Methods for synthesis of nanocrystalline 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 average crystal size of less than 150 nm are provided. Products also can have a surface area of at least 40 m #1# 2/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.
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#1# 0. 45. A composition, comprising particulate apatite having an average apatite particle size of less than 1 μm, an average apatite crystal size of less than 150 nm, a surface area of at least 40 m2/g, and wherein the particulate apatite undergoes apatite phase decomposition of less than 10% when exposed to conditions of at least 1300° C. for at least 2 hours, and wherein the crystal has an aspect ratio from spherical to about 2.9:1.
#1# 0. 1. A composition, comprising particulate apatite having an average apatite crystal size of less than 100 nm, wherein the crystal is spherical.
#1# 0. 2. The composition of
#1# 0. 3. The composition of
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#1# 0. 5. The composition of
#1# 0. 6. The composition of
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#1# 0. 9. A composition comprising particulate apatite having a surface area of at least 40 m2/g and a spherical crystal.
#1# 0. 10. The composition of
#1# 0. 11. The composition of
#1# 0. 12. The composition of
#1# 0. 13. The composition of
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#1# 0. 24. An article having a dimension of at least 0.5 cm made up of the composition of
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#1# 0. 38. The densified article of
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, 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 crystals. 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
Surface
Density
Sintered Bulk
Trial
Size (nm)
Area (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
1.88
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 consistent with hydroxyapatite. Trials 9, 5, and 10 correspond 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 gives 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 is increased. Similar to Trial 10, Trial 13 deviates from the trend established 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 (CaHPO4) and brushite (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 prescence of monetite and brushite during the synthesis of Trial 10 may give rise to the deviation in the crystallite size. Furthermore, samples prepared under similar conditions as Trial 13 have resulted in hydroxyapatite, confirming the metastability of this region.
Trial 9, 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 resulted in a particle morphology and distribution favorable toward densification.
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 distribution. Conversely, slow addition of precursors results in a more homogenous mixture of reactants 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
Surface
Density
Sintered Bulk
Trial
Size (nm)
Area (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 corresponded to the JCPDS hydroxyapatite file (9-0432) and no other phases were found. All FTIR spectra possess peaks characteristic of nanocrystalline hydroxyapatite. Trials 9 and 15 possessed a larger XRD crystallite size and a higher BET surface area than Trials 14 and 12, respectively, 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 anticipated. In addition, by using a slow addition to obtain a more uniform particle morphology and distribution, the final sintered bulk densities were enhanced. These effects were significant for Trials 9 and 14, but addition rate did not play a dominant role in Trials 15 and 12. The lesser role of addition rate at low precursor concentrations can be attributed to the difference in molar flow rates. The difference in molar rates between Trials 15 and 12 is 7.5×10−3 moles/min whereas the difference in molar flow rates between Trials 9 and 14 is 7.4×10−2 moles/min. These results confirm that crystallite size depends on the rate of 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.
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
0.100
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
Surface
Density
Sintered Bulk
Trial
Size (nm)
Area (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
31
89.71
1.74
95.6
The XRD patterns of Trials 9, 17, and 15 correspond to hydroxyapatite while 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 under a higher 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 kinetics 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 Time on Trial 16: Results
XRD
BET
Green
% Theoretical
Crystallite
Surface
Density
Sintered Bulk
Trial
Size (nm)
Area (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 possessed 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 using the synthesis 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.
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 and the molecular structure were examined. By identifying the important processing parameters and the method by which they can be controlled, the crystallite size can be reduced to enhance the mechanical properties of bulk hydroxyapatite. Furthermore, using the parameters to reduce agglomeration, to control the particle morphology and size distribution, and to control the chemical reactivity of the particles, full densification can be achieved at lower sintering temperatures. The XRD patterns of the nano-hydroxyapatite 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 amounts while intermediate NH4OH amounts were preferred at low precursor concentrations. Precursor addition rate affected the nucleation and crystal growth rates and particle morphology. Slow addition rates were preferred at both high and low precursor concentrations. 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 to prepare compacts for pressureless sintering. The nano-hydroxyapatite compact had superior sinterability when compared to conventional hydroxyapatite. The highly densified hydroxyapatite was obtained by pressureless sintering at 1100° C. Also, the dense compacts derived from nanocrystalline hydroxyapatite demonstrated excellent resistance to high-temperature decomposition, compared to the 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.
Colloidal and Hot Pressing of Nanocrystalline Hydroxyapatite
By only controlling the synthesis parameters without any subsequent powder processing, 96% theoretical bulk density was obtained, indicating the superiority of this nanocrystalline hydroxyapatite powder. To further illustrate the improvements of the nanocrystalline hydroxyapatite and its processing over the conventional hydroxyapatite and conventional 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 pressed powders, 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
% Theoretical
BET
Bull: Density
Bulk
XRD
Surface
Green
by Pressureless
Density by
Crystallite
Area
Density
Sintering
Hot Pressing
Trial
Size (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 less than 225 nm by SEM, indicating that an ultrafine microstructure was present after the sintering process. 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. Though 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 upper limit for sintering temperature and a lower limit for the applied pressure because of the slight decomposition detected in the XRD patterns. Observations indicate that densification 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 hydroxyapatite powder; without any special powder processing, full densification of hydroxyapatite can be achieved.
Colloidal Pressing
The sample (Trial 20) prepared by colloidal pressing was synthesized under the similar conditions as Trial 15. The as-synthesized hydroxyapatite gel, instead of rinsing and centrifuging with ethanol in the last two washing steps, was washed with water. A slurry was prepared, and this slurry was colloidally pressed. After careful drying, the pellet was CIPed to 300 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 by 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 by increasing 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.
Synthesis and Characterization of Hydroxyapatite-Zirconia Composites
A composite including an apatite and a structural additive was prepared, with the additive selected to enhance the mechanical properties. To further strengthen hydroxyapatite and to maintain the nanocrystallinity after sintering, the addition of a secondary 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. Hydroxyapatite-polymer composites have been developed to improve upon the mechanical reliability of conventional hydroxyapatite. Hydroxyapatite has also been used as the reinforcing phase in glass-hydroxyapatite composites. Hydroxyapatite composites formed with another secondary ceramic phase such as alumina or zirconia have been shown to significantly improve the mechanical properties of hydroxyapatite. The hydroxyapatite-alumina composites required complex processing such as glass encapsulated hot isostatic pressing. Significant improvements 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 hydroxyapatite-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 dispersiods can toughen the hydroxyapatite matrix by a transformation toughening mechanism as well as crack deflection. By using nanocrystalline materials processing, the mechanical properties can be further enhanced. The zirconia dispersion can then be used to “pin” the hydroxyapatite 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. Nanocrystalline hydroxyapatite 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.
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)2 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 washed at least three times with distilled water with a decreasing concentration of NH4OH each time and finally with ethanol. The gel was air dried at room temperature for 24 hours 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 treated in air at 550° C. with a heating rate of 10° C./min, and a dwell time of 2 hours.
A 2.00 M ZrOCl2.8 H2O (3 mol % Y2O3) stock solution is prepared from reagent grade ZrOCl2.8 H2O 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 ZrOCl2.8 H2O (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 occurs 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 conditions to quench the reaction and to remove all the chloride ions. The gel is then ground with a pestel in a preheated mortar until a fine powder is obtained. This powder is allowed to dry in a 110° C. oven overnight. Finally, the powder is calcined at 550° C. for 2 hours with a ramp rate of 10° C./min.
Proof of Concept and Initial Studies
In these series of experiments, composites formed from conventional hydroxyapatite (Aldrich), conventional zirconia (Toso), nanocrystalline 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 for 2 hours in air at sintering temperatures of 1100° C., 1200° C., and 1300° C. This dry ball milling ensured good mixing and contact between the two components without the transformations that might occur by high-energy ball milling. The XRD patterns of the nanocrystalline Y2O3-doped ZrO2 indicated the presence of zirconia as 12 nm crystallites. A PA-FTIR spectrum indicated the presence of Zr-O-Zr, H2O and ZrOH peaks. The calcined nanocrystalline Y2O3-doped ZrO2 possessed a BET surface area of 140 m2/g and an average pore size of 9 nm. After calcination at 550° C., the nanocrystalline 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 phosphate 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 temperature 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 1200° C., the nano-hydroxyapatite/nano-zirconia composite attained 98% theoretical density of hydroxyapatite while nano-hydroxyapatite/zirconia (Toso) achieved less than 70% theoretical density by 1300° C.
TEM micrographs indicated that there were no glassy 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 grain sizes, 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 nano-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 zirconia particles during either the chemical precipitation or the aging of the nanocrystalline hydroxyapatite.
The proof of concept and initial studies of the synthesis of hydroxyapatite/zirconia nanocomposite used an earlier method for the synthesis of nanocrystalline hydroxyapatite. By using the recently optimized method for the synthesis of nanocrystalline hydroxyapatite (Trial 9 or 15), an improved nanocomposite with 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 and contacting between the zirconia and hydroxyapatite can be achieved by dispersing zirconia particles during either the chemical precipitation or the aging of the nanocrystalline hydroxyapatite.
Synthesis and Characterization of Nanocrystalline Carbonate Hydroxyapatite
Since the mineral phase of human bone has recently been identified as carbonate apatite, not hydroxyapatite7, a nanocrystalline 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 mechanical properties of carbonate apatite 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 hydroxyapatite is synthesized was modified to derive nanocrystalline carbonate apatite, Ca10(PO4)6CO3 (Type A where the CO32− occupies the monovalent anionic (OH−) sites) or Ca10-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 by adding another carbonate source such as sodium bicarbonate or ammonium bicarbonate, followed by a hydrothermal treatment, in an attempt to stabilize the carbonate ion in the precipitate.
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 were 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 Ca(NO3)2 was added to 300 ml of 0.300 M aqueous (NH4)2HPO4 at 3 ml/min. A gas stream composed 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 decanting, 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 powders 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 induction period. Prior to hydroxyapatite formation, the precipitate is thought to convert from an amorphous calcium 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 samples, both Type A and Type B carbonate apatites were detected in FTIR spectra. 879 cm−1 is assigned to Type A carbonate 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 from hydroxyapatite synthesis which strongly suggests that the presence of the carbonate ions restricts crystal growth. CaCO3 became the dominant phase for aqueously aged samples when [(NH4)2HPO4]<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 sample was 65 nm, also considerably smaller than sizes measured for hydroxyapatite synthesized at similar conditions. These results suggest 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 CO2(g). The surface areas for nanocrystalline carbonate apatite is expected to be similar to or greater than the surface areas of nanocrystalline hydroxyapatite synthesized under similar conditions.
This sample 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 carbonate ions for the phosphate ions in Type B carbonate apatite can be controlled. Important parameters will be reaction and aging temperatures, carbonate source, method of carbonate introduction, precursor concentrations, aging time, and pH.
The above examples demonstrate superior processes and products resulting from densifications of nanocrystalline hydroxyapatite. 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 concentration, and grinding method were controlled, while the grain sizes of conventional hydroxyapatite were on a micron 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 sample after calcination at 550° C. has a very small surface area of 5.4 m2/g. The sample 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 substantial conversion to tricalcium phosphate and calcia by 700° C. In a sample of the invention 96% of the theoretical density was obtained at a low sintering temperature of 1100° C. by pressureless sintering for nanocrystalline hydroxyapatite which was stable up to 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 conventional sample contained large pores and microcracks. Our nanocrystalline hydroxyapatite has high purity and phase homogeneity as well as superior sinterability compared to the conventionally prepared hydroxyapatite. When our nanocrystalline hydroxyapatite was sintered using either colloidal or hot pressing, 99% theoretical bulk density with a grain size of less than 250 nm can be obtained. Dense nanocrystalline hydroxyapatite compacts further possessed a compressive strength as high as 745 MPa, while 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 chemical stability of hydroxyapatite by pinning the mobility of any intergranular and intragranular defects. A dense composite of nanocrystalline hydroxyapatite and 10 wt % nanocrystalline 3 mol % Y2O3-doped ZrO2 possessed an even higher compressive strength of 1020 MPa. With more complete characterization, the densified nanocrystalline hydroxyapatite and hydroxyapatite-zirconia composites can easily be developed into dental and orthopedic weight-bearing implants. Furthermore, the processing of nanocrystalline hydroxyapatite can be adapted to synthesize a nanocrystalline carbonate apatite 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 parameters will depend upon the specific application for which 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.
Ahn, Edward S., Ying, Jackie Y., Nakahira, Atsushi
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