Tesis doctoral de Carolina Mochales Palau
In the last years, the interest for calcium phosphates as biomaterials has increased due to their multiple clinical applications in the musculoeskeletal system: bone defect filling, bone replacement and reconstruction, coating of metal prostheses, drug delivery, etc. in addition, calcium phosphate particles are used as bioactive fillers in polymer matrix composites. These ceramics are usually composed of stoichiometric hydroxyapatite (ha) with a calcium to phosphorus molar ratio (ca/p) of 1.67, pure íY-tricalcium phosphate (íY-tcp) with a ca/p of 1.5, or a mixture of íY-tcp and ha (biphasic calcium phosphate ceramic, bcp) with 1.5 < ca/p < 1.67. hydroxyapatite is the main constituent of the mineral phase of bone. However, biological hydroxyapatite is not stoichiometric but calcium deficient and contains ionic substitutions such as as carbonates, potassium, sodium, etc. recent studies point out to dry mechanosynthesis as a possible technique to synthesize hydroxyapatite with an easy control of the ca/p molar ratio. Moreover, this technique allows the easy incorporation of different ions in the crystalline structure of the synthesized products. Mechanosynthesis is a technique based in the mechanical activation of a chemical reaction by the energy produced by milling from the collisions between the milling balls and vial (ball-ball, ball-vial). In the case that no water is added to the reactants mixture, this process is called dry mechanosynthesis. this thesis studies dry mechanosynthesis of calcium deficient hydroxyapatites (cdhas) with several ca/p molar ratios between 1.5 and 1.67 by milling a mixture of dicalcium phosphate dihydrate (dcpd) and calcium oxide (cao). The study and optimization of cdhas dry mechanosynthesis was developed in collaboration with the research centre for artificial biopolymers (crba-cnrs 5473) of the faculty of pharmacy of the university of montpellier. moreover, dry mechanosynthesis has been shown to be a new and effective method to introduce silicon into the hydroxyapatite lattice by milling dcpd and cao together with a mechanosynthesised amorphous calcium silicate hydrate (csh), resulting in a silicon substituted calcium deficient hydroxyapatite (si-cdha). Although silicon is not present in mature bone, the influence of silicon in bone formation processes has been reported by numerous biological studies. Moreover, the enhancement of the bioactive potential of calcium phosphate ceramics by the addition of silicon has been demonstrated. Thus, in the last decade, many studies are being focused on the synthesis of silicon-substituted hydroxyapatite for bone replacement applications. the chemical structure of both cdha and si-cdha obtained by this milling technique was analysed in collaboration with the dental biophysics department of the queen mary college, university of london. the effect of three milling parameters was studied for cdha mechanosynthesis in centrifugal mills: (a) mass and surface area of balls, (b) milling velocity (disc rotation speed) and (c) mill eccentricity (rd), corresponding to the distance between the center of the vial and the rotating disc. The effect of the balls used during milling were modelled by two simple linear equations, which related dcpd disappearance rate constant (kdcpd) and final reaction time (tf) with the product of the total mass (m) and total surface area (sa) of the balls used in the milling. The reaction kinetics dependence on the milling rotation speed was also modelled by quadratic equations relating kdcpd and tf with the square of the disc rotation speed (¿d 2 ). These equations allow relating two reactions performed in the same mill but using different milling balls by a simple correction factor: the product of the total mass and surface area of the balls used (m x sa), and taking into account the rotation velocities used during milling as the square of the disc rotation speed (¿d 2 ). Moreover, the reaction rates in centrifugal mills with different eccentricities (rd) were related to the ratio of the square of the eccentricity. planetary mills, with a constant ratio between vial and disc rotation speed (¿v/¿d), were shown to follow the same relation of kdcpd and tf with the square of the rotation speed. Moreover, the final reaction time required to synthesise cdhas by milling in a planetary mill was significantly lower than the one required when milling in a centrifugal mill. Thus, planetary mills were more efficient than centrifugal mills. hydroxyapatites obtained by dry mechanosynthesis consisted of agglomerates of nano sized crystals, and the crystallinity of the hydroxyapatite powder was shown to increase with milling time. Mechanosynthesis of apatites with increasing ca/p molar ratios leaded to apatitic structures with decreasing hpo4 2- groups in agreement with general cdhas formulations. Rietveld refinement of a mechanosynthesised cdha with ca/p molar ratio of 1.5, obtained after milling 40g during 1 h 15 m, showed an increase in the a lattice parameter (9.4418(20) í¥) and a decrease in the c lattice parameter (6.8745(17) í¥), in agreement with the literature. during milling some contamination came from the jar (vial) and balls, as a consequence of the collisions between them which wear them down. Contamination stemming from the jar (vial) and the balls material produced by the collisions between them during milling also depended on the milling parameters. For instance, it increased with longer milling times and decreased when higher amounts of reactants powder are milled. The effect of these two milling parameters, i.E. Milling time and mass of powder, competed during cdhas mechanosynthesis. Indeed, a longer milling time was necessary to complete the cdha mechanosynthesis reaction of a higher amount of reactants powder. Although the increase in the milling time, contamination produced along the cdhas mechanosyntheses decreased when milling higher amounts of powder reactants due to a screening effect (coverage of the balls and vial walls). Moreover, this screening effect also decreased contamination when using a planetary mill instead of a centrifugal one. In fact, for a planetary mill, porcelain and agate balls and jar produced almost insignificant amounts of contaminant, which were under the detection limit of inductively coupled plasma optical emission spectroscopy (icp-oes) technique. However, corundum jar and balls showed a higher contamination. Thus, it is very important to choose adequate balls and jar materials. In this sense, it was concluded that porcelain and agate are adecquate jar and balls materials, contrary to corundum. in the case of si-cdha with a ca/(p+si) molar ratio of 1.5, rietveld refinements showed that the reaction product was a monophasic silicon-containing apatitic phase. After 2 h of milling, lattice parameters showed a significant expansion of the c lattice parameter of 0.011 í¥ compared to mechanosynthesised cdha, while the a lattice parameter remained constant. Increasing milling times (12, 18 and 24 h) did not show any significant change in the c lattice parameter. Infrared (ir) spectroscopy of si-cdha showed typical additional bands (503, 752 and 890 cm-1 ) previously assigned in high temperature (around or over 900ºc) sintered si-has to the effect of silicate ions (sio4 4- ) substitution in its lattice. Moreover, bands corresponding to carbonate (co3 2- ) and, or hydrogenphosphate (hpo4 2- ) ions substitution were shown. Since cdha did not show carbonate bands, it was concluded that the incorporation of silicate ions into the apatite lattice is in parallel with the incorporation of carbonate ions. With increasing milling times, the additional ir band at 890 cm-1 assigned to silicate ions introduction increased and carbonate bands evolved suggesting a rearrangement carbonate ions between poorly defined sites and phosphate sites (b-type substitution). After sintering at 900ºc for 2 h, the additional infrared bands assigned in the literature to the silicate ions substitution disappeared and other additional bands not reported previously appeared. Moreover, carbonate ions almost disappeared and the remaining ones rearranged into hydroxyl (oh- ) sites (a-type substitution). Sintered si-cdha samples showed a significant decrease of the a lattice parameter in addition to the increase in the c lattice parameter, compared to mechanosynthesised cdha. All of these results point out to the silicon introduction into the cdha lattice in both sintered and non-sintered si-cdhas. Nuclear magnetic resonance (nmr) spectra corroborated that non-sintered si-cdha samples contain a few hpo4 2- groups and showed that both, sintered and non-sintered si-cdhas, contained silicate species in a different form than the initial amorphous csh. Moreover, silicate species present in si-cdha samples evolved with increasing milling times from q4 species to q0 and q2 /q3 species. This result showed the progressive reduction in bonding complexity with increased milling time. for the non-sintered si-cdhas, a possible charge compensation mechanism, which involves the increase of co3 2- groups was proposed: + 2 3 4 4 3 4 2 co sio po as well as a two-step process of the silicate introduction into the cdha lattice by means of mechanosynthesis: during the first hours of milling, co3 2- ions would be incorporated along with sio4 4- ions. For longer milling times, the increase in the incorporation of sio4 4- ions in the apatite lattice would occur simultaneously to a rearrangement of the carbonate ions already present in the apatite lattice. moreover, the subsequent sintering at 900ºc for 2h of the mechanosynthesised si-cdha, would release the the co3 2- ions substituting po4 3- groups (b-type) out of the structure or to oh- positions (a-type). The release of co3 2- ions from po4 3- positions would favour the sio4 4- ions introduction into the apatite lattice in po4 3- positions. This heat treatment could also favour the progressive introduction of the silicate ions from the crystallites in the grain surface to the ones in the grain bulk. in summary, dry mechanosynthesis was proven to be an effective method to prepare both cdha and si-cdha, and planetary mills were more efficient than centrifugal mills, since reaction milling time was reduced considerably. the characterisation techniques used showed similar indications of silicate ions substitution for mechanosynthesised si-cdha as the ones reported in the literature for high temperature sintered si-has. However, a further detailed neutron diffraction study should be done to definetively prove the substitution of phosphate ions by silicate ions and find out their position in apatitic lattice. Moreover, the degree in the chemical homogeneity of the si-cdhas grains should be studied and the effect of the heat treatment in the mechanosynthesised si-cdha should be analysed.
Datos académicos de la tesis doctoral «Dry mechanosynthesis of calcium deficient and silicon substituted hydroxyapatites«
- Título de la tesis: Dry mechanosynthesis of calcium deficient and silicon substituted hydroxyapatites
- Autor: Carolina Mochales Palau
- Universidad: Politécnica de catalunya
- Fecha de lectura de la tesis: 07/05/2009
Dirección y tribunal
- Director de la tesis
- María Pau Ginebra Molins
- Tribunal
- Presidente del tribunal: josep antoni Planell estany
- philippe Boudeville (vocal)
- ibrahim Khairoun (vocal)
- wilson Rory (vocal)