Hair straightener
Hair straightening or relaxing has become increasingly popular in view of hair styles which require relatively or perfectly straight hair. Hair generally can be divided into four categories: straight, wavy, curly, and kinky. Hair generally is formed with a cuticle (the outer layer), a cortex (an inner layer), and a medulla (the innermost layer). The spiraling nature of naturally curly hair is caused by the shaft of the hair having a flat cross-section. Additionally, the cuticle of the hair serves to maintain the curly nature of the hair through a thick and durable covering of keratin proteins. Many people with naturally kinky, curly, or even wavy hair often desire to straighten their hair. A number of combs have been designed specifically for use with curly hair, such as the Afro pick, and a comb having holes along the spine of the comb to allow the hair a space to turn and unwind. Despite these straightening actions, however, the hair is only pulled straight for an instant, and due to the curly nature of the hair, the hair returns to its original curvature. In order to keep the hair in a straightened state, a chemical relaxer is often applied to the hair. To permanently alter the natural curl of human hair, a number of types of bonds within the hair are cleaved, including salt bridges and disulfide bonds. Human hair is composed of cross-linked, .alpha.-helix protein, primarily keratin. Keratin is a complex of polypeptide chains of high molecular weight. The hair shaft will maintain its outward shape by ionic bonding or salt bridges (disulfide or cystine bridges) which pair various amino acids found in the hair. Current straightening methods break disulfide bonds into free sulfhydryls and then re-establish new disulfide bonds in the desired configuration by reduction/oxidation of the hair shaft using various chemicals. Several types of chemical hair straightening products are available. But strong alkali, especially sodium hydroxide, is generally considered the most effective agent for substantially permanently straightening kinky-curly negroid hair.
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