Titanium Foams Research Articles

P.097 Tensile properties of Polyvinyl Alcohol Cryogel (PVA-C) formulations and generation of a tissue mimicking artificial lumbar intervertebral disc prototype

Background: Current lumbar intervertebral disc prostheses provide suboptimal symptom relief with little natural load-cushioning. PVA-C is a promising biocompatible material, however previous studies from our lab show that it does not have adequate elastic modulus to mimic the annulus fibrosus. Here we present a prototype of an artificial lumbar intervertebral disc. Methods: The tensile properties of pure (5-35% PVA-C) and particle-reinforced (15% PVA-C with 5% of either Sephadex or hydroxyapatite) composite PVA-C formulations were evaluated. Simple tension and tensile stress relaxation tests were performed. Woven Teflon mesh was embedded in PVA-C and tested under compression. Endplate pull-out tests were performed. Results: Tensile testing showed that all PVA-C formulations behaved linearly for physiologic levels of strain (<20%). Tensile elastic modulus is an order of magnitude lower than the annulus fibrosus. Teflon has similar elastic modulus as collagen and compression of the hybrid Teflon-PVA-C construct revealed good biomechanical mimicry with elastic modulus of 20-25MPa at 20% deformation, similar to human data. Bonding between PVA-C and porous titanium endplate is excellent. Conclusions: A fiber-reinforced PVA-C impregnated composite adequately mimics the annulus fibrosus. Our prototype of a tissue mimicking artificial intervertebral disc utilizes a woven Teflon fiber with 20% PVA-C (+Hydroxyapatite) annulus and 5% pure PVA-C nucleus bonded to porous titanium foam endplates.

Processing and characterization of titanium dioxide grown on titanium foam for potential use as Li-ion electrode

This study investigates the processing and potential application of Ti foams to the anode of lithium-ion batteries (LIBs). Ti foam is successfully synthesized using a water-based freeze-casting process, and anatase titanium dioxide (TiO2) is formed on the surface of the Ti foam for application to the anode of LIB. The metallic Ti foam acts as a current collector “platform” with increased surface area and the TiO2 surface coating acts as an active anode material. Coin-cell test results show that the unique combination of the Ti foam and the TiO2 coating anode has highly stable cycling properties and can thus be considered promising for use as an advanced anode for LIBs that require high safety and stability. It is anticipated that the use of the unique Ti-foam-based electrode design will not only be limited to LIBs but also will be applied to other energy and environmental areas as a catalyst or filter.

Numerical modelling of the mechanical response of cellular solids made from sintered Titanium powders

Two algorithms are developed to generate virtual microstructures of Titanium foams of medium relative density, in the range 0.37 – 0.48; one of the algorithms captures the effect of pore shape and size distribution, while the other algorithm only accounts for the relative density of the material. Foams are modelled as two-phase composites consisting of a solid material and voids; the elasto-plastic and damage response of the parent material is deduced from its measured mechanical responses in tension and compression, while the material occupying the voids is assigned a plastically compressible constitutive response, to mimic the effects of pore collapse and subsequent self-contact. Finite Element (FE) simulations are conducted to predict the measured macroscopic material response in uniaxial tension and compression, as well as in pure shear; a mesh convergence study is performed and the minimum RVE size is determined. The simulations are found in good agreement with measurements and can predict accurately and effectively the material's mechanical response up to large strains, as well as the scaling of mechanical properties with relative density.

Compressive Behavior of Open-Cell Titanium Foams with Different Unit Cell Geometries

Mechanical properties of open-cell titanium foams with different cell geometries (truncated octahedron and rhombic dodecahedron cells) were examined through compressive tests. These foams were manufactured through the electron beam melting (EBM) process. The compressive behavior depends on the porosity, cell geometry and the cell orientation. Titanium foams with truncated octahedron cells showed high strength compared to those of rhombic dodecahedron cells. This is due to the short cell edges in truncated octahedron cells. In addition, the parallel and oblique cell edges against the compression direction are effective to increase the compressive strength. Macroscopic shear bands caused by ordered cell geometry were observed in some titanium foams.

Microstructure and Characteristics of Calcium Phosphate Layers on Bioactive Oxide Surfaces of Air-Sintered Titanium Foams after Immersion in Simulated Body Fluid.

We propose a simple and low-cost process for the preparation of porous Ti foams through a sponge replication method using single-step air sintering at various temperatures. In this study, the apatite-forming ability of air-sintered Ti samples after 21 days of immersion in simulated body fluid (SBF) was investigated. The microstructures of the prepared Ca–P deposits were examined by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy, and cross-sectional transmission electron microscopy (TEM). In contrast to the control sample sintered in vacuum, which was found to have the simple hexagonal α-Ti phase, the air-sintered samples contained only the rutile phase. High intensities of XRD peaks for rutile TiO2 were obtained with samples sintered at 1000 °C. Moreover, the air-sintered Ti samples had a greater apatite-forming ability than that of the Ti sample sintered in vacuum. Ti samples sintered at 900 and 1000 °C had large aggregated spheroidal particles on their surfaces after immersion in SBF for 21 days. Combined XRD, energy-dispersive X-ray spectroscopy, FTIR spectroscopy, and TEM results suggest that the calcium phosphate deposited on the rutile TiO2 surfaces consist of carbonated calcium-deficient hydroxyapatite instead of octacalcium phosphate.

Sustainable production of titanium foams for biomedical applications by Concentrated Solar Energy sintering

Porous titanium foams were successfully sintered using a solar parabolic concentrator, of vertical axis and 2kW power, in argon atmosphere. Green pellets were previously prepared following the space holder powder manufacturing route using NaCl as soluble spacer. Titanium foams were fully sintered at lower temperature and shorter time than in conventional furnace sintering. Moreover, porosity achieved ranges between 58% and 77%, with dual size range: larger porous ranging from 500 to 1000µm resulting from the NaCl particles; and smaller porous of 1–10µm due to the powder sintering. Both the porosity and the porous size obtained for titanium foams produced by solar sintering fulfil the requirements for load-bearing and scaffold applications in tissue engineering.

Characterization of Ti-Based Foams for Biomaterials Applications

Development of titanium foams with low elastic modulus has increased its scientific and technological relevance due to the evident need to avoid the stress shielding problems related to stiffness mismatch with respect to bone. In this work we studied the synthesis and characterization of foams of pure Ti which present high potential for biomedical applications. Foams were obtained using NaCl (40, 50 and 60 %v/v), with particle size ranging from 150 to 350 mm, as space-holder. Powders and space-holder were mixed together and compacted under 200 to 800 MPa stress. NaCl particles were removed from the green compacts by submerging samples in distilled water at 60 °C (5 cycles of 4h). The green compacts were sintered at 1300 °C for 3 h in Ar atmosphere. Ti-based foams were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, optical microscopy. In addition, electrochemical impedance spectroscopy (EIS) measurements were performed under potentiostatic conditions at the corrosion potential in Ringer solution. Results showed that Ti-based foams with tailored heterogeneous pore distribution may be obtained using space holder method. Using theoretical models the elastic modulus of foams was estimated between 10 and 25 GPa. The experimental EIS data revealed a CPE behavior of the foams for all exposure time studied. Not significant effect was observed in the impedance modulus at low frequency range (|Z|LF) for low compaction stress. However, the |Z|LF decreased markedly for higher compaction stress, which could be related to the increase of the stress raiser. From our results, we conclude that foams with 50 and 60 % of porosity are potential bone replacement materials due to their stiffness closer to human bone value. Ackowledgements: The authors are grateful to Fondecyt Grant 1160604 and 1161444

Strain rate sensitivity of open-cell titanium foam at elevated temperature

Compressive deformation of open-cell pure titanium foams with the porosity of 60% has been studied at different compression speeds from 0.1 to 10mm/min and temperatures from 300 to 673K. Stress-strain curves showed linear elastic, plateau and densification regions which are typical characteristics of metal foams. Both the flow stress and the energy absorption increase with increasing the strain rate and with decreasing the temperature. Strain rate sensitivity exponents of the titanium foam at 300, 473 and 673K were 0.018, 0.042 and 0.021, respectively. Apparent activation energy of the titanium foam was 72.6kJ/mol. These values are close to those of dense commercially pure titanium. These results indicate that the thermally activated kinetics of the titanium foam mainly depends on the characteristics of the base material. In addition, XRD analysis showed the excellent oxidation resistance of the titanium foam. Present study revealed that titanium foams have a potential of lightweight structural material even at elevated temperature up to 673K.

Open pore titanium foams via metal injection molding of metal powder with a space holder

Powder methods are highly applicable for the processing of more challenging metals and forms. Examples of materials that encompass both of these are metallic foams, which are advanced materials that consist of a network of interconnected or randomly spaced macropores separated by dense or microporous cell walls. These macropores can be either open or closed, or mix of those two, depending on the manufacturing process. One popular metal foam that has received a huge amount of interest in the last decade is Ti foam, due to it offering a unique combination of properties, such as high strength to weight ratio and high permeability combined with excellent biocompatibility. In this study the use of metal injection molding of titanium powder in combination with a space holder (to create large pore spaces) is examined for the production of open pore Ti foams for biomedical applications. Potassium chloride with two different particle shapes (spherical and cubic) was used as a space holder. It was found that feedstocks prepared with spherical KCl particles had a lower viscosity and better flowability compared to those made using cubic particles. Ti foams with a total porosity of 61.25% ± 0.29 were successfully produced. The structure of the foams produced was characterized using SEM and X-ray micro-computed tomography.

The Modified Maquet Procedure (MMP) in Dogs: Technical Development and Initial Clinical Experience.

The literature about tibial tuberosity advancement surgery in dogs and humans informed the development of a version of the operation using a wedge-shaped implant of titanium foam. Computer-assisted drawing and stereolithography was used to create instruments and implants that were evaluated by cadaver surgery. A trial, involving 26 client-owned dogs with lameness due to cranial cruciate ligament failure, was started. Follow-up was done by clinical and radiographic examination after 4 wk and clinical examination again 6-11 mo after surgery. The titanium foam implant maintained tibial tuberosity advancement easily and effectively. The same major complication occurred in 2 of the first 6 cases before, a slightly modified technique was used to treat 20 dogs without complication. At mid-term follow-up (6-11 mo), 20/26 dogs (77%) had returned to full function, two dogs (7.7%) had acceptable function, two dogs (7.7%) could not be evaluated due to recent contra lateral modified Maquet procedure surgery, and two (7.7%) dogs had died for reasons unrelated to the study. This is the first clinical report of the use of titanium foam in veterinary orthopaedics. Modified Maquet procedure appears to be an effective treatment for lameness due to failure of the cranial cruciate ligament in dogs.

Microstructure and Interconnections Characteristics of Titanium Foam

Titanium foams are widely used as biomaterials and potentially as a twin skinned, sandwich, structures for aerospace structures, filter or a catalyst or catalysts carrier for chemical reactions. The porosity is particularly important for tissues ingrowth and vascularity. Open porosity is essential in the case of flow-on machines. The distribution and size of pores is significant to achieve a uniform material effort and ensure to receive an appropriate hydraulic properties.The aim of this study was to determine the effect of titanium particle size and the amount of porogen on the microstructure and the size of pore interconnections in titanium foams made using saccharose as the space holder material.The paper characterizes titanium foam, made from the Grade 1 Ti powders (Alfa Aesar) with a particle sizes of 0.150 mm and 0.044 mm (separately) and spherical particles of saccharose (Pfeifer & Langen) having an average size of 0.7 ÷ 0.9 mm, as a porogen. There was prepared a mixture of powders of the proposed porosity of 50, 60 and 70%. Summarizing 6 mixtures were prepared. After sintering there were received specimens having a diameter of 8 mm and a height of 5 mm. Microstructure analysis was performed using the microtomography Skyscan 1172 (Bruker microCT) and the CTAn software (Bruker microCT).The results indicate the uniform pore distribution and size of the interconnections allowing high permeability.

Measurements and micro-mechanical modelling of the response of sintered titanium foams

Titanium foams of relative density in the range 0.35–0.50 are tested in quasi-static compression, tension and shear. The response is ductile in compression but brittle, and weaker, in shear and tension. Virtual foam microstructures are generated by an algorithm based on Voronoi tessellation of three-dimensional space, capable of reproducing the measured size distribution of the pores in the foam. Finite Element (FE) simulations are conducted to explore the mechanical response of the material, by analysing the elasto-plastic response of a statistical volume element (SVE). The simulations correctly predict the ductile compressive response and its dependence on relative density.

Microstructure and mechanical properties of open cellular Ti–6Al–4V prototypes fabricated by electron beam melting for biomedical applications

Titanium and titanium alloys with open cellular structures and foams possess low Young's modulus matching to human bone and the capability to provide space for bone tissue ingrowth to reach a better fixation, which have been thought as a good choice for the replacement of commercial dense implants. However, currently developed fabrication methods of titanium cellular alloys, such as powder sintering, reveal shortcomings like the low porosity and poor control over the size, shape and distribution of the pores. Recently, additive manufacturing using the electron beam melting method has been applied successfully to fabricate titanium cellular meshes and foams. Compared to other reported methods, this technique has the advantages of accurate control of internal pore architectures and complex cell shapes. In the present paper, the authors briefly review the fabrication, Young's modulus, mechanical properties and biocompatibility of Ti–6Al–4V cellular structures fabricated by electron beam melting techniques, along with future development trends.

Geometry anisotropy and mechanical property isotropy in titanium foam fabricated by replica impregnation method

Polyurethane (PU) foams have both geometry and mechanical property anisotropy. Metal foams, which are manufacturing by investment casting or melt deposition method and using PU foam as a template, also have mechanical property anisotropy. This work studied the mechanical properties in two directions of titanium foam with four different cell sizes fabricated using the replica impregnation method. The two directions are (1) the loading direction parallel to the foaming direction where the cells are elongated (EL direction) and (2) the loading direction perpendicular to the foaming direction where the cell are equiaxed (EQ direction). The results show that the compression responses for both EL and EQ directions are isotropy. Micrographs and X-ray micro-computed tomography show that the degree of geometry anisotropy is not strong enough to results in mechanical property anisotropy.

EMI shielding and thermal conductivity of a high porosity reticular titanium foam

A highly porous titanium foam with open cells was made by the process of impregnating polymer foam in slurry and then sintering, which displayed a three-dimensional reticular structure with interconnected pores. For this reticular titanium foam, the electromagnetic shielding performance was investigated in the frequency range of 0.3–3000MHz (covering that of radio waves). The experimental results indicated the porous samples displayed an evident effectiveness of electromagnetic shielding, especially a good performance at low frequencies. The shielding effectiveness presented an overall tendency of decrease with the increase of the frequency, and the corresponding curve shows a fluctuating region when the frequency is higher than some value. In this region, the shielding effectiveness fluctuated around an average value. In the whole range of testing frequency, the shielding effectiveness could be remarkably enhanced when the thickness increased for the sample with the same pore-parameter. In addition, this titanium foam exhibited a relatively low thermal conductivity, which was measured to be about 0.4–0.8W/(m·K) at room temperature for the sample with the porosity of 87%–89%. Besides, the thermal conductivity showed a greatly decreasing tendency when increasing the porosity of the sample.

Volume change of macropores of titanium foams during sintering

The porosity of titanium foams obtained from the space holder technique was theoretically analyzed in the cases of volume shrinking, retaining and expanding during sintering. The relationship between porosity and spacer content was compared under different conditions. The kind of volume change of macropores during sintering was discussed. The results indicate that the relationship between porosity and spacer content depends on the decreased volume of macropores and the volume of micropores in cell-walls in the first case, while the porosity will be greater than the spacer content for the other two cases. It proves that the volume change of macropores during sintering decreases based on theory and practice.

Characterization of the electromagnetic shielding and compressive behavior of a highly porous titanium foam with spherical pores

Abstract

Microstructure and mechanical properties of commercially pure titanium foam with varied cell size fabricated by replica impregnation method

The most common sacrificial material for producing metal foam using the replica impregnation method is polyurethane foam (PU) since it is readily available with different cell sizes. This work studied the microstructure and mechanical properties of commercially pure titanium with varied cell sizes. Titanium foams were fabricated using PU foam with four different cell sizes as templates. The results show that as the number of pores per inch (ppi) increased, the apparent density increased, i.e. the cell size decreased, the porosity decreased and the load bearing capacity increased nonlinearly. This work agreed with the scaling law of Gibson and Ashby for determination of the limit stress. However, the Gibson and Ashby model significantly over-predicted the value for quasi-elastic modulus.

Investigation on relationship between porosity and spacer content of titanium foams

Abstract Titanium foam has been obtained more and more attention due to its combination with porous structure and titanium. The space holder technique was widely used to fabricate this kind of material because of its low cost, easy operation and freely controllable pore structure and properties. However, how to obtain the desired porosities has been a big challenge for this technique, because they are always not equal to the expected ones. Hence, the relationship between porosity (P) and spacer content (Sc) was studied based on theory and practice in this paper. The results show that the relationship between porosity and spacer content was linear, that is P = aSc + b. It indicates that the volumetric change rate of pores (VR) is a constant and its value altered in different preparing conditions. Through the research of this paper, the porosity and mechanical properties of a foam material can be predicted by spacer content.

Analysis of clinically relevant mechanical and thermal characteristics of titanium foam spinal implants during drilling.

Although high biocompatibility promotes the use of titanium (Ti) alloy in spinal implants, this material shows high stiffness, which is an issue for removal by drilling. The recently developed, porous Ti foam implants, which have shown enhanced osteoformation, may overcome this flaw. Thus, this study aimed to compare the mechanical and thermal characteristics of Ti-foam (80% porosity) and conventional Ti alloy (0% porosity) implants drilled in clinically relevant conditions. Mechanical properties were analyzed by measuring axial and torque forces using a pressure sensor with a drill of 2.5-mm diameter at a rotation frequency of 20Hz. Thermography was used to evaluate the heat generated by a diamond burr attached to a high-speed (80,000rpm) drill. The torque and axial strengths of Ti foam (13.63±1.43 and 82.60±7.78N, respectively) were significantly lower (P=0.001) than those of Ti alloy (73.58±13.60 and 850.72±146.99N, respectively). Furthermore, irrigation reduced the area of local heating for Ti foam to 56-82% of that for Ti alloy, indicating lower thermal conductivity. These data suggest that the use of Ti foam implants may be advantageous in cases with a probability of implant drilling in the future.