Читать книгу pH of the Skin: Issues and Challenges - Группа авторов - Страница 34

Reducing skin pH is a well-known concept for treating wounds. An acidic environment created by the use of acids, such as acetic acid, alginic acid, and hyaluronic acid helps in wound healing by controlling wound infection and enhancing epithelization and angiogenesis. Most of the pathogenic bacteria associated with infected wounds in humans need a pH value >6, while their growth is inhibited by lower pH values [33]. To keep antimicrobial activity, skin pH must be low for a relatively long time, ideally until the next treatment occurs. On the other hand, too much acid is toxic for cells and reduces wound healing. Therefore, medium strong acids and the respective salts are used as buffers to keep the pH low for several hours. Acetic acid has a pKa of 4.75. Alginic acid is a complex compound and may have a pKa from 1.5 to 3.5. The pKa of HA carboxyl groups in hyaluronic acid is 3–4, meaning that all 3 compounds may form suitable buffers for reducing bacteria in wounds.

Reducing the pH of the skin has been suggested as a therapeutic or preventive strategy in skin diseases with a more superficial impairment. These treatments minimize bacterial and fungal colonization and reduce the amount of surface and secreted proteins, which contribute to inflammation and reduced epidermal barrier function. The application of acidic electrolytic water on epithelial surfaces has been shown to reduce S. aureus colonization in AD [34]. Therefore, emollients with a reasonable buffer capacity keeping the pH low for several hours should be used preferably.

As already mentioned, the skin has a fairly high buffer capacity. High pH in some soaps (pH of about 9) is well tolerated by many people and only repeated application in high concentration may lead to skin diseases in susceptible individuals [35]. Buffer capacity of the skin in relation to alkalinity or acidity (amount of alkali or acid) of a substance is crucial to determine skin toxicity. This means that a single or sporadic contact to low amounts or low concentration of strong acids or bases may be tolerated. On the other hand, contact with high concentrations of a strong and even repeated contact with weak acids or bases in high concentration may lead to skin problems, most often resulting in an irritant contact dermatitis. The buffer capacity is reduced by repeated assaults, for example, by washing out the buffer components by water and detergent.

To normalize an increased SC pH, not only the pH of a topically applied formulation, but more importantly, the buffer capacity formed by the concentration and the amount of the weak acid should be considered. It has been shown that treatment with water in oil emulsion with a citric acid/citrate buffer resulted in a faster normalization of the skin pH after washing with an alkaline soap [18]. The buffer capacity of a topical applied preparation is very important but is seldom exploited. Furthermore, many topically applied compounds even have a pH higher than that of the normal skin pH [36].

Since many studies showed that the pH is important for skin physiology, products have been developed to influence the skin’s pH. Several topical skin care products are adjusted to pH 5.4 or 5.5 aimed to preserve the “physiological” skin pH. However, adult skin exhibits a pH range of 4.1–5.8 (mean 4.9) and aged skin and skin in inflammatory diseases exhibit an increased pH (sometimes even above pH 6). Therefore, products with a more acidic pH 4 to power counteract an increased pH were introduced. pH 4 emollients have been used in acne and aged skin and favorable effects were seen. Those pH 4 emollients reduce the skin’s pH, improve skin barrier, reduce skin dryness, and improve the physiological bacterial flora as compared to formulations with a pH of 5.8–6.0 [37–39]. A glycolic acid-containing water-in-oil emulsion with pH 4 reduced the skin pH in healthy, elderly, and diabetic individuals [40]. In preliminary studies, water-in-oil formulation with pH 4 decreased skin pH, restored disrupted epidermal barrier, and improved lipid lamellae structure in aged skin (Angelova-Fischer et al. [37, 41], Kilic et al. [42]). Glycolic acid influenced desquamation-regulating proteinases cathepsin D-like (CD) and chymotrypsin-like (SCCE) proteinases in human SC [43]. Glycolic acid has a pKa value of 3.6–3.8, which makes it a very good candidate for buffers with a pH of 4 at a high buffer capacity.

The presence of impaired skin barrier in newborns is an initial step in the atopic march. The daily use of moisturizers on newborns and infants (from birth to about 6 months of age) is potentially helpful in preventing the development of atopic dermatitis [44–46]. The skin pH of newborns is also increased and thin skin should have a low buffer capacity. Thus, the usage of emollients with a low pH and an appropriate buffer capacity is also advisable during the first months of life. Also, the importance of acidification in patients already affected by atopic dermatitis has been highlighted [47].

To summarize, emollients with a pH more acidic than the “physiologic” one, for example, pH 4, and an appropriate buffer system may be suitable for several kinds of skin conditions and skin diseases that exhibit an increased pH. Dry skin, sensitive dry skin, eczema of the various types, psoriasis, and acne vulgaris should be treated with pH-adjusted and buffered emollients.