Preparation of mullite bonded porous SiC ceramics by an infiltration method

A powder compact of α-SiC and α-Al₂O₃ was infiltrated with a liquid precursor of SiO₂, which on subsequent heat treatment at 1500 °C produced a mullite bonded porous SiC ceramics. Results showed that infiltration rate could be estimated by using weight gain measurements and theoretical analysis. The bond phase was composed of needle-shaped mullite which was observed to be grown from a siliceous melt formed during the process of oxide bonding. The porous SiC ceramics exhibited a density and porosity of 2 g cm⁻³ and 30 vol%, respectively, and also a pore size distribution in a range of 2–15 μm with an average pore size of 5 μm. No appreciable degradation of room temperature flexural strength (51 MPa) was observed at high temperatures (1100 °C).     

Processing and compression testing of Ti6Al4V foams for biomedical applications

Open cell Ti6Al4V foams (60% porosity) were prepared at sintering temperatures between 1,200 and 1,350 °C using ammonium bicarbonate particles (315–500 μm) as space holder. The resulting cellular structure of the foams showed bimodal pore size distribution, comprising macropores (300–500 μm) and micropores (1–30 μm). Compression tests have shown that increasing sintering temperature increased the elastic modulus, yield and compressive strength, and failure strain of foams. The improvements in the mechanical properties of foams prepared using smaller size Ti64 powder with bimodal particle distribution were attributed to the increased number of sintering necks and contact areas between the particles. Finally, the strength of foams sintered at 1,350 °C was found to satisfy the strength requirement for cancellous bone replacement.     

Mechanical and microstructural properties of cordierite-bonded porous SiC ceramics processed by infiltration technique using various pore formers

Cordierite-bonded porous SiC ceramics were prepared by air sintering of cordierite sol infiltrated porous powder compacts of SiC with graphite and polymer microbeads as pore-forming agents. The effect of sintering temperature, type of pore former and its morphology on microstructure, mechanical strength, phase composition, porosity and pore size distribution pattern of porous SiC ceramics were investigated. Depending on type and size of pore former, the average pore diameter, porosities and flexural strength of the final ceramics sintered at 1400 °C varied in the range of ~ 7.6 to 10.1 µm, 34–49 vol% and 34–15 MPa, respectively. The strength–porosity relationship was explained by the minimum solid area (MSA) model. After mechanical stress was applied to the porous SiC ceramics, microstructures of fracture surface appeared without affecting dense struts of thickness ~ 2 to 10 µm showing restriction in crack propagation through interfacial zone of SiC particles. The effect of corrosion on oxide bond phases was investigated in strong acid and basic salt medium at 90 °C. The residual mechanical strength, SEM micrographs and EDX analyses were conducted on the corroded samples and explained the corrosion mechanisms.     

Biomorphous porous hydroxyapatite-ceramics from rattan (<Emphasis Type="Italic">Calamus Rotang</Emphasis>)

The three-dimensional, highly oriented pore channel anatomy of native rattan (Calamus rotang) was used as a template to fabricate biomorphous hydroxyapatite (Ca₅(PO₄)₃OH) ceramics designed for bone regeneration scaffolds. A low viscous hydroxyapatite-sol was prepared from triethyl phosphite and calcium nitrate tetrahydrate and repeatedly vacuum infiltrated into the native template. The template was subsequently pyrolysed at 800°C to form a biocarbon replica of the native tissue. Heat treatment at 1,300°C in air atmosphere caused oxidation of the carbon skeleton and sintering of the hydroxyapatite. SEM analysis confirmed detailed replication of rattan anatomy. Porosity of the samples measured by mercury porosimetry showed a multimodal pore size distribution in the range of 300 nm to 300 μm. Phase composition was determined by XRD and FT-IR revealing hydroxyapatite as the dominant phase with minimum fractions of CaO and Ca₃(PO₄)₂. The biomorphous scaffolds with a total porosity of 70–80% obtained a compressive strength of 3–5 MPa in axial direction and 1–2 MPa in radial direction of the pore channel orientation. Bending strength was determined in a coaxial double ring test resulting in a maximum bending strength of ~2 MPa.     

Preparation of 3-D regenerated fibroin scaffolds with freeze drying method and freeze drying/foaming technique

Although three-dimensional fibroin scaffolds have been prepared with freeze drying method, the porosity and pore sizes still can not satisfy the requirement of tissue engineering. In this article, fibroin porous scaffold with high porosity and > 100μm diameter interconnected pores was firstly prepared with freeze drying method through adjusting fibroin concentration. The morphology of different scaffolds lyophilized from different fibroin concentration was observed by SEM. A novel freeze drying improved method, freeze drying/foaming technique, was also devised to prepare fibroin scaffolds at different fibroin concentrations. Using the said method, the porosity and pore size of fibroin scaffolds prepared from 12% concentration were 85.8 ± 4% and 109 ± 20 μm respectively with yield strength up to 450 ± 6 KPa while the porosity and pore size of fibroin scaffolds prepared from 8% concentration were 96.9 ± 3.6% and 120 ± 30 μm respectively with yield strength up to 30 ± 1 KPa. The freeze drying/foaming technique produced scaffolds with a useful combination of high yield strength, interconnected pores, and pore sizes greater than 100 μm in diameter. Through adjusting fibroin concentration and thawing time, the porosity, pore sizes and mechanical properties could be controlled to satisfy the different requirements of tissue engineering. The results suggested that fibroin scaffolds prepared with the above methods could be formed for utility in biomaterial application.     

Porous yttria-stabilized zirconia ceramics with ultra-low thermal conductivity

Porous yttria-stabilized zirconia (ZrO₂-8 mol% Y₂O₃, YSZ) ceramics with ultra-low thermal conductivity (as low as 0.06 W/mK) could be fabricated by tert-butyl alcohol (TBA)-based gel-casting process with low solid loadings of 10 and 15 vol%. High porosity (52–76%) and fine pores with average pore size of 0.7–1.8 μm formed after sintering at 1350–1550 °C. These air-containing pores were believed to affect the through-thickness heat transfer propagation, resulting in low thermal conductivity. The thermal conductivity of porous YSZ ceramics with different porosities fits well with computed values derived from Effective Medium Theory (EMT).     

Effects of talc and clay addition on pressureless sintering of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Porous Si₃N₄ ceramics were successfully synthesized using cheaper talc and clay as sintering additives by pressureless sintering technology and the microstructure and mechanical properties of the ceramics were also investigated. The results indicated that the ceramics consisted of elongated β-Si₃N₄ and small Si₂N₂O grains. Fibrous β-Si₃N₄ grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. Adding 20% talc and clay sintered at 1700°C for 2 h, the porous Si₃N₄ ceramics were obtained with excellent properties. The final mechanical properties of the Si₃N₄ ceramics were as follows: porosity, P ₀ = 45·39%; density, ρ = 1·663·g·cm⁻³; flexural strength, σ _b (average) = 131·59 MPa; Weibull modulus, m = 16·20.     

Comparison between the process–structure–property relationships of silica foams prepared through two different processing routes

Cellular silica with improved framework, crosslinking, and stability properties are desirable for applications in thermal insulation. A process for the preparation of cellular silica foam with interconnected cells with tailored porosity and pore size distribution has been attempted. The silica foams have been prepared through two different methods; surfactant- and particle-based stabilization. The silica foams prepared through two different processes namely surfactant-stabilized foams (SSF) and particle-stabilized foams (PSF) have exhibited a wide range of differences in their structure which in turn have shown to affect the final properties of the foam. The cell size distributions in SSF (89 vol% porosity) and PSF (85 vol% porosity) have been found in the range of 50–250 μm (monomodal) and 4–10 μm and 50–100 μm (bimodal), respectively, whereas the cell counts of both have been found in close proximity. The microstructure of both the sintered SSF as well as PSF samples foams have shown an open and interconnected porosity with the permeability of both in the region of ~10⁻⁸ m². The mechanical (compressive) strength and Young’s modulus of the PSF are a third of that in SSF. The structure–property relationship of both the SSF and PSF and their comparison have been discussed.     

Preparation,microstructure and mechanical properties of porous titanium sintered by Ti fibres

Open-cell porous Ti with a porosity ranging from 35 to 84% was successfully manufactured by sintering titanium fibres. The microstructure of the porous titanium was observed by SEM and the compressive mechanical properties were tested. By adjusting the spiral structure of the porous titanium, the pore size can be controlled in a range of 150–600 μm. With the increasing of the porosity, compressive yield strength and modulus decrease as predicated. However, high mechanical properties were still obtained at a medium porosity, e.g. the compressive yield strength and the modulus are as high as 100–200 MPa and 3.5–4.2 GPa, respectively, when the porosity is in the range of 50–70%. It was suggested that the porous titanium be strong enough to resist handing during implantation and in vivo loading. It is expected to be used as biocompatible implant, because their interconnected porous structures permit bone tissues ingrowth and the body fluids transportation.     

多孔羟基磷灰石陶瓷的制备

以碳粉为造孔剂,研究了多孔羟基磷灰石（HA）陶瓷的制备方法及性能．采用三种不同颗粒尺寸的碳粉,以聚丙烯酸氨为分散剂,可制得气孔率30％～45％,抗弯强度＞10MPa,孔径＜200μm的多孔体．并发现随保温时间的延长,气孔率先降低后升高,抗弯强度则一直升高．通过严格控制碳粉的粒径,可有效地控制多孔烧结体的孔径大小与分布．应用XRD、SEM等技术手段,对多孔陶瓷的性能、微观结构、制造工艺及其影响因素作了分析与探讨．     

Cancellous bone from porous T{i}6Al4V by multiple coating technique

A highly porous T{i}6Al4V with interconnected porous structure has been developed in our previous study. By using a so-called “Multiple coating” technique, the porous T{i}6Al4V can be tailored to resemble cancellous bone in terms of porous structure and mechanical properties. A thin layer of T{i}6Al4V slurry was coated on the struts of base porous T{i}6Al4V to improve the pore structure. After two additional coating, pore sizes ranged from 100 μm to 700 μm, and the porosity was decreased from ∼90% to ∼ 75%, while the compressive strength was increased from 10.3 ± 3.3 MPa to 59.4 ± 20.3 MPa and the Young's modulus increased from 0.8 ± 0.3 GPa to 1.8 ± 0.3 GPa. The pore size and porosity are similar to that of cancellous bone, meanwhile the compressive strength is higher than that of cancellous bone, and the Young's modulus is between that of cancellous bone and cortical bone. Porosity, pore size and mechanical properties can be controlled by the parameters in such multiple coating processes. Therefore the porous T{i}6Al4V with the characteristics of cancellous bone is expected to be a promising biomaterial for biomedical applications. Author to whom all correspondence should be addressed.     

RETRACTED ARTICLE: Characteristics of porous zirconia coated with hydroxyapatite as human bones

Since hydroxyapatite has excellent biocompatibility and bone bonding ability, porous hydroxyapatite ceramics have been intensively studied. However, porous hydroxyapatite bodies are mechanically weak and brittle, which makes shaping and implantation difficult. One way to solve this problem is to introduce a strong porous network onto which hydroxyapatite coating is applied. In this study, porous zirconia and alumina-added zirconia ceramics were prepared by ceramic slurry infiltration of expanded polystyrene bead compacts, followed by firing at 1500°C. Then slurry of hydroxyapatite-borosilicate glass mixed powder was used to coat the porous ceramics, followed by firing at 1200°C. The porous structures without the coating had high porosities of 51–69%, high pore interconnectivity, and sufficiently large pore window sizes (300–500 μm). The porous ceramics had compressive strengths of 5·3∼36·8 MPa, favourably comparable to the mechanical properties of cancellous bones. In addition, porous hydroxyapatite surface was formed on the top of the composite coating, whereas a borosilicate glass layer was found on the interface. Thus, porous zirconia-based ceramics were modified with a bioactive composite coating for biomedical applications.     

Fabrication of porous alumina–zirconia ceramics by gel-casting and infiltration methods

Porous alumina–zirconia ceramics were obtained by infiltrating porous alumina ceramics, which were prepared by tert-butyl alcohol (TBA)-based gel-casting method. Back scattering images of the fracture surface and energy dispersive spectroscopy were performed to obtain composition profiles on the fracture surface and across sections of the sintered composites. The porosity, pore size distribution and compressive strength were also investigated. The results show that the content of zirconia can be adjusted effectively by infiltration times and it decreases with increasing distance from the surface of the samples. The porosity and compressive strength can also be controlled by the infiltration times. With increases of the infiltration times from 1 to 3 cycles, the open porosity decreases slightly from 62.43% to 56.62%, while the compressive strength of the porous alumina–zirconia ceramics increases from 13.57 ± 1.21 to 26.87 ± 2.01 MPa, indicating that the porous ceramics with high porosity and high strength can be prepared by TBA-based gel-casting method combined with the infiltration process.     

Properties of porous Si3N4 ceramic electromagnetic wave transparent materials prepared by technique combining freeze drying and oxidation sintering

A simple and low-cost technique combining freeze drying and oxidation sintering is explored to prepare Si₃N₄ ceramics with high porosity and complex shape. The effects of sintering temperature and time on the phase composition, microstructure, porosity, pore size and dielectric constant of the porous Si₃N₄ ceramics are studied. Due to the variations of phase composition and microstructure, the porous Si₃N₄ ceramics sintered at different temperature possess characteristic in flexural strength. The porous Si₃N₄ ceramics sintered at 1,300 °C for 2–3 h have the highest flexural strength of 71–74 MPa. The changes of porosity and composition have much effect on the dielectric constant of porous Si₃N₄ ceramics. Because of the high porosity and SiO₂ volume fraction, the porous Si₃N₄ ceramics sintered at 1,300 °C for 2–3 h possess low dielectric constant of 3.4–3.6 and small pore size of 0.9 μm. The porous Si₃N₄ ceramics are good structural/functional and promising electromagnetic wave transparent material.     

Preparation and characterisation of controlled porosity alginate hydrogels made via a simultaneous micelle templating and internal gelation process

Controlled porosity alginate hydrogel monoliths were synthesised by simultaneous micelle templating (MT) and an internal gelation reaction. In water, the self assembling surfactant, cetyltrimethylammonium bromide (CTAB) formed non-spherical micelles that were used as a template for pore formation. The porous microstructure was assessed by mercury intrusion porosimetry (MIP), helium pycnometry, X-ray microtomography (XMT) and scanning electron microscopy (SEM), respectively. The MT hydrogels displayed relatively monodisperse pore size distributions (with pore sizes ranging from 32.5 μm to 164.0 μm), high total pore volumes (4.5–20.3 cm³/g) and high degrees of porosity (83–97%). Some control over pore size distributions was achieved by varying the surfactant concentration; higher surfactant concentrations, led to smaller pores with lower total pore volumes. Uniaxial compression testing revealed that hydrogels made via MT are stable in cell culture media for 28 days. Fourier transform infrared (FTIR) spectroscopy data, suggested that all surfactant could be removed from the final product by washing with ethanol and water, making these hydrogels potentially suitable for tissue engineering (TE) applications.     

Fabrication and characterization of porous poly(lactic-<Emphasis Type="Italic">co</Emphasis>-glycolic acid) (PLGA) microspheres for use as a drug delivery system

In this work, Simvastatin (SIM) loaded porous poly(lactic-co-glycolic acid) (PLGA) microspheres were fabricated using the W/O/W1/W2 double emulsion and solvent evaporation method. The optimal conditions for fabricating porous PLGA microspheres were determined to be 20% distilled water (v/v), 10% PLGA (m/v), and a 4:1 ratio of internal polyvinyl alcohol (PVA) to dichloromethane (DCM). The pores size distribution of porous PLGA microspheres was varied from 0.01 to 40 μm, while their particle displayed a bimodal size distribution that had two diameter peaks at around 100 μm and 500 μm. The SIM encapsulation efficacy was found to be very high with a yield near 80% and the porous PLGA microspheres showed the excellent biocompatibility. In addition, the drug release profile was found to be significantly different from a temporal basis. Base on the combined results of this study, SIM loaded PLGA microspheres holds great promise for use in biomedical applications, especially in drug delivery system or tissue regeneration.     

Fabrication of porous poly(L-lactide) (PLLA) scaffolds for tissue engineering using liquid–liquid phase separation and freeze extraction

PLLA scaffolds were successfully fabricated using liquid–liquid phase separation with freeze extraction techniques. The effects of different processing conditions, such as method of cooling (direct quenching and pre-quenching), freezing temperature (−80°C and −196°C) and polymer concentration (3, 5 and 7 wt%) were investigated in relations to the scaffold morphology. SEM micrographs of scaffolds showed interconnected porous network with pore size ranging from 20 to 60 μm. The scaffolds had porosity values ranging from 80 to 90%. Changes to the interconnected network, porosity and pore size were observed when the method of cooling and polymer concentration was changed. Direct quenching to −80°C gave a more porous interconnected microstructure with uniform pore size compared to samples prepared using pre-quenching method. Larger pores were observed for samples quenched at −80°C compared to −196°C. Scaffolds prepared using direct quenching to −196°C had higher elastic modulus and compressive stress compared to those quenched to −80°C. The compressive elastic modulus ranged from 4 to 7 MPa and compressive stress at 10% strain was from 0.13 to 0.18 MPa.     

Indirect rapid prototyping of biphasic calcium phosphate scaffolds as bone substitutes: influence of phase composition,macroporosity and pore geometry on mechanical properties

While various materials have been developed for bone substitute and bone tissue engineering applications over the last decades, processing techniques meeting the high demands of scaffold shaping are still under development. Individually adapted and mechanically optimised scaffolds can be derived from calcium phosphate (CaP-) ceramics via rapid prototyping (RP). In this study, porous ceramic scaffolds with a periodic pattern of interconnecting pores were prepared from hydroxyapatite, β-tricalcium phosphate and biphasic calcium phosphates using a negative-mould RP technique. Moulds predetermining various pore patterns (round and square cross section, perpendicular and 60° inclined orientation) were manufactured via a wax printer and subsequently impregnated with CaP-ceramic slurries. Different pore patterns resulted in macroporosity values ranging from about 26.0–71.9 vol% with pore diameters of approximately 340 μm. Compressive strength of the specimens (1.3–27.6 MPa) was found to be mainly influenced by the phase composition as well as the macroporosity, both exceeding the influence of the pore geometry. A maximum was found for scaffolds with 60 wt% hydroxyapatite and 26.0 vol% open porosity. It has been shown that wax ink-jet printing allows to process CaP-ceramic into scaffolds with highly defined geometry, exhibiting strength values that can be adjusted by phase composition and pore geometry. This strength level is within and above the range of human cancellous bone. Therefore, this technique is well suited to manufacture scaffolds for bone tissue engineering.     

Biocompatibility and Biodegradation of novel PHB porous substrates with controlled multi-pore size by emulsion templates method

PHB porous substrates were prepared based on the mono-membrane fabricated by emulsion templates method. The key factors of the method affecting the pore size and porosity of the PHB porous substrates were studied. The surface of PHB porous substrates were investigated by scanning electron microscope (SEM), which showed the even pore size and regular arranged pore. The transect of the PHB porous substrates prepared using the templates method was good. Moreover, the effects of variation of surfactant content (P%) and water content (R) on the pore size and porosity of PHB films were discussed. Preliminary studies showed that when P% is less than 20%, the pore size made by emulsion templates ranged from 5 μm to 30 μm with the value of P increasing. As P% is up to 20%. It was interesting to see that the porous substrates had muti-pore size distribution, i.e., median pore sizes were about 5 μm and inside the wall of pore, there existed numerous micro-pores size can be controlled from 100 nm to 500 nm only by adjusting the parameter R of the microemulsion. The cell-compatibility was evaluated via Chinese Hamster Lung (CHL) fibroblast cultivation in vitro. The Cells were cultured on both the mono-pore size membrane prepared by emulsion templates and the multi-pore size membrane prepared by microemulsion templates. It can be seen that the cells cultured on multi-pore size membrane stretched their morphology and proliferated better than that of mono-pore size membrane. These results indicated that the multi-pore size membrane had better cell-compatibility and was more suitable for tissue engineering. The degradation experiment indicated that the degradation of PHB porous substrates were accelerated by enzyme in vitro and the porous configuration was favorable to its degradation.     

Proliferation and function of MC3T3-E1 cells on freeze-cast hydroxyapatite scaffolds with oriented pore architectures

Previous work by the authors showed that hydroxyapatite (HA) scaffolds with different types of oriented microstructures and a unique ‘elastic–plastic’ mechanical response could be prepared by unidirectional freezing of suspensions. The objective of the present work was to evaluate the in vitro cellular response to these freeze-cast HA scaffolds. Unidirectional scaffolds with approximately the same porosity (65–70%) but different pore architectures, described as ‘lamellar’ (pore width = 25 ± 5 μm) and ‘cellular’ (pore diameter = 100 ± 10 μm), were evaluated. Whereas both groups of scaffolds showed excellent ability to support the proliferation of MC3T3-E1 pre-osteoblastic cells on their surfaces, scaffolds with the cellular-type microstructure showed far better ability to support cell proliferation into the pores and cell function. These results indicate that freeze-cast HA scaffolds with the cellular-type microstructure have better potential for bone repair applications.