Functional Gradient Structure and Properties of a Bivalve Hinge Ligament
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Katsumichi Ono A characteristic feature of soft biological tissue is a low compression modulus at small strains. On the other hand, the compression modulus of artificial elastomers, such as vulcanized rubber, generally increases with decreasing strain, according to the theory of rubber elasticity. In this paper, I will consider that the low compression modulus of biological tissue is possibly brought about by the organic and inorganic FGM structure of the tissue. An example of a bivalve hinge ligament is given here. The tissue of the hinge ligament of a bivalve (Spisula sachalinensis) is a hybrid
material, composed of a soft cross-linked protein matrix and inorganic calcium carbonate crystals (aragonite). In this example, small rectangular strips were cut from the ligament. The aragonite can then be easily removed from the tissue by treating it with dilute acetic acid (to decal-
Figure 1. Coordinates for the bivalve hinge ligament.1
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cify the sample). At this point, mechanical anisotropy and swelling were measured and discussed, based on the anisotropic, fine structure of the aragonite. Figure 1 shows the coordinates for a bivalve hinge ligament. The x axis and y axis are the growth direction and the tangential direction against the growth line, respectively. The z axis is approximately normal in direction to the shell surface. Xray diffraction of a long strip cut along the x axis showed a typical fiber pattern characteristic of the aragonite structure. The crystallographic c axis of the aragonite coincided with the x direction. Transmission electron microscopic (TEM) studies of the tissue revealed that the aragonite crystals, dispersed in protein matrix, are hexagonal rods. The diameter of the rod used in the studies was about 800 A, and the length was 3000^000 A. All of the axes of the rods (crystallographic c axis) were aligned along the x direction (the growth direction). Thus, rods were approximately perpendicular to the shell surface. This structure enables the tissue to sustain large compressive strain. The swelling behavior of the tissue in water was interesting. The intact ligament swelled only in the y and z directions. The swelling in the x direction was negligible. On the other hand, the decalcified ligament was able to swell also in the x direction. The swelling behavior is completely consistent with the fine crystal structure observed with x-ray diffraction and TEM. Thus, the hinge ligament may be regarded as a uniaxially reinforced soft compos-
Figure 2. Uniaxial stress-strain curve for intact and decalcified hinge ligaments, a is compression ratio.'
ite material under the swelled state. Figure 2 shows the uniaxial compression stress-strain curve for intact and decalcified hinge ligaments.1 All curves showed the characteristic feature of biological soft tissue, i.e., low modulus at small strains. The behavior can be MRS BULLETIN/JANUARY 1995
Functional Gradient Structure and Properties of a Bivalve Hinge Ligament
explained by the large deformation of the protein matrix at small stra
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