SURFACTANT PENETRATION 97 is around 0.05- to 1-mm thick, directly contacts with the outside environment (13). The epidermis consists of keratinocytes and other components such as Langerhans cells, mela- nocytes, and Merkel cells (18). The keratinocytes, as one of the most important compo- nents in the epidermis, form a protective barrier to keep the skin hydrated internally (19). The epidermis can be further divided into the viable epidermis, i.e., the inner layer, in- cluding the basal cell layer, spinous cell layer, and granular layer, and the nonviable epi- dermis, i.e., the outer layer, also called the stratum corneum (SC). The distinction among the various epidermis layers depends on the mature stages of the keratinocytes. From the basal cell layer (the innermost epidermis layer) to the SC (the outermost epidermis layer), the mature stages deepen, so do the density and fl attening of the cells. One of the popular models to describe the SC structure is termed as the “brick and mor- tar.” The “brick” represents the corneocytes (20) which are the most mature stage of the keratinocytes—the dead keratin-fi lled cells (21). The corneocytes are surrounded by the cell envelopes (protein shells). The cell envelopes serve as the protective layer for the cor- neocytes. Situated at the external of the cell envelopes is a layer of bonded lipids, which forms the cornifi ed lipid envelope with the protein shells (22). The external bonded lipids build compatibility between the corneocytes and the intercellular lipids (23). The natural moisturizing factor (NMF) is an important substance in the corneocytes which can only be found in the SC. NMFs are water-soluble compounds and are mainly composed of about 40% free amino acids, 12% of sodium salt of pyroglutamic acid, 12% of lactic acid salts, 7% of urea, and 18.5% of inorganic salts (4,24). NMFs occupy 5–30% of the total dry weight of the SC, and their quantity has an infl uence on the degree of skin dryness (25,26). The “mortar” consists of two parts: one is the intercellular lipids, which is mainly made up of approximately 50% of ceramides (CERs), 15% of free fatty acids, 25% of cholesterol (CHOL), and 10% of CHOL esters (4). The intercellular lipids play an impor- tant role in maintaining the skin structure (4,20). Lipid matrix exists in both the crystal- line state and the liquid crystalline state (27,28). Liquid crystalline is the state after the substance melted or dissolved in solvents. It is the middle phase between the liquid and crystal, which loses the rigidity of solid substances and has the fl uidity of liquid (29). Because of the fl uid nature, the lipids in the liquid crystalline state are relatively perme- able. Furthermore, the solid lipid crystalline states mainly consist of two phases: the or- thorhombic (OR) and the hexagonal (HEX) phases. Their main function is to ensure the integrity of the skin barrier structure (30). The OR phase is considered as the most solid structure among the three structures (OR/HEX/liquid crystalline state), whereas the liq- uid crystalline state is the most fl uid (22). The other part of the “mortar” is the corneodesmosomes, which are proteins connecting the corneocytes together, providing strength for the skin structure (31). Consequently, the coexistence of the lipid matrix and the corneodesmosome proteins serves as an impor- tant factor that enables the SC to act as the tight barrier against the penetration of the external stimulus and other harmful substances (32). Windsor and Burch fi rst discovered that the skin barrier is located in the SC and its stability is directly linked with the states of the lipids and the proteins in the SC (23). THE IMPACT OF SURFACTANTS TO SKIN With the constant upgrade of skin-cleansing formulations, the function of the cleansers is not just limited to dirt removal. The mild cleansing and the skin feel after moisturizing

JOURNAL OF COSMETIC SCIENCE 98 recently become the important criteria for consumers to choose products. Nevertheless, some surfactants in the cleansing products may still exert certain adverse impacts to the skin. It is not uncommon that some surfactants can be detrimental to the skin barrier, thereby causing a series of skin problems, such as dryness, tightness, itch, irritation, and even infl ammation. As mentioned earlier, the integrity of the skin barrier is associated with the lipid matrix and proteins in the SC. There are usually two transepidermal path- ways for surfactants penetrating into the skin. One is the intracellular route, which allows the molecules to diffuse through the corneocytes. This route is mainly for transporting the hydrophilic or polar solutes. The other pathway is to transport the lipophilic or non- polar solutes via intercellular lipids. The transepidermal pathways suggest surfactants in skincare products could interact with the proteins and the lipids in the SC, which may change the status of the skin barrier (33,34). The two pathways described earlier are pre- sented in Figure 5. THE IMPACT OF SURFACTANTS TO INTERCELLULAR LIPIDS The impact of surfacants to the lipid matrix could be specifi ed in three possible mechanisms: (i) the detergency ability of the surfactants removing dirt and excess oil as well as the lipids from the skin surface, (ii) the mixing between the skin lipids and surfactants leads to the disorganization of the skin lipid matrix, and (iii) the lipids in the SC are extracted and solubilized into the surfactant solutions (35). To illustrate how surfactants affect the Figure 5. Two pathways existing in human SC (Reprinted (67) with permission. Copyright 2015 Elsevier).