Design and additive manufacturing of metamaterials for orthopedic implants and its mechanical characterization

Lattice (porous) structures with interconnected pores, which are also known as metamaterials, provide better osteointegration compared to solid implants and minimize the stress shielding effect by aligning the effective elastic modulus of implants closer to the natural bones. The present work focuses on developing biocompatible orthopedic implants with reduced stress-shielding, better load-bearing, and osteointegration capabilities. Triply Periodic Minimal Surface (TPMS) based Gyroid and strut-based Diamond lattice structures were designed considering optimum pore sizes for enhanced osteointegration and porosity percentages ranging from 65% to 85% to bring the stiffness of implants closer to the bone tissues. The quasi-static mechanical performance of Gyroid and Diamond lattice structures was investigated computationally by Finite Element Analysis (FEA), including size-effect analysis to obtain size-independent mechanical properties. The size effect analysis indicated that 4³ cells are sufficient to obtain bulk mechanical properties of the lattice structures. Lattice structures were fabricated using the Direct Metal Laser Sintering (DMLS) process, followed by microstructural evaluation, which revealed that the micro-defects generated during additive manufacturing (AM) drastically reduce the mechanical strength of structures. The lattice structures were tested under compressive loading, and experimental data were compared with the FEA results. The deformation behavior of both structures was analyzed to assess their suitability for load-bearing applications. Research findings indicate that the Gyroid and Diamond lattice structures offer easy control over volume fraction, thus, scaffolds’ effective elastic modulus and strength. With porosities ranging from 65% to 85%, these structures exhibit effective elastic moduli between 0.14 GPa to 3.5 GPa and yield strength from 3 MPa to 60 MPa, making them suitable for implantation adjacent to cancellous bones; however, requiring enhancement in strength to be viable for implantation near cortical bones. TPMS-based Gyroid and strut-based Diamond lattice structures were designed to reduce stress shielding and improve osteointegration in metallic implants. Finite Element Analysis (FEA) was conducted to evaluate the quasi-static mechanical behavior of lattice structures. Lattice structures were fabricated using the DMLS process and subjected to compression testing to assess their mechanical properties. The structures exhibited effective elastic moduli and strengths comparable to cancellous bones, making them suitable for implantation near cancellous bone tissue. The study investigated the influence of manufacturing defects on the mechanical properties of lattice structures.