According to cosmetic industry’s classic definition, a shampoo is a product with cleansing and foaming actions on the scalp that leaves the hair soft, shiny and easy to comb. However, this broad definition does not explain the multiple functions of this category of products or its large success. Indeed, a sub-category of the shampoo family is the anti-dandruff niche, recognized worldwide for its importance in terms of sales and volume—approximately 8 million Euros and 600 million tons respectively, as of 2011. This column reviews basic shampoo formulation strategies, then looks to dandruff and specialty ingredients and strategies used to address this condition, including new concepts on the horizon for future product development.
Shampoo formulations involve a complex mix of ingredients for various functions, including the following:
Cleansing: Ingredients are required to remove all residues of sebaceous secretions, environmental dirt of a fatty nature, and dust and solid particles derived from scalp desquamation from the hair surface. Since the entire hydrophobic hair surface is ~2,000 cm2, the detachment, with the aid of water, of all these materials as a stable suspension and/or micro-emulsion generally requires surfactants. These molecules decrease the interfacial tension between water and fatty materials with the keratin surface. At the same time, the reduction of excess microbial charge on the scalp results in visibly better hair conditions.
Odor elimination: Removing all odoriferous substances, of environmental or self-origin, absorbed onto the scalp and hair surface and into the sebum layers wetting them is also desirable.
Foaming: Although not strictly necessary, foam formation accompanies the massaging of surfactant solutions onto the scalp. Indeed, foam formation, volume, thickness and appearance contribute to the overall product acceptance. Foam massage is a form of self-reward for consumers, and the final elimination of the foam during rinsing is ritualistic, corresponding to removing not only the physical, but the psychological negative encounters of daily life. Moreover, structured foam helps to maintain the bulk of product dilute solution on the application site.
The described actions are obtained by the proper combination of surfactants and co-surfactants, at a total solid, i.e., active matter, content between 10% and 25%. In particular, molecules possessing foam-boosting properties represent 2% to 6% of the formula. A combination of distinct surfactant structures is generally used rather than a single ingredient, in order to optimize cleansing properties, product rheology and mildness for the scalp. Luxurious foam performances and hair conditioning effects, together with ease of rinsing and affordability, are also milestones in the formulation strategy.
In general, the main surfactant is 60–85% of the total surfactant content in a formula, and is usually an anionic such as sodium laureth sulfate (SLES) as the key foamer. The other surfactants work for mildness and skin and hair conditioning. In general, equilibrated blends of anionic, nonionic and amphoteric surfactants are used to optimize performances and costs.
Adequate flow: Flow refers to the movement of a product from the container, control over the amount poured into the hand and the ease with which it is distributed and massaged onto the scalp. This is achieved by the appropriate selection of foam boosters, which generally have a positive influence over viscosity or by the selection of thickeners. Polymeric thickeners are used at a maximum level of 1–2%. Sodium chloride is the cheapest possible thickening agent but its total concentration, including that from surfactant raw materials, should not exceed 2% in order to avoid harshness to the scalp and possible crystallization in cold environments. It is also important to remember, viscosity is influenced by the pH of the solution.
Anti-static action: This function is necessary to reduce electrostatic-induced fly-aways and permit better alignment of the scales along the hair axis, providing enhanced luster and better combability. To this end, polymeric or non-polymeric cationic conditioners, as well as blends of them, are often used at levels between 0.5–1.5%.
Lubricant action: To compensate for the complete loss of sebum from hair due to cleansing with shampoos and to decrease the resulting friction in hair, lubricant agents are added to form a shiny, protective film on the hair surface. This also supports the functionality of conditioners. It is important that, as the product is diluted with water during use, these ingredients deposit onto the hair surface and resist the rinse-off operation. Their use concentration is typically not higher than 3–5%.
Aesthetic additives: Color increases the sensory acceptability of products, including hiding yellowing, which frequently develops in cleansing blends. Pearling agents are added for the same reason as well and to impart the perception of richness. To address the difficulty of obtaining a satisfactory pearl effect in shampoo formulas without heating the whole bulk to 50–60°C, proprietary concentrated blends of ethoxylated and non-ethoxylated alkyl stearates generally are used. The stability of the pearl effect in the final formula is related both to the thixotropy of the blend at 1–5%, and the crystal size and stability of the pearled premix.
Perfumes are another key hedonic character in shampoo that underline the product’s identity. Their use levels in a formula usually range from 0.2–1.0%. In order to obtain long-lasting olfactory sensation, perfume ingredients should exhibit a certain degree of adhesion to hair. Their color also should not excessively interfere with the color of the finished product.
Self-preservation: Shampoos require adequate microbial preservation due to their use conditions, i.e., high humidity, and the large amounts of water in their formulae, i.e., between 70–85%. This can be achieved with the right amount and combination of traditional and/or non-traditional preservatives. Satisfactory challenge test results provide a check-point to fulfill this safety aspect.
Normalizing action: Hair cleansing implies the removal of all water-holding entities and barrier ingredients in the scalp and hair, which results in increased water loss, excess scalp dryness and itch. Therefore, a good hair cleanser should favor the quick recovery of skin’s equilibrium, normalization of the pH and a fast return to scalp homeostasis. These effects strictly depend on very low surfactant content, the pH of the formula—i.e., between 4.7 and 6.5–and its buffering during dilution with water, inhibiting the absorption of surfactant into the hair shaft and scalp, and the ease of rinse-off. A low-surfactant example is given in Formula 1.
Frequently, harshness to the skin is caused by the incomplete elimination of surfactant residues from the hair and scalp surface. Indeed, a perfect rinse-off of shampoo ingredients from this complex environment of scalp and hair “forest” does not happen as easily as one might think. Surfactant residues left on the scalp for hours exert a negative degreasing effect and hinder the re-equilibration of the skin barrier. On the hair structure, these residual ingredients also modify the distribution of sebum along the hair and weaken its mechanical strength. In relation, insoluble calcium and magnesium salts from hard water, which dull hair, can be prevented from precipitating on the hair surface by sequestering agents, used at levels between 0.05–0.15%.
Dandruff and the Scalp
Beyond the cleansing, foaming, etc., functions of a basic shampoo, dandruff shampoos address a condition. Dandruff is found in a high proportion of all populations at all times. It is defined as a chronic, non-inflammatory scaling of the scalp. Dandruff occurs when the normally invisible physiological scaling of the scalp is modified by an elevated rate of desquamation of the horny cells. The horny layer then breaks up unevenly in larger cell clumps. As this occurs, numerous intracellular lipid droplets also are entrapped.1
A dandruff-afflicted scalp exhibits an abnormal stratum corneum, related to hyperproliferation with parakeratosis and excessively disorganized intercellular lipids.2 The reversal of these stratum corneum abnormalities requires not only flake removal, but also a complete treatment of the causes of dandruff. In addition to genetic disposition, excessive proliferation of specific microorganisms such as Malassezia furfur, once referred to as Pityrosporum ovale; excess sebum; the production of irritant-free fatty acids; and some metabolic disturbances often are associated with dandruff.
In the past, coal tar was used in medicated dandruff shampoos. However, its distinct scent and dark color made consumer compliance difficult; although it was successful in dermatological prescriptions. Purified grades of coal tar then became available and were more efficient but they required the formulator to use “coal-tar type” perfumes in order to convey the message of medicated efficacy. Today’s consumer association of coal tar with salicylic acid and undecylenic derivatives enables formulators to optimize keratolytic and antiseptic activities.3
Colloidal sulfur and sulfides, which inhibit the activity of M. furfur, also were largely used in the past. In fact, in 1990, a summary judgment published in the U.S. Federal Register4 expressed selenium sulfide is still a key ingredient in the field, despite the unpleasant appearance and odor; and in 1991, a patented formula5 used it at 25%; see Formula 2 for an additional example.6
Today, special importance is given to complex compounds such as pyridine thione derivatives—including zinc pyrithione (micronized, insoluble) and sodium pyrithione (water-soluble). In one particular patent,7 the use of zinc pyrithione in small-sized platelets claimed to enhance its anti-dandruff efficacy due to better adhesion to the scalp for longer-lasting action. The adhesion of small particles to the scalp seems key to inhibiting the action of M. furfur, which is present on the hair follicle.
For the predominant approach treating yeasts as the major cause of dandruff, antimycotic agents have been employed (see Formula 3). Piroctone olamine, for example, is a good active against M. furfur, and has the advantage of being soluble in surfactant solutions. More recently, ketoconazole and miconazole are being used, although in some countries they are considered medical specialties and cannot be used in cosmetic preparations. Specifically, ketoconazol is not permitted throughout the European Union and miconazol is not allowed in Switzerland. Indeed, many of these antimicrobial ingredients have anti-mitotic activity; in other words they reduce the abnormal skin cell shedding that accompanies dandruff. In a scientific comparison,8 the three actives—i.e., piroctone olamine, ketoconazole and zinc pyrithione—gave comparable results in terms of efficacy.
Specialized vehicles could assist in depositing antidandruff actives onto the scalp. For example, multi-lamellar vesicles consisting of concentric shells of surfactant bilayers currently deliver actives and conditioners to damaged hair. These structured systems are also capable of stabilizing and delivering silicone and vegetal oils.9 In relation, one interesting paper10 describes how a particular structure of surfactants with polymers can create a coacervate upon contact with water. This transformation is said to improve the delivery of anti-dandruff active principles to the scalp. And in order to keep insoluble zinc pyrithione in a stable suspension, a patent from Henkel11 describes the use of hydrogenated castor oil as a stabilizer (see Formula 4), together with a gel-former.
Up-and-coming Anti-dandruff Ingredients
In the quest for natural ingredients, vegetal saponins arise as possible alternatives to strong anti-mycotic agents. Those extracted from the bark of the South African tree Ziziphus joazeiro recently were launched and Natrue certified.12 Another organic complexa to treat dandruff is based on Andiantum capillus veneris (maidenhair) extract and Thymus serpillum (wild thyme) in glycerol and water. Quillaja bark extract also is rich in saponins and has demonstrated anti-dandruff efficacy.13
Further, recent work14 with green tea polyphenols has shown that once they are protected from oxidation, these actives can reduce scalp scaling. Also following the “green” trend, saponins extracted from Asparagus racemosus (asparagus) act as powerful anti-fungal agents.15 The lactic acid derivative16 sodium decanoyl dodecanoyl lactylate is said to be active at 2%, especially against microorganisms such as M. furfur, which cause skin disorders.
Lastly, lysine polypeptides are a new cationic ingredient type, which interact with negatively charged microbial cell membranes to destroy the cells.17 In relation, microscopic and dermatological evaluations, together with bioengineering methods, are standard ways to quantify antidandruff activity. In fact, a recent paper18 describes an enzyme cloned from M. furfur, which was identified as a good model for studying in vitro the efficacy of anti-dandruff agents such as sulfonamides.
- EA Grice and JA Segre, The Skin Microbiome, Nat Rev Microbiol 9(4) 244-53 (Apr 2011)
- RR Warner, JR Schwartz, Y Boissy and TL Dawson, Dandruff has an altered stratum corneum ultrastructure that is improved with zinc pyrithione shampoo, J Am Acad Dermatol (45)6 897-903 (2001)
- www.activedrug.com/drug-info/coalgel-pierre-fabre (Accessed Apr 29, 2015)
- R. Holbrook, commentary on Sulfidal, manufactured by Pettbone Laboratories, Cosm & Toil 105(2) 28 (1990)
- EP 317,314, Shampoo compositions, P Columnkille and PE Cotran, assigned to Procter & Gamble company McCall (May 24, 1989)
- www.cabotcorp.com/.../brochures/.../brochure-cab (Accessed Apr 14, 2015)
- WO 1992014440 A1, Antidandruff shampoo compositions, C Winyard Cardin, J Ingram Davis, J Lynn Hart and D Grob Schmidt, assigned to Procter & Gamble (1992)
- C Piérard-Franchimont et al, Nudging hair shedding by antidandruff shampoos. A comparison of 1% ketoconazole, 1% piroctone olamine and 1% zinc pyrithione formulations, Int J Cosmet Sci (24)5, 249-56 (2002)
- D Bendejacq, C Mabille, V Picquet and E Gates, Structured surfactant systems for high performance shampoo, Cosm & Toil 125(11) 22-9 (2010)
- www.udel.edu/udaily/2013/feb/li-coacervation-021213.html (Accessed Apr 29, 2015)
- WO 2008 6.412, Antidandruff shampoo containing hydrogenated castor oil as stabilizing agent, assigned Henkel KgaA (Jan 17, 2008)
- www.ulprospector.com/en/na/PersonalCare/Detail/1024/240151/DANDRILYS (Accessed May 6, 2015)
- L Rigano, A Bonfigli and R Walther, Bioactivity evaluations of Quillaja saponaria (soap bark tree) saponins in skin and scalp sebaceous imbalances, SÖFW 138(3) 14-21 (2012)
- S Hsu et al, Green tea polyphenol induces caspase 14 in epidermal keratinocytes via MAPK pathways and reduces psoriasiform lesions in the flaky skin mouse model, Exp Dermatology (16)8, 678-84 (2007)
- C Onlom, S Khanthawong, N Waranuch and K Ingkaninan, In vitro anti-Malassezia activity and potential use in anti-dandruff formulation of Asparagus racemosus, Int J Cosmetic Sci (36)1 74-8 (2014)
- www.dr-straetmans.de/en/products/productdescription_dermosoft_decalact.php (Accessed Apr 29, 2015)
- WO 2013136040, Polypeptides and their use, D O’Neil, D Mercer and C Stewart, assigned to NovaBiotics Ltd., (May 14, 2012)
- KS Hewitson et al, Molecular cloning, characterization and inhibition studies of a beta-carbonic anhydrase from Malassezia globosa, a potential antidandruff target, J Medicinal Chemistry (55) 3513-20 (2012)