Hair is almost entirely composed of protein, making it a filamentous biomaterial. The visible hair shaft is a non-living structure extending from the skin’s surface. It is formed from cells that undergo keratinization, filling them with structural protein before they die. Consequently, hair care focuses on protecting the existing protein structure rather than cellular repair.
Keratin: The Defining Protein
The primary building block of hair is a hard, fibrous structural protein called alpha-keratin. This keratin is rich in the sulfur-containing amino acid cysteine. These cysteine residues form strong, permanent chemical cross-links known as disulfide bonds, which give keratin its characteristic toughness and insolubility. At a molecular level, the protein chains form a right-handed alpha-helix, twisting together into coiled coil dimers. These dimers aggregate into complex structures like protofilaments and intermediate filaments, establishing the hair’s foundation.
The Physical Structure of Hair
The protein assemblies are organized into three distinct layers that make up the hair shaft. The innermost layer is the medulla, a soft region often absent in finer hair types. Surrounding this is the cortex, which constitutes the main bulk of the fiber and is the primary source of its mechanical strength. The cortex is densely packed with keratin bundles and contains the pigment that determines hair color. The outermost layer is the cuticle, which serves as the hair’s protective barrier, composed of several layers of flat, overlapping, keratinized cells. When healthy, the cuticle lies flat and smooth, reflecting light and preventing damage to the inner cortex.
How Protein Loss Occurs
The structural integrity of hair protein can be compromised by external factors that lead to chemical and physical degradation. Chemical treatments, such as coloring, bleaching, and perming, intentionally lift the protective cuticle layer and alter the hair’s internal chemical bonds. Bleaching specifically oxidizes the melanin and attacks the disulfide bonds within the keratin structure. Mechanical damage and environmental stressors also play a role in protein degradation. Excessive heat from styling tools causes the denaturation of keratin, meaning the protein loses its defined structure, which exposes the inner cortex and makes the structural keratin more vulnerable to erosion.
The Functional Role of Hair Protein
The intact protein structure provides hair with its defining mechanical properties: tensile strength and elasticity. Tensile strength is the maximum force the hair can withstand before breaking, a property directly attributed to the dense, cross-linked keratin network in the cortex. Elasticity refers to the hair’s ability to stretch and then return to its original shape without permanent deformation. Healthy hair with an intact keratin structure demonstrates high elasticity. When the protein structure is compromised, the hair’s resilience is lost, leading to low elasticity, which causes the hair to feel brittle and snap easily.