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Peel Science

By: Gül Ç. Zone
Posted: January 31, 2014, from the February 2014 issue of Skin Inc. magazine.

Chemical peels remain some of the most effective, efficient and financially feasible skin care treatments available, and understanding the science behind them allows skin care professionals to most effectively manage, market and take advantage of this powerful, lucrative tool.

Skin, like any material, is subject to wear and tear, chemical and photo degradation, and absorption of chemical pollutants, not to mention microbial invasion. Fortunately, this natural “material” has the benefit of re-building itself through an influx of new cells from the basal layer of the epidermis. However, with age, this constant supply of new cells slows, and a degraded and disorganized skin surface becomes more of the norm. This is where chemical peels—both acid and nonacid—work their magic. Beginning by simply removing the top layer of the epidermis, the skin will exhibit some of its former luster and uniformity, much as the light sanding of an old wooden surface will return some of its finish and surface quality. As a peel works deeper into the epidermis, the lower skin layer loosens and the basal layer of the skin is stimulated to produce new cells. The appearance of new cells can replace aged, oxidized and hyperpigmented skin. The end result is more cellular bulk, offsetting the epidermal thinning that comes with aging; hydrating skin; and improving color uniformity and smoothness.

Peel chemistry

The most common types of chemical peels are acid peels, which work to remove the cells of the top layer of skin’s upper epidermis by breaking weak chemical bonds that hold together the epidermal skin cells, termed “corneocytes” or “keratinocytes.” Depending on the strength, pH and delivery agent of the peel product, upon application, the skin cells delaminate and lift away. (See Figure 1.) As the peel process proceeds, or upon subsequent treatment, it can penetrate further into the thickness of the epidermis to interact and weaken the bonds holding together the cells of the lower layers of the epidermis.

The strength of an acid peel is based on several factors:

  1. The concentration of the active ingredient;
  2. The type of peel agent;
  3. The pH of the product; and
  4. The delivery system.

The concentration refers to the amount of free acid found in the peel, as determined by an analytical method called titration. As the concentration of the acid increases, the strength increases. This strength is characterized by the pH of the product, a chemical parameter that defines the acidity or basicity of an aqueous solution. The lower the pH of the product, the stronger the acid, which is counterintuitive. By definition, the lower the pH of an acid system, the more it wants to release its hydrogen ion (H+), which can then interact and react with the chemical bonds that hold the skin cells together. Conversely, the higher the pH an acid exhibits, the more the acid will tend to hold its hydrogen ion, and it is considered more basic. As shown in Figure 2, the pH scale shows the range of pH values, with 0 being the most acidic, 7 being neutral and 14 the most basic. Most acid-based chemical peels have pHs ranging from 1.5–4.0.