**Structure of Hair**
1. **Cuticle Layer** — Composed of keratinized fish-scale-like cells arranged in the direction towards the hair tip. The cuticle layer of general hair consists of 6-12 layers of hair scales (fine and soft hair: 6-8 layers; coarse and hard hair: 8-12 layers). This layer protects the hair from external damage, such as structural disruption. When the hair is wet, the cuticle scales swell and become more susceptible 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's structure and is its main body. It contains the following bonds: salt bonds, sulfur bonds, and fiber-like cortical cells twisted like braids, which provide elasticity, tension, and toughness to the hair. The natural color of the hair (melanin) exists within the cortex and is composed of two pigments: blue-black pigment and red-yellow pigment. 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 vacuum structure (which determines the thickness or thinness of the hair). When the medulla layer is tightly packed, the hair becomes coarse and hard (some hairs may not have a medulla layer).
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**Four Key Bonds in Hair**
1. **Hydrogen Bonds**: Reorganize under high temperature and break when exposed to water.
2. **Salt Bonds**: Hair is healthiest when its pH level is between 4.5-5.5.
- Hydrogen bonds and salt bonds break when exposed to water but can be recombined using warm air, lasting until the next rinse (controlled by water and temperature during blow-drying).
3. **Disulfide Bonds**: These are strong bonds that can break and re-form (controlled during perming), determining whether the hair is straight or curly.
4. **Amide Bonds**: Determine the shine and elasticity of the hair (only present in coarse and hard hair, not in fine and soft hair).
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**Properties of Hair**
1. **Physical Properties**: Elasticity, Tension, Stretchability
2. **Chemical Properties**: Water Absorption, Water Repellency
- Healthy hair has weak water absorption and strong water repellency.
- Damaged hair has strong water absorption and weak water repellency.
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**Hair Texture Analysis**
- **Healthy Hair**: Untreated with perm or dye, tightly closed cuticle scales, shiny, easy to comb.
- **Oily Hair**: Overproduction of sebum.
- **Dry Hair**: Lacks oil, dry and dull, difficult to comb.
- **Neutral Hair**: Beautiful, shiny, and resilient.
- **Damaged Hair**:
- **Mildly Damaged**: Color level 5-6, dry ends.
- **Severely Damaged**: Color level 8-10, shedding cuticle scales (strong water absorption, loses water repellency if not blow-dried).
**PS**: Flipped or shed cuticle scales result in lack of shine, no elasticity, and tangling.
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**pH Levels of Hair**
- Hair pH: 4.5-5.5
- Shampoo: 6-6.5
- Perm solution: 8-10
- Conditioner: 2.8-3.5
- Mask treatment: 3.5-3.8
- Straightening cream: 11-13
- Hydrogen peroxide: 3
- Dye: 9
**PS**: Alkaline opens the cuticle scales, while acidic closes them.
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**Factors That Damage Hair in Daily Life**
1. Chemical damage (excessive perms and dyes)
2. Incorrect or low-quality shampoo (improper washing techniques)
3. Sunlight and ultraviolet radiation
4. Improper cutting and blow-drying
5. Seawater (alkaline)
6. Sulfate copper in tap water
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