Luis M. Rodríguez-Lorenzo

Influence of fluorine in the synthesis of apatites. Synthesis of solid solutions of hydroxy-fluorapatite

Hydroxy-fluorapatites (OH-FAps) occur biologically in teeth and form the basis for application as biomaterials. This work aims to synthesize a series of fluoride substituted calcium hydroxyapatites (OHAps) to determine how fluoride influences the synthesis and the resulting characteristics of solid solutions. OH-FAPs powders were synthesized with a chemical composition of Ca(10)(PO(4))(6)(OH)(2-x) F(x), with x=0.0, 0.4, 0.8, 1.2, 1.6 and 2.0. The synthesis of partially substituted OHAp yields materials with lower crystallinity and higher specific surface area than OHAp or fluorapatite (FAp). The smallest crystal size of 263A, occurs at less than 50% hydroxyl substitution with fluoride at x=0.4, and the highest surface area of 132m(2)/g occurs at x=0.8. Reaction kinetics occur faster at higher fluoride content, producing the expected Ca/P ratio of 1.67 only for x=2.0. X-ray and IR studies show that OH-FAPs are homogeneous solid solutions instead of mixtures of OHAp and FAp. The presence of a high fluoride concentration increases the driving force for crystal growth during the calcination process.

Fabrication of hydroxyapatite bodies by uniaxial pressing from a precipitated powder.

Hydroxyapatite (OHAp) has been obtained by using a precipitation method. Thermal stability of the precipitated powder has been checked to define an appropriate working range to process ceramic bodies without the appearance of odd phases. Pellets of hydroxyapatite have been manufactured by means of uniaxial pressing and convenient thermal treatments have been investigated to achieve high density OHAp bodies. Finally, factors that could affect the ultimate mechanical properties of the sintered bodies prepared by uniaxial pressing have been discussed.

Preparation and in vitro bioactivity of hydroxyapatite/solgel glass biphasic material

Hydroxyapatite/solgel glass biphasic material has been obtained in order to improve the bioactivity of the hydroxyapatite (OHAp). A mixture of stoichiometric OHAp and the precursor gel of a solgel glass, with nominal composition in mol% CaO-26, SiO2-70, P205-4, has been prepared. The amounts of components used have been selected to obtain a final relationship for OHAp/solgel glass of 60/40 on heating. Two different thermal treatments have been used: (i) 700 degrees C, temperature of solgel glass stabilisation and (ii) 1000 degrees C, lower temperature of hydroxyapatite sintering. The bioactivity of the resulting materials has been examined in vitro by immersion in simulated body fluid at 37 degrees C. The results obtained show that both materials are bioactive. The apatite-like layer grown is greater for the new materials than for the OHAp and the solgel glass themselves.

Sintered hydroxyfluorapatites. Part II: Mechanical properties of solid solutions determined by microindentation

Fluoride substitution within hydroxyapatite is an important occurrence for biological apatites and is a promising approach for the chemical modification of synthetic hydroxyapatite. Limited information on the influence of fluoride substitution for hydroxyl groups on the mechanical properties has provided the rationale for this study. Hydroxyfluorapatites with 0%, 20%, 40%, 60%, 80% and 100% replacement of hydroxyl groups with fluoride ions were assessed for hardness, elastic modulus, fracture toughness and brittleness using microindentation of sintered pellets. The production of samples with a similar grain size and density allowed the influence of fluoride on mechanical properties to be determined. It was found that the hardness remains unaffected until 80% replacement of hydroxyl groups with fluoride, after which the hardness rapidly increases. The elastic modulus increases linearly with fluoride content. Fracture toughness is improved with fluoride incorporation into the lattice and reaches a peak of 1.8 for a 95% dense sintered pellet with a 60% fluoride replacement, followed by a rapid decrease at higher fluoride concentrations. The brittleness index is lowered to a minimum at 60%, after which a rapid increase occurs. High fluoride levels are unfavourable from a mechanical perspective, are not recommended for biomaterials, and can lead to a higher incidence of fracture where sodium fluoride, for treatment of osteoporosis, may produce a highly fluoridated hydroxyapatite.

Colloidal processing of hydroxyapatite.

Reliable bioceramics are needed to implement the high requirements that living tissues demand. This work focuses on the processing steps necessary to manufacture advanced ceramics that can be used as implant devices. The influence of the heat treatment temperature on the characteristics of a precipitated hydroxyapatite (OHAp) powder was evaluated in order to obtain an appropriate specific surface area for colloidal processing. Ball milling of the calcined powders for 20 h was required to improve the rheological properties of the suspensions and the packing ability during consolidation. Different dispersing agents were tested and the first trial was made based on their effect on the zeta potential. The most promising ones were then selected and their efficiency was evaluated from rheological measurements and slip-casting performance of suspensions prepared at different solids loading. Targon 1128 was revealed to be the most efficient dispersant, enabling to prepare fluid suspensions containing 50 vol% solids and the highest green and sintered density values to be obtained.

Sintered hydroxyfluorapatites. Part I: Sintering ability of precipitated solid solution powders

The presence of fluoride within apatites occurring naturally within the body provides the basis for investigating the sintering ability of fluoride containing hydroxyapatites for use as biomaterials. Hydroxyfluorapatites were synthesized and then calcined to produce a 0%, 20%, 40%, 60%, 80% and 100% replacement of the hydroxyl ions with fluoride in the hydroxyapatite structure. Fluoride ion occupancy within the apatites was found to be about 90% of the anticipated value. Pycnometry results revealed a constant true density for powders of low to medium fluoride content followed by a rapid increase to the fluorapatite composition. Powders were uniaxially pressed, cold-isostatically pressed and sintered at 1150 degrees C, 1200 degrees C and 1250 degrees C. All hydroxyfluorapatite powders displayed a comparable ability for densification except when hydroxyl groups and fluoride ions were present at a comparable concentration. The grain size revealed that this composition also exhibited the smallest grain size and displayed the highest activation energy for grain growth. The lower diffusion created by similar amounts of fluoride and hydroxide within the lattice decreases grain growth and densification.

Composite biomaterials based on ceramic polymers. I. Reinforced systems based on Al2O3/PMMA/PLLA

Composite biomaterials with good mechanical response and a partially biodegradable character were prepared by the free radical polymerization of mixtures of alpha-Al2O3, low-molecular-weight but crystalline poly(L-lactic acid) (PLLA), and methyl methacrylate (MMA). Cylindrical specimens prepared with different composition were characterized by thermogravimetry, calorimetry, 1H-NMR spectroscopy, and x-ray diffraction (XRD). The in vitro biodegradative process was studied in different media, following variations of the pH, gravimetric weight loss of the specimens, and crystalline domain change by XRD after immersion in pure water and buffered solutions at pH 4.0 and pH 8.0 for 90 days. Formation of a relatively porous structure with good cohesion after the biodegradative treatment (confirmed by SEM) was observed. These systems can be considered for applications in orthopedic surgery as filling biomaterials and even as control drug-delivery systems.

Synthesis, characterization, bioactivity and biocompatibility of nanostructured materials based on the wollastonite-poly(ethylmethacrylate-co-vinylpyrrolidone) system

Composite materials are very promising biomaterials for hard tissue augmentation. The approach assayed in this work involves the manufacturing of a composite made of a bioactive ceramic, natural wollastonite (W) and a nanostructured copolymer of ethylmethacrylate (EMA) and vinylpyrrolidone (VP) to yield a bioresorbable and biocompatible VP-EMA copolymer. A bulk polymerization was induced thermally at 50 degrees C, using 1 wt % azobis(isobutyronitrile) (AIBN) as free-radical initiator. Structural characterization, compressive strength, flexural strength (FS), degradation, bioactivity, and biocompatibility were evaluated in specimens with a 60/40 VP/EMA ratio and ceramic content in the range 0-60%. A good integration between phases was achieved. Greater compression and FS, in comparison with the pure copolymer specimens was obtained only when the ceramic load got up to 60% of the total weight. The soaking in NaCl solution resulted in the initial swelling of the specimens tested. The maximum swelling was reached after 2-3 h of immersion and it was significantly greater for lower ceramic loads. This result makes the polymer component the main responsible for the interactions with the media. After soaking in SBF, microdomains segregation can be observed in the polymer component that can be related with a dramatic difference in the reactivity of both monomers in free radical polymerization, whereas the formation of an apatite-like layer on the W surfaces can be observed. Biocompatibility in vitro studies showed the absence of cytotoxicity of all formulations. The cells were able to adhere on the polystyrene negative control and on specimens containing 60 wt % wollastonite forming a monolayer and showing a normal morphology. However, a low cellular growth was observed.

Control of structural type and particle size in alumina synthesized by the spray pyrolysis method

Abstract Fine particles of aluminum oxide (amorphous, α, γ and θ) have been synthesized by spray pyrolysis as a function of both in situ and annealing temperatures and nature of precursors used in the preparation method. A scanning electron micrograph study shows as-received materials constituted by filled spherical particles, the ulterior annealing leading to the formation of small crystallites on the precursor spheres.

The synthesis and characterisation of strontium and calcium folates with potential osteogenic activity

Compounds having the general formula MFO·4H2O where M = Ca or Sr and FO = a folate anion were prepared and their structure and physico-chemical properties were determined by elemental and thermal TGA, DSC analysis, FTIR, and EDAX spectroscopies and DRX. The results indicate that the two compounds form stable structures where folic acid acts as a self-bridging ligand via two bidentate carboxylate groups. Moreover the two compounds showed a low toxicity in vitro response as h-osteoblast cell viability was not negatively affected by the presence of folate derivatives within the range of 0.063-0.5 mg ml-1. The results also indicate that the folate derivatives that are formed overcome the toxic effects related to free Sr2+ ions. The range of maximum cell viability corresponding with a concentration of SrFO falls within the in vitro physiologically active range for strontium while within the same range the strontium derivative showed a potential osteogenic activity as indicated by the overexpression of ALP activity.