**Structure of Hair**
1. **Cuticle Layer** — Composed of keratinized fish-scale-like cells arranged in the direction toward the hair tip. The cuticle layer generally consists of 6–12 layers of overlapping scales (fine soft hair has 6–8 layers, coarse hard hair has 8–12 layers). It protects the hair from external damage, such as mechanical stress. When the hair is wet, the cuticle scales swell and become more vulnerable to damage. (They open under alkaline high temperatures and close under acidic low temperatures.)
2. **Cortex Layer** — Made up of protein cells and pigment cells, this layer accounts for 80% of the hair structure, forming the main body of the hair. It contains the following bonds: salt bridges, sulfur bridges, and fibrous cortical cells that twist like braids, giving the hair elasticity, tension, and resilience. The natural color of the hair (melanin) resides within the cortex layer, composed of two pigments: blue-black melanin and red-yellow melanin. The ratio and arrangement of these pigments determine the color of the hair.
3. **Medulla Layer** — Located at the innermost layer of the hair, surrounded by cortical cells. It is formed by a sponge-like structure (which determines the thickness or thinness of the hair). When tightly packed, the medulla makes the hair coarse and stiff (some hairs may lack a medulla).
**Four Key Bonds in Hair**
1) **Hydrogen Bond**: Reorganizes under high temperature and breaks when exposed to water.
2) **Salt Bond**: Hair is healthiest when its pH is between 4.5–5.5.
Hydrogen bonds and salt bonds break when exposed to water and can be recombined using warm air until the next rinse (water and temperature influence blow-drying, so control these two bonds during styling).
3) **Disulfide Bond**: A very strong bond that can break and re-form (controlled during perms), determining whether the hair is straight or curly.
4) **Amide Bond**: Determines the shine and elasticity of the hair (this bond is absent in fine soft hair but exists in coarse, hard hair).
**Properties of Hair**
1. **Physical Properties**: Elasticity, Tension, Flexibility
2. **Chemical Properties**: Water Absorption, Water Repellency
*Note*: Healthy hair has weak water absorption and strong water repellency. Damaged hair absorbs water strongly and repels it weakly.
**Hair Texture Analysis**
*Healthy Hair*: No previous perming or dyeing; tight closed cuticles; good luster; easy to comb.
1. Oily Hair: Excessive sebum secretion.
2. Dry Hair: Lacks oil; dry and lacks shine; difficult to comb.
3. Normal Hair: Beautiful, shiny, and resilient.
*Damaged Hair*:
1. Mild Damage: Color level between 5–6; dry ends.
2. Severe Damage: Color level between 8–10; loss of cuticles (strong water absorption, poor drainage if not blow-dried).
*Note*: Flipped or fallen cuticles; no shine; no elasticity; tangles easily.
**pH Value of Hair**
Hair pH: 4.5–5.5
Shampoo: 6–6.5
Perm Solution: 8–10
Conditioner: 2.8–3.5
Cap Treatment: 3.5–3.8
Straightening Cream: 11–13
Hydrogen Peroxide: 3
Dye: 9
*Note*: Alkaline opens the cuticles; acidic closes them.
**Factors That Damage Hair in Daily Life**
1. Chemical Damage (excessive perming or dyeing)
2. Incorrect or low-quality shampoo (improper washing methods)
3. UV Radiation from the Sun
4. Improper cutting or blow-drying
5. Sea Water (alkaline)
6. Sulfate Copper in Tap Water
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