Optimizing mechanical stiffness of shape memory and superelastic alloys by introducing engineering porosity.

摘要

Common metals for stable long-term implants (e.g. stainless steel, Titanium, and Titanium alloys) are much stier than spongy cancellous and even stier than cortical bone. When bone and implant are loaded this stiness mismatch results in stress shielding and as a consequence, degradation of surrounding bony structure can lead to disassociation of the implant. Due to its lower stiness and high reversible deformability, which is associated with the superelastic behavior, NiTi is an attractive biomaterial for load bearing implants. However, the stiness of austenitic Nitinol is closer to that of bone but still too high. Additive manufacturing provides, in addition to the fabrication of patient specic implants, the ability to solve the stiness mismatch by adding engineered porosity to the implant. This in turn allows for the design of dierent stiness proles in one implant tailored to the physiological load conditions. This work covers a fundamental approach to bring this vision to reality. At the rst step a stiness optimization scheme is presented. Based on the ndings from this algorithm a second optimization method is developed to change the parameters of predened porosities to optimize the stiness. The suggested porous structures are then manufactured out of Titanium and Nitinol by selective laser melting. The manufactured parts tested under compression to nd the modulus of elasticity and the results compared to the results of simulation.;Simulation and experiment results show that the mechanical properties of the devices can be changed signicantly after introducing porosity. The results of this work paves the way for designing stiness tailored bone implants.