Years of wear and tear can be hard on the skin of the feet, as it is the continuous point of contact between human mass and the earth’s surface. This part of the body is frequently neglected while small, progressive and imperceptive damage accumulates in its structure. The consumer typically does nothing about foot damage until there is evidence and a cosmetic, and in some cases medical, need.
Many dermatological diseases such as athlete’s foot occur because the feet spend long spans of time in a warm, dark and humid environment, i.e., the shoe, which is ideal for the formation of the fungus. Fungal and bacterial proliferation can cause dry skin, redness, blisters, itching and peeling to occur. While there are several foot diseases related to diabetes, including gout, poor blood circulation or improperly trimmed toenails, this column will focus on the development of foot care formulae designed for cosmetic treatments.
Skin of the Feet
The skin of the feet has a structure and function similar to skin in other body areas, with some important differences. The skin on the foot is thicker than on the rest of the body; as it adsorbs the stress and strain of an upright stance, the most external layer of the epidermis on the sole hardens early in life and may reach 5 mm in thickness, which is 20 times thicker than in most other areas on the body. With this type of heavily compressed structure, it is evident that the skin of the feet, specifically the sole, requires targeted products with higher concentrations of water binders in comparison to other cosmetics.
The skin on top of the foot is not affected by the same structure-threatening conditions unique to the bottom of feet, such as corns or plantar warts; although both can be afflicted with bacterial infections due to continuous friction and a humid environment. Sunburn is a more common occurrence on the skin on top of the foot due to the neglect of this area during sunscreen distribution to the body.1 Generally, consumers do not remember that UV radiation contacts the upper part of the foot perpendicularly, allowing that skin to absorb the maximum possible amount of solar energy—with damaging consequences.
Foot Care Emulsions
In all personal care categories, emulsions play an important role but foot care emulsions have a number of specialized characteristics. First, foot care emulsions contain lower levels of lipids, 12–15%, to avoid a residual oily feeling on the feet. One could compare this amount to what is normally adopted in after-shave emulsions or in body lotions designed for oily skin. Rather, these emulsions include easily absorbed, medium emollient, non-occlusive lipids that are generally fluid at room temperature (RT). Polar esters are preferred but medium chain triglycerides also can be incorporated. Rarely are hydrocarbons used and if so, they are never used alone in order to avoid the risk of skin occlusion. A fraction of the total oil phase should be made of polyunsaturated fatty acids (PUFAs), 1–2%, for better relief of dry skin. Further, when PUFAs are utilized, the formula must be protected from rancidity due to oxidation. Also, these emulsions incorporate minimal levels of emulsifiers to avoid the perception of soapiness during the application phase and excessive skin drying in longer-term wear.
Velvety-feeling powders such as talc, aluminum starch octenylsuccinate and fumed silica gel, are often used to provide slip and easy wear of socks, but also to absorb excess sweat. In addition, keratolytic agents such as salicylic acid and allantoin are included for the elimination of shed corneocytes and skin softening. Polyols like glycerin, sorbitol and xylitol are used to fight immediate skin dryness, although they are not incorporated in large amounts, just 2–3%, so as not to interfere with the water activity and to maintain a fast evaporation of the aqueous phase. A quickly dried skin surface after cosmetic application is always an important requirement.
Urea, which is a wound healer at low concentrations but also an efficient skin humectant and keratoplastic agent, is often used in foot care emulsions at levels varying from 1–50%. Hydrolysis protectants such as pH buffers, hydrotropes, sodium ascorbyl phosphate, lactates and polyhydric compounds, are often used to slow urea decomposition, and their use amount is proportional to the amount of urea in the formula.
Bacteriostatic and anti-mycotic agents often are used as well because the feet are close to a number of microorganisms. Odor absorbers are also incorporated to catch small molecules produced by bacterial metabolism and released by shoe materials. Perfumes also are used to hide uncatchable, off-odor molecules. Finally, the usual antioxidants, bactericides and sequestering agents are incorporated into foot care emulsions. Safe, volatile solvents like water or ethanol-water blends are used for quicker distribution and fast-drying properties. Moreover, these solvents quickly supply water to the horny layer. If surfactants are required, such as in foot cleansing emulsions, they typically are mild in nature, like sarcosinates, isethionates and sucrose esters. In general, foot care emulsions may be subdivided into three main key functional categories, to: fight dryness, reduce excessive sweating and relieve tiredness, which are described herein. These key categories are often accompanied by other purposes, such as massage and anti-fungal/antibacterial benefits.
Dry, chapped skin is one of the most frequent problems on the soles of the feet. This uncomfortable condition is the result of excessive water loss from the most external part of epidermis, the horny layer, which causes shrinking and flattening of the skin cells. Following this structural change, the capability to hold water to make the skin elastic, soft and compact, decreases dramatically.
The water-holding capacity of skin in general is related to the presence of complexes of small hydrotropic molecules such as urea, glycerin and pyroglutamic acid. These molecules are carried by sweat or produced by the lysis of skin cells; by fatty materials that structure and coordinate water around the horny cells; and finally, and most importantly, by ceramides and sphingolipids that can create a spaced organization in the architecture of the water and fatty materials compartments in skin. Aging, high temperatures, irritant chemicals and type of shoes may impair such organization. The progressive slowing of the physiological mechanisms that produce water-holding lipids, a typical sign of aging, adds to all these factors.
When the epidermis lacks enough water, the sides of keratinized heels, which are under the heavy pressure of the body weight, are subject to vertical chapping and cracking due to their lack of elasticity. This can create pain during impact with the ground. In general, cosmetics developed to treat such conditions contain urea, which can be difficult to use because of its relative instability in aqueous solution and its destabilizing effect on emulsions since it is a disruptor of the water associative structure. Its stability can be improved, however, by combining it with sodium ascorbyl phosphate or alpha hydroxy acids in a solution. As urea is soluble in water, concentrations up to 50% can be reached in aqueous solutions. At such high levels, w/o emulsions are preferred for obtaining long-term stability. Urea is an efficient keratoplastic ingredient and strong water binder but its action must be accompanied by adequate ingredients in the oil phase for long-lasting effects. It is frequently associated with antibacterial agents but also with lenitive ingredients to reduce the pain due to skin chapping.
Formula 1 is an example of w/o emulsions for dry skin foot care, and its characteristics as a high internal phase w/o emulsion are evident by the type of emulsifier used and presence of magnesium sulfate. Alpha and beta hydroxy acids are present to stabilize urea and condition skin, and the buffering effect is provided by the arginine salts of the same acids. As the 10% aqueous dispersion of this emulsion has a pH 4.0, free salicylic acid is present to perform an efficient keratolytic and exfoliating action. Allantoin will help the dissolution of dead skin cells to provide smoother, more even skin. Also, as noted, low residue after application is a key requirement of foot care products, which is guaranteed in this formula by volatile silicone and hydrogenated polydecene. This type of highly branched-chain hydrocarbon, with a rounded molecular shape, has a strong lubricating action that leaves a very light, nonocclusive, final perception after application. To maximize the effect of urea, product developers have suggested applying such products to the feet before sleeping and covering the feet with a pair of special socks to enhance transepidermal diffusion and deep absorption.
Natural dry skin foot cream: An example of a foot care product for dry skin is Nuxe’s Ultra-Comfortable Foot.2 This cream is based on oils of wheat, rice hull, sesame, soybean, echium and tomato, as well as on shea butter, all enriched with acacia honey. This product has proven that in order to fight dryness for 24 hr, lipids are required, especially those rich in polyunsaturated fatty acids. In this case, the sequestering agent phytic acid is vegetal, and other hydrophilic moisturizers derived from this same source are used, including: a blend of glycerin, anhydro-xylitol, xylitol and xylityl glucoside. The antibacterial action is performed by ethylhexylglycerin, caprylyl glycol and dehydroacetic acid. As with the previously mentioned foot cream, the manufacturer recommends wearing socks after application to ensure maximum efficacy. Skin relief formula: A traditional marketing approach is to transfer a successful hand product into a successful foot product, as this company has achieved. The Neutrogena Norwegian Formula Foot Cream contains the most traditional fatty phase—cetearyl alcohol, petrolatum, cyclomethicone, dimethicone and stearic acid, combined with the emulsifier sodium cetearyl sulfate in the presence of C16–18 fatty alcohols.3 Less important actives include keratin amino acids for skin protection, menthol for a cooling sensation, and allantoin for dead cell dissolution. However, the antioxidant and ceramide-strengthening action of tocopheryl linoleate is quite interesting. It is a multiple-action molecule, providing the antioxidant power of vitamin E to protect both the skin and the linoleic chain, in addition to supporting the skin barrier with linoleic acid. Dilauryl thiodipropionate also protects the formula from oxidation. Besides such standard emulsion formulae, the market also offers hydrating mousses that provide a light perception due to their high gas content, allowing for socks and shoes to be immediately worn after product application; for instance, Dr. Scholl’s Crackling Ice Foot Spray.
Bacteria for foot exfoliation: Xerosis, or extremely dry skin, is accompanied by roughness that may require intensive exfoliating, such as on the heels and on thickened nails. This is usually accomplished by the combination of physical exfoliants including pumice or polythene spheres, together with alpha hydroxy acids. A recent patent describes the use of Pseudoalteromonas ferment extract for repairing cracked heels, which has objectively been demonstrated by in vivo trials.4
Reducing Excessive Sweat
Foot sweat can be produced by the shape and material of shoes, the structure and composition of socks, or even an individual’s predisposition. In the summer, foot sweat can lead to skin maceration for occlusion, which is usually aggravated by heavy bacterial and fungal proliferation, irritation and odor formation. Cosmetic products for sweaty feet aim to absorb excess sweat; provide a fresh, cooling perception; and neutralize bad odors. These products are typically in the form of light creams or aspersion powders, which also reduce bacterial proliferation in the shoes. Anti-odor strategies are very similar to those used in underarm deodorants, i.e., mainly performed by bacteriostatic and fungistatic agents, antiperspirant salts, enzymatic deviators, odor absorbers and perfumes.
In the Decleor Foot Care Cream, the essential oils of thyme, clary, cypress and tea tree are claimed to reduce perspiration and minimize bacterial proliferation.5 Moisturization is imparted to feet by the synthetic esters octyl-dodecyl-stearoyl stearate and octyl palmitate, together with the vegetal oils avocado, coco-caprylate/caprate, shea butter and meadowfoam seed oil. Odor-fighting agents, which are used in most cosmetic foot care products, attempt to omit traditional preservatives because their use is considered risky for a body surface like the feet, which is occluded most of the day, potentially increasing exposure. As an alternative strategy to well-known antibacterial agents, undecylenoyl collagen amino acids have been incorporated.6 Myristalkonium saccharinate, a mild cationic preservative, was used for a similar function in another case.7
Applied in past axillary deodorants, triethyl citrate is also now being used in foot care for its anti-odor activity through an interesting mechanism; it behaves as a preferential target for the hydrolytic enzymes esterase, which breaks its structure and creates free citric acid, an enzyme inhibitor. Ethyl-hexyl glycerin has appeared in a new deodorant,8 where it is unusually combined with octenidine dihydrochloride; while the association of antibacterial agents is a common practice, octenidine hydrochloride, a cationic bacterial killer that has the structure of a pyridine derivative, rarely is used. Further, new molecules are being launched with superior antibacterial and anti-odor properties, such as dimethyl phenyl 2-butanol9 and 2-methyl 5-cyclohexyl pentanol, which additionally is effective against the formation of bacterial biofilms.
The metallic salt zinc undecylenate is used in deodorant powders for the foot as well. Specifically, a composition containing talc, hydrated silica, zinc undecylenate, fragrance (parfum), triclosan and myristalkonium saccharinate is representative of this category of products.10 Talc acts as an anti-friction agent and a spreading aid. Silica provides sweat absorption and dryness, while preservatives inhibit bacterial growth and fragrance masks odors. The primary odor absorber used to date remains zinc ricinoleate, which is long-lasting and especially effective in fixing sulfur derivatives produced by bacterial growth. Also, due to the hydrolysis induced by sweat, dispersed zinc oxide microgranules will also have some antibacterial properties.
Zinc coceth-sulfate is also an interesting zinc derivative to fight odor. This surfactant is a good foamer and is used in slightly acidic systems of an approximate pH of 4.5. Aside from being more delicate than its parent sodium salt, it is slowly hydrolyzed onto the skin while rinsed with tap water to provide a finely divided, invisible zinc hydroxide precipitate on the skin surface that functions as a long-lasting, soothing odor-adsorbing ingredient. Finally, a zeolite was recently introduced to the market that can incorporate the small molecules of many odoriferous substances into its open crystalline lattice.11 Its use in foot care products has not yet been described, but it could represent an interesting new solution to odor-fighting.
Tired Foot Relief
Refreshing, lightly perfumed foot baths are often used to soothe tired and overworked feet. These baths usually contain salts that release oxygen and minerals for an immediate refreshing and relieving result. When this effect is desired via an emulsion, horse chestnut and witch hazel extracts are used to provide superficial capillary microcirculation in the skin of the feet, in addition to astringent effects. In order to simultaneously obtain a dry, velvety feel, some formulae contain talc in suspension, which is a long-lasting feel modifier.
The foaming bath salts shown in Formula 2 incorporate the mild surfactant sulfo-succinate with the soothing and capillary protecting action of two vegetal extracts. The ionic force provided by sodium sulfate reduces skin swelling, while the bicarbonate salt has a mild deodorant action and helps to neutralize volatile odorous acids. Urea softens the skin, together with allantoin, and allows for improved functioning of the vegetal principles. Rice starch performs an absorbing and soothing action, and freshness is provided by a fragrance with menthyl lactate, a menthol derivative with much lower allergenic power than its parent compound.
Refreshing gels are also commonly used to relieve and cool tired feet, with additional functionality as deodorant treatments for tired, hot feet. Some products such as the aforementioned Dr. Scholl’s Crackling Ice Foot Gel have been developed that when actuated from the aerosol can, produces a foam that crackles and sparkles. The cooling perception provided by alcohol as it evaporates is frequently encountered in foot deodorants. This provides immediate cooling action, which is often reinforced by menthol derivatives. Surprisingly, some sucrose ester emulsifiers and emulsifying polymers such as taurate derivatives provide a light, cooling sensation on the skin.
Freshness also can be obtained in an anhydrous way. In one foot balm formula, the Minty Fresh Foot Balm, shea butter softens the skin of the feet, mineral-rich seaweed (Fucus) extract nourishes this skin, peppermint oil refreshes the skin, and rosemary oil revitalizes it.12 The claim that the formula is vegan but not intended for pregnant women is an interesting one.
In Formula 3, callus-fighting activity is provided by salicylic acid and urea is stabilized by the ascorbate derivative. The osmotic protectant betaine (trimethyl glycine), the simplest amphoteric molecule, is also present, which protects living cells from imbalances in osmotic pressure inside and outside of the cell membrane. Such differences arise from differences in concentration between dissolved molecules or ions inside the living cells and the surrounding matrix. Osmotic protectants can freely cross the cell membrane in both directions and equilibrate such pressure differences, which is a relief for stressed cells and serves as a survival strategy in extreme conditions.
Another way to massage and refresh is using scrubs. Often they are made using a blend of oils loaded with solid crystals that dissolve after dilution with water. These solids can be from standard brown sugar or marine-derived scrubs such as sea salt.
Special Foot Treatments
Typical disequilibrium in foot skin is exhibited by calluses and they typically are treated by stick or pen products intended for intensive, immediately perceived action.13 Again, salicylic acid and high concentrations of urea and alpha hydroxy acids serve as the actives in these formulations. In one example, the Dr. Scholl’s Hard Skin Softening Pen, an emulsion is obtained by combining steareth-2 and steareth 21 with cetyl alcohol, castor oil, octyldodecanol and caprylic/capric triglycerides. Alpha hydroxyl acids such as tartaric, lactic and citric perform the exfoliating action; alcohol and glycerin buffer the evaporation speed of the aqueous phase; and tocopherol, ascorbyl palmitate and ascorbic acid protect the formula from oxidation.
Another formula, SVR Xerial 50 Extreme, shows an emulsion where the active principle is more abundant than the vehicle;14 urea is present at 50%, therefore water is second in the ingredient list. The fatty phase is a blend of synthetic and natural ingredients including isohexadecane, shea butter, octyldodecanol and sorbitan isostearate. Further, proteases (bacillus ferment) are included to induce the quick and deep action of the actives by weakening the bonds among the cells of the horny layer. Mud masks are becoming a popular way to rejuvenate the feet and provide sensorial enchantment as well. In a natural combination of mineral and vegetal ingredients, one example formula, Elov Whitening Nourishing Foot mask, contains cherry essence, natural bamboo, powdered vinegar essence, natural mineral powder, loquat leaves, tourmaline, chitin, vitamins A and C, natural planet essential oils, etc. While it is not clear how effective this sparse ingredient combination could be in skin-soothing, the ingredient choices are certainly evocative and therefore immediately relaxing for the brain. Masks, which are left on the feet for more than 15 min, are indeed a good means to accelerate transdermal delivery of the actives by providing immediate and important skin moisturization. Moreover, the rinse-off operation of the muddy layer creates a spa ritual, which might have psychological advantages as a rite of body purification.
Other unusual categories of foot care products belong in the field of makeup, acting in some ways like foot concealers to mask veins, blemishes, blisters and uneven skin tone, and to minimize the appearance of calluses and corns. Of course, one should select a color that suits the skin tone.15
Innovations in Foot Care?
The field of cosmetics for foot care is, formulation-wise, quite conservative, so new concepts and advanced molecules are usually considered only after their use has been proven in other cosmetics. However, new slow-release and long-lasting antimicrobial agents that can hinder microbial and fungal growth for the duration of the day are technologies that are sought. Indeed, the application of antibacterial agents typically comes to mind when the feet are already clean, and thus not needed. But as time goes by, bacterial development flourishes and the antibacterial agents have slowly disappeared, which is why slow-release antibacterial agents are required. Vegetal extracts have great potential in providing low toxicity molecules with such activity.
Also, in order to strengthen the delicate structure of the top and side skin of the feet, newly developed micro-fine apatite, which increases skin elasticity and firmness by providing calcium and phosphate ions, could be adopted. In terms of new emollients, cactus (Opuntia) seed oil is quite interesting as a new, very dry emollient that is quite oxidation-stable and adsorbed very quickly by the skin.
The Sporting Feet
A quickly developing sector that could influence the overall foot care strategy is that of sport and fitness products. Re-invigorating, pre-sport massage oils, fluid soothing emulsions and relaxing massage oils represent a new appealing challenge for innovation. Oils in such products should be light but also protective, such as is provided by Moringa oleifera oil, a powerful emollient, and by Salvia hispanica seed oil, which is rich in omega-3 fatty acids that help to rebuild the skin barrier and re-establish skin moisturization. Emulsifiers of the sucrose ester type are fully skin friendly, which is especially useful after long sport performances. In addition, polymeric emulsifiers could be adopted for the same superior compatibility, even with sensitive skin.
Skin toning can be assured by witch hazel extract, osmotic protection and water pressure equilibrium benefits that are provided by xylitol and mio-inositol, and soothing and protective functions are imparted by new vegetal extracts such as Boswellia serrata (incense) resin extract and Coleus forskohlii root extract. Escin, the active principle of horse chestnut extract, may be incorporated to stimulate microcirculation, while refreshing actions can be provided by isopulegol and menthanediol, two refreshing molecules from mint essential oil. In addition, free radicals are blocked by tocotrienols extracted from palm oil. Massage oil would be better made of a wise blend of silicone oils for easy spreading and a silky feel without residual stickiness, together with sweet almond, annatto and andiroba oils, all emollients and soothing natural oils. Further, marigold (Calendula officinalis) extract could assist the skin’s defense system as it is a good anti-inflammatory agent, even on chapped skin. Adding tocopherols to the blend would also strengthen the skin structure. However, before undertaking any sport challenge, muscles should be warmed up by massage. A synergic oil blend such as soy, which is rich in omega-3 fatty chains; and meadowfoam seed and shea butter, which are characterized by high emollient power, ease of spreading and massageability; used together with silicone oils would provide a silky final touch. Natural essential oils could topically warm skin’s microcirculation while simultaneously energizing the mental spirit with fragrance.
One example of natural fitness foot cream is shown in Formula 4. This is a clear example of an all-natural formulation strategy, with thoroughly purified vegetal oils that are well-protected from oxidation by adequate amounts of vitamin E. Catalytic iron traces are sequestered by phytic acid, and bacteria are tamed by p-anisic acid. Thickeners of natural origin are also used, and skin protection is insured by all the unsaponifiable fractions of the vegetal oils used, i.e., sterols, tocopherols and tocotrienols, together with the soothing ingredients of shea butter. Long-lasting freshness is provided by the menthol derivative. This formula is certainly more suitable for very dry skin, as the film-forming properties of beeswax provide an adequate limit layer on the skin—i.e., the minimum thickness of product that can be perceived when distributed to the skin by hand.
Having covered foot skin from head to toe—or rather, toe to heal, formulators may now take an enlightened walk in this often neglected field.
- http://americas.nuxe.com/enultra- comfortable-foot-164/reve-de-miel
- US PA 20110195103, Cosmetic or dermopharmaceutical composition containing pseudoalteromonas ferment extract, R Perez Arcas, C Oncins Bergas and J Maria Garcia Anton, assigned to Lipotec SA (Aug 11, 2011)
- www.labo-svr.com/SVR_EN/index.php?option= com_content&view=article&id=44&Itemid=90
All references accessed on Apr 30, 2012.