Permeability of 3D Printed Titanium Implants

Overview

Porous titanium foam implants are a common choice for bone augmentation, as their open porosity offers an effective void space for encouraging blood flow and nutrient transfer after implantation. This case study looks at how Selective Laser Melting (SLM) can be used to manufacture implant structure on multiple length scales with controlled pore networks, enabling evaluation of permeability through experimental and computational methods. Experiments were carried out into how adjustments to material properties can optimise titanium implant design. Simpleware software was used for segmenting the image data and then for generating high quality CFD meshes for analysis in ANSYS Fluent.

Characteristics:

  • 3D FE models from µCT images
  • Segmentation of void and solid regions
  • Placement of fluid region in +FE module
  • Setting of fluid inlet/outlet boundaries in +FE module
  • Generation of an analysis ready volume mesh in +FE module
  • Simulation of fluid flow and characterisation of permeability in ANSYS Fluent
  • Comparison of experimental and computational results

Thanks to

Imperial College London (Department of Materials), University of Liverpool (School of Engineering), Manchester X-Ray Imaging Facility, Stryker Orthopaedics:
Z. Zhang • D. Jones • S. Yue • P.D. Lee • J.R. Jones • C.J. Sutcliffe • E. Jones

Manufacturing & Experimental Testing

Titanium foam samples were created using commercially pure Ti (Grade 1) metal powder, based on CAD models using the Unit Cell (UC) approach. This involved creating a CAD model of the porous structures with its 3D geometries completely filled by cubic unit cells and a connected lattice structure by joining sets of points and vectors in 3D space to form regular octahedron. Foam structures were produced using SLM, and porosity and pore size distributions calculated and measured in relation to fluid flow analyses.

Image Processing & Meshing

For computational analysis, samples were scanned at a resolution of 9 µm per voxel with a commercial µCT unit (Phoenix v|tome|x, GE Measurement and Control, MA, USA). 3D volumetric images were reconstructed, pre-processed and modified using the SLM input file before being imported to ScanIP for segmentation and thresholding. Further adjustments were made to include an entirely fluid region, as well as fluid inlet-outlet boundaries set in Simpleware +FE, prior to a finite element mesh of the fluid flow regions being imported to ANSYS Fluent.

Simulation & Results

The exported mesh file was loaded in ANSYS Fluent, where the boundary conditions and fluid properties were set and the model was run using the contingent gradient solver. Following convergence of each model, the effective permeability was calculated using Darcy’s Law. Results from this CFD analysis were compared to experimental results, and indicated how permeability is related to the optimum implant design inputs. The study demonstrates how structure porosity, strut surface roughness and internal structure perturbation adjustments can be used to design hierarchical additive manufactured implants with tailored properties.