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Formula Anatomy Deciphered: Temporary Hair Depilation
By: Eric S. Abrutyn, TPC2 Advisors Ltd., Inc.
Posted: December 29, 2010, from the January 2011 issue of Cosmetics & Toiletries.
- Figure 1. The breakage of the disulfide bridges of keratin allows for the formation of new disulfide bonds.
- Figure 2. Thioglycolic acid
- Figure 3. X-Epil Extra Sensitive Depilation Cream
- Figure 4. Veet Hair Removal Mousse
- Figure 5. Yves Rocher Aloe Vera Essentiel Cold Wax Strips:
- Figure 6. Auchan Depil Mousse Piels Sensibles (Hair Removing Mousse for Sensitive Skin)
There are many approaches to temporary body hair removal including shaving, epilation/electrolysis, depilation, waxing, plucking and other therapeutic anti-androgen/enzymatic treatments. They all have general advantages and drawbacks. For instance, shaving requires continuous hair removal and can lead to skin irritation from abrasion. Epilation removes the entire hair shaft from the root for long-term hair removal but can be painful and can even cause scarring if not properly performed. Depilation is effective and provides extended hair reduction but due to its high pH, is potentially irritating. Waxing and plucking do not effectively or completely remove hair and can be painful. Finally, anti-androgen and enzymatic treatments require proper timing, i.e. within the hair cycle stage, to provide long-lasting hair removal.
There are also permanent approaches to hair removal that target and destroy the mechanisms regulating hair growth, including electro-epilation, photo-epilation and ultrasound-epilation. In addition, the thermo-laser approach heats hair follicles pretreated with a specific black-colored solution that, once heated, destroys the hair, resulting in the long-term retardation of hair growth. Although some permanent approaches to hair depilation can be effective, this column will focus on creating personal care formulations for the temporary depilation of hair.
Key Components of Hair Depilation
The key chemical necessary for hair depilation is based on an alkaline reducing agent that disintegrates the disulfide bonds (S-S) formed between cysteine units of keratin molecules in hair; this allows for the easy removal of hair by washing the skin surface. This depilatory action allows for the formation of new disulfide bonds (e.g., thioglycolic acid). This reaction is shown in Figure 1 on Page 24.
There are a limited number of cosmetically acceptable, strong alkaline bases available because the pH must be > 11 and they must be stable in the formulation and safe for use. The most widely used category of alkaline bases are mercaptans, known as thioglycolic acid (and salts). They are typically used in a formula around 4–6% and include a strong alkaline salt such as sodium hydroxide, lithium hydroxide or calcium hydroxide to buffer the pH to greater than 11. A key limitation of thioglycolic acid and its derivatives is susceptibility to oxidation by mechanisms such as air, high temperatures and trace metal-ions. Of these three mechanisms, trace metal-ion catalysis is the most detrimental to hair depilatory formulations. In general, calcium and lithium salts are considered the least irritating salt forms.
Depilatory actives can be ionized or nonionized as well as water-soluble or water-dispersible. Some examples of depilatory actives include: potassium or calcium thioglycolate, thioglycerol, 2-mercaptopropionic acid, monoethanolamine thioglycolic acid, homocysteine, cysteine and glutathione, etc. (see Depilatory Actives). The other components of a depilatory formula are chosen based on their compatibility with a high alkaline pH as well as skin-conditioning properties. If the formula is an emulsion, emulsifiers must meet the above conditions and fatty alcohols, alkaline-associative thickeners such as carbomer, hydroxyethyl cellulose and nonionic co-emulsifiers are considered acceptable. Formulators creating a hair depilation product typically are cautioned not to use fatty acid esters due to their instability at high pH levels. Hair shaft-penetrating swelling agents such as urea, thiourea and guanidine carbonate are also incorporated to assist in accelerating the penetration of depilatory actives.