The composition of bones grants them strength, light weight, and some flexibility via small inorganic crystals and thin collagen fibers.
"Nature has no reason for making a bone round or square. The outlines of bones, therefore, follow the stress lines or are vertical to them so that they give an indication of the pressures the bone has to withstand. But this ideal distribution of bone material along the stress lines would have been to little avail were the material itself not so well adapted to extraordinary pressure. Just like fiberglass made of synthetics threaded with glass fiber, bone tissue is made up of two constituents which greatly differ in their mechanical properties. About half the bone volume is made up of inorganic crystalline material. It consists of phosphate, calcium, and hydroxyl ions and comes very close to hydroxylapatite in structure. It appears in the bone in the form of tiny crystals, only about 200 atomic diameters in size. They are inserted between thin fiber hairs of the elastic material collagen and seem to be linked with them. Many of these parallel inorganic and organic building blocks form fascicles, which may be interwoven in various ways. The end product is a material that is considerably stiffer than collagen, though low in weight, but by far not as brittle and inelastic as pure hydroxylapatite. Besides, because of the continuous alternation between brittle and elastic material, there is little chance for a fracture to spread unchecked." (Tributsch 1984:32-33)
"Mineralized collagen fibrils are highly conserved nanostructural building blocks of bone. By a combination of molecular dynamics simulation and theoretical analysis it is shown that the characteristic nanostructure of mineralized collagen fibrils is vital for its high strength and its ability to sustain large deformation, as is relevant to the physiological role of bone, creating a strong and tough material. An analysis of the molecular mechanisms of protein and mineral phases under large deformation of mineralized collagen fibrils reveals a fibrillar toughening mechanism that leads to a manifold increase of energy dissipation compared to fibrils without mineral phase. This fibrillar toughening mechanism increases the resistance to fracture by forming large local yield regions around crack-like defects, a mechanism that protects the integrity of the entire structure by allowing for localized failure. As a consequence, mineralized collagen fibrils are able to tolerate microcracks of the order of several hundred micrometres in size without causing any macroscopic failure of the tissue, which may be essential to enable bone remodelling. The analysis proves that adding nanoscopic small platelets to collagen fibrils increases their Young's modulus and yield strength as well as their fracture strength. We find that mineralized collagen fibrils have a Young's modulus of 6.23 GPa (versus 4.59 GPa for the collagen fibril), yield at a tensile strain of 6.7% (versus 5% for the collagen fibril) and feature a fracture stress of 0.6 GPa (versus 0.3 GPa for the collagen fibril)." (Buehler 2007:1)
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