When updating a sunscreen formulation to meet today’s requirements, formulators must consider improved water-resistance as a key parameter, since sunscreens are often used outdoors near bodies of water. Reapplication of the product after each water exposure is undesirable for modern consumers who seek both comfort and ease of use from their sun care products. Moreover, the protection provided by a sunscreen can be compromised with sweating; in fact, the reduced efficacy of sun care products due to perspiration has been shown in athletes, where rates of skin cancer have grown.1, 2
The term waterproof can no longer accompany sun care products, as it provided an excessive sense of safety to consumers and increased their risk of being burned. Waterproof has been substituted by the claims water-resistant or very water-resistant, when the sun protection provided by a product is reduced by less than 50% after a lukewarm bath lasting 40 min or 80 min, respectively.3, 4 Many have tried to update this evaluation procedure, including an attempt to develop an in vitro non-human test; however, the traditional in vivo method remains the most adopted.
Water-resistant products are more likely to achieve lower transdermal delivery since their components are contained inside the applied film. This factor allows water-resistant sunscreens to provide longer-lasting protection than standard sunscreen products. In addition, the filters entrapped in the film will have low transdermal delivery to the body, reducing the risk of chemicals penetrating the skin and lowering the accumulation of said substances. Beyond maintaining the efficacy of sunscreens when they come in contact with water, there is also increasing concern related to the accumulation of trace sunscreen ingredients that could harm the resident flora and fauna in lakes, rivers and oceans.5, 6
At first sight, the formulation of a water-resistant sunscreen does not seem different from other sunscreens. In general, water-resistant formulae provide medium to high sun protection (SPF 25–50+); however, looking closer reveals their real nature and the related formulation strategy. In general, sufficient or high water-resistance can be accomplished by: preparing a fully anhydrous formulation; creating a w/o emulsion; adding water-resistant film-forming polymers to standard emulsions; adding alcohol to dissolve water-insoluble substances that deposit onto the skin from an insoluble layer; obtaining a meta-stable o/w emulsion with a low level of emulsifiers; and entrapping filters in skin-substantive liposomes or sponges. One or more of these strategies are used when formulating a water-resistant formulation.
It should be noted that water-resistance should be obtained without impacting the feel of the product, which cannot be occlusive, heavy or sticky. Moreover, water-resistant formulations can be developed to adhere to an organic/natural strategy, i.e., using mainly certified vegetal oils and zinc oxide, or using a high-tech approach, employing a wide range of modern silicones.
In theory, the simple strategy of anhydrous formulation has many advantages for water-resistant sunscreens. The absence of emulsifiers avoids subtracting the oil layer from the skin. Moreover, sunscreens are generally soluble in many polar oils and do not reach their limit solubility because of the high amount of solvent materials. Nevertheless, oil formulae are not the most popular cosmetic form for sun protection, mainly for sensorial and cost reasons. However, they are generally preferred by men, as they are considered more masculine than a cream. Anhydrous formulation is generally used for aerosol or stick sunscreens, with sticks aimed to protect specific areas like the face or lips, or used by those having dry skin.
Oils are selected according to their substantivity and adhesion to the skin. For example, cetyl octanoate forms a water-resistant film on the skin, and hydrogenated polyisobutene can be used in place of mineral oils for its lubricating action and the shine it imparts to the finished formula. Due to its low density, a combination of sucrose acetate isobutyrate and caprylic/capric triglyceridea is useful in water-resistant spray sunscreens. Phenyl trimethiconeb can also be used in anhydrous sunscreen formulations for water-resistance, including for hair care.
This type of formulation has the advantage of difficult removal from the skin surface. When water has evaporated from skin or been absorbed by it, the emulsifier is homogeneously dispersed in the oil phase, and there is not enough energy available in the oil + emulsifier layer to re-incorporate water droplets from the environment. Moreover, these systems show good compatibility with skin physiology and allow the normal evaporation of sweat. Aside from improving the affinity of oil to moist skin, the w/o emulsifier allows for solubility of the water as a microemulsion in the oil phase. Therefore, the water from sweat can be temporarily incorporated into the residual film on the skin, then evaporate into the environment.
In some hot countries, the special skin feel of w/o emulsions, which can be massaged for a long time without ever seeming to be absorbed by skin, is not popular; of course, it is the opposite in cold climates.
Frequently used w/o emulsifiers include a combination of polyglyceryl-6 polyricinoleate, polyglyceryl-2 isostearate and disteardimonium hectoritec in non-silicone formulae; or a blend of cyclopentasiloxane, PEG-10 dimethicone and disteardimonium hectorite d in silicone-based formulations. For mixed silicone/non-silicone oil phases, a balanced blend of the two emulsifiers is suggested, where the key advantage is the light feel of the obtained emulsion.
W/O sunscreen emulsions are generally stabilized by external phase thickeners, which provide residual resistance to the flow to the oil film deposited on the skin. In organic products, such rheological behavior is obtained by the cautious addition of beeswax, low enough to avoid skin occlusion. This is the strategy adopted by La Roche-Posay in its Anthelios 50 Mineral Ultra Light Sunscreen Fluid (see Ingredients: Anthelios 50 Mineral Ultra Light Sunscreen Fluid).
Adding water-resistant, film-forming polymers to sunscreens is the most common strategy for imparting water-resistance, and the market offers several polymers having a wide range of insolubilities, once dry. These provide a light, residual feel and in many cases, give a natural appearance. One example is a range of waterborne polyurethane dispersions, where polyurethane-34e offers water-resistance while providing a non-greasy, non-sticky skin feel. Cyclodextrinf and a combination of dimethicone and vinyldimethyl/trimethylsiloxysilicate/dimethicone crosspolymerg form a complex that insures long-lasting water-resistance while protecting UV filters via inclusion into the cyclodextrin ring. The film-forming anionic polymer dehydroxanthan gumh, when used in combination with acrylates/octylacrylamide copolymerj, extends water-resistance. This combination is said to be especially suitable to avoid the clumping of solid particles used in broad spectrum mineral sunscreens with titanium dioxide and zinc oxide. Acrylates/C12-22 alkylmethacrylate copolymerk can also produce very water-resistant formulae and has high compatibility with inorganic and organic sunscreens; it is especially used in sunscreen sprays or those containing alcohol.
Another interesting film-former that provides a flexible film, is completely insoluble in water when dry, and becomes viscous and slippery at humid conditions is VP/dimethiconylacrylate/polycarbamyl polyglycol ester/VP/polycarbamyl polyglycol esterm. Cyclopentasiloxane (and) diphenyl dimethiconen (Gransil C-DPDM, Grant Industries) provides skin softness, shine and enduring water-resistance. Also, grafted silicones like C20-40 alkyl dimethiconep offer an impermeable film for water-resistance while decreasing the stickiness and greasiness of the residual layer. Other waterproofing polymers include PVP derivatives, e.g., triacontanyl PVP, which are hydrophobic and efficient pigment dispersant ingredients. This list could go on indefinitely; these are but a few representative examples.
An example of the polymeric strategy for obtaining very water-resistant formulations with SPF 50+, using filters allowed in the EU, is shown in Formula 1. In this case, the polymeric agent strategy is combined with the alcohol driven “dissolving, then precipitating” action and low emulsifier effects. In this formula, the combination of sunscreens is the key for reaching a high SPF. The amount of hydrotropes is low to avoid a high affinity for water. Moreover, high amounts of humectants would increase the skin moisturization level, consequently making the skin more transparent to UV rays and decreasing the measured SPF.
The fatty phase, including fluid filters like cinnamate that act as solvents, must be tailored to dissolve the sunscreens; like any other active, when sunscreens are in a saturated-solution state in a topically applied product, the driving force for penetration into the skin is the highest. Sunscreens, of course, are required to stay layered on the skin surface. Polymeric emulsifiers in general give the advantage of reducing the re-emulsification of oils when in contact with water or sweat. The monomeric emulsifier and emulsion stabilizer are reduced to a minimum, just to allow re-emulsification in the bottle thoroughly when shaken. Alcohol helps in dissolving the substantive acrylic polymer and to deposit it after evaporation. The ethanol solution of the polymer is added when the emulsion has reached a temperature lower than 30°C, while the previous phases are carried out around 75°C.
Dissolution with Alcohol
The limited solubility of sunscreens and some water-resistant polymers in synthetic and vegetal oils poses a large hurdle for formulators. Adding ethanol to the formula can frequently solve this issue. At limited concentrations (5–20%), alcohol is safe even in a cosmetic formula for sensitive skin. This addition allows the formulator to minimize or even avoid the use of preservatives and reduce the amount of necessary emulsifiers, which increases the water-resistance of the formula and reduces its transdermal delivery. Finally, it provides a fresh sensation upon application, especially in summer, and makes the product fluid. The drawback of alcohol addition is the application of safety norms in the production plant, which could be reduced by a side preparation of dilutions of the pure alcohol with water or other hydrotropes.
In Riemann & Co.’s P20 SPF 20 sunscreen product (see Ingredients: Riemann & Co. P20 SPF 20), ethanol is used both as the main solvent of all sun filters and as an election solvent for the acrylate copolymer. As soon as alcohol evaporates, the polymer will precipitate and form a water-resistant film on the skin. The alcohol’s evaporation from this transparent fluid gel is broken by hexyldecanol, while cyclopentasiloxane works as sensorial modifier. The propoxylated alcohol will favor the skin wetting and adhesion of sunscreens to the skin, while hydroxypropyl cellulose will provide cushion and insure long lasting compatibility with sweat during the day.
Another interesting example applying alcohol in high amounts is Johnson & Johnson’s Neutrogena Wet Skin Sunblock Spray SPF 30 (see Ingredients: Neutrogena Wet Skin Sunblock Spray SPF 30). This product is intended for application on wet skin. Again, alcohol dissolves the film-forming complex of polymers octyldodecyl citrate copolymer, acrylates/octylacrylamide copolymer and acrylates/dimethicone copolymer, with the additional help of dimethyl ether. After being diluted with water, these film-formers will be insoluble and adhere to the skin as a fixed film that incorporates the sunscreens. Another example of this combined strategy is shown in Formula 2, which uses a water-repellent silicone polymer,7 a film-former and sodium stearoyl glutamate as a low emulsifier.
Meta-stable O/W Emulsion
Oftentimes, water-resistant sunscreens without a gas-propelled spray are requested. This requires thin formulae and powerful spraying valves. More fluid formulas have the higher possibility of creaming and separating, a risk dramatically increased by low amounts of emulsifier. If emulsification is poor however, its stability will be further challenged by sebum when it reaches the skin; the emulsion will separate into oil and water phases during massage, and the applied film will neither re-emulsify easily nor resist the action of water.
One further help is given when a soap, such as sodium or triethanolamine stearate, is included as an emulsifier. The acidic pH of the skin’s mantle will disassemble the soap, reduce the amount of efficient emulsifier and precipitate the stearic acid, which adds to the insolubility of the film on the skin.
Entrapping filters in liposomes is a new formulation approach. For example, CoLabs International Corp. developed a technologyq whereby sun filters are deposited onto the skin during showering or bathing. This is achieved by encapsulating the sunscreen into substantive polysaccharide film that forms microcapsules that are positively charged. When used as a shampoo, sunscreen protection is provided in one step to the body and hair, and other areas not typically protected on a daily basis, such as the scalp, ears and neck. After application, the skin is soft and smooth due to the cationic structure of the technology. Encapsulation of the sunscreen improves formulation stability over time, and ensures an efficient, photostable and adherent type of sunscreen shield. Transdermal delivery and systemic absorption of filters is also reduced. The benefit for individuals with thinning hair or for those who engage in outdoor activities is evident.
Water-resistant spray formulations are the perfect example of modern sunscreens, as they are highly efficient, come in convenient packaging, have a light touch, can be used all over the body, provide long lasting protection, impart maximum safety, and are photostable. However, new steps to advance this technology further are still sought. Future iterations could incorporate a color-changing alarm when exposed to the sun, or technologies that tailor the amount of sun protection to the level of sun exposure. Currently, the sun care industry could work toward body sprays that instantly make skin’s sun protection resistant to water.
- M Moehrle, Ultraviolet exposure in the Ironman triathlon, Official J of the Amer College of Sports Medicine (2000)
- TW Bender, Cutaneous manifestations of disease in athletes, Dermatology for the Clinician 2(1) 34–41 (Jan/Feb 2003)
- M Díaz-Cruz, M Llorca, D Barceló, Organic UV filters and their photodegradates, metabolites and disinfection by-products in the aquatic environment, TrAC Trends in Analytical Chemistry 27(10) 873–887 (Nov 2008)
- H Buser et al, Occurrence of UV filters 4-methylbenzylidene camphor and octocrylene in fish from various Swiss rivers with inputs from wastewater treatment plants, Environ Sci Technol 40 (5) 1427–1431 (2006)
- Commission Recommendation of 22 Sep 2006 on the efficacy of sunscreen products and the claims made relating thereto, Official Journal L 265 0039-0043 (Sep 26, 2006)
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