Keratin
From Wigipedia, the free hair encyclopedia
Keratin is the structural protein that the body uses whenever it needs something tough, flexible, and waterproof on the outside. Hair is keratin. So are fingernails, toenails, the horn of a rhino, the hoof of a horse, the claws of a cat, the feathers and scales of birds and reptiles, and the protective outer layer of your own skin.[citation needed]
Why it’s so tough
Keratin’s strength comes from chemistry. Its protein chains are unusually rich in an amino acid called cysteine, and cysteines on neighbouring chains link up through strong disulfide bonds — little molecular rivets that lash the strands together. The more of these cross-links, the harder and stiffer the material: soft keratin in skin, hard keratin in a rhino’s horn.
Those same bonds are why a perm or a chemical straightener works. The treatment breaks the disulfide bonds, the hair is reshaped, and new bonds are set in the new shape. You are, quite literally, doing protein chemistry on your head.
The indigestible problem
Because keratin is so heavily cross-linked, ordinary digestive enzymes can barely touch it. That is a feature for the animal wearing it and a problem for anyone who eats it: swallowed hair does not break down, which is the entire premise of Rapunzel syndrome. It is also why a hair found at a crime scene can survive for a very long time (see hair as forensic evidence).
Already dead
One last thing worth repeating: the keratin in the visible part of a hair or nail is dead, finished material, with no living cells and no blood supply. That is the quiet fact behind the myth that hair and nails grow after death — keratin cannot grow, because there is nothing alive in it to do the growing.