Formulators of personal care products for children are faced with the difficult task of creating products with maximum care and minimum risk, all while trying to be innovative. Although young infants 0–12 months are sometimes treated with emollient oils and zinc oxide emulsions, many regions of the world do not apply skin care to children this young. The needs of a child’s skin evolve and change with age; therefore, one should ideally differentiate skin care for children into three groups: infants and toddlers (1–3 years); elementary-aged children (4–10 years) and pre-teenagers (10–12+ years).1 In the youngest skin care segment, an emollient effect is the primary goal, while the goal of the second segment is to protect from dryness caused by water sports and UV exposure. In the oldest group, seborrhea and acne treatment are necessary. This column will address the requirements of the first group, with particular attention to skin care emulsions.
Infant skin is different from adult skin in many ways. First of all, it is characterized by a immature stratum corneum with a higher moisture content and looser structure, compared with adult skin. Skin in infants is also about 20–30% thinner than in adults,2 and it produces less sebum and sweat.3 In other words, it is less protected from the penetration of substances, and its pH buffering capability does not respond quickly.4
Baby skin shows extreme reactivity to a wide range of topically applied substances. Frequently it is affected by diaper rash dermatitis due to prolonged contact with urine and feces in occlusive conditions. Finally, it has a number of hair follicles per square centimeter, abundantly higher than in adult skin, which increases risk of deeper penetration of substances. Even though the amount of skin surface lipids is significantly lower in infants than in healthy adults, almost all evaluation methods show that the barrier function adequately protects the infant, even if the barrier function is lower than in adults. Any induction of toxic effects into the body via the skin is mainly due to the greater skin surface/body ratio.5
The key requirements for baby skin care creams include: a pH value to match the acid mantle (4.2–5.6); the ability to decrease water evaporation and the skin’s tendency to dry out; quick moisturizing and protective effects; benefits such as preventing and reducing inflammation; microbial purity and ability to counteract microbial growth; sensory reward, i.e., freshness, during and after application; maximum purity from all toxic, irritant and allergenic substances; no transdermal delivery enhancers; and chemical and microbial stability.
Babies cannot wait for long term efficacy. Emulsions are the best way to quickly relieve disequilibrium in infant skin while providing protection, water organization and an emollient effect. The separate structures of the main phases are discussed below.
Oil phase: The most frequently used oils in baby care vary from pure, saturated mineral oils to vegetal oils, rich in unsaturated fatty acids. For decades, mineral oil and petroleum jelly were used in baby care emulsions for their chemical inertia, soothing properties, excellent feel and water repellency effect. The detailed chemical composition of the commercial blends is largely unknown. Nevertheless, their potential of inducing skin occlusion has never been described as a problem, even though they are applied to sensitive body areas. Recently, the European Union (EU) has placed special emphasis on the requirements for high chemical purity in all petroleum-derived molecules, especially for polycyclic hydrocarbons impurities.6
Food-grade hydrogenated polydecenes and polyisobutenes have been proposed as possible alternatives to mineral oils, as they do not contain any poly-nuclear hydrocarbons. Vegetal oils and butters combine complex emollient triglycerides with their ‘train’ of natural soothing ingredients like phytosterols. One recent example of the use of vegetal oils in infant care products is Neways Generations Baby Lotion, with babassu seed oil, shea butter, meadowfoam seed oil and dog rose fruit oil in its oil phase. Other products on the market rely on sesame, sweet almond, rice, palm or sunflower oils. Inconveniently, these oils contain triglycerides with easily oxidized fatty chains, so to avoid rancidity, blends of different categories of oils with distinct polarity levels is suggested to optimize the formula’s oxidative stability, the product’s sensorial profile during massage, the supply of soothing actives, the emollient effects and water coordination power. In this respect, the use of silicone oils that possess most of these properties is increasing. Adequate antioxidants and chelating agents are always necessary to protect the shelf-life of this phase. Usually the oil phase content is included at 15–20% in o/w emulsions, while it can reach 40–45% in w/o systems.
Formula 1 is an o/w baby balm7 that uses a polymeric emulsifier, which is compatible with the pre-dispersion of zinc oxide, with the stabilizing support of a less hydrophilic emulsifier, trideceth 10, and the bulky lipophilic emulsifier polyglycerol-3 polyricinoleate. The oil phase is almost completely natural. The water phase shows the maximum possible simplicity.
Aqueous phase: The water phase of baby care emulsions is simple and based on traditional, highly safe ingredients. Glycerin is the most commonly used humectant, even if many other polyols are theoretically possible. Allantoin and D-panthenol are generally present to soothe and help skin functionality. Vegetal extracts such as aloe leaf and marigold flower provide soothing properties for their mucilaginous content. Oat as colloidal meal or its active principles are also used as an anti-irritant. Hydrophilic polymers are introduced generally at low concentrations, to accomplish rheology requirements. The spreadability of baby care creams must be high, and complex rheological profiles of the finished product are not necessary. One important reason for the selection of thickeners is the cushion and playtime they provide.
The osmotic equilibrium of this delicate skin should also be taken into account. Osmotic protectants like trehalose, betaine and ectoine may be used in the formulation strategy to avoid cell stress due to wayward concentrations of water-soluble substances. In Sebamed’s baby products, the aqueous phase provides ingredients with pH 5.5 buffering properties, e.g., lactate or citrate buffers, to help protect infant skin. Urea is often added to the water phase of baby products along with salicylic acid to treat cradle cap.
Formula 2 is a w/o fluid emulsion8 with a high amount of zinc oxide, which shows how the product rheology is dependent on the emulsifier properties. Soothing ingredients are glycyrrhetinic acid, allantoin and calendula extract. This formula is suitable for both diaper rash treatment and sensitive skin as a daily skin care and sun protection product. Hydrogenated polydecene allows for homogenous product distribution without friction and reduces the evidence of whitening, which is a usual drawback of these suspension-emulsions.
Emulsifiers: Once in contact with the skin surface, emulsifiers in a cream may interfere with the structure of the hydro-lipidic film of the epidermis, modify its bilayer emulsion organization, and increase water loss from the skin. For this reason, the criteria for choosing an emulsifying system should include maximum compatibility with the skin and minimum interference with the physiological protective emulsion of skin lipids. Vegetal fatty acids combined with polyols (e.g.sorbitan olivate) are a possible category of skin compatible emulsifiers, together with the least skin disturbing sodium stearoyl lactate. More recently, polymeric emulsifiers have found extensive applications, as they do not maintain their lipid emulsifying action once applied to the skin. The combination methyl glucose sesquistearate and PEG-20 methyl glucose sesquistearate is suggested for its low eye irritation potential, as baby products are more likely to come into contact with the eye area. Low dioxane impurities, i.e., between 4 ppm and 1 ppm, are also a key requirement from most safety conscious industries when ethoxylated emulsifiers are used.
Of course, the initial decision of the formulator rests on which type of emulsion is used, and then which polarity of emulsifier is necessary. Thick w/o emulsions are preferred for application onto diaper irritated areas for their long-lasting action and water repellency, while light o/w emulsions are preferred for general and fast emollient effects. The amount of emulsifier should be kept as low as possible. Polymeric w/o emulsifiers work well at concentrations between 2–4%.
High skin compatibility is achieved in Formula 3 by replacing traditional emulsifiers with synthetic emulsifying polymers, vegetal sterols and phospholipids. A blend of unsaturated long chain triglycerides and hydrogenated polydecene exhibits emollient properties and protects the skin. Moisturizers and skin protective ingredients soothe infant skin, which as noted, has a reduced barrier and is more sensitive to irritants.
Typical w/o baby creams for diaper rash contain zinc oxide as a soothing and lightly antimicrobial ingredient. This property is characteristic of other insoluble zinc compounds, like zinc carbonate, and its function seems related to surface activity of the crystalline lattice. It should be noted that aqueous suspensions of non-coated zinc oxide tend to provide a slightly alkaline reaction (pH 7.9–8.2) at equilibrium that cannot be buffered until solid zinc oxide is present.
Surface interactions take place between the dispersed solid and surrounding vehicle. To obtain a spreadable cream, the emulsifier amount should be higher than in solid-free emulsions. The maximum load of zinc oxide in w/o emulsions (10–20% w/w) strictly depends on the characteristics of the emulsifier used. Not only is the zinc oxide percentage important, but its particle size is also important, as its soothing activity is mainly bound to the surface extension and not to its weight in the formula. For this reason, the type of zinc oxide is different from the coated, micronized type used as UVA sunscreen.
Other zinc derivatives like zinc PCA or zinc carbonate may also be used. Soothing vegetal extracts such as chamomile, calendula, ruscus and hamamelis are frequently used, sometimes in combination. These extracts are selected for their soothing benefits as well as the fact that they commnicate beneficial effects. Selecting ingredients with a “soothing” profile in their name is also a common practice for emulsifiers, thickeners or oils in baby care, even though this action may not be their primary function. For example, sucrose esters may be selected for their “sweet” identity.
The dilemma concerning the choice between traditional and non-traditional preservatives is even more acute when dealing with baby skin. A combination of preservatives with minimal harmful potential must be selected when considering the risks of transdermal absorption. Many companies under pressure by opinion leaders try to avoid the use of formaldehyde releasers, thus alternative preservatives like capryloyl glycin, undecylenoyl glycine, higher glycols, etc., are increasingly successful. Some manufacturers are reformulating products to omit parabens, if they have not done so already. This phobia of parabens is difficult for the science-driven formulator to understand, especially when deprived of a powerful antibacterial tool. Therefore, a limited and wiser use of parabens, i.e., the shortest chain molecules, would be a better answer to market pressure.
Actives and Fragrance
Actives in baby products typically are intended as soothing and calming ingredients. Besides the aforementioned allantoin and panthenol, modern actives can be included like one recent vegetal blend that reduces skin reactivity, provides quicker recovery from irritation, and reduces redness and itching by working against hyperosmotic stress. Another vegetal extract from pine bark provides anti-irritant, antioxidant and alternative preservative actions at the same time.
Aromatherapeutic scents are usually preferred in baby care; in fact, Babo Botanicals reports that lavender notes have a calming effect on babies/children and moms. However, where there is proven or suspect contact allergy to perfume ingredients, it is better to avoid all perfume ingredients. Interestingly, non-perfumed baby products should be preferred because when off-odors develop due to formula instability, their perception by the user could discourage product use and therefore avoid skin contact with unstable ingredients.9 It is suggested that colors and pearls also be avoided for the same reason.
Since the skin compatibility of formulations cannot be evaluated on babies, the wise formulator knows the only way to test his formulae is on adult people with sensitive skin, while adopting exaggerated test conditions like repeated insult applications. In vivo evaluations must be conducted on logical, scientifically sound models. It is important to obtain reliable, reproducible results with logical interpretation. SPF evaluations are always conducted on adult humans, even if they are aimed to protect children. To involve children in in vivo evaluations is unethical, risky and difficult to standardize. The selection of a consenting adult panel of subjects having sensitive skin can be more accurately normalized and easily practiced.
The personal care market has witnessed a growth in baby care products, specifically skin care, which are often formulated with organic ingredients to ensure safety for the infant. One example is the Soothing Cream by Episencial. It is based on vegetal-derived glycerin, olive oil-derived emulsifiers and citric acid, in addition to a glutamic acid derivative, i.e., tetrasodium glutamate diacetate, as the sequestering agent. Preservation is obtained through caprylyl glycol and the mild-scented phenylethyl alcohol, found in rose essential oil. Oxidation is prevented by a non-GMO tocopherol. Ingredients derived from fermentation include xanthan gum and a yogurt derivate. All the oils and butter—safflower, shea, avocado, blueberry, etc.—are certified organic. Perfume is provided by chamomile flower/leaves extract.
Ethno-botanical ingredients traditionally used by ancient Native Americans, oils grown in Indonesia and the Pacific Islands or Swiss moisturizers are combined in lines like BabySpa. Examples are Aleuritis moluccana (Kendi) and Calophyllum inophyllum (Nyamplung) seed oils, which are exceptionally rich in omega-3 polyunsaturated fatty acids. These ingredients are efficent against skin dryness, as they help to reconstruct the impaired skin barrier. Their exotic appeal resides in the communication of rediscovered ancient traditions, the fascination of far-away countries, the appreciation of Asian folk medicine, and imagination.
Formula 410 is a good example of simple and modern o/w emollient emulsion, with reduced amount of lipids and a preferential choice for natural derived ingredients. Preservatives are of the non-standard type, and emulsifiers are reduced to the minimum possible.
In Formula 5,11 the oil phase (17.5% w/w) is an equilibrated blend of branched and linear polar compounds—octyldodecanol, propylene glycol dicaprate/dicaprylate and caprylic/capric triglyceride for the first group; and behenyl and batyl alcohol for the second. The first group is used for its ease of distribution and massage over the skin, the second for giving the sensory signal when sufficient massage and the limit layer have been reached. Providing the skin with the unsaturated lipids necessary for proper-moisture holding capacity is achieved with raspberry seed oil, with its double bonds adequately protected by the antioxidant. Moreover, this phase contains a good amount of silicone for easy spreading over the skin and a little amount of beeswax for its film-forming properties.
Emulsifiers are selected for their efficient performance as emulsion stabilizers. There is a relatively higher amount of emulsifier ingredients in this formula, as 4% would be more typical. One possible reason is that the composition of the fatty phase is heterogeneous. The alkyl chains branching would permit the sliding of emulsifier molecules at the surface of oil droplets; therefore, extra emulsifiers are required to fix them in place. One more explanation is that the suspended powder of the zinc derivative requires some extra emulsifier for its good wetting.
The processing method for Formula 5 is traditional for o/w emulsions. It evidently tries to avoid prolonged heating of the unsaturated vegetal oil. The addition procedure of xanthan gum and sclerotium gum is not thoroughly detailed, but it is suggested to disperse the gums well into the cold water and heating the blend to 40°C. Operate the turbine for 30 min at this temperature to avoid the formation of fish eyes. Some companies create a pre-dispersion of the powders in a humectant, butylene glycol in this case, then add the slurry to water under a homogenizer. Long homogenization is usually unavoidable but it was discovered that by wetting xanthan gum with 4% mio-inositol produces efficient swelling at RT without the need for heating and homogenization.
Soothing activity, besides that of the natural alpha-bisabolol, is achieved by the insoluble salt zinc PCA, which joins the anti-inflammatory properties of zinc insoluble compounds to the moisturization given by PCA. It represents a modern alternative to the common zinc oxide. In addition, allantoin plus skin-repairing vegetal extracts and inflammation reducing oligosaccharides of beta glucan complete the healing properties complex.
Thickeners of the aqueous phase are selected among the polysaccharide family for better skin compatibility and a good level of moisture retention. Sclerotium gum is not only a thickener but also a moisturizing active principle. Moreover, it cooperates with xanthan gum in obtaining an elastic network in the aqueous phase and maintaining suspension of the zinc PCA. Finally, preservatives and perfume should be selected with the previously described criteria in mind.
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