What Affects Hair Elasticity?
The interior of the hair shaft, the cortex, is the portion of the hair structure that carries the bulk of an applied load and contributes most significantly to elasticity. Although it is very important, the cuticle is only significant in this regard for its role in guarding the integrity of the inner shaft of the hair.
The cortex is an elaborate structure of clusters of fibrils of keratin protein embedded within a matrix with high water content. The individual molecules of keratin are in the alpha-helical conformation. There are many different inter- and intramolecular interactions and bonds that occur both between amino acids on the same protein strand, amino acids on adjacent protein chains, and between proteins and water molecules within the matrix.
Hydrogen bonds are weak physical bonds that occur between aqueous hydrogen and amino acid nitrogen and oxygen atoms. These interactions are easily formed and broken and are responsible for a large portion of the elastic behavior of hair. For this reason, it is very important to maintain a proper amount of moisture inside the hair shaft. Without adequate hydration, hydrogen bonding will be decreased, which adversely affects elasticity of hair strands.
Salt bonds are weak physical interactions that occur between amino acids and require hair to be maintained at an optimum pH. Cystine bonds, also known as disulfide bonds, are chemical bonds which impart a high degree of elasticity to hair by providing crosslinks between different amino acids on a single protein fiber and also between protein strands. All of these various types of bonds act to hold strands of protein together and allow them to stretch just so far and to snap back into their original shape.
Another factor that influences the elasticity of hair is its diameter. Hair of smaller diameter cannot withstand the same forces as hair of thicker diameter. Remember, stress = force per unit area, so thinner hair experiences greater stresses at the same forces. This means that those with finer hair may have more trouble with their hair losing curl, not holding styles, and developing frizz and breakage. African hair typically has the smallest diameter, with Caucasian hair having medium diameter, and Asian hair having the thickest diameter. There is no known way to overcome this, so one must take care to treat fine hair with the same care one would afford your most precious cashmere sweater.