WO2000068144A1

WO2000068144A1 - Method for the preparation of carbonated hydroxyapatite compositions

Info

Publication number: WO2000068144A1
Application number: PCT/GB2000/001738
Authority: WO
Inventors: William Bonfield; Iain Ronald Gibson
Original assignee: Abonetics Limited
Priority date: 1999-05-10
Filing date: 2000-05-05
Publication date: 2000-11-16
Also published as: AU4592100A; GB9910847D0; GB2349877A

Abstract

A process for the preparation of a carbonate-substituted hydroxyapatite, which process comprises heating a hydroxyapatite having a Ca/P molar ratio greater than 1.67 but less than approximately 1.82 at a temperature of from 600 to 1200 °C in an atmosphere comprising CO2. This process enables a substantially single phase carbonate-substituted hydroxyapatite composition to be produced which does not contain sodium or ammonium ions.

Description

METHOD FOR THE PREPARATION OF CARBONATED HYDROXYAPATITE COMPOSITIONS

The present invention relates to a method for forming carbonated hydroxyapatite compositions and, in particular, to a method for forming carbonated hydroxyapatite compositions which are substantially sodium-free and ammonium-free and which are substantially of a single phase. Synthetic hydroxyapatite Ca₁₀ (P0₄) ₅ (OH) ₂ has been reported as having been used as a bone replacement material in porous, granular, plasma sprayed and dense forms. Investigations have shown hydroxyapatite to be similar structurally to bone material. However hydroxyapatite is one of the range of stoichiometric calcium phosphate apatites. Human and animal bone mineral have been shown to contain significant amounts of from 3 to 7 wt% of carbonate. There is evidence that the carbonate group can substitute in two sites, the phosphate and hydroxyl sites, termed B and A respectively; bone mineral being predominately a B type apatite. As a result of this similarity in chemical composition, it is envisaged that carbonated hydroxyapatite will have better bioactivity than unsubstituted stoichiometric hydroxyapatite which is currently used in commercial applications, such as plasma-sprayed coatings on metallic implants and porous hydroxyapatite ceramic bone substitutes. A carbonate substituted hydroxyapatite would also find application for use in chromatography and for purification, such as the removal of heavy metal ions by adsorption.

The preparation of carbonate-substituted hydroxyapatite ceramic materials must be easy and reproducible in order to achieve commercial exploitation. Additionally, the carbonate-substituted hydroxyapatite composition must be thermally stable such that it will not decompose to undesirable secondary phases (e.g. tricalcium phosphate or calcium oxide) upon calcining/sintering. Furthermore, during this heat treatment, the carbonate-substituted hydroxyapatite must not lose the carbonate ions that have been substituted into the hydroxyapatite structure.

Up to the present time, the methods which have been reported to prepare carbonate-substituted hydroxyapatite compositions have involved one of the following procedures.

The heating of a stoichiometric hydroxyapatite ceramic composition in a C0₂ atmosphere at approximately 900°C for several days [R.Wallaeys, Silicon, Sulphur, Phosphates. Coll. Int. Union Pure Appl. Chem. Munster (1954) 183-190]. This process results in low levels of carbonate substitution, with poor control over the extent of carbonate substitution and the homogeneity of the substitution throughout the sample. Furthermore, the carbonate substitution is at the wrong site, i.e. the A site, to provide a material which is equivalent to bone.

A wet precipitation method using Na₂C0₃, NaHC0₃ or (NH₄)₂C0₃ as a source of carbonate ions [See, for example: Y. Doi, Y. Moriwaki, T. Aoba, M. Okazati, J. Takahashi and K. Joshin, "Carbonate apatites from aqueous and non-aqueous media studied by esr, IR and X-Ray Diffraction: Effect of NH,^T ions on crystallographic parameters", J. Deut . Res. 61(1982) 429-434, D.G.A. Nelson and J.D.B. Featherstone, "Preparation analysis and characterization of Carbonated apatites", Calcif. Tiss. Int. 34(1082) 569- 581] . This process results in the substitution of the additional ions, Na^" or NH₄ ^T, into the hydroxyapatite structure, poor thermal stability of the product upon calcining/sintering, the loss of large quantities of the carbonate ions upon heating, and poor control of the levels of the carbonate substitution.

Incorporation of carbonate ions by soaking carbonate-free samples in a saturated solution of carbon-dioxide. [E.G. Nordstrom and K.H. Karlsson, "Carbonate-doped hydroxyapatite", J.Mat. Sci. Mater. Med. 1 (1990) 182-184]. A soaking time of 2 months gives a carbonate content of 4 to 5.5 wt% as determined by TGA. At 1100°C a 3.3wt% C0₂ loss was registered. This method involves soaking pure, stoichiometric, sintered hydroxyapatite ceramic powder in mineral water for up to 2 months to try to ^Λion- exchange' carbonate ions from solution into the hydroxyapatite ceramic.

Sintering of stoichiometric hydroxyapatite in a C0₂ atmosphere for several days to give A-type substitution. Co-precipitating some carbonate and some calcium phosphate to give B-type substitution. Reacting fluorapatite and alkaline earth carbonate at 900°C in dry carbon dioxide gas to give B-type substitution [all described in G. Bonel, "Contribution a 1' etude de la carbonation des apatites", Ann. Chim. 7 (1972) 127-144].

Slowly adding a solution of calcium nitrate in contact with carbon dioxide gas to a solution of ammonium phosphate (pH=8.6, 80°C) . The precipitation lasts 64 hours and the suspension is maintained at the above conditions for a further 51 hours [A. Barroug, C. Rey, J.C. Trombe and G. Montel, "The synthesis in aqueous-media of AB carbonate apatite similar to dental enamel", C.R.Acad. Sc. Paris 292 [3] (1981) 303- 306] . After ageing for 51 hours the apatite contains 1.3 wt% carbonate. Chemical analysis gives a Ca/P ratio of 1.63-1.61. No heat treatment is carried out so the stability of the apatite and the carbonate content after heating/sintering is unknown.

Neutralization of a Ca(OH)₂ solution with H₃P0₄ under stirring at 37°C in presence of C0₂ [A. Bigi, F. Marchetti, A. Ripamonti, N. Roveri and E. Foresti, "Magnesium and strontium interaction with carbonate- containing hydroxyapatite in aqueous medium", J. Inorg. Bioche . 15 (1981) 317-327]. The product is stored for one week in contact with the mother solution, then filtered, washed with distilled water, and dried at 100°C. Hydroxyapatite with a Ca/P molar ratio of 1.95 is obtained. The carbonate content [FTIR] is 6 wt% in a sample dried at 100°C, decreasing to a value of 2.5 wt% in a sample heat treated at 700°C, and is undetectable at 1000°C.

EP-A-0722773 and JP-A-8225312 disclose the preparation of an A-type substituted hydroxyapatite in which the carbonate ions substitute for OH^" ions in the structure.

EP-A-0626590 discloses the preparation of a carbonate substituted apatite in which the Ca/P ratio is maintained at approximately 1.66 and sodium and carbonate ions are co-substituted into the lattice with the amount of carbonate that is substituted being controlled by the amount of sodium bicarbonate used in the reaction.

WO-A-94/08458 discloses a process for the preparation of carbonated hydroxyapatite in which the starting materials are mixed at room temperature and the material sets to form a cement at room or physiological temperature. The source of carbonate ions is solid calcium carbonate. The material produced is poorly-crystalline or amorphous apatite which contains sodium ions.

JP-A-61151011 discloses adding Ca(OH)₂ and CaC0₃ to a slurry of CaHP0. The C0₃ ions are introduced into the reaction mixture as insoluble CaC0₃ and not via solution. The ratios of Ca/P used are always less than 1.67. After sintering at 1000° to 1100°C the carbonate content of the resulting material is less than 0.1%.

It is mainly due to the problems encountered with the preparation routes discussed above that these routes have not been developed to prepare carbonate- substituted hydroxyapatite ceramic materials commercially. We have now developed a novel process for the preparation of a substantially single phase carbonate- substituted hydroxyapatite composition which overcomes the problems of the prior art methods and does not contain sodium or ammonium ions. Accordingly, the present invention provides a process for the preparation of a carbonate-substituted hydroxyapatite, which process comprises heating an hydroxyapatite having a Ca/P molar ratio greater than 1.67 but less than approximately 1.82 at a temperature of from 600 to 1200°C in an atmosphere comprising C0₂. Preferably the atmosphere also comprises H₂0. The presence of H₂0 appears to increase the maximum temperature that can be used without decomposition of the product. Carbonate-substituted hydroxyapatites, containing between 0 and 5 wt% C0₃ ^2" _, may be prepared by this simple process. Furthermore, the carbonate- substituted hydroxyapatites produced by this process are substantially single phase. This means that greater than 98%, preferably greater than 99% and most preferably approximately 100% of the structure is of a single phase. Also, the carbonate-substituted hydroxyapatites produced by this process are substantially free of sodium and ammonium ions. Carbonate-substituted hydroxyapatites prepared by this process have both A-type and B-type substitution. When the heat-treatment takes place at a lower temperature, such as 600-700°C, there is a roughly equal mix of A-type and B-type substitution and the products may be represented by the general formula

Ca₁₀ (P0₄) ₆__x (C0₃) _x (OH) ₂-_x (C0₃) _x where x is greater than 0 and less than or equal to 0.5.

Commonly however some A-type substituted carbonate is lost on heat treatment at higher temperatures, such as 900-1000°C or above, and this is most noticeable as the level of carbonate substitution is increased. Higher temperatures such as 900-1000°C are required to sinter the carbonate-substituted hydroxyapatite to near-theoretical densities, for applications such as granules and dense and porous ceramics. The loss of carbonate can be represented by the equation below:

Ca₁₀ (P0₄ ) ₆__x (C0₃) _x (OH) ₂__x (C0₃) _x

1 Ca₁₀(PO₄)₆._x(CO₃)_x(OH)₂._x(CO₃)_x._yO_y + yC0₂

Where y increases as the temperature increases. There is a point, when y increases significantly, where CaO will form. Thus, at higher temperatures, carbonate- substituted hydroxyapatites will (a) eventually decompose to a purely B-type carbonate-substituted hydroxyapatite and calcium oxide and (b) be limited in the amount of carbonate substitution that can be made. In the context of the present specification hydroxyapatites having a Ca/P molar ratio greater than 1.67 but less than approximately 1.82 will be referred to as "Ca-rich" hydroxyapatites. A Ca-rich hydroxyapatite is preferably prepared by process which comprises as an essential step a precipitation reaction between Ca(OH), and H₃P0₄. A suitable process for preparing a Ca-rich hydroxyapatite, which comprises as an essential step a precipitation reaction between Ca(OH)₂ and H₃P0₄, may comprise the steps of:

(i) providing a suspension of Ca(OH)₂ in water and a solution of H₃P0₄ in water,

(ii) adding the solution to the suspension, preferably in a dropwise manner, to provide a mixture wherein the molar ratio of Ca/P is greater than 1.67 but less than approximately 1.82,

(iii) optionally adjusting the pH of the mixture resulting from step (ii) to a value in the range of from 10 to 11, preferably by the addition of ammonia,

(iv) optionally stirring the mixture resulting from step (ii) or step (iii), preferably for a period of from 1 to 3 hours,

(v) allowing the mixture obtained from any one of steps (ii) , (iii) or (iv) to age without stirring, preferably for a period of from 5 to 24 hours, (vi) isolating solid material from the mixture resulting from step (v) , preferably by filtration,

(vii) drying the solid material isolated in step (vi) , preferably by heating at approximately 80°C for a period of from 5 to 24 hours, (viii) optionally forming the dry solid material resulting from step (vii) into a powder, said powder preferably consisting of particles substantially all of which have a particle size of less than approximately lOOμm. For the Ca-rich mixtures resulting from step (ii) , i.e. Ca/P > 1.67, the pH will generally be greater than 9, and for Ca/P > 1.70 the pH will be greater than 10, thus ideally no ammonia needs to be added. By preparing the hydroxyapatite with a Ca/P molar ratio > 1.67 but less than approximately 1.82, 'vacancies' or 'spaces' are left in the phosphate sites of the hydroxyapatite structure. The extent of incorporation of C0₃ ^2~ into the structure depends upon the number of available 'vacancies' and can therefore be controlled by controlling the Ca/P molar ratio, i.e. the higher the Ca/P molar ratio, the more carbonate ions can be substituted into the structure. Preferably the hydroxyapatite is prepared with a Ca/P molar ratio in the range of from 1.69 to 1.82, more preferably from 1.72 to 1.78. Most preferably the Ca/P molar ratio is approximately 1.76.

The calcium-rich hydroxyapatites are heat- treated/sintered at a temperature of from 600 to 1200°C, depending upon the desired carbonate content and the desired density properties of the product. As mentioned above, lower temperatures result in higher levels of carbonate substitution since carbonate is lost as C0₂ at higher temperatures. However, at higher temperatures a higher density sintered product is obtained. Thus, the choice of temperature results from a balance between the desired carbonate content and the desired density properties of the product. Preferably the heating occurs at a temperature of from 850 to 1000°C. More preferably the heating occurs at a temperature of from 900 to 950°C. Preferably a tube furnace is used for the heating process. The tube furnace is sealed at both ends with one or more gas inlets and one or more gas outlets attached at either end. The rate of heating to the desired temperature may be from 1 to 20°/min, preferably from 2.5 to 5°/min and most preferably approximately 2.5°/min.

Above these temperatures, partial decomposition to hydroxyapatite + CaO/CaC0₃ is observed. This therefore results in a product which is not substantially single phase. The C0₂ from the sintering atmosphere is incorporated into the hydroxyapatite structure as C0₃ ^2" groups which occupy, predominantly, the phosphate sites.

The concentration of C0₂ in the atmosphere may be controlled by altering the flow-rate of C0₂ gas through the heating apparatus. Preferably the flow- rate is in the range of from 0.5 to 3 1/min, more preferably from 1 to 2 1/min and most preferably approximately 1.5 1/min.

The concentration of H₂0 in the atmosphere, if included, will depend upon the concentration of C0₂ and the heating temperature. A suitable concentration of H₂0 in the atmosphere may be achieved by passing the C0₂ gas through a container of water, preferably at room temperature, prior to its entry into the heating apparatus. The heating may be carried out for a period of from 1 to 24 hours. Preferably, the heating is carried out for a period of from 1 to 8 hours. More preferably, the heating is carried out for a period of from 1 to 4 hours. Most preferably the heating is carried out for a period of from 1 to 2 hours.

By the method of the present process, a dense, crystalline, single-phase carbonate-substituted hydroxyapatite is obtained. The process allows a controlled amount of carbonate ions to be substituted into hydroxyapatite, with the amount being determined by the level of Ca/P molar ratio i.e. the higher the Ca/P molar ratio, the more carbonate ions can be substituted into the structure. A carbonate- substituted hydroxyapatite can be prepared by this method which has a carbonate content comparable to bone, with the carbonate substitution being predominantly on the B-site (the phosphate site) of hydroxyapatite. This process is extremely simple and it reduces the possibility of producing a material that will not be single-phase.

The carbonated hydroxyapatite compositions prepared in accordance with the present invention may be used in any of the applications for which hydroxyapatite is used, for example; the formation of plasma-sprayed coatings on metallic implants, the formation of porous ceramic bone substitutes, the preparation of composites with polymeric materials such as high density polyethylene, as granules or beads for packing or filling bone defects, as materials for use in chromatography or as materials for use in purification methods such as the removal of heavy metals by adsorption.

The present invention will be further described with reference to the accompanying drawings in which:

Figure 1 shows an X-ray diffraction pattern for a Ca-rich hydroxyapatite (HA) having Ca/P = 1.76 which has been heat-treated at 900°C in C0₂/H₂0. For comparison there is also shown an X-ray diffraction pattern for stoichiometric hydroxyapatite (HA) having Ca/P = 1.67 which has also been heat-treated at 900°C in C0₂/H₂0. The standard peaks of HA are marked on the diagram by black dots (from the ICDD (JCPDS) standard for HA, PDF No. 9-432) .

Figure 2 shows an X-ray diffraction pattern for a Ca-rich hydroxyapatite (HA) having Ca/P = 1.76 as precipitated. For comparison there is also shown an X-ray diffraction pattern for stoichiometric hydroxyapatite (HA) having Ca/P = 1.67 as precipitated.

The present invention will be further described by reference to the following examples (and comparative examples) which are intended to illustrate the present invention but not to limit its scope.

Examples

Initial preparation of stoichiometric and Ca-rich hydroxyapatites .

A 'calcium' suspension (suspension A) was prepared by dispersing the appropriate quantity, see

Table 1, of Ca(OH)₂ (BDH AnalaR) in 1 litre of deionised water. Suspension A was stirred for 15 minutes prior to the precipitation reaction.

Phosphoric acid, H₃P0₄, (BDH GPR 85% assay 0.3 moles 34.588g) was added to 1 litre of deionised water (solution B) . Solution B was stirred for 15 minutes prior to the precipitation reaction.

Solution B was added dropwise to suspension A, which was stirred constantly; the addition of solution B took approximately 3 hours and was performed at room temperature. After the solution B was added, the pH of the resulting mixture (mixture C) was adjusted to 10.5-11 with ammonia (BDH AnalaR) . Mixture C was then stirred for 2 hours and aged overnight without stirring. The aged mixture C was filtered and the resulting filtercake was dried at 80°C overnight. The dried filtercake was crushed and ground to a fine powder .

Table 1. Quantities of Ca(0H)₂ used to prepare Ca-rich hydroxyapatites (Ca/P = 1.72, 1.76 and 1.78) and stoichiometric hydroxyapatite (Ca/P=1.67). Comparative Example 1: Preparation of carbonate- substituted hydroxyapatite from stoichiometric hydroxyapatite .

A stoichiometric hydroxyapatite (Ca/P = 1.67) was prepared by the above method. The stoichiometric hydroxyapatite was heat-treated in a tube furnace at 900°C for 2 hours in a C0₂/H₂0 atmosphere. The C0₂/H₂0 atmosphere was provided by a flow-rate of 1.5 1/min of C0₂ through the furnace, the C0₂ being bubbled through water at room temperature prior to entering the furnace. The as-precipitated and the calcined/sintered powders of the stoichiometric hydroxyapatite (Ca/P = 1.67) have been characterised by CHN analysis, XRF, XRD and FTIR.

Comparative Example 2: Preparation of carbonate- substituted hydroxyapatite from stoichiometric hydroxyapatite .

A stoichiometric hydroxyapatite (Ca/P = 1.67) was prepared by the above method. The stoichiometric hydroxyapatite was heat-treated in a tube furnace at 900°C for 2 hours in air. The as-precipitated and the calcined/sintered powders of the stoichiometric hydroxyapatite (Ca/P = 1.67) have been characterised by CHN analysis and XRF.

Example 1: Preparation of carbonate-substituted hydroxyapatite from Ca-rich hydroxyapatite.

A Ca-rich hydroxyapatite (Ca/P = 1.76) was prepared by the above method. The Ca-rich hydroxyapatite was heat-treated in a tube furnace at 900°C for 2 hours in a C0₂/H₂0 atmosphere. The C0₂/H₂0 atmosphere was provided by a flow-rate of 1.5 1/min of C0₂ through the furnace, the C0₂ being bubbled through water at room temperature prior to entering the furnace. The as-precipitated and the calcined/sintered powders of the Ca-rich hydroxyapatite (Ca/P = 1.76) have been characterised by CHN analysis, XRF, XRD and FTIR. Results from analysis indicate that the carbonate-substitution is a combined AB-type substitution, described by the formula:

Ca₁₀ (P0₄ ) ₆__x (C0₃) _x (OH) ₂__x (C0₃) _x._y O_y

Substitution is predominantly B-type due to the higher sintering temperature of 900°C that was used, and the associated loss of some C0₂ from the A-sites.

Comparative Example 3: Preparation of carbonate- substituted hydroxyapatite from Ca-rich hydroxyapatite .

A Ca-rich hydroxyapatite (Ca/P = 1.76) was prepared by the above method. The Ca-rich hydroxyapatite was heat-treated in a tube furnace at 900°C for 2 hours in air. The as-precipitated and the calcined/sintered powders of the Ca-rich hydroxyapatite (Ca/P = 1.76) have been characterised by CHN analysis and XRF.

CHN and XRF data are summarised in Table 2. Within the experimental errors of the XRF method, the calculated and measured values are comparable (experimental errors are shown in brackets) . The CHN analysis of both the stoichiometric hydroxyapatite and the Ca-rich hydroxyapatite show that both materials in the as-precipitated stage have appreciable levels of C0₃ ^2". These levels will result from the absorption/adsorption of C0₃ ^2" during precipitation, both from solution and from the atmosphere, especially as the precipitate is at a high pH. For stoichiometric hydroxyapatite, this C0₃ ^2~ is lost during heat- treatment/sintering in C0₂/H₂0 and in air. The C0₃ ^2" content of the Ca-rich hydroxyapatite increases upon sintering in C0₂/H₂0 to a value of 3.2 wt% but decreases on sintering in air.

Table 2: Results of CHN and XRF analysis of carbonate- substituted hydroxyapatite.

Figures 1 and 2 show X-ray Diffraction data (25-40° 2Θ) for calcined/sintered (900°C C0₂/H₂0) and as-precipitated Ca-rich hydroxyapatite (HA) , respectively; stoichiometric hydroxyapatite is included for reference. No additional phases were observed in any of the diffraction patterns.

Referring to Figure 1, the X-ray diffraction patterns of calcined/sintered samples with Ca/P=1.67 and 1.76 show only peaks corresponding to hydroxyapatite (JCPDS Card no. 9-432, shown by black dots) .

The X-ray diffraction pattern of Ca-rich hydroxyapatite sintered in air at 900°C for 2 hours (not shown) gave peaks corresponding to hydroxyapatite and CaO indicating that the as-precipitated material had decomposed due to the large loss of carbonate.

Claims

1. A process for the preparation of a carbonate- substituted hydroxyapatite, which process comprises heating an hydroxyapatite having a Ca/P molar ratio greater than 1.67 but less than approximately 1.82 at a temperature of from 600 to 1200°C in a atmosphere comprising C0₂.

2. A process as claimed in claim 1 wherein the atmosphere additionally comprises H₂0.

3. A process as claimed in claim 1 or claim 2 wherein the hydroxyapatite having a Ca/P molar ratio greater than 1.67 but less than approximately 1.82 is synthesised by a process which comprises a precipitation reaction between Ca(OH)₂ and H₃P0₄.

4. A process as claimed in any one of the preceding claims wherein the Ca/P molar ratio is in the range of from 1.69 to 1.82.

5. A process as claimed in any one of the preceding claims wherein the Ca/P molar ratio is in the range of from 1.72 to 1.78.

6. A process as claimed in any one of the preceding claims wherein the Ca/P molar ratio is approximately 1.76.

7. A process as claimed in any one of the preceding claims wherein the heating occurs at a temperature of from 850 to 1000°C.

8. A process as claimed in any one of the preceding claims wherein the heating occurs at a temperature of from 900 to 950°C.

9. A process as claimed in any one of the preceding claims wherein the concentration of C0₂ in the atmosphere is controlled by providing a flow-rate of C0₂ gas through the heating apparatus in the range of from 0.5 to 3 1/min.

10. A process as claimed in any one of claims 2 to 9 wherein H₂0 is introduced into the atmosphere by passing the atmosphere comprising C0₂ through a container of water prior to its entry into the heating apparatus.

11. A process as claimed in any one of the preceding claims wherein the heating is carried out for a period of from 1 to 24 hours.

12. A process as claimed in any one of the preceding claims wherein the heating is carried out for a period of from 1 to 2 hours.

13. A substantially single phase carbonate- substituted hydroxyapatite whenever produced by a process as claimed in any one of claims 1 to 12.

14. A substantially single phase carbonate- substituted hydroxyapatite according to claim 13 containing between 0 and 5 wt% C0₃ ^2".

15. A substantially single phase carbonate- substituted hydroxyapatite according to claim 13 or claim 14 which is substantially free of sodium and ammonium ions .