Читать книгу pH of the Skin: Issues and Challenges - Группа авторов - Страница 69

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 [22, 23]. Elias elevated the stratum corneum as the key component of the diverse biological functions of the integument [24]. All stratum corneum layers contribute to its barrier properties [25]. Richter et al. [26] employed cryofixation and scanning electron microscopy to show 3 distinct hydration zones within the horny layer. 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 [27]. 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 the development of 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 the early 19th 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 see [22]. 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 [28]. “Barrier repair” creams have become recently popular moisturizers.

Fig. 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 in neutral pH buffers [29]. 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 [30, 31]. Acidic pH also impacts lipid-lipid interactions in the horny lamellar bilayers [32]. 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 (Fig. 1), which could potentially prevent the pH probe and the aqueous solution layer on the probe surface from contacting the acidic compounds on the surface of the skin. 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.

Fig. 2. Changes in measured skin surface pH after use of emollient-based and no emollient-based finish pads (a) and emollient-based and no emollient-based panty liners (b) in the Behind-the-Knee test model. * Significant difference (p < 0.05) between emollient-based finish and non-emollient-based finish pads or pantiliner. Blue bars represent baseline pH data and red bars are mean pH values after 6 h (days 1–5) of product wear.

In 2 clinical studies with human subjects using our feminine-care emollient products, it was observed that after the application of emollient-based finish pads (and pantiliners) there was a small, but statistically significant, increase in measured skin pH (Fig. 2a, b). 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.