Page 380 - Handbook of Properties of Textile and Technical Fibres

Structure and behavior of collagen fibers 353

depending on the collagen fiber orientation, rate of loading, and the quantity of other
tissue constituents (Dunn and Silver, 1983).
A number of excellent studies have been published that have helped in the interpre-
tation of the stressestrain behavior of tendon at the molecular and fibrillar levels.
Much of our current understanding of the relationship between hierarchical structure
and viscoelastic behavior of ECMs is based on studies of the mechanical properties
of developing and mature tendons (Torp et al., 1974; McBride et al., 1985, 1988).
The properties of developing tendon rapidly change just prior to the onset of locomo-
tion. The maximum total stress that can be borne by a 14-day-old embryonic chick leg
extensor tendon is about 2 MPa and increases to 60 MPa, 2 days after birth (McBride
et al., 1985, 1988). This rapid increase in tensile stress by tendon occurs without large
changes in its hierarchical structure (McBride et al., 1985, 1988). In this case, the
collagen fibril length appears to be more important for energy storage and for increased
ultimate tensile strength than fibril diameter; but the two parameters are linked together
since fibrils have been shown to grow in length by lateral fusion of fibril bundles (Torp
et al., 1974; McBride et al., 1988; Silver et al., 2003).
Mechanistically, during mechanical loading, a tensional increase in the D period is
observed with increasing strain that is associated with (1) molecular elongation at the
triple-helical level of structure; (2) increases in the gap distance between the end of one
triple-helix and the start of the next one in the microfibril (see Fig. 11.2); and (3) mo-
lecular slippage (Sasaki et al., 1999). Molecular stretching occurs at lower stresses fol-
lowed by increases in the gap spacing and molecular sliding that occur at higher
stresses (Folkhard et al., 1987).
The time-dependent behavior of tendon makes it difficult to interpret stressestrain
relationships for these tissues. However, using incremental stressestrain curves, the
elastic and viscous behaviors can be separated and analyzed in terms of tissue structure
(Dunn and Silver, 1983). The viscoelastic properties of ECMs have been obtained by
constructing incremental stressestrain curves for a variety of tissues including tendon
(Dunn and Silver, 1983; Silver, 2006) (see Fig. 11.6 top). Such incremental stress-
strain curves are derived for tendon and other ECMs by stretching the tissue in a series
of strain increments and then allowing the stress to relax to an equilibrium value at
each strain increment before another strain increment is added (Fig. 11.6 top) (Dunn
and Silver, 1983). By subtracting the elastic stress (equilibrium stress value) from
the initial or total stress value, the viscous stress is obtained. By plotting the equilib-
rium stress versus strain and the total stress minus the equilibrium stress versus strain
(Fig. 11.6 bottom) we get elastic and viscous stressestrain curves for tendon (Silver,
2006)(Fig. 11.7). From these curves and the literature, important information can be
obtained concerning the mechanism of stretching and sliding of the collagen molecules
and fibrils that make up the structure of tendon (Silver, 2006). It turns out that the slope
of the elastic stressestrain curve is proportional to the elastic modulus of the collagen
molecule (Silver et al., 2003), while the viscous stress at a particular strain is a measure
of the fibril length (Silver et al., 2003). An estimate of the elastic modulus of the
collagen molecule is obtained by dividing the slope of the elastic stressestrain curve
by the collagen content and by the ratio of the molecular strain (change in h spacing-
axial rise per amino acid residue along the molecule backbone) divided by the