INTRODUCTION
Cleansers are designed to remove dirt, sweat, sebum, and other unwanted materials from skin. Normal exfoliation of superficial dead cells is also aided by the cleansing process. Thus, cleansing is the first step in daily skincare.The cleanser actives, surfactants, are designed to remove oily materials and drive the cleansing process. The challenge is in limiting the interaction of surfactants to just removal of undesirable materials and preventing them from altering the properties of the stratum corneum barrier, leading to skin dryness, irritation, itch, and other undesirable events. It is well-known that cleansers based on alkaline soaps are more irritating to skin than those based on synthetic surfactant actives, commonly referred to as syndets, which function under neutral pH conditions.1 It has also been established that the harshness of alkaline soaps stems from their harsh cleanser active, the soap molecules, and their high pH.2,3 Early work on interaction of surfactants with stratum corneum has been reviewed extensively in the literature4-11 and the importance of milder surfactants underscored.The emergence of liquid cleansers in the 1990’s opened up opportunities for a wider range of surfactants to be explored in the cleansing arena. Synergistic combination of surfactants can lead to improved mildness.4,7 However, the challenge has been in designing ultra-mild products without compromising on consumer-desired sensory properties such as lather. In fact, dermatologists would acknowledge that compliance even among those with compromised skin is poor for products that have inferior sensory. Recent advances in cleansing surfactant systems have resulted in ultra-mild The liquid format also made it possible to develop technologies that deposit moisturizing and sensory enhancing actives during the wash process. Benefit actives from wash-off systems include humectants such as glycerol, oily materials such as triglyceride oils and petrolatum, sensory enhancers such as silicones, and skin-natural lipids such as fatty acids and sterols.4,9 Along with the progress in cleanser technologies, the ability to predict the skin irritation and drying potential of cleansers and assess the skin barrier quality also has advanced significantly.14 Interaction of Cleanser Surfactants With Stratum Corneum The human stratum corneum consists of flattened corneocyte cells embedded in a lipid matrix.15,16 While the corneocytes with their cross-linked keratins and natural moisturizing factors (NMFs) contribute to the mechanical and water holding properties of the stratum corneum (SC), the lipid matrix acts as the main barrier to water loss from the body.17,18 Recent advances in our understanding of the stratum corneum have shown that it is not only a physical barrier, but also an immunological and a microbial barrier.19, 20 Thus, any damage to the stratum corneum from use of harsh cleansing products can impact its multiple barrier functions.Interactions of Surfactants With SC Proteins and Relevance to Skin Irritation The early work on interaction of common cleanser surfactants with skin showed that in vivo irritation potential of surfactants correlated with the ability of surfactants to denature proteins such as BSA or Zein, and/or swell cross linked proteins such as collagen.4,7 Over the years, there have been attempts to use stratum corneum itself as a substrate for in vitro irritation studies.5,6,14, 21 Skin irritation potential of some of the common
