Hierarchical Structure and Mechanical Adaptation of Biological Materials

Abstract

Most stiff biological tissues are hierarchically structured, bone and wood being prominent examples. This means that the nanometer structure varies on a micrometer scale. As a consequence, the mechanical properties can be adjusted locally by the organism. Functional gradients and complex structural elements are common in natural tissues. The stiffness of dentin, for example, is graded in such a way that a minimum appears right at the dentin- enamel junction, which is important to prevent catastrophic failure (section 3.2). The flexibiilty of the material in a branch is also graded to account for the asymmetric loading due to gravitational forces (section 4.2). Mechanical adaptation also leads to age-related changes in the hierachical structure, both in bone (section 3.1) and wood (section 4.1 and 4.2). Continued research on natural hierarchical structures is necessary, not only to improve our understanding of biological tissues but also to reveal the strategies and mechanisms used by nature and which may be applied in an engineering context for improving material properties.