To facilitate the design and development of porous metals, simulation of their mechanical behavior is essential. As an alternative to complex tomography procedures, a methodology has been developed to construct a simulated microstructure that retains the essential features of the experimental material. The target material is a moderate porosity titanium foam that is being developed as a bone implant material. The methodology applies stereology theory to a foaming process based on growth of pressurized pores. Three-dimensional (3D) pore size and pore distribution information is derived from 2D sections for a sample with low porosity, early in the foaming process. A 3D microstructure is developed based on the 3D location and size distribution of the pores by use of a computational procedure. Pores are allowed to grow and coalesce in a simple simulated foaming process to achieve microstructures of higher porosity. These data have been used as inputs to write scripts of I-DEAS to create 3D finite element models which are then examined for basic global and local mechanical properties.
Successful bone formation which leads to functional osseointegration is determined by the local mechanical environment around boneinterfacing implants. In this work, a novel porous titanium material is developed and tested and then impact of porosity on mechanical properties as a function of bone ingrowth is studied numerically. A superplastic foaming technique is used to produce CP-Ti material with rounded, interconnected pores of 50% porosity; the pore size and morphology is particularly suitable for bone ingrowth. In order to understand the structure-property relations for this new material, a numerical simulation is performed to study the effect of the porous microstructure and bone ingrowth on the mechanical properties. Using ABAQUS, we create two-dimensional representative microstructures for fully porous samples, as well as samples with partial and full bone ingrowth. We then use the finite element method to predict the macroscopic mechanical properties of the foam, e.g., overall Young's modulus and yield stress, as well as the local stress and strain pattern of both the titanium foam and bone inclusions. The strain-stress curve, stress concentrations and stress shielding caused by the bone-implant modulus mismatch are examined for different microstructures in both elastic and plastic region. The results are compared with experimental data from the porous titanium samples. Based on the finite element predictions, bone ingrowth is predicted to dramatically reduce stress concentrations around the pores. It is shown that the morphology of the implants will influence both macroscopic properties (such as modulus) and localized behavior (such as stress concentrations). Therefore, these studies provide a methodology for the optimal design of porous titanium as an implant material.
We have reported previously a method to introduce bioactive nanofiber networks through selfassembly into the pores of titanium alloy foams for bone repair. In this study we evaluate the in vitro colonization by mouse pre-osteoblastic cells of these metal-peptide amphiphile hybrids containing phosphoserine residues and the RGDS epitope. The aim was to determine the effect of varying the RGDS epitope concentration within a given range, and confirm the ability for cells to infiltrate and survive within the nanofiber-filled interconnected porosity of the hybrid material. We performed proliferation (DNA content) and differentiation assays (alkaline phosphatase and osteopontin expression) as well as SEM and confocal microscopy to evaluate cell colonization of the hybrids. At the RGDS epitope concentrations used in the nanofiber networks, all samples demonstrated significant cell migration into the hybrids, proliferation, and differentiation into osteoblastic lineage.
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