Macroporous ceramics with pore sizes from 400 nm to 4 mm and porosity within the range 20%–97% have been produced for a number of well-established and emerging applications, such as molten metal filtration, catalysis, refractory insulation, and hot gas filtration. These applications take advantage of the unique properties achieved through the incorporation of macropores into solid ceramics. In this article, we review the main processing routes that can be used for the fabrication of macroporous ceramics with tailored microstructure and chemical composition. Emphasis is given to versatile and simple approaches that allow one to control the microstructural features that ultimately determine the properties of the macroporous material. Replica, sacrificial template, and direct foaming techniques are described and compared in terms of microstructures and mechanical properties that can be achieved. Finally, directions to future investigations on the processing of macroporous ceramics are proposed.

/pdf/processing-routes-to-macroporous-ceramics-a-review-1urld578zq.pdf

Processing Routes to Macroporous Ceramics: A Review

Starting from Ca(OH)2 and H3PO4 hydroxyapatite powders with three different crystallinity degrees have been prepared and characterized. Densification extent and mechanisms were studied through dilatometric measurements in isothermal regime in the range of temperature 750–1250°C: the influence of different powder features (including the effect of calcination treatment) have been evaluated. Powder characterized by the lowest crystallinity degree has the highest densification extent; overlapping phenomena occurring during the sintering treatments are responsible for unexpectedly low values found for the shrinkage rate, which can easily lead to a misidentification of the rate controlling mechanism. An interpretation of the densification mechanism, consistent with all experimental findings, is proposed.

Densification behaviour and mechanisms of synthetic hydroxyapatites

Functional gradation is one characteristic feature of living tissue. Bio-inspired materials open new approaches for manufacturing implants for bone replacement. Different routes for new implant materials are presented using the principle of functional gradation. An artificial biomaterial for knee joint replacement has been developed by building a graded structure consisting of ultra-high molecular weight polyethylene (UHMWPE) fibre reinforced high-density polyethylene combined with a surface of UHMWPE. The ingrowth behaviour of titanium implants into hard tissue can be improved by depositing a graded biopolymer coating of fibronectin, collagen types I and III with a gradation, derived from the mechanisms occurring during healing in vivo. Functionally graded porous hydroxyapatite (HAP) ceramics can be produced using alternative routes, e.g. sintering of laminated structures of HAP tapes filled with polymer spheres or combining biodegradable polyesters such as polylactide, polylactide-co-glycolide and polyglycolide, with carbonated nanocrystalline hydroxyapatite. HAP–collagen I scaffolds are an appropriate material for in vitro growth of bone. The scaffold has to be functionally graded in order to create an optimised mechanical behaviour as well as the intended improvement of the cell ingrowth.

Functionally graded materials for biomedical applications

Background

The purpose of this review is to examine the usefulness of cell culture methods to model the mechanisms of bone formation on the surfaces of candidate implant materials.



Methods

The central objective is to show that in vitro methods are uniquely valuable in providing an understanding of how new bone is formed on solid surfaces. It should be emphasized, at the outset, that the use of cell culture studies as cytotoxicity assays will not be addressed, nor is it implied that cell cultures can model all the complexities of the in vivo environment. Nevertheless, by comparison with in vivo data, which are by nature retrospective, it is shown that primary differentiating osteogenic cell cultures, derived from bone marrow, illustrate a sequence of extracellular matrix elaboration events that characterize the establishment of the interface between newly formed bone and solid surfaces. These solid surfaces either may be implant materials, or indeed previously formed bone matrix, which has been resorbed during normal bone remodeling events. In each case the first biologically derived matrix at these sites is a morphologically distinct collagen fibre-free extracellular matrix, which, in bone histology has been referred to for >100 years as a cement line.



Results

The sequence starts with secretion and adsorption to the substratum of organic components, of which the major proteins are osteopontin and bone sialoprotein. Mineralization of this matrix occurs by the seeding of nanocrystalline calcium phosphate, which precedes the appearance of morphologically identifiable collagen fibres. This is clearly contrary to the dogma that collagen is necessary for mineralization of bone, but is in agreement with specific cases of other, particularly dental, calcified connective tissues. Although collagen is synthesized by the differentiating osteogenic cells that elaborate the cement line interface, it is not adsorbed to the underlying solid surface. Following the elaboration of the cement line matrix, collagen fibre assembly occurs and is then mineralized to produce morphologically identifiable bone matrix.



Conclusions

Key elements of this sequence of events can be seen at the interface of implants retrieved from in vivo experiments, which indicates that these in vitro methods not only mimic known in vivo phenomena, but also provide a mechanistic understanding of bone elaboration at implant surfaces. However, distinction is drawn between the events of new bone formation at implant surfaces and other bone/implant morphologies, which are unrelated to de novo bone formation at the implant surface. Finally, this new information emerging from bone marrow cell culture studies demands a re-examination of the concepts of bone-bonding and nonbonding implant materials. © 1996 Wiley-Liss, Inc.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/%28SICI%291097-0185%28199606%29245%3A2%3C426%3A%3AAID-AR21%3E3.0.CO%3B2-Q

In vitro modeling of the bone/implant interface

Nanocrystalline HAp powder was synthesized using surfactant template systems. Composition of the microemulsion and synthesis parameters had significant effect on the formation of HAp nanopowder and their surface area and morphology. Powders were prepared with a surface area of 130 m2/g and particle size between 30 and 50 nm with needle shape and spherical morphology. Nanocrystalline hydroxyapatite (HAp) powder was synthesized using the reverse micelle-processing route. Cyclohexane was used as the oil phase, mixed poly(oxyethylene)5 nonylphenol ether (NP-5) and poly(oxyethylene)12 nonylphenol ether (NP-12) as the surfactant phase, and a solution of Ca(NO3)2 and H3PO4 was used as the aqueous phase. The powders were characterized by BET surface area analyzer, powder X-ray diffraction, and transmission electron microscopy. It was found that experimental conditions such as aqueous/organic phase volume ratio, pH, aging time, aging temperature, and metal ion concentration in the aqueous phase affected the crysta...

Synthesis and Characterization of Hydroxyapatite Nanopowders by Emulsion Technique

Hydroxyapatite (Ca10(PO4)6(OH)2) ceramic sheets with a wide range of porosities (up to 62%) have been prepared. The process is based on the reaction between dicalcium phosphate (CaHPO4) and calcium carbonate (CaCO3). When mixed with the appropriate Ca/P ratio, this proves to be a reliable new method for obtaining hydroxyapatite. Moreover, CaCO3 serves as a gas-forming agent (due to the evolution of carbon dioxide and water during the reaction), which leads to the development of highly porous microstructures. Alternatively, CaHPO4 and CaCO3 can be reacted by calcining at 1000°C to produce pure hydroxyapatite powders. When processed in a similar way, a dense ceramic results. By mixing 50 vol% of CaCO3 and CaHPO4 with precalcined powders, hydroxyapatite with an intermediate porosity was obtained. Moreover, it should be possible to achieve porosity control by mixing different amounts of uncalcined and precalcined powders. All of these powders are colloidally processed using tape casting to produce thin sheets 150–200 μm thick. This technique can be used to make laminates, with or without porosity gradients, up to several millimetres thick.

Synthesis and processing of hydroxyapatite ceramic tapes with controlled porosity

Chemistry and sintering behaviour of thin hydroxyapatite ceramics with controlled porosity

Bulk nucleation has been studied for glasses within the Y2O3–Al2O3–SiO2 system, using a two-stage nucleation and growth heat treatment. The crystalline phases formed have been identified. Annealing in an inert atmosphere is required in order to prevent surface nucleation from dominating the results. Despite this, the phases observed are in general agreement with those observed in previous studies done in air. The nucleation kinetics of the y-Y2Si2O7 phase have been measured and the optimum nucleation temperatures have been identified. The kinetic data have been analyzed using two existing models based on classical nucleation theory. The results of this study have been compared with crystallization results of a previous study of glass of similar composition within sintered silicon nitride samples. A two-stage heat treatment is suggested as a process that may lead to improved devitrification of grain boundary glass in such materials.

Crystallization of Yttria–Alumina–Silica Glasses

Preparation and characterization of a natural fiber-polymer resin composite

A review of the basic properties of the so-called polyelectrolyte cements is presented. Full characterization of these materials was performed by means of microscopy, Fourier transform infrarred spectroscopy, mechanical testing, etc. These studies allow to fully understand the kinetics of formation of the cements and a truly engineered material can then be designed. Some of the practical applications of these investigations are discussed as well.

Irene H. Arita

Papers

Synthesis and processing of hydroxyapatite ceramic tapes with controlled porosity

Chemistry and sintering behaviour of thin hydroxyapatite ceramics with controlled porosity

Crystallization of Yttria–Alumina–Silica Glasses

Preparation and characterization of a natural fiber-polymer resin composite

Engineering Materials: The Case of Polyelectrolyte Cements