Fabrication of compositionally and structurally gradedTi–TiO2 structures using laser engineered net shaping (LENS)

Vamsi Krishna Balla, Paul Duteil DeVasConCellos, Weichang Xue,Susmita Bose, Amit Bandyopadhyay *

W. M. Keck Biomedical Materials Research Laboratory, School of Mechanical and Materials Engineering, Washington State University,

Pullman, WA 99164-2920, USA

Received 27 August 2008; received in revised form 3 December 2008; accepted 6 January 2009

Available online 22 January 2009

Abstract

Novel structures with functional gradation in composition and structure were successfully made in Ti–TiO2 combination using laserengineered net shaping. The addition of fully dense, compositionally graded TiO2 ceramic on porous Ti significantly increased the surfacewettability and hardness. The graded structures with varying concentrations of TiO2 on the top surface were found to be non-toxic andbiocompatible. In addition, the higher wettability of surfaces with TiO2 can enhance their ability to form chemisorbed lubricating films,which can potentially lower the friction coefficient against ultrahigh molecular weight polyethylene liner, thus reducing its wear rate.These unitized structures with open porosity on one side and hard, low friction surface on the other side can eliminate the need for multi-ple parts with different compositions for load-bearing implants such as total hip prostheses.Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Functionally graded materials; Laser processing; Laser engineered net shaping (LENS); Biocompatibility; Osteoblast

1. Introduction

The high wear rate of ultrahigh molecular weight poly-ethylene (UHMWPE) liner used in traditional hip replace-ments is a cause of serious concern due to osteolysis.Osteolysis and aseptic loosening has been identified asmajor factors limiting the life of hip prostheses, with indi-cations that fine UHMWPE wear debris [1,2], generatedprimarily at the interface between the femoral head andthe acetabular cup, promotes this degradation. Thus,wear-particle-induced bone loss is one of the main limitingfactors affecting the long-term stability of UHMWPE liner-based total hip replacements and other load-bearingimplants [3,4]. Several attempts have been made to mini-mize the wear-induced osteolysis including the use ofdesign modifications [5], UHMWPE property modification

[6] and alternative bearing couples, such as metal-on-metaland ceramic-on-ceramic, eliminating the use of PE [7,8].Design modifications can only change the contact stressesand related fatigue wear of the UHMWPE; modificationof UHMWPE properties did not yield significant improve-ment in wear performance [9]. In addition, roughening andscratches on hard counterface of the femoral componentscan increase UHMWPE wear via adhesive and abrasivewear mechanisms [10,11]. An alternative and more effectivesolution is to modify the hard counter surface articulatingwith UHMWPE to reduce its wear and debris generation.Therefore, a hard counter surface providing low frictionwith UHMWPE and resisting roughening can reduce abra-sive and adhesive wear, and thereby enhances the long-term stability and survival of load-bearing metal implants.Hard ceramic surfaces appear to accomplish this long-termperformance goal most effectively [12] because of theirhigher wettability than metals/alloys. Among the ceramicsTiO2, Al2O3 and ZrO2 have highly ionic character, there-fore have higher wettability [13] than the passive oxide

1742-7061/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.actbio.2009.01.011

* Corresponding author.E-mail address: amitband@wsu.edu (A. Bandyopadhyay).

Available online at www.sciencedirect.com

Acta Biomaterialia 5 (2009) 1831–1837

www.elsevier.com/locate/actabiomat

films on stainless steels or CoCrMo alloys used for biomed-ical applications. Also, these passive oxide films on metals/alloys can break off from the substrate during articulationand do not provide sufficient wear resistance or low frictionagainst UHMWPE. It has been reported that lubricatingcompounds, such as water, readily chemisorbs to ceramicssurface than to metals and these films can effectively reducethe friction coefficient and hence the wear of UHMWPEdue to their lubrication characteristics [13–15]. Also, suchadsorption activities were found to increase with ioniccharacter of ceramics [13]. Therefore, a ceramic coatingon biocompatible metals such as Ti can provide a favorablearticulation surface with low friction coefficient and highwear resistance against UHMWPE to reduce its wear rate.

Recently there is a growing interest in the fabrication ofnovel structures to mimic multiple tissues and tissue inter-faces on the same implant, such as implants with gradientsin porosity and pore sizes that will allow on one side of theimplant high vascularization and direct osteogenesis, whilepromoting osteochondral ossification on the other. In thecurrent context, innovative designs such as functionallygraded acetabular shells with open porosity on one side(in contact with the bone) to improve cell–material interac-tions and a hard ceramic coating on the other side (in con-tact with UHMWPE liner) to reduce liner wear, cansignificantly improve the implant’s in vivo life. While creat-ing a hard/low friction ceramic surface on metal substratesseems plausible, there is a possibility of delamination orcracking due to the mismatch in elastic moduli, coefficientof thermal expansion and hardness between the ceramiccoating and the metal substrate. Although functionallygraded coatings (FGCs) can overcome such difficulties, itis difficult, if not impossible, to fabricate net shapeimplants/structures with spatial gradation in compositionand structure (porosity) with conventional processingroutes. Therefore, in this work we have used laser engi-neered net shaping (LENSTM)—a solid freeform fabricationtechnique—to fabricate compositionally and structurallygraded Ti–TiO2 structures with sound interface betweenthe two materials and high surface hardness. Such gradientstructures provide useful mechanical support for the wear-resistant, low friction exterior layers and minimize the like-lihood of localized Hertzian failure during implant service.This paper focuses on processing, coating characterizationand in vitro biocompatibility of these LENSTM-processedfunctionally/structurally graded structures.

2. Materials and methods

2.1. Fabrication and characterization

Commercially pure titanium powder (Advanced Spe-cialty Metals, Inc., NH) with particle size between 50 and150 lm and 99% pure TiO2 powder (CERAC Inc., Mil-waukee, WI) with particle size between 45 and 106 lmwas used in this study. Gradient samples with 10 mm diam-eter were fabricated on a substrate of 3 mm thick rolled,

commercially pure Ti plates using LENSTM-750 (OptomecInc., Albuquerque, NM) equipped with a 500 W Nd–YAG laser and double powder feeder system. The firsthopper was filled with Ti powder and the second hopperwith TiO2 powder. Gradient structures with 50%, 90%and 100% TiO2 on the top surface were made at 300 Wlaser power and 22.5 mm sÿ1 scan speed. These composi-tionally graded structures consisted of 100% Ti in the first10 layers. The composition in the transition region of thegradient structure was varied from 100% Ti at the firstlayer to various concentrations of TiO2 at the top layerover 5–10 layers. The first 10 layers of Ti were made at200 W to create �30 vol % porosity, which can achieve sta-ble long-term fixation due to bone cell in-growth throughinterconnected porosity [16]. The transition layers weremade at 300 W to achieve fully dense transition/top layers.In the transition zone, the powder feed rate for TiO2

increased from 0 to 23.7 g minÿ1 over the number of tran-sition layers. For Ti it was 17.5 g minÿ1 to start with andreduced to zero over the same number of layers. Thesecompositionally graded structures with gradient porosityhaving open porosity on one side of the structure canimprove cell–material interactions [16,17] and hard, low-friction coating on the other side can decreases the wearof UHMWPE liner. All samples were fabricated in a glovebox containing argon atmosphere with O2 content less than10 ppm to limit oxidation of alloys during processing.

Cross-sectional microstructures of the samples wereexamined using both optical and scanning electron micros-copy (SEM). Constituent phases in the as-received TiO2

powder and in the top surface of graded coatings were iden-tified using a SiemensD500Kristalloflex diffractometerwithCu Ka radiation. In order to track the compositional gradi-ent across the deposit, a series ofmicrohardness indentationswere placed fromone end of the deposit to the other endwithneighboring indents being separated by 0.2 mm using Vick-ers microhardness tester (Leco, M-400G3) at 300 g loadapplied for 20 s. Top surface hardness was also measured,and an average of 10 measurements on each sample isreported.

2.2. Simulated body fluid (SBF) study

Bioactivity of compositionally/structurally gradedTi–TiO2 structures was evaluated by immersing in SBFwith similar ionic composition as that of human bloodplasma, and compared with that of laser-processed pureTi control sample. The SBF solution is prepared by dissolv-ing NaCl, KCl, NaHCO3, MgCl2�6H2O, CaCl2�2H2O,Na2SO4� 10H2O and K2HPO4 into distilled water and buf-fered at pH 7.35 with tris-hydroxymethyl aminomethane(TRIS) and 1 (N) HCl at 37 °C [18]. The ion concentrations(mmol Lÿ1) in SBF thus prepared was Na+: 142.0, K+: 5.0,Ca2+: 2.5, Mg2+: 1.5, HCO3

ÿ: 4.2, Clÿ: 148.5, HPO42ÿ: 1.0

and SO42ÿ: 0.5. Samples were immersed in a glass vial con-

taining 10 ml of SBF solutions and were kept under staticconditions inside a incubator at 37 °C for 7 and 14 days.

1832 V.K. Balla et al. / Acta Biomaterialia 5 (2009) 1831–1837

The SBF solution was changed every 2–3 days for the dura-tion of the experiment. All experiments were performed induplicate, by running two independent glass vials simulta-neously. After exposure, samples were washed with dis-tilled water and then dried at 150 °C for 24 h. Surfacefeature of samples after SBF immersion were studied usinga SEM. SBF-immersed samples were also analyzed by Fou-rier transform infrared (FTIR) spectroscopy (Nicolet 6700,Thermo Fischer Scientific) to identify functional groups ofapatite precipitations on the surface.

2.3. Contact angle measurement

All the sample surfaces for contact angle measurementswere ground using series of SiC grinding papers with var-ious sizes up to 1200 grit. Samples were then polished onvelvet cloth using 1 lm Al2O3 suspended in distilledwater. Finally, just before testing, all the samples wereultrasonically cleaned in alcohol bath. This procedureensured identical surface roughness and topography onall sample surfaces. Contact angles were measured usingthe sessile drop method with a face contact angle set-upequipped with a microscope and a camera. A 0.5–1.0 lldroplet of distilled water was suspended from the tip ofthe microliter syringe. The sample surface was advancedtoward the syringe tip until the droplets made contactwith the sample surface. Images were collected with thecamera and the contact angle between the drop and thesubstrate was measured from the magnified image. Allthe measurements were carried out at 21 °C with a rela-tive humidity of 21%.

2.4. Cell culture and morphology of osteoblastic precursor

cell line 1 (OPC1)

All samples for cell culture test were sterilized by auto-claving at 121 °C for 20 min. In this study, the cells usedwere an immortalized, cloned OPC1, which was derivedfrom human fetal bone tissue [19]. OPC1 cells were seededonto the samples placed in 24-well plates. Initial cell densitywas 5.0 � 104 cells/well. One milliliters of McCoy’s 5Amedium (enriched with 5% fetal bovine serum, 5% bovinecalf serum and supplemented with 4 lg mlÿ1 of fungizone)was added to each well. Cultures were maintained at 37 °Cunder an atmosphere of 5% CO2. Medium was changedevery 2–3 days for the duration of the experiment. Samplesfor testing were removed from culture at 4, 7 and 11 daysof incubation.

All samples for SEM observation were fixed with 2%paraformaldehyde/2% glutaraldehyde in 0.1 M cacodylatebuffer overnight at 4 °C. Post-fixation was performed with2% osmium tetroxide (OsO4) for 2 h at room temperature.The fixed samples were then dehydrated in an ethanol ser-ies (30%, 50%, 70%, 95% and 100% three times), followedby a hexamethyldisilane (HMDS) drying procedure. Aftergold coating, the samples were observed under SEM forcell morphologies.

3. Results and discussion

3.1. Microstructures and phase analysis

A representative cross-sectional microstructure ofgraded Ti–TiO2 structure is shown in Fig. 1. The Ti–TiO2

graded structures exhibited good bonding between individ-ual layers without gross porosity, cracks or lack of fusiondefects. This indicates that laser parameters used in thepresent work are sufficient to ensure complete meltingand uniform mixing of TiO2 and Ti powders. However,severe cracking was observed in the structures with 100%TiO2 on the top surface and the cracking was reduced withdecreasing TiO2 concentration. Therefore, further charac-terization and testing was not performed on the sampleswith 100% TiO2 on the top surface. Using optimizedLENSTM processing parameters crack-free structures con-taining up to 90% TiO2 on the top surface were successfullyfabricated with excellent reproducibility. Interface betweenporous Ti and graded TiO2 top surface, shown in Fig. 2,also indicate complete melting of Ti and TiO2, ensuringsound interface without cracks or porosity. Also, nounmelted or partially melted powders were observedthroughout the structure’s cross-section. This indicatessmall temperature fluctuations in the liquid metal pooldue to changes in the composition across the samplecross-section. Large temperature fluctuations can result innon-uniform or inadequate intermixing of powders in thetransition region of such graded structures [17]. In the pres-ent work, the small temperature fluctuations could be dueto small difference between melting points of Ti (1668 °C)and TiO2 (1800 °C), and their laser absorption coefficients.

Top surface microstructural variation as a function ofTiO2 concentration of graded structures is shown inFig. 3. The top surface microstructures did not show clearcontrast between Ti and TiO2 phases. However, it can beseen from Fig. 3a and b that the amount of Ti phase (phasewith clear grain boundaries and grains with b needles)decreased with increasing TiO2 concentration. Lightlyetched, fine TiO2 grains in the range of 6–8 lm (measuredusing linear intercept method) can be seen from Fig. 3b.Overall scale of the solidification structure in these gradedcoatings is very fine, reflecting the extremely high coolingrates, in the range of 103–105 K sÿ1, associated withLENSTM process. This is a significant advantage of LENSTM,as the slow cooling rates associated with other processingtechniques often result in coarse-grained structures andresults in poor wear resistance. The Ti phase in Fig. 3a isfound to contain some high temperature b phase (needles).The retention of b phase at room temperature is attributedto the high cooling rates associated with laser processing.However, the amount of b phase observed in the presentsamples was considerably lower than observed in our ear-lier study [20] on LENSTM-processed porous Ti, where aconsiderable amount of b phase is retained at room tem-perature. The lower amount of b phase in the present sam-ples is presumably due to stabilization of a phase by the

V.K. Balla et al. / Acta Biomaterialia 5 (2009) 1831–1837 1833

oxygen present in top surface with varying concentrationsof TiO2. In addition, the ratio of b phase peak intensityto the a phase peak intensity found to decrease from0.252 in the as-received powder to 0.102 in the gradedstructure with 50% TiO2 on top surface. This indicates thatthe amount of b phase present in the as-received powderwas reduced due to laser processing. The presence of b

phase in the top surface of these samples was also con-firmed by XRD study. Fig. 4 presents the XRD results ofas-received powders and laser-processed gradient struc-tures. All the peaks from top surface of gradient structurescorrespond to as-received powders of Ti (JCPDS file No.011197) or TiO2 (JCPDS file No. 881172).

In order to systematically track the compositional gradi-ent across the deposit, a series of microhardness indentmarkers were placed from one end of the transition regionto the other end. The hardness gradually increased with theincrease in TiO2 concentration as shown in Fig. 5. Gradualincrease in the hardness from Ti towards the top TiO2 coat-ing validates the potential of LENSTM process in creatingunique microstructural gradients across the composition-ally graded surfaces/coatings, which are difficult to processusing traditional manufacturing routes. The hardness ofthe top surface increased from 1102 ± 140 Hv to 1122 ±116 Hv when the TiO2 concentration at the top surfacewas increased from 50% to 90% in the coating. This indi-cates that the concentration of TiO2 above 50% had little

influence on the top surface hardness. These top surfacehardness values are significantly higher than the averagehardness of the laser-deposited Ti, which was 192 ±14 Hv. Gradient coatings with only 50% TiO2 in top sur-face showed �474% increase in the surface hardness. Thefiner grain size, uniform microstructure and high hardnessof this laser-processed gradient coating can potentially pro-vide excellent wear resistance. Moreover, the porosity onthe Ti side, which will be in contact with bone, can improvecell–material interactions [16]. It is anticipated that hardTiO2 ceramic coating on the other side can decrease thewear of UHMWPE due to its higher wettability and chem-isorbed lubricating films [13–15]. Therefore, these composi-

Fig. 1. Typical cross-sectional microstructures of laser-processed, compositionally graded TiO2 coating on porous on Ti.

Fig. 2. Microstructure showing the interface between porous Ti and

graded TiO2 coating.

Fig. 3. Top surface microstructures of graded TiO2 coatings with varying

TiO2 concentration on top surface: (a) 50% TiO2, (b) 90% TiO2.

1834 V.K. Balla et al. / Acta Biomaterialia 5 (2009) 1831–1837

tionally/structurally graded Ti–TiO2 structures could pro-vide a favorable articulation surface with low friction coef-ficient and high wear resistance against UHMWPEreducing its wear rate.

3.2. SBF study

For comparison of bioactivity in terms of apatite-form-ing ability, laser-processed graded structures and Ti sam-ples were immersed in SBF for 7 and 14 days. Fig. 6shows surface micrographs of these samples after 14 daysof immersion in SBF. It can be seen from Fig. 6a thatlaser-processed pure Ti control sample surface has low apa-tite-forming ability than graded surfaces with various con-centrations of TiO2 on top. Also, significant amount ofapatite precipitation was noticed after 7 days on these com-positionally graded surfaces. The FTIR results of these pre-

cipitates, shown in Fig. 7, indicate the presence of HPO42ÿ,

PO43ÿ, OHÿ and CO3

2ÿ groups. The enhanced apatite-forming ability of TiO2 containing surface can be explainedon the basis of several reported models for apatite deposi-tion on the TiO2 surface [21,22]. The Ti–OH groups formedon the TiO2 film, upon immersion in aqueous solutions, actas preferential nucleation sites for apatite crystals andfavor deposition/precipitation on the surface. Therefore,the TiO2 in the graded structures surface enhances apatiteformation.

3.3. Contact angle

While TiO2 in the surface of graded structures greatlyimproved the apatite precipitation, it can also significantlyalter surface properties like wettability and surface energy,which has strong influence not only on cell–material inter-

20 30 40 50 60 70 80

Two Theta

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2(3

01

)

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Ti(

100

)

d

Fig. 4. XRD pattern of TiO2 powders and top surface of graded TiO2 coatings: (a) TiO2 powder; (b) coating with 90% TiO2 on top surface; (c) coating

with 50% TiO2 on top surface; (d) Ti powder.

100

300

500

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1100

1300

-4 -3.6 -3.2 -2.8 -2.4 -2 -1.6 -1.2 -0.8 -0.4 0 0.4 0.8

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Vic

ker

's H

ard

nes

s, H

V

50% TiO2 Top (Surface Hardness 1102±140 HV)

90% TiO2 Top (Surface Hardness 1121±116HV)

Titanium← increasing TiO2 concentration

Fig. 5. Hardness variation across the graded TiO2 coatings.

V.K. Balla et al. / Acta Biomaterialia 5 (2009) 1831–1837 1835

actions but also on the ability to form chemisorbed lubri-cating films. The contact angle between water and the Tisurface decreased from 62 ± 3° to 41 ± 4° due to TiO2 inthe surface of graded structures indicating that the Ti sur-face is more hydrophilic in nature. Lower contact anglealso suggests higher surface energy of graded surfaces withvarying TiO2 concentrations. The higher hydrophilicityand surface energy of these surfaces improved the apa-tite-forming ability in SBF. Besides, the higher wettabilityof surfaces with TiO2 can enhance their ability to formchemisorbed lubricating films thus lowering friction coeffi-cient against UHMWPE. Similar observations on theincrease of wettability and surface energy due to surfaceoxide layers have already been reported [23,24].

3.4. Cell–material interactions

Cell–material testing is focused on cytotoxicity analysisof these LENS-processed structures to confirm that LENS

processing does not have any toxic influence to these struc-tures. The higher wettability of TiO2 containing surfacesfound to influence the OPC1 cell attachment. Fig. 8 showsthe morphology of OPC1 cells on graded Ti–TiO2 structuresurfaces after 7 days of culture. Flattened cells with little fil-opodia extensions were observed on laser-processed pureTi samples. In contrast, the cells had numerous filopodiaextensions on TiO2 containing surfaces. Identical cell mor-phology was observed on the surface with varying concen-trations of TiO2 on the top surface, which spread and grewwell on these sample surfaces. Enhanced osteoblast cellattachment and spreading on TiO2 containing surfaces isattributed to the higher bioactivity and wettability of thesesurfaces compared to that of pure Ti surfaces. Thesein vitro results confirm that these compositionally andstructurally graded samples are non-toxic and biocompati-ble even after LENS processing.

Graded structures fabricated in this work have thepotential to replace conventional multi-piece hip implantsby introducing monoblock structures. With these struc-tures, acetabular shells can be designed with porosity inone side for bone tissue in-growth and the hard ceramiccoating on the other side to reduce liner wear. This config-uration can reduce the standard two-piece acetabular cupinto a one-piece design. With further modification, theUHMWPE liner part can also be removed, making it adirect ceramic-on-ceramic implant. Such monoblockdesigns reduce the possibilities of osteolysis from polymericdebris and also make the surgical procedure easier.

4. Conclusions

Novel, unitized structures with porous Ti on one sideand compositionally graded, hard TiO2 surface on theother side have been fabricated using LENSTM process.Gradient structures with 50% TiO2 in top surface showeda hardness of 1102 ± 140 Hv, which is approximately fourtimes higher than the hardness of laser-processed Ti. More-over, these gradient coatings decreased the contact angle by�34% indicating their enhanced wettability, cell–materialinteractions and lubrication ability. Our results indicate

Fig. 6. Apatite precipitates after 14 days of immersion in SBF: (a) Ti control; (b) 50% TiO2 on top; (c) 90% TiO2 on top surface.

θ

θ

Θ – CO 32-

-- PO 43-

66

9 c

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49

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90% TiO 2 on top

50% TiO 2 on top

-- OH-

3500 3000 2500 2000 1500 1000 500

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nsm

itta

nce

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.)

Wavenumbers (cm-1)

θ

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-- PO 43-

66

9 c

m-1

49

5 c

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10

66 c

m-1

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58 c

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16

52 c

m-1

90% TiO 2 on top

50% TiO 2 on top

-- OH-

▲

▲

▲

▲

▲

▲

▲

Fig. 7. FTIR spectra of graded TiO2 surfaces after immersion into SBF

for 14 days.

1836 V.K. Balla et al. / Acta Biomaterialia 5 (2009) 1831–1837

that compositionally/structurally graded Ti–TiO2 struc-tures with 50% TiO2 on top can provide a favorable artic-ulation surface with low friction coefficient againstUHMWPE reducing its wear rate.

Acknowledgments

The authors would like to acknowledge the financialsupport from the W. M. Keck Foundation to establish aBiomedical Materials Research Lab at WSU; also the Of-fice of Naval Research for financial support under GrantNo. N00014-1-05-0583. Authors would also like toacknowledge the financial support from the National Sci-ence Foundation under the grant number CMMI 0728348.

References

[1] Cooper RA, McAllister CM, Borden LS, Bauer TW. Polyethylene

debris-induced osteolysis and loosening in uncemented total hip

arthroplasty. J Arthroplasty 1992;7:285–90.

[2] Howie DW, Rogers SD, Haynes DR, Pearcy MJ. The role of small

polyethylene wear particles in inducing the release of mediators known

to stimulate bone resorption. Trans Orthop Res Soc 1994;19:201–8.

[3] Schmalzried TT, Kwong LM, Jasry M, Sedlacek RC, Haire TC,

Connor DO. The mechanism of loosening of cemented acetabular

components in total hip arthroplasty. Clin Orthop Rel Res

1992;274:60–78.

[4] Edidin AA, Rimnac CM, Goldberg VM, Kurtz SM. Mechanical

behaviour, wear surface morphology, and clinical performance of

UHMWPE acetabular components after 10 years of implantation.

Wear 2001;250:152–8.

[5] Bartel DL, Bicknell VL, Wright TM. The effect of conformity,

thickness, and material on stresses in ultrahigh molecular weight

components for total joint replacement. J Bone Joint Surg

1986;68A:1041–51.

[6] Ries MD, Scott ML, Jani S. Relationship between gravimetric wear

and particle generation in hip simulators: conventional compared

with cross linked polyethylene. J Bone Joint Surg 2001;83A(Suppl. 2):

116–22.

[7] Rieker C, Kottig P. In vivo tribological performance of 231 metal-on-

metal hip articulations. Hip Int 2002;12:73–6.

[8] Douglas N, Christopher PR, Javad P, Daniel JB, Stefan E, Andre B.

Metal-on-metal versus metal-on-polyethylene bearings in total hip

arthroplasty: a matched case-control study. J Arthroplasty 2004;19(7,

Suppl. 2):35–41.

[9] Davidson JA, Poggie RA, Mishra AK, Salehi A, Harbaugh M.

Increased wear and contact stress from increased polyethylene

stiffness in prosthetic knee and hip articulation. In: Goh J, Nather

A, editors. Proceedings of the 7th international conference on

biomedical engineering, Singapore; 1992. p. 152–4.

[10] Fisher J, Firkins P, Reeves EA, Hailey JL, Isaac GH. The influence of

scratches to metallic counterfaces on the wear of ultra-high molecular

weight polyethylene. Proc Inst Mech Eng [H] 1995;209:263–4.

[11] Dowson D, Taheri S, Wallbridge NC. The role of counterface

imperfections in the wear of polyethylene. Wear 1987;119:

277–93.

[12] Davidson JA. Characteristics of metal and ceramic total hip bearing

surfaces and their effect on long term ultra high molecular weight PE

wear. Clin Orthop 1993;294:361–78.

[13] Mori S. Adsorption and lubrication. J Tribol 1991;36:161.

[14] Gates R, Hsu S, Klaus E. Tribochemical mechanism of alumina with

water. STLE Trib Trans 1989;32:357.

[15] Semlitsch M, Tehmann M, Weber H, Dorre E, Willert HG. New

prospects for a prolonged functional life-span of artificial hip joints

by using the material combination polyethylene/aluminium oxide

ceramic/metal. J Biomed Mater Res 1977;11:537–52.

[16] Xue W, Balla VK, Bandyopadhyay A, Bose S. Processing and

biocompatibility evaluation of laser processed porous titanium. Acta

Biomater 2007;3:1007–18.

[17] Vamsi Krishna B, Xue Weichang, Bose Susmita, Bandyopadhyay

Amit. Functionally graded Co–Cr–Mo coating on Ti–6Al–4V alloy

structures. Acta Biomater 2008;4:697–706.

[18] Kokubo T, Kushitani H, Sakka S, Kitsugi T, Yamamuro T. Solutions

able to reproduce in vivo surface-structure changes in bioactive glass-

ceramic A-W3. J Biomed Mater Res 1990;24:721–34.

[19] Winn SR, Randolph G, Uludag H, Wong SC, Hair GA, Hollinger

JO. Establishing an immortalized human osteoprecusor cell line:

OPC1. J Bone Miner Res 1999;14:1721–33.

[20] Vamsi Krishna B, Bose S, Bandyopadhyay A. Low stiffness porous Ti

structures for load-bearing implants. Acta Biomater 2007;3:

997–1006.

[21] Jonasova L, Muller FA, Helebrant A, Strnad J, Greil P. Biomimetic

apatite formation on chemically treated titanium. Biomaterials

2004;25:1187–94.

[22] Li PJ, Ohtsuki C, Kokubo T, Nakanishi K, Soga N, Degroot K. The

role of hydrated silica, titania, and alumina in inducing apatite on

implants. J Biomed Mater Res 1994;28:7–15.

[23] Hao L, Lawrence J. Effects of Nd:YAG laser treatment on the

wettability characteristics of a zirconia-based bioceramic. Opt Lasers

Eng 2006;44:803–14.

[24] Hobbs LW, Benezra Rosen V, Mangin SP, Treska M, Hunter G.

Oxidation microstructures and interfaces in the oxidized zirconium

knee. Int J Appl Ceram Technol 2005;2(3):221–46.

Fig. 8. SEM micrographs of OPC1 cells after 7 days of culture: (a) Ti control; (b) 50% TiO2 on top; (c) 90% TiO2 on top surface.

V.K. Balla et al. / Acta Biomaterialia 5 (2009) 1831–1837 1837