Happy, healthy hair starts with strength and elasticity. Think of a rope made up of many interwoven strings. On its own, one string isn’t very strong, but when many strings come together, they form a more stable structure.

Your hair is similar, but rather than string, its strength comes from interwoven chains of the protein that we at K18 know and love—keratin.

the structure of hair

Keratin is a fibrous protein that forms long, cross-linked chains that give your hair its core strength and elasticity. Keratin proteins are the fundamental building blocks of hair.

Your hair is more complex than you might think. The hair strand is composed of separate layers—the cuticle and cortex. The outermost layer, the cuticle, is composed of overlapping tile-like cells that protect the inner layer, the cortex, from damage. 

Although the cuticle plays a significant role in protecting your hair proteins from environmental and heat damage, studies have shown that damage to the cuticle does not impact the strength of hair. The real source of strength is in the cortex, which is composed of keratin chains that assemble to form larger structures within the hair strand. 

There are other types of molecules in hair beyond keratin proteins. Lipids are fatty, water-disliking substances found on the surface of hair and inside of the cortex. Keratin-associated proteins are also present in the hair strand, working alongside keratin to add strength to your hair. 

But the biggest player in the strength and elasticity conversation is keratin itself, which is responsible for up to 95% of your hair’s total weight.

peptide bonds give hair’s length strength

Keratin proteins are joined together by a type of chemical bond called a peptide bond. A type of covalent bond, aka the strongest kind of bond, these allow keratin to form long polypeptide chains, giving hair strength along its length.

disulfide bonds stabilize hair’s structure

Your hair has a hierarchical structure. That means that the keratin chain building blocks come together to form larger structures, and those structures come together to form even larger structures.

Keratin chains form a helical shape, like a metal spring, and assemble together to form intermediate filaments. These filaments come together to form macrofibrils, and those macrofibrils then come together to form the cortical cells that make up the larger cortex layer.

This hierarchical structure is stabilized by disulfide bonds that cross-link the keratin chains in the cortex, providing your hair more strength along the width of the hair shaft. Disulfide bonds are the focus of bond building technology, which aims to restore hair’s strength and repair damage from chemical services on a singular level. But true molecular repair through the K18PEPTIDE™ happens at multiple points along the hair’s structure. 

damage decreases strength

Everyone has some level of hair damage, making the idea of “virgin hair” a myth. UV rays from the sun and pollutants from the environment damage your hair. If you frequently use heat tools like dryers, straighteners, or curling irons, those damage your hair as well. Chemical services like bleaching, curling, perming, and chemical straightening also hurt your hair. 

When we talk about damage to the keratin chains in hair, what we’re really talking about is breaking chemical bonds. All types of damage can break down the many types of bonds in your hair, weakening your hair’s core structure and strength as a result. Thankfully, these broken bonds are replaceable.