Acids and skin: principles of how chemical peels work

Sergey Ostapchuk , dermatocosmetologist, certified trainer of the Toskanicosmetics company (Spain), head of the “Peelings, contouring” department of the Karse company (Kiev)

Chemical peeling is a method based on controlled damage to the skin followed by an inflammatory response that activates reparative processes. Chemical peels allow you to correct age-related and scarring changes in the skin, help in the treatment of acne, hyperpigmentation, etc. The choice of peeling solutions available today is large. However, understanding the pathogenetic effects of a particular acid on the skin allows you to make the right choice and get good results.

AHA acids

AHA acids (alphahydroxy acids) are organic carboxylic acids that, in addition to the carboxyl group, have a hydroxyl group. These acids are found in various foods, in particular fruits, which is where their second name comes from – fruit acids.

In cosmetology, the arsenal of fruit acids is quite wide, since, in addition to the general exfoliating effect, different AHAs have different specific activities. The exfoliating effect of these acids is associated with their ability to weaken the adhesion of corneocytes in the lower, young layers of the stratum corneum due to the fact that they affect the activity of certain enzymes involved in the formation of ionic bonds. There is also evidence that exfoliation occurs as a result of increased apoptosis.

The weakening of the adhesion of corneocytes at the level of the granular layer contributes to their faster advancement into the stratum corneum and exfoliation, which is a trigger for the division and differentiation of underlying keratinocytes.

A decrease in corneocyte cohesion also affects the level of skin hydration. NMF (natural moisturizing factor of the skin) is better developed in young corneocytes - therefore, rapid renewal of the epidermis leads to an increase in the content of functionally active NMF. In addition, hygroscopic AHA molecules are able to bind water and, penetrating deep into the skin, deliver it to the deeper layers. Lactic acid has the most pronounced moisturizing effect.

Despite their exfoliating activity, ANAs have a positive effect on the barrier functions of the skin. This is due to their ability (in particular, lactic acid) to increase the total amount of ceramides (components of the intercellular lipid layers of the stratum corneum) and modulate their type. This is especially important when there is a deficiency of type 1 linoleate-containing ceramides, which results in increased permeability of the stratum corneum. Lactic acid also has a positive effect on the synthesis of type II and III ceramides.

ANAs have anti-inflammatory effects. This effect is due to the effect on inflammatory mediators and a decrease in the production of hydroxyl radicals. The anti-inflammatory activity of AHA acids depends on their antioxidant activity. The most pronounced antioxidant activity is in pyruvic acid, the least - in glycolic and lactic acid. But in combination with other antioxidants, their activity increases.

The anti-aging effect of ANA is associated with stimulation of fibroblast proliferation and activation of type 1 collagen synthesis. It has been proven that glycolic acid has the most pronounced proliferative effect, while lactic and pyruvic acids are somewhat weaker.

There is also evidence of the depigmenting effect of ANA. For example, lactic acid promotes the uniform distribution of melanin granules in corneocytes and inhibits the activity of tyrosinase.

Separately, it is worth mentioning mandelic acid . Possessing all the benefits of AHA, but containing a benzene group, it becomes lipophilic, which is important for oily, porous skin. At the same time, it causes less skin irritation due to the large molecule, so it is ideal for sensitive skin.

The structure of mandelic acid is similar to that of broad-spectrum antibiotics, which makes it indispensable in the treatment of patients with skin prone to rashes.

Salicylic acid

Salicylic acid (phenolic aromatic o-hydroxybenzoic acid) exhibits acidic properties, like its analogue phenol. It has a pronounced exfoliating effect due to a decrease in the cohesion of corneocytes as a result of disruption of sulfate, sulfite, sulfide bonds between them.

Being a BHA (beta-hydroxy acid), salicylic acid, stronger than fruit acids, stimulates the renewal of skin cells, providing long-term results, evens out the skin texture due to pronounced large-plate peeling. Salicylic acid reduces hyperkeratosis not only on the surface of the skin, but also in the mouths of the follicles, improving the outflow of sebum and reducing the number of comedones. According to one study, salicylic acid inhibits factors necessary for bacterial replication in tissues.

Thus, peeling based on salicylic acid is indicated in complex therapy for the correction of post-acne, folliculitis, seborrheic dermatitis, oily seborrhea of the face, melasma, rosacea, as well as for mature problematic porous skin.

Azelaic acid

Currently, peeling solutions with azelaic acid, which has bacteriostatic activity against P. acnes and Staphylococcus epidermidis, are actively used. Azelaic acid has an anti-inflammatory, sebum-regulating, antibacterial effect, without causing resistance of microorganisms (no addictive effect), kerato- and comedolytic effects. The antibacterial effect is associated with the active transport of azelaic acid into the bacterial cell. Also of positive properties, it is worth noting a decrease in the secretion of free fatty acids on the surface of the skin.

Azelaic acid has a good whitening effect, therefore it is actively used in the treatment of hyperpigmentation, which is due to direct inhibition of tyrosinase. Azelaic acid also has an inhibitory effect on DNA synthesis and cellular respiration of pathological melanocytes.

Trichloroacetic acid

A universal peeling agent is trichloroacetic acid , which has become a classic peeling in cosmetology. The mechanism of action of TCA is based on keratocoagulation and precipitation of proteins of the epidermis and partly the dermis. As a result, a deep inflammatory reaction occurs, causing restructuring of the dermis. It should be taken into account that the concentration of a substance directly correlates with the depth of penetration and, consequently, the depth of destruction.

It must be remembered that, regardless of the concentration, each subsequent application of TCA increases the depth of acid penetration, which is accompanied by the appearance of frost. The nature of frost and erythema are criteria for acid penetration and the degree of damaging effect of peeling.

Currently, 15% TCA is increasingly preferred to achieve significant results with minimal complications.

Phenol

Phenol is one of the peeling solutions, the use of which in practice should be the most balanced. Classic deep phenol (88%) peeling is carried out in a hospital under anesthesia and with constant monitoring of the cardiovascular system, since phenol has a toxic effect.

In the practice of cosmetologists, phenol is actively used in lower concentrations, but with a good cosmetic effect. It is worth remembering that, regardless of the concentration, the toxicity of phenol remains, so its use over large areas is unacceptable, and the selection of patients should be more careful.

Phenol peeling causes liquefaction necrosis. As a result, in the post-peeling period all stages of the inflammatory process are observed - inflammation, exudation, proliferation. Thus, a restructuring of the dermis occurs and, as a result, a qualitative improvement in the skin texture.

Retinoic peels

The peculiarity of retinoic peels is the absence of a primary damaging effect on skin structures, which makes it possible to distinguish them into a separate group of peels.

Retinoids include chemicals that interact with specific nuclear receptors (α-, β-, γ-RARs; α-, β-, γ-RXRs) to retinoic acid: tretinoin, isotretinoin, retinaldehyde, retinol palmitate, retinol acetate, b- carotene and adapalene. These components are actively involved in embryogenesis, angiogenesis, and skin homeostasis. After interaction, the receptor-retinoid complex binds to a specific DNA nucleotide sequence, resulting in an increase in the synthesis of the gene transcription regulator protein responsible for the proliferation and differentiation of keratinocytes, sebaceous glands, etc.

Although retinoic peels are superficial peels, their results are comparable to those obtained after medium peels. The mechanism of action of retinoids is associated with the influence on the functioning of cells, changes in the expression of genes responsible for the proliferation and differentiation of keratinocytes. They influence the processes of desquamation, activate the synthesis of collagen and elastin, stimulate angiogenesis and the synthesis of epidermal lipids by the cells of the spinous layer, being one of the most actively used drugs in anti-aging programs for the correction of involutional changes in the skin of various origins.

In addition, retinoids suppress the proliferation of sebocytes, which leads to a decrease in sebum secretion. Therefore, peels based on retinoic acid are the drugs of choice for acne.

The depigmenting effect of retinoic acid is associated with the suppression of tyrosinase activity, which allows the use of drugs based on it for problems associated with hyperpigmentation.

However, despite their obvious benefits, retinoids cause irritation when applied to the skin, so precursors of retinoic acid are mainly used in cosmetology: retinal, retinol, retinol esters.

Thus, peelings are used in rejuvenation programs, treatment of pigment disorders, acne and other pathological skin conditions. Understanding the pathogenetic mechanisms of peeling action allows you to choose the right treatment regimen and obtain the expected results.

First published in Les Nouvelles Esthetiques 2012/№1