Formulation and Evaluation of Self-tanners

Social norms for tanning in many parts of the world have dramatically changed in recent times. Before the Industrial Revolution, tanned skin conveyed the social status of an outdoor laborer; post-revolution, however, many jobs moved indoors and the general wealth of the public increased—along with paid vacations and modified work weeks. Thus, the public was able to enjoy leisure time, especially outdoors, and having a tan then conveyed a social status of wealth, especially during the winter months. Indeed, many college students enjoy their “spring break” in Florida, Mexico and other warm destinations. This practice has been taking place as almost a ritual for the past 50 years on US college campuses.

While more information has become available regarding the negative effects of UV exposure,^1-3 and the public is beginning to slowly understand the dangers and thereby modify lifestyle choices toward safer sun practices, the change has been slow because sun exposure behavior is in part influenced by psychological and societal factors, as well as general confusing messages from nongovernmental organizations (NGOs), bloggers, etc.^4-6 Self-tanning preparations are therefore becoming an increasingly important option for those desiring a tanned look without exposing themselves to undue harm, although there is as yet some reticence among the public for using them. This article reviews the ingredients and mechanisms of action of self-tanners, in addition to the formulating challenges of such products.

Sunless Tanning Products

Self-tanning products, or sunless tanners, are preparations that when applied topically, impart a temporary coloration to the skin to mimic the color of naturally sun-tanned skin. Depending on the formulation and active ingredients, the onset of color formation can be anywhere from immediate to several hours and can last up to one week. Self-tanning formulations were introduced in the 1960s, but consumer acceptance of them soon waned due to unattractive results such as orange hands, streaking and poor coloration. Due to these drawbacks, consumers today still associate sunless tanners with undesirable results, which is one cause behind the public’s reticence for using them. However, improved formulations have appeared on the market. Refinements in the dihydroxyacetone (DHA) manufacturing process has aided in the creation of formulations that produce a more natural looking color and better longevity.^{7, 8}

Dihydroxyacetone

DHA is a triose and the simplest of all ketoses (see Figure 1). It is the most widely used and most efficacious active ingredient in self-tanners, and is the only ingredient currently recognized as a self-tanning agent by the US Food and Drug Administration (FDA).⁹ Further, DHA-based sunless tanners have been recommended by the Skin Cancer Foundation and the American Academy of Dermatology Association.^{10, 11}

Ketones and aldehydes react with primary amines to form Schiff Bases.¹² This is similar to the Maillard reaction, also known as non-enzymatic browning, and specifically involves the reaction between carbohydrates and primary amines.¹³ DHA is able to penetrate into the epidermis due to its size. Pyruvic acid is formed from the DHA, and either can react with sterically unhindered terminal amino groups in the amino acids of epidermal proteins.

Epidermal proteins contain high concentrations of both the epsilon amino group of lysine and the guanido group of arginine, which are particularly susceptible to nucleophilic attack by the reactive carbonyl oxygen of DHA. Based on photoacoustic depth profilometry, the tanning effect of DHA initially begins in the deeper part of the stratum corneum layer (15 μm–22 μm) before expanding over the entire stratum corneum and stratum granulosum.^{14, 15} Subsequent steps of the reaction mechanism are not fully understood, but the resultant products are brown in color and collectively referred to as melanoidins.

Alternate Actives

As previously stated, U.S. federal regulations recognize only DHA as a sunless tanning agent.⁹ However, alternative technologies exist with the capability to impart an artificial tan to skin. For instance, reducing sugars other than DHA can act as Maillard reaction intermediates and therefore have the potential for use as sunless tanning agents. Reducing sugars generally have a free ketone or aldehyde functional group where the initial condensation reaction of non-enzymatic browning will occur.

The most commonly known reducing sugar is glucose. Unfortunately, a large amount of heat energy is required to trigger the glycation reaction between glucose and free amines, and this limitation renders many reducing sugars useless for sunless tanning products.¹⁵ An exception, however, is erythulose, a ketotetrose. Although this reducing sugar produces a more gradual tan than DHA, it has been utilized as a self-tanning enhancer for years. As corporations continue to aggressively pursue new sunless tanning technologies, reducing sugars may provide the next generation of self-tanning actives.

More recent advances have exploited other mechanisms of action. One such natural active is extracted from Pisum sativum (pea)^a. Interestingly, this extract was found to increase the melanin index in vivo; increase skin pigmentation in the presence of UVB, versus an untreated control, ex vivo; increase skin pigmentation with no UV exposure ex vivo; and decrease IL-1 β expression in human fibroblasts when treated both before and after UVB exposure. This could find utility in enhancing skin tone both before and after sun exposure.¹⁶ Another alternative is a peptide from Lactobacillus ferment^b. This peptide induces the production of eumelanin, i.e., dark brown melanin, while limiting the production of pheomelanin, i.e., reddish/yellow melanin. By doing so, a deep, dark tan is produced.

Formulation Challenges

Due to the established use of DHA, the challenges of formulating DHA-based self-tanners will be discussed here. First, the amount of DHA used will depend upon the desired browning intensity of the skin; normally, this amounts to a range of 4% to 8% DHA. At this level, depending on the type of formulation and skin type, a tan generally appears about two to three hours after use. During storage over time, the pH of a DHA-containing formulation will drift to about stable. In order to ensure end product stability, however, other key factors must be considered.

pH and buffers: As mentioned, the pH of DHA-containing formulation drops during storage and the resulting pH lies in the range of 3 to 4. In the past, buffering was recommended to keep the pH at a level of 4 to 6. Investigations since, however, have shown that the storage stability of DHA could be increased when formulations are kept at a pH of 3 to 4, and buffering at a higher pH in fact enhances the degradation of DHA.⁸ The pH of a formulation may be adjusted to approximately 3 to 4 by using either a small amount of citric acid or acetate buffers, as they do not affect DHA stability.^{8, 17}

Processing and storing DHA: Storage and heating of DHA to above 40°C should be avoided as it causes rapid degradation of DHA. During manufacturing processes that require heating, as in the case of emulsions, DHA should not be added until the formulation has been cooled to below 30°C. Additionally, finished products containing DHA should be sold in opaque or other UV-protective packaging, as well as re-sealable packaging to limit exposure to air.

Nitrogen-containing compounds: Amines and other nitrogen-containing compounds should be avoided in DHAcontaining formulations. This includes collagen, urea derivatives, amino acids and proteins. The reactivity of DHA toward these compounds can lead to its degradation and therefore result in a loss in efficacy and unacceptable resulting color. However, some commercial formulations combine DHA with nitrogen-containing compounds, e.g., amino acids. This combination provides a perceptual advantage to customers as it tans the skin within one hour due to the accelerated reaction between DHA and amino acids; this tan is not substantive, however, and most of it is easily washed off.¹⁷

Sunscreens: A tan achieved with DHA alone does not offer sun protection comparable to that of sunscreens. Formerly, SPF products with DHA were marketed in the United States; however, with the FDA issuing a final over-the-counter (OTC) monograph for SPF products, many marketers will be discontinuing these combination products. This is due to a labeling conflict between directions for sunscreens, i.e., “apply every two hours,” compared with directions for self-tanners, i.e., “use daily for several days until a desirable color is achieved, then three times per week thereafter.”

Delivery Vehicles

Creams and lotions: Creams and lotions tend to be the most widely used of all the vehicles for self-tanners. Internal studies have confirmed that although they are conventional, creams and lotions are preferred by consumers due to their ease of use and reduced likelihood for streaky results. This is possibly due to the extended play time, e.g., rub-in time, offered by cream and lotion vehicles. The use of nonionic emulsifiers is recommended over ionic emulsifiers for improving the stability of DHA.¹⁸ In addition, emollients play an important role in many self-tanning formulations as they impart hydration to the skin, play time during application, and a smooth and silky after feel. Types of emollients may include oils, waxes, fatty alcohols, silicone materials and certain esters.

Gels and gelées: The thickening of formulations containing DHA, particularly to produce a clear gel, is relatively difficult because many conventional thickeners are not compatible with DHA. Studies have found that hydroxyethylcellulose, methylcellulose and silica are good choices, whereas carbomers, PVM/MA decadiene crosspolymer and magnesium aluminum silicate rapidly degrade DHA.¹⁸

Silicones such as dimethicone and cyclomethicones have increased in popularity over recent years, particularly for producing water-in-silicone emulsions, typically classified as gelées. Gelées are similar in appearance to gels however, they tend to offer improved play time and skin feel over gels since they contain high levels of the silicone emollients.⁸

Product Attributes

Beyond such considerations as the individual components, vehicle and formulating challenges, the product must of course provide given attributes in order to ensure consumer acceptance.

Coloration: As noted, the onset of coloration starts at approximately 2 hr to 3 hr and will continue to darken for 24 hr to 72 hr after a single application, depending on formulation and skin type. Since DHA forms covalent bonds with epidermal proteins, the tan will not sweat off or wash away with soap or water once the bond is formed. The color gradually fades over 3 to 10 days, in conjunction with stratum corneum exfoliation. As such, any product or process that increases the rate of cell turnover or removes portions of the stratum corneum will decrease the longevity of the color. Thus, preparations containing alpha- and beta-hydroxy acids and retinoids, as well as microdermabrasion creams and the process of shaving all will decrease the longevity of coloration from self-tanning products.⁸ Prior to application of selftanners, consumers should be advised to exfoliate the skin surface to minimize any steric hindrance between the DHA and the reactive epidermal proteins by removing shedding corneocytes.⁸

Moisturization: The recent trend in cosmetic products is to be multifunctional. Moisturizing self-tanner formulations are increasing in popularity in keeping with this trend; in fact, formulations with 8-hr to 24-hr hydration claims are not uncommon. Current self-tanners are formulated into sprays, lotions, creams, gels, mousses, and cosmetic wipes. In general, there are no obstacles to obtaining satisfactory levels of hydration, though there are some compromises that may need to be made. For instance, alcohol is often incorporated to achieve quick-drying self-tanner formulations. The trade off is sacrificing some level of hydration. This can be offset with humectants such as glycerin or sodium hyaluronate.

Evaluation

Various spectrophotometric methods can be used to evaluate the coloration parameters of self-tanners such as the onset and longevity of color. The most popular is the L*a*b* standard from Commission Internationale d’Eclairage (CIE). The three coordinates of CIELAB represent the lightness of the color (L*), its position between red/ magenta and green (a*), and its position between yellow and blue (b*). The total color difference between any two colors in L*a*b* can be approximated by treating each color—with their respective L*, a*, and b* components—as a point in a three-dimensional space and taking the Euclidean distance between them (ΔE). Delta E (ΔE) is calculated as the square root of the sum of the squares of ΔL* + Δa* + Δb*.¹⁹ It is generally recognized that about 1 ΔE unit is the minimal difference detectable to the naked eye. Comparisons to baseline readings can yield the onset of tanning, i.e., usually readings at 30 min, 60 min, etc., and the longevity of tanning, i.e., readings at 48 hr, 72 hr, etc.

Regulatory Considerations

The FDA considers sunless tanning actives as color additives since they impart color to the skin. According to 21CFR70,²⁰ color additives are defined as “…a dye, pigment, or other substance…that, when added or applied to a food, drug or cosmetic or to the human body or any part thereof, is capable (alone or through reaction with another substance) of imparting a color thereto.”²¹ The actives permitted, which as noted, only include DHA in sunless tanning products in the United States, are limited to those approved for such use. Labeling requirements are also specified under current FDA guidelines. All sunless tanning products that do not contain SPF protection must be labeled with the following warning statement, per the US Code of Federal Regulations: “Warning—This product does not contain a sunscreen and does not protect against sunburn. Repeated exposure of unprotected skin while tanning may increase the risk of skin aging, skin cancer and other harmful effects to the skin even if you do not burn.”²² Self-tanners using DHA are permitted for use in the EU as cosmetics. On the other hand, self-tanner formulations containing DHA are not permitted in China.

Trends in Sunless Tanning

Daily use moisturizers/glow: Face and body moisturizers with low levels of DHA have grown in popularity over the past five years. Though not new to the market, the concept of using a daily moisturizer that imparts gradual color was particularly well-received by those who were afraid of making mistakes and/or “turning orange,” such as occurred with traditional sunless tanners. Typically formulated with 1% to 3% DHA, glow moisturizers are easy to apply and, depending on the formulation and user’s skin tone, may impart a darker shade to the skin after 1 to 3 applications.

No-rub mists: No-rub sunless tanning mists have been sought as the expensive alternatives to the airbrushing trend. These multi-angle applicator systems allow for simple, even and oftentimes hands-free application. The formulation base systems are typically hydro-alcoholic or aqueous solutions, therefore allowing for quick-drying properties.

Sunless tanning with UV protection: As discussed earlier, the tan imparted by sunless tanners is not adequate to protect against UVB and UVA damage. Sunless tanners must therefore carry the required FDA warning statement.¹⁷ Sunless tanning products that do contain sunscreen may be removed from the market due to conflicting use instructions in light of the 2011 SPF Monograph issued by the FDA.

Summary

With an increasing awareness of the harmful acute and chronic effects of UV damage, sunless tanning use remains a popular alternative to tan seekers. Modern day formulations are efficacious, well-tolerated, easy-to-use and provide natural-looking results. A probable increase in consumer compliance of safe sun practices can therefore be anticipated.

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