Nanostructured Diamond Coating of Nitinol Stents

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Nanostructured Diamond Coating of Nitinol Stents

• Stephen Evans, Dr. Aaron Catledge,
• Dr. Yogesh Vohra,
• Jerry Sewell

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What are Stents?

• Biomedical implants used to support collapsed or
  narrowed arteries
• Allow the blood to flow normally
• Function to prevent heart attacks and strokes

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What is Nitinol?

• Nickel-Titanium Naval Ordnance Laboratory
• Intermetallic compound of Nickel and Titanium
• Created by the Navy in the early 60s
• 55 percent Nickel by weight
• Known for its very peculiar properties

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Properties of Nitinol

• Super-Elastic Property
• Shape-Memory Effect
• Provides a Constant Force
• Kink Resistant
• Good Biocompatibility
• Radiopaque

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Super-Elasticity of Nitinol

• Allows Nitinol to be deformed and reassume its
  original shape
• Stress results in a reversible phase change of
  Nitinol from austenite to martensite
• Austenite Phase
• Rigid\Hard
• Cubic lattice structure
• Martensite
• Manipulative\Soft
• Rhombohedral lattice structure
• Upon the relief of stress Nitinol returns to its
  austenite phase and its original shape

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Super-Elasticity of Nitinol
Schematic presentation of lattice structure
changes caused by outer stress in Nitinol.
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Properties of Nitinol

• Super-Elastic Property
• Shape-Memory Effect
• Provides a Constant Force
• Kink Resistant
• Good Biocompatibility
• Radiopaque
• Very good material for making stents

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Applications of Nitinol
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Nitinol Stents and Dissolution of Nickel

• Can dissolve Nickel into the body
• Nickel is an essential element in the body
• Can be harmful if deposited in large quantities
• May cause allergenic or toxic reactions
• Can this be avoided?

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Nitinol with CVD Diamond The Perfect Material
for Stents

• Chemical Vapor Deposition (CVD) of Diamond
• Process used to deposit thin diamond films onto
  non-diamond substrates
• Can avoid the release of nickel from Nitinol
  stents
• Diamond is biocompatible and extremely hard
• Can prevent stress corrosion cracking of stents

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Research Goal

• To identify the conditions needed to grow a
  continuous and adherent crystalline carbon
  coating on a Nitinol substrate using CVD

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The Deposition Process

• Mechanical Polishing
• Annealing step
• Seeding
• Ultrasonic Agitation
• Dry Seeding
• CVD

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Annealing

• Gas Flow Rates
• Hydrogen 100 ccm
• Nitrogen 50 ccm
• Average Temperature- 600C to 900C
• Time- 30 minutes
• Forms an oxide layer on the Nitinol surface
  allowing for faster growth of the diamond film

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The Deposition Process

• Mechanical Polishing
• Annealing step
• Seeding
• Ultrasonic Agitation
• Dry Seeding
• CVD

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Chemical Vapor Deposition

• Flow Rates
• Hydrogen 500 ccm
• Nitrogen 8.8 ccm
• Methane 88 ccm
• Average Temperature- 600C to 900C
• Number of Fringes- 2-5
• Film thickness measured by the number of fringes

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Optical Interference

• Is detected as fringes in the time vs.
  temperature plot during CVD
• Caused by destructive and constructive
  interference to the pyrometers detection of
  light waves emitted from the substrate

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Research Considerations

• Substrate temperature during the annealing step
• Ratio of the intensity of the Austenite Phase to
  the intensity of the Oxide Layer on the substrate
• Substrate temperature during CVD

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Substrate Annealing Temperature and Austenite to
Oxide Ratio (AO)

• May have a very large effect on the condition of
  a diamond film grown on Nitinol
• There is Negative correlation between the
  substrate annealing temperature and AO
• The lower the substrate temperature during
  annealing the higher the AO on the surface

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Substrate Annealing Temperature and Austenite to
Oxide Ratio (AO)
High Temperature Annealing
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Substrate Annealing Temperature and Austenite to
Oxide Ratio (AO)
Low Temperature Annealing
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Substrate Annealing Temperature and Austenite to
Oxide Ratio (AO)
Correlation between Substrate Mean Annealing
Temperature and Austenite to Oxide Ratio After CVD
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Substrate Austenite to Oxide Ratio and Diamond
Film Condition

• There is a correlation between the AO and the
  film condition
• The most adherent and continuous diamond films
  are grown on substrates that have high AO

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Substrate Austenite to Oxide Ratio and Diamond
Film Condition
A low AO yields a poorly adhered and
discontinuous film
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Substrate Austenite to Oxide Ratio and Diamond
Film Condition
A high AO yields an adherent and largely
continuous film
400x
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Substrate Deposition Temperature and Diamond Film
Condition

• The substrate temperature during CVD has a
  correlation to the condition of the resulting
  film
• Diamond films grown with a mean substrate
  temperature below 800ºC are poor in condition
• Diamond films grown with a mean substrate
  temperature above 900ºC are also poor in quality
• Diamond films grown between 800ºC and 900ºC are
  the best adhered and closest to continuity

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Substrate Deposition Temperature and Diamond Film
Condition
Shows a film after growth below 800ºC
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Substrate Deposition Temperature and Diamond Film
Condition
Shows a film after growth above 900ºC
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Substrate Deposition Temperature and Diamond Film
Condition
Shows a film after growth between 800ºC and 900ºC
843
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Conclusions

• The best diamond films are grown when
• The substrate annealing temperature is low and
  the resulting AO is high
• The substrate CVD temperature is between 800ºC to
  900ºC

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Questions??????