Elisabetta Foresti

Magnesium influence on hydroxyapatite crystallization

Abstract X-ray diffraction, infrared absorption, and chemical investigations have been carried out on hydroxyapatite synthesized in the presence of different magnesium concentrations in solution. Magnesium inhibits the crystallization of hydroxyapatite through a reduction of Ca/P molar ratio and crystal sizes of apatite. The reduction of the crystal sizes is also very great for very low magnesium content and increases on increasing magnesium concentration in solution up to 35 Mg atom percent with respect to the total metal ions. The samples are completely amorphous between 35 and 50 Mg atom percent. For higher magnesium concentration different crystalline phases are formed. The results of the x-ray powder pattern fitting indicate that the HA crystal structure at most hosts magnesium amounts of about seven percent. Magnesium substitutes only the calcium atoms which form the channels containing the hydroxy ions. Since magnesium content is much smaller than that found in the solid phase, the greatest amount of magnesium must not be lattice bound. The extent of hydroxyapatite conversion into magnesium substituted β-tricalcium phosphate on heat treatment appears strongly related to magnesium content of the apatitic solid phase. 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 role of magnesium on the structure of biological apatites

SummaryX-ray diffraction, infrared absorption spectroscopy, and chemical investigation have been carried out on deproteinated samples of turkey leg tendon at different degrees of calcification. The inorganic phase consists of poorly crystalline B carbonated apatite. On increasing calcification, the apatite crystal size, as well as its thermal stability, increase while the relative magnesium content is reduced. On the other hand, synchrotron X-ray diffraction data clearly indicate that apatite lattice parameters do not change as the crystals get larger. At the last stage of calcification the crystal size, chemical composition, and thermal conversion of the apatite crystallites approximate those of bone samples, which have been examined for comparison. The results provide a quantitative relationship between relative magnesium content and extent of apatite conversion into B-tricalcium phosphate by heat treatment. Furthermore, they suggest that the smaller crystallites laid down inside the gap region of the collagen fibrils are richer in magnesium than the longer ones that fill the space between collagen fibrils.

Inhibiting effect of zinc on hydroxylapatite crystallization

Abstract X-Ray diffraction and spectrophotometric analysis have been used to investigate the role of zinc on hydroxylapatite (HA) crystallization. The presence of zinc in solution strongly inhibits the crystallization of hydroxylapatite, which can be synthesized as a unique crystalline phase only up to zinc concentration of about 25 atom %. This phase exhibits a reduction of Ca/P molar ratio and crystal sizes with increasing zinc concentration. Although the Ca/Zn ratio in the solid phase is almost equivalent to that in solution, the values of the cell parameters of the apatitic phase indicate that zinc cannot appreciably substitute for calcium in HA structure. Therefore, zinc must be assumed to be adsorbed on the surface of apatite crystallites and/or in the amorphous phase. The extent of thermal conversion of HA into s-tricalcium phosphate (s-TCP) increases with increasing zinc concentration in the solid phase, either when it is obtained by means of synthesis in solution or after cyclic pH fluctuation. The decrease of the lattice constants of s-tricalcium phosphate with increasing zinc concentration in the solid phase indicates that zinc partially replaces calcium in this structure. The inhibiting effect of zinc on HA crystallization and its preference for s-TCP structure closely resembles the behavior previously observed for magnesium.

Isomorphous substitutions in β-tricalcium phosphate: The different effects of zinc and strontium

Abstract X-ray diffraction and infrared absorption analyses have been carried out on zinc-substituted and strontium-substituted β-tricalcium phosphate prepared by solid-state reaction. Zinc can substitute calcium up to 20 atom %, inducing a nonlinear variation of the lattice constants and an increase in degeneracy of the PO 4 3− infrared absorption bands. On the other hand, up to 80 atom % of strontium can enter into the crystal structure of β-tricalcium phosphate, causing a linear enlargement of the unit cell, in agreement with its greater ionic radius compared to that of calcium. Furthermore, strontium incorporation provokes the shift of the PO 4 3− absorption bands toward lower frequencies. On the basis of the data previously obtained on magnesium-substituted β-tricalcium phosphate, the different behaviours exhibited by zinc and strontium could be attributed to a different distribution into the cationic sites of the β-tricalcium phosphate structure. The results allow us to relate the effect of bivalent ions on the structure and relative stability of calcium phosphates with their ionic radius, and can be utilized to interpret the role of ionic composition on the properties of biological phosphates.

Surface enamel remineralization: biomimetic apatite nanocrystals and fluoride ions different effects

A new method for altered enamel surface remineralization has been proposed. To this aim carbonate-hydroxyapatite nanocrystals which mimic for composition, structure, nanodimensions, and morphology dentine apatite crystals and resemble closely natural apatite chemical-physical properties have been used The results underline the differences induced by the use of fluoride ions and hydroxyapatite nanocrystals in contrasting the mechanical abrasions and acid attacks to which tooth enamel is exposed. Fluoride ions generate a surface modification of the natural enamel apatite crystals increasing their crystallinity degree and relative mechanical and acid resistance. On the other hand, the remineralization produced by carbonate-hydroxyapatite consists in a deposition of a new apatitic mineral into the eroded enamel surface scratches. A new biomimetic mineral coating, which progressively fills and shadows surface scratches, covers and safeguards the enamel structure by contrasting the acid and bacteria attacks.

Rietveld structure refinements of calcium hydroxylapatite containing magnesium

The crystal structures of four hydroxylapatite (HA) samples prepared from solutions in the presence of 10, 15, 25 and 30 Mg-atom-% have been investigated by X-ray powder pattern fitting. The total magnesium content of the solid samples, as determined by chemical analysis, was 4.9, 14.1, 20.4 and 30.6 Mg-atom-%, respectively. Rietveld analysis was performed using the computer program PREFIN implemented with routines which allow the refinements of the average crystallite sizes. Different refinement procedures were carried out in order to evaluate the effect of the amorphous and background profiles on the occupancy factor data. For comparison, magnesium-free hydroxylapatite was refined with the same strategies. The results of the different approaches indicate that the degree of magnesium substitution for calcium in the HA structure can be at most ~ 10 atom-%. Magnesium substitutes calcium preferentially at the 6(h) site. The broadening of the diffraction peaks increases on increasing the total magnesium content in the solid phase, which is always significantly higher than the amount incorporated into the HA structure. The excess is probably located in the amorphous phase and/or on the crystallite surface.

Self-organizing functional materials via ionic self assembly: porphyrins H- and J-aggregates on synthetic chrysotile nanotubes.

Positively charged, synthetic chrysotile nanotubes act as inorganic tectons supporting H- and J-type aggregates of anionic porphyrins with markedly different optical properties, making these nanohybrid materials of interest for application in nanotechnology.

Synthetic Biomimetic Carbonate-Hydroxyapatite Nanocrystals for Enamel Remineralization

New biomimetic carbonate-hydroxyapatite nanocrystals (CHA) have been designed and synthesized in order to obtain a remineralization of the altered enamel surfaces. Synthesized CHA mimic for composition, structure, nano dimension and morphology bone apatite crystals and their chemical-physical properties resemble closely those exhibited by enamel natural apatite. CHA can chemically bound themselves on the surface of natural enamel apatite thanks to their tailored biomimetic characteristics. The remineralization effect induced by CHA represents a real new deposition of carbonate-hydroxyapatite into the eroded enamel surface scratches forming a persistent biomimetic mineral coating, which covers and safeguards the enamel structure. The experimental results point out the possibility to use materials alternative to fluoride compounds which is commonly utilized to contrast the mechanical abrasions and acid attacks. The apatitic synthetic coating is less crystalline than enamel natural apatite, but consists of a new biomimetic apatitic mineral deposition which progressively fills the surface scratches. Therefore the application of biomimetic CHA may be considered an innovative approach to contrast the acid and bacteria attacks.

Structural and chemical characterization of inorganic deposits in calcified human mitral valve

X-ray diffraction, i.r. absorption, and chemical analyses have been carried out on the mineral deposits of calcified human mitral valves and glutaraldehyde-preserved porcine aortic grafts. The mineral deposits isolated from highly calcified mitral valves and porcine aortic grafts are constituted of type B-carbonate apatite. Magnesium substituted beta-tricalcium phosphate is present, together with an apatitic phase similar to dahllite, in the ashes of poorly calcified mitral valves. The contraction of the unit cell of beta-tricalcium phosphate due to magnesium incorporation is compared with the variation of the lattice constants of synthetic beta-tricalcium phosphate at different degree of magnesium substitution for calcium. The results reveal the important role of magnesium on the calcification of human valves. In fact, the apatitic phase deposited at the beginning of the calcification process, when there is a high magnesium content, converts completely into beta-tricalcium phosphate by heat treatment at 1,000 degrees C. On the other hand, when the calcification becomes massive, magnesium content appears highly reduced, and the deposited apatitic phase is characterized by a high thermal stability.

Thermal behavior of bone and synthetic hydroxyapatites submitted to magnesium interaction in aqueous medium.

The thermal behavior of the products obtained from magnesium interaction with powdered femoral bone and carbonate containing synthetic hydroxyapatite under conditions of pH fluctuation in aqueous medium has been investigated. The products, heat treated at different temperatures from 100 to 1300 degrees C, have been characterized by infrared spectroscopy and X-ray diffraction technique. The results show that the interaction with magnesium ion destabilizes the apatitic structure and favours its thermal conversion into beta-tricalcium phosphate (beta-TCP). The replacement of magnesium with calcium in the beta-TCP crystal lattice hinders its subsequent thermal conversion into the alpha form. The influence of magnesium on the thermal stability is much more evident for carbonate-containing synthetic hydroxyapatite than for bone apatite.