Synthesis of Calcium Hydroxyapatite–Tricalcium Phosphate (HA–TCP) composite bioceramic powders and their sintering behavior

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Synthesis of Calcium Hydroxyapatite–Tricalcium Phosphate (HA–TCP) composite bioceramic powders and their sintering behavior Powered By Docstoc
					Journal                                                                                                                     J. Am. Ceram. Soc., 81 [9] 2245–52 (1998)

                       Synthesis of Calcium Hydroxyapatite–Tricalcium Phosphate (HA–TCP)
                                Composite Bioceramic Powders and Their Sintering Behavior
                                                                                                             Nezahat Kivrak and A. Cuneyt Tas*
                       Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06531, Turkey

Composite (biphasic) mixtures of two of the most important                               stability of the HA / -TCP ceramics in vitro. Jarcho,10 Dries-
inorganic phases of synthetic bone applications—namely,                                  sens,11 LeGeros,12 and de Groot13 have claimed that HA ce-
calcium hydroxyapatite (Ca10(PO4)6(OH)2 (HA)) and tri-                                   ramics can be considered as bioactive and nonbiodegradable
calcium phosphate (Ca3(PO4)2 (TCP))—were prepared as                                     bone-replacement materials. However, the nonbiodegradability
submicrometer-sized, chemically homogeneous, and high-                                   of HA, when used for alveolar ridge augmentation, is a disad-
purity ceramic powders by using a novel, one-step chemical                               vantage to the host tissue that surrounds the HA. As a result,
precipitation technique. Starting materials of calcium ni-                                 -TCP ceramics have been developed as a biodegradable bone
trate tetrahydrate and diammonium hydrogen phosphate                                     replacement.14 A material with ideal biodegradability would be
salts that were dissolved in appropriate amounts in distilled                            replaced by bone as it degraded. However, when used as a
water were used during powder precipitation runs. The                                    biomaterial for alveolar ridge augmentation, the rate of biodeg-
composite bioceramic powders were prepared with compo-                                   radation of -TCP has been shown to be too fast. To slow the
sitions of 20%–90% HA (the balance being the TCP phase)                                  rate of biodegradation, biphasic calcium phosphate (BCP) ce-
with increments of 10%. The pellets prepared from the                                    ramics (i.e., composite ceramics that consist of mixtures of
composite powders were sintered to almost full density in a                              both the HA and -TCP phases) have been studied.15–21 How-
dry air atmosphere at a temperature of ∼1200°C. Phase-                                   ever, there is almost no information, in the relevant literature,
evolution characteristics of the composite powders were                                  on the systematic procedures of HA–TCP composite powder
studied via X-ray diffractometry as a function of tempera-                               manufacture.
ture in the range of 1000°–1300°C. The sintering behavior                                   To our knowledge, the study presented here becomes the
of the composite bioceramics were observed by using scan-                                first systematic step taken in this field of biomaterials technol-
ning electron microscopy. Chemical analysis of the compos-                               ogy, and it specifically describes the chemical powder synthe-
ite samples was performed by using the inductively coupled                               sis procedures of ‘‘HA–TCP composite bioceramics.’’ The
plasma–atomic emission spectroscopy technique.                                           ‘‘bioresorbable’’ nature of TCP in the chemically homoge-
                                                                                         neous HA–TCP composite precursors and powders may serve
                                                                                         as the initial-matrix bioimplant material that will later induce
                            I.      Introduction                                         and undergo an in situ generation of different levels of porosity
                                                                                         and areas of new bone formation upon contact with the bodily
S   YNTHETICALLY produced calcium phosphate ceramics and
    implants have an important position among other biomate-
rials, because they are considered to be almost fully biocom-
                                                                                         fluids, because of the complete resorption of the TCP phase.
                                                                                         The control to be gained over the phase constitution and bio-
patible with living bodies when replacing the hard bone tissues.                         degradability of a calcium phosphate-based bioceramic implant
Calcium hydroxyapatite (Ca10(PO4)6(OH)2 (HA)) and trical-                                establishes the main driving force for this study.
cium phosphate (Ca3(PO4)2 (TCP)) are currently recognized as
ceramic materials that significantly simulate the mineralogical                                           II. Experimental Procedure
structure of bone. HA and TCP bioceramic powders can be                                  (1) Preparation of 0.40M Ca(NO3)2 4H2O Stock Solutions
synthesized by using techniques such as co-precipitation or                                  Calcium nitrate tetrahydrate powder (94.460 g) (99+%,
acid–base titration from aqueous solutions that contain calcium                          Merck, Darmstadt, Germany) was dissolved in 700 mL of dis-
nitrate (Ca(NO3)2) and diammonium hydrogen phosphate                                     tilled water at room temperature, and the solution was then
((NH4)2HPO4). HA powders have been synthesized by using a                                stirred for a few minutes until the nitrate salt completely dis-
wet-chemical method, in aqueous solutions, by several re-                                solved. The working volume was later increased to 1 L via the
searchers. 1–6 Aqueous solutions of Ca(NO 3 ) 2 4H 2 O and                               addition of the required amount of distilled water.
(NH4)2HPO4 were selected as the starting materials in all the
above HA-precipitation studies. Single-phase TCP powders                                 (2) Preparation of 0.156M (NH4)2HPO4 Stock Solutions
have also been synthesized successfully by Tas et al.,6 Akao et
                                               ¸                                             Diammonium hydrogen phosphate (20.614 g) (99+%,
al.,7 and Jarcho et al.8                                                                 Merck) was placed into a 1000 mL beaker at room temperature.
   Biphasic, composite calcium phosphate ceramics, until re-                             The powder was then readily dissolved by stirring in 700 mL
cently, have only accidentally been encountered during the                               of distilled water for a few minutes. The working volume was
synthesis studies of pure HA or pure -TCP (whitlockite)                                  later increased to 1 L via the addition of the required amount of
phases by the previous researchers. Masaki et al.9 studied the                           distilled water.
                                                                                         (3) Chemical Precipitation
                                                                                             The precipitation procedures that are used in the synthesis of
  D. J. Green—contributing editor
                                                                                         HA–TCP composite bioceramic powders are described in de-
                                                                                         tail in the process flowcharts22 given in Figs. 1(A) (composi-
                                                                                         tions of 20%–50% HA) and (B) (compositions of 60%–90%
                                                                                         HA). Diammonium hydrogen phosphate solutions of the proper
  Manuscript No. 190968. Received May 27, 1997, approved November 18, 1997.              concentration were added to the solutions of calcium nitrate at
  Supported in part by the Turkish State Planning Agency (Project No. ODTU-AFP-          the rate of 2 mL/min under continuous stirring in all the pre-
DPT-95K120491) and the Turkish Scientific and Technical Research Foundation
(Project No. Misag-58).                                                                  cipitation runs. Table I shows the values used in the assessment
  *Member, American Ceramic Society.                                                     of the preceding flowcharts.
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                                            Journal of the American Ceramic Society—Kivrak and Tas                        Vol. 81, No. 9

Fig. 1. Chemical precipitation flowcharts used in the synthesis of HA–TCP composite bioceramic powders (A) for compositions of 20%–50% HA
and (B) for compositions of 60%–90% HA.

                     Table I.        Chemical Precipitation Parameters Used in HA–TCP Composite Power Synthesis
                                                  Volume (mL)                               Temperature (°C)
       Composition              V1          V2        V3         V4       V5        T1       T2         T3      T4        pH1        pH2
20%HA      +   80%TCP       185            170       170         9         2       40       40          55      65        8          4
30%HA      +   70%TCP       198            170       170         7         2       40       40          55      65        7.5        6
40%HA      +   60%TCP       198            160       160         7.5       6       40       45          55      65        7.5        6.5
50%HA      +   50%TCP       120             85       170        99         2       55       55          55      65        9.5        9.5
60%HA      +   40%TCP        54             77        70         1        11       40       40          65                8         10.5
70%HA      +   30%TCP        54            115        70         1        11       40       40          65                8.5       10.7
80%HA      +   20%TCP        54            115        70         4        11       40       40          65                9.3       10.7
90%HA      +   10%TCP        54             77        70         7        11       40       40          65                9.7       10.7

(4) Characterization of Composite Bioceramic Powders                     ducibility in powder precipitation conditions and phase com-
   The phase purity and constitution of the synthesized com-             positions), on the collected X-ray data were achieved by using
posite bioceramic powders were checked by powder X-ray dif-              the automated, built-in computer software of the X-ray diffrac-
fractometry (XRD) (Model D-Max/B, Rigaku Co., Tokyo, Ja-                 tometer used. The indexing and lattice parameter runs on the
pan). The samples of synthesized bioceramics were initially              samples were performed by the personal-computer-based least-
ground to a fine powder in an agate mortar under isopropyl               squares cell refinement algorithm of Appleman and Evans.23
alcohol prior to laying them flat onto a glass sample holder.               The samples were investigated for their microstructural and
Monochromated FeK radiation was used at operating values                 morphological features by using a scanning electron micros-
of 40 kV and 20 mA. The XRD data were collected at a                     copy (SEM) microscope (Model JSM-6400, JEOL, Tokyo, Ja-
temperature of 22° ± 2°C over the 2 range of 10°–90° at a step           pan) that was operated at a typical accelerating voltage of 20
size of 0.02° and a count time of 1 s. The determination of the          kV and equipped with an energy-dispersive X-ray spectroscopy
2 values and peak heights, as well as the semiquantitative               (EDXS) analyzer (Kevex, Valencia, CA) for semiquantitative
phase analysis (duplicated for three samples to ensure repro-            phase analysis. Prior to SEM examination, all the samples were
September 1998                           Synthesis of HA–TCP Composite Bioceramic Powders                                           2247

Fig. 2. XRD spectra of 20%HA–80%TCP composite powders (sintered for 5 h in air). Peaks labeled ‘‘1,’’ ‘‘2,’’ and ‘‘3’’ correspond to -TCP
(ICDD File Card No. 9-169), HA (ICDD File Card No. 9-432), and -TCP (ICDD File Card No. 9-348), respectively.

sputter-coated by a gold–palladium alloy layer ∼250 Å thick, to        green pellets were heated in a stagnant-air atmosphere in the
minimize any possible surface charging effects. Microstruc-            temperature range of 200°–1300°C, with soaking times of 5 h
tural and morphological characteristics of the sintered samples        at the peak temperatures, to investigate the densification char-
were studied on either the fracture or as-sintered surfaces of         acteristics of the bioceramic samples. The heated samples were
pellets. The rather-flat surfaces of the as-sintered pellets were      furnace cooled to the ambient temperature.
used in EDXS analysis without further polishing.
   Chemical analyses to determine the Ca:P atomic ratios in                             III. Results and Discussion
synthesized ceramics, on selected samples, were performed.                A chemical precipitation procedure for the synthesis of
Inductively coupled plasma–atomic emission spectrometry                phase-pure HA and TCP powders that was similar in general
(ICP–AES) (Model Plasma-1000, Perkin Elmer Corp., London,              strategy to those procedures described in this study has previ-
U.K.) was used.                                                        ously been demonstrated by us elsewhere.6 The manufacture of
   To study the sintering behavior, the synthesized powders of         HA–TCP composite (biphasic) powders via our procedure is
HA–TCP composites were pressed uniaxially into pellets 3 mm            dependent on the simultaneous, in situ precipitation of both
thick and 10 mm in diameter under typical stresses of 145–170          phases as a function of the overall solution pH and cation
MPa in a hardened steel die. The die walls were lubricated by          concentrations achieved. The boiling step added to the process
using a liquid solution of 4 wt% stearic acid in ethanol. The          flowchart for higher HA compositions (Fig. 1(B)) ensures
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                                      Journal of the American Ceramic Society—Kivrak and Tas                              Vol. 81, No. 9

Fig. 3. XRD spectra of 40%HA–60%TCP composite powders (sintered for 5 h in air). Peaks labeled ‘‘1,’’ ‘‘2,’’ and ‘‘3’’ correspond to -TCP
(ICDD File Card No. 9-169), HA (ICDD File Card No. 9-432), and -TCP (ICDD File Card No. 9-348), respectively.

conversion of the TCP phase to HA, as discussed in the pre-            present in powder bodies were almost impossible to differen-
vious literature.6,24,25                                               tiate from each other just by investigating their morphology via
   The phase purity and constitution of all the HA–TCP                 the SEM micrographs.
samples synthesized in this study were analyzed and monitored             We have still conformed, during this study, to the erroneous
via powder XRD runs. The selected powder XRD spectra                   notation that was heavily used in the previous literature for
of composite (biphasic) powder samples of 20%, 40%, and                the designation of the low and high polymorphs of the TCP
60% HA (the balance being the TCP phase) are shown in                  phase, as we labeled the phases in Figs. 2–4. The previous
Figs. 2, 3, and 4, respectively. The HA–TCP powders were               researchers designated the low-temperature hexagonal form
heated over a temperature range of 1000°–1300°C for 5 h in a           of TCP as -TCP, and the high-temperature orthohombic
dry-air atmosphere so that the phase-evolution schemes in              form of TCP as -TCP. We hereby would like to propose a
these precursors could be followed. The precipitates recovered         change in this notation in the following way: the low-
from the mother liquors were not amorphous, after drying at            temperature polymorph of TCP to be designated by -TCP,
75°C for 24 h. The composite powders also possessed the same           and the high-temperature polymorph of TCP to be designated
HA:TCP ratio as those of 1000°C-heated samples when they               by -TCP. The transformation from the low-temperature
were heated in the temperature range of 700°–900°C. The dried          form to the high-temperature form occurs readily in a dry-
(75°C, 24 h) composite powders consisted of submicrometer-             air atmosphere at ∼1180°C. This fact was strictly confirmed
sized (with a typical particle size of 0.6–0.7 m), roughly             in the XRD data given in Figs. 2–4, and the TCP phase present
spherical particles. The two phases—i.e., HA and TCP—                  in our biphasic, composite samples has accomplished this
September 1998                                     Synthesis of HA–TCP Composite Bioceramic Powders                                         2249

Fig. 4. XRD spectra of 60%HA–40%TCP composite powders (sintered for 5 h in air). Peaks labeled ‘‘1,’’ ‘‘2,’’ and ‘‘3’’ correspond to -TCP
(ICDD File Card No. 9-169), HA (ICDD File Card No. 9-432), and -TCP (ICDD File Card No. 9-348), respectively.

phase transformation when they were heated to and above                          posites as a function of temperature are tabulated in Table II.
1200°C.                                                                          The values reported in this table were the averages of three
   In general terms, the synthesized HA–TCP composites dis-                      duplicate samples that received the same treatments. The fol-
played three different compositional behaviors during the sin-                   lowing observations could be drawn from the data presented in
tering process. The amounts of each phase for different com-                     Table II:

                      Table II.      Variation in Phase Assemblage of HA–TCP Composites as a Function of Temperature†
                                                                                  Composition after heating
        composition                          1000°C                     1100°C                                1200°C               1300°C
20%HA        +   80%TCP                18%HA   +   82%TCP         24%HA   +   76%TCP                32%HA      +   68%TCP   50%HA    +   50%TCP
30%HA        +   70%TCP                36%HA   +   64%TCP         52%HA   +   48%TCP                70%HA      +   30%TCP   53%HA    +   47%TCP
40%HA        +   60%TCP                44%HA   +   56%TCP         45%HA   +   55%TCP                20%HA      +   80%TCP   33%HA    +   67%TCP
50%HA        +   50%TCP                48%HA   +   52%TCP         26%HA   +   74%TCP                10%HA      +   90%TCP   23%HA    +   77%TCP
60%HA        +   40%TCP                61%HA   +   39%TCP         66%HA   +   34%TCP                65%HA      +   35%TCP   64%HA    +   36%TCP
70%HA        +   30%TCP                71%HA   +   29%TCP         63%HA   +   37%TCP                72%HA      +   28%TCP   71%HA    +   29%TCP
80%HA        +   20%TCP                82%HA   +   18%TCP         85%HA   +   15%TCP                83%HA      +   17%TCP   78%HA    +   22%TCP
90%HA        +   10%TCP                89%HA   +   11%TCP         87%HA   +   13%TCP                90%HA      +   10%TCP   88%HA    +   12%TCP
     After heating at temperature for 5 h.
2250                                                                                                  ¸
                                                 Journal of the American Ceramic Society—Kivrak and Tas                                        Vol. 81, No. 9

                              Table III.      Variation in Lattice Parameters of HA and -TCP Phases in Composites
                                                               Calcined at 1050°C for 5 h
                                                            Lattice parameters of HA phase (Å)          Lattice parameters of -TCP phase (Å)
                                 Composite                       a                     c                     a                       c
                          20%HA     +   80%TCP               9.4302                 6.8882               10.4525                 37.7127
                          30%HA     +   70%TCP               9.4287                 6.8625               10.4476                 37.9837
                          40%HA     +   60%TCP               9.4494                 6.8784               10.4655                 37.4916
                          50%HA     +   50%TCP               9.4115                 6.8829               10.4500                 37.7500
                          60%HA     +   40%TCP               9.4337                 6.8926               10.4541                 37.7364
                          70%HA     +   30%TCP               9.4425                 6.8944               10.4623                 37.5407
                          80%HA     +   20%TCP               9.4344                 6.8926               10.4359                 37.7000
                          90%HA     +   10%TCP               9.4208                 6.8893               10.4467                 37.4821

   (1) For 20% HA composite powders, the amount of HA                                         Pellets prepared from composite bioceramic powders that
increased monotonically as the temperature increased from                                  had different HA:TCP ratios showed different densification
1000°C to 1300°C.                                                                          and phase-evolution behavior over the studied phase-mixture
   (2) For 30% HA composite powders, the amount of HA                                      range. The porosity of composite samples were directly read
steadily increased through 1000°–1200°C, reaching a maxi-                                  from the surfaces of the heated pellets via the SEM micro-
mum at 1200°C, and then decreased when heated at 1300°C.                                   graphs by using a lineal analysis procedure. It should be re-
   (3) For 40% HA and 50% HA composite powders, the                                        membered that these micrographs only showed the surface mi-
amount of HA initially decreased as the temperature increased                              crostructures, which may or may not be the same as the bulk.
from 1000°C to 1200°C, passing through a minimum at                                        Although we have not examined the microstructures of any
1200°C, and then slightly increased when temperature reached                               polished sections of sintered composite pellets in this study, the
1300°C.                                                                                    densities measured by the Archimedes technique (in xylene)
   (4) For 60% HA composite powders, the amount of HA                                      did not essentially differ from those determined by assessing
almost remained stable at all sintering temperatures.                                      the surface porosity. The apparent porosity on the surfaces of
   This complex pattern of behavior therefore demonstrates that                            all the composite pellets was observed to be in the range of
control of the phase content of the chemically precipitated                                17%–23% for the 1000°C-heated pellets. The biphasic pellets
powders (after heating at 1000°C) does not give control over                               heated at 1300°C for 5 h had surface-porosity values in the
the phase content of the densified bioceramics.                                            range of only 1%–2%. The SEM photomicrographs given in
   The composite bioceramics that contain 60% (by weight)                                  Figs. 5 and 6 display the densification behavior of the 20% and
of HA were not significantly affected by increasing the sinter-                            70% HA pellets, respectively.
ing temperature from 1000°C to 1300°C. In other words, the                                    The phase constitution and the chemical homogeneity of the
initially prescribed HA:TCP phase ratios were precisely pre-                               composite samples of this study were further examined by
served within these samples during the entire sintering process.                           quantitative chemical analysis via inductively coupled plasma
One may hypothesize from this observation that, as the amount                              (ICP) spectroscopy on some selected samples. The Ca:P atomic
of TCP phase in the composites increases beyond a certain                                  ratios determined by ICP analysis are given in Table IV. The
critical level ( 50%), the presence of the TCP phase in com-                               Ca:P atomic ratios presented in this table also confirmed the
posite powders, at temperatures in excess of 1200°C, acts as a                             accuracy of the results obtained during this study from the
driving force for the further decomposition of the HA phase                                semiquantitative technique of determining the phase constitu-
through dehydroxylation. This decomposition is believed to                                 tions of the composite samples through the comparison of the
occur according to the reaction                                                            XRD peak intensities of the terminal phases.
                                                                                              It should be remembered that the HA phase represents the
       Ca10(PO4)6(OH)2 → 3Ca3(PO4)2 + CaO + H2O ↑                            (1)           ‘‘bioinert’’ component and the TCP phase is the ‘‘bioresorb-
This decomposition reaction is only accompanied by a 1.8%                                  able’’ agent in these composite bioceramics. Implants manu-
weight loss; thus, it is difficult to detect via thermogravimetric                         factured from such composite bioceramics should provide a
analysis. Moreover, the most-intense XRD (FeK ) peaks (i.e.,                               uniform and homogeneous way of obtaining desired amounts
the peaks at 40.793° and 47.451°) of the CaO product phase                                 of porosity under in vivo conditions following the implantation.
(ICDD† Powder Diffraction File No. 37-1497) coincided                                      It is hereby expected that the bioresorbable TCP phase would
with those of HA (ICDD Powder Diffraction File No. 9-432)                                  undergo extensive resorption within the body in a certain pe-
and -TCP (ICDD Powder Diffraction File No. 9-169),                                         riod of time and thus provide uniform porosity within the ‘‘ag-
respectively.                                                                              ing implant.’’ From this perspective, the results of this study,
   The decomposition of HA to TCP occurred in accordance                                   especially those for the composites that contain 60% HA, are
with the polymorphic transformation observed in the TCP                                    believed to make it possible to manufacture calcium phosphate
phase. In other words, when the temperature was <1200°C, the                               bioceramic implants with controlled amounts of a resorbable
HA present in the mixture decomposed to -TCP, and when                                     phase and porosity (in the range of 10%–40%) in the form of
the temperature was >1200°C, it decomposed to the -TCP                                     ‘‘turning-porous, aging implants.’’
   The lattice parameters of the HA phase in the composite                                                         IV.    Conclusions
powder samples heated at 1050°C in dry air for 5 h remained
almost constant. The lattice parameters of the -TCP phase in                                  Submicrometer-sized calcium hydroxyapatite–tricalcium
the composite powder samples heated at 1050°C in dry air for                               phosphate (HA–TCP) composite bioceramic powders have
5 h also displayed only a small variation. The experimental                                been synthesized, for the first time, from aqueous solutions by
values of both parameters a and c of the phases present in the                             using a novel chemical precipitation process. The phase evo-
composite powders are given in Table III.                                                  lution and sintering behavior of the composite powders were
                                                                                           investigated by using X-ray diffractometry and scanning elec-
                                                                                           tron microscopy, as a function of temperature in the range of
     International Centre for Diffraction Data, Newtown Square, PA.                           The HA–TCP composite powders have been successfully
September 1998                                   Synthesis of HA–TCP Composite Bioceramic Powders                                                     2251

         Fig. 5.      SEM photomicrographs of 20%HA–80%TCP pellets ((A) sintered at 1100°C for 5 h and (B) sintered at 1300°C for 5 h).

         Fig. 6.      SEM photomicrographs of 70%HA–30%TCP pellets ((A) sintered at 1100°C for 5 h and (B) sintered at 1300°C for 5 h).

Table IV. Chemical Analysis (ICP) of Pure HA, Pure TCP,                       References
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70%HA + 30%TCP                  39.56          18.94           1.615             5
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temperature range of 1000°–1300°C.                                               8
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                                                                                   F. C. M. Driessens, ‘‘Formation and Stability of Calcium Phosphates in
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step chemical precipitation technique.                                        materials, 1, 47–50 (1980).
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                                              Journal of the American Ceramic Society—Kivrak and Tas                                          Vol. 81, No. 9
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