Hair

:Hair is also a musical: see Hair (musical) and Hair (movie)

Structure

Hair is a biological polymer; over 90% of its dry weight is made up of proteins called keratins. Under normal conditions, human hair contains around 10% water, which modifies its mechanical properties considerably. Hair proteins are held together by disulfide bonds, from the amino acid cysteine. These links are very robust; for example, virtually intact hair has been recovered from ancient Egyptian tombs. Different parts of the hair have different cysteine levels, leading to harder or softer material.

Related Topics:
Keratin - Water - Disulfide bond - Amino acid - Cysteine

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Structurally, hair consists of an inner cortex, comprising spindle-shaped cells, and an outer sheath, called the cuticle. Within each cortical cell are many fibrils, running parallel to the fibre axis, and between the fibrils is a softer material called the matrix. It grows from a hair follicle.

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The cuticle is responsible for much of the mechanical strength of the hair fibre. It consists of scale-shaped layers. Human hair typically has 6-8 layers of cuticle. Wool has only one, and other animal hair may have many more layers.

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Hair responds to its environment, and to its mechanical and chemical history. For example, hair which is wetted, styled and then dried, acquires a temporary 'set', which can hold it in style. This style is lost when the hair gets wet again. For more permanent styling, chemical treatments (perms) break and re-form the disulphide links within the hair structure.

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The diameter of a human hair ranges from about 18 µm to 180 µm. In people of European descent, blond hair and black hair are at the thinner end of the scale, while red hair is the thickest. The hair of people of Asian descent is typically thicker in diameter than the hair of other groups.

Related Topics:
18 µm - 180 µm

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Cross-section shape of human hair is typically round in people of Asian descent, round to oval in European descent, and nearly flat in African peoples; it is that flatness which allows African hair to attain its frizzly form. In contrast, hair that has a round cross-section will be straight. A strand of straight round cross-section hair that has been flattened, for example, with an edge of a coin, will curl up into a micro-afro.

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The speed of growth is roughly 11 cm/yr = 0.3 mm/day = 300 nm/s.

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The cells at the base of the hair follicle divide and grow extremely rapidly. This is why people under chemotherapy sometimes lose their hair; the treatment targets any rapidly-dividing cell, not just the cancerous ones.

Related Topics:
Chemotherapy - Cancerous

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Hair is strong. A single strand can hold 100g (3.5oz) of weight. A head of hair could support 12 tonnes. It is equivalent in strength to aluminium or Kevlar. Wet hair, however, is very fragile.

Related Topics:
Aluminium - Kevlar

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