Characterization and Dissolution Behavior of Sputtered Calcium Phosphate Coatings After Different Postdeposition Heat Treatment Temperatures

CHARACTERIZATION AND DISSOLUTION BEHAVIOROF SPUTTERED CALCIUM PHOSPHATE COATINGSAFTER DIFFERENT POSTDEPOSITION HEATTREATMENT TEMPERATURESRESEARCHY. Yang, PhDC. M. Agrawal, PhDK.-H. Kim, PhDH. Martin, BSK. Schulz, PhDJ. D. Bumgardner, PhDJ. L. Ong, PhDKEY WORDSCalcium phosphate coatingHeat treatmentX-ray diffractionCrystallinityX-ray photoelectron spectroscopyY. Yang, PhD, is with the Department ofRestorative Dentistry, Division of Biomaterials,University of Texas Health Center at SanAntonio, and C. M. Agrawal, PhD is withthe Center for Clinical Bioengineering,University of Texas Health Science Center andthe College of Engineering, University of Texasat San Antonio, San Antonio, TX.K.-H. Kim, PhD, is with the College ofDentistry and Institute of BiomaterialsResearch and Development, Department ofDental Biomaterials, Kyungpook NationalUniversity, Jung-Gu, Daegu, Korea.K. Schulz, PhD, is with the Dave C. SwalmSchool of Chemical Engineering and theBiomedical Engineering Program, and J. D.Bumgardner, PhD, is an associate professorwith the Biomedical Engineering Program andthe Agricultural and Biological EngineeringDepartment, Mississippi State University,Mississippi State, MS.J. L. Ong, PhD, is professor in the Center forClinical Bioengineering, University of TexasHealth Science Center, and the Department ofRestorative Dentistry, Division of Biomaterials,MSC 7890, University of Texas Health Centerat San Antonio, 7703 Floyd Curl Drive, SanAntonio, TX 78229-3900. Correspondenceshould be addressed to Dr Ong.270 Vol. XXIX/No. Six/2003There is a lack of correlation between specific properties of hydroxyapatitecoating surfaces, osseointegration processes, and implant success. The aim of thisstudy was to evaluate the relationship between well-characterized structural andchemical properties of radio-frequency sputtered calcium phosphate (CaP)coatings and their dissolution behavior. Sputtered CaP coatings were evaluatedas-sputtered (non-heat treated) or after 1 hour of postsputter heat treatments at4008C or 6008C. All coatings were characterized by X-ray diffraction, X-rayphotoelectron spectroscopy, Fourier transform infrared spectroscopy, and contactangle measurement. The dissolution behavior of CaP coatings in the presenceand absence of proteins was also investigated. It was observed from this studythat as-sputtered CaP coatings were amorphous. The 4008C heat-treated CaPcoatings exhibited low crystallinity (1.9% 6 0.4%), whereas the 6008C heattreatedCaP coatings were highly crystalline (67.0% 6 2.4%). The increase ofCa/P ratio, PO4/HPO4 ratio, and the number of PO4 peaks were observed to beconsistent with the increase in heating temperature and the degrees of coatingcrystallinity. Phosphorus ions released from CaP coatings decreased with theincrease of crystallinity of CaP coatings. In addition, immersion of CaP coatingsin media containing proteins resulted in an increase in P ions released ascompared with coatings immersed in media without proteins. It was concludedthat the degree of CaP coating crystallinity can be controlled by varying thepostdeposition heat-treatment temperature. It was also concluded that, aside fromcoating crystallinity, dissolution and reprecipitation of the coatings can becontrolled by knowing the presence of proteins in the media and PO4/HPO4 ratiowithin the coatings.INTRODUCTIONydroxyapatite (HA) andcalcium phosphate (CaP)coatings are successfullyused in dental and orthopedicimplant therapies.1,2 However, thereare still many concerns about the resorptionof HA coatings in a biologicalenvironment. It was suggested thatdissolution of HA coatings may play arole in influencing early osseointegration,3 thereby resulting in the increasedinterfacial strength through early skeletalattachment and increased bonecontact with implant surfaces.4,5 However,it was also reported that exceedinglyrapid dissolution may result inpoor bone bonding to implant surfacesand coating disintegration, which mayresult in particulate debris formation.6,7Dissolution is an important variableand is affected by the characteristicsof both the HA coating and thephysiological solution.8 The criticalcoating specifications include purity(phase composition), crystallinity,Ca/P ratio, microstructure, porosity,surface morphology, surface roughness,coating thickness, and coating adhesionto implant surfaces. These variables,depending upon the processingparameters and the method of deposition,affect dissolution and reprecipitationof CaP materials, which will ultimatelyaffect their short- and longtermclinical performance. Unfortunately,the performance-enhancingadvantages of plasma-sprayed HA aremired by considerable controversy becauseof the lack of correlation betweenspecific properties of the HA surfaceand implant success. This has led tomany conflicting animal and clinicalobservations and, as a result, no consensusexists on the optimum HA coatingproperties required for the optimumrate of development of osseointegration.9,10Several experimental depositionprocesses exist for producing HA andCaP coatings, including electrophoreticcodeposition,11 sputter deposition,12,13and high-velocity oxy-fuel combustionspray deposition.14 These experimentalprocesses have been developed in anattempt to improve the adhesive, compositional,and structural properties ofthe coatings. Regardless of the depositionprocess used, it is the surface ofthe coating that is especially crucial forthe fixation mechanism because of itsdirect contact with bone and body fluidson implantation. As such, characterizationof the coatings before use iscritical. In this study, the relationshipbetween structural and chemical propertiesof CaP coatings and their dissolutionbehavior was evaluated. Characterizationof radio-frequency sputtered-CaP coatings after postdepositionheat treatment was performed byX-ray diffraction, X-ray photoelectronspectroscopy (XPS), Fourier transforminfrared (FITR) spectroscopy, and contactangle measurements. The dissolutionbehavior of sputtered coatings afterdifferent treatments was evaluatedin the presence and absence of proteinfor a period of 12 days.MATERIALS AND METHODSMaterials preparationTitanium (Ti) grade 2 (Metal Samples,Munford, Ala) disks, 15.30-mm diameterand 2-mm thick, were used in thisstudy. The Ti surfaces were preparedby wet grinding with 240-, 400-, and600-grit silicon (Si) carbide paper to asurface roughness of ra 5 0.37 6 0.01mm, as measured by profilometry (Surtronic3, Taylor-Hobson, UK). Thesesurfaces were ultrasonically degreasedin acetone and ethanol for 10 minuteseach, with deionized water rinsing betweenapplications of each solvent. Passivationwas accomplished by exposingthe Ti samples to a 40% nitric acid solutionat room temperature for 30 minutes(ASTM F86-76) and rinsing withdeionized water, which was followedby air drying. The cleaned Ti diskswere sputter coated with a thin CaPlayer. In addition, glass slides wereused as substrates for sputtering.Sputter coatingSputtering of CaP onto the cleaned Tiand glass slide surfaces was performedwith a CMS-18 radiofrequency magnetronsputtering system (Kurt J LeskerCompany, Clariton, Pa). The targetused in the sputtering process was a10.16 cm diameter sintered HA (0.635cm thick) on a copper backing (TargetMaterials Inc, Dayton, Ohio). The basepressure in the sputtering chamberwas #5.0 3 1026 torr. Sputter depositionof all samples was accomplishedin a single batch with a process pressureof 1.0;1.5 mbar and a sputteringpower of 200 W for 7 hours at a coatingYunzhi Yang et alrate of 60 nm per hour. After sputtering,the coatings of 0.42 mm thick (20samples each group) were left as-sputteredor subjected to a postdepositionheat treatment at either 4008C or 6008Cfor 1 hour in a Thermolyne 48000 furnace(Barnstead International, Dubuque,Iowa).X-ray diffractionA D8 advanced X-ray diffractometer(Bruker, Madison, Wis) was used tocharacterize the structure of the CaPcoatings. The X-ray diffractometer wasequipped with a single Gobel mirror toyield a diffracted parallel beam whileremoving the Kb radiation. By using agrazing incidence attachment, a 0.358soller slit, and a LiF (100) flat crystalmonochromator to improve resolutionand peak-to-background ratios, triplicatecoatings were analyzed with CuKa radiation at 40 KV and 30 mA.Triplicate coatings per treatment groupwere scanned from 258 to 358 2u at ascan rate of 0.18 per minute. Crystallinepeak area in the range of 258 to 358 wascalculated, and the crystallinity percentageof the coatings was quantifiedby correlating the crystalline peak areato an HA crystallinity standard curve.The HA crystallinity standard curvewas derived by mixing known quantitiesof 100% crystalline and amorphouscommercial HA powder (HitemcoMedical Applications Inc, Old Bethpage,NY). Crystallinity percentage of thecoating was analyzed by analysis ofvariance (ANOVA), and differences incrystallinity were considered signifi-cant at P , .05.X-ray photoelectron spectroscopyElemental and chemical composition ofthe surface layer of the disks was determinedby XPS with a Physical ElectronicsModel 1600 surface analysissystem (Phi, Eden Prairie, Minn). Thesystem used an Mg electrode (Ka radiationat 1253.6 eV) at 15 kV and 300W as the X-ray source. Triplicate sampleswere positioned at a 458 take-offangle with respect to the analyzer. Twospots, approximately 800 mm in di-Journal of Oral Implantology 271EFFECT OF HEATING TEMPERATURE ON CALCIUM PHOSPHATE COATINGSFIGURE 1. Representative X-ray diffraction patterns of sputtered calcium phosphate (CaP)coatings (1) after a 4008C heat treatment and (2) after a 6008C heat treatment. The arrowindicates the (300) peak.ameter, were evaluated on each sample.Survey spectra were averaged from 10scans with a pass energy of 46.95eV.Survey spectra were used to identifysurface elements and to calculate theirrelative composition in atomic percent.High-resolution spectra of elementswere averaged from 15 scans with apass energy of 23.5eV. The high-resolutionspectra were used to identify thechemical states and to estimate percentageof chemical species present.Quantitative analyses were based onpeak areas and atomic sensitivity factorswith the Spectral Data Processorv2.3 software (XPS International,Mountain View, Calif). The Ca/P ratioof coatings from different treatmentswas analyzed by ANOVA, and differencesin Ca/P ratios were consideredsignificant at P , .05.Fourier transform infraredspectroscopyStructural and molecular compositionof coatings and sputtering target wereevaluated with a model 550 Magna-IRFTIR spectroscopy (Thermo Nicolet,Madison, Wis). By using a resolution of1 cm21 and a scan number of 32, triplicatecoatings per treatment groupwere analyzed from 400 cm21 to 4000cm21. Control Ti disks were used forbackground collection. For the CaPcoatings, data collection was per-272 Vol. XXIX/No. Six/2003formed with an 808 grazing-angle accessory.Contact anglesContact angles for 5 surfaces per groupwere measured with a VCA 2000 videocontact angle system (Advance SurfaceTechnology, Billerica, Mass). The contactangles, determined with drops ofultrapure distilled water, cell culturemedia (Dulbecco's modified Eagle'smedium) and 1 mg/mL albumin solution,were measured at room temperature.The image of the water dropletwas captured within 10 seconds of delivery.The results were analyzed byANOVA, and differences in contact angleswere considered significant at P ,.05.Dissolution studyThe heat-treated and non-heat-treatedCaP coatings were immersed in a 1.0M Tris (Fisher Chemical, Fair Lawn,NJ) buffer containing 80 mM NaCl(Fisher Chemical) in the presence andabsence of 0.1 mg/mL bovine plasmafibronectin (Sigma Chemical Company,St Louis, Mo). The pH of solutions wasbalanced at 7.4 before the dissolutionstudy. The experiment was performedin a sterile and humidified 95% air, 5%carbon dioxide atmosphere at 378C. Itis imperative that the pH of each solutionis achieved after all chemicalcomponents are added to the mediaand brought to 378C. A Tris buffer wasused to maintain a constant pH of 7.4around each sample during the incubationperiod. The specimens were incubatedin buffer solution at 378C for12 days. The buffer media werechanged daily to ensure all parameterswere equal with respect to the constitutionof the media between each of thetest specimens, especially with respectto quantities of calcium (Ca) and phosphate.As the buffer media were collectedeach day, the volume withdrawnand pH were recorded. Each dissolutionsample was saved for subsequentanalysis of phosphate release.Measurement of inorganic phosphorusThe amount of phosphorus (P) ions releasedin solution each day was measuredcolorimetrically utilizing the reactionof ammonium molybdate andascorbic acid with the inorganic phosphateto obtain a molybdenum bluecomplex. The reaction was done in a96-well microtiter plate. Each samplewas diluted 10-fold to make a 100-mLsolution. Solution A was made by combining2 parts double distilled water, 1part 5.0 N sulfuric acid (Baker Analyzed,Phillipsburg, NJ), 1 part 0.01 Mammonium molybdate tetrahydrate(Sigma) in water, and 1 part 10% ascorbicacid (Sigma). Solution A wasmade fresh for each assay. To the 100-mL sample dilution, a 100-mL solutionA was added. The complex was allowedto form for 1 hour at room temperature.It subsequently was read at655 nm on a Benchmark microplatereader (Bio-Rad Laboratories Inc, Hercules,Calif). Each P concentration wascalibrated with standard curve of P vsabsorbance. ANOVA analyses for inorganicP ions released were carried outon the absorbance data, and differenceswere considered significant if P ,.05.RESULTSX-ray diffractionThe as-sputtered CaP coatings wereobserved to be amorphous, whereasAtomic percentage (mean 6 SD; n 5 3) of elements on the surface of Ti and CaP-coated Ti samplesC Sample58.2 6 4.345.7 6 3.557.8 6 5.2Ti controlCaP, not heat treatedCaP, heat treated at 4008C52.2 6 5.7 CaP, heat treated at 6008CTi indicates titanium; CaP, calcium phosphate; C, carbon; O, oxygen; N, nitrogen; Ca, calcium; P, phosphorus; Si, silicon.*Values with the same superscript are not statistically different for the CaP-coated samples at P # .05.crystalline peaks were observed for theheat-treated coatings (Figure 1). A singlepeak was observed for the 4008Cheat-treated CaP coatings, whereaspeaks matching apatite-type structurewere observed for the 6008C heat-treatedCaP coatings. Crystallinity of the4008C and 6008C heat-treated CaP coatingswere 1.9% 6 0.4% and 67.0% 62.4%, respectively.X-ray photoelectron spectroscopyRepresentative XPS survey spectra ofthe as-sputtered CaP coatings andheat-treated CaP coatings indicated thepresence of Ca, P, and oxygen (O). Adventitiouscarbon (C) and Si from thegrinding process were also detected.The relative surface composition of theelements for the samples is shown inTable 1. The C concentrations on thesurfaces of as-sputtered CaP coatings,4008C heat-treated CaP coatings, and6008C heat-treated CaP coatings were45.7% 6 3.5%, 57.8% 6 5.2%, and52.2% 6 5.7%, respectively. Significantdifferences in C concentrations betweenas-sputtered CaP coatings and4008C heat-treated CaP coatings (P 5.0008) and as-sputtered CaP coatingsand 6008C heat-treated CaP coatings (P5 .037) were observed.The as-sputtered CaP coatings exhibiteda near-HA stoichiometric ratioof Ca/P on the surface, whereas bothof the heat-treated CaP coatings exhibiteda significant enrichment of Ca relativeto P (P # .0001). There was nodifference between the surface Ca/Pratio for the heat-treated samples (P ..05).Representative high-resolutionspectra for P from the CaP coatingsYunzhi Yang et alTABLE 1Ca/P* Ti Si P Ca N O1.9 6 0.61.6 6 0.2a2.6 6 0.3b2.7 6 0.61.1 6 1.11.0 6 1.15.43 6 0.93.0 6 0.71.3 6 1.68.4 6 0.87.6 6 1.31.4 6 2.01.1 6 1.40.4 6 1.134.6 6 3.638.2 6 2.430.1 6 4.5 2.5 6 0.2b 2.2 6 0.7 3.5 6 0.6 8.6 6 1.2 0.3 6 0.8 33.1 6 4.6FIGURE 2. Representative high-resolution 2p spectra of calcium phosphate (CaP) coatingswith X-ray photoelectron spectroscopy. The 2p were curve-fitted showing 2 peaks at 132.7eV and 133.7 eV.TABLE 2Estimate of percentage (mean 6 SD; n 5 3) of (PO4)23 and H(PO4)22 species fromP high-resolution spectra of CaP-coated Ti samples*RatioH(PO4)22[;133.7 eV](PO4)23[;132.7 eV] Sample2.2 6 0.72.5 6 1.43.4 6 2.332 6 532 6 1228 6 1267 6 567 6 1272 6 12CaP, not heat treatedCaP, heat treated at 4008CCaP, heat treated at 6008C*P indicates phosphorus; CaP, calcium phosphate; Ti, titanium.4008C heat-treated CaP coatings (P 5.003) and 6008C heat-treated CaP coatings(P 5 .037).to (PO FTIR spectroscopyshowed that 2 peaks were found to fitto the 2p peak (Figure 2). The peak atapproximately 132.7 eV corresponded4)23 type species, and the peakat approximately 133.7 correspondedto H(PO4)22 type species. As shown in No hydroxyl (OH) group peak was ob-Table 2, the PO4/HPO4 ratio increased served in as-sputtered CaP coatingsand 4008C heat-treated CaP coatings(Figure 3). For 6008C heat-treated CaPcoatings, an OH peak at 3572 cm21 wasobserved. The number of PO4 peakswas observed to increase with increaswiththe heat treatment temperature.However, no statistical differencescaused by the different heat treatmentswere found between the 2 phosphatespecies peak areas (P 5 .7). Significantdifferences in O concentrations also ing heat treatment temperature. TwoPO4 peaks at 547 cm21 and 458 cm21were observed in as-sputtered CaPwere observed on surfaces of as-sputteredCaP coatings as compared withJournal of Oral Implantology 273EFFECT OF HEATING TEMPERATURE ON CALCIUM PHOSPHATE COATINGSFIGURE 3. Representative fourier transform infrared spectra for as-sputtered calcium phosphate(CaP) coatings, 4008C heat-treated CaP coatings, and 6008C heat-treated CaP coatings.The arrow indicates the OH and phosphate peaks.FIGURE 4. Contact angles for as-sputtered calcium phosphate (CaP) coatings, 4008C heattreatedCaP coatingsh and 6008C heat-treated CaP coatings.coatings and 4008C heat-treated CaP treated CaP surfaces. Significant differcoatings.In addition to these peaks, ences of contact angles on as-sputteredadditional PO4 peaks at 1084 cm21, 964 CaP coatings, 4008C heat-treated CaP significant increase in the degree ofcoatings, and 6008C heat-treated CaP cm21, 617 cm21, and 585 cm21 were observedfor 6008C heat-treated coating.Other peaks (3365 cm21, 3735 cm21,1253 cm21, 1158 cm21, 915 cm21, 812cm21, 656 cm21) were also observed forall coatings.Contact angleAs shown in Figure 4, significantly differentcontact angles were observed foras-sputtered CaP coatings, 4008C heattreatedCaP coatings, and 6008C heattreatedCaP coatings. Highest contactangle was observed for the 4008C heat-274 Vol. XXIX/No. Six/2003coatings were observed regardless ofthe media used.Dissolution studyPhosphorous ions released in the presenceand absence of protein for assputteredCaP coatings, 4008C heattreatedCaP coatings, and 6008C heattreatedCaP coatings are shown in Figures5a through b. It was observed thatin the presence and absence of protein,a significantly lower amount of P ionswas released from the 6008C heat-treatedsurfaces. The 4008C heat-treatedCaP coatings were observed to releasemore P ions compared with the 6008Cheat-treated CaP coatings, and the asdepositedCaP coatings were observedto have the greatest amount of P ionsreleased. Significant differences werefound in released P ions levels on thefirst day between as-sputtered CaPcoatings and 6008C heat-treated CaPcoatings (P 5 .00025). Samples immersedin media containing proteinswere observed to release significantlyhigher concentrations of P ions as comparedwith coatings immersed in mediawithout protein.DISCUSSIONAlthough HA and CaP coatings havebeen successfully used in dental andorthopedic implant therapies, manyconflicting animal and clinical observationsabound regarding their clinicaloutcomes.9,10 It is generally known thatvariation in the chemical and physicalcharacteristics of the CaP and HA coatingaffect early bone cell activity, therebyinfluencing long-term success.1 Assuch, it is critical to characterize thesurface of the coatings before biologicalevaluations in vitro and in vivo.In this study, the as-sputtered CaPcoatings were observed to be amorphous,indicating that the sputteringprocess resulted in a loss of the apatite-type structure found on the HAtarget. Postdeposition heat treatmentallowed CaP coatings to crystallize,forming an apatite-type structure. Acoating crystallinity was observed asthe heating temperature was increased.Amorphous-to-crystalline phase transformationtemperature has also beenreported during the post-plasmasprayedheat treatments of D-Gun-sprayed HA coatings.15Surface analysis with XPS indicatedthe presence of adventitious C andsilica contaminations on surfaces ofsputtered CaP coatings. Significant differencesin C concentrations betweenas-sputtered CaP coatings and 4008Cheat-treated CaP coatings and as-sput-FIGURE 5. Phosphate release in a 1.0 M Tris buffer in the absence (a) and presence (b) ofprotein for as-sputtered calcium phosphate (CaP) coatings, 4008C heat-treated CaP coatings,and 6008C heat-treated CaP coatings.tered CaP coatings and 6008C heattreatedCaP coatings were observed,with the highest C concentrations observedon surfaces of the 4008C heattreatedCaP coatings. It was suggestedthat the C contaminants were mainlyin the form of carboxyl groups (CHx) face. No significant difference between presence of protein, coatings were obratherthan the carbonate groups the surface Ca/P ratio of the 4008C and served to release significantly more P(CO3).16-18 This observed significant 6008C heat-treated samples was ob- ions compared with coatings imserved.It was suggested that differenc- concentration difference in surface carboxylcontaminant was confirmed withcontact angle measurements, showingsignificant differences in contact anglesbetween the different heat-treated CaPcoatings. In addition, differences incontact angle measurements on coatingsmay be attributed to the signifi-cant differences in O concentrations onthe CaP surfaces of different heat treatments.Compositional analysis of the assputteredcoatings with XPS also indicateda near-HA stoichiometric ratio ofCa/P on the surface, whereas heattreatedCaP surfaces were observed toexhibit a significantly enriched Ca suresin Ca/P ratio between the as-sputteredand heat-treated CaP coatingsmay have been the result of Ca ion(Ca21) diffusion from bulk-to-surfaceduring the heat treatments.19XPS analysis indicated an increaseof PO4/HPO4 ratio as postdepositionheat-treatment temperature was increased.This increase in PO4/HPO4 ra- In addition, it was observed from XPStio was consistent with the increasingYunzhi Yang et aldegree of coating crystallinity as a resultof increasing heat-treatment temperature.In addition, the increasingnumber of PO4 peaks observed duringthe FTIR analysis as a result of increasingpostdeposition heat-treatment temperaturewas also consistent with theincreasing degree of coating crystallinity.It was suggested that HPO4, tosome degree, indicated crystal imperfection.20,21 By using high resolutiontransmission electron microscopy, otherinvestigators have shown that higherannealing temperature not only increasedgrain size or crystallinity, butalso improved crystal formation byminimizing the number or preventingthe formation of crystal defects.22Generally, it was reported that CaPcoatings immersed in media wouldfirst experience dissolution followed byachievement of a dissolution and reprecipitationequilibrium.23 Such changesin factors influencing the equilibriumwould change the dissolution behavior.In this study, the release of P ions fromthe coatings was measured. In thepresence and absence of protein, a significantlylower amount of P ions wasreleased from the 6008C heat-treatedsurfaces. The 4008C heat-treated CaPcoatings were observed to release moreP ions as compared with the 6008Cheat-treated CaP coatings, and the asdepositedCaP coatings were observedto have the greatest amount of P ionsreleased. Significant differences in Pions released or dissolution was suggestedto be attributed to the differencesin CaP crystallinity.1,8 In themersed in the absence of protein. It hasbeen reported that some organic substanceshave a high affinity for HA orCaP surfaces and that the growth ofHA crystals was often inhibited by theadsorption of these organic substances.23-26 As a result of the high affinityfor CaP surfaces to adsorb proteins, reprecipitationof CaP is often inhibited.that the heat-treated CaP coatings ex-Journal of Oral Implantology 275EFFECT OF HEATING TEMPERATURE ON CALCIUM PHOSPHATE COATINGSgess JO, Windeler AS. The advantagesof coated titanium implants prepared4/HPO4 ratio. It has been sug- by radiofrequency sputtering from hydroxyapatite.J Prosthet Dent. 1992;67:93-100.hibited higher PO4/HPO4, whereas theas-sputtered CaP coatings exhibitedlower POgested that the higher the PO4/HPO4ratio, the lower the dissolution rate.23,27As such, it was suggested that in additionto coating crystallinity, the PO4/HPO4 ratio may be an additional importantfactor influencing CaP dissolutionand reprecipitation.CONCLUSIONSIn this study, amorphous CaP coatingswere produced with radiofrequencymagnetron sputtering. The increase inthe degree of coating crystallinity wasconsistent with the increasing numberof PO4 peaks observed as a result ofdifferent postdeposition heat treatments.By varying the postdepositionheat-treatment temperature, the degreeof CaP coating crystallinity was controlled.It was also concluded that asidefrom coating crystallinity, dissolutionand reprecipitation of the coatings canbe controlled by knowing the presenceof proteins in the media and PO4/HPO4 ratio within the coatings.ACKNOWLEDGMENTThis study was funded by grants fromthe National Institute of Health (Grant1RO1AR46581 and 1S10RR016879).REFERENCES1. Yang Y, Bessho K, Ong JL. CommercialHA-coated and TPS-coated implants.In: Wise DL, ed. BiomaterialsHandbookAdvanced Applications of BasicSciences and Bioengineering. NewYork, NY: Marcel Dekker Inc; 2003:541-563.2. Yang YZ, Ong JL. Bond strength,compositional, and structural propertiesof hydroxyapatite coating on Ti,ZrO2-coated Ti, and TPS-coated Tisubstrates. 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