Integrated Biomaterials Science

Soft Tissue 349
10.1.2. Mechanical Properties
The constituents of soft tissue are organized into many different kinds
of structures in order to perform in specific functions; as a result the
mechanical properties are different. Collagen and elastin are organized so as
to optimize mainly the strength and elasticity of the tissue while the ground
substances (proteoglycans, water, hyaluronic acid, cells, etc.) are mostly
responsible for the viscoelastic properties.
10.1.2.1. Stress–Strain Properties
All soft tissues present a peculiar behavior when they are tested in a
uniaxial mode as a simple elongation at constant strain rate. The related
diagrams of load versus elongation, for tensile rather than compressive test,
show an upward concavity with an initial low modulus region (toe region)
followed by a progressive increase in modulus up to a “linear” load–strain
characteristic. The linear behavior held up to the failure of the tissue. The
toe region is usually the physiological range in which the tissue normally
functions, and generally depends on the composition of the ground substances
and its interaction with collagen fibers and their orientation. For
tendons and ligaments with parallel fiber, the toe region is less extended
than the one obtained from a more random structure such as the skin. At
the end of the toe region the collagen fibers begin to become straightened,
resisting deformation and generating load up to failure (Viidik, 1980; Lanir,
1978).
This nonlinear behavior has been usually described with a fiberrecruitment
model in which the progressive alignment of the collagen fibers
along the direction of the load is modeled by several exponential constitutive
equations (Kenedi et al., 1964):
or (Ridge and Wright, 1964):
The stress–strain behavior of intervertebral disc is best described by a
cubic relationship (Skaggs et al., 1994; Ebara et al., 1996):