Showing papers by "Francisco Medina published in 2010"

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

Abstract: 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 alpha-phase (hexagonal close-packed) microstructure by creating mixtures of alpha/alpha' (martensite) observed by optical and electron metallography. Microindentation hardness measurements are characteristic of these microstructures and microstructure mixtures (alpha/alpha') and sizes.

Characterization of Ti–6Al–4V open cellular foams fabricated by additive manufacturing using electron beam melting

Abstract: Ti–6Al–4V open cellular foams were fabricated by additive manufacturing using electron beam melting (EBM). Foam models were developed from CT-scans of aluminum open cellular foams and embedded in CAD for EBM. These foams were fabricated with solid cell structures as well as hollow cell structures and exhibit tailorable stiffness and strength. The strength in proportion to the measured microindentation hardness is as much as 40% higher for hollow cell (wall) structures in contrast to solid, fully dense EBM fabricated components. Plots of relative stiffness versus relative density were in good agreement with the Gibson–Ashby model for open cellular foam materials. Stiffness or Young's modulus values measured using a resonant frequency-damping analysis technique were found to vary inversely with porosity especially for solid cell wall, open cellular structure foams. These foams exhibit the potential for novel biomedical, aeronautics, and automotive applications.

Fused deposition modeling of patient‐specific polymethylmethacrylate implants

Abstract: Purpose – The purpose of this paper is to investigate the use of medical‐grade polymethylmethacrylate (PMMA) in fused deposition modeling (FDM) to fabricate porous customized freeform structures for several applications including craniofacial reconstruction and orthopaedic spacers. It also aims to examine the effects of different fabrication conditions on porosity and mechanical properties of PMMA samples.Design/methodology/approach – The building parameters and procedures to properly and consistently extrude PMMA filament in FDM for building 3D structures were determined. Two experiments were performed that examined the effects of different fabrication conditions, including tip wipe frequency, layer orientation, and air gap (AG) (or distance between filament edges) on the mechanical properties and porosity of the fabricated structures. The samples were characterized through optical micrographs, and measurements of weight and dimensions of the samples were used to calculate porosity. The yield strength, s...

Comparison of Microstructures and Mechanical Properties for Solid and Mesh Cobalt-Base Alloy Prototypes Fabricated by Electron Beam Melting

Abstract: The microstructures and mechanical behavior of simple, as-fabricated, solid geometries (with a density of 8.4 g/cm3), as-fabricated and fabricated and annealed femoral (knee) prototypes, and reticulated mesh components (with a density of 1.5 g/cm3) all produced by additive manufacturing (AM) using electron beam melting (EBM) of Co-26Cr-6Mo-0.2C powder are examined and compared in this study. Microstructures and microstructural issues are examined by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD), while mechanical properties included selective specimen tensile testing and Vickers microindentation hardness (HV) and Rockwell C-scale hardness (HRC) measurements. Orthogonal (X-Y) melt scanning of the electron beam during AM produced unique, orthogonal and related Cr23C6 carbide (precipitate) arrays (a controlled microstructural architecture) with dimensions of ~2 μm in the build plane perpendicular to the build direction, while connected carbide columns were formed in the vertical plane, parallel to the build direction, with microindentation hardnesses ranging from 4.4 to 5.9 GPa, corresponding to a yield stress and ultimate tensile strength (UTS) of 0.51 and 1.45 GPa with elongations ranging from 1.9 to 5.3 pct. Annealing produced an equiaxed fcc grain structure with some grain boundary carbides, frequent annealing twins, and often a high density of intrinsic {111} stacking faults within the grains. The reticulated mesh strut microstructure consisted of dense carbide arrays producing an average microindentation hardness of 6.2 GPa or roughly 25 pct higher than the fully dense components.

Extraordinary Transmission Through Arrays of Slits: A Circuit Theory Model

Abstract: Extraordinary transmission and other interesting related phenomena for 1-D periodic arrays of slits (compound diffraction gratings) have recently been the object of intense research in the optics and solid state physics communities. This case should be differentiated from the extraordinary transmission through arrays of small apertures on metal screens since small holes only support below-cutoff modes, whereas slits can also support transverse electromagnetic modes without cutoff frequency. In this paper, an equivalent-circuit approach is proposed to account for the most relevant details of the behavior of slit-based periodic structures: extraordinary transmission peaks, Fabry-Perot resonances, and transmission dips observed in compound structures. The proposed equivalent-circuit model, based on well-established concepts of waveguide and circuit theory, provides a simple and accurate description of the phenomenon that is appropriate for educational purposes, as well as for the design of potential devices based on the behavior of the structures under study.

Circuit modeling of the transmissivity of stacked two-dimensional metallic meshes

Abstract: This paper presents a simple analytical circuit-like model to study the transmission of electromagnetic waves through stacked two-dimensional (2-D) conducting meshes. When possible the application of this methodology is very convenient since it provides a straightforward rationale to understand the physical mechanisms behind measured and computed transmission spectra of complex geometries. Also, the disposal of closed-form expressions for the circuit parameters makes the computation effort required by this approach almost negligible. The model is tested by proper comparison with previously obtained numerical and experimental results. The experimental results are explained in terms of the behavior of a finite number of strongly coupled Fabry-Perot resonators. The number of transmission peaks within a transmission band is equal to the number of resonators. The approximate resonance frequencies of the first and last transmission peaks are obtained from the analysis of an infinite structure of periodically stacked resonators, along with the analytical expressions for the lower and upper limits of the pass-band based on the circuit model.

Circuit model for a periodic array of slits sandwiched between two dielectric slabs

Abstract: This paper proposes an equivalent circuit model that uses lumped elements and transmission lines to explain the transmission of electromagnetic waves through a conducting screen periodically perforated with slits and sandwiched between two different dielectric slabs. The present model relies on the impedance-matching point of view, previously introduced by some of the authors, rather than on the surface plasmon polariton concept. Thus, the model constitutes a simple and insightful framework that easily leads to accurate qualitative and quantitative predictions about the nature of the transmission spectrum of such structures.

Study of Extraordinary Transmission in a Circular Waveguide System

Abstract: Extraordinary transmission through periodic distributions of sub-wavelength holes made in opaque screens has been demonstrated and exhaustively studied along the last decade. More recently, extraordinary transmission has also been predicted and experimentally observed through electrically small diaphragms located inside hollow pipe waveguides. This last phenomenon cannot be explained in terms of surface waves excited along the periodic system (the so-called surface plasmon polaritons). Transverse resonances can be invoked, however, as a sound explanation for extraordinary transmission in this kind of systems. In this paper, a simple waveguide system, exhibiting exactly the same behavior previously observed in periodic 2-D arrays of holes, is analyzed in depth. Analogies and differences with the periodic case are discussed. The theoretical and experimental results reported in this paper provide strong evidence in favor of the point of view emphasizing the concept of impedance matching versus surface wave excitation. The role of material losses is discussed as an important practical issue limiting the maximum achievable subwavelength transmission level. Most of our conclusions can be applied to both periodic arrays of holes and diaphragms in closed waveguides.

Broadband extraordinary transmission in a single sub-wavelength aperture

Abstract: Coordinate transformation is applied to design an all-dielectric device for Extraordinary Transmission (ET) in a single sub-wavelength slit. The proposed device has a broadband feature and can be applied from microwave to visible frequency bands. Finite-Difference Time-Domain (FDTD) simulations are used to verify the device’s performance. The results show that significantly increased transmission is achieved through the sub-wavelength aperture from 4 GHz to 8 GHz when the device is applied. In contrast with previously reported systems, the frequency sensitivity of the new device is very low.

Analysis of Bonding Methods for FDM-Manufactured Parts

Abstract: The fused deposition modeling (FDM) additive manufacturing (AM) technology has been valuable for producing a variety of concept models, functional prototypes, end-use parts and manufacturing tools using a range of durable thermoplastic materials. The largest individual component that can be produced in FDM depends on the dimensions of the build chamber for the specific FDM system being used, with a maximum build chamber size available of 914 x 610 x 914 mm. This limitation is not unique to FDM as all AM systems are constrained by a build chamber. However, by using thermoplastic materials, individual components can be bonded together using different methods to form a single piece. Bonding can be used to help reduce building time and support material use, and also allows for the fabrication and assembly of final products larger than the build chamber. This work investigated different methods for bonding FDM-manufactured parts, including the use of five different adhesives and solvents as well as two different welding techniques (hot air welding and ultrasonic welding). The available FDM materials investigated included acrylonitrile butadiene styrene (ABSi, ABS-M30, ABS-M30i), polycarbonate (PC, PC-ABS, PC-ISO), polyphenylsulfone (PPSF), and ULTEM 9085. Bonding strengths were characterized by comparing ultimate tensile strengths at break and analyzing the mode of failure. Overall, the bonding method of hot air welding produced the strongest bond for all the materials investigated except for ULTEM 9085 for which the strongest bond was achieved with the two-part epoxy adhesive Hysol E-20HP.

In Vitro Validation of Finite Element Model of AAA Hemodynamics Incorporating Realistic Outflow Boundary Conditions

Abstract: Numerical methods have become a powerful tool to quantify hemodynamic forces in the cardiovascular system. Studying these forces enables us to understand cardiovascular disease mechanisms, as well as better evaluate and design cardiovascular medical devices. However, much work remains to validate numerical methods against experimental data. Vignon-Clementel and colleagues have shown that the boundary conditions (BCs) prescribed at the outlets of numerical domains dramatically influence the flow and pressure computational results [1]. While many in-vitro studies have implemented simple BCs to allow for acceptable validations of numerical methods, none has implemented BCs that provide physiologically realistic flows and pressures.Copyright © 2010 by ASME

Parallel coupled CPW filters for spurious band suppression

Abstract: Modified coplanar waveguide (CPW) edge-coupled band-pass filters with floating conductor strips printed below the coupled-line sections are proposed. These structures allow one to suppress the first undesired spurious passband associated with the nonhomogeneous nature of the dielectric in CPW structures. Also, tight coupling levels can be easily achieved. An optimization code based on a fast quasi-TEM solver is used to optimize the dimensions of the filter sections, in such a way that final dimensions are obtained in less than 1 min. A couple of filters have been built and measured to illustrate the theory. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1094–1097, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25135

Advances on circuit modeling of extraordinary transmission

Abstract: Extraordinary optical transmission (EOT) is a phenomenon which has received extraordinary attention since its discovery, twelve years ago. The phenomenon takes place at any frequency range — not only at the optical regime —, and it has been explained in terms of the excitation of surface waves along the periodically structured metal screens used in the experiments. EOT also takes place, however, through slit gratings for a polarization precluding the excitation of such surface waves. It can be also observed through electrically small diaphragms located inside hollow pipe waveguides. Along the last three years, some of the authors have developed models explaining the phenomenon in terms of simple concepts extracted from conventional waveguide theory. The models lead themselves to simple circuit equivalents which help the understanding of the underlying physics. This paper summarizes the main results obtained by the members of the group and some other collaborators concerning the capabilities of the proposed approach.

Analytical modelling and experimental verification of Fabry-Pérot resonances in multilayer sub-wavelength partially-reflecting surfaces

Abstract: Transmission through stacked perforated metallic screens (2-D metallic meshes) printed on thin dielectric layers is studied at microwave frequencies using simple circuit models. The physical mechanisms of the transmission resonances are clearly explained and the most relevant parameters of the circuit are accurately determined. The results of the circuit model are tested by a proper comparison with previously obtained numerical and experimental results. The observed transmission peaks and lower/upper frequency band edges of the pass-band are explained in terms of the behaviour of a finite number of strongly coupled Fabry-Perot resonators using the circuit model.