What Makes Hair Curly or Straight?

Hair has three main structures: the medulla, the cortex, and the cuticle.

The three main structures of a human hair

The cuticle is the outer most layer of the hair. It consists of layers of flat overlapping cells. The cuticle is generally resistant to chemicals and is the area most often damaged by daily activities, such as brushing hair.

The cuticle cells of a human hair

The bulk of each hair’s mass is contained in the cortex. The cortex contains the various proteins and bonds which determine the curliness or straightness of the hair.

A damaged human hair revealing the cortex. The cuticle has been ripped off revealing the mass of proteins and other sub structures comprising the cortex.

The final structure contained in hair is the medulla. In human hair, the medulla contains only a small fraction of each hair’s mass. The medulla is not believed to have any effect on the straightness of human hair. The medulla is more prevalent in stiff hairs, such as like those comprising a porcupine’s quills or a horse’s tail.

Temporary hair straightening can be achieved by breaking the hydrogen bonds located in the cortex. These bonds kink the hair, causing it to curl. As hydrogen bonds are heated, their resistance to imposed strain decreases. Therefore if hair is heated and a tensile stress is applied, these hydrogen bonds break and the hair looses its curl. However if the hair is exposed to water or humidity, the hydrogen bonds can use the water to reform.