Disulfide bonds, protein-moisture balance and what bond-building treatments actually do at a molecular level.
What Is a Hair Fibre Made Of?
A single strand is keratin protein arranged in three layers: the cuticle, a shingled outer sheath; the cortex, which carries most of the strength and all of the pigment; and in thicker hair a medulla at the centre. Damage is almost always a story about the first two.
- Cuticle: 6 to 10 overlapping scale layers; when they lift, hair snags, dulls and loses water
- Cortex: bundled keratin chains held by disulfide, hydrogen and salt bonds, the source of tensile strength
- Medulla: a hollow core present mainly in coarse hair, with little structural role
Why Are Disulfide Bonds the Ones That Matter?
Hydrogen and salt bonds break every time you wet your hair and reform as it dries, that is why a blow-dry holds a shape until the next wash. Disulfide bonds are covalent, roughly twenty times stronger, and do not reform on their own. Bleach, perms and relaxers break them chemically; heat above about 175°C breaks them thermally. Once broken, the fibre is permanently weaker unless something re-links them.
Every claim in hair repair comes down to one question: does the product re-link disulfide bonds, or does it coat the outside of a fibre that is still broken?
How Do Bond Builders Rebuild Hair?
Bond builders carry small molecules with two reactive ends that find severed sulfur groups in the cortex and bridge them. Olaplex uses bis-aminopropyl diglycol dimaleate; K18 uses a bioactive peptide that slots into the keratin chain. Because the repair happens inside the cortex and is chemical rather than adhesive, it survives shampooing, the practical test that separates repair from coating.
What Is Protein-Moisture Balance?
Protein treatments fill gaps in a damaged cuticle and stiffen the fibre. Moisture treatments soften it. Too much protein and hair snaps dry; too much moisture and it stretches and stays limp. Alternating them is not a marketing ritual, it is how you keep a damaged fibre both strong and flexible while new growth comes through.
Can Damaged Hair Be Fully Repaired?
No. Bond builders restore a measurable share of tensile strength, and sealing treatments restore the feel, but hair is not living tissue and cannot heal. Everything below the mid-lengths is already-formed fibre. The realistic goal is to stop further breakage and let healthier growth replace it, at roughly a centimetre a month.
Keep reading: Top shampoos for damaged hair · Complete Guide to Repairing Damaged Hair · Take the Hair Quiz.
Compare Damaged Hair Products
Bond builders and protein cleansers for breakage, split ends and over-processed lengths.
Questions
Frequently asked
01 What are disulfide bonds in hair?
Covalent sulfur-to-sulfur links between keratin chains in the cortex. They are roughly twenty times stronger than the hydrogen bonds that reform when hair dries, and unlike those they do not reform on their own once broken.
02 Do bond builders work on all hair types?
Yes, because they act on keratin chemistry rather than on texture. The difference is the carrier: fine hair needs a lightweight formulation, coily hair tolerates a richer one, but the repair mechanism is identical.
03 Is heat damage permanent?
Above roughly 175°C, yes. Thermal energy severs disulfide bonds and denatures keratin, and neither reverses. Bond builders restore part of the tensile strength; nothing restores the original fibre.
Sources & further reading
- Robbins, C., Chemical and Physical Behavior of Human Hair, 5th ed., Springer. Cuticle and cortex structure, bond types and tensile behaviour.
- Olaplex patent US 9,668,954 B2, published chemistry of bis-aminopropyl diglycol dimaleate bond repair.
- Gavazzoni Dias, M.F., Hair Cosmetics: An Overview, International Journal of Trichology, 2015. Protein and moisture treatment function.