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Hair
strength
Hair is surprisingly strong: a single hair can support a load of
about 100 grams without breaking. You could even spin rope out of
hair!
The keratin protein of the cortex is responsible
for this unusual strength. The long keratin molecules in the cortex
are compressed to form a regular structure, which is not only strong
but also flexible.
We saw earlier that proteins are made up of long
chains of amino acids. Each chain takes up a helical or coiled form,
rather like a long spring, or the cable of a telephone handset.
Most protein chains are made up of various mixtures
of the same 20 or so amino acids. Keratin is unique in that its
chains contain high concentrations of a particular amino acid called
cystine. The proteins in the matrix of the hair contain the
highest levels of cystine.
Every cystine unit contains two cysteine
amino acids in different chains which have come to lie near to each
other and are linked together by two sulphur atoms, forming a very
strong chemical bond known as a disulphide linkage. Many
disulphide bonds form down the length of the keratin chains, joining
them together like the rungs of a ladder.
The disulphide bond is one of the strongest bonds
known anywhere in nature. This cross-linking by disulphide
linkages between the keratin chains accounts for much of the strength
of hair.
Chemical bonds within the hair maintain its shape
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How
hair gets its shape
The shape of a hair depends on several factors, including the shape
of the hair follicle and its opening; these vary from one
person to another and also between races. As keratin is hardening
it is compressed into the shape of the hair follicle. The hair is
then held in shape by the pattern of the chemical bonds within it.
Of these, the disulphide bonds are the strongest. They can only
be changed by chemical methods such as perming or relaxing.
But within each hair the keratin chains are also
linked by bonds of a different kind, called hydrogen bonds.
There are far more hydrogen bonds than disulphide linkages. The
hydrogen bonds are much weaker than the disulphide linkages and
more easily broken, and they give hair its flexibility. Hydrogen
bonds are broken apart whenever the hair is wetted, and form again
as it dries. When they break the shape of the hair changes. If the
wet hair is then wound on to rollers it will form a new shape, and
if it is dried on the rollers it will keep this shape.
This is the basis of the setting process.
The change in shape is only temporary. It is lost when the hair
is dampened, because the new hydrogen bonds are broken again.
Hair dimensions
People describe their hair as being thick or thin, coarse or fine.
What they are usually talking about is the amount of coverage their
hair gives to the head. This coverage depends on two things: how
many hairs there are, and the thickness (diameter) of each hair
shaft.
People vary a lot in how many hairs they have,
and also in how closely together they grow. The 'average' person
has around 100,000 hairs, but people with very dense hair may have
as many as 150,000.
The diameter of the hair shaft varies too. It
is usually around 57-90 µm in Europeans. This is much less
than in Asians, in whom it can be 120 µm. (These are general
figures, representing a wide range of values.)
Hair that is both dense and thick looks completely
different from that of someone who has fewer and finer hairs.
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