Acids at the Service of Cosmetics - The Complete Guide

Acids are among the most active and effective ingredients in cosmetic products. Hyaluronic acid and alpha and beta hydroxy acids are the most prominent and well-known acids, but they are certainly not the only ones. The following article reviews various acids used in treatments administered at the cosmetic salon and in the field of dermatology as well. 

Hyaluronic Acid

Hyaluronic acid is a natural substance produced in our skin, which fulfills a key role in the skin’s ability to preserve moisture. As it is a large molecule, its capacity for adsorbing and storing water is high. It can adsorb thousands of water molecules— approximately 1000 times its weight. It operates to repair the skin tissue and to accelerate the processes of healing from injuries and infections. Studies from recent years indicate that hyaluronic acid fulfills a central role in the communication between the cells, in the cell division and migration, and operates as a sort of “courier” that assists in the message transmission between the cells and various substances. This role is extremely important in tissue regeneration and the maintaining of youthful skin. For example, one of the important messages, transmitted by the hyaluronic acid to the fibroblasts, is the need to produce collagen and other components essential for the regeneration of the skin.

Although researchers were already able to successfully isolate hyaluronic acid in the 1930s and realized that it holds a central role in preserving the volume and moisture of the skin, its use in aesthetic medicine began only in 1996. In 2003, the FDA approved the first hyaluronic acid-based dermal filler, and since then, it has become the most common dermal filler in the world in this field.

While the natural hyaluronic acid in our bodies is decomposed in the skin within a few days by an enzyme called hyaluronidase, the hyaluronic acid used in aesthetic medicine to fill wrinkles is relatively durable (6-12 months). The durability is created due to the manufacturing processes that create the cross-linking of the molecules. Such linking decelerates the decomposition of the molecules by enzymes in the skin and enables long-term results.

When the acid is injected into the wrinkles, it operates as a sort of “putty” that fills the wrinkles, and its water-binding ability grants the wrinkles additional volume. In this manner, it stimulates the skin to create collagen in the area of the injection.

In addition to this type of cross-linked hyaluronic acid, there is another type, non-crossing linked hyaluronic acid, also used in aesthetic medicine. This substance is used in mesotherapy treatments: it is inserted into the skin using a tiny needle in a superficial injection and grants an immediate result of vital and radiant skin. Since the acid does not undergo manufacturing processes, it remains in the skin for only several days. However, the injection of small amounts in numerous spots enables its distribution in the skin on a large contact surface, thus creating a substantial reservoir of water in the skin, which remains even after the substance itself vanishes. 

It is important to note that the mere injection and presence of the hyaluronic acid in the skin causes the fibroblasts, the cells that produce the collagen in the skin, to awaken and operate. Besides, today, hyaluronic acid can be found in moisturizers, and this, thanks to its efficiency in the preservation of the moisture between the skin cells in the epidermis. 

Alpha Hydroxy Acids (AHAs)

In recent decades, alpha-hydroxy acids have been proven to be among the most effective cosmetic ingredients for the regeneration and rejuvenation of the skin. Today, peels and products based on these acids hold a central role in the dermatological treatment and the advanced cosmetic treatments for the rejuvenation of the skin, blurring wrinkles and lightening age spots. These acids are produced from various plants and fruit and include glycolic acid (produced from sugar canes), citric acid (produced from citrus fruits), malic acid (produced from apples), tartaric acid (produced from grapes)— or synthetically produced acids based on the natural acids. Another common alpha hydroxy acid is lactic acid, produced from milk.

Many studies that examined the operation of alpha hydroxy acid have demonstrated that when applied to the skin, they encourage the exfoliation of the dead cells on the stratum corneum—the outermost skin layer—through the attachment to and neutralization of the calcium ions found in the connections between those cells. The reduction of the calcium ions’ concentration loosens the ties between the cells and enables their exfoliation. The result is the shedding of dull and rough skin and the encouragement of the cells’ regeneration. Also, it has been found that the ability to attach to and neutralize the calcium ions between the epidermis cells also enables the neutralization of the free radicals and various polluters— which means these acids have antioxidation and antipollution properties. Moreover, it was found that these acids improve the function of the skin barrier, rehabilitate the skin’s moisture levels, and are even able to penetrate the deeper layer of the skin, into the dermis layer, where they improve the quality of the collagen and elastin fibers and increase their density, encourage the increase of the gene expression of collagen and hyaluronic acid in the dermis, and thicken the skin. These processes lead to a significant improvement in the appearance of the wrinkles, in the skins’ moisture level and the cutaneous characterizations, such as skin elasticity and tone.

Thus, for example, a clinical study published in the Journal of the American Academy of Dermatology reported that after four weeks of treatment with lactic acid in a concentration of 12%, a 19% increase in the thickness of the epidermis was found, and in addition, an increase in the production of the collagen in the dermis was detected, as well as an increase in the production of glycosaminoglycans— polysaccharides, whose important property is their ability to bind large amounts of water, thus providing moisture and granting the skin volume.

The two most common alpha hydroxy acids in aesthetic treatments are glycolic acid and lactic acid. Lactic acid is gentler in its influence than glycolic acid and irritates the skin. Furthermore, thanks to the fact that it constitutes one of the components of the NMF—the skin barrier—it increases the skin’s ability to preserve water in the stratum corneum and thus contributes to it moisturizing. Therefore, it is especially suitable for dry and dehydrated skin types, for sensitive skin, and for people who suffer from seborrhea and dermatitis, and it can be found in the active products that offer a solution for these skin problems, as well as in specific moisturizers for skin rehabilitation.

Salicylic Acid (BHA)

Salicylic acid, also called beta hydroxy acid, belongs to the aspirin family, and is found naturally in the bark of the willow tree. It is very similar to alpha-hydroxy acids, but while alpha hydroxy acids are water-soluble, salicylic acid is fat-soluble. This structure enables it to penetrate the skin through fat follicles. Thus, other than the exfoliation of the outer skin layer, it can also penetrate the pores and sebaceous glands, exfoliating the skin cells that accumulate within the walls of the pores and normalizing the activity of the glands. Besides, salicylic acid has anti-inflammatory and antiseptic properties, which operate against bacteria and fungi. Therefore, it is used in products that treat skin inflammations, psoriasis and oily seborrhea (characterized by the secretion of fat), and in antibacterial products (in a concentration of up to 1%), as well as in peeling products for the treatment of oily and acnatic skin (in a concentration of 2%). 

Trichloroacetic Acid (TCA)

This acid has been used in peelings for more than five decades. It is used in treatments to achieve medium to deep exfoliation, to blur wrinkles, refine post-acne scars and lighten age spots and hyper pigmentation.

Relatively high concentrations of the substance—15% - 50%—are used in the professional treatment, which is only performed by dermatologists or plastic surgeons. 

TCA acid penetrates the skin and causes the congelation of the proteins, which causes the appearance of a white residue on the skin surface and the full destruction of the epidermis cells, and leads to a forceful inflammation. As a result, new skin cells are created. The treatment necessitates a healing period at home. It should be noted that it is not suitable for dark-skinned people, since it damages the melanocytes (the cells that produce the pigment melanin), and therefore may cause the appearance of white spots. Furthermore, it is not designated for the areas of the neck and décolleté, due to the risk of scarring.

Ascorbic Acid (Vitamin C)

Ascorbic acid is the most senior acid of all. The beauty industry has been using it for many decades. This fact is unsurprising in light of the central role held by vitamin C in our bodies and skin. First and foremost, vitamin C is not only considered as one of the most potent antioxidants in nature, but also as one found in the largest amounts in our skin, and it influences the skin’s health and appearance, both when swallowed and applied. As such, it protects the skin from free radicals, formed as the result of the skin’s exposure to various harmful environmental factors. Since it is water-soluble, the vitamin operates in the watery areas of the skin cells. It protects it from the oxidization process — a state in which the cells cannot cope with the number of free radicals — by contributing electrons that neutralize these destructive molecules. A very important advantage of vitamin C as an antioxidant is the fact that even its oxidized forms, after finishing its activity against free radicals, are not active, and thus do not damage the skin.

Moreover, vitamin C is essential for the production of collagen and elastin. Clinical studies have shown that its application onto the skin increases their production in older skin as well, and encourages the production of hyaluronic acid and essential enzymes in the skin. Besides, it operates to lighten the skin by blocking the operation of the enzyme tyrosinase, responsible for the production of melanin during the exposure to the sun. Furthermore, it operates as an anti-inflammatory ingredient since it impedes the production of the protein NF Kappa B — a protein that controls the operation of hundreds of genes involved in inflammatory processes, and responsible for the activation of several inflammation-encouraging cytokines, such as TNF alpha. Hence, it can be used as an efficient ingredient for the treatment of skin conditions such as acne vulgaris and rosacea, and also encourages the healing of lesions, and prevents the formation of post-inflammation spots.

The active concentration for cosmetic purposes is 5% and up.

Azelaic Acid

Azelaic acid inhibits the enzyme tyrosinase, and thus impedes the production of melanin in the skin. It is also used often with additional skin lighteners to lighten the skin, and thus facilitates the achievement of a quicker result in the melanin suppression and dark spot reduction. Additionally, azelaic acid is used as a therapeutic ingredient in conditions of acne, psoriasis, and oily seborrhea. This is thanks to its ability to reduce the pace of the division of cells in the skin and the cells of the sebaceous glands, which secrete fat (sebum) on the skin surface. By this, it assists in reducing the accelerated division pace of the keratin cells in conditions of psoriasis and in decreasing the excess fat secreted from the sebaceous glands, which leads to the clogging of the hair follicles and the creation of a fertile ground for the P-Acne bacterium. 

The concentrations used in aesthetic treatments vary from 10% to 20%. 

Kojic Acid

This acid is a by-product of the fermentation process of rice, and it is also used in skin-lightening treatments. It is considered to be relatively safe and gentle. Like ascorbic acid, it also operates to inhibit the production of the pigment melanin by reducing the efficiency of the enzyme tyrosinase. It is used in concentrations of 1%-3%, often combined with additional skin-lighteners — a combination that creates synergy between the substances and intensifies their operation. 

Urea 

Urea, also known as carbamide, is a water-soluble substance found in the skin’s stratum corneum, which has an important role in the preservation of the skin’s moisture. It is one of the NMFs—the natural moisturizing factors—which form the skin barrier. As such, it preserves the moisture in the skin cells and protects the skin from the damaging influence of detergents like SLS. Also, urea has anti-inflammatory and anti-bacterial properties. Healthy skin contains 1% of urea, but in conditions of dryness and skin conditions such as contact dermatitis, the skin suffers from a urea deficiency. 

In the cosmetics industry, urea is used as an important ingredient in many products, including moisturizers, products that protect against skin irritation, and products for the rehabilitation of irritated and injured skin (a concentration of 1% - 3% for moisturizing purposes, and 4% - 5% for the repair of damaged tissue, cracks in injured skin and the healing of inflammatory lesions). Studies have found that the regular use of urea as part of cosmetics products assists to increase the level of moisture in the skin and reduce the water loss from the stratum corneum, reduces the skin’s sensitivity to detergents and accelerates its healing and rehabilitation following such exposure. Besides, urea adds volume to the stratum corneum, and therefore, despite being part of the skin barrier, it increases the skin’s penetrability to additional ingredients in the product.

However, urea also bears a keratolytic effect, and in a concentration of 6% and up, it is used for a powerful exfoliation, renewal of the skin, and even for the removal/dissolution of ingrown nails. In cosmetic products designated for skin regeneration, it is found in these concentrations.