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Corrosion Testing for Medical Device Validation

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Presentation on theme: "Corrosion Testing for Medical Device Validation"— 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:
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 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-2t-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 (Er) Breakdown potential (Eb) 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 (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 Cyclic Potentiodynamic Polarization
No breakdown Good resistance to localized corrosion Vertex Potential, Ev Potential V (SCE) Rest Potential, Er Current mA cm-2

11 Cyclic Potentiodynamic Polarization
Breakdown observed Breakdown potential Breakdown Potential, Eb Potential V (SCE) Rest potential Repassivation potential Rest Potential, Er 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 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: Device Er Ezc Eb Ep
Eb-Er Ep-Er Test 1 -305 -337 809 420 - 1104 725 Test 2 -410 -442 NB 1200 Test 3 -376 -393 Test 4 -311 -350 Test 5 -420 -449 740 332 1160 752 Test 6 -431 -452 Average -404 775 376 1132 739 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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