Moisturizers are an important category of personal care products,1 and such formulas are designed to add moisture to the skin. Developing a good moisturizer requires carefully balancing the ingredients in a formula so that, upon application, the product maintains proper water content in the skin, i.e., 10–30%, to maintain its plasticity and barrier integrity. Insufficient water content can lead to the thickening or thinning of skin; fissure development, which produces chapped, rough and cracked skin; and the loss of pleasing skin aesthetics. Therefore, choosing the right moisturizer requires knowledge of its chemical, physical and performance properties and how to best utilize it against the targeted performance claims and consumer expectations. In addition, it requires knowledge of the skin to which it will be applied.
In general, skin conditioning needs can be classified based on skin health: normal, varying degrees of dry, oily, sensitive, dermatologically damaged and mature. There are other ways to classify skin types, most notably by the well-known and modified Fitzpatrick systems, all of which are more centered on the physiological health of skin and effects of sun exposure. Skin discomfort is unacceptable to consumers, thus moisturizers containing emollients and humectants are used to help alleviate the symptoms of skin discomfort. Since moisturizing lotions provide temporary relief of these symptoms, they are applied to the skin as a constant routine for skin management, to help to provide a healthy skin feeling. Following is an abbreviated explanation of the performance criteria of moisturizers, humectants and emollients.
Approaches to Moisture
Moisturizers: Moisturizer is a generic term used to signify an ingredient that adds moisture to the skin; to refer to humectants, which maintain skin hydration; and to describe emollients that soften skin.2–4 Unfortunately, these terms are used interchangeably but each of these materials provides different performance benefits to skin and is quantified by different clinical methods. Moisturizer typically describes the function of a finished product and tends to include polar materials that are hygroscopic and able to hold water in place. The skin’s natural moisturizing factor (NMF) also plays an important role in moisturizing the stratum corneum (SC).5 The average NMF composition consists of approximately: 40% amino acid; 12% PCA; 12% lactate salt; 7% urea; 1.5% glucosamine/creatinine; 0.5% citrate salts; 18.5% Na, K, Ca or Mg phosphate; chlorine; and 8.5% sugars, organic acids and peptides. Thus, moisturizing ingredients are chosen based on their ability to substitute, replenish or maintain skin’s natural moisturizing components.
Classic moisturizers: Classic ingredients used as moisturizers include petrolatum as an occlusive barrier to hold moisture in the epidermis and SC; dimethicone (250–5000 cps) to modulate moisture in the skin; lanolin and its derivatives; vegetable butters/oils and their derivatives, e.g. shea butter, avocado oil, olive oil, etc.; high molecular weight linear fatty acid esters such as arachidyl propionate and cetyl palmitate; potassium lactate, alkyl methyl siloxanes, e.g. C30-45 alkyl dimethicone; and film-forming polymers.
Humectants: Humectants are ingredients that have hygroscopic properties and can thus modulate moisture and retard moisture loss from skin.3, 6 They typically are associated with skin moisturization and can be divided into two classes of chemistry—inorganic, e.g. calcium chloride, etc., and organic, including polyols, organic salts and amino acids. Popular humectants include glycerin, urea, pyrrolidone carboxylic acid (PCA) and others (see Commonly Used Humectants). They provide good water-binding capacity with good skin substantivity.
Formulating with humectants: One noted polyol combination mixes various polyols with glycerin in a ratio of glycerin/sorbitol/propylene glycol/water at 12/9/10/59 (40/20/40 glycerin/sorbitol/propylene glycol) to provide good skin hydration along with good skin aesthetics to aqueous-based hydrating moisturizers. Also, diglycerol and triglycerol are good additions to glycerin to slow the absorption of water from the skin and provide a better equilibrium of moisture externally and internally to skin, equating to better moisture modulation. They also tend to be recognized as contributing less tackiness to a cosmetic formula.
Finally, polysaccharides are also a class of humectants that can provide good skin hydrating properties, and in some cases, they impart the added benefit of desquamation of flaky skin. Hyaluronic acid, i.e. glycosaminoglycan, and chitosan, the deacetylation of chitin, are two popular ingredients. Hyaluronic acid is ubiquitous with the intercellular matrix of connective tissue.
Emollients: Emollient is a complex and misused term.7 It is most often associated with substances that have the ability to plasticize, soften and smooth the skin, usually by filling void spaces on the skin surface and replacing lost lipids in the SC. This term was originally used to describe skin plasticization based on NMF to slow transepidermal water loss (TEWL) based on the theory that moisture retention can keep the SC flexible; therefore, water loss prevention maintains softer and smoother skin. Emollients can also provide protection and lubrication on the skin surface to minimize chafing and enhance skin’s aesthetic smoothness and softness.
Emollient properties are described analytically by interfacial tension/spreading coefficients, coefficient of friction, solubility and polarity characteristics of the molecule. Interesting concepts such as those based on cascading emolliency,8 surface tension, polarity and solubility have been used to develop synergistic blends of emollient ingredients with differing skin aesthetics, spreading characteristics and a solubility similar to the SC.
Formulating with emollients: The chemistry of emollients is diverse and covers most fields of chemistry. The most popular emollients are based on hydrocarbons and their derivatives; mineral oil, fatty acid esters, vegetable oils, synthetic triglycerides and polymers (see Figure 1 and Commonly Used Emollients). Formulators should note the viscosity of a mineral oil, which is based on complex mixtures of varying carbon chain length-hydrocarbons purified to varying degrees, is a key determinant to the average molecular weight of the hydrocarbon mixture.
Another popular hydrocarbon emollient that is used as a moisturizer is petrolatum. It is based on linear and branched aliphatic and aromatic chain hydrocarbons and waxes. A sub-class of hydrocarbons is isoparaffins—aliphatic branched chain hydrocarbons of high purity; e.g. isoeiconsane, isohexadecane, etc. There is a variety of pure and blended hydrocarbons with carbon chain lengths greater than 10 that provide excellent emolliency to both skin and hair.
Fatty acid derivatives are generally sourced from triglycerides and waxes, and can also be made synthetically. The most popular carbon chain length fatty acids are C12-22, as well as unsaturated C18 fatty acids, but there are a number of examples using C3-8. Common derivatives used as emollients are: esters of mono and polyhydric alcohols, both linear and branched, which were first developed in the 1950s as porosity esters to duplicate properties of the NMF;9, 10 dicarboxylic and tricarboxylic acid derivatives such as esters; carbohydrate derivatives including starch; and methylsiloxane derivatives (see Figure 2).
Popular carbohydrates used as emollients are sucrose, glucose and methylglucoside esters, which were first developed in the early 1970s. In addition, polymers such as polyesters, polyethers, polyacrylates and others are used extensively as emollients and film-forming moisturizers. Finally, ethers and carbonates are finding acceptable application as emollients in skin and hair applications.
Objective Skin Moisture Assessments
It is important for product developers to understand how to measure11 skin moisturization so as to determine whether the functional component of a formula is acting as a moisturizer, humectant or emollient. There is a critical balance between clinical measurements and the consumer’s perception of performance. By most accounts, consumers make an initial assessment of a moisturizer’s performance based on how the product applies to their skin, in addition to perceptions later in the day of relief from dry skin symptoms. Consumers will continue to use a product if their symptoms are continually reduced and healthier skin is maintained longer, ideally greater than 24 hr.
Clinical evaluations can assess the ability of a formula to alleviate dry skin symptoms within a reasonable and acceptable time frame. The basic methods to assess clinical dry skin and skin hydration include TEWL, conductance, corneometry, dryness and desquamatry.
TEWL: TEWL measures the occlusivity of water on skin as it migrates to the surface.12, 13 Healthy skin will have low TEWL while damaged skin will have high TEWL. Occlusive films such as petrolatum can disguise the real health of skin because they create an occlusive barrier and reduce the ability to predict the actual transpiration rate of moisture evaporation from the SC. TEWL reduction lasting for several hours, i.e. from 4–24 hr after application of a moisturizer, can be used to predict the ability of the formulation to reduce water loss from the SC. Thus, when a product applied to skin shows prolonged TEWL reduction, this measurement can infer the product’s ability to reduce water loss from the skin. Instruments used for the measurement of TEWL include evaporimeters and TEWL probes operating on vapor pressure gradient open chamber principles.
Conductance: Conductance14, 15 uses the Galvanic skin response to measure electrical charges in the skin. These charges relate to the water-binding capacity of the SC.
Corneometry: Corneometry16, 17 is a capacitance measurement based on changes in the dielectric constant due to variations in skin surface hydration. Water and other materials on the skin’s surface will have a different capacitance reading. Measurement comparisons using this method are limited to in-study comparisons.
Observer dryness: Observer dryness assesses skin health through subjective, trained clinician grading. This is usually conducted in conjunction with panelist dryness grading (e.g., panelists’ self-assessment of their skin). The best approach is to perform a seven-day mini regression,18, 19 applying test products daily on days one through four, and comparing untreated with treated skin. On days five through seven, the skin is allowed to regress without further treatment, which allows for the evaluation of the longer term effects of the treatment sample. In this scenario, measurements are taken initially and on days two, four and seven. A dryness scale is used to score subjects’ skin: 0 = no dryness; 1 = slight flaking; 2 = moderate flaking/scaling; 3 = marked scaling/slight fissuring; and 4 = severe scaling/fissuring.
Desquamation: Desquamatory methods utilize skin sampling discsa to collect cells from the superficial SC that are visually scored. The cells are then stained and their color evaluated with a colorimeter (C* value). This method can be used to measure the degree of flaking before and after use of a test skin treatment to provide a good determination of whether skin is alleviated.20, 21
Other measurement tools that provide insight into the performance of a moisturizer include the following.
Irritation/redness: This measurement utilizes skin patch methodology to determine the ability of an ingredient to cause skin irritation and/or countenance of redness.
Skin flexibility/elasticity: This approach determines the elastic and plastic characteristics of skin. Most measurements are made with a cutometer that vacuum sucks a defined area of the skin surface, recording visco-elastic properties.
Permittivity: Permittivity measures polarity; emollients with high permittivity are highly polar, exhibiting greater affinity for water.22 By matching the formula’s permittivity closely to the permittivity of healthy skin, formulators can stabilize all the ingredients in the formula to achieve optimum performance on the skin. This is particularly important for multicomponent color cosmetics. Some high permittivity ingredients include lauryl lactate, diisopropyl sebecate and isostearyl hydroxystearate, and low permittivity ingredients include isoparaffin, isododecane and dimethicone.
Contact angle: Contact angle (CA) measures the angle formed by a liquid droplet on a solid surface at the air/solid/liquid contact point, which determines the degree to which a liquid wets or spreads on a solid; CA = 0 degrees indicates complete wetting, and CA = 180 degrees indicates complete non-wetting. Low spreadability emollients work well for an active requiring enhanced substantivity and low wash-off characteristics, e.g., eye treatments, sunscreens and lip products. Low contact angle ingredients include cyclosiloxanes, isoparaffin, isododecane and mineral oil. High contact angle ingredients include diisostearyl maleate, hydrogenated castor oil, dimethicone and dimethiconol. The addition of hydroxyl groups allows for hydrogen bonding interactions between the emollient and other more polar materials, as well as between emollient molecules.
Interfacial tension: Interfacial tension (IFT) is an indicator of the ease with which emulsions are formed. Knowing the density of the two phases, researchers can calculate the IFT by measuring the curvature of the droplet. High IFT ingredients include mineral oil, isododecane and hydrogenated polyisobutene; low IFT ingredients include lauryl lactate, neopentyl glycol dicaprate and diisostearyl maleate.
Lubricity: Lubricity impacts skin feel and is a measure of the coefficient of friction (CofF) between two surfaces with the emollient of interest between them. High CofF ingredients include triisostearyl citrate and diisostearyl maleate, and low CofF ingredients include isostearyl neopentanoate, dimethicone and C12-15 alkyl benzoate.
Humectants exist in various different chemistries. They provide topical moisture to the skin surface that can equilibrate the moisture in the SC, providing a reservoir of moisture on an as-needed basis. When combined with occlusive agents and other emollients, humectants can minimize skin moisture from desorbing to the hygroscopic film on the skin surface.23 As noted, among the most popular ingredients is glycerin, which is still the most effective.24, 25 It has a very safe and long history of use, high hygroscopicity, is effective at low levels (4%), can provide some desquamation of the skin, and is easy to formulate, with consumer-acceptable aesthetics. Other humectants are listed in the sidebar, Commonly Used Humectants.
Occlusive agents: Occlusive agents are ingredients that can reduce the porosity of the skin’s surface by providing an envelope over the skin, thus reducing or preventing evaporation of moisture. Common occlusive agents include petrolatum and alkyldimethicones.
Emollients: Emollients used in cosmetic and personal care products are typically in the family of fats and oils (lipids). Optimum-performing synthetic emollients are capable of mimicking natural ingredients. Some natural emollients are: water, aloe vera, seed and nut oils, fruit butters, fruit and vegetable juices. These materials prevent water loss from the dermis and epidermis and have been reported to repair wounds due to their enhancement of epidermal proliferation. Other humectants are again listed in the sidebar, Commonly Used Emollients.
Key properties of emollients include good spreading properties, low toxicity/skin irritation, and good oxidative stability. Additional important formulating considerations are as follows.
- It is important for formulators to understand that unsaturated oils such as olive oil have double bonds that can react with oxygen, especially when heated. This oxidation process can produce off colors and odors in lipids, causing them to go rancid and render them unusable.
- Silicones such as cyclopentasiloxane and dimethicone can be added to increase slip and emolliency.
- Oils that contain high levels of essential fatty acids (EFAs) can be used to supplement loss of the lipids (oils) found naturally within the skin; linoleic acid is an example of an EFA.
- Long-chain alcohols, also called fatty alcohols, are useful as emollients and emulsion stabilizers. Their polar hydroxyl groups orient to the water phase and their fatty chains orient to the oil phase. Balancing the length of the carbon chain—i.e., C16 vs. C18 vs. C22—with the alkoxylated derivative can dramatically change the play time of a lotion during rubout and the perception of moisturization on the skin post dry-down.
- Lanolin, derived from sheep’s wool and often called wool grease, has been used for centuries for its unique composition of complex sterols, fatty alcohols and fatty acids. Cholesterol, a cyclic molecule called a sterol, is a major component of lanolin. The polar hydroxyl groups of sterols and alcohols enable the lanolin to absorb and hold water.
- Waxes are composed primarily of long-chain esters that are solid at room temperature. Common waxes used in cosmetics are beeswax, candelilla, carnauba, polyethylene and paraffin. The melting point of waxes varies widely depending on their unique composition and chain length. By combining waxes having different properties, such as high shine, flexibility and brittleness, optimal cosmetic performance can be achieved. Oftentimes waxes are combined with compatible oils to achieve the desired softness.
- Combining esters and hydrocarbons have several effects on the bulk properties; isododecane/diisopropyl adipate combo increases polarity of hydrocarbon, which is reflected in a higher permittivity and lower IFT. Intermolecular interactions due to the ester groups also cause an increase in viscosity, which leads to a decrease in spreading (indicated by an increase in the CA). Another factor affecting bulk behavior of esters is the degree of branching on the hydrocarbon moieties. The length of the hydrocarbon chains attached to the ester groups can affect the properties of an ester.
- Esters having good spreading properties are often useful for dispersing pigments and improving the incorporation of micro-fine oxides into sun products. They can also improve the smoothness and lightness of oils and reduce tackiness without affecting emollient characteristics.
Following are some ingredient declarations taken from commercial moisturizer products on the market. They are provided merely as examples from the above discussion. Examples of emollients, humectants and moisturizers are denoted as follows: * = emollient, † = humectant and ‡ = moisturizer. Readers should note that freedom from patent infringement is not to be inferred.
Sally Hansen Lip Exfoliator and Moisturizer: Ingredients: Ricinus Communis (Castor) Seed Oil‡, Octyldodecanol, bis-Diglyceryl Polyacyladipate-2, Microcrystalline Wax, Isopropyl Palmitate*, Ethylhexyl Palmitate*, Ozokerite, Euphorbia Cerifera (Candelilla) Wax, Polybutene*, Stearyl Alcohol, PEG-8 Beeswax, Stearic Acid, Sorbitan Isostearate, Tocopheryl Acetate, Diisostearyl Malate*, Flavor, Propylparaben. May contain (+/-): Citrus Aurantium Amara (Bitter Orange) Peel, Red 7 Lake, Red 27 Lake.
Ulta Smoothies Banana Split Body Whip Cream Body Moisturizer: According to the manufacturer, this product is light, fluffy and soft to the touch. This body lotion reportedly provides moisture that melts into the skin, leaving a “delicious” scent. Ingredients: Water (aqua), Myristyl Alcohol, Propane, Glycerin, Propylene Glycol†, Butane, Cetrimonium Chloride, Isobutane, Amodimethicone*, Trideceth-12, Diazolidinyl Urea, PEG-40 Hydrogenated Castor Oil, Fragrance (parfum), Sodium PCA†, Citric Acid.
Éminence Organic Skincare Immuni-Tea: Guava and Bamboo Age-Defying Moisturizer: Ingredients: Guava Juice, Bamboo Leaf Extract, Coco-Caprylate/Caprate*, Cetearyl Glucoside, Shea Butter‡, Corn Germ Oil*, Natural Source Cetearyl Alcohol, Natural Source Glyceryl Stearate, Marigold Oil, Vegetable Glycerin†, Tara Tree Extract, Methyl Glucose Sesquistearate, Guava Essence, Xanthan Gum, Biocomplex (Vitamin A, Vitamin C Ester, Vitamin E, Coenzyme Q10, Alpha Lipoic Acid), Salicylic Acid, Sodium Benzoate, Potassium Sorbate, Sodium Salicylate.
Kao Brands Curel Itch Defense Skin Balancing Moisture Lotion: Ingredients: Water (aqua), Glycerin†, Petrolatum‡, Tapioca Starch, Cetearyl Alcohol, PEG/PPG-17/6 Copolymer, Behentrimonium Chloride*, Dimethicone*, Isopropyl Palmitate*, Propylene Glycol Isostearate*, Cyclopentasiloxane*, PPG-15 Stearyl Ether, Cetyl-PG Hydroxyethyl Palmitamide‡, bis-Methoxypropylamido Isododecane, Panthenol, Tocopheryl Acetate, Butyrospermum Parkii (Shea Butter) ‡, Olea Europaea (Olive) Fruit Oil‡, Methylparaben, Propylparaben, Benzalkonium Chloride.
Skin hydration and aesthetic improvement are important aspects of skin care. The consumer continues to seek out better skin hydration and barrier repair products in hope of curing damaged skin and maintaining healthy skin longer. The trend in R&D will be to continue to find better ways to deliver optimized skin hydration, improved skin barrier and consumer acceptable aesthetics. Unfortunately, moving too far afield from where the industry is today moves this category closer to cosmeceutical and drug-implied physiological actions, possibly creating a situation where such products will no longer be considered cosmetic.
In addition, the trend toward more naturally sourced ingredients fits well with skin care moisturization and hydration because many good skin hydrating agents come from natural sources; specifically saccharride chemistry and glycerin split from natural oils. Reproduction of the article without expressed consent is strictly prohibited.
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