Much is being said and written today about how skin peeling treatments act on our skin, but there’s little literature which explains the chemistry of peelings and how they actually work.
This week, we address the topic of the strength of facial peels: the aim is to explain and help decipher the codes of peeling acids and thus be more informed when choosing which peel to do.
Some quick facts about the Skin, ph and peelings
Considerations about skin pH can be divided into two parts: the outside and inside skin pH. The pH on the surface of a healthy, undamaged adult skin is slightly acidic, varying from ph4 to 6. It is important to realize that it is impossible to assign only one pH-value to the skin. Variations in outside skin pH depend on many endogenous and exogenous factors such as anatomical site, sex, age, race, circadian rhythm, temperature, humidity, etc. In addition, the skin has a pH-gradient through the epidermis, changing from acidic values on upper layer of the epidermis to a near-neutral pH of around ph7.4 in viable epidermis.
Chemical peelings act by temporarily reducing the ph of the skin by applying caustic agents to induce a controlled kerato-coagulation and denaturation of the proteins within the epidermis and dermis. This results in the release of pro-inflammatory cytokines and chemokines. Such targeted inflammation activates the normal healing signal cascade, including stimulation, development and deposition of new dermal collagen and elastin, reorganization of structural scaffold proteins and dermal connective tissue, and regeneration of new keratinocytes.
This results in the rejuvenation and thickening of the epidermis and an increase in dermal volume. Simultaneously, the kerato-coagulation and subsequent exfoliation result in an improvement in superficial and medium-depth dyspigmentation.
While there might be subtle variability between the types of chemical agents used and their intended cosmetic outcome (i.e., reduction of redness vs. dyspigmentation vs. scarring), the general objective of all chemical peels is to improve the clinical appearance of skin by decreasing the quantity and quality of rhytides and/or acne scars, reducing inflammatory and noninflammatory acne lesions, improving dyspigmentation, and producing an overall more youthful appearance.
Some quick facts about Acids and Bases
Acids by definition can accept an electron pair or donate a hydrogen ion or a proton in a chemical reaction, while bases can donate an electron pair or accept hydrogen or a proton. Any aqueous (water-based) liquid can be classified as an acid, base, or neutral. Oils and other non-aqueous liquids cannot be classified as acids or bases.
Acids and bases are characterized as strong or weak. A strong acid or strong base completely dissociates into ions in water. If the compound does not completely dissociate, it’s classified as a weak acid or a weak base.
“How corrosive an acid or a base is does not relate to its strength or percentage in the solution.”
Some quick facts about ph
We hear a lot of marketing around the concentrations of acids but an important factor in determining its strength is also the ph of the solution.
pH is a logarithmic measure of the hydrogen ion concentration of an aqueous solution:
pH = -log[H+] this describes how acidic or alkaline an aqueous solution is.
The pH scale generally runs from 1 to 14 (although minus and much higher values exist). Each whole pH value below 7 (the pH of pure water) is ten times more acidic than the higher value and each whole pH value above 7 is ten times less acidic than the one below it. For example, a pH of 3 is ten times more acidic than a pH of 4 and 100 times (10 times 10) more acidic than a pH value of 5. So, a strong acid may have a pH of 1-2, while a strong base may have a pH of 13-14. A pH near 7 is considered to be neutral.
So one would imagine that as the ph of the skin ranges from ph5-6, any peel under ph 4 or 5 should be effective, in inducing a chemo-exfoliation right? Well no it’s not that simple.
Some quick facts about pka
The percentage concentration of acids in the solution and ph are not the only determining factors in determining the strength of a peel. A chemistry law that largely unspoken of in marketing brochures but which is inherent to all molecules, because it determines the relative strength of all acids or bases, is what known as the pka value.
Ka is the acid dissociation constant or base dissociation constant, that indicates the relative strength of an acid or base. This is used to calculate the pKa, an logarithmic constant: pka = – log10 Ka.
“The higher the pKa value, the weaker the acid. The lower the pka value the stronger the acid”
Pka values are fixed and constant for every molecule, it never changes no matter what the concentration of the molecule in a solution, no matter what the ph of a solution is.
In practice when the ph of a solution is equal to the pka of the acid, this means that only 50% of the acid is in its active acid form and 50% of the acid will be available in its basic form. For 100 % or the free acid value to be active the ph must -2 points lower than the pka value/s of the acid/s used.
Some concrete examples
Let’s take the examples of pka, ph and % concentrations for Glycolic Acid and Trichloroacetic acid.
Glycolic acid is structurally similar to TCA, but with a lower acidity thanks to the presence of the -OH Hydroxyl group in its molecule.
Glycolic acid small molecular size at 76g/mol means that it penetrates quickly into the epidermis. While its acid pka value is (3.8). For a Glycolic Acid peel of 35%, when the ph is 1.6 it means that it contains 100% Free acid value, i.e. the full 35% is active.
For example in France beauticians are legally not allowed to used peelings with a ph under 3. So a trick used by many brands is to chemically alter the ph using buffering agents. If the pH is higher than desired, it can be lowered by adding a hydrochloric acid solution. If the pH is lower than desired, it can be raised by adding a sodium hydroxide solution. So while the peeling can still contain 35% Glycolic acid for marketing purposes, the ph can be altered to bring it closer to neutral pH 7, but of course this decreases its efficacy.
The Trichloroacetic acid molecule is 163 g·mol−1 twice the size of Glycolic acid, but the pka of TCA is (0.66), 6 times lower than the pka of Glycolic acid. The frosting the comes with a TCA is due to the caustic nature of the TCA which effectively coagulates the kerantinocytes as it penetrates quickly, causing a necrosis or death of the keratinocytes. The GA is less acidic, therefore less caustic, but because it is small it penetrates through all the layers of the epidermis without causing a necrosis of the cells.
Let’s consider a TCA 20% with ph1. This means that 20% of the total solution contains TCA. But TCA 20% with ph of 1, means that less than 10% of the TCA is actually in its free acid or active form. With its pka of 0.66 in order for 100% of the 20% TCA to be in its free acid form, the ph of the peel would have to have a minus value of ph -1.
The same rules to all acids including Vitamin C (ascorbic acid), in order for 100% of the concentration of Vitamin C contained in the formulation to be available in its free acid active form, it requires the formulation to have a ph of less than 3.
Some other examples of popular acids, their molecular size, pka and ph required for free acid value worth noting are summarized in the table below:
TABLE 1: Popular Acids used in Aesthetic Medicine
Source : https://pubchem.ncbi.nlm.nih.gov
THE TAKE AWAY MESSAGE
Unfortunately, most doctors and patients are aware of these chemistry formulation rules and therefore are generally misguided by marketing claims. The efficacy and actual strength of a peeling formulation is highly complex. It depends not only on the structure and size of the molecules, the concentration of the molecule, the pka of each molecule, the ph of the global formulation relative to each of the pka of each of the active molecules and the mechanism of action of each of the molecule taking into consideration the minimum and maximum recommended doses for safety. Efficacy is not only a question of the ingredients on the packaging, efficacy is also a question of balance and respect of chemistry rules.
In the aesthetics industry Research and Development timelines don’t generally go well with marketing timelines and customer aspirations; new products have to come out every year, but true scientific research on new ingredients takes time. Marketing dynamics used by many well- known brands are often based on shortcuts or partial truths and renaming of old ingredients to better boost sales and keep the risks of new products intentionally low.
However today attitudes are changing, there is a growing interest and demand from both patient’s and health professionals to differentiate between marketing communication gimmicks, between scientific facts regarding skin treatments, ingredients and their claims. Nothing is more valuable than respecting our health capital and being careful in the products we chose to use, to respect and maintain healthy looking skin not just short term but for our lifetime.