Adaptive Anisotropy: An Example in Living Bone

Summary

Abstract

Many natural materials adjust to their environmentally applied loads by changing their microstructure. These microstructural changes induce changes in the mechanical anisotropy of the material. Thus, the anisotropy of the material adapts to the applied loads. Materials that adapt their anisotropy by mechanical means include soft living tissue, rocks and all granular materials. Materials that adapt their anisotropy by chemical means include living bones and trees and saturated sandstones and limestones. In this paper an example of adaptive anisotropy in living bone is presented.

Introduction

Many natural materials possess mechanisms by which they adjust the degree of their anisotropy in order to carry, more efficiently, the load to which they are being subjected. By the degree of anisotropy we mean the relative stiffness or compliance of the material in different directions. For example, some fibrous composites have a ratio of Young's modulus in their fiber direction to Young's modulus in their transverse direction of 200. These materials are said to be strongly anisotropic. On the other hand bone can be described as mildly anisotropic because the ratio of Young's moduli in different directions generally exceeds two. However, bone has a mechanism by which it changes the degree of its anisotropy to adjust to its environmental load and manmade fibrous composites do not generally possess these adaptive mechanisms. As an example of the effect of this adaptive mechanism in bone one can note the fact that the ratio of Young's moduli near the mid-shaft of the human femur is about two, but it decreases to near one (almost isotropic) near the joints.