TECHNICAL FIELD The present disclosure relates to broadly to methods for making hydroxyapatite.

BACKGROUND

Hydroxyapatite (Ca^o(Pθ4)5(OH)2) is widely used in biomaterials, and various techniques have been developed to synthesize hydroxyapatite powders. The most common methods to synthesize hydroxyapatite particles have been to use an inverse emulsion or microemulsion route. These routes have proven to be expensive and produce low quantities of materials. Emulsions and microemulsions are known to be susceptible to environmental changes, such as temperature, which can lead to different composition and/or particle size if the conditions are not rigorously controlled. Calcite has previously been converted to hydroxyapatite under pressure and at elevated temperature using hydrothermal techniques. However, efforts to date have not been able to achieve rapid conversion of calcite to hydroxyapatite in high conversion yield at modest or lower temperatures.

SUMMARY

In one aspect, the present disclosure provides a method of making hydroxyapatite, the method comprising: preparing a dispersion comprising calcite particles in water having phosphate ions dissolved therein, wherein the calcite particles have an average primary particle diameter of less than or equal to 500 nanometers and wherein the dispersion has a pH value in a range of from 3 to 11; and converting, at a temperature at or below 100 degrees Celsius, at least a portion of the calcite particles to particles comprising hydroxyapatite.

In some embodiments, the particles comprising hydroxyapatite have an average primary particle size of less than or equal to 500 nanometers. In some embodiments, the particles comprising hydroxyapatite comprise particles consisting essentially of hydroxyapatite. The term "consisting essentially" as used in this context means that the particles are at least 99 percent hydroxyapatite as determined by X-ray diffraction analysis.

In some embodiments, the temperature is less than 80 degrees Celsius, or even less than 50 degrees Celsius. In some embodiments, the calcite particles are present in an initial amount of from 0.1 to 5 percent of a total weight of the dispersion. In some embodiments, the solution further comprises ammonium ions. In some embodiments, a weight ratio of calcite to phosphate ion is in a range of from 1.5 to 1.7. In some embodiments, the dispersion has a pH in a range of from 5 to 7. In some embodiments, the dispersion has a substantially constant pH over time. In some embodiments, substantially all of the calcite particles are fully converted to hydroxyapatite particles in less than one hour. In some embodiments, the method is carried out in a continuous manner.

Advantageously, the present disclosure provides a relatively quick method for manufacturing hydroxyapatite particles (typically nanoscale) at modest temperatures (for example, ambient or near ambient temperatures) under conditions favorable for industrial production.

As used herein:

"average primary particle size" refers only to primary particles, and not to aggregates or agglomerates of such primary particles; "phosphate ion" refers to a P(=O)(O ^' )3 ion, commonly expressed as PO-^ ^' in the chemical arts;

"primary particle" refers to a particle of uniform composition that is not composed of discrete constituent particles;

"substantially constant pH" means that over time the pH varies by no more than 0.2 pH units; and

"water" refers to liquid water.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an XRD analysis scattering plot of dried compositions of Comparative Example A and Examples 1 to 4 after heating and stirring; and

Fig. 2 is an XRD analysis scattering plot of dried compositions of Examples 5 to 7 after heating and stirring.

DETAILED DESCRIPTION

Useful calcite particles may be prepared according to established methodologies such as, for example, precipitation, milling and classification of calcite mineral, or they may be purchased from commercial suppliers (for example, as a dispersion in a liquid vehicle). Examples of commercial suppliers include: NanoMaterials Technology Pte Ltd. of Singapore; SkySpring Nanomaterials, Inc. of Houston, TX; and Solvay SA of Brussells, Belgium.

The calcite particles may have a minimum average primary particle size of at least 5 nm, at least 10 nm, at least 20 nm, or even at least 40 nm, or more, although smaller particles may also be used. The calcite particles may have a maximum average primary particle size of less than or equal to 250 nm, less than or equal to 100 nm, or even less than or equal to 60 nm, although larger particles may be used. The calcite particles may consist of aggregated or agglomerated primary particles (of calcite) or the entire calcite particle may constitute one primary particle. Any primary particles that may constitute the calcite particles may have any size distribution such as, for example, a monomodal (for example, a Gaussian distribution) or a polymodal distribution.

The dissolved phosphate ion may be provided by any suitable means including, for example, by dissolving a soluble phosphate salt (for example, an alkali metal phosphate; ammonium phosphate; or an organoonium phosphate such as, for example, a tetraalkylammonium phosphate) or neutralizing phosphoric acid, or a monobasic or dibasic conjugate base thereof, with an appropriate base (for example, ammonium hydroxide or an alkali metal hydroxide).

The dispersion may further comprise additional components (for example, surfactants, colorants, bactericides, and/or thickening agents). In some cases, it may be desirable that the dispersion is essentially free of (that is, contains less than 0.01 percent by weight) of metal ions (for example, alkali metal ions such as Li ⁺ , Na ⁺ , and K ⁺ ^ alkaline earth ions such as Mg^ ⁺ , Ca^ ⁺ , and Ba^ ⁺ ; and transition metal ions).

In some embodiments, the dispersion may further comprise a surface-modifying agent that is incorporated onto the converted particles comprising hydroxyapatite according to the present disclosure. Examples of suitable surface-modifying agents include hydrophilic surface treatments such as, for example, alkoxyalkoxylated organophosphonic acids (for example, as CH3θCH2CH2θCH2CH2P(=O)(OH)2) and salts thereof, and hydrophobic surface treatments such as, for example, alkylphosphonic acids (for example, C ₈ H ₁₇ P(=O)(OH) ₂ or Ci ₈ H ₃₇ P(=O)(OH) ₂ ) and salts thereof. Surface-modifying agents may be added to the dispersion prior to during or after conversion of the calcite articles to the particles comprising hydroxyapatite.

Typically, the dispersion includes from 0.01 to 10 percent by weight of calcite particles by weight based on the total weight of the dispersion, although other amounts may also be used. More typically, the dispersion has from 0.1 to 5 percent by weight of calcite particles based on the total weight of the dispersion.

The phosphate ion concentration typically varies with the amount of the calcite particles in the dispersion. Typically, the weight ratio of calcite particles to phosphate ion is in a range of from 0.5 to 5, more typically 0.6 to 3.3, still more typically 1.5 to 1.7, and most typically about 1.6, although other weight ratios may also be used. The dispersion may be made by any suitable means including, for example, adding phosphate ions to a dispersion comprising calcite particles in water or by dispersing calcite particles in water having dissolved phosphate ions. Simple stirring is typically sufficient to effectively mix the components of the dispersion.

Advantageously, the conversion of the calcite particles to particles comprising hydroxyapatite (for example, hydroxyapatite particles) can be effected according to the present disclosure over a broad pH range. The pH of the dispersion is in a range of from 3 to 11, more typically 5 to 7. The pH may be held essentially constant during the conversion of the calcite particles to hydroxyapatite particles. In some embodiments, the pH of the dispersion may vary over time and/or throughout the dispersion.

Advantageously, the conversion of the calcite particles to particles comprising hydroxyapatite can be effected according to the present disclosure at relatively low temperatures. Accordingly, it is practiced at or below 100 degrees Celsius ( ⁰ C). In some embodiments, the conversion is carried out at a temperature of less than or equal to 80 ⁰ C, less than or equal to 60 ⁰ C, less than or equal to 40 ⁰ C, or even less than 25 ⁰ C. The conversion may be carried out isothermally or the temperature may be varied over time and/or throughout the dispersion. The dispersion should be maintained at appropriate temperature and pH for sufficient time to convert at least a portion, typically a major portion (for example, at least 60, at least 70, at least 80, or even at least 90 volume percent), of the calcite nanoparticles to hydroxyapatite particles. Typically, this may be achieved in about 5 hours, 2 hours, or even an hour or less depending on the process conditions (for example, temperature, reactant concentrations, pH, and particle size distribution). Unexpectedly, it is presently discovered that conversions carried out isothermally at elevated temperature typically result in larger hydroxyapatite particles than the corresponding conversions carried out on identical dispersion and for the same time interval wherein the initial temperature is reduced to a lower temperature after and initial period of time. Accordingly, the dispersion may be stirred for an additional period of time (for example, 3 to 6 hours) at a lower temperature than the initial temperature, during which additional conversion typically occurs. The conversion of the calcite particles to particles comprising hydroxyapatite may be carried out as a batch process or in a continuous fashion (for example, using a flow tube reactor). Once the process has been carried out to a desired level of conversion the resultant product may be used as is. Typically, particles in the converted dispersion are isolated (for example, by filtration) then washed and dried. In the event that calcite particles remain in the isolated product they can be conveniently removed by addition of a concentrated water-miscible carboxylic acid (for example, glacial acetic acid) that is effective to dissolve the calcite followed by rinsing with water and drying, resulting in hydroxyapatite particles. Objects and advantages of this disclosure are further illustrated by the following non- limiting examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight.

In the following Examples, water (>18 MOhm) was purified by filtering through a Milli-Q Synthesis AlO purifying system from Millipore of Bedford, MA.

Nano precipitated calcium carbonate (NPCC) (calcite with average primary particle size of 40 nm, narrow particle size distribution, BET surface area >40 m2/g) was obtained from NanoMaterials Technology Pte Ltd., Singapore.

Analysis Method (X-ray diffraction)

The presence of hydroxyapatite (HAP) and its level were determined by X-ray diffraction analysis (XRD analysis). Specimens were prepared and placed on zero background specimen holders composed of single crystal quartz or glass inserts. Reflection geometry data were collected in the form of a survey scan by use of a Bruker D8 Advance Diffractometer, copper K _n radiation, and Vantec detector registry of the scattered radiation. The diffractometer was fitted with variable incident beam slits and fixed diffracted beam slits. The survey scan was conducted in a coupled continuous mode from 10 to 80 degrees (2Θ) using a 0.015 degree step size and an 8 second dwell time. X- ray generator settings of 40 kV and 40 mA were employed. Additional reflection geometry data were collected in the form of a survey scan by use of a Philips vertical diffractometer, copper K _n radiation, and proportional detector registry of the scattered radiation. The diffractometer was fitted with variable incident beam slits, fixed diffracted beam slits, and graphite diffracted beam monochromator. The survey scan was conducted from 10 to 60 degrees (2Θ) using a 0.04 degree step size and 8 second dwell time. X-ray generator settings of 45 kV and 35 mA were employed. The following descriptions were used to indicate HAP level in the reaction mixture. "Total" indicates 100% conversion. "Major" indicates that the HAP/calcite ratio is equal to or greater than 1/1. "Moderate" indicates that the HAP/calcite ratio is greater than 10/90 and less than 50/50. "None" indicates that no HAP was identified in the reaction mixture.

Preparation of Phosphate Salt Solution Purified water was placed into a beaker equipped with a magnetic stirrer.

Ammonium hydroxide, sodium hydroxide, or Ca(OH)2 was added while stirring, followed by addition of phosphoric acid. The resultant mixture was stirred until it was homogeneous (about 10 seconds).

COMPARATIVE EXAMPLE A AND EXAMPLES 1 -7

Calcium carbonate in an amount specified in Table 1 was placed into a 500-mL round-bottom flask equipped with a mechanical stirrer and a water-cooled condenser. Then the prepared phosphate salt solution, in an amount specified Table 1 was added to the reaction flask and the mixture was heated with stirring at a temperature and time reported in Table 2. The reaction mixture was then allowed to cool to room temperature with stirring for the time reported in Table 2. The reaction mixture was filtered and rinsed with purified water to obtain a white solid. This white solid material was then transferred to an evaporation dish and placed in an oven at 100 ⁰ C for 60 minutes. The resultant dried material was then pulverized and analyzed by X-ray diffraction. Results are reported in Table 2 and Figs. 1 and 2. Figs. 1 and 2 show the result of XRD analysis of dried compositions of Comparative Example A and Examples 1 to 7 after heating and cooling.

TABLE 1

OO

TABLE 2

Objects and advantages of this disclosure are further illustrated by the following non- limiting examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure.