Gregory E. Hilmas

Mechanical properties of bioactive glass (13-93) scaffolds fabricated by robotic deposition for structural bone repair

There is a need to develop synthetic scaffolds to repair large defects in load-bearing bones. Bioactive glasses have attractive properties as a scaffold material for bone repair, but data on their mechanical properties are limited. The objective of the present study was to comprehensively evaluate the mechanical properties of strong porous scaffolds of silicate 13-93 bioactive glass fabricated by robocasting. As-fabricated scaffolds with a grid-like microstructure (porosity 47%, filament diameter 330μm, pore width 300μm) were tested in compressive and flexural loading to determine their strength, elastic modulus, Weibull modulus, fatigue resistance, and fracture toughness. Scaffolds were also tested in compression after they were immersed in simulated body fluid (SBF) in vitro or implanted in a rat subcutaneous model in vivo. As fabricated, the scaffolds had a strength of 86±9MPa, elastic modulus of 13±2GPa, and a Weibull modulus of 12 when tested in compression. In flexural loading the strength, elastic modulus, and Weibull modulus were 11±3MPa, 13±2GPa, and 6, respectively. In compression, the as-fabricated scaffolds had a mean fatigue life of ∼10(6) cycles when tested in air at room temperature or in phosphate-buffered saline at 37°C under cyclic stresses of 1-10 or 2-20MPa. The compressive strength of the scaffolds decreased markedly during the first 2weeks of immersion in SBF or implantation in vivo, but more slowly thereafter. The brittle mechanical response of the scaffolds in vitro changed to an elasto-plastic response after implantation for longer than 2-4weeks in vivo. In addition to providing critically needed data for designing bioactive glass scaffolds, the results are promising for the application of these strong porous scaffolds in loaded bone repair.

Fabrication of 13-93 bioactive glass scaffolds for bone tissue engineering using indirect selective laser sintering.

Bioactive glasses are promising materials for bone scaffolds due to their ability to assist in tissue regeneration. When implanted in vivo, bioactive glasses can convert into hydroxyapatite, the main mineral constituent of human bone, and form a strong bond with the surrounding tissues, thus providing an advantage over polymer scaffold materials. Bone scaffold fabrication using additive manufacturing techniques can provide control over pore interconnectivity during fabrication of the scaffold, which helps in mimicking human trabecular bone. 13-93 glass, a third-generation bioactive material designed to accelerate the bodys natural ability to heal itself, was used in the research described herein to fabricate bone scaffolds using the selective laser sintering (SLS) process. 13-93 glass mixed with stearic acid (as the polymer binder) by ball milling was used as the powder feedstock for the SLS machine. The fabricated green scaffolds underwent binder burnout to remove the stearic acid binder and were then sintered at temperatures between 675 °C and 695 °C. The sintered scaffolds had pore sizes ranging from 300 to 800 µm with 50% apparent porosity and an average compressive strength of 20.4 MPa, which is excellent for non-load bearing applications and among the highest reported for an interconnected porous scaffold fabricated with bioactive glasses using the SLS process. The MTT labeling experiment and measurements of MTT formazan formation are evidence that the rough surface of SLS scaffolds provides a cell-friendly surface capable of supporting robust cell growth.

Aqueous‐based freeze‐form extrusion fabrication of alumina components

Abstract : Fabricating ceramic materials into complex 3-D components is typically a complicated, costly, and time-consuming process that involves diamond machining to achieve its final shape. In most cases, the initial processing is powder-based, followed by densification (sintering) at elevated temperatures. Only a few circumstances, such as fuse casting and thermal spraying, can ceramics be directly fabricated into near net-shape, fully dense ceramic components. However, these techniques require an extremely high temperature to melt the ceramic.

Freeze-form extrusion fabrication of ceramic parts

A novel, environmentally friendly solid freeform fabrication method called freeze-form extrusion fabrication (FEF) has been developed for the fabrication of ceramic-based components. The method is based on deposition of ceramic pastes using water as the media. The ceramic solids loading can be 50 vol.% or higher and initial studies have focused on the use of aluminum oxide (Al2O3). The FEF system components and their interaction are examined, and the main process parameters affecting part geometry defined. Three-dimensional shaped components have been fabricated by extrusion deposition of the ceramic paste in a layer-by-layer fashion. The feasibility of this process has been demonstrated by building components having a simple geometry, such as cylinders and solid or hollow cones. Hollow cones have also been fabricated to demonstrate the ability to build structures with sloped walls.

Investigation of laser sintering for freeform fabrication of zirconium diboride parts

This paper presents a study wherein the Laser Sintering (LS) process is used to perform additive manufacturing of zirconium diboride (ZrB2) parts. Experiments were conducted to determine values of LS process parameters (laser power, scan speed, energy density, line spacing, and layer thickness) that can be used to improve building of ZrB2 parts. A sacrificial plate with a proper number of layers was first constructed in order to reduce thermal gradients in building the main part. The sacrificial plate was found to eliminate cracks that would otherwise occur in the bottom of the main part. The fabricated green parts went through post-processing steps, including binder burnout and sintering at appropriate heat treatment schedules, to remove the binder and sinter the ZrB2 particles. The test bars after sintering had an average relative density of 87% and an average flexural strength of 250 MPa.

Rheological behavior of coextruded multilayer architectures

Utilizing a thermoplastic extrusion process, a multilayered architecture was fabricated. Thermoplastic blends of 55 vol% X7R dielectric and 50 vol% nickel powder were prepared by high shear mixing. Sheets pressed from this material were cut, stacked, and laminated to produce multilayered blocks. The blocks were extruded through a slotted spinneret to reduce layer thickness. The relation between viscosity and shear rate is relatively well understood for two- or three-layered polymer coextrusion. This behavior has not been studied for heavily loaded multi-component systems, such as might be used for MLCCs and other multilayered devices. A correlation was observed between the flow behavior during extrusion and that observed during mixing. Results show how control of the rheological behavior of highly loaded systems can control extrusion defects.

Oxidation of ZrB2 and ZrB2-SiC Ceramics with Tungsten Additions

Abstract : The effect of tungsten additions on the oxidation behavior of zirconium diboride-based ceramics was studied. Four mole percent tungsten carbide was added to ZrB2. The oxidation behavior was studied using thermal gravimetric analysis and isothermal testing in flowing air. Upon heating to 1500 degrees C, the mass gain decreased from approx. 14 milligrams/(square centimeter) for nominally pure ZrB2 to approx. 4.5 milligrams/(square centimeter) for tungsten containing ZrB2. After heating to 1500 degrees C for three hours, the scale thickness on nominally pure ZrB2 was approx. 500 micrometers compared to approx. 100 micrometers for tungsten containing ZrB2. Tungsten additions improved the oxidation resistance of ZrB2 by modifying the morphology of the ZrO2 scale by liquid phase sintering, making it a better barrier to oxygen diffusion.

In vitro assessment of laser sintered bioactive glass scaffolds with different pore geometries

Purpose – The purpose of this paper is to utilize the selective laser sintering (SLS) process to fabricate scaffolds with complex pore shapes and investigate the effects of pore geometry in vitro. The pore geometry of scaffolds intended for use in bone repair is one of the most important parameters used to determine the rate of bone regeneration. Design/methodology/approach – Scaffolds with five different architectures, having approximately 50 per cent porosity, were fabricated with silicate (13–93) and borate (13–93B3)-based bioactive glasses using the SLS process. An established late-osteoblasts/early-osteocytes cell line was used to perform cell proliferation tests on the scaffolds. The cell-seeded scaffolds were incubated for two, four and six days followed by MTT assay to quantify the metabolically active cells. Findings – The results indicated that the cells proliferate significantly more on the scaffolds which mimic the trabecular bone architecture compared to traditional lattice structures. The su...

Computational study of micromechanical damage behavior in continuous fiber-reinforced ceramic composites

A comprehensive numerical analysis of micromechanical damage behavior in a continuous fiber-reinforced ceramic composite is presented. A three-dimensional micromechanical finite element modeling procedure is developed for effective elastic property estimation and damage evaluation by the example of a composite consisting of a silicon carbide matrix unidirectionally reinforced with silicon carbide fiber (SiC/SiCf). The effect of a fiber/matrix interface on predicted elastic properties of the SiC/SiCf composite is considered. Representative volume element (RVE) models are developed for an SiC/SiCf composite with damageable interfaces. Statistically equivalent RVE models with randomly distributed fibers are generated using a developed algorithm. The statistical variability of fiber and matrix strengths is considered in developing RVE models and assumed to follow a Weibull probability law. A user-material subroutine with an adaptive material constitutive law is developed to predict damage behavior in the RVE. The predicted uniaxial stress versus strain behavior and damage in the composite are discussed.

Modeling of Thermal and Mechanical Behavior of ZrB2-SiC Ceramics after High Temperature Oxidation

The effects of oxidation on heat transfer and mechanical behavior of ZrB2-SiC ceramics at high temperature are modeled using a micromechanics based finite element model. The model recognizes that when exposed to high temperature in air ZrB2-SiC oxidizes into ZrO2, SiO2, and SiC-depleted ZrB2 layer. A steady-state heat transfer analysis was conducted at first and that is followed by a thermal stress analysis. A “global-local modeling” technique is used combining finite element with infinite element for thermal stress analysis. A theoretical formulation is developed for calculating the thermal conductivity of liquid phase SiO2. All other temperature dependent thermal and mechanical properties were obtained from published literature. Thermal stress concentrations occur near the pore due to the geometric discontinuity and material properties mismatch between the ceramic matrix and the new products. The predicted results indicate the development of thermal stresses in the SiO2 and ZrO2 layers and high residual stresses in the SiC-depleted ZrB2 layer.