Depilatories act like a chemical razor that dissolves the hair fiber, causing it to separate easily from the skin surface with results lasting up to two weeks [5,17]. Two forms of depilatories, chemical and enzymatic, are described in the literature and are commercially available. However, only the chemical-based depilatories are effective and has majority of the market share (>95%). Chemical depilation can be achieved by agents such as alkali – metal sulfites and sulfides, amines, and mercaptans to cause the hair to lose its tensile strength and deteriorate. The use of sulfides as depilatories goes back to the1880s [12]. Though these depilatories are most efficacious, they have a high odor and their use is linked to skin toxicity. Depilatory products containing barium sulfide (Magic Shave) have been specially formulated for use by African-American men who have pseudofolliculitis barbae (PFB), and are unable to shave. On the other hand, the sulfite depilatories have low irritancy potential, do not have the strong odor of sulfides, but are also slow-acting and inefficient [13,14].
Patents covering the use of mercaptans for use in depilatories were first issued in the late 1930s [13,14]. The use of nonpolar aliphatic mercaptans (methyl, butyl, and benzyl mercaptans) is limited to the leather industry because of their strong smell [12-14]. The odor of the polar mercaptans is not as strong, and can be more easily masked. The sulfhydryls that fall into this class are thioglycolic acid, thiolactic acid, beta-aminoethyl mercaptan, thioglycerol, beta-mercapto-alkanesulfonic acids, and dithiothreitol [13,14]. These compounds react more slowly with the hair but have less odor. Another advantage of the mer – captans is that they are safer and can be used on the face [15].
Currently, the most commonly used chemical depilatories are mercaptans, particularly salts of thioglycolic acid. Cosmetically elegant alkaline creams containing thioglycolates were first patented in 1940s for human use (by Nair), and they remain the standard chemical depilatories used today. The active ingredient in nearly all commercially successful chemical depilatories is calcium thioglycolate, which is most often used in conjunction with calcium hydroxide. Thioglycolate depilatories work by hydrolyzing (reducing) disulfide bonds [17]. Hair strength is a function of the disulfide bonds between cysteine molecules. Cysteine forms 15% of the keratin protein in hair fiber and about 1-2% of the keratin in skin. This differential provides for the preferential hydrolysis of hair keratin over skin. There is a rapid reaction between alkaline thioglycolate and hair keratin, that is pH-dependent and reversible [12,16].
The thioglycolate-based compounds have a low systemic toxicity and are stable at the concentrations (2.5-4%) at which they are effective [12,15]. The depilatory preparations are able to produce optimum effect in 5-15 min, depending on the pH of the preparation. The pH must be at least 10, with quickest depilation occurring at a pH of about 12.5. In general, preparations designed for facial use are milder (lower pH and/or low thioglycolate concentration) than those intended for use on the limbs. The thioglycolates are claimed to be safe at concentrations of up to 15%, if used infrequently. Depilatories can work well, but they can also cause serious skin irritation and even chemical burns, and possibly scarring, if the formula is too strong, or is left on for too long [15].
Recent developments in depilatory technologies include a better masking of the mercaptan odor, hands-free application technology, addition of agents such as antiinflammatory agent, antiirritants, emollient oils, skin-soothing agents, vitamins, hair regrowth inhibitors, and other such agents that provide skin benefits. Because of the high pH of the preparations and a strong ‘reducing’ ability of mercaptan, compatibility with additives also needs to be addressed. Attempts have been made to accelerate the depilatory action of thioglycolates by including reagents that swell the hair fibers. Sodium metasilicate and a blend of N-vinyl lactam, esters of dicarboxylic acid, melamine or dicyanamide are reported to enhance the action of thioglycolate. Urea, imidazolidinone, guanindines and surfactants have also been used for this purpose [15,16]. Others have the thioglycolate mixed with reagents such as mercaptocarboxylic acid, dimethylisosorbide, and long-chain alkylamines for enhancement in activity. Methods to increase the speed and efficacy of depilatories by providing heat to the treated area have also been reported. Work has therefore continued toward developing an ideal depilatory that is highly efficacious, rapid acting, safe for frequent and longterm use, nonirritating, odorless, and can be mass marketed. Though literature is filled with such claims, a product with these attributes is yet to be marketed.