Multi-scale Characteristics of Bone Toughness

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Bone is both strong and tough because of its unique composition and structure. Bone is composed of two main building blocks: a hard mineral and a soft protein. These building blocks are organized in distinct ways from the molecular level up to the whole bone level, forming a hierarchical composite. Alterations of the composition or organization of the building blocks results in bones that are weak or brittle, but the specific way this happens is not known. The objective of our research is to understand how alterations across levels of hierarchy affect the strength and toughness of bone. The insights gained will help define the critical characteristics of bone toughness, from which we can determine why bone fails in aging and disease. In the scope of this project we will bring the excitement of bones and materials research to the public through 1) adult community education outreach seminars, 2) involving high school summer students in the research, and 3) developing an elementary school science module on "why things break." The objective of this research is to determine the structural, compositional, and mechanical properties of bones at the whole bone (mm), tissue (micron), fibril (nm), and molecular (Angstrom) length scales. We will examine various mouse strains that have altered proteins (transgenic, genetic mutations, and knock-out models) compared to wild type mice. The altered proteins may be the building blocks themselves (such as collagen defects in brittle bone disease) or proteins that are essential to organization and mineralization processes (such as crosslinking proteins) that affect strength and toughness of the bone. At the whole bone level we will determine resistance curves, which characterize both initiation and propagation toughness, from notched 3-point bending. At the tissue level we will examine mineral density with quantitative backscatter scanning electron microscopy and map the elastic modulus to define heterogeneity of material properties. At the fibril level we will examine the mechanics of unmineralized collagen (from tail tendon of the same mice) with dynamic mechanical analysis and mineralized collagen fibrils with small angle x-ray scattering. At the molecular level we will measure crosslinking and non-collagenous proteins. This multi-scale analysis of mechanical, structural, and compositional properties will identify the factors contributing to bone strength and toughness.