Iranian Journal of Pharmaceutical Sciences Winter 2008: 4(1): 63-68www.ijps.ir

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Original Article

Electrophoretic Deposition of Hydroxyapatite

Aliasghar Behnamghadera,*, Narjes Bagheria, Babak Raissia, Tahmineh Foratib, Dionysia Ntentopoulouc, Ehsan Marzbanrada

aMaterials and Energy Research Center, Tehran, Iran,bDepartment of Biomedical Engineering, Islamic Azad University, Science and Research Branch,

Tehran, Iran,cCenter of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran,

Tehran, Iran

AbstractThe purpose of this study was to investigate the deposition of the hydroxyapatite

(HA) coating via the electrophoresis procedure. The HA deposition was performedin an ethanol, methanol, acetone and isopropanol suspension. Methanol was foundto be the best deposition media. Among the different environmental conditions,including the encapsulation of the samples under two vacuum types of pressure(10-5-10-4 and 2×10-2 Torr) and also the purge of the argon gas in the tube-likefurnace, the optimum environment was the one demonstrating the encapsulation underthe vacuum pressure of 2×10-2 Torr (washing with argon gas of 99.9% purity). Afterthe examination of 3 sintering temperatures (1020, 1050 and 1100 ºC), the sinteringtemperature at 1050 ºC illustrated the most desired results. The samples sintered underthese conditions were apparently intact, most of the interfacial part of the coatingwas found to be attached to the substrate surface irregularities and no single crackswere observed.

Keywords: Coating; Electrophoretic deposition; Hydroxyapatite; Sintering.Received: December 4, 2007 Accepted: June 10, 2008

1. IntroductionThe characteristics of the bone tissue,

related to its strength and its viscoelasticbehavior, are attributed to the presence of thenanometric crystallites, rich in phosphorusand calcium. Hydroxyapatite (HA), the mostimportant member of the calcium phosphatesfamily, demonstrates a good biocompatibility

behavior and encourages the bone formation.These advantageous characteristics lead to theworldwide use of this type of biomaterialsfor hard tissue applications [1].

The HA coatings are coated on metallicsubstrates by different methods such as plasmaspraying process, electrophoresis deposition(EPD), sol-gel process and others. Thecommercial method for the preparation ofthese coatings is the plasma spraying process,being inexpensive and highly rapid in theproduction. However, the employment of this

*Corresponding author: Aliasghar Behnamghader, Materials andEnergy Research Center, P.O. Box 14155-4777, Tehran, Iran. Tel (+98)261-6204131-3. E-mail: a-behnamghader@merc.ac.ir.

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method unfortunately presents somedrawbacks. For instance, the preparation ofuniform coatings is a hard task, especiallyfor complicated shapes. Another drawbackis the presence of various amorphous phasesand residual stresses in the product, due to thehigh cooling speed and the high temperatureof the process [2-4].

The suitability for the preparation of thecomplicated shapes, the ability of thethickness control and the low potentiallyresidual stressing are some of the mostsignificant characteristics of the EPD method[5]. The purpose of this study is to investigatethe optimum conditions of the media for theelectrophoresis deposition of hydroxyapatiteand the sintering of the coated materials. Inthe following sections, after the description ofthe used experimental equipment andmaterials, the obtained results will bediscussed.

2. Materials and MethodsThe deposition set-up consisted of the

power supply, the amperometer, thevoltameters and a timer. The cathode was a Ti-6Al-4V alloy (GOST BT6, Russia) with athickness of 3 mm, a width of 10 mm and alength of 25 mm. Of this length, only 10 mmwere used for the deposition. The glass plateanode was coated with gold on both sidesand was placed against the cathode. Thedeposition was performed at a 100 ml beakerand the voltage was set at the value of 100 V.

Because of the high sensitivity and accuracyof the electrophoretic deposition process, thecomponents used in this process were washedcompletely. In detail, this equipment wasfirstly washed with tap water, then withdistilled water and, finally, with pure acetone.

The surface roughness parameter (Ra) wasmeasured by the mechanical contact method(Surface Roughness Tester-TR100). Themicro-structural analysis of the samples wasmade by scanning electron microscopy (SEM,Cambridge Stereoscan 360). For the micro-structural analysis of the coatings, after thesurface preparation, a gold-palladium layerwas coated on the samples. The phase analysiswas carried out by an X-ray diffractometer(Siemens D-500). The measurement of thecalcium-phosphorous ratio was conductedwith the Inductively Coupled Plasma method(ICP, ARL 340, Switzerland).

3. Results and discussionThe XRD analysis showed the powder

used in this work (Merck code, 2196)composed of a crystalline HA. The calcium-phosphorous ratio was 1.66. It should benoted that the accuracy of the ICP techniquefor the major elements measurement is low[6]. The analysis of the particle sizedistribution (Figure 1) and its observation bySEM (Figure 2) revealed that the powderdemonstrated the particle size distributionranging from submicrometer to severalmicrometers.

Figure 1. The particle size distribution of Hydroxyapatite. Figure 2. The SEM image of the Hydroxyapatite particles.

Electrophoretic deposition of hydroxyapatite

3.1. Deposition conditionsIn order to analyze the effect of the

suspension media, to which only a limitednumber studies have been reported [7-11],the HA deposition was performed in anethanol, methanol, acetone and isopropanolmedia. The primary ratio of the HAsuspension was fixed 0.6 (Wt%) and thesubstrate roughness (Ra) value was 2-4microns.

The deposition in isopropanol was tooslow and there was no considerable depositionyield. The creation of the electrophoreticlayer in acetone was more significant butduring the sample extraction from thesuspension, a lot of particles were dispersedagain in the suspension. This phenomenoncould be attributed to the acetone evaporation.The ethanol suspension was better than thetwo previous suspensions, due to the HAlayer creation, even if they were thin andobviously inhomogeneous. However, the bestdeposited samples, obtained in methanolsuspension, were intact and uniform and therewas a small dispersion in the suspensionwhile extracting the sample.

At first, the deposition speed in methanolwas high, but when the time elapsed, therewas a speed decrease (Figure 3). Theconcentration decrease of the suspension andthe voltage drop due to the thickening of thelayer, have been known as the two influentialfactors on this phenomenon [2].

Although in the beginning of the depositionall suspensions were opaque, afterwards thetransparency gradually increased. After theinitiation of the deposition in methanol, ittook less than 1 min. for the suspension tobecome transparent until the completion of theprocess. This observation is in agreementwith the mathematical predictions of the elec-trophoretic deposition. According to the

mathematical prediction, at first the relationbetween the weight of the deposited materialand the deposition time should be linear,while afterwards this relation becomesexponential [5].

3.2. Sintering conditionsSintering is the final preparation stage for

the coated ceramic materials. An undesirablesintering can lead to the substrate deformation,the coating cracking or even thesubstrate/coating separation. Taking intoaccount the available data [3, 8, 10], thefollowing procedure was followed toinvestigate the appropriate sinteringconditions.

3.2.1. Sintering environmentBecause of the sensitivity of titanium

surface to oxidation at high temperature,sintering in an environment in the presence ofair would destroy the interface between thesubstrate and the coating. To overcome thisproblem, sintering was carried out under 3

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Table 1. The environment characteristics and the conditions for the sample in the sintering.Environment Sample position Vacuum pressure (Torr)Argon vacuum In capsule 10-4-10-3 and 2×10-2

Argon purging Tube-like furnace Atmosphere

Figure 3. The deposition thickness variation during the elec-trophoretic deposition.

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various conditions; purge of the argon gasinto a tube-like furnace and the encapsulationof the samples under two different vacuumenvironments (Table 1). The temperature andtime for sintering were fixed at 1050 ºC andfor 1 h, respectively. The thickness of theraw coating in all samples was 60±15 μm.

The coating was not absolutely attached tothe substrate in the samples, sintered under theargon purge and the use of the tube-likefurnace (Figure 4). This phenomenon wasrelated to the destructive influence of theavailable oxygen, even when the argon gashad been previously purified.

The capsule samples were made of quartztubes with a length of 200 mm and a diameterof 18 mm. The capsule tube was washedtwice with argon gas of 99.9% purity. Topurify the argon gas from the oxygen impurity,a capsule of red copper powder was used.The samples sintered under the vacuum

pressure of 10-5-10-4 and 10-2 Torr wereapparently intact.

The microscopic observations of thecoating/substrate interface exhibited that mostof the coating interfacial part was attached tothe substrate surface irregularities and nosingle cracks were noticed (Figures 5 and 6).The crack growth at the substrate/coatinginterface has been known as a major cause ofthe interfacial fracture of the coated materials [3].

Two substrates were encapsulated at thesame conditions to compare the oxygen effectwhich might be present in the two usedcapsule. Despite of the substrate sintered at10-5-10-4 Torr, very thin layers of largetitanium oxide crystals were observed visuallyand identified by XRD on the one sintered at10-2 Torr. Apart from the hydroxyapatite as themajor phase component of the coating, variouscompounds such as CaHPO4, Ca(OH)2 andCa(H2PO4)2 were also recognized (Figure7).Among the calcium phosphate components,HA is the most stable phase and exhibits thelowest dissolution degree in the simulationbody fluid. Dicalcium phosphate (DCP,CaHPO4), monocalcium phosphate [MCP,Ca(H2PO4)2] and monocalcium phosphatemonohydrate [MCPM, Ca(H2PO4)2.H2O],due to their characteristics, present manyapplications as precursors in the synthesis ofcalcium phosphate powders and/or in bonecement applications [12].

Figure 5. The SEM image of the cross section surface of theencapsulated sample sintered under the vacuum pressure of2×10-2 Torr and at 1050 ºC.

Figure 6. The outer surface of the encapsulated coatingsintered under the vacuum pressure of 2×10-2 Torr and at 1050ºC (optical microscopy).

Figure 4. Detachment of the coating from the substrate in thesamples sintered in tube-like furnace (optical microscope).

Electrophoretic deposition of hydroxyapatite

Except for HA, other identified phasescan also be the outcome of sintering atvacuum environment and, subsequently, adifferent effect will be noticed on the substrateat the interfacial part of the coating. Theseparameters lead to the phosphorous diffusionfrom the coating to the substrate, thecrystalline allotropic transformation, thecrystalline transformation to an amorphouscomponent and the change of the phase trans-formations temperature [8, 9, 12-15]. Theexamination of the coating chemical analysisrevealed that the phosphorous amount in theouter surface of the coating was smallercompared with that in the interfacial part ofthe coating.

Taking into account the correspondingresults attained from sintering under thevacuum pressure of 10-5-10-4 and 10-2 Torr andconsidering the cost and the feasibility aspectsfor the preparation of an environment typewith high vacuum, the vacuum pressure of10-2 Torr would be preferred.

3.2.2. Sintering temperatureSamples were sintered at the temperatures

of 1020, 1050 and 1100 ºC for 1h, toinvestigate the effect of the sinteringtemperature. The sample sintered at 1100 ºCwas deformed slightly. As depicted in Figure8, the microstructural discontinuity and thedefects in the samples sintered at 1020 ºCwere greater than the ones sintered at 1050 ºC.

4. ConclusionBased on the results obtained in this

research, the methanol proved to be the mostsuitable media for the electrophoreticdeposition of hydroxyapatite with respect toethanol, acetone and isopropanol media. Thebest sintering condition was concluded to bethe encapsulation under the vacuum pressureof 2×10-2 Torr (washing with argon gas of99.9% purity) with the sintering temperatureof 1050 ºC. The samples, sintered under theseconditions, were clearly intact, the coating wasattached to the substrate and no gap wasobserved at the coating/substrate interface.

AcknowledgementsThis research is a part of the internal

Project, supported by the Material and EnergyResearch Center. The authors are grateful forthe co-operation of this Center, particularly theThermal Processing, Vacuum preparation andmaterials characterization laboratories.

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Figure 7. The X-ray pattern of the coating sintered under thevacuum pressure of 2×10-2 Torr.

Figure 8. The outer surface of the encapsulated coatingsintered under the vacuum pressure of 2×10-2 Torr and at 1020ºC (optical microscopy).

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