Design exploration of 3D-printed triply periodic minimal surface scaffolds for bone implants

Triply Periodic Minimal Surface (TPMS) scaffolds have recently received considerable attention because of their potentials to be used as internal structures of additively manufactured bone implants. Advantages of TPMS-based implant were from their implicit natures, allowing for precise geometric modification. As a result, many physical characteristics such as surface-to-volume ratio, pore size, elastic properties, and fluid behaviors became controllable parameters. Therefore, TPMS structures offered an opportunity to design bone implants, which were both mechanically and biologically optimized. Nonetheless, the interconnection among structures, mechanical properties, biological performances, and manufacturing limitations was not clearly understood. As a result, the present study utilized an analytical approach, FE modeling, and CFD analysis to examine the synergistic effects of six different TPMS structures including Primitive, Gyroid, Diamond, Neovius, FRD, and IWP on their applicability as bone substitute structures. According to our analysis, we found that pore size, elastic properties, and flow behaviors were highly dependent on the choices of TPMS models. Also, TPMS structures could exhibit both isotropic and anisotropic elastic properties, in which Primitive and Neovius were among the most anisotropic structures. In addition, Gaussian curvature and fluid-induced wall shear stress were suggested to be considered for both mean values and local distributions when evaluating different TPMS models. Ultimately, the design map was constructed to display designable regions, where geometries, preferable pore sizes, mechanical properties, and manufacturing constraints were fulfilled. We found that there was no single optimal structure since optimal geometries varied with different given constraints. Nonetheless, Gyroid, Diamond, and IWP were shown as promising candidates thanks to their flexible design space, isotropic elastic property, adequately uniform Gaussian curvature and fluid-induced wall shear stress, as well as high permeability.