Hydroxyapatite powders characterized by ionic substitutions both in anionic and cationic sites were successfully prepared by synthesis in aqueous medium. The process parameters were set up to allow the simultaneous substitution of the foreign ions, namely carbonate, magnesium and silicon in the crystallographic site of calcium and phosphorus, keeping in count the competition which arises between atoms destined to occupy the same crystallographic site.The chemico-physical properties of the powders were investigated through several analytical techniques, i.e. X-ray diffraction, infrared spectroscopy, atomic emission spectroscopy and thermo-gravimetric analysis. The results show that the utilization of sodium hydrogen-carbonate as a reactant allows the entering of carbonate into the HA structure, mainly in phosphate position, while sodium is eliminated during the process of the powder washing. The entering of silicon in the HA structure progressively reduces its crystallinity, as also carbonate ions do. Silicate and carbonate ions can enter simultaneously into the HA structure, in biological-like amounts, although they compete for the occupation of the phosphate site; the powder crystallinity is strongly reduced as the content of the two substituting ions increases, so that a limit molar concentration exists where the apatite structure collapses and an amorphous phase forms with the simultaneous formation of crystalline calcium carbonate. Solubility tests, carried out at physiological conditions, reveal an increased calcium release in the HA powders containing silicon compared to the silicon-free HA; the solubility behaviour of the multi-substituted HA powders at physiological conditions makes these materials promising as bioactive bone scaffold, as they are able to continuously supply ions which are essential for the process of bone reconstruction. (C) 2006 Elsevier B.V. All lights reserved.

The preparation of dense MgB2 bodies, undoped and doped with different atomic species (Na, Ag, Y), was performed via reactive sintering by uniaxial and isostatic hot pressing, starting from the pure elements, and compared with undoped samples obtained by commercial MgB2 powder. The superconducting characteristics of the obtained materials, namely critical temperature (Tc) and current (Jc), were obtained through ac susceptibility measurements and compared to their structural features, like phase purity and secondary phases formation and distribution in the MgB2 matrix. Both the adopted hot pressing techniques gave rise to undoped MgB2 pieces exhibiting phase purity in the range 85–95% and relative density above 80%; although in most cases the doped samples underwent higher extents of phase decomposition and lesser densification, they all exhibited higher critical temperature and current compared to the corresponding undoped material, indicating a net influence of the doping on the superconducting behaviour of MgB2, particularly Ag and Y. An opportune quality factor was adopted, to obtain a more reliable comparison between the different MgB2 samples and evaluation of the samples goodness, in relation to their superconducting characteristics. It was put in evidence that low amounts of doping can improve the superconducting behaviour of MgB2 and that this influence can be addressed in terms of pinning centres, as there was no experimental evidence of an actual atomic substitutions in the MgB2 crystal.

The toughness increment occurring in Si 3 N 4 -based composites due to the addition of MoSi 2 particles was compared to the predictions of theoretical models based on the combination of residual stresses and crack deflection toughening mechanisms. A direct application of theoretical models led to a substantial discrepancy between predicted and observed values. For this reason, the basic parameters of the theoretical models were experimentally evaluated. The residual stresses were assessed by measuring the strain in the reinforcing particles by X-ray diffraction. Moreover, the MoSi 2 interparticle distance was calculated by image analysis and the crack paths were analyzed in order to check the actual extent of crack deflection. The overall toughness increase recalculated as the sum of the newly estimated values of residual stresses and crack deflection contributions, was shown to be in good agreement with the experimental results.

The toughness increment occurring in Si3N4-based composites due to the addition of MoSi2 particles was compared to the predictions of theoretical models based on the combination of residual stresses and crack deflection toughening mechanisms. A direct application of theoretical models led to a substantial discrepancy between predicted and observed values. For this reason, the basic parameters of the theoretical models were experimentally evaluated. The residual stresses were assessed by measuring the strain in the reinforcing particles by X-ray diffraction. Moreover, the MoSi2 interparticle distance was calculated by image analysis and the crack paths were analyzed in order to check the actual extent of crack deflection. The overall toughness increase recalculated as the sum of the newly estimated values of residual stresses and crack deflection contributions, was shown to be in good agreement with the experimental results.

Poorly crystalline apatite has been directly nucleated on self-assembling alginate chains by neutralization synthesis to obtain a biomimetic artificial bone-like composite. It has been observed that in preparing HA/alginate composites, Ca2+ ions present on the apatitic surface cross-link the alginate chains to produce a material with different morphology and thermal stability, both functions of the HA/alginate weight ratio. In vitro tests were performed on different samples in terms of both the HA/alginate ratio and synthesis temperature. All the samples were cultured for seven days with MG63 osteoblast-like cells and then underwent morphological and biochemical analyses (MTT and ALP tests). Scaffolds showed a different solubility into the culture media, which was related to the temperature of synthesis and to the HA/alginate ratio. All our data confirm the ability of the tested materials to favour cell growth and to maintain their osteoblastic functionality, at least during the examined period.

The incorporation of magnesium ions into the hydroxyapatite structure, which is of great interest for the developing of artificial bone, was performed starting from a wet chemical synthesis using magnesium chloride as Mg source. Different doping extents were attempted, four powders were produced and characterized in term of morphology, composition, solubility, thermal resistance,etc. in comparison with stoichiometric HA. In vitro tests with mesenchimal stem cells (MSCs) and human osteoblast like cells MG-63 cells were performed with the powder characterized with a biological-like doping of 5%Mg. The same powder was used, in form of granules, to carry out in vivo test by filling a defect in the femur on New Zealand White rabbits. All the tests showed better performance of the Mg doped apatite compared to stoichiometric HA, in agreement with the chemico-physical features of the material.

Hydroxyapatite is an elective material for bone substitution. In this outline of our recent activity the crucial role of nanostructured ceramics in the design and preparation of ceramic scaffolds will be described, focussing on our more recent interest in biomimetic apatites, in particular apatites containing HPO 4 2− CO 3 2− and Mg 2+ which are similar to the mineral component of bone. The paper describes such nanostructured products and, in particular, innovative synthetic techniques capable of yielding powders with higher reactivity and bioactivity. However, so far the characteristics of artificial bone tissues have been shown to be very different from those of natural bone, mainly because of the absence of the peculiar self-organizing interaction between apatites and the protein component. This causes modification of the structure of apatites and of the features of the overall composite forming human bone tissue. Therefore, attempts to mimic the features and structure of natural bone tissue, leading toward so-called bio-inspired materials , will be speculated upon. New techniques used to reproduce a composite in which a nanosize blade-like crystal of hydroxyapatite (HA) grows in contact with self-assembling fibres of natural polymer will be presented. In this specific case, the amazing ability of biological systems to store and process information at the molecular level, nucleating nanosize apatites (bio-inspired material), is exploited.

A series of synthetic apatites bulk samples, characterized by different atomic substitutions were polarized by field of 1 and 2 kV·cm-1 at 300 °C. The thermally stimulated depolarization currents (TSDC) were measured and the stored electric charge density evaluated. The recent development in the synthesis of non-stoichiometric HA powders, whose chemical composition resembles that of the natural bone, is promising for the realization of improved biomimetic implants for bone substitution; therefore, electrical polarization applied on the ceramic pieces could further enhance the already strong bioactivity of these materials. The depolarization processes in the various material were evaluated by the Arrhenius’ method and related with the corresponding chemical features; finally, SEM observations of some polarized samples immersed in SBF at 37 °C revealed a wide growth of HA particles within a few days in all the investigated materials.

Stoichiometric and magnesium-substituted synthetic hydroxyapatite (HA) powders with different Mg contents were characterized by Raman and cathodoluminescence (CL) spectroscopies. The substitution of Ca ions by Mg is presently of great interest because it may improve activity in the first stage of the bone remodeling process. In this paper, we show new evidence that CL spectroscopy has the capability to detect the presence of crystal defects, related to the presence of magnesium substituting calcium in Mg-doped HA powders. The dependence of CL spectra of stoichiometric and magnesium-doped HA powders on their chemical composition was studied, and the results are compared with Raman analysis and data previously collected by other analytical tools. All the investigated powders showed five distinct CL bands; moreover, in magnesium-doped HA, an additional band at higher energy was found. The intensity ratios between selected CL bands showed some relationships with the powder crystallinity and the estimated amount of magnesium both in the HA lattice and in the amorphous surface layer; moreover the band observed only in magnesium-substituted powders could be directly related to the amount of magnesium entered into the HA lattice. Such results can contribute to improve the knowledge of the crystallographic structure of Mg-substituted hydroxyapatite.

Residual stresses in ceramic particle electroconductive composites were investigated by Raman microprobe spectroscopy and X-ray diffraction. The composites were ternary electroconductive ceramics in the system AlN+SiC+(ZrB 2 ,MoSi 2 ). Due to the poor definition of the reinforcing phase peaks, only the matrix residual stress could be evaluated by Raman spectroscopy, whilst the residual stress in the reinforcing phase was calculated by the equilibrium conditions. These calculated values were compared with those experimentally obtained by X-ray diffraction. The agreement between Raman and X-ray results was quite satisfactory. The values of residual stress calculated by the composite theory were in good agreement with those measured by Raman and X-ray diffraction for the MoSi 2 -containing composite. For the ZrB 2 -containing composite, the value calculated by the composite theory falls between the values measured by Raman and X-ray diffraction.

A new method to synthesise carbonate-substituted hydroxyapatite (CHA) powder has been set up introducing a CO2 flux, as a source of carbonate, in the HA synthesis process based on the neutralisation reaction. The reactants are abundant and inexpensive. The yield is good compared to other CHA powder synthesis. The reaction may be performed at low temperature and without pH control and does not produce any by-products.The influence of the synthesis parameters (temperature, H3PO4 solution dropping rate, i.e. reaction time, CO2 flux, ageing time) has been tested to optimise the process conditions in order to obtain the highest carbonation degree and favour the B-type CHA precipitation with respect to A-type one.The prepared powder (5.8 wt% of total carbonate with an A/B ratio of 0.78) was thermally treated at various temperatures in the range 500-1400degreesC in different atmospheres (air, wet and dry carbon dioxide).The thermal treatments were performed with a double aim, to eliminate selectively the carbonate groups in A-position maintaining the B-type substitution, and to evaluate the thermal stability of the CHA and the total loss of carbonate as a function of temperature. The thermal treatment at 900degreesC in wet CO2, gave the best result in terms of a high carbonate residue and a low A/B ratio.We also investigate the use of different techniques (inductively coupled plasma, TGA, Fourier transformed infrared spectroscopy, X-ray diffraction) for characterising CHA and calculating sensitivity and accuracy in the quantification of carbonate ions for each molecular site. (C) 2003 Elsevier Ltd. All rights reserved.

The effectiveness of synthetic body fluids (SBF) as biomimetic sources to synthesize carbonated hydroxyapatite (CHA) powder similar to the biological inorganic phase, in terms of composition and microstructure, was investigated. CHA apatite powders were prepared following two widely experimented routes: (1) calcium nitrate tetrahydrate and diammonium hydrogen phosphate and (2) calcium hydroxide and ortophosphoric acid, but using SBF as synthesis medium instead of pure water. The characteristics of the as-prepared powders were compared, also with the features of apatite powders synthesized via pure water-based classical methods. The powder thermal resistance and behaviour during densification were studied together with the mechanical properties of the dense samples. The sponge impregnation process was used to prepare porous samples having morphological and mechanical characteristics suitable for bone substitution. Using this novel synthesis was it possible to prepare nanosized (approximately equal to 20 nm), pure, carbonate apatite powder containing Mg, Na, K ions, with morphological and compositional features mimicking natural apatite and with improved thermal properties. After sintering at 1250 degrees C the carbonate-free apatite porous samples showed a surprising, high compressive strength together with a biomimetic morphology.

A new method to synthesize the carbonate substituted hydroxyapatite (CHA) powder has been set up introducing a CO2 flux, as a source of carbonate, in the HA synthesis process based on the neutralization reaction between Ca(OH)2 and H3PO4; the Ca/P molar ratio of the starting reactants has been fixed at the stoichiometric value of hydroxyapatite (1.67). The reactants are abundant and inexpensive, the yield is good compared to other CHA powder synthesis, the reaction may be performed at low temperature and without pH control and does not produce any by-products that have to be disposed off: these are all things of great interest for an industrialisation of the process. The influence of the synthesis parameters (temperature, H3PO4 solution dropping rate, i.e. reaction time, CO2 flux, aging time) has been tested to optimise the process conditions in order to obtain the highest carbonation degree and favour the B-type CHA precipitation in respect to A-type one.

Recently the replacement of bone tissue by graft materials and products of tissue engineering with composition, structure and biological features very similar to natural tissue is the main topic of research in biomaterials. A biomimetic synthesis has been performed to prepare new bone like composites constituted of hydroxyapatite nanocrystals and self-assembled type 1 collagen fibers. We have used a biological inspired approach which proves that the biological systems store and process information at the molecular level. Direct nucleation of hydroxyapatite (HA) into reconstituted collagen fibers during their assembling was employed. The new preparation technique was experimented then the composites thoroughly characterized and compared : composites showed an intimated interaction of the inorganic and proteic components which modified the apatitic phase and made its composition, morphology and structure similar to the mineral component of natural bone.

Hydroxyapatite (HA) represents a good model system for biological apatites which are characterized by poor crystallinity and non stoichiometry due to the presence of significant amount of foreign ions. Some of these ions are known to enter into the HA crystal lattice, others have been suggested to be only absorbed on the apatite surface.X-ray diffraction, infrared absorption, and chemical investigations have been performed on hydroxyapatite synthesized in the presence of different magnesium concentration. Magnesium inhibits the crystallization of hydroxyapatite through a reduction of Ca / P molar ratio and crystal size of apatite; contemporarily it increases the nucleation rate of HA causing a trapping Of CO32- ions into the apatitic structure mainly in B position. The samples become completely amorphous between 35 and 50 Mg atom percent, at higher magnesium concentration different crystalline phases form. The results of the X-ray powder pattern fitting indicate that the HA crystal structure at most hosts magnesium amount of about 13% mol. Magnesium substitutes preferentially one of the two different calcium atoms positions: in our case the favourite substitution is in Ca (1), i.e. the Ca atoms lying within the phosphate groups columns (4f).On the basis of these results, the key role of magnesium on the crystallization, crystal growth, and thermal stability of hydroxyapatite has been used to explain the relevant properties of biological apatites.

The design and synthesis of porous phosphate-gelatine composite implant which mimicks the structure of natural bone and has drug delivery function is proposed. Gelatine reproducing the proteinaceous part of bone was cross-linked in order to modulate its solubility in the physiologic fluids. The kinetic of gelatine release from ceramic matrix was also evaluated as model of the release of any therapeutic compound which can be loaded into gelatine.

B-carbonateapatite (CHA) powder was synthesized starting from calcium nitrate tetrahydrate, diammonium hydrogen phosphate and sodium hydrogen carbonate. The powder was fully characterized in terms of phase purity, stoichiometry, morphology, specific surface area and particle size distribution. The thermal stability of the powder in air and CO2 atmosphere also was evaluated by thermal analysis. Electroacoustic analysis of the water based suspension of the CHA powder was used to determine the stability of the slurry. Porous bodies of CHA were prepared by impregnation of cellulose sponges with a proper slurry of the powder and optimizing the subsequent sintering. The fired samples were characterized in terms of phase purity and carbonate content, microstructure and pore size distribution. The compressive strength also was evaluated, resulting in 6.0±0.5 MPa. First results of in vivo tests on New Zealand White rabbits showed good biocompatibility and osteointegration of the CHA implant, with higher osteoconductive properties and earlier bioresorption, compared to HA samples, used as control.

The densification behaviour of MgB2 was studied when commercial powder was hot-pressed in the temperature range 1070–1190 °C. The mechanisms active during sintering were investigated by continuously recording the shrinkage vs. time and elaborating the data on the basis of Kingery’s model for liquid phase sintering. XRD and SEM analysis on final dense bodies were used to evaluate secondary phases formation during sintering and the effect of magnesium sublimation. AC magnetic susceptibility measurements were also performed to correlate the microstructural and morphological modifications induced by hot-pressing and the superconducting properties.