Surfactant interaction with skin

L.D. Rhein and F.C. Simion, Surfactant Interactions with Skin, in Interfacial Phenomena in Biological Systems, Surfactant Science Ser., Vol. 39, Marcel Dekker, New York, 1991. 

Rieger, M. Surfactant interactions with skin. Cosmet. Toil. 110 31-50, 1995. 

Studies involving animals have also been modified in the attempt by researchers to develop tests more and more respectful of the animal s welfare. However, the Sixth Amendment to the European Union s Cosmetic Directive called for an end to animal testing by January 1, 1998, unless acceptable alternative methods are not available. In the absence of acceptable methods, such a decision has been recently reviewed it has been maintained for finished cosmetic products, barring exceptional cases, and postponed for tests on ingredients up to June 1, 2000. Thus, animal models for studying surfactant interaction with skin will continue to rapidly decline, except for specific purposes where no alternative exists. 

The study of skin irritation is probably still more complex than that of eye irritation. Surfactants interact with epidermal tissues, proteins, and enzymes causing local effects. Singer and Pittz [369], Cooper and Berner [370], and Schwuger and Bartnik [371] presented excellent explanations and reviews on these interactions. 

SC. Surfactant interaction with lipids and proteins leads to a fundamental breakdown of biological processes that underpin skin health. Mild surfactants have lead to cleansers with significantly reduced drying and damaging potential but only within the last decade have truly moisturizing cleansers begun to emerge. 

In recent years, protein-based surfactants are finding some applications in personal care products due to their abilities to interact with skin and hair." A-acyl polypeptide condensates (protein derivatives) belong to the mild surfactant category with foaming performance inferior to alkyl sulfates, but they produce creamy lather and leave hair feeling soft and manageable. However, it appears that extra attention and care should be taken to ensure preservation of these protein derivatives due to compatibility with other anionic surfactants when present in shampoo formulations. 

Surfactant interactions with proteins have been extensively studied by cosmetic scientists, with special interest in the effects of tensides on skin and hair keratin. Despite the increasing availability of highly skin-compatible surfactants that still retain excellent detergent properties, the adverse reactions potentially caused by these ingredients have never been underestimated by dermatologists and cosmetic chemists, and the subject persists as one of the key topics of cosmetology. The physicochemical aspects of protein-surfactant interactions have been investigated with a more theoretical approach in noncosmetic contexts by many scientists who exhaustively explored the behavior of binary systems of anionic tensides and native proteins (55-59). 

As a result of their diverse physico-chemical properties, surfactants interact with the skin in various ways [61]. Not surprisingly, the outcomes of these interactions are just as diverse [62]. In the following section, the more dominant interactions will be reviewed. However, the reader should note that because of their long history of use and dominance in consumer products, most research efforts have focused on understanding the interactions of anionic surfactants with the skin [63]. 

Amphoteric surfactants by definition are chimeric, exhibiting anionic character in alkaline solution, nonionic character near their isoelectric point, and cationic character in acidic solution [73]. As a result of their complex charge characteristics, their interactions with interfaces must be examined iudividually and as a function of pH. For example, the adsorption of alkyl betaines firom solution onto wool keratin is much greater at acidic than alkalide pH values. Although amphoteric surfactants are used extensively to improve the cosmetic attributes of many consmner products [74], their interactions with skin have received little attention. 

Models for Studying Surfactant Interactions with the Skin... 

Animals have been used in the past to investigate the interaction of surfactants with the skin in vivo. However, due to differences between animal and human skin [47], animals have mainly been used for governmental or safety purposes rather than for understanding the mechanisms of surfactant interaction with the skin. 

This test is based on the extraction of a fluorescent marker, dansyl chloride, previously bound to the amino groups of proteins inside the full thickness of the stratum comeum [76]. Fluorescent labeling of the skin is performed 1 week prior to the application of the surfactants. When surfactants are applied at 2% of active ingredients and under occlusion, two or three applications of 30 min each are sufficient to detach and extract all or part of the fluorescent dye from the skin in the absence of visible irritation. The level of dansyl chloride extraction has been shown to correlate with the irritation potential of the surfactants as determined in separate soap chamber tests. This test demonstrates that surfactants interact with the skin surface proteins very quickly and are able to release molecules that would be linked to these proteins. The evaluation of fluorescence extraction is done visually by a trained assessor on a 0-4 scale, 0 being no fluorescence extraction and 4 being complete fluorescence extraction. 

Labeling of the skin with dansyl chloride has also been extensively used by dermatologists to investigate the turnover rate of the epidermis [77]. After labeling with the fluorescent dye, the effect of test products on the epidermis proliferation rate is measured by the kinetics of fading of the fluorescent from the skin. However, this method has to be used very carefully when surfactants or surfactant-based products are tested, as many of them are able to quickly remove the fluorescence by direct interaction with skin proteins. 

When an irritating surfactant interacts with the skin surface, it partially or totally removes the hydrolipidic film, disorganizes the lipidic barrier of the stratum comeum, and... 

The assessment of the self-perception of surfactant interaction with the stratum cor-neum is performed by means of questionnaires in which several skin attributes are evaluated. Table 9 summarizes some of the attributes usually asked to and easily perceived by the subjects ... 

Azone (laurocapram) is used extensively as a transdermal permeation enhancer, and has also found use in buccal drug delivery. It is a lipophilic surfactant in nature (Figure 10.4). Permeation of salicylic acid was enhanced by the pre-application of an Azone emulsion in vivo in a keratinized hamster cheek pouch model [35]. Octreotide and some hydrophobic compounds absorption have also been improved by the use of Azone [36], Azone was shown to interact with the lipid domains and alter the molecular moment on the surface of the bilayers [37], In skin it has been proposed that Azone was able to form ion pairs with anionic drugs to promote their permeation [38],... 

Hofland, H.E., et al. 1991. Interactions of non-ionic surfactant vesicles with cultured keratinocytes and human skin in vitro A survey of toxicological aspects and ultrastructural changes in stratum corneum. J Control Release 16 155. 

The basic function of a cleanser is to promote health and hygiene of skin by removing excess dirt, sebum, and bacteria from skin and promoting exfoliation. However, as explained earlier, cleanser surfactants also interact with SC proteins and lipids, causing damage to the SC barrier, leading to a net loss in SC hydration. 

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