my hairstyle made with a curling stick at home doesn't last long, but in a barbershop, after using a "potion", I can make my hair curl or straighten for a long time. What happened during the perm?

in fact, a long-lasting perm is a chemical reaction. Through the treatment of "potion", the chemical bond structure in the hair has been changed.

most of the human hair is made up of the same protein-keratin. This is a very strong protein that forms not only the hair, but also the protective layer of the nails and skin. Human hair is quite tough and tensile, and can stably maintain a straight or curly shape, which is closely related to the disulfide bonds formed between keratin molecular chains. This chemical bond is formed between the cysteine residues of the protein and keratin binds firmly together.

and a perm is actually a process of breaking disulfide bonds and letting them regenerate in a new location. The potion used for perming is divided into two parts: softener and setting agent. The common active ingredient in softeners is mercaptoacetic acid, which may appear in different forms, but for the same purpose: it reacts with disulfide bonds in proteins, breaking these connections. After the softener treatment, the hair without disulfide bond will become soft and difficult to maintain its original shape.

next, the stereotype will be responsible for promoting the formation of new disulfide bonds. The active ingredients in the setting agent are some oxidants, such as hydrogen peroxide. Under the action of oxidants, disulfide bonds are re-formed between the cysteines of keratin. The newly formed disulfide bond not only restores the toughness of the hair, but also permanently fastens the hair to the shape curled by the hairdresser. In fact, the principle of straightening hair in a barbershop is the same, except that it is fixed to a straight shape in the same way.

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(a schematic diagram of the perm process, by Christine Herman. I won't redraw _ (: hair "∠) _)

the perm directly affects the chemical structure of hair keratin, so it will cause some damage to the hair more or less. In order to reduce the damage, on the one hand, people will improve the chemical composition of the perm agent, on the other hand, it is necessary to carefully control the amount of the potion applied, and control the heating temperature during the perm. After all, the more controllable the reaction conditions, the more chemical reactions can be carried out as we expect. It is also for this reason that we rarely see perm agents that can be used at home, and it is better to give these potions to professionals.

in fact, there is another beauty product that uses basically the same chemical principle, that is, hair removal cream. After applying depilation cream for a few minutes, the hair will soften and lose its toughness, and it will break as long as it is gently scraped with a plastic scraper. This is actually the same chemical principle as the first step of a perm: Mercaptoacetic acid is also added to the hair removal cream, which breaks the disulfide bond and destroys the structure of the hair. It's just that this time we don't need to re-form the disulfide bond.

when using hair removal cream, you may notice that it has a strange special smell, which smells exactly like a perm in a barbershop, which is emitted by mercaptoacetic acid, which also reflects the chemical similarities between the two processes. Of course, although the core principle is the same, there are still many differences in the specific formula of hair removal cream and perm agent.

read more: how does hair removal cream work?