So how are curls and kinks formed? It’s all down to two things: the keratin fibres that make up the strand and that wonder of chemistry: the chemical bond! (I bet you wish that you had listened in chemistry class now!) The building block of the hair is the amino acid, and a couple of these join together to form a peptide (another name for a small bit of protein). These peptides then link up to make several long strand chains called fibrils. Several of these fibrils then accumulate several times over producing the cortex of the hair1. There are three different bonds that keep all this stuff together: the permanent disulphide bond (which means two sulphur atoms joining) and two weaker bonds: the ionic (or salt) bond and hydrogen bond.
The disulphide bonds in the strand are due to one amino acid: Cysteine (there are only two amino acids that contain sulphur anyway). Disulphide bonds are permanent meaning they cannot be dissolved by water, they are only destroyed by heat, force or a strong chemical and once destroyed cannot be reformed. They occur between the individual fibrils across the strand and in between the scales of the cuticle1. The number of bonds formed determines if your hair will grow to be curly or straight as well as the strength of your hair (also they cause that horrible distinctive burning-hair smell when you straighten your hair). The more disulphide bonds you have the more your fibrils twist around each other leading to a wavy, curly or kinky strand.
|This is actually collagen, but it's a good illustration of how fibrils twist around each other causing a curl.|
The ionic and hydrogen bonds are the reason why your hair looks different when wet and when dry. Both are also the reason why hair has the ability to change shape using twists, braids, rollers, straighteners, blowdryers (basically lots of things) as they interact with water molecules2, (this is why rain and humidity can ruin a good hairstyle!) Hydrogen bonds are the most numerous and occur in between the peptide chains, their soluble nature allows us to set our hair in a style when wet, then as the hair dries the water evaporates and new hydrogen bonds are formed meaning the dry hair remains in the style. Slightly stronger ionic bonds occur down the length of the chain, they can dissolve in water (but in hair usually stay strong) but are totally dissolved by acids and alkalis2, this is why we may need hair products to keep our hair in a style (this is also how those ionic blow-dryers and straighteners that claim to strengthen the hair work).
Next time I'll talk specifically about the structural weakness of afro hair.
- Franbourg, A. et al (2003) Current research on Ethnic Hair Journal of American Academic Dermatology, Volume 48, S115-9
- Wolfram, L.J, (2003) Human Hair: A Unique Physicochemical Composite, Journal of American Academic Dermatology, Volume 48, S106-14
- Images from Current research on Ethnic Hair (2003) and European Bioinformatics Institute