GB2433257A

GB2433257A - Preparation of hydroxyapatite

Info

Publication number: GB2433257A
Application number: GB0525704A
Authority: GB
Inventors: Thomas Campbell Prentice; Keith Taylor Scott; David Leslie Segal
Original assignee: Accentus Medical PLC
Current assignee: Accentus Medical PLC
Priority date: 2005-12-19
Filing date: 2005-12-19
Publication date: 2007-06-20
Also published as: GB0525704D0

Abstract

A hydroxyapatite of a Group 2 element such as calcium is made by treating a phosphate of the Group 2 element (e.g. Ca3(PO4)2) under basic conditions (e.g. using NaOH solution) to extract sufficient phosphorus to give rise to the hydroxyapatite. The hydroxyapatite may be in the form of a powder that can be thermally sprayed without being substantially degraded. When so-sprayed (e.g. by plasma spraying) onto a substrate, a hydroxyapatite coating is produced to give a coated substrate with application, for example, in making a biological implant material when the hydroxyapatite is calcium hydroxyapatite.

Description

Material Preparation This invention relates to a method of preparing a

hydroxyapatite, for example in the form of a powder, suitable for use in providing coatings on substrates by thermal spraying.

Calcium hydroxyapatite (stoichiometric formula Ca10(P04)6(OH)2) has a structure similar to that of naturally occurring bone and tooth minerals with which it is biocompatible. It therefore has potential as a biological implant material, for example in the form of a porous coating on the femoral stems of metallic hip implants.

Plasma spraying is a known method of applying coatings and its potential in providing coatings of calcium hydroxyapatite is discussed by K.A. Cross, G.H.

Haddad and C.C. Berndt; "Thermally sprayed coatings for orthopaedic applications", Proc. Twelfth mt. Conf. on Thermal Spraying London, June 1989, Paper 119. However, a problem in such plasma spraying is to provide a calcium hydroxyapatite powder that can give rise to a satisfactory coating, e.g. without suffering degradation during plasma spraying.

The invention provides a method of making a hydroxyapatite of a Group 2 element of the Periodic Table which comprises treating a phosphate of the Group 2 element under basic conditions such as to extract sufficient of the phosphorus from the phosphate to give rise to the hydroxyapatite.

It is to be understood that the term "hydroxyapatite" does not imply precise stoichiometry.

As is common in the art, the term also includes materials departing to a small extent therefrom.

The hydroxyapatite may be produced as, or converted into, a powder that is capable of being thermally sprayed to give a hydroxyapatite coating on a substrate without the hydroxyapatite being substantially degraded. The hydroxyapatite may have a mole ratio of Group 2 element to phosphorus approaching that of stoichiometric hydroxyapatite (1.667), for example about 1.6 or greater.

Preferably, the Group 2 element in the hydroxyapatite is calcium when the proportion of calcium by weight may be 37% or greater and when the proportion of phosphorus by weight may be 18% or greater.

Preferably the phosphate is treated with an aqueous solution of a strong base such as potassium hydroxide, sodium hydroxide, or tetramethylammonium hydroxide. This may be of concentration above 0.1 M, preferably above 0.3 M, so it is at least pH 12. It may be at an elevated temperature, for example above 40 C, such as 50 C. The treatment may be carried out for several hours, for example over 12 hours.

The material made by the present method may be used, for example, in the manufacture of bulk ceramics or, as indicated, in the preparation of coatings. Thus, it has been found to be particularly effective in powder form as a material that can be thermally sprayed successfully to give coatings of essentially the same phase as the powder, i.e. hydroxyapatite. The phosphate used in the method is preferably calcium orthosphate (Ca3(P04)2) when the hydroxyapatite produced will be calcium hydroxyapatite. The basic conditions of the method can comprise treatment with an aqueous alkali, for example an aqueous hydroxide such as NaOH solution, under such conditions of time and temperature as to produce a hydroxyapatite. The conditions of the present method may have to be selected with regard to the use of the material produced. Thus, if a biological implant application is intended (when the hydroxyapatite is calcium hydroxyapatite), care has to be taken to ensure that toxic materials such as heavy metal ions are not introduced. The present invention has, in fact, been found capable of giving rise to materials of high purity which can be thermally sprayed to give rise to high purity coatings.

To produce coatings using material made by the present method, in powder form, it may be necessary to mix the powder with a binder such as starch or polyvinyl alcohol to generate flow properties for enabling the powder to be thermally sprayed. After mixing with a binder, spray drying may be used to produce a powder suitable for thermal spraying. The spraying parameters may be varied to control the properties of the coating.

For example, it may be desirable to select parameters to induce limited degradation of the hydroxyapatite to improve the adhesion and integrity of the coating.

Generally, it has been found that the coating quality is improved by using higher power (Ar) or higher enthalpy (Ar/He) plasmas.

Thermal Spraying' is a generic term for a method of spraying a solid material by melting the material and projecting the molten material with high kinetic energy at a target which, in this invention, is the substrate.

The term includes plasma spraying and flame spraying, the former being preferred in the practice of this invention.

In plasma spraying, the material in powder form is rapidly heated in a DC arc-plasma and then projected at high velocity in the molten or plastic state onto the substrate surface where it quenches and adheres to produce the coating. For example, the heat source in the plasma process may be an electric arc struck between two electrodes, the anode of which serves as a nozzle. By suitable design, the arc is constricted and stabilised within the latter, generating temperatures of up to 20,000K with the ionised gases being ejected at exit velocities of several hundred metres per second. The arc plasma is a source of high thermal and kinetic energy to entrained material. The apparatus used for plasma spraying is called a plasma spray gun -such guns may use nitrogen, argon or helium as the main plasma gas and operate at power levels between 20 and 80 kW.

The substrate may be metallic or non-metallic, preferably metallic such as a component of a biological implant as aforementioned.

The invention will now be particularly described, by way of example only, as follows:

Example 1

Preparation of Powder Ca3(P04)2 (500g) was digested in an aqueous solution of NaOH (50g in 3L of demineralised water) over 3 hours with continuous stirring to give a slurry. The slurry was then placed in an oven at 52 C for 24 hours and stirred at regular intervals. The slurry was removed from the oven, the liquid decanted, and the residual slurry washed repeatedly with demineralised water until the pH of the supernatant liquid was 7. The solid was filtered off and finally rinsed to give a powder.

The above-prepared powder was analysed and found to contain 37.1% Ca and 18.0% p (both by weight) which corresponds to a mole ratio of Ca:P of 1.60 (cf. the theoretical mole ratio in calcium hydroxyapatite is 1.667).

The bulk of the above calcium hydroxyapatite was mixed with polyvinyl alcohol (M.W. 49,000; lOg), added to water (1L), and spray dried to yield a free flowing powder, 90% of which fell within the size range of 10 pm to 90 pm.

Preparation of coating A FECRALLOY (Registered Trade Mark) yttrium-containing aluminium-bearing ferritic steel substrate (50 x 20 x 1 mm) was vapour degreased and grit blasted and the free flowing powder plasma sprayed thereon using a Metco 7MB spray gun using argon/helium as the plasma gas, a spray rate of 20g min1, 12 kW power and a spray distance of 45 mm. This is an unusually low power plasma, but is needed to avoid overheating the hydroxyapatite. The gas mixture and the nozzle design are such as to ensure high temperature and high speed gas flow, respectively, to minimize the time particles remain in the plasma. However, the above parameters may require optimisation to produce the best coatings.

The resulting coating was hard, of about 0.15 millimetres thickness, and shown by X-ray diffraction to be substantially calcium hydroxyapatite.

EXAMPLES 2 -4

The procedure of Example 1 was repeated with the exception that, in the preparation of the powder, batches of the slurry were placed in the oven for 48, 72 and 96 hours respectively. The weight % Ca, weight % P and Ca:? mole ratios were found to be as follows for powders so prepared: Treatment time % Ca % P Ca:? mole hours (weight) (weight) ratio 48 37.8 18.2 1.61 72 37.6 18.0 1.62 96 37.4 18.1 1.61 The results show that extending treatment time had little effect.

Claims

Claims 1. A method of making a hydroxyapatite of a Group 2 element of

the Periodic Table which comprises treating a phosphate of the Group 2 element under basic conditions such as to extract sufficient of the phosphorus from the phosphate to give rise to the hydroxyapatite.

2. A method as claimed in claim 1 wherein the hydroxyapatite has a mole ratio of Group 2 element to phosphorus which is at least 1.6.

3. A method as claimed in claim 2 wherein the Group 2 element is calcium.

4. A method as claimed in any one of the preceding claims wherein the phosphate is treated with an aqueous solution of a strong base whose initial pH is at least pH 12.

5. A method as claimed in any one of the preceding claims wherein the phosphate is treated at an elevated temperature for several hours.

6. A method of making a hydroxyapatite of a Group 2 element of the Periodic Table substantially as hereinbefore described.

7. A method of forming a coating comprising making a hydroxyapatite by a method as claimed in any one of the preceding claims, obtaining a powder of the hydroxyapatite, and thermally spraying the powder to form the coating.

8. A method of forming a coating as claimed in claim 7 wherein the powder is mixed with a binder and spray-dried to form a free-flowing powder predominantly of particle size in the range 10-90 pm, and is then plasma sprayed to form the coating.