Cortex

The cortex is the structural core of a hair fiber: tightly packed bundles of keratin macrofibrils, crosslinked by disulfide bonds, that account for ~90% of fiber volume and most of its mechanical strength. Repairing damaged hair at the structural level means operating inside the cortex.

Background

Cortex anatomy: alpha-keratin chains fold into intermediate filaments, intermediate filaments bundle into macrofibrils, macrofibrils pack into cortical cells, and cortical cells make up the cortex. Disulfide bonds crosslink keratin chains throughout the structure.

When the cortex is intact, hair is strong, elastic within limits, and recovers shape after stretching. When the cortex is damaged (bleach, oxidation, repeated heat), the disulfide network breaks, the fiber loses lateral cohesion, and the cortex responds to stretch with permanent deformation rather than elastic recovery.

Tensile-strength testing (the SGS Proderm 24.0172-96 method) measures cortex integrity directly: a single fiber is pulled to break, and the maximum force recorded is the breaking force. Bleached hair breaks at lower force than virgin hair because the cortex's crosslink network is compromised.

Cortex damage is not visible on the surface. A fiber can have an intact cuticle and a thoroughly compromised cortex; it will look fine, feel mostly normal, and still break under low load. The 135% tensile strength gain ANATOMY publishes (15.2 → 35.8 cN on bleached hair) is a direct measurement of cortex-level recovery via click-chemistry crosslink formation.