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Carolyn Lahti, R&D Materials Engineer

Surface Quality of Incoming Nitinol Wire Used to Manufacture Braided Implants

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Carolyn Lahti, R&D Metallurgical Engineer at Boston Scientific Materials Testing, Analysis, and Characterization (MTAC)

B.S. in Materials Science and Engineering Michigan Technological University

MTAC •  In-house service lab •  Analyze a wide variety of products

–  Interventional Cardiology –  Peripheral Interventions

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Nitinol Manufacturing Process

The Methods of Preparation of Ti-Ni-X Alloys and Their Forming Radim Kocich, Ivo Szurman and Miroslav Kursa (http://cdn.intechopen.com/pdfs-wm/44019.pdf)

Surface finishing: chemical etch,

EP, passivation, coating

Sent to customer

Supplier surface finishing techniques: •  Mechanical finish

•  Diamond draw (shiny polish) •  Sandblast (matte surface)

•  Chemical finish •  Etch/pickle •  Electropolish (EP)

•  Oxide surface •  Coating •  Plating

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Importance of Surface Quality Surface quality influences performance

From Noll and Lipson, "Allowable Working Stresses", Society for Experimental Stress Analysis, Vol. III, no. 2, 1949

http://www.andersonmaterials.com/medical-devices.html

http://www.snipview.com/q/Dimensional_instruments http://sme.org/MEMagazine/Article.aspx?id=49609&taxid=1418!

Compromising factors

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Goal of Study

Analyze the surface quality of incoming wire and EP material of nitinol wire •  Characterize and optimize EP

http://www.imaging-git.com/products/3d-laser-confocal-microscope-simplifies-surface-analysis

http://www.witec.de/products/raman/rise-raman-imaging-scanning-electron-microscopy/

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Surface Roughness Measurements

Ra: arithmetic average of absolute surface roughness values Rp: maximum peak height Rv: maximum valley depth Rz: average distance between the highest peak and lowest valley in each sampling length

https://upload.wikimedia.org/wikipedia/commons/3/39/Mechanical_filtering_of_surface_finish_trace.svg

http://www.wisetool.com/surfacefinish.htm

Rz Rv

Rp

Ra

http://www.olympus-ims.com/en/metrology/ols4000/

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Experimental Setup

Mechanical Finish

Chemical Etch Finish

Proprietary Finish

Supplier A Supplier C Supplier B

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3 4

2

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High purity nitinol wire

Nitinol wire

356 micron wire

25 micron material removal (radial)

50 micron material removal (radial)

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SEM Images – Incoming Material

Mechanical Finish

Chemical Etch Finish

Proprietary Finish

Supplier A Supplier C Supplier B

356 micron wire

25 micron material removal (radial)

50 micron material removal (radial)

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Material         Incoming  Surface  Condi4on   Observa4ons   Peak    

Rp  (μm)        Valley  Rv  (μm)        

Range  Rz  (μm)        

Mean  Ra  (μm)            

1   Mechanically  polished       Heavy  surface  scratching   11.89   8.70   20.59   2.16  

2   Mechanically  polished   Mostly  smooth   2.89   3.56   6.46   0.15  

3   Chemically  etched   Smooth  with  small  draw  lines   10.83   11.97   22.79   1.65  

4   Chemically  etched   Smooth  with  small  defects     8.57   10.08   18.65   1.59  

5   Proprietary  finish     Smooth  with  small  draw  lines   4.63   4.32   8.95   0.97  

Peak    (Rp)      

Valley    (Rp)  

Range    (Rz)    

Mean    (Ra)  

               

               

               

               

               

Surface Roughness – Incoming Material

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SEM Images – 25 µm Material Removal

Mechanical Finish

Chemical Etch Finish

Proprietary Finish

Supplier A Supplier C Supplier B

356 micron wire

25 micron material removal (radial)

50 micron material removal (radial)

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Surface Roughness – 25 µm Material Removal

Material         Incoming  Surface  Condi4on  

Incoming  Material  Observa4ons  

Peak    Rp  (μm)        

Valley  Rv  (μm)        

Range  Rz  (μm)        

Mean  Ra  (μm)            

1   Mechanically  polished       Heavy  surface  scratching  

2   Mechanically  polished   Mostly  smooth   0.60   0.54   1.15   0.24  

3   Chemically  etched   Smooth  with  small  draw  lines   0.54   0.46   1.01   0.17  

4   Chemically  etched   Smooth  with  small  defects     1.98   2.47   4.45   0.29  

5   Proprietary  finish     Smooth  with  small  draw  lines   0.73   0.53   1.27   0.20  

Peak    (Rp)      

Valley    (Rp)  

Range    (Rz)    

Mean    (Ra)  

               

               

               

               

               

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SEM Images – 50 µm Material Removal

Mechanical Finish

Chemical Etch Finish

Proprietary Finish

Supplier A Supplier C Supplier B

356 micron wire

25 micron material removal (radial)

50 micron material removal (radial)

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Surface Roughness – 50 µm Material Removal

Material         Incoming  Surface  Condi4on  

Incoming  Material  Observa4ons  

Peak    Rp  (μm)        

Valley  Rv  (μm)        

Range  Rz  (μm)        

Mean  Ra  (μm)            

1   Mechanically  polished       Heavy  surface  scratching   3.27   3.37   6.64   0.92  

2   Mechanically  polished   Mostly  smooth   1.10   0.88   1.98   0.26  

3   Chemically  etched   Smooth  with  small  draw  lines   0.69   1.12   1.81   0.21  

4   Chemically  etched   Smooth  with  small  defects     2.65   3.28   5.93   0.28  

5   Proprietary  finish     Smooth  with  small  draw  lines   2.45   1.70   4.15   0.21  

Peak    (Rp)      

Valley    (Rp)  

Range    (Rz)    

Mean    (Ra)  

               

               

               

               

               

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Max Peak to Valley Distance

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Mean Surface Roughness

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Summary

•  Quantification of surface finish variation throughout a collection of suppliers was assessed via SEM and roughness measurements

•  Large variation in incoming surface quality observed across the different suppliers and incoming surface finishes

•  The same electropolishing process works for a broad range of nitinol materials and results in a smoother surface

•  Overall, incoming condition was less critical to the final EP condition •  However, incoming condition may be important to processing wire prior to

EP (i.e. winding wire over fixtures) •  Smoother surfaces from suppliers may allow for a reduction in material

removed during EP