Silk, placenta, ginseng and pearl are recognized as the four major cosmetic additives in the world. Silk beauty has a long history in China. The “Compendium of Materia Medica” recorded that, silk fibroin could remove skin dark spots and treat the record of suppurative dermatitis.

In recent years, more and more research has been done on the cosmetic effects of silk fibroin, and a number of new nutritional cosmetics containing silk peptides have been developed. As a skin care cosmetic application, silk fibroin needs to decompose an amino group into an oligopeptide, and the molecular weight is generally required to be 1000-2000 Da. Silk fibroin has water solubility and can be obtained by controlling hydrolysis conditions.

Silk protein can be used as a good material for making cosmetics because:

1）Silk protein is similar to human keratin, structure, and has good affinity for dialysis keratin protein.

2) Silk protein is rich in nutrients. Silk is a precious natural protein fiber. 97% is pure protein and has a nutritive effect on the skin. Therefore, it is superior to talc powder and titanium dioxide which are usually used in cosmetics.

Silk protein has a complete range of amino acids and consists of 18 amino acids. Among them, leucine, serine, threonine and glycine are the nutrient elements of skin, while the side chain hydrophilic amino acids (aspartic acid, glutamic acid) are in human body. When the nutrient elements of silk fibroin enter the cortex, they are organically bound to human cortical cells and absorbed by epidermal cells, especially for the elasticity of damaged epidermal cells. Long-term use of cosmetics with silk protein can promote skin metabolism, prevent premature aging of the skin, and increase the vitality of skin. The silk protein contains more than ten amino acids similar to the amino acids contained in skin.

3) Silk protein has good film-forming properties and is beneficial for maintaining the normal function of skin surface film. On the outermost side of human skin epidermis, there is a film of several mm thick, usually called sebum film, which can prevent the external factors from irritating, prevent the evaporation of water, and protect the skin. Silk protein has good film-forming properties, and silk fibroin film formed after drying the silk protein solution has a unique effect.

When silk fibroin is used as a cosmetic, the key is the molecular weight. In the range of 300-4000 Da, the smaller molecular weight, the better absorption and stretch ability on the skin and hair surface.

4) Silk protein has good hygroscopicity, can maintain a certain water content of the skin, and has the function of natural humidity-regulating factor. The test shows that in cosmetics, just a small amount of silk protein can obtain good moisturizing.

Hygroscopic properties, it is because the molecular conformation of silk fibroin is a random coil structure, the peptide chain is loose and disordered, and the hydrophilic group is on the surface of molecular space structure. These hydrophilic groups can transfer water in human body to the stratum corneum of epidermis. In combination with it, it keeps the moisture content in skin, make skin elastic, smooth and soft, and effectively prevent dryness, wrinkles and aging of skin.

According to research, silk powder has the function of preventing ultraviolet radiation. Adding about 10% of silk fibroin powder to cosmetics can effectively absorb ultraviolet rays, inhibit the formation of a large amount of melanin in skin, and prevent the formation of black spots and freckles.

Skin care application properties of silk proteins include:

1) Natural moisturizing and nourishing skin properties

Silk fibroin has good moisture absorption and desorption properties, and its molecular structure has many hydrophilic genes on the surface of molecular three-dimensional structure. This structure indicates that silk fibroin is a natural moisturizing factor and has a good moisturizing effect. The experiment showed that the moisture absorption rate of silk protein solution with a concentration of 2.5% was equivalent to the moisture absorption rate of 48% glycerol at a temperature of 27 ℃, a relative humidity of 86% RH and a moisture absorption time of 12 hours.

That is, a small amount of silk protein solution is added to the cosmetic to obtain a good moisturizing type and hygroscopicity. Cosmetics containing silk protein as a main additive have been introduced in the market, such as moisturizers, facial cleansers, facial cleansers, shower gels, shampoos and conditioners.

2）Skin melanin inhibition

Japanese scientists found that silk fibroin has the property of inhibiting tyrosinase activity. In a phosphate buffer containing L 3,4-dihydroxyphenylalanine (DOPA) and mushroom tyrosinase, 0.5% and 1.0% silk protein solution were added respectively. Compared with the control, the activity of tyrosinase was reduced to 76% and 50%, respectively. The mechanism may be due to the complexation of a high proportion of hydroxy amino acids in silk fibroin with trace elements such as copper and iron, thereby affecting the normal function of tyrosinase activity.

Blackening of human skin is due to the tyrosinase in melanocytes, which oxidizes tyrosine in the keratin component, followed by 3,4 dihydroxyphenylalanine (referred to as dopa), dopaquinone and 5, 6 Dihydroxyanthracene, finally polymerized into melanin. Ultraviolet light promotes this oxidation reaction, so in summer, human skin is often blackened when exposed to sunlight. If the silky protein-containing cosmetics are often applied to the skin, it can effectively inhibit the formation of melanin in skin and keep skin white and tender.

Tyrosine, tryptophan and phenylalanine in silk protein can effectively absorb ultraviolet rays. In addition, the Institute of Sericulture of Zhejiang Academy of Agricultural Sciences passed the following experiment: using silkworm blood thin solution as the reference solution, adding different concentrations of silk protein solution, the amount of melanin produced from the solution was determined by spectrophotometer to determine the inhibition rate of melanin production ( Taking distilled water as a control, it was found that when the silk protein concentration was 25%, the inhibition rate for producing melanin was about 70%, which is unmatched by other cosmetic additives.

3) Promote skin tissue regeneration, prevent cracking and chemical damage

Silk protein is very effective in promoting skin wound healing. A wound was punctured on both sides of the spine of a rabbit, and silk fibroin and physiological saline were respectively applied. The results showed that silk fibroin was non-irritating and accelerated wound healing. Cosmetics containing silk protein also have a good effect on preventing skin splitting.

At the same time, silk fibroin can also resist the erosion of sunlight, microwave, acid and alkali chemicals and atmospheric pollutants on the skin, which can inhibit the occurrence of reactive oxygen species and prevent skin wrinkling and aging.

4) Inhibition of skin cancer

It was found by mouse experiments that silk fibroin has a good inhibitory effect on skin cancer. First, the hair on the back of mouse was shaved, and 7.12-dimethylbenzindole (DMBA) as a carcinogen was applied to the exposed portion of skin. Then, the mice were divided into three groups, a control group, a silk fibroin group A, and a silk fibroin group B.

The silk fibroin-coated group was coated with silk fibroin on the mice at a rate of two times a week after applying DMBA for one week, and the amount of silk protein applied per time was: group A 2.5 mg. ; Group B 5.0mg. After silk fibroin was applied, 12-0-tetradecanoyl-phorbol-13-acetone (TPA) was used as a carcinogenic promoter over 1 hour.

Then, the skin condition of the mouse was observed over 16 weeks, and when the number of skin tumors was counted, it was found that there was a tendency of tumor growth in the control group, but for each application of 5.0 mg silk fibroin group, no skin tumors were found.

5) Antioxidant function of silk protein

Mistol, which produces chlorination with active oxygen, has important medical value for common diseases such as cancer, arteriosclerosis, and diabetes. Professor Kato of the Bioproduction Science of Hiroshima University experimented with the brain homogenization product of rats. The brain contains many polyunsaturated fatty acids. If it is hatched, it is easily oxidized by active oxygen to produce an aldehyde as a reaction product. By reacting this aldehyde with thiobarbital acid, it can be quantified and become an indicator for detecting the quality of lipid after oxidation (TBA method).

The experimental results showed that the lipid was oxidized at 37 ° C for 120 minutes, and the oxidation amount of silk protein or BSA was measured. The control value of the amount of oxidation is 100%, expressed as a relative value. As the amount of silk protein added increases, the oxidized lipid decreases, confirming that the silk protein has antioxidant capacity.

Further, silk protein and ascorbic acid were added under heat treatment conditions at 100 ° C for 120 minutes, and when the amount of oxidation was confirmed under the same conditions, although ascorbic acid lost its antioxidant ability due to heat, silk fibroin exhibited thermal stability. Moreover, silk protein is better than vitamin C with antioxidant function.