Skin tissue can be divided into the Epidermis, Dermis and Subcutaneous Tissue from the inner to the outer. Besides, the Epidermis can be also divided into the Stratum Corneum, Transparent Layer, Granular Layer, Spinous Layer and Base Layer. Stratum Corneum is the first layer of skin tissue which is composed by brick-like keratinocytes, containing 15%-20% Natural Moisturizing Factor (NMF) for regulation of skin hydration and the keratinocytes was filled with cement like intercellular lipid which was composed by Triglyceride, Ceramide and Free Fatty Acid to protect the water loss from the skin, external stimulants and bacteria. In addition, the Sebaceous Glands can produce the sebum on the surface of the skin to form a hydrolipdic film which is consist of Squalene, Cholesterol, Cholesterol Ester, Wax Ester and Triglyceride, giving a hydration and protection for the skin.
Besides, the balance of water, lipid and NMF of skin will be influenced due to the excessive cleansing or improper usage of products. The skin will become peeling and sensititive when the water content of skin is below 10%.