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Acta Biomaterialia 4 (2008) 1553–1559

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Initial exploration of Ti–Ta, Ti–Ta–Ir and Ti–Ir alloys:Candidate materials for coronary stents

Barry O’Brien a,*, Jon Stinson b, William Carroll a

a National Centre for Biomedical Engineering Science, National University of Ireland Galway, Irelandb Boston Scientific Corporation, One Scimed Place, Maple Grove, MN 55311, USA

Received 19 October 2007; received in revised form 8 February 2008; accepted 6 March 2008Available online 20 March 2008

Abstract

The objective of this study was to explore titanium alloys with increased elastic modulus and improved radiopacity, with a view toutilizing titanium in balloon-expandable coronary stents. Ti–50Ta, Ti–45Ta–5Ir and Ti–17Ir alloys were prepared using arc-melting tech-niques. Microstructural and tensile properties were evaluated in solution-treated conditions for each alloy, and also in aged conditionsfor the Ti–17Ir. An elastic modulus of 128 GPa was recorded for the Ti–17Ir alloy and this high value is attributed to the stiff Ti3Ir phasepresent in the eutectoid structure observed. The mechanical properties recorded, in addition to improved radiopacity, make the Ti–17Iralloy more suitable for stent applications than commercially available titanium materials. Corrosion resistance and biocompatibility havenot been assessed but the noble characteristics of iridium suggest that these aspects will be acceptable.� 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Titanium; Elastic modulus; Stent; Iridium; Eutectoid

1. Introduction

Titanium alloys have now built up a long history of suc-cessful applications in medical devices, mainly attributableto their excellent corrosion resistance and superior biocom-patibility. Usage has been primarily for orthopaedicimplants where the relatively low elastic modulus of tita-nium, compared to stainless steel or cobalt–chromiumalloys, is a further significant benefit. In these applications,it is desirable to match the stiffness of the implant materialas close as possible to that of adjacent bone, in order tominimize stress shielding and bone resorption. Table 1shows some typical tensile property data for pure titaniumand Ti–6Al–4V in comparison to 316L stainless steel andcobalt–chromium alloy MP35N [1–3]. The advantage ofthe titanium materials from an elastic modulus perspectivecan be further realized when these values are comparedagainst an elastic modulus value of 10–40 GPa for bone

1742-7061/$ - see front matter � 2008 Acta Materialia Inc. Published by Else

doi:10.1016/j.actbio.2008.03.002

* Corresponding author. Tel.: +353 87 2934292.E-mail address: J.OBrien9@nuigalway.ie (B. O’Brien).

[4,5]. However, despite their widespread use for orthopae-dic implants, titanium alloys have not been fully exploitedin the area of coronary stents. Titanium is a constituent ofnickel–titanium alloys, which are widely used for periphe-ral vascular stents, but in this context it is the shape mem-ory and superelastic characteristics of the material that areprimarily of interest [6].

There are a number of reasons why titanium alloys havenot been used in the manufacture of balloon-expandablecoronary stents. Foremost amongst these is in fact thelow elastic modulus, which is so attractive for other appli-cations. This low elastic modulus combined with a shorterplastic deformation range, compared to stainless steels orcobalt–chromium alloys, leads to high elastic recoil afterstent deployment and also increases the risk of tensile frac-tures during deployment. Another significant challenge isthe low radiopacity of titanium alloys. During implanta-tion, the location and deployment of coronary stents ismonitored using X-ray fluoroscopy. The visibility of thestent is therefore dependent on the material’s abilityto attenuate X-rays, and in the case of titanium this

vier Ltd. All rights reserved.

Table 1Tensile mechanical properties for Ti materials compared to stainless steel and cobalt–chromium alloys

Material Ultimate tensile strength (MPa) 0.2% yield strength (MPa) Elongation (%) Elastic modulus (GPa) References

Ti 240–331 170–241 30 103 [1]Ti–6Al–4V 900–993 830–924 14 114 [1]316L 595 275 60 193 [2]MP35N 930 414 45 233 [3]

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attenuation is much lower than for traditional stainlesssteel or cobalt–chromium alloys. Finally, the compositionof the workhorse Ti–6Al–4V material itself also poses achallenge. While Ti–6Al–4V was considered to be accept-able as a first-generation implantable titanium material,an increasing awareness of the effects of metal ion leachinghas resulted in some concern about various metals, includ-ing vanadium and aluminium [7].

Although its usage in coronary stents may have beeninhibited, successful vascular system applications for tradi-tional titanium alloys have included inferior vena cava fil-ters [8] and intracranial aneurysm clips [9]. While themechanical requirements for these devices differ fromstents, the introduction of titanium aneurysm clips isdirectly attributable to the material’s low magnetic suscep-tibility; this results in improved safety and reduced artefact,compared to stainless steel and cobalt–chromium alloys,during magnetic resonance imaging (MRI) [10]. This bene-fit of improved MR safety and reduced artefact is now alsobecoming of interest in the field of coronary stenting, dueto the increased use of MRI procedures and the continuedgrowth in stent implantations. With the low magnetic sus-ceptibility of titanium in mind, this paper reports on an ini-tial study into the possibility of increasing both radiopacityand elastic modulus in titanium alloys, with a view toachieving properties suitable for coronary stentapplications.

2. Titanium alloy systems and selection

Titanium alloys are generally categorized according tothe crystal structure that is present at room temperature.Pure titanium has a hexagonal close-packed (hcp) structureup to 882 �C, designated as the a phase. Above this temper-ature it transforms to the body-centred cubic (bcc) struc-ture, designated as the b phase; the transformation pointis known as the b transus temperature. The addition of fur-ther elements to pure titanium will then either increase ordecrease this transus temperature such that the b phase,or additional phases, may be present at room temperature.The Ti–6Al–4V workhorse alloy was initially developed toprovide increased strength over pure titanium, primarily inaerospace applications. The structure that results, andtherefore the alloy classification is a + b, due to the factthat aluminium is an a-stabilizing element and vanadiumis b-stabilizing. In Ti–6Al–4V, the majority of the a + bstructure is typically a phase [11].

The desire to further improve biocompatibility, and alsoto reduce the elastic modulus to be closer to that of bone,stimulated further development of new alloy systems, andin particular, exploration of b-type alloys has increased inrecent years. These materials have sufficient b-stabilizingelements added to ensure that a fully b-phase structureexists at room temperature after fast cooling from abovethe b transus. This structure is typically in a metastablecondition, and aging treatments are then used for strength-ening. In general, b alloys provide good formability com-bined with a wide range of possible strength levels, butmost importantly from an orthopaedic implant perspective,b alloys typically have a lower elastic modulus than puretitanium (103 GPa) or Ti–6Al–4V (114 GPa). One of thefirst such alloys to be commercially developed was Ti–12Mo–6Zr–2Fe, also known as TMZF, which has an elas-tic modulus of approximately 80 GPa and has widelyaccepted biocompatibility [12]. Among the many other balloys are Ti–15Mo with a modulus of 78 GPa [13] andTi–35Nb–5Ta–7Zr with one of the lowest reported modu-lus values of 55 GPa [4].

As indicated earlier, in the current study the objectivewas to explore the possibility of increasing both the radio-pacity and elastic modulus of titanium. One of the firstselection criteria for alloying additions was that theyshould be b-stabilizers; the presence of b phase is desiredin order to confer improved ductilty and formability onthe material. Secondly, the additions should have highradiopacity and be biocompatible. All these requirementsare readily met by the widely used elements niobium, tan-talum, molybdenum and iron. However, as indicatedabove, virtually all of the b alloys with these additions haveresulted in a material with reduced elastic modulus. Whilethe contribution of each element towards stabilizing bphase is understood, the impact that various additions haveon elastic modulus is more complicated, as it depends onsubsequent heat treatments and precipitation reactions[14]. As an initial approach, it was decided to explore theaddition of elements which had high density, high elasticmodulus and high melting points. This was on the rationalethat the smaller inter-atomic spacing of higher-density sin-gle-phase materials generally results in high elastic modu-lus. Table 2 shows these characteristics and the X-rayattenuation values for some of the main candidate ele-ments. Niobium and molybdenum have been ruled outdue to their relatively low radiopacity (X-ray attenuation),bearing in mind the need for significant enhancement of the

Table 2Physical properties for titanium and candidate alloying elements

Density(g cm�3)[1]

Elasticmodulus(GPa) [1]

Meltingpoint(�C) [1]

X-ray linearattenuation coefficientat 100 keV (cm�1) [15]

Titanium 4.50 103 1668 1.2Tungsten 19.25 411 3410 85.5Molybdenum 10.22 324 2610 11.20Tantalum 16.60 186 2996 71.4Iridium 22.65 517 2447 109.9Niobium 8.57 103 2468 8.9

B. O’Brien et al. / Acta Biomaterialia 4 (2008) 1553–1559 1555

low value for titanium. Tungsten may be a good candidatebut has been ruled out due to practical challenges of arc-melting and mixing with the less dense and lower meltingtitanium. This leaves tantalum and iridium, both of whichare explored in the form of Ti–Ta, Ti–Ta–Ir and Ti–Iralloys. In order to increase the radiopacity of titanium sig-nificantly, a Ti–50Ta alloy was initially examined, followedby a Ti–45Ta–5Ir alloy, i.e. substituting some of the tanta-lum with the higher modulus and more radiopaque iridium.All compositions are in wt.% values. Finally, a Ti–17Iralloy was explored in order to assess the potential benefitsof a eutectoid structure at this composition, as shown bythe Ti–Ir phase diagram [16].

3. Materials and methods

An arc-melting furnace (Materials Research FurnacesABJ-900) was used to produce ingots of approximately50 g. The raw materials for the melt were all obtained fromGoodfellow Cambridge Limited (UK). Melting was carriedout at a maximum current of 400 A, under argon gas pro-tection to prevent contamination. Each ingot was remeltedthree times in order to obtain good melt mixing. A homog-enization treatment at 1200 �C in vacuum was then per-formed; a duration of 1 h was used for the Ti–50Ta andTi–45Ta–5Ir materials, while a duration of 4 h was usedfor the Ti–17Ir material. Ingot dimensions were approxi-mately 75 mm long � 18 mm wide � 7.5 mm thick andthese were then machined to a thickness of 5 mm for striprolling. A series of cold-rolling reductions and anneal treat-ments were then carried out to provide strips with a typicalthickness of 0.60 mm. Cold-rolling was performed using aflat rolling mill (Pepe Tools model 189.00) and all heattreatments were carried out using a vacuum heat treat fur-nace (AVS Model VMM-4-4-1500). The initial interstageanneal for the Ti–50Ta and the Ti–45Ta–5Ir was at1200 �C for 1 h, while all subsequent interstage annealswere at 650 �C for 1 h. The Ti–50Ta showed the lowestformability, enduring typical cold-rolling thickness reduc-tions of 10% compared to the Ti–45Ta–5Ir, which allowedreductions of approximately 50% before requiring anneal-ing. The Ti–17Ir material was hot rolled at 900 �C. Thestrip samples were then used for heat-treatment experi-ments and for tensile testing of selected heat-treatmentconditions.

The Ti–50Ta samples were given b solution treatments;temperatures of 685, 750 and 850 �C were explored in orderto identify the effect of treatment temperature on themicrostructure. Treatments were carried out for 10 min attemperature, followed by furnace cooling, maintained atvacuum. The cooling rate ranged from 14 �C min�1 forthe 685 �C treatment to 19 �C min�1 for the 850 �C treat-ment. The Ti–45Ta–5Ir samples were solution-treated attemperatures of 680, 700, 720 and 850 �C, each also for10 min. Cooling was carried out using an argon backfillwith rates ranging from 32 �C min�1 for the 680 �C treat-ment to 38 �C min�1 for the 850 �C treatment. The eutec-toid Ti–17Ir composition was initially given a b solutiontreatment at 900 �C for 1 h. This was followed by agingtreatments at 650 �C for 17 and 24 h in order to explorethe eutectoid structure formation. The cooling rate from900 to 650 �C was 23 �C min�1 under vacuum and the cool-ing rate from the aging temperature was approximately13 �C min�1, also under vacuum.

Samples for metallographic examination were removedfrom the heat-treated strips and prepared by mounting inepoxy resin, ground with silicon carbide paper and finallydiamond polished. Etching was performed on all samplesusing Kroll’s reagent (100 ml H2O, 5 ml HNO3, 3 mlHF). Samples were examined using an Olympus IX70microscope and digital images of the microstructure wererecorded.

Tensile test samples were cut from the strip in accor-dance with the geometry recommended in ASTM E8 forsub-size specimens (Standard Test Methods for TensionTesting of Metallic Materials). Gauge dimensions were25.4 mm length � 6.25 mm width � 0.60 mm thickness.Testing was performed at room temperature using an0.5 in. (12.7 mm) extensometer gauge length. A strain rateof 0.005 min�1 was used up to the 0.2% off-set yieldstrength and a speed of 0.02 in. min�1 was used fromthis point up to failure. Tensile testing of strips wasrestricted to one sample from the Ti–50Ta material andthree samples each from the Ti–45Ta–5Ir and Ti–17Irmaterials.

4. Results

4.1. Microstructural examination

The microstructures for the Ti–50Ta alloy are shown inFig. 1a–c for the three solution treatment temperatures of685, 750 and 850 �C respectively. At 685 �C a predomi-nantly fine grain structure is visible (dark phase) with asmall proportion of a larger grain structure (light phase).Since X-ray diffraction has not been performed, the exactnature of these phases has not been confirmed. However,work by Zhou et al. on similar compositions reported thepredominant structure to be orthorombic martensite (a00),with a needle-like structure [17]. The lighter phase is mostlikely to be b phase as its volume fraction increases pro-gressively at the higher solution treatment temperatures

Fig. 1. Ti–50Ta microstructure after solution treatment for 10 min. (a) 685 �C treatment showing fine a00 and b structure with some larger developing bgrains indicated; (b) 750 �C treatment with some of the increasing amount of b grains highlighted; (c) 850 �C treatment showing predominantly b grains,with small region of original fine a00 and b structure remaining.

Fig. 2. Ti–45Ta–5Ir microstructure after solution treatment for 10 min at720 �C.

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of 750 and 850 �C. At 850 �C, the structure is primarily bphase with just a small amount of the original a00 still pres-ent. The structure from the 850 �C treatment also hadtraces of a needle-like structure as seen in the lower sectionof Fig. 1c. This may be retained martensite phase or possi-bly acicular a phase.

The microstructure for the Ti–45Ta–5Ir alloy showedvery little change over the solution treatment range exam-ined, confirming that all treatments from 680 to 850 �Cwere well above the b transus temperature for the material.Fig. 2 shows a representative image of the b phase structurefrom the 720 �C treatment.

In the solution-treated condition, the microstructure ofthe Ti–17Ir material was similar to the other alloys, i.e. afully b phase structure. However, after aging, the materialexhibited a lamellar structure, confirming that the 650 �Caging treatment has promoted the eutectoid reaction.Fig. 3 shows this structure for the sample aged for 17 hwhile the sample aged for 24 h had a similar appearance.

The microstructural examinations revealed that all threematerials had homogeneous structures. This can be attrib-uted to the triple arc-melting procedures, the homogeniza-tion treatment and to the subsequent extensivethermomechanical processing employed to convert themelted materials to strip samples.

4.2. Tensile properties

Tensile data for the three materials in various conditionsis shown in Table 3. The Ti–50Ta material has relativelyhigh strength and low ductility but with a low elastic mod-ulus of 90 GPa. The Ti–45Ta–5Ir alloy has a better balanceof strength and ductility but has an even lower elasticmodulus of 72 GPa. In the solution-treated condition, the

Fig. 3. Ti–17Ir microstructure after solution treatment at 900 �C for 1 h,followed by aging at 650 �C for 17 h.

Table 3Tensile properties for the Ti–50Ta, Ti–45Ta–5Ir and Ti–17Ir alloy

Material Condition Ultimatetensilestrength(MPa)

0.2%yieldstrength(MPa)

Elongation(%)

Elasticmodulus(GPa)

Ti–50Ta ST 750 �C,10 min

1151 931 3 90

Ti–45Ta–5Ir ST 850 �C,10 minsample 1

875 641 18 72

ST 850 �C,10 minsample 2

875 717 15 72

Ti–17Ir ST 900 �C,1 hNo aging

1448 1337 a 97

ST 900 �C,1 h aged650 �C,17 h

1137 1103 a 128

ST 900 �C,1 h aged650 �C,24 h

958 903 14 109

a Value not recorded as sample broke outside gauge length.

Fig. 4. Typical stress–strain curves for the three materials.

B. O’Brien et al. / Acta Biomaterialia 4 (2008) 1553–1559 1557

Ti–17Ir material has an exceptionally high yield strength of1337 MPa. Although an elongation value was not recorded(due to fracture outside the guage length) it is most likelythat this sample had relatively low ductility. Interestingly,even in this solution-treated b phase condition, the Ti–17Ir exhibited the highest elastic modulus of all three mate-rials, with a value of 97 GPa recorded. Upon aging, todevelop the eutectoid structure, the strength values havebeen reduced and most importantly there is a substantialincrease in elastic modulus in both aged conditions. Recog-nizing that these are single data points, with no statisticalsignificance, the elastic modulus value of 128 GPa,recorded for the 17 h aging treatment, is promising and isevidence of the benefit of the two-phase eutectoid structure.

Fig. 4 shows representative stress–strain plots for the solu-tion-treated Ti–50Ta and Ti–45Ta–5Ir and for the solu-tion-treated and aged Ti–17Ir. The Ti–17Ir shows thebest balance of strength, ductility and increased elasticmodulus.

5. Discussion

This study set out to explore the possibility of increasingboth the elastic modulus and the radiopacity of titanium,with a view to creating titanium materials that could beused for coronary stents. Maintaining the good biocompat-ibility and low magnetic susceptibility of titanium wereadditional considerations. Whilst reducing the elastic mod-ulus has long been an objective for titanium developmentin the orthopaedic implant industry, it appears that the vastmajority of alloying additions have readily provided thisreduction, with values in the range 44–97 GPa reportedby Niinomi for different alloy systems [18]. Some scopedoes exist to further tune the elastic modulus through agingtreatments as clearly demonstrated by Jablokov et al. for aTi–15Mo alloy, for which modulus values in the range 57–115 GPa were reported, depending on aging time and tem-perature [19]. It is also worth noting that the upper value of115 GPa appears to be one of the highest reported for balloys and compares well with 103 GPa for pure titaniumand 114 GPa for Ti–6Al–4V, as shown in Table 1. Thisvalue was, however, associated with a low elongation of4.7% and it is also most likely that the 15% molybdenumcontent would not improve radiopacity to the level desiredfor coronary stents. Relying on aging treatments alonetherefore clearly has limitations, as any modulus increaseis usually associated with excessive strength and reducedductility. The strategy in this study was therefore to firstmaximize modulus in the b phase solution-treated condi-tion. This approach of intentionally shifting elastic modu-lus to higher values by alloying has not been studied

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elsewhere and certainly appears to be a more challengingobjective than reducing modulus.

The Ti–50Ta alloy investigated here proved to have arelatively low modulus of 90 GPa in the solution-treatedb condition, despite the high modulus, high melting pointand high density of tantalum itself. While few other studiesof this system have been reported, Zhou et al. have alsoshown reductions in elastic modulus for a range of Ti–Tacompositions [17]. This study examined modulus in a num-ber of microstructural conditions and proposed thatdecreased modulus could be attributed to increases in theunit-cell volume, as tantalum content increases.

Moving from the Ti–50Ta to the Ti–45Ta–5Ir alloy, itcan be seen that the substitution of 5% tantalum by 5%iridium actually resulted in a further decrease in elasticmodulus to 72 GPa, for the b phase solution-treated condi-tion examined. Bearing in mind that the elastic modulus ofpure iridium is one of the highest of all metals at 517 GPa,it is again evident that the modulus of individual elementsis not a practical indicator of elastic modulus of the finalalloy. Nevertheless, in a study of Ti–Nb–Hf alloys, Honet al. report that an addition of 5% Hf to a Ti–40Nb com-position resulted in a modulus shift from 57 up to 67 GPa,even though just single b phase structure was observedoptically [20]. In this regard, the Ti–40Nb–5Hf alloy is sim-ilar to the Ti–45Ta–5Ir alloy of this study; a low modulus bphase structure with a high modulus element being added.However, in the case of the Ti–40Nb–5Hf, X-ray diffrac-tion analysis showed that x phase precipitates had formedand the increase in modulus was attributed to these. Per-haps an aging treatment of the Ti–45Ta–5Ir would alsohave produced some modulus enhancement, but the start-ing point of 72 GPa was considered too low to merit suchtreatments.

Exploration of the eutectoid Ti–17Ir alloy was thereforean alternative approach to achieving significant modulusimprovements. The phase diagram for the Ti–Ir system,which has been established by Okamato [16], shows aeutectoid point at 17 wt.% iridium (5 at.%), indicating thatthe eutectoid structure should consist of a phase Ti and theTi3Ir compound. The eutectoid temperature is shown to be720 �C. It was hypothesized that if the elastic modulus ofthe Ti3Ir compound was sufficiently higher than titanium,then a composite stiffening effect may be obtained from aeutectoid structure, thereby increasing the elastic modulusof the material. The elastic modulus of Ti3Ir was not avail-able; however, work by Fleischer and Zabala on a varietyof titanium intermetallic compounds has shown Ti3Ir tobe one of many such high stiffness compounds and notesthat the modulus of these is typically higher than that ofthe metals from which they are formed [21]. Furthermore,early work by Kandarpa et al., which explored the Ti–Irsystem in the region of the eutectoid composition, reportedthat a high stiffness was observed after aging treatments[22]. Although modulus values were not presented, thisgives further support to the potential for significant stiff-ness increases from the Ti3Ir compound.

The data for the Ti–17Ir of the current study, presentedin Table 3, shows that an exceptionally high strength wasobtained, even in the solution-treated condition. Whilstthis high strength is likely to be associated with a low duc-tility, it is promising to see that the elastic modulus of97 GPa, for the b phase structure, is the highest of thethree compositions studied. The aged samples show areduction in solid solution strengthening as the Ti3Irphase has precipitated out to give the classical lamellareutectoid structure, as shown in Fig. 3. The elastic modu-lus of 128 GPa for the 17 h aging is clear evidence thatthis structure does indeed have a composite stiffeningeffect, and this value is potentially one of the highest everrecorded for a titanium alloy. Aging for 24 h has shownreduction in both strength and modulus; this may berelated to a coarsening of the lamellar structure or possi-bly due to a loss of coherency at the Ti–Ti3Ir interface.Clearly some compromise on strength, ductility or modu-lus may be needed in order to obtain an optimum balanceof material properties, but the higher level of the modulusnow makes this titanium material a more attractive candi-date for stent applications.

The radiopacity of the Ti–17Ir alloy has not been mea-sured; however, using a simple rule-of-mixtures calcula-tion, the X-ray linear attenuation coefficient (LAC) forthe material can be calculated as shown

LACTi–17Ir ¼ ½ðLACTi � atomic%TiÞ þ ðLACIr

� atomic%IrÞ�=100 ð1Þ

Using the LAC data presented in Table 2 for titanium(1.2 cm�1) and iridium (109.9 cm�1) at 100 keV, and using5 at.% iridium, gives a value of 6.63 cm�1 for the Ti–17Ir al-loy. Taking the X-ray LAC for iron of 2.9 cm�1 to be a goodapproximation for that of 316L stainless steel shows that theTi–17Ir is even more radiopaque than stainless steel at100 keV. This improved radiopacity should allow the mate-rial to be readily visualized under X-ray fluoroscopy, evenon such small devices as coronary stents. It may also makethe material an attractive candidate for devices such asaneurysm clips. Ti–6Al–4V is already being used for theseclips, but improved X-ray visibility of such small compo-nents, by using Ti–17Ir, may be of benefit. In addition, itshould be noted that iridium has a lower magnetic suscepti-bility than titanium, and therefore Ti–17Ir will also behighly MRI compatible, similar to titanium.

Biocompatibility has not been assessed, but based on thecomposition it is most likely that the material should per-form well in this regard. Iridium is a highly noble metaland is already used – as a pure metal, in oxide form andas a constituent of Pt–Ir alloys – in a diverse range ofimplant applications. In a neural electrode application,Lee et al. demonstrated that the biocompatibility of irid-ium surfaces was similar to that of pure titanium [23]. Irid-ium oxide has been used for both its electricalcharacteristics and biocompatibility in several electrodeapplications, including implantable defibrillator electrodes

B. O’Brien et al. / Acta Biomaterialia 4 (2008) 1553–1559 1559

[24]. The use of iridium oxide coatings on coronary stents isalso being explored, with an initial implant study reportedby Di Mario et al. [25]. While recognizing that the iridiumin the Ti–17Ir alloy is present as Ti3Ir lamellae, rather thanas oxides, the corrosion resistance and biocompatibility ofthis phase is also likely to be high and comparable to thatof titanium.

6. Conclusions

This feasibility study showed that Ti–50Ta and Ti–45Ta–5Ir alloys have lower elastic moduli than pure titaniumwhile Ti–17Ir has a superior elastic modulus with valuesup to 128 GPa recorded. This increased modulus, comparedto titanium and commercial titanium alloys, is based on thestiffening effect provided by the Ti3Ir phase in the eutectoidstructure. Significant exploration of aging heat treatments isneeded to optimize mechanical performance, but from theinitial data collected here, the Ti–17Ir alloy is a practicalcandidate material for balloon-expandable stent applica-tions. It is recommended that future work explore themechanical properties that may be provided by the full spec-trum of microstructures from b solution-treated through topartially and fully transformed conditions. In addition, fur-ther experimentation with hypoeutectoid and hypereutec-toid compositions may provide further options onmaterial properties suitable for stent design.

The significant level of iridium in the alloy has also dra-matically improved the radiopacity of the material. Thisimproved radiopacity further enhances the alloy as apotential stent material but could also be an advantagefor other small devices such as aneurysm clips. Corrosionbehaviour and biocompatibility have not yet been evalu-ated, but based on the alloy composition, these character-istics should be acceptable for vascular device applications.

Acknowledgements

The authors thank Matt Cambronne (Mounds View,MN, USA) for his significant input on heat treatmentsand metallography.

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