M.I. Lopez

Next-generation biomedical implants using additive manufacturing of complex, cellular and functional mesh arrays

In this paper, we examine prospects for the manufacture of patient-specific biomedical implants replacing hard tissues (bone), particularly knee and hip stems and large bone (femoral) intramedullary rods, using additive manufacturing (AM) by electron beam melting (EBM). Of particular interest is the fabrication of complex functional (biocompatible) mesh arrays. Mesh elements or unit cells can be divided into different regions in order to use different cell designs in different areas of the component to produce various or continually varying (functionally graded) mesh densities. Numerous design elements have been used to fabricate prototypes by AM using EBM of Ti-6Al-4V powders, where the densities have been compared with the elastic (Young) moduli determined by resonant frequency and damping analysis. Density optimization at the bone–implant interface can allow for bone ingrowth and cementless implant components. Computerized tomography (CT) scans of metal (aluminium alloy) foam have also allowed for the building of Ti-6Al-4V foams by embedding the digital-layered scans in computer-aided design or software models for EBM. Variations in mesh complexity and especially strut (or truss) dimensions alter the cooling and solidification rate, which alters the α-phase (hexagonal close-packed) microstructure by creating mixtures of α/α′ (martensite) observed by optical and electron metallography. Microindentation hardness measurements are characteristic of these microstructures and microstructure mixtures (α/α′) and sizes.

Advanced metal powder based manufacturing of complex components by electron beam melting

Abstract Electron beam melting of Ti–6Al–4V powder (∼30 μm diameter) to create complex, three-dimensional components layer by layer using CAD is described along with the characterisation of these products (builds) by optical and electron microscopy, and mechanical testing. Build defects, including porous (unmelted or unsintered) zones, inclusions and gas bubbles trapped in the atomised powder particles and retained in the build, are illustrated. Reticulated mesh geometries and their applications are described along with examples having biomedical applications. Microstructures of solid components and mesh arrays are described. Powder chemistries and solid build chemistries are also examined and shown to be constant for up to 40 cycles of powder reuse, but there was a 10–15% reduction in Al content in the solid builds at optimised build conditions. Quality control and related issues are also described using duplicate products for destructive testing correlated with removable quality control tabs on the builds.

Additive Layered Manufacturing of Reticulated Ti-6Al-4V Biomedical Mesh Structures by Electron Beam Melting

Porous coatings, notably thin, porous bead coatings, sintered mesh arrays, thermal-spray coatings and metallic foams have been incorporated into biomedical devices and appliances for several decades to improve bone compatibility, stability, and bone ingrowth. In this research program, we are concerned with the fabrication of reticulated mesh arrays as integral components of monolithic products using Ti-6Al-4V powder to build complex, 3D structures by electron beam melting (EBM). Utilizing software capable of building lattice-truss or cellular lattices with high symmetry, 3D-periodic reticulated arrangements such as hip stem and knee component prototypes have been fabricated with complete mesh arrays or with solid stems with a surrounding mesh structure completely fabricated as a monolithic product. The 3D mesh structures begin with so-called lattice elements which can be designed, computed, and attached to a CAD program for additive layered manufacturing by EBM. Mesh arrays with cortical bone density (1.9 g/cm3) can be fabricated with various lattice-truss structures and truss dimensions tailored to stress-strain and stiffness properties to optimize porous bone-replacement implants, including craniofacial replacements, etc. Mesh-to-mesh structures and functionally graded structures are also explored. Metallographic analysis of these structures using optical and electron microscopies illustrate their microstructural characteristics in association with measured mechanical properties such as microindentation hardness which can be related linearly to residual stress.

Metallographic Characterization of Additive-Layer Manufactured Products by Electron Beam Melting of Ti-6Al-4V Powder

Abstract Optical metallography and electron metallography (SEM and TEM) techniques are applied to the characterization of additive-layer manufactured products and prototypes by electron beam melting (EBM) of Ti-6Al-4V atomized precursor powder. In addition, microstructure-property comparisons are made by hardness and tensile test measurements which demonstrate prospects for controlled property-performance variations by varying process parameters including temperature profiles and cooling rates in manufactured prototypes.