Hair is indeed a polymer, specifically a protein polymer. Many natural substances, including those that form our bodies, are also polymers. Understanding this fundamental aspect of hair’s composition helps explain its unique characteristics and behaviors.
Understanding Polymers
A polymer is a large molecule made up of many smaller, repeating units called monomers. Both natural and synthetic materials, from DNA to rubber, are examples of polymers. These large molecular structures give polymers distinct physical properties, such as toughness and elasticity.
Keratin: The Core Polymer of Hair
The primary polymer found in hair is a protein called keratin. Keratin is a fibrous structural protein composed of amino acids. These amino acids are the monomers that link together to create the long protein chains of keratin. In mammalian hair, the predominant form is alpha-keratin, which is also a main component of nails, horns, and the outer layer of skin. Keratin is insoluble and provides structural support and protection.
The Hierarchical Structure of Hair
The keratin polymers within a hair strand are organized in a complex, hierarchical manner. Individual keratin protein chains first twist into a right-handed alpha-helix. Two of these alpha-helices then coil together to form a left-handed helical structure known as a coiled-coil dimer. These dimers then aggregate to form protofilaments, which further combine into protofibrils, and then microfibrils.
Hundreds of these microfibrils bundle together to form macrofibrils, which are the main components of the hair’s cortex. The cortex forms the bulk of the hair fiber and is responsible for its strength. The outermost protective layer, the cuticle, is also composed of keratinized cells that overlap like scales.
Properties Rooted in Hair’s Polymeric Nature
Hair’s polymeric composition and intricate structure directly influence its physical and chemical properties. The strong bonds within keratin polymers, particularly disulfide bonds formed between cysteine amino acids, provide hair with significant strength and resilience. These bonds contribute to hair’s ability to resist daily stresses like brushing and washing. The organized arrangement of keratin also grants hair elasticity and flexibility, allowing it to stretch and return to its original shape.
This polymeric nature also explains how chemical treatments interact with hair. Perming, for example, involves breaking the disulfide bonds in the hair’s cortex using a reducing agent, allowing the hair to be reshaped. A neutralizer then reforms these bonds in the new configuration, setting the curl or straightness. Similarly, hair dyes penetrate the cuticle and cortex, interacting with the hair’s internal structure to change its color. The ability of hair to undergo these changes is a direct consequence of its complex, yet modifiable, polymeric framework.