The use of acids to improve the appearance of the skin has a long history as a cosmetic peeling process. However, there are still uncertainties regarding the effects and side effects. A look at the chemistry reveals what is behind the substances and which substance is best suited for which indication or for which skin condition.
Before chemical peelings found their way into cosmetics, they were already used in medicine for a long time. The dermatologist Dr McKnee was the first to treat acne scars with phenol and modified phenol solutions in 1950. The first scientific publication on TCA peeling procedures in dermatology appeared in 1960.
In the 1980s, the effects of deep peelings and derm-abrasion became known. In the meantime, from a cosmetics perspective, a much more exciting group of active ingredients has found its way into dermatology – alpha hydroxy acids (alpha hydroxy acids, or AHA for short).
Chemical bases: AHA
The first scientific overview of the use of AHAs was published in 1989 by dermatologist Van Scott. Both the keratolytic effects at low concentrations and the detachment of the epidermis, epidermolysis, at higher concentrations were described. Due to their flexible use and their low potential for side effects compared to other peeling substances, AHAs are still the most commonly used peeling substance in Aesthetics. Alpha hydroxy acids are organic carboxylic acids with a hydroxyl group in the α-position, that is, on the carbon atom closest to the carboxyl group.
From a chemical point of view, the smallest AHA is glycolic acid (2-hydroxyethanoic acid). It has its natural origin in numerous plants, for example in unripe grapes, sugar beets and sugar cane. Other AHAs include lactic acid, malic acid, tartaric acid, citric acid, and mandelic acid. Due to its small molecular size, glycolic acid can penetrate the skin particularly well and, in addition to its exfoliating properties, also has a moisturizing effect. It is the most commonly used peeling substance, both in dermatology and in cosmetics.
Dermatology and cosmetics
In dermatology, concentrations of up to 70% are used, resulting in a loosening of the epidermis.
In cosmetics, glycolic acid is used both in topical preparations, i.e. in creams, and in peelings. It is found in creams in concentrations of between 4% and 10%, and studies show that it moisturizes very well, has an exfoliative (scaly) effect and can support the skin renewal process. In addition, it can improve the penetration of other active ingredients. From a concentration of 10% glycolic acid is used as a peeling substance.
In addition to glycolic acid, lactic acid is a widely used AHA in cosmetics. It is part of our skin's natural moisturizing factor (NMF) and is therefore very well tolerated, which is why it is contained in numerous creams as a moisturizing component, usually in a concentration of around 5%. Due to its keratolytic and sebum-regulating properties, it is particularly suitable for impure, seborrheic skin. Since it is slightly larger than glycolic acid due to its molecular size, it is less able to penetrate the skin. For this reason, it is used in higher concentrations of 30 % or more. In medicine, melasmas are even successfully treated with almost pure lactic acid (92 %).
Mechanisms of action of AHAs
All AHAs are based on the same mechanism of action. They split the cell network of the stratum corneum by breaking the intracellular calcium ion bond. The result is reduced corneocyte cohesion, which leads to keratolysis.
The following applies: the higher the concentration of the acid and the lower the pH value of the product used, the faster keratolysis is induced.. This formula is based on the fact that the so-called acid value depends on the pH value of a product: the lower the pH value, the higher the acid value.
During keratolysis, a signal is sent to the basal cell layer, causing an increased rate of mitosis and skin renewal. In vitro studies show, for example, that glycolic acid can increase vitality and cell division activity by more than 40%.
In view of the fact that the cell renewal rate decreases with increasing age and the skin cells no longer effectively slough off by themselves, chemical peelings are a very sensible active component.
In addition, the stimulated cell division joy of the keratinocytes leads to a thickening of the epidermis. The result is more resistant and more even skin.
Indications for AHAs
Indications in both cosmetics and dermatology are the improvement of skin roughness, superficial scars and epidermal hyperpigmentation.
Impure skin prone to acne is also an indication, especially if there are closed or open comedones. In addition, the formation of new impurities is counteracted, since dead skin cells and too much sebum are removed, which can clog the pores. In the long term, pores are visibly refined. In the case of inflammatory lesions, the application should be avoided.
AHAs should also be used with extreme caution on sensitive skin, as they are irritating and can cause severe redness. Chemical peels with AHAs, on the other hand, have proven their worth as an anti-aging treatment. Due to the skin-renewing effect and an expected increase in the thickness of the epidermis, pigment shifts can be improved, the skin topography optimized and small wrinkles improved.. After several applications, the water-binding capacity of the skin is increased, which is reflected in an improved glow.
PHA Polyhydroxy acids
In addition to the widespread AHAs, there are more and more peeling products on the market that contain polyhydroxy acids (polyhydroxy acids, PHA for short). From a purely chemical point of view, PHAs also belong to carboxylic acids but have two or more hydroxyl groups compared to AHAs. Their use in cosmetics is not new. PHAs are found in numerous cream preparations.
The best-known PHAs include lactobionic acid and gluconolactone. Lactobionic acid is made from milk, more precisely from lactose. Chemically, it is a sugar acid, a disaccharide made from gluconic acid and galactose. It has shown its effectiveness and tolerability in studies.9 Gluconolactone is a lactone of gluconic acid and is also very well tolerated.
Comparison with AHAs
Compared to AHAs, PHAs hydrate the stratum corneum more effectively and sustainably and have a positive effect on the skin barrier. In addition, they bring antioxidant properties by neutralizing free radicals and thus protect against environmentally induced skin aging.
The biggest difference, however, lies in the compatibility. Due to their molecular size, PHAs cannot be absorbed as well by the skin and are therefore far less irritating. Therefore, they represent a real alternative for sensitive and hypersensitive skin.
Like AHAs, PHAs act as peeling substances against light-induced skin aging by reducing fine lines and hyperpigmentation, refining pores and thus optimizing the complexion. In addition, they strengthen the skin's own barrier function, reduce redness and reduce sensitivity to external irritation. Therefore, they are also a real alternative for rosacea patients. Similar to AHAs, PHAs are used in different concentrations. They still have a caring effect even at higher concentrations of around 30 %
Combinations of AHAs and PHAs can also represent an alternative to pure AHAs and have already proven themselves in practice.