concept was the designation of the stratum corneum as the barrier to percutaneous absorption. Some compounds such as low-molecular-weight alcohols were studied; the barrier properties of the isolated stratum corneum were demonstrated for these chemicals. It was then assumed that the stratum corneum is the primary barrier for all compounds. The need to study percutaneous absorption stems from concerns of dermatotoxicity. by which compounds pose a threat to human health, and dermatopharmacology, since drugs need to be delivered into and through the skin to treat disease both locally (skin disease) and systemically (transdermal delivery). Most compounds and defined drugs that are of interest and concern in dermatotoxicology and dermatopharmacology are lipophilic. The stratum corneum, which is the first and often most important barrier to percutaneous absorption, is a lipid-saturated tissue that is like a sink to topically applied lipotropic materials; its function more closely approximates that of a sponge capable of absorbing a quantity of material, limited only by the solubility of the chemical in sebaceous and epidermal lipids. The chemical and physical properties of the topical vehicle and the barriedsink properties of the living stratum corneum determine the initial absorption of compounds into the skin. The vitality of the living skin will determine the metabolism, distribution, and excretion of the compounds through the skin and the body. This paper briefly defines and discusses some factors that determine the nature of in vivo percutaneous absorption. We discuss only some of the methodology and its limitations and point out those steps of in vivo percutaneous absorption that we presently understand.