Surface Conditioning of 316LVM Slotted Tube Cardiovascular

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Surface Conditioning of 316LVM Slotted TubeCardiovascular Stents

Ankur Raval, Animesh Choubey, Chhaya Engineer, Devesh Kothwala

To cite this version:Ankur Raval, Animesh Choubey, Chhaya Engineer, Devesh Kothwala. Surface Conditioning of316LVM Slotted Tube Cardiovascular Stents. Journal of Biomaterials Applications, SAGE Publi-cations, 2005, 19 (3), pp.197-213. �10.1177/0885328205046564�. �hal-00570755�

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Surface Conditioningof 316LVM Slotted TubeCardiovascular Stents

ANKUR RAVAL,* ANIMESH CHOUBEY,CHHAYA ENGINEER AND DEVESH KOTHWALA

Research and Development Division

Sahajanand Medical Technologies

Surat-GJ 395 003, India

ABSTRACT: The surface quality of coronary stents has a significant influenceon its biocompatibility. Therefore, surface polishing is of paramount importancein the production and application of stents. In the present study, electropolishingis performed on 316LVM steel slotted tube coronary stents. Additionally, acidpickling, as a pretreatment of electropolishing, is also conducted. Gravimetricanalysis of the stents (weight loss and strut width change) in the process of acidpickling and electropolishing are done. Qualitative roughness measurements aremade to evaluate the stent surface. Electropolished stents are passivated causingchromium enrichment on the surface of the material, thereby enhancing itscorrosion resistance. Passivated and electropolished samples are examined usingenergy dispersive spectrometry. Balloon expanded and crimped profiles of thepassivated stents are qualitatively analyzed.

KEY WORDS: coronary stents, electropolishing, passivation, balloonexpandability.

INTRODUCTION

Stents are tubular intravascular devices, which are placed withinthe blood vessels to structurally hold open the vessel. The device

can be used to maintain the patency of a blood vessel, immediately after

*Author to whom correspondence should be addressed. E-mail: [email protected]

JOURNAL OF BIOMATERIALS APPLICATIONS Volume 19 — January 2005 197

0885-3282/05/03 0197–17 $10.00/0 DOI: 10.1177/0885328205046564� 2005 Sage Publications

intravascular treatments, and to reduce the likelihood of the develop-ment of restenosis [1,2]. Figure 1 shows a typical coronary stent profilefeaturing the main strut, the subsidiary strut, and the link. It is highlydesirable for the surface of the stent to be extremely smooth so that itcan be inserted easily and experience low-friction travel through thetortuous vessel pathway prior to implantation. A roughened outersurface may result in increased frictional obstruction during insertionand excess drag during travel to the stenotic site as well as damaging theendothelium lining of the vessel wall [3]. A rough surface may causefrictional resistance to such an extent as to prevent travel to the desireddistal locations. It may also cause damage to the underlying inflationballoon [3]. Surface roughness also influences the amount of proteinadherence as it determines the contact area [4]. Immediately followingand in concert with the adsorbed proteins after stent implantation, thestent surface properties will determine platelet and leucocyte adherence.Fibrogen, immunoglobulins, complement factors, and coagulationfactors, all influence platelet activation and aggregation, as well asneutrophil and macrophage adhesion and activation [5]. As such, thesurface properties determine thrombogenicity, inflammation, andvascular wound-healing.

As the surface characteristics influence the biocompatibility of thestents directly, a smooth surface obtained after acid pickling andelectropolishing can minimize thrombosis effectively, and also has a

Figure 1. Expanded stent profile showing: (a) main strut; (b) subsidiary strut; and

(c) link.

198 A. RAVAL ET AL.

potential beneficial effect on neointimal hyperplasia [6]. Electropolishingis a process in which a metallic surface is smoothened by polarizing itanodically in an adequate electrolyte. It is classified into two processes:anodic leveling and anodic brightening [7]. Anodic leveling results fromthe difference in the dissolution rate between the peaks and valleys on arough metal surface depending on the current distribution or mass-transport conditions. On the other hand, anodic brightening is associatedwith the suppression of the influence of the metal microstructure onthe dissolution rate. A smooth electropolished surface, which appearsbright to the naked eye, results from a combination of these two factors[7]. The aim of the current research work is to investigate and deter-mine the optimal conditions for acid pickling and electropolishing of316LVM coronary stents. Also, an attempt has beenmade to characterizestents by weight and strut width measurements. Passivation ofelectropolished stents is done to increase its corrosion resistance insevere blood-contacting environments. Stents must have a high expand-ability. This can also be a high ratio between crimped and expandeddiameters to allow for the smallest diameter of the system. Hence,crimped and expanded balloon profiles of the passivated stents are alsoobserved.

MATERIALS AND METHODS

Materials

The original material used in this study was 316LVM (low carbonvacuum remelted steel) slotted tube coronary stents produced from astainless steel tube by laser precision cutting (Sahajanand LaserTechnologies Pvt. Ltd., India). The stents were approximately 16mmlong. Their outer diameter was about 1.72mm. The wall thickness of thestent was 110 mm. The as-received samples were initially cleaned usingdistilled water in an ultrasonic agitation bath for 15min and dried in air.Percutaneous Transluminal Coronary Angioplasty (PTCA) ballooncatheter used during the study was procured from Arthesis, France(Figure 2). It was made of Nylon-12 material with PTFE-coatedstainless steel hypotube shaft. Two radiopaque markers were locatedunderneath the balloon, which fluoroscopically marked its workinglength. The dimensions of the balloons used were 3.0� 17mm and3.5� 17mm. Of these dimensions, 3.0 and 3.5 designates the balloondiameter (in mm), when expanded at recommended expansion pressure(6 atm) provided by the manufacturer and 17mm indicates the length ofthe balloon.

Surface Conditioning of 316LVM Slotted Tube Cardiovascular Stents 199

Acid Pickling

Laser processing is done at high temperature and this often leavesdebris and slag material attached to the stent. Acid pickling is aneffective method for the chemical removal of surface oxides and othercontaminants from metallic materials by immersion in an aqueous acidsolution [8]. Alkaline soap wash by a spray of soap water (approximately2mL of concentrated alkaline liquid cleaner mixed in 200mL ofdeionized water) was given to the laser-cut stents to remove dustparticles, oil, grease, and slag. The stents were then cleaned with a sprayof deionized water to remove soap. To remove any residual grease or oilfrom the stent surface, it was dipped in 25mL of trichloroethylene (ARgrade) in a culture tube for 11–12h. After degreasing, the stents werewashed with deionized water using an ultrasonic cleaner for 10minfollowed by drying in an oven at 90–95�C. For acid pickling, twodifferent bath compositions were explored (Table 1a). The samples werepickled for a different amount of time for both the processes and by theobservation of the change of the surface state of these specimens, a timewas chosen which caused the complete removal of oxide scales from theinner and outer surfaces and the cutting zone. In acid pickling withHNO3 bath, a glass beaker containing the stents in acid solution wasplaced on a thermostat, maintaining the temperature at 80–90�C. Instep one, the stents were pickled for a varying amount of time 1, 3, and5 h and washed with a spray of deionized water. In step two, for the

Figure 2. Balloon used for expanding stents in the arteries (Arthesis, France).

200 A. RAVAL ET AL.

further removal of loosened oxide scales, electrolysis (see Table 1b) wascarried out at room temperature for 20–45 s.

In the second process (Table 1a), the pickling of stents was performedby immersing the stents in an acid solution containing HF, HNO3, andH2O. The acid beaker was kept in an ultrasonic cleaner for 10min toallow degassing. The temperature of the water bath in the ultrasoniccleaner was maintained at 35–40�C throughout the process. The glassbeaker containing the stents dipped in acid pickling solution wassubjected to ultrasonic agitation. After the specified time (5, 10, and15min) of cleaning, the heat controller, timer and ultrasonic cleanerwere switched off. Pickled stents were given a deionized water wash anddried by means of a hot air drier (Philips Flex-1000, Japan).

Electropolishing

The electropolishing device was self-designed, as illustrated inFigure 3. A 1000mL glass beaker was used as a cell. A DC rectifier(Testronix 92D) was used for power supply (30V and 10A maximum).Electropolishing is a process by which metal is removed from aworkpiece by the passage of electric current when the workpiece isimmersed in a liquid medium, i.e. electrolyte. The workpiece (316LVMstents) were connected to the anodic terminal and 304 steel ring wasused as the cathode. Both anodic and cathodic terminals weresubmerged in the solution, forming a complete electrical circuit. Thecurrent applied is a direct current. The composition of the electrolyte issummarized in Table 2. The optimized parameters for electropolishing

Table 1a. Composition of pickling solutions.

Component Amount (mL)

1 HNO3 1502 HF (48%) 1

HNO3 (70%) 9Deionized water 90

Table 1b. Composition of electrolysis solution.

Component Amount

H2SO4 20mLDeionized water 180mLNH2CS.NH2 (thiourea) 0.5 g


are listed in Table 3. The temperature was maintained between 70 and80�C and the bath was stirred for about 40min with a magnetic stirrer(Remi 1MLH) during the preparation of the solution for the completedissolution of the chemicals. Voltage and current are co-related by theOhm’s law. When a current passes through an electrolyte, a liquid layerof anodic dissolution products is formed on the surface of the anode.

Figure 3. Self-designed electrochemical polishing device.

Table 2. Composition of electrolyte for electropolishing.

Component Amount

H3PO4 (ortho-phosphoric acid) 325mLH2SO4 125mLDeionized water 50mLNa5O10P3 (sodium tripolyphosphate) 1.0 g

Table 3. Parameters for electropolishing.

Parameter Value

Temperature range 70–80�CVoltage 8.0 VCurrent 0.4 ATime 80 s

202 A. RAVAL ET AL.

This layer has a higher viscosity and greater electrical resistivity thanthe bulk of the electrolyte. The thickness of the liquid layer on a roughsurface differs from one area to the next. The current density isnonuniform as a result of such a nonuniform liquid layer; i.e. it is higheron peaks than crevices. Thus, peaks dissolve more rapidly than crevices,this, therefore produces a surface leveling effect on the stents [8]. Thestents during electropolishing were handled carefully using latex gloves(sterilized with ethylene oxide) and were stored in an airtight container.

Gravimetric Analysis of Stents

Weights of the as-cut, acid pickled, and electropolished stents weremeasured with an electronic analytical balance (Citizen CX-265).Measurement of widths of the stent strut was carried out before andafter electrochemical polishing using a light optical microscope (SZX-12,Olympus-Japan). Six measuring sites of the stent struts were arbitrarilychosen to conduct the width measurement, and the mean value wascalculated. Percentage weight loss and width reduction of the stentswere also estimated.

Passivation

316L stainless steel has an intrinsic corrosion resistance because ofits ability to form a protective oxide or passive layer on its surface byalloying >12% chromium with iron, nickel, and molybdenum [9]. Thisprotective layer is thought to consist of a heterogeneous chromium oxidefilm (Cr2O3) along with elemental iron, nickel, and their respectiveoxides. In general, this passive layer protects the steel from oxidation.However, when subjected to extremely corrosive environments, theheterogeneous film is attacked and chemical reduction occurs. When theoxide film layer is destroyed beyond regeneration, localized corrosiontakes place in the bulk alloy and reaction by-products are generated[9,10]. The corrosion resistance at the steel surface can be enhanced byincreasing the amount of chromium that can form Cr2O3 in the presenceof an oxidizing environment.

In the present study, passivation solution was made by adding 35mLof nitric acid (AR grade) to 70mL of deionized water in a glass beaker.The beaker containing the solution was heated to 75�C (�3�C) using athermostat. Stents were dipped in the solution after the desiredtemperature was achieved and kept there for 30–35min. A carbonatewash was provided to (30 g of sodium carbonate added to 200mL ofdeionized water) neutralize the nitric acid followed by the cleaning of


stents with deionized water to remove any traces of carbonate. Thestents were kept in isopropyl alcohol solution to avoid any microbialcontamination on the surface and dried with a hot air drier in the filterarea. Care was taken to keep the drier in such a position that it drawsair from the laminar airflow supply. Passivated surfaces were examinedby SEM-EDAX (Philips XL30, Japan).

Balloon Expandability of Stents

To ensure uniform expandability and mechanical strength of thestents after surface treatment, they were subjected to balloon expand-ability test. Passivated stents were crimped on the balloon by means ofautomatic crimping machine (Machine Solutions, MSI-500, USA). Thecrimped stents were expanded by expansion device (Medtronic, Skimed-Sedat, USA) used by interventional cardiologists during angioplasty.The pressure gauge was attached with the expansion device to measurethe pressure applied to the balloon. Sterile fluid was pumped in to assistballoon expansion. Balloons of diameters 3 and 3.5mm were used.Stents crimped on these balloons were expanded at a rated pressure of6 atm. The balloon was kept dilated on manufacturers’ recommendedpressure for approximately 45–60 s and then deflated. The procedurewas repeated twice with a 1–2min rest period between each dilation.The crimped and dilated stent diameters were measured with amicrometer and their profile was inspected by stereooptical microscope(SZX-12, Olympus-Japan) and scanning electron microscopy.

RESULTS AND DISCUSSION

As-received Stents

Figure 4 shows the morphology of the laser-cut zone of as-cut stent ‘asreceived’. The slag is clearly visible in the scanning electron micrograph.Considerable roughness was observed in the laser-cut zone.

Pickling of Stents

Figure 5 shows the scanning electron micrographs of stents pickled byHNO3þ electrolysis (Figure 5(a)) and HFþHNO3 process (Figure 5(b)).The slag in the laser-cut zone was not removed totally in the case ofpickling with HNO3þ electrolysis process; also the oxides produced bythe laser cutting process still adhered to the surface of the stents.Immersion time (5 h) could not be exceeded with this solution to avoid

204 A. RAVAL ET AL.

excessive stock removal. Whereas, after pickling for a short period of15min with HFþHNO3, the slag could be effectively removed from thelaser-cut zone. Therefore, it can be considered that pickling withHFþHNO3 is more suitable for 316LVM slotted tube coronary stents.Acid pickling with both the processes could remove the productscovering the outer surface of the stent formed while producing thetubing, thus resulting in a rough outer surface. It has been reported thatnitric–hydrofluoric acid solution is used for removing both metalliccontamination, welding, and heat-treating scales [11]. The mixture ofHFþHNO3 directly penetrates through the crevices of the scale layerand starts dissolving it. By combining the effect of ultrasonic waves, thepickling efficiency increases and complete removal of the scale can beobserved in a short span of time. Ultrasonic waves loosen and removecontaminants from deep recesses and other inaccessible areas [12].Cavitation in the pickling solution produced by the high-frequencysound waves causes micro agitation of the cleaning media in even tinyrecesses of the stent, making the scales to remove completely from theentire surface area.

Electropolishing

Figure 6 shows scanning electron micrographs of electropolishedstents after acid pickling in HFþHNO3 bath. Evidently, after electro-polishing a considerably smooth surface was revealed (Figure 6(a)).

Figure 4. Surface topography of laser-cut zone and outer surface.


The surface quality was largely improved compared to the pickledstent. The roughness on the laser-cut zone has been entirely removed(Figure 6(b)). Therefore, from a comparison of the polished andas-received stents, it can be considered that this selected condition ofelectropolishing is suitable for the improvement of the surface rough-ness of the stents.

(a)

(b)

Figure 5. Surface topography of stents pickled by: (a) HNO3þ electrolysis; and

(b) HFþHNO3 process.

206 A. RAVAL ET AL.

Removal of Stent Material

Table 4 presents the average weight and percentage weight loss of theas-received, acid pickled, and electropolished stents. Table 5 illustratesthe width and percentage width reduction of stent struts. Theas-received stent has a weight of 27.8mg and a strut width of about150� 5mm. After pickling in the selected acid solution (HFþHNO3) at45�C for 15min, the weight loss was 7.91%. The strut width decreased to

(a)

(b)

Figure 6. Surface topography of stents electropolished after pickling in HFþHNO3 bath.


145� 5 mm. The width reduction was thus 3.33%. After polishing usingan electropolishing bath (Table 2) at the condition presented in Table 3,the weight of the stent became 20.7mg and the strut width was reducedto 110� 5 mm. Figure 7 gives a plot between the weight of stent and thenumber of stents. It shows a variation of weight after acid picking,electropolishing, and weight loss in electropolishing, for 24 stentsexamined.

024

68

101214

16182022

242628

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

No. of Stents

Wei

gh

t (m

g)

After acid pickling

After electropolishing

Weight loss in electropolishing

25

Figure 7. Plot showing variation of stent weight.

Table 5. Width and %width reduction of stent strut beforeand after electropolishing.

Sample Width (mm)Width

reduction (%)

As-received 150� 5 –Acid pickled 145� 5 3.33Electropolished 110� 5 24.14

Table 4. Weight and % weight loss of stents before andafter electropolishing.

SampleAverage

weight (mg)Weightloss (%)

As-received 27.8 –Acid pickled (HFþHNO3) 25.6 7.91Electropolished 20.7 19.18

208 A. RAVAL ET AL.

Passivation

The SEM-EDAX analysis provided data on the elemental and oxidesurface chemistries of the treated samples. Table 6 compares thechromium to iron (Cr/Fe) ratios for surfaces modified by electropolishingand passivation for five samples. The ratios for the passivated sampleswere higher than the values obtained for the electropolished samples.Specifically, the average Cr/Fe ratio for the passivated samples was0.41 versus 0.29 for the electropolished samples. The higher surfacechromium on the passivated samples might contribute to a highersurface corrosion resistance than that of the electropolished samplesbecause the removal of surface iron and metallic contaminants reducesthe potential for reactive-site corrosion.

Balloon Expandability of Stents

Figure 8 shows the stent crimped on a balloon. The scanning electronmicrograph shows a perfect crimping profile. Overlapping of any of thestruts was not observed. Links and subsidiary struts are found to be

Figure 8. Scanning electron micrograph showing a stent crimped on a balloon.

Table 6. Cr/Fe ratio for electropolished and passivated stent samples.

Sample 1 2 3 4 5

Electropolished (Cr/Fe ratio) 0.28 0.30 0.34 0.29 0.27Passivated (Cr/Fe ratio) 0.42 0.40 0.41 0.40 0.43


absolutely straight. For the stent mounted on 3.0 and 3.5-mm diameterballoon, the crimped diameter of the stent was 1.016 and 1.041mm,respectively. Figure 9(a) and (b) illustrate the expansion profile of thestents. The figures reveal that, at 6 atm of applied pressure the stentswere fully expanded to a diameter of approximately 3.0mm (using theballoon of 3mm diameter) and 3.5mm (using the balloon of 3.5mmdiameter). Appropriate straightening of the alternate link between twosubsequent cells could be observed. An expanded profile also revealed

(a)

(b)

Figure 9. Scanning electron micrographs showing the profile of a stent dilated to (a) 3mm

and (b) 3.5mm.

210 A. RAVAL ET AL.

that none of the struts were disproportionately dilated. Proper openingof the main and subsidiary struts at an expansion pressure of 6 atmcould also be observed from the scanning electron micrograph. Figure 10shows the optical micrographs of as-received stent and dilated stentsof 3 and 3.5mm diameter. No foreshortening (reduction in length uponexpansion) was observed after balloon expansion to these diameters.Foreshortening was measured by comparing the length of the dilatedstents to that of the undilated ones. No distinguishable reduction in

(a)

(b)

(c)

Figure 10. Optical micrographs showing (a) as-received stent; (b) 3mm dilated and

(c) 3.5mm dilated stent geometry.


length of the expanded stents was observed. This can be attributed tolow stress concentration in the longitudinal direction and uniform stressdistribution, owing to smooth stent surface topography obtained afterelectropolishing.

CONCLUSIONS

Scanning electron micrographs revealed that acid pickling withHFþHNO3 caused a considerable decrease in roughness of the cuttingzone. The slag was effectively removed and a clean stent surface wasobtained for electropolishing. Surface quality of 316LVM slotted tubecoronary stent is largely improved by means of electropolishingperformed with optimized parameters. Compositional analysis ofelectropolished and passivated stent surfaces showed a higher Cr/Feratio for passivated stents, indicating enhanced corrosion resistanceof these stents in severe blood contacting human body environments.To ensure the mechanical strength of the stent after material removalduring surface treatment, balloon expanded and crimped profiles of thestents were observed. The original laser-cut stents were found to be toorigid to be balloon expandable, whereas, passivated coronary stentcould very easily be crimped on the balloon and subsequently dilated.Uniform expansion profile was observed in scanning electron micro-graphs. Optical images depicted almost zero foreshortening of thedilated stents.

ACKNOWLEDGMENT

The authors wish to thank Mr. Rajesh Vaishnav, Technical Director,Sahajanand Medical Technologies Pvt. Ltd. for providing the technicalsupport. They also express their sincere gratitude to Mr. DhirajlalKotadia, Chairman, Sahajanand Group of Companies, for providingfinancial assistance to carry out the research work.

REFERENCES

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2. Puel, J., Juilliere, Y. and Betrand, M.E., et al. (1988). Early and LateAssessment of Stenosis Geometry after Coronary Arterial Stenting, AmericanJournal of Cardiology, 61: 546–553.

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3. Williams, D.F. (1996). Surface Interactions, In: Sigwart, U. (Ed.),Endoluminal Stenting, W.B. Saunders, London, pp. 45–51.

4. Andrade, J.D. and Hlady, V. (1987). Plasma Protein Adsorbtion: The BigTwelve, In: Leonard, E.F., Turitto, V.T. and Vroman, I. (Eds), Blood inContact with Natural and Artificial Surfaces, Annals of the New YorkAcademy of Sciences, New York, pp. 153–172.

5. Spector, M., Cease, C. and Tong-Li, X. (1989). The Local Tissue Response toBiomaterials, Critical Reviews in Biocompatibility, 5: 269–295.

6. de Scheerder, I., Sohier, J. and Wang, K., et al. (2000). Metallic SurfaceTreatment Using Electrochemical Polishing Decreases Thrombogenicityand Neointimal Hyperplasia of Coronary Stents, Journal of InterventionalCardiology, 13: 179–185.

7. Maganio, S., Matlosz, M., Landolt, D. (1993). An Impedance Study ofStainless Steel Electropolishing, Journal of Electrochemical Society, 140:1365–1373.

8. Hudson, R.M., Joniec, R.J. and Shatynski, S.R. (1982). Pickling of iron andSteel, In: Cubberly, W.H. et al (Ed.), Metals Handbook, 9th edn, AmericanSociety for Metals, Metals Park, Ohio, 68 pp.

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Surface Conditioning of 316LVM Slotted Tube Cardiovascular