Abstract

The physiological skin surface pH is just below 5. With age the skin surface pH increases up to 6. An increased pH correlates with reduced barrier integrity/cohesion. The present pilot study assesses possible normalization of an increased skin surface pH of the elderly and improvement of barrier function via application of ≈pH 4.0 skin care products. Baseline skin surface pH was determined in elderly (80+ years old; n = 15) compared to middle aged adults (31 - 50 years old; n = 15). The effect of o/w emulsions at pH-values of 3.5, 4.0, 4.5 and 5.5 on the skin surface pH was determined in both groups. Further, the effect of a 4-week treatment with a pH 4.0 skin care product on the skin surface pH, skin hydration and barrier integrity was assessed. Thirteen elderly females were involved in this home-in-use test. Increased baseline skin surface pH of the elderly normalizes to the physiological pH of 4.5 - 5.0 over 7 hours after single application of o/w-emulsions with a given pH of 3.5 or 4.0. A 4 week treatment employing the pH 4.0 skin care product improves the epidermal barrier integrity of the elderly significantly (p = 0.005). Reduction of the increased baseline skin surface pH of the elderly is accompanied by improved epidermal barrier integrity. Skin care products for the elderly have to be adjusted in the pH range of 3.5 to 4.0.

Keywords

Skin Aging, Skin Care, Skin Surface pH, Epidermal Barrier Function

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1. Introduction

The physiological skin surface pH has been evaluated repeatedly. Even though the skin surface pH is still defined to be 5.4 - 5.9 [1], recent multicenter studies postulate a skin surface pH just below 5.0 appeared in 330 or 222 subjects [2-3] . The acid pH of the skin surface, known as the “acid mantle” [4], and the pH gradient [5] of the stratum corneum (SC), regulates at least three epidermal functions: antimicrobial barrier, permeability barrier and barrier integrity/cohesion [6]. The relevance of an acid skin surface pH as an antimicrobial barrier has been demonstrated repeatedly [2,7-10] . At a pH of 5.0 growth of pathogenic bacteria, like S. aureus, is inhibited in vitro and in vivo. Species of the normal resident flora are positively affected by the physiological, acidic milieu [7-10] . Further, the dissociation of endogenous bacteria from skin surface is enhanced at alkaline conditions [2].

More recent studies provide insight into the skin surface pH performing a key role in SC functions as permeability barrier homeostasis and SC integrity/cohesion (the converse of desquamation) [6]. Permeability barrier homeostasis depends on the pH gradient throughout the SC. When acute perturbed skin sites are exposed to a neutral pH buffer [11] or to “superbases” [12], barrier recovery is delayed. These pH-induced barrier abnormalities are associated with an inhibition of two lipid processing enzymes, β-glucocerebrosidase (β‑GlcCer’ase) and acid sphingomyelinase (aSMase), which exhibit low pH optima and transfer polar lipids as, glucosylceramide and sphingomyelin, to the non-polar barrier organization [13-16] .

Furthermore, SC integrity/cohesion depends on the activity of pH-dependent serinproteases (SP) of the kallikrein family, such as kallikrein‑5 (KLK5, SC tryptic enzyme, SCTE), kallikrein‑7 (KLK7, SC chymotryptic enzyme, SCCE) and kallikrein-8 (KLK8, neuropsin) [17]. KLK5 and KLK7 play a key role in integrity/cohesion in desquamation, which exhibit neutral pH optima [18-19]. Desmoglein 1 (DSG1), desmocollin 1 (DSC1) and corneodesmosin (CDSN) are extracellular protein structures of corneodesmosomes. Desquamation is linked to the degradation of DSG1, DSC1 and CDSN by kallikreins [20] . The acidic SC pH regulates KLK5 and KLK7 by reducing, but not completely inhibiting, their activities and by maintaining physiological desquamation. Because of the pH-gradient from 7 (Stratum granulosum (SG)/SC interface) to below 5 (skin surface) the activities of KLK5 and KLK7 change accordingly [6,17] .

An increased skin surface pH has been described repeatedly in the elderly: Zlotogorski et al. [21] showed significant higher skin ph values in the over-80 group in comparison to the younger groups. Thune et al. [22] demonstrated a higher pH (5.7 ± 0.15) in the elderly (67 ‑ 95 years, mean age 81) and revealed a positive correlation between age and pH. These results are in line with those of Wilhelm et al. [23], who found higher values on the ankles of elderly subjects (70.5 ± 13.8 years). Man et al. [24] retrieved pH values of approx 6.0 on the forearms and foreheads of male and female subjects in the age group over-70. Moreover, in aged mice an elevated pH was demonstrated at all levels throughout the SC by Choi et al. [25] .

While an elevated pH inhibits the activity of the lipid-processing enzymes β‑GlcCer’ase and aSMase, which results in inadequate formation of lamellar lipid bilayers, the activity of KLK5 and KLK7 is increased, which results in premature degradation of corneodesmosomes. The functional consequences are 1) delayed barrier recovery [11-12] ; 2) reduced barrier integrity/cohesion [12,26] and 3) negative effects on the skin microbiota [2]. In the context of aged skin, delayed barrier recovery [25,27] and an impaired barrier integrity [25, 27-28] has been demonstrated in the elderly.

The aim of our pilot study was to investigate possible normalization of an increased skin pH of the elderly and improvement of barrier function by application of skin care products with a pH 3.5 - 4.0.

2. Materials and Methods

2.1. Volunteers

Primarily, the effect of a single application of a given o/w emulsion on the skin surface pH was assessed in ten elderly females (range 81 ‑ 95, mean 85.3 ± 4.8) and ten young females (31 ‑ 48, 39.1 ± 6.4). A time course of 7 hours after single application of the given o/w emulsion was performed on five elderly females (81 ‑ 87, 82.6 ± 2.5) compared to five young females (31 ‑ 49, 38.2 ± 8.2). Secondly, the effect of a 4-week treatment with a pH 4.0 skin care product on the skin surface pH, skin hydration and barrier integrity was assessed. Thirteen elderly females (80 ‑ 90, 84.5 ± 3.9) were involved in this homein-use test.

The study was approved by the local ethics committee of the University of Osnabrück (Germany) which gave it unanimous approval. Written, informed consent was obtained from all volunteers. The volunteers were healthy without any skin diseases or lesions on the test sites.

2.2. Material

Skin surface pH was measured with a flat glass electrode (Mettler-Toledo, Giessen, Germany) attached to a pH meter (Skin-pH-Meter® PH905, Courage & Khazaka, Cologne, Germany) and SC hydration was determined by a capacitate-based corneometer (Corneometer® CM825, Courage & Khazaka, Cologne, Germany). Furthermore, barrier function was determined by measuring transepidermal water loss (TEWL). TEWL was measured using an open chamber device (Tewameter® TM300, Courage & Khazaka, Cologne, Germany). For the evaluation of barrier integrity strippings on the volar forearm were necessary, for which we used a commercially available adhesive tape (3M Blenderm surgical tape, 3M Deutschland, Neuss, Germany).

2.3. Test Products

The different pH values of the tested o/w emulsions were adjusted with citric acid (<0.5%). The test products were A (pH 3.5), B (pH 4.0), C (pH 4.5) and D (pH 5.5). According to INCI ingredients were aqua, arachis hypogaea oil, prunus amygdalus dulcis oil, simmondsia chinensis seed oil, pentylene glycol, cetearyl alcohol, glyceryl stearate, glycerin, oenothera biennis oil, persea gratissima oil, persea gratissima oil unsaponifiables, prunus domestica seed oil, potassium palmitoyl hydrolyzed wheat protein, rosmarinus officinalis leaf extract, panthenol, xanthan gum, citric acid, tocopherol.

2.4. Experimental Design

The present study was divided into two sequential parts. The first part (skin surface pH time curves) employed the evaluation of a single application of a given o/w emulsion with different pH values on the skin surface pH of the volar forearm of elderly (80+ years old) in comparison to middle aged adults (31 - 50 years old). The second part (functional assessment) focused on the evaluation of the long-term effect of a pH 4.0 o/w emulsion on skin pH, skin hydration and barrier integrity after a 4‑week treatment. The assessment of the skin pH time curves was carried out in a nursing home in Osnabrück (Germany) at room temperature (21.7˚C ± 1.4˚C; relative humidity 46.2% ± 3.8%) and the functional assessment at the volunteer home (temperature 20.2˚C ± 2.6˚C; relative humidity 49.4% ± 6.5%).

2.5. Skin pH Time Curves

There was a 12‑hour pre-trial period, during which volunteers were requested to refrain from applying any skin care products and tap water to the test sites. The control and treated sites (50 × 60 mm) were attributed at random to each volunteer on the midvolar aspect of each forearm with a 4-cm distance to the cubital fossa. To get skin pH time curves we first measured the basal pH on the test sites on the volar forearm. We next applied 0.09 g (= 3 mg/cm²) of the test products to the respective test site with a plastic syringe (Omnifix®‑F, B. Braun Melsungen AG, Melsungen, Germany). The cream was spread with a gloved finger in a standardized way. For the 2‑hour period, the skin surface pH was measured every twenty minutes after application. For the 7‑hour period, the skin surface pH was measured after twenty minutes and thereafter every 2 hours. The point of measuring of every test area was changed to avoid a wash‑out effect from the water drop from the skin pH meter.

2.6. Functional Assessment

To evaluate the functional effects (functional assessment) of an acid cream on aged skin, i.e. aged permeability barrier, a 4‑week home-in-use-test was performed. The volunteers were instructed to use the test product (o/w cream, pH 4.0) twice daily on the test sites. In this study the volar aspect of the forearm was used to determine barrier integrity before and after application of the test product. To assess changes in skin surface pH and skin hydration the forehead (photoaged skin, pa) and the medial upper arm (chronological aged skin, ca) were involved and compared. Before any product treatment the basal skin surface pH and skin hydration was measured (Basis). On the forearm the barrier integrity, i.e. number of strippings to disturb the barrier, was evaluated according to Gunathilake et al. [29]. TEWL values were measured after every five tape strippings and the strippings were repeated until TEWL increased by three fold. The practical procedure of the stripping itself was performed according to Dickel et al. [30] . Tape strips were gently pressed downward by fingertips for about 2 seconds and tape was removed on one quick movement at an angle of 45˚ in the direction of adherence. For each single strip a new tape cut was used and positioned on exactly the same skin area. The basal barrier integrity was assessed on randomized change of left or right forearm. The barrier integrity after the 4‑week treatment (day 28) was assessed on both forearms.

2.7. Statistical Analysis

Statistical analysis was performed using SPSS software version 18.0 (SPSS, Chicago, IL, USA). Differences between the test sites were tested for their statistical significance using the Mann‑Whitney U‑test for not normally distributed non-paired data. To compare the nonparametric paired values of each test site the Wilcoxon signed‑rank test was used. The significance level was set at p ≤ 0.05. All measured and calculated parameters are reported as arithmetic mean and standard deviation (mean ± SD). The Skin surface pH of each volunteer was converted to the hydrogen ion concentration. The arithmetic mean of the hydrogen ionic concentration was calculated for the respective group and this in turn converted to a pH [31].

3. Results

3.1. Skin pH Time Curves

Baseline skin pH values, which are significantly (p < 0.006) higher in the elderly compared to middle aged adults, are presented in Figure 1. In the context of a 2‑hour measurement ten very old females were compared with ten middle aged adults and in the context of the 7‑hour measurement five very old were compared with five middle aged females. The baseline skin surface pH of the volar forearm was 5.50 ± 0.54 in the fifteen elderly and 4.98 ± 0.39 in the fifteen younger volunteers (Figure 1).

Figure 2(a) reveals skin surface pH values over the duration of 2 hours after single application of an o/w emulsion with a pH of 3.5, 4.0, 4.5 and 5.5 on the volar

Conflicts of Interest

The authors declare no conflicts of interest.

References