Biomimetic apatite sintered at very low temperature by spark plasma sintering: Physico-chemistry and microstructure aspects

Nanocrystalline apatites analogous to bone mineral are very promising materials for the preparation of highly bioactive ceramics due to their unique intrinsic physico-chemical characteristics. Their surface reactivity is indeed linked to the presence of a metastable hydrated layer on the surface of the nanocrystals. Yet the sintering of such apatites by conventional techniques, at high temperature, strongly alters their physico-chemical characteristics and biological properties, which points out the need for "softer" sintering processes limiting such alterations. In the present work a non-conventional technique, spark plasma sintering, was used to consolidate such nanocrystalline apatites at non-conventional, very low temperatures (T° < 300 °C) so as to preserve the surface hydrated layer present on the nanocrystals. The bioceramics obtained were then thoroughly characterized by way of complementary techniques. In particular, microstructural, nanostructural and other major physico-chemical features were investigated and commented on. This work adds to the current international concern aiming at improving the capacities of present bioceramics, in view of elaborating a new generation of resorbable and highly bioactive ceramics for bone tissue engineering

Thermodynamic basis for evolution of apatite in calcified tissues

Bone remodeling and tooth enamel maturation are biological processes that alter the physico-chemical features of biominerals with time. However, although the ubiquity of bone remodeling is clear, why is well-crystallized bone mineral systematically replaced by immature nanocrystalline inorganic material? In enamel, a clear evolution is also seen from the first mineral formed during the secretory stage and its mature well-crystalline form, which then changes little in the adult tooth. This contribution provides the thermodynamic basis underlying these biological phenomena. We determined, for the first time, the energetics of biomimetic apatites corresponding to an increasing degree of maturation. Our data point out the progressive evolution of the enthalpy (ΔHf°) and free energy (ΔGf°) of formation toward more negative values upon maturation. Entropy contributions to ΔGf° values remained small compared to enthalpy contributions. ΔHf° varied from –12 058.9 ± 12.2 to –12 771.0 ± 21.4 kJ/mol for maturation times increasing from 20 min to 3 weeks, approaching the value for stoichiometric hydroxyapatite, –13 431.0 ± 22.7 kJ/mol. Apatite thermodynamic stability increased as its composition moved toward stoichiometry. These findings imply diminishing aqueous solubility of calcium and phosphate ions as well as decreased surface reactivity. Such thermodynamically driven maturation is favorable for enamel maturation since this biomineral is intended to resist external aggressions such as contact with acids. In contrast, maintaining a metastable highly reactive and soluble form of apatite is essential to the effective participation of bone as a source of calcium and phosphate for homeostasis. Therefore our data strongly suggest that, far from being trivial, the intrinsic thermodynamic properties of apatite mineral represent a critical driving force for continuous bone remodeling, in contrast to current views favoring a purely biologically driven cycle. These thermodynamic data may prove helpful in other domains relating, for example, to apatite-based biomaterials development or in the field of (geo)microbiology

Nanocrystalline apatites: The fundamental role of water

Bone is a natural nanocomposite. Its mineral component is nanocrystalline calcium phosphate apatite, whose synthetic biomimetic analogs can be prepared by wet chemistry. The initially formed crystals, whether biological or synthetic, exhibit very peculiar physicochemical features. In particular, they are nanocrystalline, nonstoichiometric, and hydrated. The surface of the nanocrystals is covered by a non-apatitic hydrated layer containing mobile ions, which may explain their exceptional surface reactivity. For their precipitation in vivo or in vitro, for their evolution in solution, for the 3D organization of the nanocrystals, and for their consolidation to obtain bulk ceramic materials, water appears to be a central component that has not received much attention. In this mini-review, we explore these key roles of water on the basis of physicochemical and thermodynamic data obtained by complementary tools including FTIR, XRD, ion titrations, oxide melt solution calorimetry, and cryo-FEG-SEM. We also report new data obtained by DSC, aiming to explore the types of water molecules associated with the nanocrystals. These data support the existence of two main types of water molecules associated with the nanocrystals, with different characteristics and probably different roles and functions. These findings improve our understanding of the behavior of bioinspired apatite-based systems for biomedi- cine and also of biomineralization processes taking place in vivo, at present and in the geologic past. This paper is thus intended to give an overview of the specificities of apatite nanocrystals and their close relationship with water

Développement de nouvelles biocéramiques par consolidation à basse température d'apatites nanocristallines biomimétiques

Biomimetic nanocrystalline apatites (BNA) Ca10-x-Z(PO4)6-(HPO4)x(OH)2-x-2Z, (H2O)n were synthesized by precipitation in aqueous medium, then consolidated by Spark Plasma Sintering (or SPS). They are constituted of nanocrystals involving an apatitic core and a surface phospho-calcic hydrated layer containing "non-apatitic" phosphate, hydrogenphosphate and calcium ions, highly "labile" (easily exchangeable), responsible for their high reactivity. The chemical composition, structure and morphology of the nanocrystals of BNA evolve upon maturation in solution, and they tend toward greater thermodynamic stability. Although the amount of non-apatitic chemical species decreases upon maturation, their presence is still significant after a long maturation. Low temperature (150°C) SPS sintering of maturated BNA allowed us to obtain highlycohesive, porous ceramics. The sintering phenomenon observed in such conditions suggests a "crystal fusion" consolidation process, involving the high surface reactivity of the nanocrystals by way of their hydrated layer. The mechanical properties (elastic modulus between 12 and 35 GPa, flexure strength close to 10 MPa) of the ceramics obtained are close to those of bone mineral. Moreover, the nanometer-scale dimensions of the crystals, beneficial to bioresorption after implantation in osseous site, as well as the presence of labile nonapatitic ionic species, favorable to bioactivity, are preserved after SPS. These properties confer to BNA ceramics a particularly promising potential in view of applications in the field of bone tissue engineering.Des apatites nanocristallines biomimétiques (ANB), de formule Ca10-x-Z(PO4)6-x(HPO4)x(OH)2-x-2Z, (H2O)n, ont été synthétisées par précipitation en milieu aqueux puis consolidées par frittage flash (Spark Plasma Sintering, SPS). Elles sont composées de nanocristaux munis d'un coeur apatitique entouré d'une couche phosphocalcique hydratée de surface contenant des ions phosphate, hydrogénophosphate et calcium "non-apatitiques" mobiles et facilement échangeables, qui leur confère une forte réactivité. La composition chimique, la structure et la morphologie des nanocristaux synthétisés évoluent avec le vieillissement en solution et ils tendent vers une plus grande stabilité thermodynamique. Bien que la teneur en espèces chimiques non-apatitiques diminue dans la couche hydratée, leur présence reste importante même après une longue maturation. Le procédé de frittage par SPS à basse température (150°C) de ces ANB a permis d'élaborer des céramiques poreuses fortement cohésives. Le phénomène de frittage ainsi observé suggère une consolidation de type "fusion cristalline" qui met à contribution la forte réactivité de surface des nanocristaux via leur couche hydratée de surface. Le module d'élasticité (12 à 35 GPa) et la résistance à la rupture en flexion (environ 10 MPa) de ces céramiques sont voisins de ceux du minéral osseux. De plus, la taille nanométrique des cristaux, bénéfique à la biorésorption après implantation en site osseux, et la présence d'espèces ioniques nonapatitiques mobiles favorable à la bioactivité sont préservées après SPS. Ces propriétés offrent aux céramiques d'ANB un potentiel particulièrement intéressant pour des applications en ingénierie tissulaire osseuse

Développement de nouvelles biocéramiques par consolidation à basse température d'apatites nanocristallines biomimétiques

Biomimetic nanocrystalline apatites (BNA) Ca10-x-Z(PO4)6-(HPO4)x(OH)2-x-2Z, (H2O)n were synthesized by precipitation in aqueous medium, then consolidated by Spark Plasma Sintering (or SPS). They are constituted of nanocrystals involving an apatitic core and a surface phospho-calcic hydrated layer containing "non-apatitic" phosphate, hydrogenphosphate and calcium ions, highly "labile" (easily exchangeable), responsible for their high reactivity. The chemical composition, structure and morphology of the nanocrystals of BNA evolve upon maturation in solution, and they tend toward greater thermodynamic stability. Although the amount of non-apatitic chemical species decreases upon maturation, their presence is still significant after a long maturation. Low temperature (150°C) SPS sintering of maturated BNA allowed us to obtain highlycohesive, porous ceramics. The sintering phenomenon observed in such conditions suggests a "crystal fusion" consolidation process, involving the high surface reactivity of the nanocrystals by way of their hydrated layer. The mechanical properties (elastic modulus between 12 and 35 GPa, flexure strength close to 10 MPa) of the ceramics obtained are close to those of bone mineral. Moreover, the nanometer-scale dimensions of the crystals, beneficial to bioresorption after implantation in osseous site, as well as the presence of labile nonapatitic ionic species, favorable to bioactivity, are preserved after SPS. These properties confer to BNA ceramics a particularly promising potential in view of applications in the field of bone tissue engineering.Des apatites nanocristallines biomimétiques (ANB), de formule Ca10-x-Z(PO4)6-x(HPO4)x(OH)2-x-2Z, (H2O)n, ont été synthétisées par précipitation en milieu aqueux puis consolidées par frittage flash (Spark Plasma Sintering, SPS). Elles sont composées de nanocristaux munis d'un coeur apatitique entouré d'une couche phosphocalcique hydratée de surface contenant des ions phosphate, hydrogénophosphate et calcium "non-apatitiques" mobiles et facilement échangeables, qui leur confère une forte réactivité. La composition chimique, la structure et la morphologie des nanocristaux synthétisés évoluent avec le vieillissement en solution et ils tendent vers une plus grande stabilité thermodynamique. Bien que la teneur en espèces chimiques non-apatitiques diminue dans la couche hydratée, leur présence reste importante même après une longue maturation. Le procédé de frittage par SPS à basse température (150°C) de ces ANB a permis d'élaborer des céramiques poreuses fortement cohésives. Le phénomène de frittage ainsi observé suggère une consolidation de type "fusion cristalline" qui met à contribution la forte réactivité de surface des nanocristaux via leur couche hydratée de surface. Le module d'élasticité (12 à 35 GPa) et la résistance à la rupture en flexion (environ 10 MPa) de ces céramiques sont voisins de ceux du minéral osseux. De plus, la taille nanométrique des cristaux, bénéfique à la biorésorption après implantation en site osseux, et la présence d'espèces ioniques nonapatitiques mobiles favorable à la bioactivité sont préservées après SPS. Ces propriétés offrent aux céramiques d'ANB un potentiel particulièrement intéressant pour des applications en ingénierie tissulaire osseuse

Développement de nouvelles biocéramiques par consolidation à basse température d'apatites nanocristallines bionimétiques

Des apatites nanocristallines biomimétiques (ANB), de formule Ca10-x-Z(PO4)6-x(HPO4)x(OH)2-x-2Z, (H2O)n, ont été synthétisées par précipitation en milieu aqueux puis consolidées par frittage flash (Spark Plasma Sintering, SPS). Elles sont composées de nanocristaux munis d un cœur apatitique entouré d une couche phosphocalcique hydratée de surface contenant des ions phosphate, hydrogénophosphate et calcium "non-apatitiques" mobiles et facilement échangeables, qui leur confère une forte réactivité. La composition chimique, la structure et la morphologie des nanocristaux synthétisés évoluent avec le vieillissement en solution et ils tendent vers une plus grande stabilité thermodynamique. Bien que la teneur en espèces chimiques non-apatitiques diminue dans la couche hydratée, leur présence reste importante même après une longue maturation. Le procédé de frittage par SPS à basse température (150C) de ces ANB a permis d'élaborer des céramiques poreuses fortement cohésives. Le phénomène de frittage ainsi observé suggère une consolidation de type "fusion cristalline" qui met à contribution la forte réactivité de surface des nanocristaux via leur couche hydratée de surface. Le module d'élasticité (12 à 35 GPa) et la résistance à la rupture en flexion (environ 10 MPa) de ces céramiques sont voisins de ceux du minéral osseux. De plus, la taille nanométrique des cristaux, bénéfique à la biorésorption après implantation en site osseux, et la présence d'espèces ioniques non-apatitiques mobiles favorable à la bioactivité sont préservées après SPS. Ces propriétés offrent aux céramiques d'ANB un potentiel particulièrement intéressant pour des applications en ingénierie tissulaire osseuse.Biomimetic nanocrystalline apatites (BNA) Ca10-x-Z(PO4)6-x(HPO4)x(OH)2-x-2Z, (H2O)n were synthesized by precipitation in aqueous medium, then consolidated by Spark Plasma Sintering (or SPS). They are constituted of nanocrystals involving an apatitic core and a surface phospho-calcic hydrated layer containing non-apatitic phosphate, hydrogenphosphate and calcium ions, highly labile (easily exchangeable), responsible for their high reactivity. The chemical composition, structure and morphology of the nanocrystals of BNA evolve upon maturation in solution, and they tend toward greater thermodynamic stability. Although the amount of non-apatitic chemical species decreases upon maturation, their presence is still significant after a long maturation.Low temperature (150C) SPS sintering of maturated BNA allowed us to obtain highly-cohesive, porous ceramics. The sintering phenomenon observed in such conditions suggests a crystal fusion consolidation process, involving the high surface reactivity of the nanocrystals by way of their hydrated layer. The mechanical properties (elastic modulus between 12 and 35 GPa, flexure strength close to 10 MPa) of the ceramics obtained are close to those of bone mineral. Moreover, the nanometer-scale dimensions of the crystals, beneficial to bioresorption after implantation in osseous site, as well as the presence of labile non-apatitic ionic species, favorable to bioactivity, are preserved after SPS. These properties confer to BNA ceramics a particularly promising potential in view of applications in the field of bone tissue engineering.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

Thermoelectric Materials: A New Rapid Synthesis Process for Nontoxic and High-Performance Tetrahedrite Compounds

International audienceThe feasibility to synthesize, in large quantity, pure, and nontoxic tetrahedrite compounds using high-energy mechanical-alloying from only elemental precursors is reported in this study for the first time. Our processing technique allows a better control of the final product composition and leads to high thermoelectric performances (ZT of 0.75 at 700 K), comparable to that reported on sealed tube synthesis samples. Combined with spark plasma sintering, the production of highly pure and dense samples is achieved in a very short time, at least 8 times shorter than in conventional liquid–solid–vapor synthesis process. The process described in this study is a promising way to produce high-performance tetrahedrite materials for cost-effective and large-scale thermoelectric applications

Thermoelectric materials: a new rapid synthesis process for non-toxic and high-performance tetrahedrite compounds

The feasibility to synthesize, in large quantity, pure and non-toxic tetrahedrite compounds using high-energy mechanical-alloying from only elemental precursors is reported in the present paper for the first time. Our processing technique allows a better control of the final product composition and leads to high thermoelectric performances (ZT of 0.75 at 700 K), comparable to that reported on sealed tube synthesis samples. Combined with spark plasma sintering, the production of highly pure and dense samples is achieved in a very short time, at least 8 times shorter than in conventional liquid-solid-vapor synthesis process. The process described in this paper is a promising way to produce high performance tetrahedrite materials for cost-effective and large-scale thermoelectric applications

Low temperature consolidation of nanocrystalline apatites. Toward a new generation of calcium phosphate ceramics

Biomimetic nanocrystalline apatites analogous to bone mineral can be prepared by different ways. These non-stoichiometric compounds possess a high reactivity related to the presence of a metastable hydrated layer on the surface of the nanocrystals. The processing of such unstable phases by conventional techniques at high temperature strongly alters their physico-chemical and biological properties. Therefore, several low temperature routes have been investigated taking advantage of the structural characteristics of these compounds. Self-setting, injectable cements leading to nanocrystalline apatites have been developed. Solid mesoporous ceramic-like materials can also be obtained at low temperature by drying aqueous suspensions of nanocrystalline apatites. Among the most promising routes, however, are pressure sintering and spark plasma sintering at temperatures lower than 300°C. These techniques produce ceramics retaining most of the characteristics of the nanocrystals. The consolidation mechanism is thought to implicate the high mobility of ions within the hydrated layer. This work should be helpful for the preparation of a new generation of resorbable highly-reactive bioceramics