Probing the effect of bone microstructure via 3D-printing

Hierarchical materials are abundantly found in nature, and recently object of biomimetic and engineering research studies. Indeed, the fascinating mechanisms behind the outstanding mechanical properties of natural materials inspire many researchers to deeply investigate such materials, with the goal of designing and manufacturing new materials or improve the existing ones. Among these, an interesting load bearing material is bone, with a lightweight hierarchical structure. The optimal combination of mechanical properties makes it very attractive for research. In particular, its fracture behavior and its significant toughness, make bone an interesting material from an engineering point of view, since it can be used as a biomimetic model for the design of new tough composite materials. By using a biomimetic approach, we design and realize, by additive manufacturing, a new polymer composite material with an internal structure inspired to the microstructure of cortical bone. The new material is characterized by a repeating structural unit with a cylindrical shape, designed to mimic the osteons. To investigate the effect of the geometry of the microstructural features, we design two materials: i) characterized by perfectly circular osteons and, ii) characterized by elliptical osteons. In both cases, we maintain the osteon volume ratio constant and equal to that measured in mid-age human bone. We characterize the material under tensile loading and we investigate the fracture behavior by performing tests in presence of a crack. The results of the tests prove how the osteon geometry and their arrangement affect the crack path and the overall behavior of the composite. By observing the failure mode, we also notice several similarities with the microscale toughening mechanisms occurring in cortical bone, such as crack deflection and branching, constrained microcracking and fibril bridging. This confirms the validity of the bone-inspired design to mimic the fracture behavior of cortical bone at microstructural level.