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Instant insight: Hair is the news
19 June 2007
Crisan Popescu and Hartwig Höcker, researchers of the DWI (German Wool Research Institute), Aachen University (RWTH), Germany, put hair under the microscope.
Biologically, hair is the filamentous appendage on the skin of mammals. Chemically, it is a composite material in which both the reinforcing fibres and the matrix are made of proteins.
All kinds of hairs look alike under a light microscope; they are strands covered with scales overlapping like roof tiles, oriented from root to tip. Under the higher magnification of a scanning electron microscope, differences of geometry and scale height can clearly be recognised. These, and other criteria, serve to identify the source of a hair.
Hair fibres, roughly cylindrical with diameters ranging from 10µm to 100µm, are multicellular tissues. The heart of the fibre is surrounded by the cuticle, made of plate-like overlapping cells whose heights can reach up to 1µm. Each cuticle cell has four layers: the epicuticle; the a-layer; the exocuticle; and the endocuticle. Inside the cuticle, the cortex contains spindle-like interlocking cortical cells, with cell membrane complex in-between. Each cortical cell is composed of macrofibrils embedded in an intermacrofibrillar material. Each macrofibril consists of microfibrils, called the intermediate filaments (IF), themselves embedded in an intermicrofibrillar matrix composed of intermediate filament associated proteins (IFAP). Thus, hair is a composite material with a complex dual structure at all levels.
The IFs consist of chains of the tough, insoluble protein alpha-keratin. Driven by the hydrophobic effect, two of these chains combine to form a coiled coil. Further packing into dimers and tetramers gives IFs with 32 chains in their cross section.
Dry hair consists of around 95% proteins and 2% lipids, a substantial amount of which are present as 18-methyl eicosanoic acid at the surface. The rest of the hair is made of nuclear remnants, carbohydrates and inorganic salts.
Hair - a sophisticated composite material
Total hydrolysis of hair fibre leads to the 20 common alpha-amino acids, ammonia, and small amounts of thiocysteine, cysteic acid and lanthionine. While the overall amino acid composition is similar for hairs from different mammals, it is strongly differentiated among the morphological components.
Elemental analysis of hair shows, remarkably independently of hair origin, 50wt% carbon, 7wt% hydrogen, 22wt% oxygen, 16wt% nitrogen and 5wt% sulfur. The sulfur comes from the high cystine content, specific to alpha-keratin fibres. Cystine disulfide bonds provide the cross-links that lead to hair's insolubility and thermal stability.
Hair absorbs moisture due to the polar amino acid residues of the inside; yet, hair's surface is hydrophobic, due to the lipid content of the epicuticle. This contradictory behaviour is known as the hair paradox. Increasing hair's moisture content from 0 to 33% results in a longitudinal swelling of only 2% but a radial swelling of 16%.
Natural hair colour is caused by pigment granules of black to brown eumelanin and yellow to red pheomelanin, generated from tyrosine. Unpigmented hair results from an interruption of the synthesis chain of melanin. Bleaching is achieved by oxidation with hydrogen peroxide, which leads to a disintegration of the melanin granules.
A variety of functional (thiol, amino, hydroxyl) groups make hair eligible to chemical reactions important for the textile and cosmetic industries. The cleavage (with thioglycolic acid or bisulfite) and re-formation (upon oxidation with hydrogen peroxide) of disulfide cross-links after shaping is the key to the permanent wave.
Unfortunately, the growth of hair from the follicle, connected with the commonly desired cure for baldness, is far from being understood today.
Read Popescu and Höcker's review 'Hair - the most sophisticated biological composite material' in a forthcoming issue of Chemical Society Reviews.
Link to journal article
Hair—the most sophisticated biological composite material
Crisan Popescu and Hartwig Höcker, Chem. Soc. Rev., 2007, 36, 1282