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Corrosion Testing for Medical Device Validation

Corrosion Testing for Medical Device Validation. Effect of Corrosion on the Body. Compatibility Tissue response Leach rates Toxicity. Corrosion Testing. Two aspects of in vivo corrosion: How susceptible is implant material to corrosion in vivo ?

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Corrosion Testing for Medical Device Validation

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  1. Corrosion Testing for Medical Device Validation

  2. Effect of Corrosion on the Body • Compatibility • Tissue response • Leach rates • Toxicity

  3. Corrosion Testing Two aspects of in vivo corrosion: • How susceptible is implant material to corrosion in vivo? • What is the effect of any corrosion (even very small amounts) on the body?

  4. Device Susceptibility: Corrosion Performance Validation Selected corrosion tests used to validate medical devices: • ASTM F 1801- Practice for Corrosion -Fatigue Testing of Metallic Implant Materials • ASTM F 1875 – Practice for Fretting Corrosion Testing of Modular Implant Interfaces: Hip Femoral Head-Bore and Cone Taper Interface • ASTM F 2129 – Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implants • ASTM G71 - Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes • ASTM F 746 – Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials

  5. Corrosion Testing • Rest Potential • Cyclic Polarization • Galvanic • Fretting

  6. Rest Potential Monitoring • Addressed by several standards • ISO 16429:2004 • Implants for surgery – Measurements of open-circuit potential to assess corrosion behaviour of metallic implantable materials and medical devices over extended time periods • ASTM F 2129-06 • Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices • Alternative standards • ISO 10271:2001 for dental materials • ISO 10993-15:2000

  7. Rest Potential Monitoring • Provides an opportunity to measure release of leachable substances, e.g., Ni, Cr, Co • Periodic solution analysis by ICP-MS Nickel Leach Rate (μg cm-2t-1) Immersion time (hours)

  8. Cyclic Potentiodynamic Polarization • Preferred test method • ASTM F 2129-06 • Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices • Extract potential data • Rest potential (Er) • Breakdown potential (Eb) • Alternative test methods • ISO 10271:2001 for dental materials • ISO 10993-15:2000 - not recommended

  9. ASTM F 2129 General Procedure: • Typically performed in saline environment at 37°C • PBS, 0.9% NaCl, simulated bile, etc. • Monitor rest potential (Er) for 1 hour • Potentiodynamic polarization to 0.8 or 1 volt vs. SCE • If breakdown, record potential (Eb) • Reverse potentiodynamic polarization • record repassivation potential (Ep) • reformation of the passive layer

  10. Vertex Potential, Ev Rest Potential, Er Cyclic Potentiodynamic Polarization • No breakdown • Good resistance to localized corrosion Potential V (SCE) Current mA cm-2

  11. Breakdown Potential, Eb Rest Potential, Er Cyclic Potentiodynamic Polarization • Breakdown observed Breakdown potential Potential V (SCE) Rest potential Repassivation potential Current mA cm-2

  12. Interpreting the Results • Cyclic Potentiodynamic Polarization • ASTM F 2129-06 is a deliberately aggressive test • General consensus that no breakdown up to 0.8 V (SCE) will provide sufficient resistance to localized corrosion in vivo • But if breakdown has been observed • How do we treat the data? • How good is good enough?

  13. Interpreting the Results • Neither ASTM F 2129, nor the FDA (or other regulatory agencies) provide specific guidance as to what constitutes an acceptance criterion • Two approaches using Eb • Compare with threshold for ‘optimum corrosion resistance’ • Criterion is independent of material and environment • Compare with that of a predicate device • Assumes suitable device is available • The breakdown potential alone, however, is not a good measure of localized corrosion resistance

  14. Interpreting the Results • Er and Eb are not intrinsic properties of a metal or alloy • For a given alloy, Eb and Er are influenced by - • The environment, e.g., pH, solution chemistry, temperature • Surface finish, e.g., mechanical polish vs. electropolish • Immersion time • Eb is also influenced by the test method • Potentiodynamic scan rate • Faster scan rates can increase the measured value of Eb

  15. Interpreting the Results • Consider the gap between the breakdown potential and the rest potential • Thus, a measure of an alloy’s susceptibility to localized corrosion is given by Eb - Er • The gap Eb - Er can be used to evaluate both pitting and crevice corrosion for a finished device • Because breakdown will occur at the most susceptible location whether it be a crevice or a pit-initiation site

  16. ASTM F 2129 Example of Typical Data Presentation: All potential values are in mV Er = rest potentialEzc = zero current potentialEb = breakdown potentialEp = repassivation potentialEv = vertex potentialNB = no breakdown

  17. Galvanic Corrosion • Perform ASTM G 71 tests on galvanic couples and individual anodes • Measure and compare steady corrosion rates (current densities) • Current increases of more than an order of magnitude are considered signficant • Also can compare coupled and un-coupled leach rates in longer-term leaching tests

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