The Effect of Material Removal on the Corrosion Resistance of Laser-cut and Wire-Form Products Jennifer Fino Decker Christine Trépanier, Alan R. Pelton, Lot Vien SMST 2010 – May 17, 2010 Surface Engineering, Corrosion and Biological Response
NiTi Medical Device Manufacturing First Steps are:
Laser-cutting
or
Cordis Corp. S.M.A.R.T.® Stent
Shape-setting
AMPLATZER® Vascular Plug
• Both undergo similar heat treatments that causes oxidation • Thermal oxides create nickel-rich phases near the surface * L. Zhu, et al., SMST-2003, p.357 © 2010 NDC.
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Passivation Processes Passivation significantly improves the corrosion resistance of NiTi
Testing conducted in Hank’s Solution at 37°C * Trepanier et. al, J. Biomed. Res., Part B, 43, 1998, p. 433
How does the amount of material removed affect the corrosion resistance of NiTi? © 2010 NDC.
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Materials and Methods Laser-cut Z-stent
• SE508 Nitinol Tube – 0.455mm wall thickness – Ground Surface
• SE508 Nitinol Wire – 0.450mm diameter – Bright Surface
Wire-form Z-stent
• Final Device Properties – Af = 25°C ± 5°C – Outer Diameter = 28mm
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Materials and Methods • Electrochemical Passivation Process Group
Weight Loss
Low
< 5%
Medium
< 10%
High
< 25%
• Each device was crimped onto 6mm pin and allowed to return to its final Outer Diameter
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Corrosion Testing • Per ASTM F2129, with the exception of NO reverse scan – – – – – –
Test Solution: Phosphate Buffered Saline (PBS) Temperature: 37°C ± 1°C 1 hour potential monitoring Scan rate: 0.167 mV/sec Sample Size: 3 – 12 stents Results Presented • Breakdown potential (Eb) or • Oxygen evolution potential (Eox ev)
* “Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices,” F 2129-08 © 2010 NDC.
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Nickel Ion Release Tests • Dissolution of nickel is normal in a physiological environment • Nickel is a known toxic element • Conducting nickel ion release tests determines biocompatibility – Equipment: ICP-MS instrument – Test Solution: Phosphate Buffered Saline (PBS) • 1 mL solution / 1 cm2 of surface area exposed
– Test Duration: 7 days – Sample Size: 3 wire-forms were required for each data point – Results Presented • Average mg/day of Nickel Ions Released
* Handbook on Toxicity of Inorganic Compounds, H.G. Seiler et al., 1988 © 2010 NDC.
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Corrosion Behavior of Laser-cut Z-stents
Weight Loss
Average Eb (mV v. SCE)
Standard Deviation (mV v. SCE)
# of # of Devices Devices that Reached Tested Eox ev
< 5%
630
319
9
3
< 10%
609
211
12
8
< 25%
1101
11
3
3
An increase in weight loss results in: Corrosion Resistance
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Decrease in Variability
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Corrosion Behavior of Wire-form Z-stents
Weight Loss
Average Eb (mV v. SCE)
Standard Deviation (mV v. SCE)
# of Devices Tested
# of Devices that Reached Eox ev
< 5%
176
120
3
0
< 10%
597 *
n/a
6
5
< 25%
1062
19
3
3
* Determined to be an outlier
Corrosion Resistance < 5% Weight Loss
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<
< 10% Weight Loss
â&#x2030;&#x2C6;
< 25% Weight Loss
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Corrosion Behavior: Laser-cut vs. Wire-form
• More material removal required for laser-cut Z-stents to achieve high corrosion resistance • More variability in < 5% and <10% weight loss groups for the laser-cut Z-stents – May be related to presence of the heat affected zone (HAZ) © 2010 NDC.
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Surface Properties Increasing Weight Loss
Laser-Cut
Wire-form
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Oxide Thickness Characterization Performed on wire-form devices
< 5% Weight Loss 216 nm
< 10% Weight Loss 3.7 nm
< 25% Weight Loss 6.0 nm Oxide layer thickness measured using full width at half maximum method. Š 2010 NDC.
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Biocompatibility Results
The lowest weight loss group of wire-forms released ten times more nickel than the other weight loss groups. Š 2010 NDC.
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Conclusions • For both laser-cut and wire-form Z-stents, as the amount of weight loss increases: Corrosion Resistance
&
Decrease in Variability
• Higher weight loss is required for laser-cut Z-stents to achieve comparable corrosion resistance with wire-form Z-stents • Smoother surface finish = Better corrosion resistance • Higher weight loss leads to thinner oxides and less nickel ion release © 2010 NDC.
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Future Work Determine how Material Removal Affects: • Oxide thickness & release of nickel for laser-cut Z-stents • Size of Heat Affected Zone • Fatigue Life
© 2010 NDC.
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Thank You!
Jennifer Fino Decker jennifer.fino@nitinol.com Senior Process Engineer
Š 2010 NDC.
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