Disulfide bonds are strong chemical, covalent bonds. They are not broken by water and they account for about 1/3 of the hair’s strength. Disulfide bonds are in all three main parts of the hair: the cuticle, cortex and medulla.
Keratin proteins make up the hair, forming polypeptide bonds that are kept in alignment by hydrogen, salt and disulfide bonds.
The overall internal structure resembles ladders that have been twisted into a shape resembling a multi-fiber cable called microfibrils. These then combine to form macrofibrils which are contained in the cortex. These “cables” stay in alignment, resisting twisting and bending, as a result of disulfide bonds. They are held together in the cortex with a microcellular cement that also contains an abundance of disulfide bonds.