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Corrosion Testing for Medical Device Validation. Effect of Corrosion on the Body Compatibility Tissue response Leach rates Toxicity.

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Presentation on theme: "Corrosion Testing for Medical Device Validation. Effect of Corrosion on the Body Compatibility Tissue response Leach rates Toxicity."— Presentation transcript:

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: 1.How susceptible is implant material to corrosion in vivo? 2.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 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 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 :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 -2 t -1 ) Immersion time (hours)

8 Cyclic Potentiodynamic Polarization Preferred test method –ASTM F Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices Extract potential data –Rest potential (E r ) –Breakdown potential (E b ) Alternative test methods –ISO 10271:2001 for dental materials –ISO : 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 (E r ) for 1 hour Potentiodynamic polarization to 0.8 or 1 volt vs. SCE –If breakdown, record potential (E b ) Reverse potentiodynamic polarization –record repassivation potential (E p ) –reformation of the passive layer

10 Cyclic Potentiodynamic Polarization No breakdown Good resistance to localized corrosion Rest Potential, E r Vertex Potential, E v Potential V (SCE) Current mA cm -2

11 Cyclic Potentiodynamic Polarization Rest Potential, E r Breakdown Potential, E b Breakdown observed Breakdown potential Repassivation potential Rest potential Potential V (SCE) Current mA cm -2

12 Interpreting the Results Cyclic Potentiodynamic Polarization –ASTM F 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 E b –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 E r and E b are not intrinsic properties of a metal or alloy For a given alloy, E b and E r are influenced by - –The environment, e.g., pH, solution chemistry, temperature –Surface finish, e.g., mechanical polish vs. electropolish –Immersion time E b is also influenced by the test method –Potentiodynamic scan rate –Faster scan rates can increase the measured value of E b

15 Interpreting the Results Consider the gap between the breakdown potential and the rest potential Thus, a measure of an alloys susceptibility to localized corrosion is given by E b - E r The gap E b - E r 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: Device ErEr E zc EbEb EpEp EvEv E b -E r E p -E r Test Test NB Test NB Test NB Test Test NB Average All potential values are in mV Er = rest potential Ezc = zero current potential Eb = breakdown potential Ep = repassivation potential Ev = vertex potential NB = 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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