Completed notes of the course

Pagina 1

1 Chiara Moreschini Notes of MECHANICS OF BIOLOGICAL STRUCTURES Prof. Pasquale Vena – AA 2020/21 1. THE BONE TISSUE (HARD TISSUE) MECHANICS The bone tissue is the most important hard tissue in the body and it is a hierarchical multiscale tissue, which means it is composed of many levels, each with its own particular mechanical properties. Let’s see in detail every level: 1. Macroscale level: cortical and cancellous or trabecular bone The macrostructure is made of two types of bone, the cortical bone or the trabecular bone (this division holds only at this level); these structures are in the order of millimeters. 2. Microsctucture level: osteons The cross section of cortical bone is made of osteons, which are concentric cylindric structures with a diameter of some hundred microns. 3. Sub-microstructure level: lamellae Osteons are made of concentric layers called lamellae, made of collagen, with a thickness of a few microns. Because of these strictures the bones are also called lamellar bones. 2 4. Nanostructure level: collagen fiber and fibril The lamellae are made of collagen fibers, which are also made of fibrils with a diameter of 0.5 micron. 5. Sub-nanostructure level: collagen molecule and hydroxyapatyte Finally, at this level we arrive to the basic constituents of bones, which are bone crystals (hydroxyapatyte) and collagen molecules, with a diameter of 1 nanometer. Starting from the mechanical properties of the microscopical level we can discover also the properties of higher levels. The elastic modulus of the bone crystals is in order of 110 GPa, so this is a very stiff material, while the elastic modulus for collagen molecule is in the order of some MPa to 2-3 GPa (it’s variable and depends on the amount of water in the molecule). ECB = 110 GPa ECM @ MPa ÷ 3…