Effects of Applied Voltages on Hydroxyapatite Coating of Ti by EPD

Effects of Applied Voltages on Hydroxyapatite Coating ofTitanium by Electrophoretic Deposition

Xianwei Meng,1,2 Tae-Yub Kwon,1,3 Yunzhi Yang,4 Joo L. Ong,4 Kyo-Han Kim1,3

1 Institute for Biomaterials Research & Development, Kyungpook National University, Daegu, South Korea

2 Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, People’s Republic of China

3 Department of Dental Biomaterials, College of Dentistry, Kyungpook National University, Daegu, South Korea

4 Department of Biomedical Engineering, University of Tennessee Health Science Center, Memphis, Tennessee

Received 25 March 2005; revised 1 October 2005; accepted 1 October 2005Published online 16 December 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30497

Abstract: Hydroxyapatite (HA) coatings were deposited on titanium substrates by electro-phoretic deposition (EPD) at constant voltage and dynamic voltage, respectively. Varioussurface morphologies were observed under different type of voltages. Under a constant voltageof 20 V, a dense HA coating could be prepared. Under a constant voltage of 200 V, big HAparticles were deposited and the coating was porous. Under a dynamic voltage, a continuousgradient HA coating could be obtained. HA coatings were characterized with a field emission-scanning electron microscopy (FE-SEM) and an X-ray diffraction (XRD). XRD indicated nosignificant HA decomposition when the coatings were sintered for 2 h at 800°C. © 2005 WileyPeriodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 78B: 373–377, 2006

Keywords: hydroxyapatite; coatings; electrophoretic deposition; voltage; microstructure

INTRODUCTION

Hydroxyapatite (HA) coatings contribute excellent biocom-patibility to implants, while titanium substrates provide fa-vorable mechanical properties to the entire implant.1–5 Toobtain the maximum benefit from such coatings, it is neces-sary to control the composition, thickness, and microstruc-tures of the deposited HA layers.6 Among alternative coatingtechniques, there is a growing interest in electrophoreticdeposition (EPD). EPD presents several advantages such asability to control thickness and morphology of the depositedlayer by controlling the electrochemical parameters, ability todeposit relatively uniform coatings on complex shapes, ahigher deposition rate as compared with most coating pro-cess, and the low cost to purchase the equipment.7

The EPD process involves the migration and coagulationof ceramic particles included in the suspension on the elec-trode surface by an impressing electric field.8 Positivelycharged particles undergo cataphoresis (deposition on thecathode), whereas negatively charged particles undergo ano-phoresis (deposition on the anode).9 Parameters that deter-mine the characteristics of this process are those related to thesuspension and the process, including physical parameters

such as the electrical nature of the electrodes and the electri-cal conditions (voltage/intensity relationship, depositiontimes, etc.).10 Suspension for EPD is a complex system inwhich each component has a substantial effect on EPD effi-ciency.11 Once a well-dispersed suspension is prepared, theapplied voltage becomes a critical parameter.

The purpose of this study was to evaluate the effect ofapplied voltages on HA coatings, by EPD process. By ad-justing applied voltage, we could obtain HA coatings withdifferent microstructures.

MATERIALS AND METHODS

Substrate Treatment

In this study, titanium (Ti) sheets of 30 � 10 � 0.5 mm wereabraded with 600, 800, and 1000 grit SiC sandpapers. Thesheets were cleaned completely with distilled water and thenwith alcohol for 10 min each, in an ultrasonic bath. Thesubstrates were etched in a solution containing nitric acid andhydrofluoric acid (3:1), rinsed with distilled water, and thendried again.

Preparation of Suspensions

Suspensions for EPD were prepared by ultrasonic agitation ofHA powder in ethanol. The HA concentration in the suspen-

Correspondence to: K.-H. Kim (e-mail: [email protected])

© 2005 Wiley Periodicals, Inc.

373

sions was 0.1%. Polyvinyl alcohol (3%) and N,N-dimethyl-frmamide (DMF) (10%) were added to the suspension foradherence, strength, and crack-proof improvement.

Electrophoretic Deposition

A glass cell of 1000 cm3 equipped with two platinum counterelectrodes (3-cm apart) was used for deposition. The EPDwas done in pH 4.0 suspensions, at a constant voltage of 20V, a constant voltage of 200 V, and a dynamic voltagebetween 0 and 200 V, respectively, using a power supply(HBPS-1A600V Hobang Electronics Co. Ltd., Korea). Thedynamic voltage process consisted of three increments, wherethe primary voltage between 0 and 10 V was applied at a rateof 0.0333 V/s, followed by a voltage between 10 and 50 V ata rate of 0.1 V/s, with a final voltage between 50 and 200 Vapplied at a rate of 1 V/s. The green coatings were then airdried for 24 h, before being sintered at a rate of 10°C/min inan argon atmosphere, and held at the temperature of 800°Cfor 2 h to allow densification of the coating.

The microstructure of the substrates, green HA coatings,and sintered HA coatings were examined by FE-SEM (S-4300, Hitachi, Japan). Samples sputter-coated with platinumfor FE-SEM were observed at 2000�, 10,000�, and50,000� magnification. The sintered HA coatings were char-acterized by X-ray diffraction (XRD) (X’pert Pero MRD,Philips, Holland), using Cu K� radiation and grazing inci-dence 2� scans (� � 1°) from 20° to 60° at a speed of 0.08°

s�1. The generator tension is 40.0 kV, and the current is 30.0mA. At least 5 samples were prepared for measurements, 4samples for FE-SEM, and 3 for XRD.

RESULTS

Figure 1 shows the surface morphology of the HA coatingsdeposited by EPD. Figure 1(a,b) is the representative surfacemorphology of coatings deposited at a constant voltage of 20and 200 V for 10 min, respectively. The HA coatings, ob-tained at a constant voltage of 20 V, were found to consist offine HA particles (150–200 nm in size), with a few largeparticles sprinkled on these fine particles. The surface wasfound to be uniform. For coatings prepared at a constantvoltage of 200 V, bigger particles (above 400 nm) wereobserved. In addition to the big particles, surface porositiesgreater than 500 nm in size were also noticed. Figure 1(c)shows the representative surface morphology of coatingsdeposited at dynamic voltage ranging from 0 to 200 V. TheHA coating obtained by dynamic voltage was observed toconsist of 300–400 nm particles and are uniform.

As shown in Figure 2(a–c), continuously gradient coatingswere produced by applying dynamic voltage during EPD.The coating layer closest to the substrate was found to bedense, whereas the outer layer was found to be porous.

Figure 3 shows the microstructure of the coatings depos-ited at a constant voltage of 20 and 200 V, and a dynamic

Figure 1. Surface morphology by FE-SEM of coatings obtained at 0.1% HA particle concentration atdifferent applied voltage: constant voltage (a) 20 V, (b) 200 V during 10 min, and (c) dynamic voltage.The magnification is 10,000�.

Journal of Biomedical Materials Research Part B: Applied BiomaterialsDOI 10.1002/jbmb

374 MENG ET AL.

voltage of 0–200 V. The coatings were sintered in an argonatmosphere at 800°C for 2 h, at a heating rate of 10°C/min.Sintered HA coatings turned to be denser and more adhesiveto the substrates than unsintered ones. As shown in Figure4(b), big agglomeration and cracks can be found on thesintered coatings obtained at a constant voltage of 200 V. Thecoating exhibited bad sinterability. Figure 3(c) shows a FE-SEM micrograph of the sintered coatings deposited at adynamic voltage of 0–200 V. The coatings, obtained usingdynamic voltage, were found to be very uniform. Neithercrack nor big particles were found on the surface.

Figure 4 shows the XRD pattern of sintered HA coatingson Ti substrate at dynamic voltage. The XRD pattern indi-cates well-defined peaks of HA, with no formation of othercrystalline phase.

DISCUSSION

Among other variables, during EPD, the electrophoretic ve-locity (�) is reported to be related to the applied voltage andparticle size, governed by the following equation:12

� � QE/4�r� (1)

where Q and r represent the charge and particle radius,respectively. E is the potential difference applied to thesuspension of the electric field, and � is the suspension

viscosity. The � can be considered constant because the HAparticle concentration in suspension is relatively low. Underthis condition, the electrophoretic velocity is mainly a func-tion of the electric field and the particle size.

As the HA particles in the suspension used for EPDusually have a distribution of particle sizes, particles withdifferent r value have different electrophoretic mobility,thereby resulting in the segregation effects observed duringthe EPD process, when E is constant. It is inferred that thesmallest particles gained the highest electrophoretic velocity,accordingly, they are the first to be deposited. This specula-tion corresponded to the deposits obtained at a constantvoltage of 20 V [Fig. 1(a)]. As the applied potential isincreased to 200 V [Fig. 1(b)], elctrophoretic velocity isincreased, thereby, bigger particles are deposited. Moreover,fast deposition rate allow little time for the particles to rear-range, as a result, quite some bigger porosities are formed inthe microstruture.

It has been reported that a macroporous HA layer has asimilar crystalline morphology to that of bone apatite, so that,the layer maintains a high potential for accommodating tobone but low cohesive strength.13–18 In this study, interestingcontraries are found on the coatings, those obtained at aconstant low voltage of 20 V produced high bond strength onTi substrates but low biocompatibility, while the coatingsobtained at a constant high voltage of 200 V yield lowadhesion to Ti substrates but satisfactory osteoinduction tobone. In an attempt to optimize the coating property, appli-

Figure 2. Surface morphology of the coating prepared by the dynamic voltage: (a) is the intermediatecoating finished the first step, (b) is the intermediate coating finished the second step, (c) is the finalcoating. The magnification is 50,000�.


375HA COATING OF Ti BY EPD

cation of dynamic voltage was evaluated in this study. It wasobserved that small particles built up in the inner layerdensely and uniformly [Fig. 2(a,b)] and bigger particlesformed the outer porous layer [Fig. 2(c)]. This graded particlesize and difference in coating density on the inner and outerlayer were achieved as a result of electrophoretic velocity andits dependence on electric field and the particle size. It can bededuced from Figure 2 and Eq. (1) that a continuously gradedHA coating was obtained using dynamic voltage.

Theories related to Eq. (1) helped explain the phenomenonobserved in this study, which told the production of a porousand rough coating at a higher constant electric field, and theproduction of a dense coating of finer particle size at a lowerconstant electric field. In addition, using a dynamic voltage, acontinuous gradient HA coating was prepared.

HA coatings obtained on Ti with various applied voltageswere sintered at 800°C for 2 h. Figure 3 shows the superficialcharacteristics of the sintered coatings. Big deposited parti-cles and higher porosity between particles in green coatingsare responsible for the cracks and big particles in the sinteredcoating prepared at a constant voltage of 200 V. The coatingsobtained through dynamic voltage were observed to be free ofcracks. Good packing of submicron particles obtained atdynamic voltage gets the credit.

It is also noted that high temperature treatment may lead tothe deterioration of desirable HA structures and thereby mayreduce the biocompatibility of the material.19–21 In this study,all coatings showed well-defined peaks of HA (Fig. 4). Theblack squares in the figure indicated standard HA peaks, withno decomposition of HA observed. It was hence suggestedthat EPD is a good processing method to form a HA coating.

CONCLUSIONS

Various microstructures of HA coatings on Ti can be pre-pared by EPD, using constant applied voltage and dynamicapplied voltage. The HA coating prepared at a constant lowvoltage of 20 V consisted of fine HA particles and was dense.The HA coating prepared at a constant high voltage of 200 Vconsisted of big HA particles and was porous. The HA

Figure 3. Surface morphology by FE-SEM of sintered HA coatings at 800°C, obtained by: (a) constantvoltage 20 V, (b) constant voltage 200 V, and (c) dynamic voltage 0–200 V. The magnification is2000�.

Figure 4. XRD patterns of HA coatings on Ti at dynamic voltage aftersintered: (f) HA, (E) rutile.


376 MENG ET AL.

coating prepared at a dynamic voltage consisted of continu-ously gradient particles, with small particles building up theinner layer while bigger particles forming the outer layer. Thedeposits prepared using a dynamic voltage followed by sin-tering at 800°C turned out a high packing of the HA particles.In addition, coatings retained HA structure after sintering.

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377HA COATING OF Ti BY EPD

Effects of Applied Voltages on Hydroxyapatite Coating of Ti by EPD

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