TTCP Paper

J. Am. Ceram. Soc., 88 [12] 3353–3360 (2005)

DOI: 10.1111/j.1551-2916.2005.00623.x

r 2005 The American Ceramic Society

Synthesis of HA-Seeded TTCP (Ca4(PO4)2O) Powders at 12301C from

Ca(CH3COO)2 . H2O and NH4H2PO4

Sahil Jalota, A. Cuneyt Tas,w and Sarit B. Bhaduri

School of Materials Science and Engineering, Clemson University, Clemson, South Carolina 29634

display extensive twinning and suspected to have a high-tem-perature orthorhombic polymorph.5 The positions of the P at-oms and the Ca and oxide ions lie close to those required by anorthorhombic space group, Pmcn. This probably explains theappreciable twinning exhibited by Ca4(PO4)2O.4

How can TTCP transform into apatite? Ca4(PO4)2Ocontainsa crystallographic layer similar to an ‘‘apatitic layer’’ present inboth Ca10(PO4)6(OH)2 and Ca8H2(PO4)6 5H2O (octacalciumphosphate, OCP). As a consequence of this layer, an epitaxialrelationship between TTCP and HA, as well as between TTCPand OCP, may be present.4 Epitaxy is often thought of as beinggoverned chiefly by the metric fits of crystallographic networksbased on the unit cell translations.4 The combination of goodmetric and chemical fits between HA and OCP makes the epi-taxy between these two compounds possible.6 Undetected epi-taxy between HA and TTCP would increase the Ca/P molarratio of an ‘‘apparent’’ apatite above 1.667, and on the micro-scopic scale the material would resemble a solid solution. HA

is,therefore, an unusual substance in that it has two closely relatedsalts with which it may enter into epitaxial relationships.One salt, Ca8H2(PO4)6 5H2O, is more acidic, and the

other,Ca4(PO4)2O, is more basic than the bone mineral, HA.4

TTCP is the most soluble compound among all the calciumphosphates.1 Production of Ca4(PO4)2O in an aqueous environ-ment is extremely difficult, if not impossible, because of theoxygen ion in its formula. In aqueous solutions hydroxyl (OH)ions incorporate themselves into the formed precipitates. A car-bonate- and/or hydroxyl ion-containing apatitic product willform readily if one attempts the synthesis of calcium

phosphatesin a basic aqueous solution with a Ca/P molar ratio of 2. Forthis reason, synthesis of TTCP, which is nowadays used as oneof the main components of self-setting orthopedic and dentalcements, has only been limited to solid-state reactive firing

prac-tices.7–9 TTCP was typically synthesized by the solid-state reac-tion of calcium carbonate (CaCO3) and dicalcium phosphateanhydrous (CaHPO4) powders, mixed with one another at a

Ca/P molar ratio of 2. Powders prepared by Brown and Chow,7

Chow and Takagi,8 and Matsuya et al.9 were usually heated at14501–15001C for 6–12 h, and then rapidly quenched to roomtemperature. Slow cooling of TTCP, instead of quenching,

fromhigh temperatures resulted in the formation of undesired sec-ondary phases, such as HA, CaO, CaCO3, and b-Ca3(PO4)2.Greish et al.10 very recently synthesized TTCP powders by mix-ing CaCO3 and Ca(H2PO4)2 H2O powders in n-heptane, fol-lowed by heating and quenching from 13101C. As of now, it

hasbeen impossible to produce TTCP powders without quenchingfrom a temperature in excess of 13001C. Rapid cooling is es-sential in order to prevent the total decomposition of TTCP

intoHA and CaO/Ca(OH)2.11 For use as a constituent of a cement,

sintered bodies of TTCP need grinding to a particle size less than

45 mm.9 The use of higher synthesis temperatures, which furtherpromotes sintering and grain growth, places a significant eco-nomic burden on the manufacturer in the form of costly grind-ing operations. Such grinding practices may also be regarded assources of further contamination.

Sargin et al.12 studied, for the first time, the influence of dif-ferent starting materials on the synthesis of TTCP powders. In

3354 Journal of the American Ceramic Society—Jalota et al. Vol. 88, No. 12

this important study, Sargin et al.12 showed that by usingCaCO3 and NH4H2PO4 as reactants, it was possible to decreasethe temperature of synthesis of single-phase TTCP down toabout 13501C. An interesting feature reported in Sargin et al.12

was that, for the first time, separate sources of Ca and P wereused. In Brown and Chow,7 Chow and Takagi,8 Matsuya et al.,9

Greish et al.,10 and Bian et al.13 calcium needed to form TTCPcame from both CaCO3 and CaHPO4 or from CaCO3 andCa(H2PO4)2 H2O. During heating, CaCO3 decomposes byevolving CO2 at around 9001C, and CaHPO4 first transformsinto Ca2P2O7.14 Therefore, in Chow’s solid-state approach, hightemperatures (49001C) were deemed necessary to even start thereactions between the starting materials with one another.7–9,13

The discovery that the basic TTCP reacts with acidic dical-cium phosphate anhydrous, CaHPO4, to form pure HA led tothe development of a self-setting calcium phosphate cement(CPC) by Brown and Chow.7,8,15 TTCP-containing self-settingCPCs are now commercially available (Norian SRS , SkeletalRepair System, Synthes Corp., Oberdorf, Switzerland; Biopex ,Mitsubishi Materials Corp., Tokyo, Japan)16 and have a proventrack record in clinical/surgical use.

In the present study we investigated a new chemical synthesisroute to prepare HA-seeded TTCP powders. One major aim wasto decrease the synthesis temperature even below that reportedby Sargin et al.12, i.e., 13501C. Formation of TTCP, from thestarting chemicals, was monitored by using X-ray diffraction,thermogravimetric/differential thermal analysis (TG/DTA),scanning electron microscopy, and Fourier-transform infraredspectroscopy. The powder synthesis route we present here sim-ply consisted of physical mixing of the stoichiometric amountsof calcium acetate monohydrate (Ca(CH3COO)2 H2O) and am-monium dihydrogen phosphate (NH4H2PO4) powders, followedby heating at 12301C in air in a box furnace for 12 h andquenching. This process enabled us to obtain powders at thelowest temperature ever reported for the formation of TTCPphase. Any significant decrease in the synthesis temperature ofsuch self-setting orthopedic cement constituents is extremely im-portant, because well-sintered TTCP samples produced by soak-ing between 14001 and 15001C need to be ground into a powderprior to their use as a biomaterial. Grinding of TTCP powderswould damage their stoichiometry and consequently alter theirin vivo performance.17 Biomimetic (i.e., with the help of anaqueous solution mimicking the ion concentrations of humanblood plasma at 371C and pH 7.4) formation of carbonatedapatitic calcium phosphates on these powders was tested andconfirmed by immersing these into a synthetic body fluid (SBF)from 36 to 96 h.18

II. Experimental Procedure

(1) Powder SynthesisHA-seeded TTCP powders were synthesized in small batches.First, 0.77 g of NH4H2PO4 (499%, Fisher Scientific, Fairlawn,NJ) was ground into a very fine powder by using an agate mor-tar and pestle. 2.45 g of Ca(CH3COO)2 H2O (499%, Fisher)was then added to it. Two powders were dry mixed in the mortarfor about 45 min without applying too much force. The powdermixture was then heated at 3001C in air for 30 min, followed byre-mixing in a mortar for 15 min. Powders were then heated at8001C in air for 1 h, followed by a homogenization mixing in anagate mortar. These powders had a gray/bluish tint, and theywere heated (with a heating rate of 21C/min) at 12301C in air for6 h. At the end of 6 h of heating at 12301C in a box furnace, thered-hot alumina boat containing the powders was taken out andimmediately placed into a desiccator. Obtained powders wereonly lightly ground and homogenized in an agate mortar for fewminutes and re-fired at 12301C for another 6 h, followed bysimilar quenching to room temperature. Powders obtained wereeasy to grind in the agate mortar. This firing scheme was onlysuitable for the synthesis of HA-seeded TTCP powders. Wewere careful not to over grind the resultant TTCP powders, be-

December 2005 Synthesis of HA-Seeded TTCP (Ca4(PO4)2O) Powders 3355

Fig. 1. (a) X-ray diffraction (XRD) spectrum of pure Ca(CH3COO)2 H2O powders. (b) Thermogravimetric and differential thermal analyses (TG/DTA) traces for pure Ca(CH3COO)2 H2O powders (in air). (c) XRD spectrum of pure NH4H2PO4 powders. (d) TG/DTA traces for pure NH4H2PO4

powders (in air).

traces of water or other adsorbed gases that may be present onthe powder surfaces.

III. Results and Discussion

XRD and TG/DTA data for the pure starting chemicals(Ca-acetate monohydrate and ammonium dihydrogen phos-phate) of powder synthesis are given in Fig. 1. The FTIR trac-es of the starting chemicals, i.e., pure NH4H2PO4 and Ca(CH3

COO)2 H2O, are given in Fig. 2. The FTIR trace for the un-heated powder mixture, which has the stoichiometry of TTCP, isalso given in Fig. 2 for comparison purposes. NH4H2PO4 meltsat around 1901C and forms an acidic phosphate liquid.19

NH4H2PO4 was chosen here over (NH4)2HPO4 mainly becauseof (1) its lower ammonium content and (2) its definite meltingpoint. This phosphate liquid immediately reacts with Ca(CH3

COO)2 H2O, which prior to reaching the melting point ofNH4H2PO4 (1901C) would have already lost its water of hydra-tion and transformed into calcium acetate anhydrous. The fol-lowing reactions took place until the temperature reached 2001C:

CaðCH3COOÞ2 H2OðsÞ ! CaðCH3COOÞ2ðsÞ

þ H2OðgÞ

NH4H2PO4ðsÞ ! NH4H2PO4ðlÞ

Equation (1) envisages a weight loss of 10.22%. Calcium ac-etate anhydrous started its decomposition at around 4001C re-

leasing volatiles, acetone in an air atmosphere, leaving behind


for ammonium dihydrogen phosphate, on the other hand, canbe given by the following equation:

2 NH4H2PO4ðsÞ ! 2 NH3ðgÞ þ 3 H2 OðgÞ þ P2O5ðsÞ (4)

Equation (4) indicated a total weight loss of 38.3%, and itexplained well the weight loss observed in the pure NH4H2PO4

samples (Fig. 1(d)) until the 6801–6901C TGA shoulder. With afurther increase in temperature, the P2O5 already formed startsevaporating, bringing the total weight loss in pure NH4H2PO4

samples to around 96%.The main purpose in heating the powder mixtures first to

3001C followed by grinding was the homogenization of the re-action products of molten ammonium phosphate and the solidphase of partially decomposed calcium acetate. Upon

continuedheating to 8001C, volatilization of NH3 from NH4H2PO4, fol-lowed by its condensation to highly reactive P2O5 was complet-ed, whereas acetone, carbon dioxide, and water vapor were themain decomposition products of Ca-acetate. At this point, thesamples were ground into a fine powder, followed by heating tothe final temperature of 12301C.

Figure 3(a) showed a typical TG/DTA trace for the Ca(CH3

COO)2 H2O and NH4H2PO4 mixtures (i.e., TTCP mix) up to

12001C. To summarize the thermal events seen in this TG/DTAchart, first, NH4H2PO4 melted at around 1901C (endothermicpeak) engulfing the calcium acetate particles; second, the

abruptdecomposition of Ca-acetate accompanied by the drastic crys-tallization of CaCO3 (indicated by the sharp exothermic peak inFig. 3(a)) took place at about 4001C; and third the decarboxyla-tion of CaCO3 was observed in the vicinity of 7501C. Crystal-lization of CaCO3 from the decomposing calcium acetate par-ticles, which are embedded in a molten phosphate bath,occurred through an exothermic reaction. X-ray diffraction

trac-es given in Fig. 3(b) depict the stepwise phase evolution in

thesemixtures, with the help of isothermal heatings (2 h at the peaktemperatures, with 51C/min heating and cooling rates) at indi-vidual temperatures indicated on this chart. Table II lists thephases assigned to the individual peaks shown in Fig. 3(b).TTCP was not detected even in samples quenched from

10001C.FTIR data of the TTCP powder mixtures were also useful in

determining the nature of phase evolution sequence in thesesamples. Figure 3(c) shows the IR spectra of samples isother-mally heated (2 h at peak temperatures, 51C/min heating andcooling rates) at the temperatures indicated (RT to 10001C)in the chart. Experimentally determined IR bands are given inTable III together with the corresponding phases.

An amorphous intermediate (consisting of long chains ofcondensed phosphate of the form (PO3)n) is expected to form asa result of the reactions between an acidic phosphate liquid andcalcium acetate, which subsequently converts itself into

partiallycrystalline b-Ca(PO3)2 (calcium metaphosphate) with an in-crease in temperature to around 2501–4001C.21 XRD andFTIR results both confirmed this, and after an isothermal heat-ing at 3501C, calcium metaphosphate, calcium hydrogen phos-

Table II. Phases Identified (by XRD) in the IsothermalHeatings of Ca-Acetate Monohydrate1NH4H2PO4 Mixed

Powders

Phase name Phase formula ICDD-PDFw

Dicalcium hydrogen CaHPO4 9-0080phosphateCalcium pyrophosphate b-Ca2P2O7 9-0346Calcium carbonate (calcite) CaCO3 5-0586

Calcium metaphosphate Ca(PO3)2 50-0584Tricalcium phosphate b-Ca3(PO4)2 9-0169Calcium hydroxyapatite Ca10(PO4)6(OH)2 9-0432Calcium hydroxide Ca(OH)2 44-1481Calcium oxide CaO 37-1497

ICDD–PDF, International Centre for Diffraction Data—Powder DiffractionFile.


cannot be detected by XRD. Below 3501C, calcium acetate (i.e.,COO bands) and ammonium phosphate (i.e., orthophosphateand acid phosphate groups) IR bands and XRD peaks werevisible. Therefore, a step of 3001C calcination (followed bygrinding) was just necessary to homogenize the powder

samplesprior to the crystallization of any calcium phosphates. Calciummetaphosphate disappeared upon heating the powder mixturesto 4301C, and the samples consisted of a biphasic mixture ofCaCO3 and Ca2P2O7 at this temperature. However, upon heat-ing to 7001C, the presence of CaCO3 significantly diminishedwith the simultaneous appearance of b-tricalcium phosphate,Ca3(PO4)2 (TCP), and HA. Because of the overall stoichiometryof the initial powder mixtures (Ca/P molar ratio 5 2), detectionof HA or TCP in the powders should automatically call for thepresence of Ca(OH)2 or CaO in those. Major peaks of CaO andCa(OH)2 are at 37.361 and 34.171 2y, respectively. Most HApeaks overlap with those of TTCP; however, the significant

peak(i.e., 002 reflection) of HA was encountered at 25.881 2y. Thepresence of Ca(OH)2 was unmistakably detected by the IR

spec-tra, with its characteristic hydroxyl stretching band observed at3640 cm 1 in the samples heated at 7001C and higher. Samplesheated at 10001C contained the phases of carbonated HA,

TCP,Ca(OH)2, and Ca2P2O7. Complete elimination of TCP andCa2P2O7 and the formation of TTCP were observed at412001C.

The characteristic XRD diagram of TTCP powders obtainedafter the second step of 12301C firing/quenching is given inFig. 4(a). These powders contained approximately 5–6 wt%(median: 5.43, estimated standard deviation: 0.26) of HA

phase,together with Ca(OH)2. The amounts of HA in these powders

Fig. 5. Morphology of hydroxyapatite-seeded TTCP powders synthe-sized at 12301C.


were determined, on three batches of preparation, by quantita- Figs. 5(a) and (b) had a BET surface area of 3274 m2/g. Con-tively comparing the peak intensities of TTCP and HA phases. ventional TTCP powders prepared by 15001C heating (for 6 h)The FTIR pattern of the HA-seeded TTCP powders is given in of CaCO3 and CaHPO4 are first ground in ball mills to reduceFig. 4(b). This experimental pattern precisely matched those their particle sizes to around 13–20 mm (coarse TTCP), and thenpreviously reported for tetracalcium phosphate by Sargin et al.12 further ground for 24 h to reduce their particle sizes to less thanand Posset et al.,22 respectively. Vibrational wavenumbers were 5 mm (fine TTCP).27

assigned by the above-mentioned researchers. In the data of SBF solutions are able28 to induce apatitic calcium phosphateFig. 4(b), threefold degenerate stretching (1105–989 cm 1) mode formation on metals, ceramics, or polymers (with proper surface(n3), symmetric stretching (962–941 cm 1) mode (n1), and three- treatments) soaked in them. It is also known that an amorphousfold degenerate deformational (620–571 cm 1) mode (n4) vibra- calcium phosphate (ACP) precursor is always present during thetions were all visible.12,22 The inset in Fig. 4(b) displays precipitation of apatitic calcium phosphates from highly super-the characteristic OH stretching vibration (3640 cm 1), origi- saturated solutions.29 Posner and Betts30 proposed that thenating from Ca(OH)2.12 On the other hand, the weak band at process of ACP formation in solution involved the formation872 cm 1 belongs to the carbonate groups, and these are con- of Ca9(PO4)6 clusters, which then aggregated randomly to pro-sidered to be associated with the carbonated HA seeds present in duce the larger spherical particles or globules, with the inter-the samples. cluster space filled with water. Such clusters, we believe, are the

The addition of HA seeds to TTCP powders has previously transient solution precursors to the formation of carbonatedbeen tested by Hamanishi et al.;23 however, they had intention- globules with the nominal stoichiometry of Ca10 x(HPO4)x

ally added about 40 wt% precipitated HA into a cement mixture (PO4)6 x(OH)2 x, where x might be converging to 1.31 Onumaof TTCP–CaHPO4 2H2O. The level of HA seeding in the and Ito32 have demonstrated, by using dynamic light scattering,present study is obviously much lower than this. the presence of calcium phosphate clusters from 0.7 to 1.0 nm in

Particle morphology of the TTCP powders in this study is size even in clear simulated body fluids. They reported the pres-given in Figs. 5(a) and (b). Powders displayed a unique vermi- ence of calcium phosphate clusters in SBF even when there wascular microstructure (less than 5 mm in size), which was indic- no visible precipitation. Therefore, one only needs some suitableative of the vapor- and liquid-phase reactions that took place. surface to be immersed into an SBF solution, which wouldThe remnants of the amorphous-looking chunks of Ca-doped probably trigger the hexagonal packing32 of those nanoclusterscondensed phosphates (i.e., P2O7, (PO3)n, and P2O5 in compar- into globules or spherulites of apatitic calcium phosphates. SEMison with PO4)24–26 were observed in the form of larger chunks photomicrographs given in Figs. 6(a)–(d) exhibited the rapidof about 40 mm. The as-quenched powder samples depicted in formation of those apatitic calcium phosphate globules after

Fig. 6. (a) Thirty-six hours in synthetic body fluid (SBF) at 371C (twinning; towards lower left-hand corner). (b) Seventy-two hours in SBF at 371C.(c, d) Ninety-six hours in SBF at 371C.


Fig. 9. Change in surface pH of HA-seeded TTCP powders in SBF.

This was ascribed to the transformation of HA-seeded TTCPpowders into carbonated apatitic calcium phosphates formedunder biomimetic conditions provided in vitro by the SBF solu-tions. On the other hand, for comparison purposes, pure,stoichiometric, crystalline HA has a pH value of about 9.5. Af-ter 96 h of soaking in SBF, recorded pH value was 9.5670.04

atRT. It is reasonable to assume that this pH value would havecontinued to drop toward the physiological pH if we had con-tinued the soaking beyond 4 days.34,35 The pH diagram ofTTCP powders given in Fig. 9 may very well serve the practi-tioner to precisely tailor the ‘‘amount of an acidic calcium

phos-phate (such as DCPA, CaHPO4; DCPD, CaHPO4

2H2O;MCPM, Ca(H2PO4)2 H2O; and MCPA, Ca(H2PO4)2) thatneeds to be blended’’ with these powders in designing aTTCP-based cement formulation having a neutral pH at thevery start of the cement-forming process.

Being the calcium phosphate compound of highest solubilityat the physiological pH, the hydrolysis of TTCP should be re-garded as a process of initial dissolution followed by the pre-cipitation of a less soluble phase. In an SBF environment (i.e.,371C and pH 7.4), the precipitating phase can only be nanosize,carbonated HA, i.e., the bone mineral. The presence of HAseeds in TTCP simply accelerates this in vitro biomineralizationprocess. Nevertheless, it is yet to be seen if these powders can

bemade into useful self-setting CaP cements.

IV. Conclusions

(1) Five to six weight percent HA-seeded TTCP (Ca4

(PO4)2O) powders were synthesized at 12301C, by using thestarting chemicals of calcium acetate monohydrate and ammo-nium dihydrogen phosphate, for the first time, followed byquenching from the synthesis temperature.

(2) This is the lowest temperature ever reported for the syn-

thesis of TTCP powders, and such low synthesis temperaturescan readily be achieved in inexpensive electrical resistance-heat-ed furnaces.

(3) HA-seeded TTCP powders underwent apatitic transfor-mation when they were brought into contact with SBF

solutionsof pH 7.4 at 371C, within the first 36–96 h. The HA seeds canperhaps be seen as having a catalytic function for the TTCPtransformation.

Acknowledgments

This work was a part of the MSc thesis of Mr. Sahil Jalota.

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