Hair fiber and its properties

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Hair fiber properties

The hair fiber is the most visible product of the hair follicle. It is composed of keratin proteins, which are organized as a two phase intracellular composite consisting of the keratin fibers embedded in an amorphous matrix. The recent breakthroughs in genome analysis has helped in identification of more human keratins - the number of identified human keratins are currently 54, and they are grouped into acidic type I and basic-to-neutral type II intermediate filaments (11 of the 28 type I keratins and 6 of the 26 type II keratins are hair keratins).

A new consensus nomenclature for mammalian keratins was recently proposed to accommodate the growing number of keratin genes. It will also allow the incorporation of keratins from other mammalian species (Schweizer 2006, J Cell Biol 174, 169-174). The matrix proteins, also known as keratin-associated proteins, are a major component of the hair fiber. They are crucial in formation of a strong hair shaft through a cross-linked network with keratin intermediate filaments. Generally, division occurs into high-sulphur proteins, ultra-high sulphur proteins and high-glycine-tyrosine proteins. These divisions are expressed by the terms such as cortex, cuticle, and matrix. So far as the raw elements are concerned, typical hair is composed of 50% carbon, 20% oxygen, 17% nitrogen, 6% hydrogen, and 5.0% sulphur. Trace amounts of magnesium, arsenic, iron, chromium and other metals and minerals are also present in the hairs.

Three layers – the cortex, cuticle, and medulla - make up the visible hair shaft seen in terminal hair follicles. The central medulla contains polygonal cells with a sponge-like appearance. The medulla may be absent or fragmented in the terminal hairs of childhood. In fine vellus hair fiber the medulla is always absent. The cortex forms a layer of cornfield fibrous cells with a longitudinal orientation around the medulla. When fully developed, these cells are packed with keratin filaments. The hair fiber cortex contains color determining melanosomes. Homogenous oval eumelanin granules and lamellar pheomelanin granules, in variable composition and density, form the wide spectrum of dark to fair hair. The outermost layer of the hair fiber, the cuticle, consists of multiple layers of corneocytes. This layer is thin and translucent thereby allowing light to penetrate to the cortex pigments.

The biometric measurements of hair are wide ranging including diameter and structure of the hair shaft. Most of these measurements are genetically programmed. Terminal hair diameter and cross sectional shape are key differentiating features. The potential cross sectional area and shape of terminal hair fibers can be predicted partly by knowing the hair colour and ethnicity. While average hair fiber diameters are higher in African and Asian populations, Caucasians typically have terminal scalp hair with a cross sectional diameter ranging from as low as 40 micrometers in blonde haired people to 90 micrometers in dark haired individuals. Hair cross sectional shape also varies with ethnicity. While the hair growth tendency in Caucasians is typically elliptical and a cross sectional shape consistent with straight or wavy hair, Asian hair is typically circular in cross section consistent with straight hair. In contrast, the terminal hair fiber of African populations is characteristic of very elliptical or even ribbon-like cross sectional shape. The ribbon-like shape makes hair relatively inflexible across the long cross sectional axis. On the other hand, curly hairs are very flexible across the short axis. The shape of the hair shaft is partly determined by the shape of the hair follicle. The position of the hair bulb relative to the rest of the hair follicle, the size and shape of the dermal papilla, and the curvature of the hair follicle along its length together determines the overall individual appearance of the hair fibers. These are also the parameters which determine the diameter and cross sectional shape of the hair shaft and the number of twists and windings (curl) per unit length of each individual hair. These properties in the most part are defined by the hair follicle that produces the hair fiber. The environmental factors from chemical exposure to friction may also modify the nature of the hair fiber.

Hair fibers are the most durable material; they have the potential to survive thousands of years. The proof lies with the Egyptian mummies. Hair is also very strong; the overall strength of the hairs is determined by the cross sectional area of hair fiber and the structural integrity of the cortex. Healthy hair fiber has a tensile strength around 180-190 MPa. This means hair is almost as strong as copper wire of the same diameter. Hair fiber also has elastic properties. It is possible to extend it by 10% of its original length under tensile stress without causing any damage. Extension of 35-50% is also possible, but that may cause permanent damage to the inner fiber structure.

Hair strength and quality get affected by hair shaft abnormalities due to either genetic changes in shape, diameter, surface cuticle defects, and structural composition, or due to environmental manipulation such as physical or chemical damage to the cuticle and cortex. In the case of curly hair, the thinner layers of cuticle of the outer curl are more fragile and consequently sensitive to exogenous stress such as heat or chemicals. The overall strength of the hair weakens due to disruption and loss of the hair cuticle - either through genetic defects or via environmental factors. This ultimately results in splitting and breakage. More limited changes in the structure of the cuticle can affect the water retention capacity of the underlying cortex with loss of humidity. At least 17% of humidity is required by the hair fibers to maintain a healthy appearance. Up to 35% of water retention is possible. Hair length may vary up to 2% depending on the air humidity.

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