The pH plays an important physiological role in nature and humans. pH varies from 1 to 8 in human organs with tight regulation in blood and epithelia of barrier organs. The physiological pH of the stratum corneum is 4.1-5.8 and several mechanisms contribute to its formation: filaggrin degradation, fatty acid content, sodium-hydrogen exchanger (NHE1) activation and melanosome release. First, the acidic pH of the stratum corneum was considered to present an antimicrobial barrier preventing colonization (e.g. by Staphylococcus aureus and Malassezia). Later on, it was found that the pH influences skin barrier function, lipid synthesis and aggregation, epidermal differentiation and desquamation. Enzymes of ceramide metabolism (e.g. β-glucocerebrosidase or acid sphingomyelinase) as well as proteases (e.g. chymotryptic enzyme or cathepsin D linked to epidermal differentiation and desquamation) are regulated by the pH. Experimental disruption of the physical barrier leads to an increase of pH, returning to normal levels only after many hours. Inflammatory skin diseases and diseases with an involvement of the epidermis exhibit a disturbed skin barrier and an increased pH. This is known for atopic dermatitis, irritant contact dermatitis, ichthyosis, rosacea and acne, but also for aged and dry skin. Normalizing the pH by acidification through topical treatment helps to establish a physiological microbiota, to repair skin barrier, to induce epidermal differentiation and to reduce inflammation.

pH in nature, humans and skin.

A detailed review of the literature is presented in attempts to emphasize several advantages of solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC) for cosmetic applications. Examples of several actives are given and the main features of the solid core of SLN and NLC for topical delivery of cosmetics are discussed. Lipid nanoparticles have been more and more explored in pharmaceutical technology, showing superior advantages for topical purposes over conventional colloidal carriers.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1468-2494.2008.00433.x

Cosmetic features and applications of lipid nanoparticles (SLN, NLC).

Metal oxides based electrochemical pH sensors: Current progress and future perspectives

The process of cutaneous wound healing comprises three overlapping major phases: inflammation, proliferation and tissue remodelling. However, while mechanisms are studied scientifically on the cellular and subcellular level, there is still a lack of knowledge concerning basic clinical parameters like wound pH or pO2. It could be proven that wound healing is affected by wound pH changes as they can lead to an inhibition of endogenous and therapeutically applied enzymes. Besides, the conformational structure of proteins and their functionality in wound healing is altered. Furthermore, the likelihood of bacterial colonization, which is a common problem in chronic wound pathogenesis, is affected by wound pH alterations. However, wound pH is rarely taken into account in current wound therapy strategies. A routinely performed monitoring of the wound pH and a subsequently adapted wound therapy would most possibly improve chronic wound therapy.

The impact of the pH value on skin integrity and cutaneous wound healing

The complex structure of human skin and its physicochemical properties turn it into an efficient outermost defence line against exogenous factors, and help maintain homeostasis of the human body. This role is played by the epidermal barrier with its major part - stratum corneum. The condition of the epidermal barrier depends on individual and environmental factors. The most important biophysical parameters characterizing the status of this barrier are the skin pH, epidermal hydration, transepidermal water loss and sebum excretion. The knowledge of biophysical skin processes may be useful for the implementation of prophylactic actions whose aim is to restore the barrier function.

https://www.termedia.pl/Journal/-7/pdf-24287-10?filename=Structural and biophysical.pdf

Structural and biophysical characteristics of human skin in maintaining proper epidermal barrier function.

Variable skin pH values are being reported in literature, all in the acidic range but with a broad range from pH 4.0 to 7.0. In a multicentre study (N = 330), we have assessed the skin surface pH of the volar forearm before and after refraining from showering and cosmetic product application for 24 h. The average pH dropped from 5.12 +/- 0.56 to 4.93 +/- 0.45. On the basis of this pH drop, it is estimated that the 'natural' skin surface pH is on average 4.7, i.e. below 5. This is in line with existing literature, where a relatively large number of reports (c. 50%) actually describes pH values below 5.0; this is in contrast to the general assumption, that skin surface pH is on average between 5.0 and 6.0. Not only prior use of cosmetic products, especially soaps, have profound influence on skin surface pH, but the use of plain tap water, in Europe with a pH value generally around 8.0, will increase skin pH up to 6 h after application before returning to its 'natural' value of on average below 5.0. It is demonstrated that skin with pH values below 5.0 is in a better condition than skin with pH values above 5.0, as shown by measuring the biophysical parameters of barrier function, moisturization and scaling. The effect of pH on adhesion of resident skin microflora was also assessed; an acid skin pH (4-4.5) keeps the resident bacterial flora attached to the skin, whereas an alkaline pH (8-9) promotes the dispersal from the skin.

Natural skin surface pH is on average below 5, which is beneficial for its resident flora

Characterization of the skin's natural state is very important for understanding skin functions and describing disturbances of skin function. In the field of cosmetics and dermatologicals the natural pH of the skin surface plays an important role in the development of products with a skin neutral pH. In a multicenter study the natural pH of the skin surface (exception of underarm and genital areas), was to be measured. It was to be determined whether gender-specific differences in the pH occur. The skin surface pH was measured on the inside of the forearms in 222 volunteers (147 female, 75 male) using the skin pH meters of the respective institutes. The pH was measured in triplicate on adjacent areas 24 h after the last contact of the measuring site with water, topical applications and an intensive washing procedure. The values of the skin surface pH were 4.9 (arithmetic mean). The 95% interval was between 4.1 and 5.8. The pH on the forearm of the male volunteers was slightly lower than that of the females. The difference was statistically significant (p < 0.01; 4.8 to 5.0). The authors suggest that the GDCh recommendations for the claims »skin neutral pH«, »ideal pH« or »skin-friendly pH« should be revised and propose a pH range of 4.1 to 5.8 for the main body areas excluding the underarm and genital areas.

Multicenter study on measurement of the natural pH of the skin surface

The UVA protection delivered by sunscreens is an issue of increasing importance due to the increasing knowledge about UVA-induced skin damage. In Europe there is no officially accepted method available to determine the degree of UVA protection. Therefore, the objective of the present study was to design a protocol combining the merits of an in vitro model, which are simple and reproducible, with aspects known to be relevant from in vivo studies. The principle is: an UV-transparent support to which the test product is applied, a (pre)irradiation and a transmission measurement. Transpore(R) tape (standard support for SPF determinations) was found to be incompatible with many preparations on prolonged contact times. Roughened quartz was adopted as a suitable alternative. Transmission measurements on this support are not reliable with a layer of 2 mg cm(-2) (standard for SPF) due to detection limitations of spectrophotometers, hence a reduced layer of 0.75 mg cm(-2) was adopted. Overall, it is very difficult to apply products in a reproducible thin layer on appropriate substrates. As a consequence, absolute parameters derived from the transmission profile show relatively large dispersion, whereas relative parameters, such as critical wavelength lambda(c)[1] or UVA/UVB ratio are much less sensitive to unavoidable variations in layer thickness. An increase in deviations was observed when the samples were irradiated before measurement. It is crucial to control the output carefully (spectral distribution and even more importantly, irradiance and dose delivered) of the light source. By doing so and also taking into account the previous learning steps, a protocol was drafted and tested in a ringtest (four samples in six laboratories). The results are encouraging and show that if relative parameters (e.g. lambda(c), UVA/UVB ratio) are considered, the intra- as well as interlaboratory reproducibility is clearly better than can be obtained in vivo. In general, we describe a suitable method, which can be considered in any future official discussions about the methodology to determine UVA protection.

P. Finkel

Papers

Natural skin surface pH is on average below 5, which is beneficial for its resident flora

Multicenter study on measurement of the natural pH of the skin surface

In vitro testing to assess the UVA protection performance of sun care products.