Hair

The hair cuticle: why a good shampoo does not “open” it

11 September 2026

Hair marketing still repeats the myth that a good product “opens the cuticle to deliver nutrients”. In practice, opening the cuticle is what we should protect hair from — not a care step.

How hair is built

Hair has three layers. Outside, the cuticle — 6–10 layers of flat, overlapping cells, laid like roof tiles or pine-cone scales, pointing toward the tips. Beneath that, the cortex, where keratin, the bonds that give hair its strength, and melanin sit. In thicker hairs, a medulla at the centre.

The cuticle has one job: protect the cortex. It is dead and does not regenerate. Every injury is permanent until a new hair grows.

What pH does

Keratin’s isoelectric point is around pH 3.7. Below that, the fibre’s net charge is positive; above it, negative — and more strongly negative as pH rises.

That becomes two physical effects:

Swelling. In an alkaline environment hair takes up water and swells. Scales lift, their edges stand off the shaft. A path into the cortex opens — and the same path lets components wash out.

Electrostatic repulsion. Strongly negatively charged scales push each other apart and lose adhesion. Hence frizz, static and the crunch of combing.

A lifted cuticle has one more effect that is rarely mentioned — friction. Hair with a closed cuticle is smooth; strands slide. Hair with a raised cuticle catches on its neighbours. During brushing, sleep, tying — scales snap off mechanically. The worst damage is not in the wash itself, but in the hours after.

Why hair looks dull

Shine is light reflecting off a smooth surface. A closed cuticle reflects directionally — we see a band of gloss. A lifted cuticle scatters light in every direction. Hair looks matte, and after colouring the shade looks shallower and fades faster, because pigment washes out of a leaky cortex more quickly.

Practical takeaways

• —Shampoo should have an acidic pH. Comparisons of shampoos at different pH show more frizz and damage from alkaline products. The wash formula matters, not only the conditioner after it.
• —A conditioner with citric acid helps close the cuticle. That is not a slogan — lowering pH reduces swelling and negative charge, so cationic ingredients can deposit where they should.
• —Do not brush wet hair hard. Wet hair is swollen and most vulnerable to mechanical cuticle tear. Detangle with a wide-tooth comb, from the ends up.
• —Water temperature matters. Hot water increases swelling. Lukewarm is enough to cleanse.
• —Colouring and bleaching work at pH 9–11 — technologically necessary, because pigment must enter the cortex. It also means every chemical service starts by deliberately opening the cuticle. After colour, the priority is closing and protecting.