Biomedical Engineering Reference
In-Depth Information
TABLE 6.4
Mechanical Properties of Some Nonmineralized
Human Tissues
Tensile Strength
(MPa)
Ultimate Elongation
(%)
Tissues
Skin
7.6
78.0
Tendon
53.0
9.4
Elastic cartilage
3.0
30.0
Heart valves (aortic)
Radial
0.45
15.3
Circumferential
2.6
10.0
Aorta
Transverse
1.1
77.0
Longitudinal
0.07
81.0
Source:
Adapted from Park, J.B. and Lakes, R.S. 1992.
Biomaterials:
An Introduction
, 2nd ed., pp. 185-222, Plenum Press, New York.
molecules which form a gel in which the collagen-rich molecules entangled. They can affect the
mechanical properties of the collagen by hindering the movements through the interstices of the col-
lagenous matrix network.
The joint cartilage has a very low coefficient of friction (<0.01). This is largely attributed to the
squeeze-film effect between cartilage and synovial fluid. The synovial fluid can be squeezed out through
highly fenestrated cartilage upon compressive loading, and the reverse action will take place in ten-
sion. The lubricating function is carried out in conjunction with
glycosaminoglycans
(GAG), especially
chondroitin sulfates.
The modulus of elasticity (10.3-20.7 MPa) and tensile strength (3.4 MPa) are quite
low. However, wherever high stress is required, the cartilage is replaced by purely collagenous tissue.
Mechanical properties of some collagen-rich tissues are given in Table 6.4 as a reference.
6.1.2.2 Physiochemical Properties
6.1.2.2.1 Electrostatic Properties
A collagen molecule has a total of ~240 ε-amino and guanidino groups of lysines, hydroxylysines, and
arginines and 230 carboxyl groups of aspartic and glutamic acids. These groups are charged under
physiological conditions. In a native fibril, most of these groups interact either intra- or intermolec-
ularly forming salt linkages providing significant stabilization energy to the collagen fibril (Li et al.,
1975). Only a small number of charged groups are free. However, the electrostatic state within a colla-
gen fibril can be altered by changing the pH of the environment. Since the p
K
a
is about 10 for an amino
group and about 4 for a carboxyl group, the electrostatic interactions are significantly perturbed at a pH
below 4 and above 10. The net result of the pH change is a weakening of the intra- and intermolecular
electrostatic interactions, resulting in a swelling of the fibrils. The fibril swelling can be prevented by
chemically introducing covalent intermolecular crosslinks. Any bifunctional reagent which reacts with
amino, carboxyl, and hydroxyl groups can serve as a crosslinking agent. The introduction of covalent
intermolecular crosslinks fixes the physical state of the fibrillar structure and balances the swelling
pressures obtained from any pH changes.
Another way of altering the electrostatic state of a collagen fibril is by chemically modifying the elec-
trostatic side groups. For example, the positively charged ε-amino groups of lysine and hydroxylysine
can be chemically modified with acetic anhydride, which converts the ε-amino groups to a neutral
acetyl group (Green et al., 1953). The result of this modification increases the number of the net negative
charges of the fibril. Conversely, the negatively charged carboxyl groups of aspartic and glutamic acid
can be chemically modified to a neutral group by methylation (Fraenkel-Conrat and Olcott, 1944). Thus,
