J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 2 ( 2 0 0 9 ) 2 1 0 – 2 1 6

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journal homepage: www.elsevier.com/locate/jmbbm

Research paper

Microstructural changes within similar coronary stentsproduced from two different austenitic steels

Sabine Weiss∗, Andreas Meissner, Alfons Fischer

University Duisburg-Essen, Materials Science and Engineering, Germany

A R T I C L E I N F O

Article history:

Received 29 April 2008

Received in revised form

15 December 2008

Accepted 19 December 2008

Published online 31 December 2008

A B S T R A C T

Coronary heart disease has become the most common source for death in western

industrial countries. Since 1986, a metal vessel scaffold (stent) is inserted to prevent the

vessel wall from collapsing [Puel, J., Joffre, F., Rousseau, H., Guermonprez, B., Lancelin, B.,

Valeix, B., Imbert, G., Bounhoure, J.P, 1987. Endo-prothéses coronariennes autoexpansives

dans la Préevention des resténoses apés angioplastie transluminale. Archives des Maladies

du Coeur et des Vaisseaux, 1311–1312]. Most of these coronary stents are made from

CrNiMo-steel (AISI 316L). Due to its austenitic structure, the material shows strength and

ductility combined with corrosion resistance and a satisfactory biocompatibility. However,

recent studies indicate that Nickel is under discussion as to its allergenic potential. Other

typically used materials like Co-Base L605 or Tantalum alloys are relatively expensive and

are not used so often. Newly developed austenitic high-nitrogen CrMnMoN-steels (AHNS)

may offer an alternative. Traditional material tests revealed that strength and ductility,

as well as corrosion resistance and biocompatibility, are as good as or even better than

those of 316L [Vogt, J.B., Degallaix, S., Foct J., 1984. Low cycle fatigue life enhancement of

316L stainless steel by nitrogen alloying. International Journal of Fatigue 6 (4), 211–215,

Menzel, J., Stein, G., 1996. High nitrogen containing Ni-free austenitic steels for medical

applications. ISIJ Intern 36 (7), 893–900, Gavriljuk, V.G., Berns, H., 1999. High nitrogen steels,

Springer Verlag, Berlin, Heidelberg]. However, because of a strut diameter of about 100 µm,

the cross section consists of about five to ten crystal grains (oligo-crystalline). Thus very

few, or even just one, grain can be responsible for the success or failure of the whole

stent. During implantation, the structure of coronary artery stents is subjected to distinct

inhomogeneous plastic deformation due to crimping and dilation.c© 2009 Elsevier Ltd. All rights reserved.

.

d

1. Introduction

Most commercially available coronary stents aremade of 316Ltype CrNiMo-steels (e.g. DIN EN 1.4441). Due to its austeniticstructure, thematerial shows a good combination of strength,

∗ Corresponding author. Tel.: +49 203 379 1263; fax: +49 203 379 4374E-mail address: sabine.weiss@uni-due.de (S. Weiss).

1751-6161/$ - see front matter c© 2009 Elsevier Ltd. All rights reservedoi:10.1016/j.jmbbm.2008.12.008

ductility, corrosion resistance, and biocompatibility (Polak

et al., 1993; Bannard and OMalley, 1983; Windelband et al.,

1997). Other conventional materials in use are cobalt-chrome-

molybdenum-, platinum or tantalum-alloys. Nevertheless,

due to its low costs and easy production, steel is still the

.

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most frequently used material. But nowadays the high nickelcontent of this steel has become a point of discussionbecause of the possibility of allergic reactions (Hildebrandet al., 1989; Hochorteler et al., 2000; EuropeanParliament,1994). Therefore, new materials with similar properties butwithout nickel were developed. One possibility could be highnitrogen steels (AHNS-steel). Besides a good biocompatibility,stents have to tolerate a distinctly inhomogeneous plasticdeformation due to crimping and dilation, leading to residualstresses in the material. These stresses are superimposedon those stresses generated by cyclic heart beats during thepatient’s life. Despite the fact that stenting is a fairly commonprocedure, being used in more than 70% of all percutaneouscoronary intervention (PCI) procedures (vanBuuren et al.,2005), there is only limited knowledge available about themicrostructure development during dilation and use. Due tothe small dimensions of stents, the material has an oligo-crystalline structure (only a few grains distributed over thecross section of a stent strut). These structures can in factneither be described as multi-crystalline materials, nor canthey be treated as single crystals (Li and Laird, 1994a,b).With regard to the importance of the orientation parameter,the Electron BackScatter Diffraction (EBSD) technique hasbeen used to compare the crystallographic orientation of thegrains in both materials after several different deformationstates. By means of transmission electron microscopy (TEM),the deformation appearances can be investigated. For thisstudy, the microstructural alterations under monotonic andcyclic loading of CrMnMoN high nitrogen steel stents wereinvestigated in comparison to those of 316L (You et al., 2006).

2. Materials and methods

For this purpose, bulk material and wires (diameter: 100 µm)from both materials, 316L type CrNiMo-steel (e.g. DIN EN1.4441) and AHNS type CrMnMoN-steel (e.g. DIN EN 1.4452,similar to ASTM F2229-02) were investigated. Furthermore,commercially available 316L coronary artery stents (meangrain size 20 µm), and prototype stents of equal designproduced – specially for our investigations – from the highnitrogen steel AHNS (mean grain size 100 µm)were compared.The stent design is displayed in Fig. 3 and representativemeasurement locations are marked. For determination ofthe mechanical properties of bulk material and wires, axialtensile tests were carried out. To analyze the deformationbehavior of the wires, in-situ tensile tests inside a scanningelectron microscope (SEM) with orientation determinationby means of EBSD-technique were chosen. The standardstent test procedure for fatigue deformation was improvedaccording to DIN EN 14299 (2004). In order to simulate thecorrosive environment, the stents were implanted insidethe artificial vessels of a commercially available EnduraTEC(Bose R© Corporation, Minnetonka, USA) stent/graft tester filledwith a physiological solution. The solution was pumpedthrough the artificial vessels. By varying the pressure insidethe tubes, a periodic change of the tube diameter associatedwith a cyclic fatigue of the stent was achieved. Stents of bothmaterials were tested for 50 million load cycles at 45 Hz.Microstructure characterization of dilated and cyclically

Fig. 1 – Typical tensile curves of bulk specimens and wiresof both materials.

loaded stents was carried out by means of scanning electronmicroscopy (SEM) as well as TEM. Furthermore, single grainorientation determination EBSD was performed, in order toreveal microstructural alterations during deformation.

No further metallographic preparation was necessary forreaching EBSD pattern, because the final stent productionstep is electrochemical polishing. Some geometrical difficul-ties had to be taken into account; further information onthis field is published by the authors (Weiss and Klement,2004). An SEM Gemini 1530 (Zeiss, Oberkochen, Germany)with an EBSD system Crystal (Oxford Instruments, Wies-baden, Germany) and calculation software Channel 5 fromHKL-Technology (Hobro, Denmark) was used. An accelerat-ing voltage of 20 kV and a working distance of 24 mm wereapplied for obtaining single grain orientations as well as ori-entation maps.

For investigation of stents by means of TEM, parts ofinterest were cut out with special preparation scissors. Theseparts were inserted into a grid net. Stent and net were gluedtogether using a suitable adhesive (M-Bond 610, Gatan GmbH,Munich, Germany). Afterwards it was ground down to athickness of 80 µm and further thinned from both sides usinga dimple grinder (Model 656, Gatan GmbH) and a water cooledion mill (Model DuoMill; Gatan GmbH, Munich, Germany).TEM investigations were performed with a Philips — EM 400TEM (FEI- Company, Eindhoven, The Netherlands) using anaccelerating voltage of 120 kV.

3. Results

The experiments started with a comparison of the mechani-cal properties of bulk material and wires of 100 µm diameterfrom both materials, 316L type CrNiMo-steel and AHNS typeCrMnMoN-steel. The results of quasi static tensile tests areavailable from Fig. 1. The comparison of both bulk materi-als reveals that the AHNS-steel reaches much higher stressand strain levels than 316L-steel. The deformation of the thinwires also leads to much better values for the AHNS-steelthan for 316L. But in contrast to the bulkmaterial, much lowerelongations were found for the wires of both materials. Thestrain varies in a relatively wide range. The following exper-iments can give possible reasons for this behavior. In situdeformation of oligo-crystalline wires (grain size within the

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Fig. 2 – (a) Orientation mapping of a 316L wire (a) beforeand (b) after deformation (c) Legend.

magnitude of wire diameter) can demonstrate that the defor-mation is concentrated on only a few, or even just one, grains(Fig. 2). In Fig. 2(a) a representative orientation mapping ofa 316L wire before and after 10% deformation is shown. Eachcolor represents a certain crystallographic orientation accord-ing to the legend in Fig. 2(c). As available from this figure, onespecially oriented grain in the center of the image was elon-gated by more than 30%. But the total elongation of the struc-ture remained small because this grain was representing onlya small part of the structure. In general, this single grain elon-gation was combined with a grain rotation, resulting in an ori-entation change. In the mapping, color changes are indicativeof orientation changes.

To consider the multi axial deformation behavior of stents,static and cyclic deformation were investigated directly onthe stent structure. Representative parts of the stent structuresimilar to the measured regions were marked in Fig. 3.Fig. 4 shows a comparison of typical dilation curves of bothmaterials. The pressure needed for stent expansion is printedagainst the stent diameter. During dilation, a much higherpressure was required for dilation of the AHNS-steel stentthan for the 316L stent.

After dilation, both materials exhibited regions of highdeformation, with characteristic deformation structures likeslip traces on the surface. In contrast to the materialprior to load (Weiss and Meissner, 2006) which hadrevealed completely homogeneous orientation mappingswithout any orientation gradient measured, now regionswith large misorientations occur. Typical grain orientationdeterminations at highly deformed regions are shown in

Fig. 3 – Representative stent design for both materials.

Fig. 4 – Typical dilation curves of stents.

Fig. 5(a) and (b). These orientation mappings compare stentbows after dilation for both materials. At first, the differentgrain size of the two materials became obvious (316L: 20 µm,AHNS: 100 µm). Thus, the already oligo-crystalline structureof steel stents was even more pronounced for the AHNS-steel. The orientation data is represented parallel to thelongitudinal direction of the stent. The coloring of themapping is according to the legend in Fig. 2(c), too. In thecorresponding local misorientation maps (Fig. 6(a) and (b)),low misorientations occurred in light gray, and increasingmisorientations were represented by dark gray shades. Aconcentration of large orientation differences in the inner andouter region of the stent bow became obvious for the 316Lstent (Fig. 6(a)), whereas nomisorientation concentration wasobserved for the AHNS stent.

The orientation gradient along a line from one grainboundary to the opposite one was determined. Thecomparison between the two different steels indicated acompletely different behavior. For 316L, the evolution ofa steady rise to a really large orientation gradient up to8 degrees for representative grains could be measured.The largest orientation gradient was observed near thegrain boundary. In AHNS-steel stents, only small gradientsbetween 1 and 2 degrees occurred, but there are considerablefluctuations between neighboring measuring points, for

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Fig. 5 – (a) Orientation mapping of a 316L stent bow and (b) AHNS-steel stent bow after dilation, the coloring of themapping is according to the legend in Fig. 2(c).

Fig. 6 – (a) Local misorientation mapping of a 316L stent bow and (b) AHNS-steel stent bow after dilation

example 1.5 degrees along a distance of only one micrometer.Fig. 7(a) and (b) show representative orientation gradients ofthese two materials.

After cyclic deformation, several differences between thedifferent materials were observed as well. As availablefrom representative orientation gradients (Fig. 8(a) and (b),316L exhibited a large orientation gradient but no moreconcentration on the grain boundary region. The gradient ofthe AHNS-steel increased to the same level as 316L but thecharacteristic fluctuations between neighboring measuringpoints still remained.

4. Discussion

The current investigation has shown significant differencesbetween poly- and oligocrystalline material of both steels.Due to the small grain size/sample size ratio, deformationbehavior comparable to the one of single crystals occurred.For wires with predominantly “hard” orientations (strongsymmetry and multiple slip) a high strength and a lowductility were observed. The opposite behavior appears for“soft” orientations (low symmetry, single slip). But such auniaxial deformation behavior was much more pronouncedfor the 316L than for the AHNS material. This result will bediscussed later, together with the TEM results.

Based on experimental results using wire specimens,it was calculated by several authors that the mechanicalproperties of stents were size dependent (Murphy et al., 2003;Savage et al., 2004; Harewood and McHugh, 2007; Cuddy et al.,2003; Murphy et al., 2006; Connolley et al., 2005).

In his investigations of the microstructure by means ofscanning electron microscopy, Murphy demonstrated thatthe coalescence of voids could be responsible for the failureof the material. Based on this observation, he developed amodel that shows a good agreement with measured values.Further development of the model together with Savage,considered the strain localization in grains of preferredorientation for slip. The newest paper of Harewood andMcHugh represents a more complete follow-on study of theSavage et al. work on 316L. In spite of a simplification ofthe grain structure to hexagonal shape and the deformationto uniaxial tension, their conclusion is in good agreementwith our experimental results for wires and 316L stents.We can also determine a concentration of deformation ingrains preferentially oriented for single slip. Interestingly,none of the available models provide an explanation for thedifferent deformation behavior of the high-nitrogen steel. Torender more information about the possible reason for thesedifferences, the microstructure was investigated by meansof transmission electron microscopy. During deformation,certain deformation appearances occurred. These should be

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Fig. 7 – (a) Orientation gradient of a 316L grain and (b) AHNS-steel grain after dilation.

Fig. 8 – (a) Orientation gradient of a 316L grain and (b) AHNS-steel grain after cyclic load.

characteristic for each material as well as for certain stentregions or different deformation states. In Fig. 9(a) and (b),representative microstructures of 316L stents are presented.Fig. 9(a) shows the microstructure after dilation but prior toload, and Fig. 9(b) after cyclic load. After dilation, a randomlyoriented dislocation structure with a higher dislocationdensity close to the grain boundaries became obvious. Thisobservation is in good agreement with the evolution ofthe orientation gradient for the corresponding grains. FromFig. 7(a) it is shown, that in the grain center there is onlya small increase in orientation gradient, while close to thegrain boundary the shape of the curve changes into a stronginclination due to the high concentration of dislocations (darkparts of the image). After cyclic load of the material 316L,in general, a dislocation cell network occurs. This so-calledwavy slip is also available from Fig. 9(b). According to theorientation gradient in Fig. 8(a), this microstructural evolutionis not associated with a further increase in the magnitudeof the gradient, but with a more constant inclination.During cyclic load, there was a movement of dislocations invarious directions on different slip systems. This migrationresulted in a rearrangement and worked in opposition to theconcentration of dislocations.

Because of the larger grain size of the AHNS-steel a largerorientation gradient could be expected, too. As seen from

Fig. 5, many fewer grains were distributed across the stentbow. Thus, there would have been more freedom for a grainto rotate. But an opposite behavior was found (Fig. 7). Apossible explication is the typical deformation behavior ofhigh nitrogen steels, the primary planar slip. Due to theirlow stacking fault energy and nitrogen-induced short rangeorder of the material, slip is concentrated on only a few slipsystems. There is nearly no possibility for the dislocationsto leave their slip plane by cross slip or climbing. Fig. 10(a)and (b) present the microstructures of an AHNS-steel stentbow after dilation. A characteristic deformation structureconsisting of parallely-oriented deformation bands becamevisible. A structure like this could be an explanation for theconsiderable orientation fluctuations between neighboringmeasuring points observed in the orientation gradientdistribution in Fig. 7(b). In Fig. 10(b) two deformation bands,a brighter one in the upper left, and a darker one in thelower right, are visible. The width of these bands, about 1micrometer, was in exact correlation with the spacing ofthe oscillating orientation gradient. An atomistic reflectionof the bands showed a partly hexagonal structure, the so-called ε-martensite. Because of the affinity of the slip planesin hexagonal and cubic face centered lattices, the alterationof the two phases appeared as an orientation scattering. This

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Fig. 9 – (a) Deformation structures in a 316L stent bow afterdilation and (b) after cyclic load.

Fig. 10 – Deformation structures in an AHNS-steel stentbow after dilation.

phase transformation in the bands could result in a relaxationof the lattice structure, and the build up of a large orientationgradient can be inhibited.

According to the literature (e.g. Gavriljuk and Berns (1999))after cyclic load of materials with planar slip, the occurrenceof stacking faults and dislocation fields is characteristic. Amicrostructure appearance like this is available from Fig. 11(a)(dislocation fields) and 11(b) (stacking fault). An atomisticreflection of the material reveals a decrease in hexagonalstructure, back to the typically cubic face centered material.A retransformation of the ε-martensite to a cubic matrixwith hexagonal substructures could be detected. Due to thecyclic load, the lattice shearing now results in a domain-like structure. The width of the deformation characteristicscorresponded to the remarkable behavior of the orientationgradient. Within the domain structure, lots of dislocationscan be enclosed. Dislocation pile-ups can activate newslip systems in neighboring domains, and thus a largerorientation gradient can be built up.

The question should be how these different deformationbehaviors of the two materials, combined with characteristicmicrostructure evolution, influence the behavior of such

Fig. 11 – Deformation structures in an AHNS-steel stentbow after cyclic load.

stents during application. The higher pressure which isrequired for dilation of the AHNS-steel stent may beconsidered as a sign for higher radial stability of thehigh nitrogen steel stent. Otherwise, the higher orientationgradient within the 316L grains appears as a contradictionto this thesis. The magnitude of the orientation gradient canbe considered as an indication for the number of dislocationsand, therefore, also for the hardening induced by dislocations.The hardening of the AHNS-steel is based on the deformationmechanism of phase transformation. Based on this fact, theresults of the dilation curves conform to the microstructuralobservations. So if the AHNS-steel stent has a higher radialstability, it would be advantageous for clinical use. However,the higher dilation pressure may be a problem. To avoid thisdisadvantage, the thickness of the stent struts should bereduced. The results after cyclic load lead to the conclusionthat, after a phase transformation induced hardening ofthe material after static deformation, a cyclic softeningtakes place that results in a stable deformation state.Therefore, comparable good cyclic deformation behavior canbe expected, as for 316L.

But taking into account that the grain size is much largerfor the AHNS-steel, single crystalline parts can occur withinthe stent structure. The microstructural results indicate thatthe grain size would not be of much importance, because thedevelopment of the domain structure implies a reduction ofthe effective grain size. For mechanical behavior, the smalldimensions of stents, especially for AHNS-steel stents, canbe considered as an advantage. Despite the unfavorable X-ray opacity, the current investigation would suggest that afurther reduction of the strut diameter is recommended forfurther enhancement of the mechanical properties of stents.Nevertheless this criterion has to be tested carefully. Furtherinvestigations on this field are ongoing.

5. Conclusions

The present study will give a more comprehensive un-derstanding of the influence of the stent material on thestructure property relationship under monotonic and cyclicdeformations, as a basis for ongoing development of new

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materials for stent optimization. Therefore stents of equal de-sign produced from 316L, and fromhigh nitrogen steel as well,were investigated. Fatigue deformation is simulated using acommercially available stent tester. Microstructure character-ization by means of scanning electron microscopy, as well astransmission electron microscopy and single grain orienta-tion determination, show the microstructure and microtex-ture evolution during deformation. The comparison revealslarge differences in mechanical and microstructural proper-ties between the two materials. Because of the small grainsize/sample size ratio, deformation behavior comparable tothe one of single crystals occurred. But such a uniaxial de-formation behavior was much more pronounced for the 316Lthan for the AHNS material. The investigation of microstruc-ture indicates that these differences are caused by thetypical deformation behavior of high nitrogen steels, the pri-mary planar slip. The results after cyclic load lead to theconclusion that after a phase transformation, induced hard-ening of the material after static deformation a cyclic soft-ening takes place, that results in a stable deformation state.Therefore, comparable good cyclic deformation behavior canbe expected for AHNS, as for 316L.

As a first result, AHNS-steel stents can be considered assuitable for clinical use, but the research is still ongoing.Further experiments, for example with regard to the grainsize to strut size relationship of the stents, are in progressand will be published in the near future.

Acknowledgments

Thanks to the Abbot Vascular Instruments DeutschlandGmbH, Rangendingen for the disposal of stents. The authorswould like to thank Prof. Dr. Uta Klement (ChalmersUniversity Gothenburg, Sweden) for cooperation, Mr. TimSchnauber for performance of some of the experimentsand Mrs. Birgit Gleising for her assistance with specimenpreparation. In addition we are indebted to the “DeutscheForschungsgemeinschaft” (DFG) for financial support undercontracts Fi451/9-1, 9-2, WE 2671/1-3 and 436 POL 17/2/06.

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