Introduction

Although there are over 80 minerals present in biological systems, the most important are hydroxyapatite (HAP) (mammals and fishes), calcium carbonates (shells, arthropods, corals), and silica (diatoms, sponges). These minerals seem to have evolved from approximately 560 million years ago.

Minerals are essential for providing compressive strength to biological systems, whereas biopolymers are primarily responsible for tensile strength. The combination of minerals and biopolymers leads to the formation of biological materials with mechanical properties tailored in terms of hardness, toughness, and anisotropy. The formation of minerals involves nucleation and growth, both mediated by biological components. The organic matrix mediates nucleation in many ways: by providing nucleation sites and by controlling the polymorphs. The growth is also mediated by organic compounds, and illustrative examples are given in Chapters 6 and 7. For instance, the rapid direction of growth for aragonite is the c-direction, and long needles are formed. In nacre, this growth is regulated by the periodic deposition of organic layers. In bone, the HAP crystals nucleate in the interstices of collagen fibrils and growth is also regulated: they reach sizes on the order of nanometers: 40–60 nm long, 20–30 nm wide, and platelets of a few nanometers thickness are formed in such a fashion.