Page 379 - Handbook of Properties of Textile and Technical Fibres

352 Handbook of Properties of Textile and Technical Fibres

into microfibrils with diameters that are multiples of 4 nm. The flexibility of collagen
molecules and fibrils allows both load transmission and energy storage (Silver, 2006).
The ability of collagen molecules to self-assemble into fibrils occurs not only in tis-
sues but also in vitro. The self-assembly of collagen molecules from solutions of type I
collagen is a model that has been used to understand the structure and mechanical
behavior of collagen fibers in tissues. Results of self-assembly studies in vitro have
shown that crosslinking is a very important step in converting collagen fibers that
deform primarily in a viscous fashion into energy storing elements (Silver et al., 2000).

11.6 Viscoelastic behavior of collagen fibers

Collagen fibers are viscoelastic and exhibit time-dependent mechanical behavior
(Fig. 11.5). Viscoelasticity may be important in resisting impact loads especially in
the musculoskeleton; however, it complicates the understanding of ECM behavior
since most real-time measurements made on these tissues contain both elastic and
viscous contributions (Dunn and Silver, 1983). The elastic behavior varies from as
high as about 75% of the total stress for tendon to as low as about 50% for skin

× 10 5

5
4
Stress (Pa) 3

2

1

0
0 0.02 0.04 0.06 0.08 0.1 0.12
Strain
Figure 11.5 Tensile stressestrain curve for decellularized dermis. The loading (upper curve)
and unloading curves (lower curve) for decellularized human dermis tested uniaxial tension are
shown. The loading curve is above the unloading curve due to energy loss during the cycle.
The unloading curve does not recover immediately but returns to zero strain after a recovery
time of about 30 min. This time dependence of the recovery strain is a manifestation of the
viscoelasticity of human dermis. The time dependence of the mechanical properties of ECMs is
manifested by the strain-rate dependence of the stressestrain curve, the recovery time for the
sample to achieve zero strain, and the dependence of the modulus on the strain-rate. The
modulus of collagenous tissues is time and strain-rate dependent and varies from about
0.5 MPa for cardiovascular tissue to over 20 GPA for bone.