Space Holder Method Research Articles

Deformation mechanisms of spherical cell porous aluminum under quasi-static compression

Quasi-static compression tests for a novel type of spherical cell porous aluminum prepared by the space-holder method were conducted. Formation and evolution mechanisms of deformation bands on the macro and micro scale were monitored and discussed by using a digital image correlation procedure. Effect of micro strain concentrations on the formation process and morphology of deformation bands at the macro level was analyzed in detail. In total three deformation modes of spherical cells and four failure modes of cell membranes including bending, shear, combined compression-shear, and eventually tearing were identified.

Novel nanostructured baghdadite-vancomycin scaffolds: In-vitro drug release, antibacterial activity and biocompatibility

One of the most important therapeutic and economic concerns regarding surgery, is the occurrence of post-operative infections which leads to an increase in premature failure rate. Therefore, novel nanostructured baghdadite-vancomycin (Ba-Vac) scaffolds were prepared using the space holder method with good mechanical properties and controlled drug release to inhibit post-surgery infections. The results showed that the (Ba-Vac) scaffolds were attained with the pore size of 300–400µm and total porosity of 80–82% with compressive strength of 0.86–0.88MPa. In drug release profiles, a burst release was observed for 6h, followed by a sustained release. Ba-Vac scaffolds presented good antibacterial activity toward Staphylococcus aureus (S. aureus). More attachment and spreading of MG-63 osteoblast cells on the Ba and Ba-(1-3wt%)Vac scaffolds was also observed in comparison with the Ba-5wt%Vac scaffold. Therefore, the Ba-(1–3wt%)Vac scaffold is a good candidate for inhibiting post-surgery infections, as well as for bone tissue engineering.

A survey of fabrication and application of metallic foams (1925–2017)

This review focuses on contemporary metal foams, their fabrication techniques and commercial applications. Different type of foam synthesis techniques like open cell and closed cell foams using solid, liquid, ions or vapor metals are discussed along with their characteristic process parameters. A vast body of research has been established for producing metallic foams for many decades. Space holder methods, sponge replication method, vapor deposition method, hot isostatic pressing as well as powder metallurgy process have been extensively reported in available literature. Techniques like sponge replication have the capability to produce up to 98% porosity with considerable amount of strength. However, metallic foam fabrication of certain metals still poses many challenges, which need to be explored. Metallic foams have enormous industrial applications, like closed cell foams have structural applications, open cell foams can be used for structural as well as biomedical applications. Often open cell metal foams have encountered low fatigue strength that might need alloying as well heat treatment operations for strengthening. Moreover, it is important to understand the physics and techniques of foaming for fabricating a good quality foam, which is also discussed in this article. The focus of the review finally shifts to open cell metallic foams for biomedical applications.

Measurement of tortuosity of porous Cu using a diffusion diaphragm cell

Open-cell porous metals have many potential applications, such as heat exchangers and flow batteries, due to good permeability for fluid flow. Porous metals manufactured by space holder methods have controllable porous structure, but the fluid pathways and channels are tortuous. The tortuosity can directly influence the fluid permeability and turbulence, which in turn affect the performance of the porous metal. This paper measured the tortuosity of porous Cu, manufactured by Lost Carbonate Sintering (LCS), directly by a diffusion method using a diaphragm cell. The tortuosity of the samples with pore sizes 250–1500µm and porosities 0.56–0.84 was in the range of 1.33–1.78. It decreased with porosity and increased with pore size. The measured values were comparable to those reported in literature, obtained by the acoustic absorption method.

Mass Transfer Performance of Porous Nickel Manufactured by Lost Carbonate Sintering Process

Open cell porous metals are excellent electrode materials due to their unique electrochemical properties. However, very little research has been conducted to date on the mass transport of porous metals manufactured by the space holder methods, which have distinctive porous structures. This paper measures the mass transfer coefficient of porous nickel manufactured by the Lost Carbonate Sintering process. For porous nickel samples with a porosity of 0.55–0.75 and a pore size of 250–1500 μm measured at an electrolyte flow velocity of 1–12 cm s−1, the mass transfer coefficient is in the range of 0.0007–0.014 cm s−1, which is up to seven times higher than that of a solid nickel plate electrode. The mass transfer coefficient increases with pore size but decreases with porosity. The porous nickel has Sherwood numbers considerably higher than the other nickel electrodes reported in the literature, due to its high real surface area and its tortuous porous structure, which promotes turbulent flow.

Controlled gentamicin- strontium release as a dual action bone agent: Combination of the porous titanium scaffold and biodegradable polymers

Titanium is considered a promising biomaterial for load bearing implantation in orthopedics. However, its biologically inert behavior makes the surface treatment essential to improve the bioactivity. In this study, a porous titanium scaffold was fabricated using the space holder method and its surface was treated by the direct oxidation method. Strontium containing gelatin micro spheres (SrGMS) were synthesized using the water in oil method and deposited on the surface of the surface treated porous titanium scaffold (Ti-SrGMS). A thin layer of gentamicin containing poly vinyl alcohol (PVA) was deposited on the surface of Ti-SrGMS for two purposes: first, to postpone strontium release and second, to initially release gentamicin. Raman spectroscopy revealed rutile formation due to the thermal oxidation of titanium at 600 °C. Scanning electron microscope observation also indicated that SrGMS particles were deposited uniformly on the surface of the titanium scaffold. Fourier transform infrared (FTIR) spectra showed that strontium ion was loaded in the gelatin micro sphere successfully. In vitro drug release measurement using inductively coupled plasma-mass spectrometer (ICP-MS) also revealed that by using a layer of PVA with different concentrations on SrMGS particles, it would be possible to control the strontium release as it was needed for the surrounding tissue.

Numerical Simulation of Heat Transfer in Porous Metals for Cooling Applications

Porous metals have low densities and novel physical, mechanical, thermal, electrical, and acoustic properties. Hence, they have attracted a large amount of interest over the last few decades. One of their applications is for thermal management in the electronics industry because of their fluid permeability and thermal conductivity. The heat transfer capability is achieved by the interaction between the internal channels within the porous metal and the coolant flowing through them. This paper studies the fluid flow and heat transfer in open-cell porous metals manufactured by space holder methods by numerical simulation using software ANSYS Fluent. A 3D geometric model of the porous structure was created based on the face-centered-cubic arrangement of spheres linked by cylinders. This model allows for different combinations of pore parameters including a wide range of porosity (50 to 80 pct), pore size (400 to 1000 µm), and metal particle size (10 to 75 µm). In this study, water was used as the coolant and copper was selected as the metal matrix. The flow rate was varied in the Darcian and Forchheimer’s regimes. The permeability, form drag coefficient, and heat transfer coefficient were calculated under a range of conditions. The numerical results showed that permeability increased whereas the form drag coefficient decreased with porosity. Both permeability and form drag coefficient increased with pore size. Increasing flow rate and decreasing porosity led to better heat transfer performance.

Preparing diopside nanoparticle scaffolds via space holder method: Simulation of the compressive strength and porosity

In the present study, diopside nanopowders were prepared via mechanical milling with eggshell as the calcium source. The space holder method (compaction of ceramic powder and spacer) as one of the most important methods to produce ceramic/metal scaffolds was used to produce diopside scaffolds. For the first time, the effect of the spacer size on mechanical properties and porosity of the obtained scaffolds was experimentally discussed. According to the results obtained, the NaCl particles (as the spacer) with the size of 400–600µm maintained their original spherical shape during the compaction and sintering processes. As a new work, the most important parameters including the spacer type, spacer concentration, spacer size, and applied pressure were considered, and their effects on mechanical properties and porosity of diopside scaffolds were simulated. Gene Expression Programming (GEP), as one of the most branches of the artificial intelligence, was used for simulation process. By using the GEP, two equations were introduced to predict the compressive strength and porosity of the obtained scaffolds with the lowest error values. The 3D diagrams extracted from the model were used to evaluate the combined effect of the process parameters on the compressive strength and porosity of the scaffolds. The GEP model presented in this work has a very low level of error and a high level of the squared regression for predicting the compressive strength and porosity of diopside scaffolds.

The Impact of Composition and Sintering Temperature for Stainless Steel Foams (SS316L) Fabricated by Space Holder Method with Urea as Space Holder

Metal foams are a cellular structure that has a solid matrix made of metal and has pores in their structure. Metal foams offer excellent combination of properties which led researchers interested in investigation in recent years. Closed-cell stainless steel (SS316L) foams for biomedical application were prepared by space holder method and the physical and morphological properties of SS316L foams were studied. Stainless steel (SS316L) powders as metallic material, polyethylene glycol (PEG) as a binder and Urea as a space holder material were mixed homogenously to avoid the particle wrecked. This mixture was compacted using uniaxial pressing machine and pressurized to 8 tons to formed the green body. By using tube furnace, the SS316L foams was two-stage sintered, the first phase at 600°C for 2 hours to decompose the urea, and the second phase at 1000°C, 1100°C, and 1200°C respectively to sinter the steel. The porosity and density test was carried out by applying Archimedean principles, while morphological observation was done by using Field Emission Scanning Electron (FESEM). The samples with 40wt.% SS316L composition and sintered at temperature of 1100°C, leads to porosities of about 44.539% and show the potential as the best metal foams.

Elevated temperature compressive properties and energy absorption response of in-situ grown CNT-reinforced Al composite foams

Carbon nanotube (CNT) reinforced Al composite foams were successfully fabricated by the combination of an in-situ chemical vapor deposition (CVD), short-time ball-milling and space-holder method. The CNTs are homogeneously dispersed and embedded in the Al foam matrix after 90min ball-milling while maintaining the structural integrity. Both compressive properties and energy absorption capacity of the composite foams increase with the increment of CNT content but decrease with the temperature rising between 25 and 250°C. The compressive yield strength and the plateau stress of 3.0wt%-CNT/Al composite foams maintain 16.8 and 20.2MPa at 150°C, respectively, which are much higher than the corresponding yield stress (5.7MPa) and plateau stress (8.6MPa) of the pure Al foam. Especially, the energy absorption capacity of the 3.0wt%-CNT/Al composite foams reaches 19.8 MJ/m3 at 150°C, which is ~2.5 times higher than that of pure Al foam. Fracture analysis shows that the failure mode of the Al foam changes from ductile type to brittle type combined with ductile type, as a result of the CNT addition in the matrix.

Diametral compression behavior of biomedical titanium scaffolds with open, interconnected pores prepared with the space holder method

Scaffolds with open, interconnected pores and appropriate mechanical properties are required to provide mechanical support and to guide the formation and development of new tissue in bone tissue engineering. Since the mechanical properties of the scaffold tend to decrease with increasing porosity, a balance must be sought in order to meet these two conflicting requirements. In this research, open, interconnected pores and mechanical properties of biomedical titanium scaffolds prepared by using the space holder method were characterized. Micro-computed tomography (micro-CT) and permeability analysis were carried out to quantify the porous structures and ascertain the presence of open, interconnected pores in the scaffolds fabricated. Diametral compression (DC) tests were performed to generate stress-strain diagrams that could be used to determine the elastic moduli and yield strengths of the scaffolds. Deformation and failure mechanisms involved in the DC tests of the titanium scaffolds were examined. The results of micro-CT and permeability analyses confirmed the presence of open, interconnected pores in the titanium scaffolds with porosity over a range of 31–61%. Among these scaffolds, a maximum specific surface area could be achieved in the scaffold with a total porosity of 5–55%. DC tests showed that the titanium scaffolds with elastic moduli and yield strengths of 0.64–3.47GPa and 28.67–80MPa, respectively, could be achieved. By comprehensive consideration of specific surface area, permeability and mechanical properties, the titanium scaffolds with porosities in a range of 50−55% were recommended to be used in cancellous bone tissue engineering.

Fabrication and characterization of baghdadite nanostructured scaffolds by space holder method

Porous baghdadite scaffold has received great attention as a candidate for bone tissue engineering application due to its remarkable bioactivity, biocompatibility, and good bone formation ability. A few studies have been focused on improving the mechanical properties of baghdadite scaffolds. Recently, space holder method has been introduced as a new and viable technique to prepare bioceramic scaffolds with interconnected pores and suitable mechanical properties. In this study, for the first time, 3D baghdadite scaffolds with interconnected porosity were produced using space holder method. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were utilized to characterize various specimens. The baghdadite scaffolds were sintered at various temperatures in the range of 1250–1350°C for 3h. The compressive strength and compressive modulus measured to be in the range of 0.05–0.52MPa and 2.1–121.5MPa, respectively. The results showed that nanostructured baghdadite scaffolds with a crystallite size of about 32nm, 75% porosity and pores size in the range of 200–500µm can be successfully fabricated after sintering at 1350°C for 3h. Simulated body fluid (SBF) was used to evaluate the apatite formation ability of the scaffolds. The results showed the formation of an apatite layer on the scaffold surface which can be considered as a bioactivity criterion.

On the synthesis of nanostructured akermanite scaffolds via space holder method: The effect of the spacer size on the porosity and mechanical properties

In the present study, for the first time, the space holder method was used to prepare akermanite scaffolds with high porous structures, high interconnectivity, and high compressive strength, while the role of different spacer sizes on the akermanite scaffold properties was also evaluated. The results showed that the increase in the NaCl particle size which was used as spacer leads to an increase of the pore size and interconnectivity and a decrease of compressive strength. When the size of the spacer was 420–600µm and more than 600µm, a total porosity of 82 and 83% and a compressive strength of 0.86 and 0.82MPa were obtained, respectively. These values are higher than those reported in previously studies and provide a great potential for akermanite to be used as bone substitute in tissue engineering. The in vitro bioactivity of the obtained akermanite scaffolds was also investigated.

The improvement of corrosion resistance, biocompatibility and osteogenesis of the novel porous Mg-Nd-Zn alloy.

A magnesium scaffold is a promising biodegradable bone repair material. However, its poor corrosion resistance limits its clinical application. In this study, we improved the corrosion resistance, biocompatibility and osteointergration ability of magnesium by alloying it with neodymium (Nd) and zinc (Zn), then fabricated a novel open-porous Mg-Nd-Zn (P-MNZ) alloy using a titanium wire space holder (TWSH) method. An in vitro corrosion experiment showed that corrosion resistance was enhanced. Both in vitro and in vivo experiments were performed to evaluate the biocompatibility and osteointergration ability of P-MNZ. The cell viability, cytoskeleton staining, ALP activity and osteogenic related gene expression confirm that the P-MNZ scaffold exhibits better biocompatibility and osteoblast differentiation properties in vitro. As for the in vivo experiment, the analysis of micro-CT scanning, Van Gieson staining and sequential polychrome labelling demonstrated that P-MNZ stimulates new bone formation and enhances the corrosion resistance of the P-MNZ scaffold. The results indicate that the P-MNZ alloy is a promising biodegradable bone repair material.

Open-Cell Metallic Porous Materials Obtained Through Space Holders—Part II: Structure and Properties. A Review

This work presents an overview of structural characteristics and basic mechanical properties of the open-cell metallic foams obtained by different space-holder methods, which are discussed in Part I of the same review. The presentation is arranged with respect to foam material, and the structure and properties are compared for different space holders and production techniques. In order to have more clear information for the structures obtained and their relation with production techniques, many images are provided and discussed. Compressive behavior of the foams is shown, and stress–strain curves are analyzed with respect to the energy absorption characteristics. The analysis are made on the basis of different porosities and processing parameters. Some applications of the open-cell metallic foams are discussed in the end of the article.

Synthesis, characterization and in vitro behavior of nanostructured diopside/biphasic calcium phosphate scaffolds

A significant challenge in bone tissue engineering is the development of 3D constructs serving as scaffolds to fill bone defects, support osteoblasts, and promote bone regeneration. In this paper, highly porous (∼79%) nanostructured diopside/biphasic calcium phosphate (BCP) scaffolds with interconnected porosity were developed using various diopside contents via space holder method. X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques were utilized to evaluate different samples. Furthermore, the effects of scaffold composition on mechanical properties, bioactivity, biodegradability, and cytotoxicity were studied as well. The results showed that the produced scaffolds had an average pore size and density of 200–340 μm and 2.5 ± 0.3–1.8 ± 0.3 gr/cm3, respectively, depending on the diopside content. Besides, increasing the diopside content of scaffolds from 0 to 15 wt% enhanced the bioactivity, biodegradability, and compressive strength from 1.2 ± 0.2 to 3.2 ± 0.3 MPa, respectively. In addition, MTT assay also confirmed that the BCP15 scaffold (containing 15 wt% diopside) significantly promoted cell viability and cell adhesion compared to BCP0 scaffold. Overall, our study suggests that nanostructured diopside/BCP scaffolds with improved biological and mechanical properties could potentially be used for bone tissue engineering application.

Fabrication and evaluation of silica-based ceramic scaffolds for hard tissue engineering applications

In recent decades, bone scaffolds have received a great attention in biomedical applications due to their critical roles in bone tissue regeneration, vascularization, and healing process. One of the main challenges of using scaffolds in bone defects is the mechanical strength mismatch between the implant and surrounding host tissue which causes stress shielding or failure of the implant during the course of treatment. In this paper, space holder method was applied to synthesize diopside/forsterite composite scaffolds with different diopside content. During the sintering process, NaCl, as spacer agent, gradually evaporated from the system and produced desirable pore size in the scaffolds. The results showed that adding 10wt.% diopside to forsterite can enormously improve the bioactivity, biodegradability, and mechanical properties of the composite scaffolds. The size of crystals and pores of the obtained scaffolds were measured to be in the range 70–100nm and 100–250μm, respectively. Composite scaffolds containing 10wt.% diopside showed similar compressive strength and Young's modulus (4.36±0.3 and 308.15±7MPa, respectively) to that of bone.

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

Anisotropic Porous Ti6Al4V Alloys Fabricated by Diffusion Bonding: Adaption of Compressive Behavior to Cortical Bone Implant Applications

In this work, porous Ti6Al4V alloys with 30%–70% porosity for biomedical applications were fabricated by diffusion bonding of alloy meshes. Pore structure was characterized by Micro-CT and SEM. Compressive behavior in the out-of-plane direction and biocompatibility with cortical bone were studied. The results reveal that the fabricated porous Ti6Al4V alloys possess anisotropic structure with square pores in the in-plane direction and elongated pores in the out-of-plane direction. The average pore size of porous Ti6Al4V alloys with 30%–70% porosity is in the range of 240–360 µm. By tailoring diffusion bonding temperature, aspect ratio of alloy meshes and porosity, porous Ti6Al4V alloys with different compressive properties can be obtained, for instance, Young's modulus and yield stress in the ranges of 4–40 GPa and 70–500 MPa, respectively. Yield stress of porous Ti6Al4V alloys fabricated by diffusion bonding is close to that of alloys fabricated by rapid prototyping, but higher than that of fabricated by powder sintering and space-holder method. Diffusion bonding temperature has some effects on the yield stress of porous Ti6Al4V alloys, but has a minor effect on the Young's modulus. The relationship between compressive properties and relative density conforms well to the Gibson–Ashby model. The Young's modulus is linear with the aspect ratio, while the yield stress is linear with the square of aspect ratio of alloy meshes. Porous Ti6Al4V alloys with 60%–70% porosity have potential for cortical bone implant applications.

Rapidly sintering of interconnected porous Ti-HA biocomposite with high strength and enhanced bioactivity

In this work, interconnected porous Ti-HA biocomposites with enhanced bioactivity, high porosity and compressive strength were prepared by spark plasma sintering (SPS) and space holder method. Pore characteristics, mechanical properties, corrosion behaviors and in vitro bioactivity of the porous Ti-HA were investigated. Results showed that porous Ti-HA with 5–30wt% HA contents possessed not only low elastic modulus of 8.2–15.8GPa (close to that of human bone) but also high compressive strength (86–388MPa). Although the HA partially decomposed and formed secondary phases, the sintered porous Ti-HA can still be good bioactivity. The homogeneity and the thickness of apatite layer increased significantly with the increase of HA. But with the thickness of apatite layer increased, micro-cracks appeared on the surface of porous Ti-30%HA. A model was built to discuss the current distribution and sintering mechanism of HA on Ti matrix during SPS process. It indicated that the excessive addition of HA would deteriorate the sintering quality, thus decreasing the mechanical properties and corrosion resistance. However, the combination of interconnected pore characteristics, low elastic modulus, high compressive strength and enhanced bioactivity might make porous Ti-HA biocomposites prepared by SPS a promising candidate for hard tissue implants.