Ninety years ago - in 1928, the term acid mantle was coined by the physicians Heinrich Schade and Alfred Marchionini in Kiel, Germany. A decade later Marchionini and several coworkers published 5 scientific communications in the Klinische Wochenschrift on "Der Säuremantel der Haut und Bakterienabwehr" (acid mantle and defense against bacteria). They described experimental detail, documented age- and body site as well as skin disease-dependent skin pH shifts, and discussed the significance of the pH and bacterial growth on the skin. In their fourth and fifth communication, they made the first connection between the altered quantitative and qualitative bacterial growth in pathologically modified skin and the shifted skin pH and attributed it partly to the gap in the acid mantle (pathologische Lücke des Säuremantels). They also investigated the pH of several topical dermatologic preparations and concluded that their benefit can at least partly be attributed to their acid character and recommended the systematic investigation of acid treatments in dermatology. At that time, the physiologic role of the acid skin surface was thought to be a protective mechanism against invading organisms. Hence, it seemed reasonable to allocate protection to an easy and conceivable term such as "mantle." Today, "acid mantle" as a term is still a very suitable metaphor to illustrate the protective quality of the "acid" in the skin and the term has become part of colloquial speech. In the meantime, our understanding of the skin pH has broadened, and we know that the acid character and its gradual change within the skin also help to orchestrate epidermal differentiation and corneocyte shedding. For many more biochemical processes within the skin, the compartmental pH is crucial, for example, in pigmentation, ion homeostasis, epidermal (stem) cell behavior, and so on. The often existing difference between the H+ concentration of extra- and intracellular as well as subcellular compartments establishes an ionic, electric, and/or osmotic driving force; hence, H+ concentration per se acts as an extra-, intra,- and subcellular signaling modality affecting and controlling many cellular functions. One may even consider pH a universal signal and effector. It is therefore also no surprise that skin pH shifts have been observed in various skin pathologies. More recently, in carefully controlled trials (acne, atopic dermatitis, incontinence-associated dermatitis, aged skin), the benefits of targeted skin acidification have become evident and the use of topical preparations with reduced pH may be recommended. The currently prevailing formulation concepts for direct acidification are based on a reduced pH of the hydrophilic product phase in combination with a buffer with a sufficiently high buffering capacity within the vehicle.
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