The chemistry of human hair was recently discussed by Whewell (1)
who related the setting characteristics of hair to its chemical structure.
The supermolecular or fine structure of animal fibers in general was
recently summarized by Rebenfeld (2) and reviewed more extensively
by Lundgren and Ward (3). In brief, keratin fibers are visualized as
being composed of high molecular weight polypeptide chains which are
intramolecularly stabilized into a-helices. The polypeptide chains are
composed of some 21 amino acids of which the disulfide-containing
amino acid, cystinc, is certainly one of the most important. The ahelices
are further wound around each other in the form of a multistranded
rope which is referred to as a protofibril. These elements of
structure are aggregated into microfibrils, which are considered to be the
key structural elements in keratin fibers. The protofibrils are arrayed
in each microfibril in a characteristic "9 + 2" arrangement wherein 9
protofibrils are disposed in the periphery of each microfibril and two
protofibrils are in the center. The microfibrils are considered to be
composed of fully crystalline a-helices; their structure is stabilized by a
variety of intramolecular and intermolecular forces, including hydrogen
bonds, van der Waals forces, dip olaf interactions, and hydrophobic bonds.
The microfibrils are embedded in a matrix which is amorphous and
presumably composed of randomly coiled polypeptide chains. It is
felt that a large proportion of the sulfur-containing proteins are in the
matrix, which is therefore highly cross-linked through the amino acid
cystinc. The microfibrils, which may be further associated into macrofibrils,
are the actual building blocks of which the ortho- and paracortical
cells are composed.
The mechanical properties of a fiber or a hair are most easily determined
in axial tension, in view of its geometric shape. The stressstrain
curve of any material is a graphical record of the stress which
develops in the material as it is deformed under closely specified experimental
conditions. The shape of the stress-strain curve frequently
provides a ready means of predicting the behavior of the material under
use conditions and of interpreting the structure of the material in relation
to its mechanical properties.
When a keratin fiber is stretched in water the stress-strain curve