Page 384 - Handbook of Properties of Textile and Technical Fibres

Structure and behavior of collagen fibers 357

molecules in order to facilitate tensile force transmission during stretching (Silver,
2006) (see Fig. 11.2). The tensile properties of tendon and crosslinked self-
assembled collagen fibers are very similar. The major difference is that the diameters
of self-assembled collagen fibrils are much smaller than the diameters of collagen
fibrils found in tendon. The transmission of tensile forces by tendon is attributable
to direct stretching of the triple helix; energy loss occurs through the sliding of fibrils
and bundles of fibrils during tensile deformation. The behavior of other ECMs is a bit
more complicated than that of self-assembled collagen fibers because of the presence
of additional components including elastic and smooth muscle fibers and differences in
collagen fiber orientation (Silver, 2006).
Morphologic studies on collagen fibers suggest that mechanical deformation leads
to collagen fibril alignment as well as development of fibril substructure that affect the
failure mechanisms of collagen fibers (Pins et al., 1997). Stretching during self-
assembly results in collagen fibers, with the outer fibrils containing additional axial
orientation and a more prominent fibrillar substructure compared to fibrils found in
the center of the fiber (Pins et al., 1997). Stretching during self-assembly also leads
to increased ultimate tensile strengths and axial alignment of the collagen subfibrils
(Pins et al., 1997). Stretched uncrosslinked collagen fibers require more energy to
pull to failure and the failed fiber surface shows more plastic deformation than does
the fracture surfaces of unstretched collagen fibers (Pins et al., 1997).

11.8 Collagen fiber failure

The effects of cycling on the structure of collagen fibrils in tendon have been reported
in the literature (Torp et al., 1974). Repeated mechanical cycling leads to the forma-
tion of voids within collagen fibrils (Torp et al., 1974). Dissociation of fibrils starts in
spots where voids are seen and results in the observation of 15 nm in diameter sub-
units that lack the collagen D-period (Torp et al., 1974). Fissures become apparent
under repeated tensile loading at sites where fibroblasts are in contact with the
collagen fibrils. Collagen fibril failure is reported to occur primarily by progressive
dissociation into subfibrils, 15 nm in diameter, with some loss of the axial alignment
of the fibrils. In addition, a secondary mechanism of failure is observed in rats
1.5 months and older. Fibroblasts in tendons from this age group appear to develop
fissures that eventually extend to other fibroblasts leading to tendon failure
(Torp et al., 1974). The improved failure stress observed with increased age appears
to be a result of increased crosslinking that leads to reduced slippage between the
subfibils and microfibrils (Torp et al., 1974).
While macroscopic fiber failure is quite apparent since visible fiber breakage can be
observed, subfiber failure has been observed and attempts have been made to quanti-
tatively analyze these changes based on observed behavioral differences. Subfiber fail-
ure can be used to examine a number of altered mechanical phenomena in tendons and
ligaments including increased laxity (Panjabi et al., 1996; Provenzano et al., 2002a,b)
and decreased stiffness (Panjabi et al., 1999; Provenzano et al., 2002a,b). Subfiber fail-
ure leads to collagen disorganization (Torp et al., 1974; McBride et al., 1985, 1988),