Biomedical Engineering Reference
In-Depth Information
6.1.2 Properties of Collagen-Rich Tissue
The function of collagenous tissue is related to its structure and properties. This section reviews some
important properties of collagen-rich tissues.
6.1.2.1 Physical and Biomechanical Properties
The physical properties of tissues vary according to the amount and structural variations of the col-
lagen fibers. In general, a collagen-rich tissue contains about 75-90% of collagen on a dry weight basis.
Table 6.2 is a typical composition of a collagen-rich soft tissue such as skin. Collagen fibers (bundles of
collagen fibrils) are arranged in different configurations in different tissues for their respective functions
at specific anatomic sites. For example, collagen fibers are arranged in parallel in tendon (Figure 6.4b)
and ligament for their high-tensile strength requirements, whereas they are arranged in random arrays
in skin (Figure 6.4c) to provide the resiliency of the tissue under stress. Other structure-supporting
functions of collagen such as transparency for the lens of the eye and shaping of the ear or tip of the nose
can also be provided by the collagen fiber. Thus, an important physical property of collagen is the 3-D
organization of the collagen fibers.
The collagen-rich tissues can be thought of as a composite polymeric material in which the highly
oriented crystalline collagen fibrils are embedded in the amorphous ground substance of noncollage-
nous
polysaccharides
,
glycoproteins
, and elastin. When the tissue is heated, its specific volume increases,
exhibiting a glass transition at about 40°C and a melting of the crystalline collagen fibrils at about 56°C.
The melting temperature of crystalline collagen fibrils is referred to as the
denaturation temperature
of
collagenous tissues.
The stress-strain curves of a collagenous tissue such as tendon exhibit nonlinear behavior (Figure 6.6).
This nonlinear behavior of stress-strain of tendon collagen is similar to that observed in synthetic
fibers. The initial toe region represents alignment of fibers in the direction of stress. The steep rise in
slope represents the majority of fibers stretched along their long axes. The decrease in slope following the
steep rise may represent the breaking of individual fibers prior to the final catastrophic failure. Table 6.3
summarizes some mechanical properties of collagen and elastic fibers. The difference in biomechanical
properties between collagen and elastin is a good example of the requirements for these proteins to serve
their specific functions in the body.
Unlike tendon or ligament, skin consists of collagen fibers randomly arranged in layers or lamellae.
Thus skin tissues show mechanical anisotropy (Figure 6.7). Another feature of the stress-strain curve
of the skin is its extensibility under small load when compared to tendon. At smaller loads, the fibers
are straightened and aligned rather than stretched. Upon further stretching, the fibrous lamellae align
with respect to each other and resist further extension. When the skin is highly stretched, the modulus
of elasticity approaches that of tendon as expected of the aligned collagen fibers.
Cartilage is another collagen-rich tissue which has two main physiological functions. One is the
maintenance of shape (ear, tip of nose, and rings around the trachea) and the other is to provide
bearing surfaces at joints. It contains very large and diffuse proteoglycan (protein-polysaccharide)
TABLE 6.2
Composition of Collagen-Rich Soft Tissues
Component
Composition (%)
Collagen
75 (dry), 30 (wet)
Proteoglycans and polysaccharides
20 (dry)
Elastin and glycoproteins
<5 (dry)
Wa t e r
60-70
Source:
Adapted from Park, J.B. and Lakes, R.S. 1992.
Biomaterials: An Introduction
, 2nd ed., pp. 185-222, Plenum
Press, New York.
