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 decalcify 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. X-ray 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.