Human hair consists of alpha-keratin, which is an animal protein. Silk protein also belongs to α-keratin, which has a similar structure to hair.  According to the principle of “homogeneous correspondence”, silk protein molecules should be easily absorbed by hair molecules and form relatively stable chemical bonds (or hydrogen bonds) without being easily washed away by water. The chemical properties of hair proteins are inactive and stable, but they are still sensitive to heat, acid and alkali. In addition, it will also stretch and deform under mechanical force. Hair is damaged easily during combing and washing, especially during perm and hairdressing. The disulfide bond in hair is easily reduced by the sulphydryl compounds (the main component of chemical cold agent), resulting in the destruction of molecular structure of hair surface, causing hair gloss to deteriorate, dry and easy to split, the elasticity and strength of the hair are also reduced, become fragile.

The protective function of silk protein depends on its absorption, film formation and moisturizing properties.

A series of experiments have shown that silk protein is more easily absorbed by hair than other hydrolyzed proteins (collagen, bone collagen, gelatin, etc.). Radiation tracking technology shows that the absorption of skin and hair by silk peptides is mainly dependent on osmosis. When silk concentration increases, the amount of silk protein absorbed by hair increases accordingly. In the case of damaged hair, the absorption of low molecular weight silk protein is greater, and the corresponding effect of silk protein on hair repairing and protection is more obvious.

Silk fibroin with a molecular weight of 1000-2000 has a good film forming property on hair, and the film has a characteristic luster, hair has a good hand feeling and elasticity.  X-ray diffraction showed that the film formed by silk fibroin was β-type crystal, consequently, the film formed on silk fibroin was not easily washed away by water. Experiments have shown that silk fibroin forms a film with a certain strength on the surface of hair and can also be used as a hair styling agent. Compared with PVP, collagen peptide and starch aqueous solution, silk fibroin is used as a styling agent under 80% RH moist conditions, which can keep hair well for a long time.