Biomedical materials

Abstract

A synthetic calcium phosphate-based biomedical material comprising gadolinium. The material may comprises a compound having the general chemical formula: Ca.sub.10−yGd.sub.y(PO.sub.4).sub.6−x(SiO.sub.4)x(OH).sub.2−x+y where 0<x<1.3 and 0<y<1.3.

Claims

1. A process for the synthesis of a calcium phosphate-based material comprising gadolinium and silicon, the process comprising: providing calcium or a calcium-containing compound, a gadolinium-containing compound, a phosphorus-containing compound and a silicon-containing compound; and forming a precipitate by reacting the compounds in an aqueous phase at an alkali pH; thereby synthesizing a calcium phosphate-based material comprising gadolinium, wherein at least some of the gadolinium is in the form of Gd.sup.3+ ions substituted in the calcium phosphate lattice, and wherein the material comprises a compound having the general chemical formula:
Ca.sub.10−yGd.sub.y(PO.sub.4).sub.6−x(SiO.sub.4).sub.x(OH).sub.2−x+y wherein 0.5<x<1.1 and 0<y<1.3.

2. The process of claim 1, wherein the calcium-containing compound comprises a calcium salt.

3. The process of claim 2, wherein the calcium salt is selected from one or more of calcium hydroxide, calcium chloride, calcium nitrate and/or calcium nitrate hydrate.

4. The process of claim 1, wherein the gadolinium-containing compound is selected from one or both of gadolinium chloride and/or gadolinium nitrate.

5. The process of claim 1, wherein the phosphorus-containing compound is selected from one or both of a phosphate salt and/or a phosphoric acid.

6. The process of claim 5, wherein the phosphorus-containing compound is selected from one or both of ammonium phosphate and/or phosphoric acid.

7. The process of claim 1, wherein the silicon-containing compound comprises a silicate.

8. The process of claim 7, wherein the silicate is selected from one or both of tetraethyl orthosilicate (TEOS) and/or silicon acetate.

9. The process of claim 1, wherein the pH is from 8 to 13.

10. The process of claim 1, wherein an alkali is added to adjust the pH of the solution to the desired pH.

11. The process of claim 10, wherein the alkali is ammonium hydroxide or concentrated ammonia.

12. The process of claim 1, wherein after the precipitate has been formed it is dried, heated and/or sintered.

13. The process of claim 1, wherein the silicon-containing and the gadolinium-containing compounds are supplied in equimolar quantities with respect to the amount of silicon and the quantity of the gadolinium cation.

14. The process of claim 1, wherein the silicon-containing compound is supplied in a greater quantity than the gadolinium compound, with respect to the quantity of silicon and the quantity of the gadolinium cation.

15. The process of claim 1, wherein the process further involves diluting the thus formed calcium phosphate-based gadolinium-containing material with a gadolinium-free material.

16. The process of claim 1, wherein the phase purity of the material, as measured by X-ray diffraction using the whole pattern method, is at least 95%.

17. The process of claim 1, wherein 0.001<y <1.1.

18. The process of claim 1, wherein 0.1<y <1.3.

19. The process of claim 1, wherein 0<y <0.001.

20. The process of claim 1, wherein 0<y <0.0005.

21. The process of claim 1, wherein x≥y.

Description

EXAMPLE 1

Synthesis of Gd.SUP.3+./SiO.SUB.4..SUP.4− .Co-Substituted hydroxyapatite (x=y=1.0)

(1) The following method describes the synthesis of approx. 10 g of a Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite by an aqueous precipitation method with the following substitution mechanism:
Ca.sub.10−yGd.sub.y(PO.sub.4).sub.6−x(SiO.sub.4).sub.x(OH).sub.2−x+y
where x=y =1.0

(2) CaCO.sub.3 was first de-carbonated overnight (16 hours) in a furnace at 900° C. The resulting CaO was then removed from the furnace and placed in a dessicator to cool. 5.0543 g CaO was added to a beaker containing approx. 100 ml deionised water in an ice bath. After complete addition of the CaO, the beaker was removed from the ice bath and placed on a stirrer. The suspension was left to stir for approx. 10 minutes; the CaO will undergo hydration to from Ca(OH).sub.2.

(3) Meanwhile, 4.5141 g Gd(NO.sub.3).6H.sub.2O (GNH) was added to a beaker containing approx. 100 ml deionised water and mixed well until the GNH had completely dissolved. The GNH solution was then slowly poured into the Ca (OH) .sub.2 suspension and this suspension was left stir for approx. 15 minutes.

(4) 5.7664 g H.sub.3PO.sub.4 (85% assay) was diluted with approx. 100 ml deionised water. This solution was poured into a dropping funnel and added drop-wise to the stirring Ca(OH).sub.2/GNH suspension over a period of approx. 70 minutes. After complete addition of the H.sub.3PO.sub.4 solution, 2.127 g Si(OC.sub.2H.sub.5).sub.4 (TEOS) was diluted with approx. 100 ml deionised water and this solution was poured into a dropping funnel and added drop-wise to stirring Ca(OH).sub.2/GNH/H.sub.3PO.sub.4 mixture over a period of approx. 70 minutes. The pH of the stirring solution was monitored throughout the addition of the H.sub.3PO.sub.4 and TEOS solutions and was maintained at pH12 by the addition of concentrated ammonia solution; in total, approx. 100 ml was added. After complete addition of the TEOS solution, the total mixture was left to stir for a further 2 hours before being left to age and precipitate overnight (approximately 16 hours). The Precipitate was then filtered, dried at 80° C. for 24 hours, and ground to form a fine powder. Approximately 3 g of the dried powder was placed in a platinum crucible and sintered in a furnace at 1200° C. for 2 hours, using heating and cooling rates of 5 and 10° C./min respectively. The sintered powder was then analysed using X-ray diffraction to confirm the phase purity. A Bruker D8 diffractometer was used to collect data from 25 to 40° 2 θ with a step size of 0.02° and a count time of 9.5 secs/step. The diffraction pattern obtained was compared with the ICDD (#09-0432) standard pattern for hydroxyapatite. All the diffraction peaks for the sintered Gd .sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite matched the peaks of the ICDD standard, with no additional peaks observed, indicating that the composition produced by this method was a single-phase material with a hydro structure (see FIG. 1).

(5) 0.2503 g of this Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite was diluted by mixing, in a pestle and mortar, with 4.7505 g of a SiO.sub.4.sup.4− substituted hydroxyapatite to give a powder containing 5 wt % Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite. Similarly, 0.0124 g of the Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite was mixed with 4.9879 g of a SiO.sub.4.sup.4− substituted hydroxyapatite to give a powder containing 0.25 wt % Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite. Approx. 5 ml deionised water was added to each of these powders to form pastes. Magnetic Resonance Imaging (MRI) was then used to assess the effect of Gd.sup.3+ substitution on the MRI activity of the hydroxyapatite materials. Materials that have little or no MRI activity appear as darkened areas in the resulting image. Those that are MRI active will in contrast appear bright. The sample containing 5 wt % Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite was found to have an improved contrast, i.e. appeared brighter, than a pure hydroxyapatite. However, the contrast of the 0.25 wt % Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite was found to be even greater than that of the 5 wt % Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite.

EXAMPLE 2

Synthesis of Gd.SUP.3+./SiO.SUB.4..SUP.4− .Co-Substituted hydroxyapatite (x=1.0, y=0.5)

(6) The following method describes the synthesis of approx. 10 g of a Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite by an aqueous precipitation method with the following substitution mechanism:
Ca.sub.10−yGd.sub.y(PO.sub.4).sub.6−x(SiO.sub.4).sub.x(OH).sub.2−x+y
where x=1.0, y=0.5

(7) CaCO.sub.3 was first de-carbonated overnight (16 hours) in a furnace at 900° C. The resulting CaO was then removed from the furnace and placed in a dessicator to cool. 5.3284 g CaO was-added to a beaker containing approx. 100 ml deionised water in an ice bath. After complete addition of the CaO, the beaker was removed from the ice bath and placed on a stirrer. The suspension was left to stir for approx. 10 minutes; the CaO will undergo hydration to from Ca(OH).sub.2. Meanwhile, 2.2589 g Gd(NO.sub.3).6H.sub.2O (GNH) was added to a beaker containing approx. 100 ml deionised water and mixed well until the GNH had completely dissolved. The GNH solution was then slowly poured into the Ca(OH).sub.2 suspension and this suspension was left to stir for approx. 15 minutes.

(8) 5.7645 g H.sub.3PO.sub.4 (85% assay) was diluted with approx. 100 ml deionised water. This solution was poured into a dropping funnel and added drop-wise to the stirring Ca(OH).sub.2/GNH suspension over a period of approx. 90 minutes. After complete addition of the H.sub.3PO.sub.4 solution, 2.1330 g Si(OC.sub.2H.sub.5).sub.4 (TEOS) was diluted with approx. 100 ml deionised water and this solution was poured into a dropping funnel and added drop-wise to stirring Ca(OH).sub.2/GNH/H.sub.3PO.sub.4 mixture over a period of approx. 75 minutes. The pH of the stirring solution was monitored throughout the addition of the H.sub.3PO.sub.4 and TEOS solutions and was maintained at pH12 by the addition of concentrated ammonia solution; in total, approx. 50 ml was added. After complete addition of the TEOS solution, the total mixture was left to stir for a further 2 hours before being left to age and precipitate overnight (approximately 16 hours). The precipitate was then filtered, dried at 80° C. for 24 hours, and ground to form a fine powder.

(9) Approximately 3 g of the dried powder was placed in a platinum crucible and sintered in a furnace at 1200° C. for 2 hours, using heating and cooling rates of 5 and 10° C./min respectively. The sintered powder was then analysed using X-ray diffraction to confirm the phase purity. A Bruker D8 diffractometer was used to collect data from 25 to 40° 2 θ with a step size of 0.02° and a count time of 9.5 secs/step. The diffraction pattern obtained was compared with the ICDD (#09-0432) standard pattern for hydroxyapatite. All the diffraction peaks for the sintered Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite matched the peaks of the ICDD standard, with no additional peaks observed, indicating that the composition produced by this method was a single-phase material with a hydroxyapatite-like structure (see FIG. 2).

EXAMPLE 3

Synthesis of Gd.SUP.3+./SiO.SUB.4..SUP.4− .Co-Substituted hydroxyapatite/tricalcium Phosphate Biphasic Mixture (x=1.1, y=1.1)

(10) The following method describes the synthesis of approx. 10 g of a Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite/tricalcium phosphate biphasic mixture by an aqueous precipitation method with the following substitution mechanism:
Ca.sub.10−yGd.sub.y(PO.sub.4).sub.6−x(SiO.sub.4).sub.x(OH).sub.2−x+y
where x=1.1, y=1.1

(11) To produce a gadolinium/silicate co-substituted biphasic composition, rather than a single phase gadolinium/silicate co-substituted hydroxyapatite composition (described in Example 1), a deficient amount of Ca(OH).sub.2 and GNH was added, which promoted the precipitation of a gadolinium/silicate co-substituted cation-deficient apatite composition. This will form a phasic gadolinium/silicate co-substituted composition on heating.

(12) CaCO.sub.3 was first de-carbonated overnight (16 hours) in a furnace at 900° C. The resulting CaO was then removed from the furnace and placed in a dessicator to cool. 4.9978 g CaO, was added to a beaker containing approx. 100 ml deionised water in an ice bath. After complete addition of the CaO, the beaker was removed from the ice bath and placed on a stirrer. The suspension was left to stir for approx. 10 minutes; the CaO will undergo hydration to form Ca(OH).sub.2. Meanwhile, 4.9696 g Gd(NO.sub.3).sub.3.6H.sub.20 (GNH) was added to a beaker containing approx. 100 ml deionised water and was mixed until the GNH had completely dissolved. The GNH solution was then slowly poured into the Ca (OH) .sub.2 suspension and this suspension was left to stir for approx. 15 minutes.

(13) 5.6500 g H.sub.3PO.sub.4 (85% assay) was diluted with approx. 100 ml deionised water and this solution was poured into a dropping funnel and added drop-wise to the stirring Ca(OH).sub.2/GNH suspension over a period of approx. 80 minutes. After complete addition of the H.sub.3PO.sub.4 solution, 2.3402 g Si(OC.sub.2H.sub.5).sub.4 (TEOS) was diluted with approx. 100 ml deionised water and this solution was poured into a dropping funnel and added drop-wise to stirring Ca(OH).sub.2/GNH/H.sub.3PO.sub.4 mixture over a period of approx. 75 minutes. The pH of the stirring solution was monitored throughout the addition of the H.sub.3PO.sub.4 and TEOS solutions and was maintained at pH12 by the addition of concentrated ammonia solution; in total approx. 50 ml was added. After complete addition of the H.sub.3PO.sub.4 and TEOS solutions, the total mixture was left to stir for a further 2 hours before being left to age and precipitate overnight (approximately 16 hours). The precipitate was then filtered, dried at 80° C. for 24 hours, and ground to form a fine powder. Approximately 3 g of the dried powder was placed in a platinum crucible and sintered in a furnace at 1200° C. for 2 hours, using heating and cooling rates of 5 and 10° C./min, respectively.

(14) The sintered powder was then analysed using X-ray diffraction to confirm the phase purity. A Broker D8 diffractometer was used to collect data from 25 to 40° 2 θ with a step size of 0.02° and a count time of 9.5 secs/step. The diffraction pattern obtained was compared with the ICDD (#09-0432) standard pattern for hydroxyapatite and (#09-0359) standard pattern for alpha-tricalcium phosphate. All the diffraction peaks for the sintered Gd.sup.3+/SiO.sub.4.sup.4− co-substituted hydroxyapatite matched the peaks of the ICDD standard, matching both the hydroxyapatite phase and the alpha-tricalcium phosphate phase, indicating that the composition produced by this method was a biphasic materials (see FIG. 3). Comparing the intensities of the most intense peaks of the hydroxyapatite and the alpha-tricalcium phosphate phases, the amount of hydroxyapatite is approximated as 84%, and the amount of tricalcium phosphate as 16%.

EXAMPLE 4

Use of a Gd.SUP.3+./SiO.SUB.4..SUP.4− .Co-Substituted hydroxyapatite (x=y=1.0) in a Calcium Phosphate Composition to Enhance MRI Contrast in a Bone Graft Material

(15) To enable clinicians to visualise tissue repair within a macroporous calcium phosphate implant, or an implant with a cavity or void, with MRI, a surgeon would have to introduce a gadolinium contrast agent, normally by the injection of a gadolinium-containing compound that will pass throughout the patient's blood stream. The present invention provides for the synthesis of a calcium phosphate composition that contains gadolinium ions that are actually incorporated into the calcium phosphate crystal structure. The amount of gadolinium ions that are required in the calcium phosphate material to enable it to act as a contrast agent in MRI is quite low. The materials and methods described herein can therefore be used in one of two ways. Firstly, a calcium phosphate material such as that described in Example 1 can be prepared but with a very small value of x, typically x=0.0001 to. 0.05; this material could then be used to construct the entire implant, and would contain sufficient gadolinium to act as a contrast agent in MRI. Alternatively, a calcium phosphate material such as that described in Example 1 can be prepared with a large value of x, such as x=1. This material could then be mixed with an appropriate amount of gadolinium-free calcium phosphate implant material, to effectively dilute the gadolinium-substituted calcium phosphate (with x=1) to an appropriate final concentration of gadolinium, for example in a 1:1000 ratio by weight of gadolinium-substituted calcium phosphate (with x=1) to gadolinium-free calcium phosphate implant material. Alternatively, the gadolinium-substituted calcium phosphate (e.g. with x=1) could be mixed with another synthetic bone replacement material, or with autograft or allograft bone, or with another bone replacement material of natural or synthetic origins.

(16) As an example, the following compositions were prepared:

(17) Samples (5 g), as sintered powders with particle sizes ranging from 10 to 500 microns, were placed in a small plastic sealable bag, and 7.5 g of water was added to form a paste like consistency. The bag was sealed, rolled up to form a cylindrical shaped sample, then covered in cling film. The samples were then fixed with tape onto a plastic bottle containing a dilute nickel chloride phantom solution, and placed in a 1 Tesla MRI. Gadolinium-substituted calcium phosphate with x=1 (as described in Example 1) was mixed with gadolinium-free calcium phosphate to provide a final concentration of Gd of approximately 0.25 wt % (A) As a comparison, a Gadolinium-substituted calcium: phosphate. (as described in Example 1) was prepared with x=0.02, corresponding to a gadolinium content of approximately 0.25 wt % (B). As controls, samples were prepared of gadolinium-free calcium phosphate (C) and Gadolinium-substituted calcium phosphate with x=1 (as described in Example 1) mixed with gadolinium-free calcium phosphate to provide a final concentration of Gd of approximately 5 wt % (D). The contrast observed for samples A and B were similar and were significantly enhanced compared to the two control samples C and D.

(18) The present invention enables gadolinium to be substituted (or co substituted with silicon) into the calcium phosphate (e.g. hydroxyapatite or apatite) lattice. As a consequence, the MRI contrast of the material can be improved. The synthetic biomedical material still closely matches the chemical composition of bone mineral. The present invention also enables the production of phase-pure, biphasic and multiphase materials.