Simulation and physical validation of metal triply periodic minimal surfaces-based scaffolds for bioengineering applications

Abstract

Metallic scaffolds are used as implants to help heal bones. Sheet-based Triply Periodic Minimal Surfaces (TPMS) are of interest due to their high surface-to-volume ratio (S/V) and customisable stiffness. They can be realised using Additive Manufacturing (AM). Other studies investigate porosity and pore size of scaffolds, but they frequently overlook S/V, which is critical for cellular response. Additionally, the limitation of AM (esp. Selective Laser Melting (SLM)) resides in the discrepancies between as-designed and as-built physical and mechanical properties of those structures, and this also needs addressing. This work investigates three types of pure Titanium TPMS scaffolds, with an emphasis on as-designed vs as-built discrepancies and the significance of S/V. As-designed scaffolds reported 70-75% porosity and 25-35 cm-1 S/V, and stiffness was measured using finite element analysis (FEA) obtaining 6.7-9.3 GPa. The as-built scaffolds had 59-70% porosity and 33-42 cm-1 S/V. Laboratory compression testing revealed an effective Young's modulus of 5-9 GPa, comparable to bone tissue. Image-based simulation methods were employed on the as-built samples which reported the stiffness range of 8.3-15 GPa, overestimating it by 54%. It is hypothesised that these discrepancies stem from the secondary roughness on the surfaces, cracks and entrapped voids created during the SLM process, causing reduction in porosity, yet not contributing to structure’s strength. The cyber-physical validation methods presented in this work are a good way to quantify these discrepancies, allowing feedback to the design stages for more predictable as-built structures.