iluvia Science
SCIENCE
The science of hair, applied to Indian water and weather.
How hair is built, what damages it, and how water hardness and pH affect it. These pages summarise published research and explain our design thinking. They are general information, not medical advice.
In this section: Hair basics and keratin · Scalp, sulfates and what damages hair · Hard water · Hair and scalp pH
1 · Hair basics: a strand has three layers and a waterproof skin
The cuticle is a layer of overlapping scales on the outside. The cortex beneath it holds the keratin proteins that give strength and elasticity. The two layers use different forms of keratin, explained below. Some hair also has a central medulla.1
The surface of each cuticle scale carries a thin lipid layer, the F-layer, built around a fatty acid called 18-MEA. It makes healthy hair water-repellent (hydrophobic). High-pH (alkaline) processes such as colouring, bleaching and smoothening can strip the F-layer away, so hair takes up more water.1
1b · Two forms of keratin: the cuticle and the cortex are built differently
Keratin is a fibrous structural protein that forms hair, nails and the outer layer of the skin.2 The human body makes 54 kinds of keratin, and 17 of them are hair keratins.2 Keratin takes two main forms, set by how its protein chains are arranged.
| ALPHA-KERATIN | BETA-KERATIN | |
|---|---|---|
| Shape of the chains | Winds into a helix, like the thread of a screw, held by hydrogen bonds within the chain.2 | Chains lie side by side as flat, parallel sheets, held by hydrogen bonds between the chains.2 |
| Feel of the structure | Fibrous, springy and elastic. | Tough and rigid.2 |
| Where it is found | Hair, nails, the outer layer of the skin and horns of mammals.2 | Feathers, claws, beaks and scales of birds and reptiles.2 |
Hair as a whole is classed as alpha-keratin. Every keratin fibre contains some of both forms, and the springiness of keratin comes from the interplay between its helices and sheets. The two layers of a hair strand lean in different directions: the cortex towards the helix, the cuticle towards the sheet.3
In short. The cortex is 85 to 90% of the fibre. Its keratin is alpha-helical: long, coiled, spring-like chains that let hair stretch and spring back.3 The cuticle is largely beta-sheet keratin: flat, layered sheets that suit a thin protective armour.3 The two layers fail in different ways, so they need different care.
| CUTICLE (OUTSIDE) | CORTEX (INSIDE) | |
|---|---|---|
| Main keratin form | Largely beta-sheet keratin. X-ray diffraction shows the signature of alpha-keratin is missing in the cuticle and present in the cortex.3 | Alpha-helical keratin. Pairs of helices twist into coiled coils, which bundle into intermediate filaments set in a cystine-rich matrix. |
| Structure | Overlapping, layered scales in a well-ordered layered structure.3 | Long spindle-shaped cells packed with filaments, aligned along the fibre.3 |
| Behaviour | Flat, thin and protective. It forms a barrier, controls friction and shine, and carries the water-repellent F-layer. | Springy. The helices can extend under stretch and recover, which gives hair its elasticity and most of its strength. |
| What stresses it | Friction, heat, alkaline products that lift the scales, and loss of 18-MEA.4 | Perms and relaxers, bleach and strong heat, which can disrupt the helix and break disulfide cross-links.4 |
| How the care differs | Smooth the scales, restore the water-repellent surface and reduce friction. | Support the internal structure and its cross-links so hair keeps its strength and elasticity. |
Why the difference matters
A spring and a shield. The helical cortex works like a spring: it stretches and recovers, so it carries the strength and elasticity. The sheet-like cuticle works like a flat shield: it protects the cortex and decides how the surface feels, shines and repels water.3
Different damage. Chemical services disrupt the helical structure: in permanent waving, part of the alpha-helix changes to a random coil, not to beta-sheet. The cuticle is damaged from the outside, by friction, heat and alkaline lifting of the scales.4
A mix inside the cortex too. The filaments are mostly alpha-helical, while beta-sheet structure is concentrated in the amorphous matrix between them.5 Pulling hair hard can also turn helix into sheet, one way hair copes with stretching.
Keratin questions, answered
What is the difference between alpha-keratin and beta-keratin?
In alpha-keratin the protein chain winds into a helix, like the thread of a screw. In beta-keratin the chains lie side by side as flat, parallel sheets, which makes the structure tough and rigid. Alpha-keratin is found in the hair, nails, outer skin and horns of mammals, and beta-keratin in the feathers, claws, beaks and scales of birds and reptiles.
Is human hair alpha-keratin or beta-keratin?
Human hair is classed as alpha-keratin, because the cortex, which is most of the fibre, is made of alpha-helical keratin. An X-ray study found that the outer cuticle is largely beta-keratin, so hair holds both forms, in different layers.
Why does the cortex stretch while the cuticle protects?
The cortex is built from helical keratin that acts like a spring: it extends under stretch and recovers. The cuticle is built from flat, layered, sheet-like keratin that suits a thin protective shield. Together they give hair both elasticity and a protective surface.
References
1. Gavazzoni Dias MF. Hair cosmetics: an overview. Int J Trichology. 2015;7(1):2–15. PMC4387693
2. Keratin. Cleveland Clinic Health Library (medically reviewed, last updated 9 June 2022; read 4 October 2026). my.clevelandclinic.org
3. Stanic V, et al. Local structure of human hair spatially resolved by sub-micron X-ray beam. Sci Rep. 2015;5:17347. PMC4663634
4. Gavazzoni Dias MF. Hair cosmetics: an overview. Int J Trichology. 2015;7(1):2–15. doi:10.4103/0974-7753.153450
5. Fellows AP, et al. Chemically characterizing the cortical cell nano-structure of human hair using atomic force microscopy integrated with infrared spectroscopy (AFM-IR). Int J Cosmet Sci. 2022. doi:10.1111/ics.12753
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