Hair is composed of more than 80% keratin. Keratin is a fibrous, structural protein. Alpha-keratin is found in animal hair and wool, while beta-keratin is found in feathers, animal hooves, and baleen.

Keratin has a helical structure consisting of three intertwined strands that form keratin molecules. These strands are connected through different types of chemical bonds — hydrogen, ionic, and disulfide bonds — forming protofibrils, which are then organized with the help of an inter-fibrillar cementing substance into larger structural units.

This organization creates a strong, rope-like structure held together by an internal spiral and external cross-links.

This is why human hair is a remarkably strong structure that does not dissolve in water at any temperature, even under strong bleaching or other chemical exposure. However, hair can naturally become damaged by chemical and thermal treatments as well as environmental factors.

The bonds within the hair are relatively labile. Breaking them requires significant chemical or physical impact, but repeated exposure can compromise the integrity of the hair structure.

A keratin fiber contains approximately 20 amino acids, creating a wide variety of structural components. To replenish components lost from the hair, scientists can obtain keratin from sheep’s wool and process it in a way designed to preserve the integrity of its amino acids, allowing them to participate in interactions with the hair’s keratin structure.

The quality of the process used by cosmetic manufacturers to obtain keratin while preserving amino-acid integrity can ultimately be evaluated through professional practice and by comparing different brands and formulations.

When a formulation contains amino acids that retain sufficient functional activity and can interact with the keratin structure of the hair, the hair can undergo reconstruction and conditioning.

In various hair reconstruction treatments, keratin molecules are intended to penetrate areas of the hair where structural gaps need to be filled. Smaller molecules can reach deeper layers, while larger molecules primarily remain within the more superficial layers.

Formulations containing keratin commonly use hydrolyzed keratin (HYDROLYZED KERATIN). Its production involves the hydrolysis of keratin proteins into smaller components, including amino acids and peptides.

Important amino acids associated with keratin include cysteine, lysine, arginine, and methionine. Methionine and lysine are essential amino acids, while cysteine and arginine are generally classified as conditionally essential or non-essential depending on physiological context.

The amino acids commonly identified in human hair proteins include:

Glycine, glutamic acid, proline, serine, tyrosine, cysteine, valine, isoleucine, leucine, lysine, methionine, threonine, tryptophan, phenylalanine, alanine, arginine, asparagine, and histidine.