HAIRDRESSING SCIENCES

One of a series of articles in which Mr D Dane FTTS MAE FRMS
(a clinical trichologist, microscopist, Fellow and Past President of The Trichological Society)
addresses scientific and practical issues associated with certain hairdressing procedures:

Chemical/physical changes - from the point of view of handling it - or using chemicals to bend or stretch it.

Alive or dead? : Both hair and skin are made of 'cells', as are all the tissues and organs of the body.  
A special protein in our outer skin and hair cells (formed between the deeper germinating layer which is alive, and the dead surface layer) is called keratin. Keratin is characterised by its high sulphur content, as compared with other proteins. Sulphur gives keratin its strength. The surface layer of our skin is of soft keratin, whereas both hair and nails are of hard keratin because they contain even more sulphur.   The whole process of skin development from living cells within the dermis to their final emergent state as keratin is known as keratinisation .   During this process of keratinisation, which takes place in the lower third of the hair follicle, most of the original cysteine existing in the developing hair at formation within that follicle is converted to cystine - and strong elastic disulphide bonds are formed whose importance is explained below:   ______________________________________________________________________________________ In the composition of hair there are at least 19 different amino acids (certain groups of nitrogenous organic chemical compounds which also include atoms of oxygen, hydrogen, carbon and sulphur) coded-for by our DNA, joined together end to end in various combinations.   A chain of such amino acids is called a polypeptide or protein, and in the creation of a hair these chains of amino acids wind around each other to form fibres of increasing size that together form a hair.   In the germinal matrix of each hair at the root within the scalp the amino acid chains first form a helix (spiral-like coil). These helices then twist around each other to form protofibrils which in turn twist together forming, in increasing size, microfibrils then macrofibrils and finally corticofibrils (cortical fibres or long cells of the cortex of hair).   The cortical fibres are embedded in a high-sulphur protein known as 'intercellular cement' or 'protein matrix', and this entire protein complex of the cortex is compressed and surrounded by an outer layer of flattened overlapping cells called cuticle scales.   The process of changing the shape of hair using chemical products may be termed 'chemical re-formation'.   Simply speaking, re-formation refers to the fact that the hair's chemical bonds are broken and then reformed with the hair in a new shape.   Three bond types are the links which give structure to the hair, imparting to it its strength and elasticity .   Any chemical process that is then administered to hair will weaken its structure porosity and tensile strength to some extent and it is a hairdresser's or hair-cosmetic manufacturer's responsibility to minimise that weakening or 'damage' as much as possible. The protein of hair is called keratin .   This keratin is held together by 'bonds'.   Molecularly speaking, the chemical bond that joins one amino acid to another (end-to-end in the chain) is called a peptide bond.   Thus a poly peptide refers to a chain of amino acids - a sequence that makes one protein (or chain-configuration) different from another. The most important bonds in hair from a hair 'dressing' point of view are those that cross between one polypeptide chain to another.   There are three different types of bonds in all and in order of their strength they are: the disulphide bonds, the ionic (or salt) bonds, and the hydrogen bonds.   The chemistry of re-formation focuses particularly upon one type of these cross-linking chemical bonds, the disulphide bonds.   Hydrogen bonds and salt bonds are weaker than disulphide bonds though between them those two types account for two-thirds of the cross-structural strength of the hair. They are the bonds which give 'elasticity' to the hair.   They happen both to be easily broken when only water is applied to the hair but then re-form as the hair dries. Thus straight hairs may be temporarily 'set' around curlers with water which, when dry the hairs retain their new-found curl (wave).   Until of course natural humidity or another wetting then causes those hairs to 'drop' straight again!   Oh, weak watery bonds!   But the disulphide bonds of hair are a different matter and are the strongest and most stable.   They can only be broken (other than by excessive heat!) by applying certain chemicals to the hair. Most of an initially large amount of an amino acid called cysteine ( pronounced sistane ) produced by the germinal matrix cells of the hair bulb (the hair growth structure at the base of the follicle in the skin which we call the root) has been converted as above-mentioned as the cells moved up the hair follicle to become another amino acid called cystine ( pronounced sisteen ). When this happened, the strong disulphide bond was formed.   Once these strong disulphide bonds formed between and linked the adjacent polypeptide chains which constitute each fibre or long cell within the cortex of each hair, the chains could no longer move freely lengthwise independently of each other.   Thus the emergent hairshaft has both elasticity and strength .   If for example by bending the hair, a stretching tension is applied to some polypeptide chains on one side of a particular cell, this tension is transmitted through the disulphide bonds to polypeptide chains on the other side, which might then feel compressed and the bend will naturally want to straighten out again.   This tension can be relieved however by 'chemical re-formation' and the bend will then remain bent. By applying an active chemical that can temporarily 'dissolve' the disulphide links within all the cells of the millions of such cells of cells that make up a hairshaft we are able to have a 'permanent-wave' whereby the hair's internal chemistry is re-formed (using first a process of chemical 'reduction' to temporarily break the disulphide bonds, followed by a process of chemical 'oxidation' which then re-fixes them in their new un-tensioned position).                                                                                                                © 2003 D Dane

                                                         

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