Research Article

Skin Surface pH and Topical Emollient: Fact or Artifact?

Miranda A. Farage^1*, William Hood¹, Mauricio Odio¹, Enzo Berardesca² and Howard Maibach³

¹The Procter & Gamble Company, Cincinnati, Ohio, USA
²San Gallicano Dermatological Institute, Rome, Italy
³Department of Dermatology, University of California, San Francisco, USA

*Corresponding Author:   Dr. Miranda A. Farage, The Procter & Gamble Company, Cincinnati, Ohio, USA, Tel: + 1 513 634 5594; Fax: + 1 (866) 622-0465; Email: farage.m@pg.com Submitted: 10-20-2015 Accepted: 11-20-2015  Published: 12-01-2015

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Article

Abstract The body skin pH can usually vary from 4.0 to 7.0 depending on location. The presence of natural acidic compounds on the skin surface helps maintain the skin’s physicochemical properties as well as its protective functions. Since the slightly acidic pH of the skin is extremely important for the skin’s protective function, the skin is widely known as “acid mantle.” Factors such as age, race, gender, body sites, biochemical differences, and even washing affect the pH of the stratum corneum. Recent clinical studies using an emollient-base finish product using the traditional way of measuring skin pH produced results that indicated an apparent increase in skin pH. The apparent pH increase with these products is most probably an artifact of the skin pH measurement technique. Our findings show that certain petrolatum-based emollients and components could create a protective barrier and help maintain the healthy acidity of the skin. Our work provides new evidence of emollients helping to stabilize skin pH in its natural balanced state rather than affecting it. This new learning should be taken into consideration by other researchers in the area of skin pH as well as in clinical studies to avoid misleading results. Keywords: Skin pH; Acid Mantle; Stratum Corneum; Emollient Barrier; Permeability Barrier; Protective Barrier; Under the Skin Introduction Skin, the largest organ that covers the exterior of the body, forms a protective barrier against the environment and its overall physiology is maintained by its physicochemical properties such as structure, hydration, temperature, pH, oxygen and carbon dioxide gradients. Skin surface pH influences the stratum corneum’s lipids composition and hydration, the skin’s microbiota, and barrier function. When a pH electrode is brought into contact with normal skin surface, the pH of the liquid on the electrode becomes more acidic, with an average pH reading of 5 to 6. Schade and Marchonini underlined acidity and the skin’s protective feature and called it the “acid mantle” in 1928 [1]. Endogenous and exogenous factors such as eccrine and sebaceous secretions, anatomic sites, moisture, proton pumps, genetic predispositions, and age influence the skin pH [2, 3]. Active proton pumps (e.g. NHE1 – sodium/hydrogen anion exchange proteins) acidify the intracellular space in the lower stratum corneum [3, 4]. Lactic acid produced, by passive processes, acidifies the superficial layers of the skin [2]. Other important components of passive metabolic processes such as free fatty acids generated by lipases, cholesterol  ulfate, urocanic acid, and pyrrolidone carboxylic acid contribute to the acid mantle [2, 3]. Oil and sweat excreted by sebaceous and sudoriferous glands on the pores of the skin also have pH of about 5.5 [5]. The dry and densely packed top layer of the skin is the first line of defense against many bacteria. Salty secretions from sweat glands create a hyperosmotic environment which is unfavorable for bacteria. Acidic mantle also protects against colonization of pathogenic bacteria, such as Staphylococcus aureus, that grow best at neutral pH [6]. Factors, such as age, race, gender, body sites (see Table 1), and biochemical differences affect the overall pH of the skin. Newborn baby’s body skin pH is usually around 7 (neutral) and, within a month, becomes acidic similar to adult skin. Higher skin pH in infants may be due to the different chemical composition of the skin lipids [7]. Normal skin pH remains unaltered between 18 and 60 years of age. In older age group, skin pH increases in both men and women [8, 9].   Cutaneous pH plays an important role in maintaining the normal bacterial flora of the skin and preventing pathogenic invasions [10]. Under normal skin pH of 5.5, growth of Propionibacterium acnes is at its minimum; however a slight increase in the pH results in increased growth of P. acnes [11]. Changes in skin pH from acidic to alkaline are also related to development of candidal infections [12], atopic dermatitis, and increased colonization of S. aureus [13, 14]. The higher physiological pH in the axilla region promote growth of local flora, which in turn creates underarm odor [15, 16]. Application of a deodorant product reduces the axillary pH leading to inhibition of growth of indigenous bacteria [15]. EMOLLIENTS/DERMATOLOGICAL PRODUCTS Stratum corneum has attracted the most attention in the area of cutaneous biology. Stratum corneum maintains skin homeostasis by providing an impermeable barrier to the inward and outward diffusion of substances, especially toxic exogenous chemicals [17, 18]. Elias elevated the stratum corneum as the key component of the diverse biological functions of the integument [19]. All stratum corneum layers contribute to its barrier properties [20]. Richter etal employed cryofixation and scanning electron microscopy to show three distinct hydration zones within the horny layer [21]. The outermost zone, where desquamation occurs, showed massive swelling; whereas, the innermost granular layer swelled to more than double its normal thickness, mainly due to extensive water inclusions between adjacent cell layers. The middle zone, which remained compact without any water pools, was believed to be the permeability barrier. Permeability can be manipulated by chemical and physical techniques [22]. The primitive topical therapies were considered to be efficacious only when they would stain, stink, and/or sting. Over recent decades, understanding the intricacies of the barrier properties of stratum corneum has revolutionized topical therapies. They are more effective and safe; free of allergens, fragrances and preservatives; and with less stinging, burning, and irritant effects. Emollients, widely used since early nineteenth century, refer to an oily substance, such as ointment or cream, used to treat rough, scaling, xerotic, erythematous, sometimes pruritic conditions to make the skin flexible, soft, and agreeable to touch and sight [17]. More recently, the term “moisturizer” is used for topical preparations that moisten and hydrate dry skin. The terms “emollient” and “moisturizer” are used interchangeably for a variety of formulations, including the ones that go beyond moistening and softening. Traditional moisturizers, such as lanolin and petrolatum, remain trapped in the stratum corneum and do not reach viable epidermis. Newer formulations, which contain physiologic ceramide-dominant lipids that penetrate the epidermis, get into keratinocytes and are then secreted into the intercellular lipid domains of the stratum corneum, thereby repairing the leaky barrier [23]. “Barrier repair” creams have become recently popular moisturizers.     Figure 1. Schematic presentation of emollient/lotion on the skin and its barrier skin effect. Importance of pH for permeability barrier homeostasis of stratum corneum was first witnessed by the delay in barrier recovery when disrupted skin sites were immersed inneutral pH buffers [24]. An acidic pH is critical for barrier homeostasis. Two key lipid-processing enzymes, β glucocerebrosidase (which generate ceramides from glucosylceramide) and acidic sphingomyelinase (the sphingomyelin precursor), exhibit low pH optima [25, 26]. Acidic pH also impacts lipid-lipid interactions in the horny lamellar bilayers [27]. Thus, an acidic stratum corneum clearly promotes integrity and cohesion of the skin. Hence, an increased skin surface pH could adversely affect permeability barrier homeostasis and stratum corneum’s integrity and cohesion [5]. A major component in the emollient-based finish used on the products is petrolatum. Petrolatum is hydrophobic and imparts a lower surface energy to the skin, thereby repelling aqueous solution at its surface. Emollient from the product gets transferred onto the skin and forms a barrier (See Figure 1), which could potentially prevent the pH probe and the aqueous solution layer on the probe surface from contacting the acidic compounds on surface of the skin. Figure 2. Changes in measured skin surface pH after use of emollient-based and no emollient-based finish pads (Fig. 2A) and emollient-based and no emollient-based pantiliners (Fig. 2B) in the Behind-the-Knee test model. * significant difference (p<0.05) between emollient-based finish and non-emollient-based finish pads (mean after 6hr treatment for emollient based finish pad vs. non-emollient pad were 5.4±0.08 and 6.01 ± 0.07, respectively) or pantiliner (mean after 6hr treatment for emollient based finish pantiliner vs. non-emollient pantiliner were 5.6 ± 0.07 and 5.00 ± 0.07, respectively). Blue bars represent baseline pH data and red bars are mean pH values after for each mean(days 1-5) of product wear. If the acidic materials on the surface of the skin cannot dissolve in the aqueous solution layer on the pH electrode, then the measured pH would not be reflective of the skin pH, but rather of the emollient layer on the surface of the skin or the neutral aqueous solution last used to rinse the probe. In two clinical studies with human subjects using our feminine-care emollient products, it was observed that after application of emollient-based finish pads (and pantiliners) there was a small, but statistically significant, increase in measured skin pH (Figures 2A and 2B). Even though the pH values after emollient-base finish product use were small and still in the normal physiological range, this was an interesting finding that we chose to investigate. We hypothesize that the measured pH after emollient-product use may not indicate the actual skin pH. Materials and Methods We conducted laboratory experiments to test our hypothesis and investigated the effect of emollient on the measured pH of skin, skin-mimic collagen film, and solutions in contact with emollient. (See Figure 3). The same emollient (Fem- Care) was used all through the listed experiments. The pH meter was Skincheck™ meter available from HANNA Instruments, 584 Park East Drive, Woonsocket, RI, 02895, USA. Figure 3. Emollient on skin acting as a hypothetical barrier to electrode- aqueous solution-skin contact. Emollient Barrier Experiments with pH Paper and Human Skin To test the emollient barrier hypothesis, we first compared the behavior of a pH indicator paper coated with emollient to the behavior of uncoated indicator paper (See Figure 4). We then applied the emollient to a portion of the forearm of test subjects (n=4) in our laboratory. This experiment was also performed in reverse order, where a drop of lime juice was placed on a fresh area of forearm skin and then, this area was covered with emollient and pH measured. A total of 13 sites were used. (See Figures 5 and 6)   To visually demonstrate the effect of the emollient barrier, an innovative technique was used where blackberry juice was applied to the forearm as a natural pH indicator. As pictured in Figure 7, half of the berry juice-dyed skin was covered with emollient, and the other half remained intact. Then, a drop of (highly alkaline aqueous solution was placed on each half. Berry juice dye is red at acidic pH and turns blue when the pH is above 7 (neutral), and this is what happened on the skin not protected by the emollient. Figure 4. Emollient Barrier Experiment with pH Paper. To test the emollient barrier hypothesis, pH on pH indicator paper coated with emollient was compared to the pH of uncoated pH indicator paper. Figure 5. Emollient Barrier Experiment with Human Skin. Skin pH was measured before and after application of the emollient solution to the forearm Figure 6. Close view of a rinsed glass electrode about to make a skin pH measurement. The image on the right shows electrode being lifted from skin and the small amount of aqueous solution that is between the glass electrode and skin during the pH measurement. On the side where the emollient was applied (upper part of Figure 7), alkaline aqueous solution was stopped by the emollient barrier and did not change the color of the berry juice on the skin. This demonstrated that the emollient creates a protective barrier that can help skin maintain a healthy, low pH, even in the presence of an alkaline chemical irritant. Figure 7. Emollient Visualization of Barrier Effect of Emollient on Human Forearm Skin using Blackberry Juice. Blackberry juice was applied to the forearm as a natural pH indicator to demonstrate visually the effect of the emollient barrier. Effect of Coated Vials with Emollient on the pH of Saline Solution or Skin-Mimic Film The objective of these experiments was to determine if the emollient material itself changes the pH of surfaces in contact with it. A laboratory experiment was done to determine how the pH of aqueous solutions kept in contact with the  emollient is affected. The insides of four glass vials were completely coated with 2 g of melted emollient; then the vials were filled with 12.4 g of 0.9% saline solution and capped. After incubation at 37oC for 20 hours, the pH of the saline solution in the vials was measured and compared to the pH of the same saline solution kept in two control vials with no emollient. (See Figure 8) Figure 8. The vial on the left has the interior surface coated with emollient vs. the right vial with no emollient. Both vials contained 12.4 g saline solution. The pH of the control sample on the right is being measured. Effect of Emollient on the pH of Skin-Mimic Film After measuring the baseline pH of untreated skin-mimic collagen film surface, emollient was uniformly applied to two new pre-weighed 80 cm2 pieces of skin-mimic film. Based on measured weight gain, the two emollient application rates were 0.26 mg/cm2 and 0.17 mg/cm2. Collagen skin-mimic film samples were large enough to make multiple pH measurements in different locations (Coffi brand, 25 micrometer; Naturin Viscofan, D-69469 Weinheim, Germany). The Skincheck pH meter was used to measure the nonemollient side of the skin-mimic film initially and then at time intervals up to 24 hours with the emollient skin-mimic film pieces stored at 37oC. This essentially simulates making a pH measurement 25 micrometer below the surface of skin that has topsheet emollient applied to the surface of the skin. A non-emollient collagen skin-mimic film control was also measured at each time interval. Interaction of Emollient, Skin-Mimic Film, and Under Skin pH We wanted to devise an experiment to determine the pH of skin under a protective emollient layer. In other words, we wanted to answer the question “What is the skin pH under the emollient layer?” This provided technical challenges since commercial pH electrodes are not designed to penetrate skin and an in vitro experiment would require a large effort and expense. However, we designed an alternative experimental approach using a skin analog. This emollient allowed us to answer the question in a fairly direct and cost-effective manner. We chose to mimic the skin using a collagen film (Coffi brand, 25 micrometer; Naturin Viscofan, D-69469 Weinheim, Germany). Collagen is a major component of skin tissue and has been used as a skin substitute in other experiments. Figure 9. Measuring the surface pH of collagen skin-mimic film while the opposite side of the collagen skin-mimic film is in contact with buffer pH= 10 solution. Experiment measured the barrier effect of emollient on the skin mimic. We know that the collagen is semi-permeable like skin and with the collagen skin-mimic film we could measure the surface pH on the top side or bottom side using a conventional pH electrode designed for skin. Collagen skin-mimic film was placed on top of an open vial filled with pH buffer 10 solution. Emollient had been applied beforehand to one side of the collagen skin-mimic film. The side of the collagen skinmimic film with the emollient was placed in contact with the buffer pH 10 solution. A wet pH electrode was placed in contact with the opposite side of the collagen skin-mimic film from above (See Figure 9). This indirectly answers whether the pH under the emollient skin would change. The pH of the collagen skin-mimic film surface at the electrode was monitored as a function of time. Results Results are presented below for emollient barrier with pH paper and human skin; with saline solution and skin-mimic film; and the interaction of emollient, skin-mimic film, and skin pH. Emollient Barrier Experiments with pH Paper and Human Skin The application of a drop of acid or alkaline (pH 10) liquid resulted in a quick color change for the uncoated pH paper. In the lower part of the picture which was coated with emollient, the pH paper behaved differently (Figure 4). The liquid did not spread and change color on the paper. The emollient was an effective short-term barrier to penetration of an aqueous solution into the paper. Acidic and basic solutions applied to pH paper with and without emollient demonstrated barrier effect. In the study of the test subjects in our laboratory, prior to emollient application, pH of the skin was 4.96 ± 0.30 (mean ± SD). After emollient application, the measured pH rose to 6.20 ± 0.32 (mean ± SD). This is the same as the pH of the electrode rinse solution which was measure at 6.30. This confirmed the findings of the previously completed clinical studies and showed that the pH skin effect was due to emollient which was easily reproduced in the lab. Adding a drop of dilute citric acid on top of the emollient area on the subject’s skin caused the measured pH to drop to 2.8. This showed that acid on the surface of the emollient could be measured with the pH probe. This low pH indicates the emollient is not highly buffered and does not change the pH of the citric acid applied to the emollient surface. This previous experiment was done in reverse order. When a drop of lime juice was placed on a fresh area of forearm skin, the measured skin pH reduced to 3.5. Then, this area was covered with emollient, and the superficial pH rose above pH 6. This demonstrates that either the emollient served as an effective barrier once covering the lime juice or else the emollient contains a chemical that can neutralize acid. A separate experiment described later in this paper was done to prove that the emollient does not increase the pH of aqueous liquids in contact with the emollient. So, the emollient is acting as a barrier. Effect of Coated Vials with Emollient on the pH of Saline Solution or Skin-Mimic Film The pH of the liquids was not significantly changed by the extended exposure emollient. Data in Table 2 show pH of aqueous solutions is not significantly affected by contact with the emollient. All solutions remained close to a neutral pH of 7 even after exaggerated exposure to the emollient. Same results were achieved when the experiment was repeated with the vial coated with 1 gm of melted emollient and filled with 20 gm of 0.9% saline solution. The measured pH in the coated vs. uncoated vials were 6.91 and 7.26 respectively Table 2:   Note: see Figure 8 for visual experiment. Effect of Emollient on the pH of Skin-Mimic Film The starting pH of untreated skin-mimic collagen film surface was measured in multiple locations using the flat Skinchek electrode giving a pH range of 4.1 to 4.5. Data in Table 3 show there was no significant change in the acidic pH of the skin-mimic film when emollient is applied to the opposite side. Any pH variations in these data sets are similar to pH variation found when measuring different locations in untreated collagen skin-mimic film. Interaction of Emollient, Skin-Mimic Film, and Skin pH Without emollient (Control), the liquid from the vial permeates the collagen skin-mimic film and the pH increases rapidly like the blue curve in Figure 10. Figure 10. Collagen skin-mimic film diffusion test: Rate of pH change slows with increasing emollient. This demonstrates that skin pH could be measured effectively.   Table 3:   Table 4:   The more emollient applied, the slower the pH increases at the electrode side. The kinetic results prove that emollient acts as a barrier and slows the diffusion rate of a high pH liquid into and through collagen skin-mimic film skin analog. The change in pH as a function of time in these experiments was done in triplicate (or duplicate in one case) and the average results were plotted. The data is shown in Tables 4 and 5. The overall average standard deviation for replicate measurements was 0.65 pH units. It is believed that much of the variability comes from location-dependent variation of the collagen skin-mimic film material. Conclusion It was believed that some emollient-based finish products increase the skin pH. Recent investigative results showed that the traditional method of measuring the pH on the emollient-coated skin does not give an accurate measure of the surface pH of the skin. In fact, the tested petrolatumbased emollient creates a protective barrier and helps maintain the healthy acidity of the skin. Thus, our work provides important evidence that certain emollient-based skin products do not alter the pH of the skin and can also help maintain a healthy “acid mantle”. This is a previously unrealized benefit to consumers and this new learning about “coated “ skin should be taken into consideration by other researchers in the field when measuring skin pH. From our current learnings, researchers and clinicians measuring skin pH in clinical studies ought to add an exclusion criteria of “no use of any lotion/emollient/wash/etc. prior clinic visit” when skin pH is planned to be measured.”   Table 5:   Acknowledgements The authors would like to thank Ms. J. Ogle, Ms. W. Qin and Ms. S. Carpenter for their laboratory assistance; Ms. B. Wang for statistical support, in memory of beloved Ms. P. Fith for illustrations and to Ms. Z. Schwen and Dr. N. Dave (Strategic Regulatory Consulting, OH, USA) for assistance with this manuscript.

References

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Cite this article: M.A. Farage et. al. Skin Surface pH and Topical Emollient: Fact or Artifact?. J J Expt Derm Res. 2015, 1(4): 019.