How does a perm work?

The year 2009 marked the 100th anniversary of the permanent wave, also known as the “perm.” In 1909, Charles Nessler submit­ted a patent to the U.K. Patent Office for “A New or Improved Process of Waving Natural Hair on the Head.” Since then, the perm has become a staple of hairstylists around the world.

Yet the perm is not just an art. Rather, it relies on the funda­mental chemical structure of hair for all of its effects. So how do a few chemicals and some heat produce those glorious curls?

To answer that question, we must first understand what holds hair together. About 95% of hair is a single protein, keratin, which has a long, helical shape. Individual keratin molecules aggregate into larger helices called protofibrils, which in turn compose microfibrils and macrofibrils, forming the superstructure of an individual hair.

Keratin molecules are rich in the amino acid cysteine, which contains reactive sulfur atoms. Two cysteine residues on two mol­ecules of keratin can form a disulfide bond, a strong connection that links the keratin molecules, preventing them from slipping past each other.

This connection is permanent until acted upon by strong external forces. The disulfide bonds are key players for the curls that a perm produces. Though it is a strong bond, the disulfide bond is still weaker than the bonds within each keratin molecule; it can be broken quite easily by reducing agents, whose electrons attack the bond.

In a standard “cold” perm, hair is put into curlers and the reducing agent ammonium thioglycolate is added. The disulfide bonds break and keratin molecules are now free to move around and adjust to the shape of the curl. Then a “neutralizer,” such as hydrogen peroxide, is added to reverse the effect of the reducing agent. New disulfide bonds form so the keratin molecules are locked into the shape of the curls.

In the last 30 years, variations on the perm have entered the market. For example, the “acid perm” uses a different reducing agent in combination with heating, resulting in better curling and less damage to the hair. Recently, companies in Asia have taken the acid perm even further and created the “digital perm,” in which the temperature of the hair is controlled by a computer.