| MadSci Network: Engineering |
Question: how does a curling iron curl hair?
Hair is made almost entirely of protein. The particular kind
of protein
that makes up hair is called a-keratin
(alpha-keratin). This protein takes on a shape known as a
helix, in
which the protein strand twists upon itself.
Many proteins adopt this shape, but the particular property of
a-keratin
is that the twisting doesn't stop there. Two helices coil around each
other to form what we call a "coiled coil." And it doesn't
stop
there. Two or more of these coiled coils wrap around each other to
form a filament,
and these filaments wrap around each other to form a structure very much
like a rope,
something like this:
What is holding all of this together? Answer: two types of chemical bond. The weaker kind are called hydrogen bonds. The stronger are known as disulfide bonds, and these are the kind involved when a curling iron is used. Disulfide bonds are bonds between two sulfur (S) atoms on two adjacent strands of the "rope." When hair curls into the shape characteristic for a particular person, this shape is held in place by the disulfide bonds. When you use a curling iron, you break the disulfide bonds. You then wrap the hair around curlers and form into into a shape you desire. New disulfide bonds form to hold this new shape together. When the heat is removed, this shape remains.
As for the exact temperature at which hair starts to curl and so forth, I'm afraid I can't help you with that - you need to see a hairdresser.
You can find more information about this subject in standard biochemistry textbooks, including:
David Reibstein, Associate Dean, Albert Dorman Honors College, New Jersey Institute of Technology
Try the links in the MadSci Library for more information on Engineering.