Bone and Cartilage – its Structure and Physical Properties - Biomechanics of Hard Tissues - page 61

Collagen has well-defined mechanical properties (strength, reversible extensi-
bility through only a small range) that make it suited to the special goals for
which it is present in the different parts of animal body. Collagen characteris-
tics, in particular, the specific shape of stress-strain curve, are important while
considering the properties of a number of tissues, like bones, tendons, or arter-
ies. The experimental trials to determine the mechanical properties of collagen
molecule were performed by several groups of researchers, using the X-ray diffrac-
tion technique and Brillouin light scattering, beginning from Cowan et al . in 1955
[256, 257]. Despite its importance to the biological function of collagen, there
is still a lack of understanding of the correlation between the specific shape
of the stress-strain curve and the deformations of collagen at the molecular
level.
Contemporary efforts to determine the shape of the stress-strain curve of
collagen widely exploit synchrotron X-ray diffraction. Synchrotron radiation studies
of fibril behavior in tissues have been critical to the investigation of structural
details and changes, since they allow transient structural features to be monitored
in realistic timescale.
Three experiments using this radiation, by Misof et al ., Puxkandl et al ., and
Gupta et al ., should be mentioned here [258-261].
Misof et al . have performed in situ synchrotron X-ray scattering experiments,
which show that the amount of lateral molecular order increases upon stretching
of collagen fibers. In strain cycling experiments, the relation between strain and
diffuse equatorial scattering was found to be linear in the ''heel'' region of the
stress-strain curve [258].
The stress-strain curve of collagen is characterized by a region of relatively low
elastic modulus at small strains (''toe region'') followed by an upward bend of the
curve (''heel region'') and, finally, a linear region with high elastic modulus at large
strains. The toe region of the stress-strain curve had been linked to a macroscopic
crimp with a period of about 100

m, found in unstretched collagen fibers by
polarized light microscopy, cf. [262, 263].
In tendons, this crimp disappears upon stretching at extensions of the order
of 4% - a value that may depend on the age of the animal. At larger strains,
X-ray diffraction measurements of the axial molecular packing were interpreted
as a side-by-side gliding of the molecules, accompanied by a stretching of the
cross-linked telopeptide terminals and a stretching of the triple helices themselves,
as was shown by Mosler et al . [264].
Although the axial packing of collagen fibrils is regular, as was shown in 1963 by
Hodge and Petruska [265], there is a disorder in the structure, in the lateral packing
of the molecules.
The following form of the stress-strain relation (

µ

σ = σ ε

( )) was proposed in [258]

by Misof et al .

K ε
1 − ε/ε 0

σ =