Normal skin surface pH is 4∼6, slightly acidic. The dense structure of stratum corneum cells, keratin and lipids are arranged in a close and orderly manner, which can form a natural barrier to resist the invasion of the skin by various external physical, chemical and biological harmful factors. Excessive use of exfoliating products or excessive application of cleansers to alter the skin’s slightly acidic environment can weaken the skin’s barrier function.
The keratinocytes in the stratum corneum of the skin can absorb a large amount of short-wave purple rays (180∼280 nm), and the melanosomes synthesized by keratinocytes in the spinous cell layer and melanocytes in the basal layer can absorb a large amount of short-wave purple rays (180∼280 nm). Absorbs long-wave UV rays to form a sunscreen. Sun exposure can damage the stratum corneum and interfere with the stratum corneum, forming cells to decompose to form natural moisturizing factors, acting on silk protease, stimulating collagen synthesis, increasing collagen degeneration or rupture and epidermal cell division. Therefore, sun protection can delay skin aging and reduce the incidence of skin cancer.
The stratum corneum is the main part of the skin to absorb foreign substances, accounting for more than 90% of the total absorption capacity of the skin. Because the stratum corneum space is dominated by lipids, and the stratum corneum absorbs fat-soluble substances, most of the topical drugs and cosmetic products researched and developed in dermatology are emulsions and creams.
The lipids and NMF in the normal stratum corneum maintain a certain water content in the stratum corneum, and a stable hydration state is a necessary condition to maintain the normal physiological function of the stratum corneum. The stratum corneum can maintain the percutaneous water loss of only 2∼5 g (h.cm2), making the skin smooth, flexible and elastic. Because 3 key lipids, namely ceramides, cholesterol and fatty acids, are necessary for skin moisturization and barrier repair. Therefore, the development of moisturizing products formulated in proportion to these 3 key lipids is more effective than non-physiological lipid substances (such as lanolin, petrolatum) to correct the corresponding diseases, such as atopic dermatitis, ichthyosis biochemical abnormal.
Smooth and hydrated skin is regularly reflected by light, and the appearance of the skin is shiny, plump and elastic. Changes in the normal number of layers of the stratum corneum, or abnormal keratinization of the cells of the stratum corneum, can lead to changes in skin color and radiance. In some skin diseases that cause dryness and scaling, such as ichthyosis and atopic dermatitis, the stratum corneum reflects light in a non-specular form, making the skin dull and dull. Excessive stratum corneum thickness for any reason can make the skin appear rough, dull and dull. If the stratum corneum is too thin, such as excessive “exfoliation”, “peeling” or frequent use of alkaline detergents, etc., the barrier function of the skin will be weakened, and external adverse factors will easily cause the skin to be sensitive and abnormally pigmented, such as Skin redness, telangiectasia erythema, pigmentation and skin aging, and even cause skin diseases.
Further reading：The difference between RF microneedling and fractional laser treatment