The pursuit of hygiene leaves the largest footprint on the skin’s surface, since cleansing procedures are the most oft-performed. From eight to 10 applications of hand soap per day, to bathing and shampooing, proper cleansing practices are essential to keep possible skin problems at bay; or in the case of atopic dermatitis (AD) management, to remove debris, crusts, exudates and microbial agents that can cause skin infection and trigger irritation.
Surely, during the entire cleansing routine, the correct use of products can prevent much of the skin damage often attributed to cleansing. This includes using the right amounts; adequate rubbing force and duration; and care devoted to the rinse-off phase in order to avoid excessive adsorbtion of surfactant residue onto the skin.
To create the “right” cleanser, one must question: Are there ingredients that must absolutely be omitted from formulas? Or, is it more that the complex of ingredients as a whole requires proper balancing? Depending on the product goals and condition of the targeted skin, it will be important to consider which formulation strategy will provide the “ideally correct” blend.
Skin disease is among the leading causes of global disease burden, affecting millions of people worldwide.1 Aging, trauma and environmental and genetic factors can result in the onset of several skin diseases, with more than 3000 entities identified in literature.2, 3
Some facts and figures on AD and other skin diseases include:
- The prevalence of AD is estimated to be 15-20% in children and 1-3% in adults; additionally, its incidence has increased by two- to threefold in industrialized countries during recent decades.
- The International Study of Asthma and Allergies in Childhood (ISAAC) can provide some of the most valuable prevalence and trend data concerning AD; it covers close to 2 million children in 100 countries, and is the largest and only allergy study to have taken a truly global approach.4
- Nearly 85 million Americans (27% of the population, or more than one in four individuals) complained of a skin disease and were seen by a physician in 2013.5
- In most industrially advanced markets, acne is generally thought to be the most common dermatosis, affecting approximately 85% of the population at some point in life.6
Moreover, eczema is a common dermatologic condition characterized by erythema, desquamation, papulation, lichenification, excoriation and itching. It affects children as well as adults, both males and females, and is related to a defective skin barrier, most commonly due to damage to the intercellular lipids caused by the improper selection of skin cleansers and moisturizers.
Hygiene procedures are frequent during the day and as stated, the overuse of cleansers can lead to epidermal barrier damage, increasing transepidermal water loss. This, in turn, has been associated with itch intensity in patients affected by eczema.7
Surfactants and Skin
Surfactants, in particular, are the major ingredients of body soaps and cleansers, and harsh varieties have been suggested to damage the skin. Thus, while the stratum corneum (SC) is rich in intercellular lipids, its structure can be easily disrupted by surfactants, consequently impairing barrier function of the skin.
Numerous studies and reviews describing the interaction between surfactants and the skin8-11 have sought to link the physicochemical properties of surfactants to their toxicity. Such studies have mainly been performed using ionic surfactants, generally known to be more irritating than nonionic surfactants.12-14 Indeed, ambiguity still exists over which physical characteristics of a surfactant system are the most important for obtaining mildness and fair skin compatibility.
Publications detailing the effects of surfactants on membrane permeability describe an apparent concentration-dependent biphasic action. An increase in membrane permeability occurs at low surfactant concentrations but this decreases at higher concentrations, generally above the critical micelle concentration (CMC) of the surfactant.15
Micelles were discovered to be capable of penetrating the SC; therefore, both monomeric and micellar surfactant species should be considered.16, 17 The presence of surfactant micelles in solution is necessary for the solubilization of lipid material and, in comparison with other assembled structures, e.g., cylinder, lamels, etc., spherical micellar solutions usually solubilize a small amount of fatty materials. The micelle size and surface charge18 are also important physical characteristics, as they influence the micelle stability and their power to affect skin lipids.
The pursuit of hygiene leaves the largest footprint on the skin’s surface.
The amounts of solubilized lipids are dependent upon:
a) the individual strengths of hydrophilicity and lipophilicity of the amphiphile. Surfactants having the same HLB can solubilize larger amount of lipids when they have longer alkyl chains; and
b) size. The micelle size increases with the increase in the alkyl chain length, even with the same HLB.19
We could simplify these interactions to three possible profiles of membrane alteration induced by surfactants, as a function of surfactant concentration (see Figure 1). Profiles A and B represent the situation where alteration is increased at premicellar concentrations. The formation of micelles decreases or maintains the capability of the surfactant to penetrate the SC. Profile C represents the case where the surfactant increases its ability to affect the integrity of the skin membrane, both before the micelle formation and after reaching its CMC.
In practice, between the mid-1980s and early 1990s, scientists realized the proper combination of different surfactants can significantly reduce protein damage and skin and irritation due to cleansing. As an example, adding amphoteric surfactants at about 20-30% of the concentration of an accompanying anionic surfactant will reduce the charge density, and thereby stabilize micelles of anionic surfactants. According the micelle theory, as mixed micelles are less likely to break apart, less surfactant monomer is able to enter the SC and damage lipids and proteins. On the other side, it should be noted that the equilibrium of micelles-monomers requires less than a few seconds to occur; in other words, it is not just the micelle or monomer state that strictly determines skin aggressivity. The most promising combinations of surfactants adopt balanced blends of the three categories: i.e., anionics, amphoterics and nonionics (see Formula 1).
One additional factor to consider is that some surfactants (i.e., anionics) are harsh toward skin proteins such as keratin, while others (i.e., nonionics and amphoterics) are aggressive toward lipid materials. Balanced surfactant combinations seem to be the best formulation strategy in respect to the overall skin barrier.
Indeed, the addition of C16 and C18 fatty acids to a sodium cocoyl isethionate-based bodywash was found to reduce both protein and lipid damage. The fatty-acid-containing bodywash also reduced skin dryness and TEWL in human volunteers.20 In relation, more recently, the reputed mildness of nonionic surfactants was discussed, highlighting that the difference in charge between the polar head group and lipophilic chain is not the only parameter to be considered.21
Today, formulations are often developed with surfactants that are partially derived from renewable sources—these include plant oils chemically modified with an added polar group to create “natural surfactants.” It is important to note that these surfactants are not inherently milder and suffer the same trade-offs between mildness and performance as other surfactants.22
Besides surfactant combinations, another approach to stabilizing surfactant micelles is to add polymers to the formulation, such as hydrophobically modified polymers (HMPs). HMPs have multiple hydrophobic groups that bind to surfactants at their hydrophobic tails. Through these contacts, HMPs stabilize the micelle core and increase its size. HMPs also can improve foaming by stabilizing the air-water interface of bubbles.23
Additional studies have lead to the inclusion of high levels of emollients (20-50%), such as petrolatum or triglycerides, to formulas; see Formula 2 and Formula 3. This is because emollients were found to improve the mildness and moisturization of cleansers. In particular, bodywashes containing about 20% triglyceride oils were shown in vivo to deposit approximately 10 µg cm−2 of triglycerides onto skin during washing.24 Thus, the adequate use of a “traditional” formulation could impart good results in terms of mildness (see Figure 2).
Finally, the use of acrylates copolymer in a cleanser might improve skin discomfort after washing, thanks to its ability to reduce friction between the skin and clothing.25
Skin cleansing is often viewed as the starting point of any skin care treatment. Indeed, cleansing products often state claims very similar to those of leave-on skin care formulae. Shampoo systems exist from which the deposition of conditioning materials such as silicone oils onto hair is achieved by means of polymers; e.g., polysaccharides, quaternary, acrylates copolymer, etc. Inspired by these technologies, the same polymers can enable skin treatment during the rinsing phase by the deposition and delivery of emollients, moisturizers and functional ingredients.
In spite of the frequent and abundant use of cleansers, the fine mechanisms taking place during skin cleansing are still largely unknown.
Typically, the most-used polymers are polyquaternium grade (P-10, P-7);26 acrylates copolymer;23 polysaccharides (hyaluronic acid, Opuntia ficus indica, Tamarindus extract, etc.);27 and/or their combination—along with a broad list of active ingredients such as vitamins (niacinamide, tocopherol, etc.); ceramides, humectants (glycerin, urea, etc.); re-epithelizing agents (panthenol, aloe vera, etc.); or trace elements including copper, zinc, manganese, etc., with antibacterial efficacy. Examples 1 and 2 in the Commercial Ingredient Listings sidebar provide sample INCI listings of products marketed for sensitive or problematic skin. Quite evident in both formulae is the combination of different types of surfactants and polymers, in order to reduce the aggressive action of the product; and polysaccharides polysaccharide derivates from vegetal origin; plus active ingredients such as vitamins, humectants and trace elements.
Taking a more extreme approach, some brands have started to formulate very mild products using nonfoaming formulas—similar in structure to the emulsions used as makeup removers or hair conditioners. These formulas can effectively cleanse and hydrate without affecting the skin barrier simply based on the solvent properties of the oil phase and the washing characteristics of high-polarity emulsifiers (see Commercial Ingredient Listings sidebar, Example 3). Also, in this case, the introduction of acrylates copolymer and quaternary compounds enhances the adhesion of the actives onto the skin.
Evaluating the mildness of formulated products after their development is, clearly, also very important. Similar to sunscreens, the mildness claims of these products must be verified in vivo using the correct testing protocols, in order to avoid serious damage to the most sensitive consumers. Simply using consumer self-evaluations, as is often claimed, is not enough. Indeed, it is strongly recommended that the subsequent combination of primary tolerability tests—a patch test (48 hr or 72 hr) followed by repeated arm wash tests28, 29—should be carried out, when possible, on volunteers with sensitive skin, together with long-term “n-use” tests under dermatologist control.
In spite of the frequent and abundant use of cleansing products, the fine mechanisms taking place during skin cleansing are still largely unknown. The chemistry and structure of surfactants play a major role, as do their association phenomena in solution and their behavior when diluted and massaged onto the skin. Moreover, their ease of rinsing and the complete release of traces of surfactant residual within skin wrinkles are also key factors, associated with water hardness and temperature.
The short length of time cleansing surfactants are in contact with skin is only partly apparent; residues in concentrated form can remain for hours on the skin surface and have the potential to disrupt the barrier in the interval between washes. Since the cleansing process is a multistage event, wherein the elimination of skin surface materials cannot easily be controlled without some type of impairment to the assembly of skin lipids beneath, studies are still required to understand this complex phenomenon and the distribution of molecules on the different levels of skin layers.
- Hay, R. J., et al. (2014). The global burden of skin disease in 2010: An analysis of the prevalence and impact of skin conditions, J Invest Dermatol 134, 1527-1534.
- Segre, J. A. (2006). Epidermal barrier formation and recovery in skin disorders, J Clin Invest 116, 1150-1158
- Lynch, P. J., (1994). Dermatology, House Officer Series. Philadelphia: Williams & Wilkins.
- Nutten, S., (2015). Atopic dermatitis: Global epidemiology and risk factors, Ann Nutr Metab 66(suppl 1), 8-16.
- Lim, H. W., et al. (2017). The burden of skin disease in the United States, JAAD 75(5), 958-972.
- Ballanger, F., et al. (2006). Heredity: A prognostic factor for acne, Dermatology 212, pp. 145-149.
- Lee, C.H., et al. (2006). Transepidermal water loss, serum IgE and beta-endorphin as important and independent biological markers for development of itch intensity in atopic dermatitis, Br J Dermatol 154, pp. 1100-1107
- Mehling, A., Kleber, M. & Hensen, H. (2007). Comparative studies on the ocular and dermal irritation potential of surfactants, Food and Chemical Toxicology 45, pp. 747-758.
- Rhein, L.D., et al., (1986). Surfactant structure effects on swelling of isolated human stratum corneum, J Soc Cosmet Chem 37, pp. 125.
- Rieger, M.M. (1995). Surfactant interactions with skin, Cosm & Toil 110(4), pp. 31-50.
- Rhein, L.D., (1997). Review of properties of surfactants that determine their interaction with stratum corneum, J Soc Cosmet Chem 48, pp. 253-274.
- Lansdown, A.B.G. & Grasso, P., (1972). Physico-chemical factors influencing epidermal damage by surface active agents, Br J Dermatol 86, pp. 361-378.
- Effendy, I., & Maibach, H. I. (1995). Surfactants and experimental irritant contact dermatitis, Contact Dermatitis 33, 217-225.
- Walters, K. A., Bialik, W., & Brain, K. R. (1993). The effects of surfactants on penetration across the skin, Int J Cosmet Sci 15, 260-271.
- Corazza, M., et al. (2010). Surfactants, skin cleansing protagonists, J Eur Acad Dermatol Venereol 24, 1-6.
- Moore, P. N., Puvvada, S., & Blankschtein, D. (2003). Challening the surfactant monomer skin penetration model: Penetration of sodium dodecyl sulfate micelles into the epidermis, J Cosmetic Sci 54(1), 29-46.
- Moore, P. N., et al. (2003). Penetration of mixed micelles into the epidermis: Effect of mixing sodium dodecyl sulfate with dodecyl hexa(ethylene oxide), J Cosmetic Sci 54(2), 143-159.
- Lips, A., et al. (2007). Role of surfactant charge in protein denaturation and surfactant-induced skin irritation, in Surfactants in Personal Care Products and Decorative Cosmetics, Third Edition, Rhein, L. D., Schlossman, M., O’Lenick, A. & Somasundaran, P., eds. Boca Raton, FL U.S.: CRC Press.
- Nakajima, H. (1999). Microemulsions in Cosmetics, IFSCC—
Monograph No. 7.
- Ananthapadmanabhan, K. P., et al. (2009). A novel technology in mild and moisturizing cleansing liquids, Cosmetic Dermatology 22(6), 307-316.
- Lémery, E., et al. (2015). Skin toxicity of surfactants: Structure/toxicity relationships, Colloids and Surfaces A: Physiochemical and Engineering Aspects 469, 166-179.
- Walters, R. M., et al. (2008). Designing cleansers for the unique needs of baby skin, Cosm & Toil 123(12), 53-60.
- Fevola, M. J., LiBrizzi, J. J., & Walters, R. M. (2008). Reducing irritation potential of surfactant-based cleansers with hydrophobically-modified polymers, Polymer Preprints 49(2), 671-672.
- Ananthapadmanabhan, K. P. (2004). Cleansing without compromise: The impact of cleansers on the skin barrier and the technology of mild cleansing, Dermatol Ther 17(suppl 1), 16-25.
- Mizukoshi, M., et al. (2015). Improvement of Winter Season-Related Skin Itchiness With Soap-Based Body Wash Containing Acrylates Copolymer and Its Mechanism. Poster presented at the 2015 IFSCC Conference, Zurich, Switzerland.
- Wickett, R. R., Sramek, J., & Trobaugh, C. (1992). Measurement of the adhesive strength of hair-hairspray junctions, Journal of the Society of Cosmetic Chemists 43, 169-178.
- Schmidgall, J., Schnetz, E., & Hensel, A. (2000). Evidence for bioadhesive effects of polysaccharides and polysaccharide containing herbs in an ex vivo bioadhesion assay on buccal membranes, Planta Med 66, 48-53.
- Ertel, K. D., et al. (1995). A forearm controlled application technique for estimating the relative mildness of personal cleansing products, J Soc Cosmetic Chem 46, 67-76.
- Strube, D. D., et al. (1989). The flex wash test: A method for evaluating the mildness of personal washing products, J Soc Cosmetic Chem 40, 297-306.