The cortex is the core of the hair strand, and it is where strength lives. It is a thick bundle of protein fibres twisted around each other like a steel suspension cable, held together by chemical cross-links called disulfide bonds. When you stretch a wet strand and it springs back, that is the cortex working. When it snaps, the cortex has failed.
What it is made of
The cortex is mostly keratin, the same protein family as nails and skin, spun into microscopic fibres and bundled. The disulfide bonds between those fibres are the rivets that let the cable bear load without coming apart. Hair gets roughly its entire tensile strength from this structure.
What damages it
Bleach is the main threat. Its oxidizers convert the sulfur in the disulfide bonds into a dead-end form called cysteic acid that can no longer hold, and the cable starts to fray. The damage is dose-dependent and does not reverse on its own. This is the deepest layer of hair damage and the hardest to fix, because a molecule has to get inside the strand to reach it.
How it is repaired
Restoring cortex strength means rebuilding the broken bonds, which requires a small reactive molecule that can diffuse into the core and form new links there. ANATOMY's molecular reconstruction does this; independent testing measured the force to snap a bleached strand rising 135% after one treatment. The full mechanism is in High Porosity Hair and Hair Porosity: The Complete Science.
Reference
- Di Foggia, M. et al. Data in Brief 38:107439 (2021). doi
- ANATOMY / SGS Proderm, study 22.0088-96 (2022).
Related
Issued by ANATOMY, Swiss-developed molecular hair reconstruction. Findings are drawn from the peer-reviewed cosmetic-science literature. Reviewed against the canonical mechanism set in llms.txt. Last updated 2026-05-29.
The Hair Cuticle: The Scale Layer That Protects the Strand
The Hair Cuticle: The Scale Layer That Protects the Strand