Nanocrystalline apatites and composites, prostheses incorporating them, and method for their production

Abstract

Methods for synthesis of nanocyrstalline apatites are presented, as well as a series of specific reaction parameters that can be adjusted to tailor, in specific ways, properties in the recovered product. Particulate apatite compositions having aveage crystal size of less than 150 nm are provided. Products also can have a surface area of at least 40 m²/g and can be of high density.

Hydroxyapatite material is investigated in particular detail. Compositions of the invention can be used as prosthetic implants and coatings for prosthetic implants.

Description

RELATED APPLICATION

, or spherical to needle-like, with increasing pH, for example to aspect ratios ranging from about 2.3:1 to 5.9:1. Tanahashi et al. reported that the solution pH greatly influenced the growth rate and morphology of hydroxyapatite and that fibrous hydroxyapatite could be prepared at high pH. Hydroxyapatite synthesized through hydrothermal treatment at a pH of 11 to 12 also resulted in nanometer-sized rod-like crsytals. However, the addition of glycerin during the synthesis confounded the relationship between high pH and the synthesis of rod-like hydroxyapatite, with the effect of additives on the synthesis of rod-like hydroxyapatite. The synthesis conditions of the calcined hydroxyapatite powders used to determine the effect of NH₄OH are presented in Table 9.

TABLE 9
Effect of NH4OH Concentration: Synthesis Conditions
	Aging		Ca(NO3)2		CaN		NHP
	Time	Rxn/Aging	Addition Rate	Grinding	Concentration	Amount	Concentration	Amount	NH4OH
Trial	(hr)	Temp (° C.)	(ml/min)	Method	(M)	(ml)	(M)	(ml)	Amount (ml)
9	12	25	2	Wet	0.500	300	0.300	300	10
5	12	25	2	Wet	0.500	300	0.300	300	30
10	12	25	2	Wet	0.500	300	0.300	300	100
11	100	25	3	Wet	0.167	900	0.100	900	30
12	100	25	3	Wet	0.167	900	0.100	900	90
13	100	25	3	Wet	0.167	900	0.100	900	300

TABLE 10
Effect of NH4OH Concentration: Results
	XRD	BET	Green	% Theoretical
	Crystallite Size	Surface Area	Density	Sintered Bulk
Trial	(nm)	(m2/g)	(g/cc)	Density
9	50	72.58	1.59	94.3
5	44	58.52	1.31	82.6
10	52	59.30	1.68	81.0
11	40	72.16	1.58	87.3
12	33	89.71	1.89	95.3
13	Not HAP	Not HAP	Not HAP	Not HAP

The XRD patterns show that all of the calcined hydroxyapatite samples, except for Trial 13, have good crystallinity and a pure hydroxyapatite phase. The peaks of the FTIR spectra were also consisted with hydroxyapaptite. Trials 9, 5, and 10 correpond to 10 ml, 30 ml, and 100 ml of NH₄OH at high precursor concentrations. The XRD results of Trials 9 and 5 suggest that the addition of more NH₄OH give rise to smaller XRD crystallites, which is consistent with the effect of increased pH which decreases solubility, favoring nucleation. However, the XRD crystallite size of Trial 10 is larger than Trial 5. This phenomenon can be explained by examining Trials 11, 12, and 13 which correspond to 30 ml, 90 ml, and 300 ml of NH₄OH at low precursor concentrations. The XRD crystallite sizes of Trials 11 and 12 decrease as pH as increased. Similarly to Trial 10, Trial 13 deviates from the trend estalbished by Trials 11 and 12. Instead of the anticipated further decrease in XRD crystallite size, as-synthesized Trial 13 is not hydroxyapatite but a combination of monetite (CaHOP₄) and burshite (CaHPO₄.2H₂O) Trial 10 may occur in a similar metastable state as Trial 13, though not as pronounced because of its shorter aging time and higher precursor concentrations. Thus, the possible presence of monetite and brushite during the synthesis of Trial 10 may give rise to the deviation in the crystallite size. Furthermore, samples prepared udner similar conditions as Trial 13 have resulted in hydroxyapatite, confirming the metastability of this region.

Trial 19, the hydroxyapatite derived with 10 ml of NH₄OH, resulted in the highest surface area and the highest theoretical sintered bulk density under a high precursor concentration synthesis. A low pH at high precursor concentrations produces a particle morphology and distribution favorable towards densification since the addition of NH₄OH is known to affect particle morphology. Conversely, at low precursor concentrations, the highest surface area and highest % theoretical sintered bulk density occurred at an intermediate pH, indicating that this amount of NH₄OH resulte din a particle morphology and distribution favorable toward densification.

EXAMPLE 8

Effect of Addition Rate

By varying the precursor addition rate, nucleation and crystal growth rates can be controlled. Rapid addition of precursors results in localized high concentrations of precursors, exceeding the solubility of hydroxyapatite in those regions, which favors nucleation and formation of small particles. However, rapid addition is also expected to result in a nonuniform particle morphology and distriubtion. Conversely, slow addition of precursors results in a more homogenous mixture of reaction favoring crystal growth and formation of larger particles. Furthermore, slow addition of precursors is anticipated to result in a uniform particle morphology and distribution. Thus, relatively few nuclei will be formed by adding Ca(NO₃)₂ slowly; crystal growth removes the precursors as fast as it is added. Adding Ca(NO₃)₂ quickly yields more and smaller particles. The synthesis conditions of the experiment investigating the effect of addition rate are presented in Table 11.

TABLE 11
Effect of Addition Rate: Synthesis Conditions
	Aging		Ca(NO3)2		CaN		NHP
	Time	Rxn/Aging	Addition Rate	Grinding	Concentration	Amount	Concentration	Amount	NH4OH
Trial	(hr)	Temp (° C.)	(ml/min)	Method	(M)	(ml)	(M)	(ml)	Amount (ml)
5	12	25	2	Wet	0.500	300	0.300	300	10
6	12	25	15	Wet	0.500	300	0.300	300	10
7	100	25	3	Wet	0.167	900	0.100	900	90
8	100	25	48	Wet	0.167	900	0.100	900	90

TABLE 12
Effect of Addition Rate: Results
	XRD	BET	Green	% Theoretical
	Crystallite Size	Surface Area	Density	Sintered Bulk
Trial	(nm)	(m2/g)	(g/cc)	Density
5	67	73.58	1.59	94.3
6	54	65.20	1.52	91.8
7	33	89.71	1.74	95.6
8	31	65.35	1.91	95.3

The XRD patterns of Trials 9, 14, 15, and 12 correspond to the JCPDS hdyroxyapatite file (9-0432) and no other phases were found. All FTIR specra possess peaks characteristic of nanocrystalline hydroxyapaptite. Trials 9 and 15 possessed a larger XRD crystallite size and a higher BET surface area than Trials 14 and 12, respectivley, and gave rise to higher sintered densities. The larger XRD crystallite sizes of Trials 9 and 12 compared to 14 and 12 suggest that a slower addition rate favors crystal growth as, anticipate. In addition, by using a slow adidtion of obtain a more unifiorm particle morphology and distribution, the final sintered bulk densities were enhanced. These effects were signficiant for Trials 9 and 14, but addition rate did not play a dominant role in Trials 15 and 12. The lesser role of additoin rate at low precursor concentrations can be attirbuted to the difference in more flow rates. The difference in molar rates between Trials 15 and 12 is 7.5×10⁻³mols/min whereas the difference in molar flow rates between Trials 9 and 14 is 7.4×10⁻² moles/min. These results confirm that crystlalite size depnedson the rateof addition with slower rates of addition resulting in larger crystallites, but to observe this effect at low precursor concentrations, a much higher flow rate should be used. To obtain a densified nanocrystalline hydroxyapatite ceramic, Ca(NO₃)₂ should be added slowly to the basic (NH₄)₂HPO₄ solution.

EXAMPLE 9

Effect of Precursor Concentration

By varying the precursor concentration, the synthesis of nanocrystalline hydroxyapatite can be further controlled by affecting the kinetics of hydroxyapatite synthesis. By reducing the precursor concentration, the kinetics of the reaction are slowed. The synthesis conditions of the hydroxyapatite gels used to determine the effect of precursor concentration are presented in Table 13.

TABLE 13
Effect of Precursor Concentration: Synthesis Conditions
	Aging		Ca(NO3)2		CaN		NHP
	Time	Rxn/Aging	Addition Rate	Grinding	Concentration	Amount	Concentration	Amount	NH4OH
Trial	(hr)	Temp (° C.)	(ml/min)	Method	(M)	(ml)	(M)	(ml)	Amount (ml)
9	12	25	2	Wet	0.500	300	0.300	300	10
16	12	25	3	Wet	0.167	900	0.300	900	30
17	100	25	2	Wet	0.500	300	0.300	300	30
15	100	25	3	Wet	0.167	900	0.100	900	90

TABLE 14
Effect of Precursor Concentration: Results
	XRD	BET	Green	% Theoretical
	Crystallite Size	Surface Area	Density	Sintered Bulk
Trial	(nm)	(m2/g)	(g/cc)	Density
9	67	73.58	1.59	94.3
16	46		1.82	85.1
17	41	65.68	1.70	80.1
15	33	89.71	1.74	95.6

The XRD pattens of Trials 9, 17, and 15 correspond to hydroxyapatite whil the XRD pattern of Trial 16 corresponds to monetite (CaPO₃OH). The FTIR spectra of Trials 9, 17, and 15 also showed the characteristic hydroxyapatite nanocrystalline peaks. By reducing the precursor concentration in Trial 9 to the precursor concentration of Trial 16, hydroxyapatite synthesis enters an intermediate state where monetite is the product. In Table 8, “Effect of Aging Time,” Trials 7 and 8 were both found to be hydroxyapatite regardless of aging time, but unlike Trial 16, Trials 7 and 8 were synthesized udner ahihger pH. Tables 15 and 16 present the synthesis conditions and results proving that Trial 16 is an intermediate state, observable because of the shorter aging time, low precursor concentration and low pH; under the same conditions as Trial 16, except with longer aging times, Trial 11 was determined to be hydroxyapatite. Thus, the effect of lowering precursor concentration at the synthesis conditions of Trials 9 and 16 is to slow the kientics of the reaction.

TABLE 15
Effect of Aging Time on Trial 16: Synthesis Conditions
	Aging		Ca(NO3)2		CaN		NHP
	Time	Rxn/Aging	Addition Rate	Grinding	Concentration	Amount	Concentration	Amount	NH4OH
Trial	(hr)	Temp (° C.)	(ml/min)	Method	(M)	(ml)	(M)	(ml)	Amount (ml)
16	12	25	3	Wet	0.167	900	0.100	900	30
11	100	25	3	Wet	0.167	900	0.100	900	30

TABLE 16
Effect of Aging on Trial 16: Results
	XRD	BET	Green	% Theoretical
	Crystallite Size	Surface Area	Density	Sintered Bulk
Trial	(nm)	(m2/g)	(g/cc)	Density
16	Not HAP	Not HAP	Not HAP	Not HAP
11	40	72.16	1.58	87.3

At longer aging times and higher pH (Trials 17 and 15), a kinetic effect is also observed. Because of the low precursor concentration, the rate of reaction is expected to be slower for Trial 15 than Trial 17 as confirmed by the smaller XRD crystallite size of Trial 15. Furthermore, the slower kinetics of Trial 15 compared to Trial 17 resulted in a higher surface area, and a particle morphology and size distribution favoring densification.

Two synthesis conditions, Trial 9 and 15, were determined to give rise to the optimal hydroxyapatite powders as assessed by % theoretical sintered bulk density. Trial 15 pressed the highest pressurelessly sintered bulk density of all trials investigated. The 95.6% theoretical sintered bulk density was obtained using a low precursor concentration, 100 hour aging time, an aging temperature of 25° C., 3 ml/min Ca(NO₃)₂ addition rate, 90 ml of NH₄OH, and wet grinding. A high theoretical density of 94.3% was obtained usign the synthesis of conditions of Trial 9: high precursor concentration, 12 hour aging time, an aging temperature of 25° C., 2 ml/min addition rate, 10 ml of NH₄OH, and wet grinding. Thus, optimal conditions were determined for the precursor concentrations investigated.

SUMMARY OF THE EXAMPLES 1-9

Nanocrystalline hydroxyapatite was synthesized successfully by chemical precipitation. The effects of NH₄OH amount, aging time, aging temperature, grinding method, precursor concentration, and Ca(NO₃)₂ addition rate on the crystallite size, agglomeration, morphology, crystallinity andthe molecular structure were examined. By identifying the important processing parameters and the method by which htey can be controlled, the crystallite size can be reduced to enhance the mechanical properties of bulk hydroxyapatitic. Furthermore, using the parameters to reduce agglomeration, to control the particle morphology and size distribution, and to contorl the chemcial reactivity of the particles, full densification can be achieved at lower sintering temperatures. The XRD patterns of the nanohydroxyapatite precursor gel were in good agreement with the JCPDS hydroxyapatite file (9-432); the peaks were substantially broadened due to the nanocrystalline nature of hydroxyapatite. The grinding method affected the surface area and the state of agglomeration with wet grinding being favored. Reaction and aging temperatures during precipitation affected the crystal growth rate with room temperature favored. Aging time affected the conversion of the precipitate into a crystalline hydroxyapatite, the crystallite size, and the particle morphology and size distribution. Short aging times were preferred by high precursor concentrations and long aging times were preferred by low precursor concentrations. Amount of NH₄OH affected the solubility of hydroxyapatite and the particle morphology and size distribution. Low NH₄OH amounts were preferred at high precursor concentrations favored low NH₄OH aounts while intermediate NH₄OH amounts were preferred at low precursor concentrations. Precursor additon rate affected the nucleation and cyrstal growth rates and particle morphology. Slow addition rate were preferred at both high and low precursor concentration. Precursor concentration affected the rate of reaction of hydroxyapatite. Optimal conditions were determined for both precursor concentrations. The nano-hydroxyapatite precursor gel heat treated at 550° C. gave an ultrafine grain size of 40 nm by TEM observation. This high-purity nano-hydroxyapatite also had higher B.E.T. surface areas than samples heat treated to 700° C. or 900° C. and was used toprepare compacts for pressureless sintering. The nano-hydroxyapatitic compact had superior stability whencompared to conventional hydroxyapatitic. The highly densified hydroxyapaptitc was obtained by pressureless sintering at 1100° C. Also, the dense compacts derived from nanocrystalline hydroxyapaptitic demonstrated excellent resistance to high-temperature decomposition, compared to te conventional hydroxyapatite. This should give rise to superior properties in bioceramic applications. The nano-hydroxyapatite synthesized in this study was resistant to thermal decomposition into β-TCP and CaO up to 1300° C.

EXAMPLE 10

Colloidal and Hot Pressing of Nanocrystalline Hyudroxyapatite

By only controlling the synthesis parameters without any subsequent powder processing, 95% theoretical bulk density wasobtained, indicating the supriority of this nanocrystalline hydroxyapatite powder. To further illsutrate the improvements of the nanocrystalline hydroxyapatite and its processing over the conventional hydroxyapaptite and conventioanl processing and to exceed the 96% theoretical bulk density obtained from pressureless sintering, the nanocrystalline powders were densified by colloidal and hot pressing.

Table 17 presents the synthesis conditions of the hot pressedpowders, and Table 18 illustrates the effect of hot pressing on the sintered densities and compares the densities obtained from hot pressing to those obtained from pressureless sintering at 1100° C. All powders were hot pressed at a pressure of 54 MPa and at a ramp rate of 10° C./min and with a dwell time of 30 minutes at 1100° C. After hot pressing, the pellets were polished with 600 grit and 800 grit SiC. Densities were measured by Archimedes' method in water.

TABLE 17
Effect of Hot Pressing: Synthesis Conditions
	Aging		Ca(NO3)2		CaN		NHP
	Time	Rxn/Aging	Addition Rate	Grinding	Concentration	Amount	Concentration	Amount	NH4OH
Trial	(hr)	Temp (° C.)	(ml/min)	Method	(M)	(ml)	(M)	(ml)	Amount (ml)
18	12	0	2	Wet	0.500	300	0.300	300	30
9	12	25	2	Wet	0.500	300	0.300	300	10
19	12	25	13	Wet	0.167	900	0.100	900	90

TABLE 18
Effect of Hot Pressing: Results
				%
				Theoretical	%
				Bulk	Theoretical
	XRD	BET		Density by	Bulk
	Crystallite	Surface	Green	Pressureless	Density by
	Size	Area	Density	Sintering	Hot Pressing
Trial	(nm)	(m2/g)	(g/cc)	at 1100° C.	at 1000° C.
18	36	53.70	1.56	67.7	3.05 g/cc
9	67	72.58	1.59	94.3	98.5
19	38	70.77	1.49	91.1	99.0

From the results presented in Table 18, hot pressing is observed to have a dramatic impact on the sintering of the hydroxyapatite powder. Hot pressing increased the % theoretical bulk density of the powder from Trial 9, one of the optimal conditions determined in the previous section, to 98.5% and enabled Trial 19 to achieve 99% theoretical density. The pellets of Trial 9 and 19 possessed a glassy finish and were slightly translucent. The β-TCP decomposition products, barely detectable by XRD, were found in the XRD patterns of the hot pressed powders from Trials 9 and 19. Furthermore, the grain sizes of the sintered pellets were found to be les sthan 225 nm by SEM, indicating theat an ultrafine microstructure was present after the sintering proces. Remarkably, even with a powder with poor pressureless sintering characteristics such as that of Trial 18, the bulk density can be increased from 2.14 g/cc to 3.05 g/cc through hot pressing. Through this sample decomposed significantly into β-TCP, this pellet was pore-free as indicated by the transparency of the pellet. The operating conditions presented for hot pressing provide an upepr limit for sintering temperature and a lower limit for the applied pressure becuase of the slight decomposition detected in the XRD patterns. Observations indicate that desnification stops before 1000° C., and that 900° C. or 800° C. may be the preferred sintering temperature. By hot pressing, the sintering temperature can be reduced by 200° C. or 300° C. Increasing the applied pressure is also anticipated to facilitates the sintering process. The most dramatic results from hot pressing are associated with a less crystalline and a more amorphous hydroxyapatite starting powder. Hot pressing seems to favor powders synthesized under either low temperature or low precursor concentration conditions. The results from hot pressing are a further demonstration of the superiority of the nanocrystalline hydroxyapaptite powder; without any special powder processing, full densification of hgydroxyapatite can be achieved.
Colloidal Pressing

The sample (Trial 20) prepared by colloidal pressing was synthesized under the similar conditions as Trial 15. The as-synethesized hydroxyapatite gel, instead of rinsing and centrifuging with ethanol in the last two washing steps, was wahsed with water. A slurry was prepared, and this slurry was colloidally pressed. After careful drying, the pellet was CIPed to 400 MPa and sintered to 1100° C. for 2 hours at 5° C./min. A highly translucent pellet was obtained with a 95.8% theoretical density. However, slight decomposition was detected in the XRD patterns. These data do strongly suggest that the hydroxyapatite prepared the method described in previous section is well suited to colloidal pressing as indicated by the translucent pellet. A mild hydrothermal treatment of the precipitate prior to colloidal pressing may improve sintering byincreasing the crystallinity of the material and by reducing the reactivity of the as-synthesized gel; the hydroxyapatite phase will be more stable and decomposition will be reduced. Furthermore, by controlling the pH and ionic strength of the slurry (e.g. by the addition of NH₄NO₃), the state of agglomeration and particle morphology can be controlled to enhance densification.

EXAMPLE 11

Synthesis and Characterization of Hydroxyapatite-Zirconia Composites

A composite including an apattie and a structural additive was prepared, with the additive selected to enhance the mechanical properties. To further strengthen hydroxyapatite and to maintian the nanocrystallinity after sintering, the adidtion of a seconary component is proposed. Many types of hydroxyapatite composites have been developed to take advantage of both the properties of hydroxyapatite and of the secondary phases. Hydroxyapaptite-polymer composites have been developed to improve the uponthe mechanical reliability of conventional hydroxyapatite. Hydroxyapatite has also been used as the reinforcing phase in glass-hydroxyapaptite composites. Hydroxyapatite composites formed with another secondary ceramic phase such as alumina or zironica have been shown to significantly improve the mechical properties of hydroxyapatite. THe hydroxyapapte-alumina composites required complex processing such as glass encapsulated hot isostatic pressing. Significant iprovements in mechanical properties were observed when vol % alumina in the composite increased above 50%. However, as the volume % of alumina is increased, the bioactivity of the composite decreases. The mechanical properties of the hydroxyapaptite-zirconia composites are expected to match or exceed the hydroxyapatite-alumina composites while using a smaller volume % of zirconia. This is because zirconia has more mechanisms by which it can provide mechanical reinforcement than alumina. Zirconia dispersions can toughen the hydroxyapatite matrix by a transformation toughening mechanism as well as crack deflection. By using nanocrystalline materials processing, the methcanical properties can be further enhanced. The zirconia dispersion can then be used to “pin” the hydropxyapatite grains suppressing grain growth during calcination and sintering to preserve nanometer-sized crystallites.

In trying to develop a composite with the optimal mechanical properties, the effects of the grain sizes of the hydroxyapatite and zirconia, dopant concentration, milling time, and milling intensity were investigated. Nanocyrstalline hyroxyapatite and zirconia were synthesized by chemical precipitation. Through the previous studies on the synthesis and characterization of hydroxyapatite, the processing parameters can be controlled to obtain a specified grain size and particle morphology and sintered density.

Synthesis of Nanocyrstalline Hydroxyapatite

Aqueous solutions of 0.300 M (NH₄)₂HPO₄ and 0.500 M Ca(NO₃)₂ were prepared so that the Ca:P ratio was 10:6 and were mixed with a magnetic stirrer. The pH of the (NH₄)₂HPO₄ aqueous solution was varied by adding 30 ml of concentrated NH₄OH. 300 ml of a 0.500 M solution of Ca(NO₃)₃ was added to 300 ml of 0.300 M aqueous (NH₄)₂HPO₄ at 10 ml/min. The combined solutions were magnetically stirred for 12 hours and aged at room temperature. The white precipitate was collected by filtration with a Buchner funnel and wahsed at least three times with distilled water with a decreasing concentraiton of NH₄OH each time and finally with ethanol. The gel was air dried at room temperature for 24 horus and then dried in a 150° C. oven for 12 hours. The gel was then finely ground with an alumina mortar and pestle. The ground powders were then heat trated in air at 550° C. with a heating rate of 10° C./min, and a dwell time of 2 hours.

Synthesis of Nanocyrstalline Zirconia

A 2.00 M ZrOCl₂.8H₂O (3 mol % Y₂O₃) stock solution is prepared from reagent grade ZrOCl₂.8H₂O and Y₂O₃ and deionized water. THe stock solution is allowed to stir for 24 hours prior to use. 25 ml of the 2.00 M ZrOCl₂.8H₂O (3 mol % Y₂O₃) is pipetted 225 ml of ethanol under constant stirring. This working solution is allowed to stir for 30 minutes. Next, a base solution is prepared by pipetting 100 ml of ammonium hydroxide into 250 ml of ethanol under constant stirring and by allowing the solution to stir for at least 15 minutes. The precipitation reaction occurrs when the 0.200 M working solution is added to a base solution at 15 ml/min under constant stirring. The solution is allowed to stir and age for 24 hours. Next, the solution is centrifuged at 1500 rpm for 20 minutes and decanted. The resulting gel is redispersed in ethanol and centrifuged 4 more times under the same condition to quench the reaction and to remove all the chloride ions. The gel is then ground with a pestle in a preheated mortar until a fine powder is obtained. This powder is allowed to dry in a 110° C. oven overnight. finally, the powdeer is calcined at 550° C. for 2 hours wiht a ramp rate of 10° C./min.

Proof of Concept and Initial Studies

In these series of experiments, compsoties formed from conventional hydroxyapatite (Aldrich), conventional zirconium (Toso), nanocyrstalline hydroxyapatite, and nanocrystalline zirconia heat treated at 550° C. were investigated. The composite was formed by dry milling the hydroxyapatite with 10 vol % of zirconia for 24 hours, CIPing at 300 MPa for 3 minutes, pressureless sintering or 2 hours in air at sintering temperatures of 1100° C., 1200° C., and 1300° C. This dry ball milling ensured good mixing and ocntact between the two components without the transformations that might occur by high-energy ball milling. The XRD patterns of the nanocyrstalline Y₂O₃-doped ZrO₂ indicated the processes of zirconia as 12 nm cyrstallites. A PA-FTIR spectrum indicated the presence of Zr-O-Zr, H₂O and ZrOH peaks. The clacined nanocrystalline Y₂O₃-doped ZrO₂ possessed a BET surface area of 140 m²/g and an average pore size of 9 mm. After calcination at 550° C., the nanocyrstalline hydroxyapatite had a XRD crystallite size of 32 nm and a BET surface area of 66.8 m²/g.

The XRD patterns of the sintered nano-hydroxyapatite/nano-zirconia composite indicated that the composite was thermally stable up to 1200° C., and that significant phase transformation of hydroxyapatite and zirconia into tricalcium phsopahte and monoclinic zirconia, respectively, occurred at 1300° C. When comparing the sinterability of nano-hydroxyapatite and zirconia reinforced hydroxyapatite, the composite required a higher sintering temperautre of 1200° C. to achieve full densification while the pure nano-hydroxyapatite required 1100° C. to achieve full densification. The nanocrystalline composite possessed better sinterability than any composite containing a conventional hydroxyapatite and/or ZrO₂ powder. By 120° C., the nano-hydroxyapatite/nano-zirconia composite attained 98% theoretical density of hydroxyapatite while nanohydroxyapatite/zirconia (Toso) achieved less than 70% theoretical density by 1300° C.

TEM micrographs indicated that there were no glossy phases at the grain boundaries showing that the nanocomposite achieved good densification without the precipitation of undesirable secondary phases. Zirconia grains were intragranularly dispersed within the hydroxyapatite matrix. With smaller grains izes, a more mechanically robust material is obtained. The pure nanocrystalline hydroxyapatite possessed a compressive strength of 745 MPa while the conventional micron-sized hydroxyapatite possessed a compressive strength of 150 MPa. Further reinforcement of the nanocrystalline hydroxyapatite with a secondary dispersoid of nanocrystalline zirconia resulted in an even higher compressive strength of 1020 MPa. This improvement in compressive strength is believed to be due to the intragranular toughening of the nanocrystalline hydroxyapatite matrix by the nana-ZrO₂ dispersoids.

Another method for the synthesis of nanocrystalline hydroxyapatite yields an improved nanocomposite with an even higher compressive strength, a lower sintering temperature and greater thermal stability. The method of producing the composite uses ajar mill to disperse the zirconia into the hydroxyapatite. Recent experiments suggests that better mixing and contacting between the zirconia and hydroxyapatite can be achieved by co-precipitation, or by dispersing zirzonia particles during either the chemical precipitation or the aging of the nanocrystalline hydroxyapatite.

The proof concept and intiial studies of the synthesis of jhydroxyapatite/zirconia nanocomposite used an earlier method for the syntehsis of nanocrystalline hydroxyapatite. By using the recently optimized method for the synthesis of nanocrystalline hydroxyapatite (Trial 9 or 15), an improved nanocomposite wioth an even higher compressive strength, a lower sintering temperature and greater thermal stability may be produced. The method of producing the composite reported above used a jar mill to disperse the zirconia into the hydroxyapatite. Recent experiments suggest that better mixing can be achieved by dispersing zirconia particles during either the chemical precipitation or the aging of the nanocrystalline hydroxyapatite.

EXAMPLE 12

Synthesis and Characterization of Nanocrystalline Carbonate Hydroxyapatite

Since the mineral phase of human bone has recently been idenitfied as carbonate apatite, not hydroxyapatite⁷, a nanocrystallilne carbonate apatite can be used as a reactive layer on a bioceramic to enhance bioactivity for bone growth on the surfaces of the implant. Because the poor mechnaical properties of carbonate apattie prevent it from being used as a structural material, the focus of this work will be the synthesis and the characterization of nanocrystalline carbonate apatite powder. With the ability to synthesize a high surface area carbonate apatite powder, the bioactivity of artificial bone crystals can be controlled.

To further illustrate the versatility of the preparative technique developed for synthesis of hydroxyapatite, the chemical precipitation process in which nanocrystalline hydropxyapatite is synthesized was modifed to derive nanocrystalline carbonate apatite, Ca₁₀(PO₄)₆CO₃ (Type A where the CO₃²⁻ occupies the monovalent anionic (OH⁻) sites) or CA_10-x(PO₄)_6-2x(CO₃)_2x(OH)_2(1-x) (Type B where the CO₃²⁻ occupies the trivalent anionic (PO₄³⁻) sites). Type A carbonate apatite is a well-defined class of compounds normally synthesized at elevated temperatures. In contrast, Type B carbonate apatite is a poorly defined class of compounds typically synthesized at low temperatures under aqueous conditions. Carbonate apatite can be generated by either saturating the reaction solution with carbon dioxide or byadding another carbonate source such as sodium bicarbonate or ammomium bicarbonate, followed by a hydrothermal treatment, in an attempt to stabilize the carbonat ion in the precipitate.

Synthesis of Nanocrystalline Carbonate Apatite

Aqueous solutions of 0.075 M to 0.300 M (NH₄)₂HPO₄ and 0.500 M Ca(NO₃)₂ were prepared so that the Ca:P ratio varied from 6.67 to 1.67 and where mixed with a magnetic stirrer. The pH of the (NH₄)₂HPO₄ aqueous solution was adjusted by adding 10 ml of concentrated NH₄OH. 300 ml of a 0.500 M solution of Ca₂(NO₃)₂ was added to 300 ml of 0.300 M aqueous (NH₄)₂HPO₄ at 3 ml/min. A gas stream copmsoed of 5% CO₂ and 95% N₂ was bubbled through the precipitate immediately after addition or 6 hours after addition for 18 hours. Some trials were magnetically stirred for 100 hours and aged at room temperature, while others were aqueously aged for 50 hours followed by 50 hours of hydrothermal treatment at 180° C. The white precipitate was collected by centrifugation at 1500 rpm for 15 minutes. After decantiong, the precipitate was redispersed in a solution of distilled water and NH₄OH by magnetically stirring for 20 minutes; this procedure was repeated two more times with decreasing amounts of NH₄OH, and two times with ethanol. The gel was air dried at room temperature for 24 hours, and then dried in a 150° C. oven for an additional 24 hours. The gel was then finely ground with an alumina mortar and pestle. The ground powder were then heat treated in air at 550° C., 700° C. and 900° C. with a heating rate of 10° C./min, and a dwell time of 2 hours.

Proof of Concept and Initial Studies

The synthesis of hydroxyapatite is known to undergo an indcution period. prior to hydroxyapatite formation, the precipitation is thought to convert from an amorphous caclium phosphate to an octacalcium phosphate and then to hydroxyapatite. Furthermore, the induction period increases with increasing pH. By synthesizing the hydroxyapatite at a low pH, higher solubility of hydroxyapatite is anticipated to aid the incorporation of the carbonate ion. In this initial study, the effect of carbonate substitution during pre- and post HAP formation, the effect of varying the Ca/P ratio, and the effect of aqueous aging versus hydrothermal treatment were examined. In all samples, a mixed phase of hydroxyapatite, Type A and Type B carbonate apatite was detected. Introducing CO₂ immediately after the addition of Ca(NO₃)₂ was found to minimize the formation of CaCO₃, as determined by the XRD patterns and FTIR spectra. If CO₂ was added 6 hours after Ca(NO₃)₂ addition was completed, significant CaCO₃ formed because more calcium cations were in solution as a result of the reprecipitation process, while calcium was bound in the precipitate immediately after Ca(NO₃)₂ addition. In both aqueous aging and hydrothermal treatment, CaCO₃ was detected in the XRD patterns when [(NH₄)₂HPO₄]<0.224 M. For aqueously aged sampels, both Type A and Type B carbonate apatites were detected in FTIR spectra. 879 cm⁻¹ is assinged to Type A carboante apatite, and 873 cm⁻¹ is assigned to Type B carbonate apatite. Type A is favored over Type B for aqueously aged samples when [(NH₄)₂HPO₄]=0.3 M. For this sample, the XRD crystallite size was determined to be 25 nm. This is considerably smaller than the XRD crystallite size determined form hydroxyapatite synthesis which strongly suggests that the presence of the carbonate ions restricts crystal growth. CaCO₃ became the dominant phase for adequately aged samples when [(NH₄)₂HOP₄]<0.224 M. For hydrothermally aged samples, both Type A and Type B carbonate apatites were detected in the FTIR spectra but the relative intensity of the Type A and Type B peaks suggests that hydrothermal treatment is more selective towards Type A carbonate apatite formation. Hydrothermal treatment stabilized the apatite phase with CaCO₃ becoming the dominant phase when [(NH₄)₂HPO₄]<0.075 M. In a subsequent experiment, carbonate apatite was synthesized under the following conditions: (1) 300 ml 0.5 M Ca(NO₃)₂, (2) 300 ml 0.3 M (NH₄)₂HPO₄, (3) 10 ml NH₄OH, (4) 80° C. reaction and aging temperature, (5 ) 3 ml/min Ca(NO₃)₂, and (6) immediate introduction of CO₂ after Ca(NO₃)₂ was added at 3 ml/min. The XRD pattern was identified as an apatite, with the FTIR spectra detecting Type A and Type B with Type A slightly favored. The XRD crystallite size for this samplew as 65 nm, also considerably smaller than sizes measured for hydroxyapatite synthesized at similar conditions. These results sugges that Type B will be favored when synthesized at temperatures below 25° C. Furthermore, the introduction of carbonate into the apatite structure may be more carefully controlled by using NH₄HCO₃ instead of CO_2(g). The surface areas for nanocrystalline carbonate apatite is expected to be similar to or greater than the surface areas of nanocrystalline hydroxapatite synthesized udner similar conditions.

This example illustrates the versatility of the process developed for synthesizing nanocrystalline hydroxyapatite and the benefits of carefully controlling the process parameters. By introducing a carbonate source and controlling the processing parameters, a nanocrystalline carbonate apatite, both Type A and Type B, was synthesized. Through further refinement, Type A and Type B carbonate apatite can be selectively synthesized, and the degree of substitution of carboante ions for the phosphate ions in Type B carbonate apattie can be controlled. Important parameters will be reaction and aging temperatures, carbonate source, method of carbonate introduction, precursor concentrations, aging time, and pH.

SUMMARY OF EXAMPLES

The above examples demonstrate superior processes and products resulting from densification of nanocrystalline hydroxyapaptite. The grain sizes of calcined samples varied from 30 nm to 100 nm depending how pH, aging time, reaction and aging temperature, Ca(NO₃)₂ addition rate, precursor concentraiton, and grinding method were controlled, while the grain sizes of conventional hydroxyapatite were on a microcon scale. For example, the surface area of one sample of the invention after calcination at 550° C. is 159.5 m²/g while the conventional sampel after calcination at 550° C. has a very small surface area of 5.4 m²/g. The samle of the invention retained phase uniformity after calcination at 550° C., but the conventional sample began to transform into tricalcium phosphate at 550° C. with substanital conversion to tricalcium phosphate and calcia by 700° c. In a sample of the invention 96% of the theoretical density was obtianed at a low temperature of 1100° c. by pressureless sintering for nanocrystalline hdyroxyapatite which was stable upto 1300° C. However, the conventional sample achieved only 70% of the theoretical density at 1200° C. with decomposition into tri-calcium phosphate. Furthermore, the densified conveitonal sample contained large pores and microcracks. Our nanocrystalline hydroxyapatite has high purity and phas ehomogeneity as well as superior sinterability compared to the conventionally prepared hydrocyapatite. When our nanocrystalline hydroxyapaptite was sintered using either colloidal or hot pressing, 99% theoretical bulk density with a grain size of les sthan 250 nm can be obtained. Dense nanocrystalline hdyroxyapatite compacts further processed a compressive strength as high as 745 MPa, whil the conventional micron-sized hydroxyapatite compacts from a similar pressureless sintering treatment possessed a compressive strength of 150 MPa. Additionally, further reinforcement of the hydroxyapatite can be accomplished by introducing a secondary dispersoid such as zirconia which would greatly improve the toughness and chemicalstability of hydroxyapatite bypinning the mobility of any integranular and intragranular defects. A dense composite of nanocrystalline hydroxyapatite and 10 wt % nanocrystalline 3 mol % Y₂O₃-doped ZrO₂ possessed an even higher compressing strength of 1020 MPa. With more complete characterization, the densified nanocrystalline hydroxyapatite and hydroxyapatite-zirconia compsoites can easily be devleoped into dental and orthopedica weight-bearing implants. Furthermore, the processing of nanocrystalline hydroxyapatite can be adapted to synthesize a nanocrystalline carbonate apatitic illustrating the versatility of our process. THis process can also be used to selectively synthesize Type A and Type B carbonate apatite as well as to control the degree of substitution of the carbonate ion into the apatite structure.

Those skilled in the art would readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameterw will depend upon the specific application for hwihc the methods and apparatus of the present invention are used. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described.

Claims

1. A composition, comprising particulate apatate having an aveage apatite crystal size of less than 100 nm, wherein the crystal is a spherical.

2. The composition of claim 1 comprising particulate apatite having an average apatite crystal size of less than 50 nm.

3. The composition of claim 1 comprising partiuclate apatite having an average apatite crystal size of less than 30 nm.

4. The composition of claim 1 comprising partiuclate apatite having an average apatite crystal size of less than 30 nm.

5. The composition of claim 1 wherien the partiuclate apatite is densified.

6. The composition of claim 1 comprising apatite having an average particle size of less than 1 μm.

7. The composition of claim 1 comprising apatite having an aveage particle size of less than 0.5 μm.

8. The composition of claim 1 comprising apatite having an average particle size of less than 0.25 μm.

9. A composition comprising particulate apatite having a surface area of at least 40 m²/g and a spherical crystal.

10. The composition of claim 7 comprising particulate apatite having a surface area of at least 100 m²/g.

11. The composition of claim 9 comprising particulate apatite having a surface area of at least 150 m²g.

12. The composition of claim 9 that undergoes apatite phase decomposition of less than 10% when exposed to conditions of at least 1000° C. for at least 2 hours.

13. The composition of claim 12 that undergoes apatite phase decomposition of less than 5% when exposed to conditions of at least 1000° C. for at least 2 hours.

14. The composition of claim 12 that undergoes apatite phase decomposition of less than 3% when exposed to conditions of at least 1000° C. for at least 2 hours.

15. The composition of claim 12 that undergoes apatite phase decomposition of less than 10% when exposed to conditions of at least 1100° C. for at least 2 hours.

16. The composition of claim 12 that undergoes apatite phase decomposition of less than 5% when exposed to conditions of at least 1100° C. for at least 2 hours.

17. The composition of claim 12 that undergoes apatite phase decomposition of less than 3% when exposed to conditions of at least 1100° C. for at least 2 hours.

18. The composition of claim 12 that undergoes apatite phase decomposition of less than 10% when exposed to conditions of at least 1200° C. for at least 2 hours.

19. The composition of claim 12 that undergoes apatite phase decomposition of less than 5% when exposed to conditions of at least 1200° C. for at least 2 hours.

20. The composition of claim 12 that undergoes apatite phase decomposition of less than 3% when exposed to conditions of at least 1200° C. for at least 2 hours.

21. The composition of claim 12 that undergoes apatite phase decomposition of less than 10% when exposed to conditions of at least 1300° C. for at least 2 hours.

22. The composition of claim 12 that undergoes apatite phase decomposition of less than 5% when exposed to conditions of at least 1300° C. for at least 2 hours.

23. The composition of claim 12 that undergoes apatite phase decomposition of less than 3% when exposed to conditions of at least 1300° C. for at least 2 hours.

24. An article having a dimension of at least 0.5 cm made up of the composition of claim 1.

25. The article of claim 24 wherein the particulate apatite is consolidated.

26. The article of claim 24, formed into the shape of a prosthesis.

27. The article of claim 24 that is a prosthesis.

28. The article of claim 24 comprising an exterior coating on a prosthesis.

29. The article of claim 28 comprising an exterior coating, on a prosthesis, of at least 0.5 micron in thickness.

30. The article of claim 24 wherein a theoretical density of at least 90%.

31. The article of claim 24 having a theoretical density of at least 95%.

32. The article of claim 24 having a theoretical density of at least 98%.

33. An article having a dimension of at least 0.5 cm made up of the composition of claim 9.

34. The article of claim 33 having a porosity of at least 20%.

35. The article of claim 33 having a porosity of at least 50%.

36. The article of claim 33 having a porosity of at least 50%.

37. The article of claim 33 having a porosity of at least 75%.

38. The densified article of claim 33 having compressive strength of at least about 150 MPa.

39. The densified article of claim 38, having a density of at least about 98%.

40. The densified article of claim 33 having compressive strength of at least about 500 MPa.

41. The densified article of claim 33 having compressive strength of at least about 700 MPa.

42. The densified article of claim 38, having a density of at least about 90%.

43. The densified article of claim 38, having a density of at least about 95%.

44. The article of claim 24 that is a part of a prosthesis.

45. An article comprising a densified apatite structure having a dimension of at least 0.5 cm and an average xrd crystal size of less than 250 nm, wherein the apatite structure undergoes phase decomposition of less than 10% when exposed to conditions of at least 1300° C. for at least 2 hours and has a compressive strength of at least 150 MPa.

46. The article of claim 45, wherein the densified apatite structure has a density of at least about 90%.

47. The article of claim 46, wherein the densified apatite structure has a density of at least about 98%.

48. The article of claim 45, wherein the densified apatite structure has a compressive strength of at least about 500 MPa.

49. The article of claim 48, wherein the densified apatite structure has a compressive strength of at least about 700 MPa.

50. The article of claim 45, wherein the phase decomposition is less than 5%.

51. The article of claim 45, wherein the densified apatite structure has an average xrd crystal size of less than 150 nm.

52. The article of claim 45, wherein the densified apatite structure comprises an auxiliary strcutural additive.

53. The article of claim 52, wherein the auxiliary structure additive comprises a ceramic additive.

54. The article of claim 53, wherein the ceramic additive comprises a metal oxide.

55. The article of claim 54, wherein the metal oxide comprises zirzonia.

56. The article of claim 52, wherein the auxiliary structural additive is nanocrystalline.

57. The article of claim 52, wherein the auxiliary structural additive is a metal or alloy.

58. The article of claim 52, wherein the auxiliary structural additive is present in an amount of between about 1% and about 50% by volume.

59. The article of claim 45, wherein the densified apatite structure comprises carbonated apatite.

60. The article of claim 45, wherein the article is at least part of a prosthesis.

61. The article of claim 60, wherein the article is a prosthesis.

62. The article of claim 45, wherein the article comprises an exterior coating on a prosthesis.

63. The article of claim 45, wherein the article is a bioactive implant.

64. The article of claim 63, wherein the bioactive implant is an orthopedic or dental implant.

65. The article of claim 59, wherein the carbonated apatite is a reactive layer on a bioceramic capable of enhancing bioactivity for bone growth.

66. An article comprising an apatite structure having a dimension of at least 0.5 cm, a density of at least about 98%, a compressive strength of at least about 500 MPa, an average xrd crystal size of less than 250 nm, and wherein the apatite structure undergoes phase decomposition of less than 10% when exposed to conditions of at least 1300° C. for at least 2 hours.

67. The article of claim 66, wherein the apatite structure has a compressive strength of at least about 700 MPa.

68. The article of claim 66, wherein the phase decomposition is less than 5%.

69. The article of claim 66, wherein the apatite structure has an average xrd crystal size of less than 150 nm.

70. The article of claim 66, wherein the article is a prosthesis.

71. The article of claim 66, wherein the article is at least part of a prosthesis.

72. The article of claim 66, wherein the article comprises an exterior coating on a prosthesis.

73. The article of claim 66, wherein the article is a bioactive implant.

74. The article of claim 73, wherein the bioactive implant is an orthopedic or dental implant.

75. The article of claim 66, wherein the apatite structure comprises an auxiliary structural additive.

76. The article of claim 75, wherein the auxiliary structural additive comprises a ceramic additive.

77. The article of claim 76, wherein the ceramic additive comprises a metal oxide.

78. The article of claim 77, wherein the metal oxide comprises zirconia.

79. The article of claim 75, wherein the auxiliary structural additive is nanocrystalline.

80. The article of claim 75, wherein the auxiliary structural additive is a metal or alloy.

81. The article of claim 75, wherein the auxiliary structural additive is added in an amount of between about 1% and about 50% by volume.

82. The article of claim 66, wherein the apatite structure comprises carbonated apatite.

83. The article of claim 82, wherein the carbonated apatite is a reactive layer on a bioceramic capable of enhancing bioactivity for bone growth.

84. An article comprising a consolidated apatite structure having a dimension of at least 0.5 cm, an average xrd crystal size of less than 250 nm, and a porosity of at least 20%, wherein the apatite structure undergoes phase decomposition of less than 10% when exposed to conditions of at least 1300° C. for at least 2 hours.

85. The article of claim 84, wherein the consolidated apatite structure has a porosity of at least 50%.

86. The article of claim 85, wherein the consolidated apatite sturcture has a porosity of at least 75%.

87. The article of claim 84, wherein the consolidated apatite structure has a compressive strength of at least about 150 MPa.

88. The article of claim 84, wherein the phase decomposition is less than 5%.

89. The article of claim 84, wherein the consolidated apatite structure has an average xrd crystal size of less than 150 nm.

90. The article of claim 84, wherein the consolidated apatite structure comprises an auxiliary structural additive.

91. The article of claim 90, wherein the auxiliary structural additive comprises a ceramic additive.

92. The article of claim 91, wherein the ceramic additive comprises a metal oxide.

93. The article of claim 92, where in the metal oxide comprises zirconia.

94. The article of claim 90, wherein the auxiliary structural additive is nanocrystalline.

95. The article of claim 90, wherein the auxiliary structural additive is a metal or alloy.

96. The article of claim 90, wherein the auxiliary structural additve is present in an amount of between about 1% and about 50% by volume.

97. The article of claim 84, wherein the consolidated apatite structure comprises carbonated apatite.

98. The article of claim 97, wherein the carbonated apatite is a reactive layer on a bioceramic capable of enhancing bioactivity for bone growth.

99. The article of claim 84, wherein the consolidated apatite structure is formed from an admixture of particulate apatite with an organic species.

100. The article of claim 99, wherein the organic species is a self-assembling surfactant or a polymer.

101. The article of claim 84, wherein the article is at least part of a prosthesis.

102. The article of claim 101, wherein the article is a prosthesis.

103. The article of claim 101, wherein the article comprises an exterior coating on a prosthesis.

104. The article of claim 84, wherein the article is a bioactive implant.

105. The article of claim 104, wherein the bioactive implant is an orthopedic or dental implant.