Surface Characterization of Electro-polished Nitinol Devices by Auger Analysis

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Surface Characterization of Electro-polished Nitinol Devices by Auger Analysis Siobhan Carroll Boston Scientific Structural Heart Division, Los Gatos, California Jay Yang

1 Surface Characterization of Electro-polished Nitinol Devices by Auger Analysis Siobhan Carroll Boston Scientific Structural Heart Division, Los Gatos, California Jay Yang Independent Nitinol Consultant, Los Gatos, California John Moskito Evans Analytical Group, Santa Clara, California Patrick Schnabel Evans Analytical Group, Santa Clara, California 1

2 Purpose of Study Auger analysis is part of a test suite to show medical devices demonstrate acceptable corrosion resistance and passivity ensuring structural integrity and biocompatibility. Reference FDA Guidances, EN ISO

3 Auger Electron Spectroscopy (AES) AES is a popular surface sensitive analytical technique that provides elemental information from the first few atomic layers of a material The discovery of the Auger Effect is widely accredited to Pierre Auger, 1920s. (Image courtesy EAG Labs) 3

4 Auger Spectrometer Source Electrons bombard the sample. Emitted electrons are directed and collected by the detector and results are processed by the image analyzer. An optional ion sputtering source is used for depth profiling. 4 (Image source Wikimedia Commons)

5 Sample Size of Auger Spectroscopy Auger can be used for very localized analyses of different types of areas even on the same sample. Incorporating Ion Sputtering with Auger gives Depth Profiling. 5 (Image courtesy EAG Labs)

6 Auger Electron Spectroscopy (AES) Typical Auger depth profile data (annotated) Titanium-rich oxide layer à Oxide Layer 80 C0CGR pro Ni1 TiTi(NiTi) O1.ls1 C1.ls1 Atomic Concentration (%) Oxygen Nickel-rich sub-oxide region Nickel Titanium Oxide layer thickness can be defined several ways Be consistent! Adsorbed hydrocarbon layer à (Due to exposure to air) Carbon (Image courtesy EAG Labs) Sputter Depth (Å ) 6

7 Summary of Study Conditions Electro-polished Wet Chemical Sterilization Post-Fatigue Rod / Wire Difficult EP Easy EP N = 12 Rod / Wire Difficult EP Easy EP N = 16 Rod / Wire Contact Non-Contact N = 16 Fatigue Conditioning: million cycles (2-6 months real-time) 37 degc Saline 7

8 Examples of Sample Extremes Exposed areas with simple geometries are the easiest to electro-polish. Difficult areas to Electro-polish are wire intersections, holes and complex geometries 8

9 Data Summary by Condition The oxide layer thickness is consistent prior to fatigue and varies greatly post-fatigue. Interval Plot of Oxide Thickness (A) O xide Thickness (A) Electro- polish 53 EP + Sterilization Condition Post Fatig ue 9

10 Data Summary by Sample Type The type of sample does not have a large effect on the passivation layer thickness. 10

11 Effect of Sampling Location Individual Value Plot of Oxide Thickness (A) 95% CI for the Mean 1000 O xide Thickness (A) Consistent at the EP process extremes Large variation in oxide layer with fatigue contact Continued passivation layer development 0 Area Condition Difficult EP Easy EP Electro- polish Difficult EP Easy EP EP + Sterilization Contact Non- contact Post Fatig ue Individual standard deviations were used to calculate the intervals. 11

12 Passivation Layer Thickness by Condition The surface oxide is thinnest directly after electro-polishing and continues to develop as a result of real-time immersion in saline, simulating postimplantation surface behavior. 12

13 Passivation Layer Thickness post-fatigue Post-Fatigue Contact is defined as physical dynamic contact nitinol-nitinol or nonmetallic-nitinol such as organic materials. Contact variables are materials and type of contact 13

14 Complete Data There is the most variation post-fatigue. Individual Value Plot of Oxide Thickness (A) Oxide Thickness (A) Area Difficult EP Easy EP Difficult EP Easy EP Difficult EP Easy EP Difficult EP Easy EP Contact Non- contact Contact Non- contact Sample Type Condition Rod Electro- polish Wire Rod Wire EP + Sterilization Rod Post Fatig ue Wire After EP + Chemical Sterilization + Fatigue 14

Conclusions AES can be useful as a process development tool to assess process capability. It compliments other corrosion and metallic leaching assessments.

15 Conclusions AES can be useful as a process development tool to assess process capability. It compliments other corrosion and metallic leaching assessments. Since it contains microscopic location-specific information, it can also be used to evaluate surface condition variations The oxide layer thickens in areas of contact during fatigue. The continued development of the passive oxide layer in simulated in vivo conditions may result in improved corrosion behavior after implantation. This is a area of continued research. 15

16 Thank you! Questions? 16

Corrosion Resistance and Biological Response to Nitinol Christine Trepanier

Corrosion Resistance and Biological Response to Nitinol Christine Trepanier Nitinol Devices & Components, Fremont, CA Outline 1. Background 2. Surface properties 3. Corrosion resistance 4. Biocompatibility