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MedicalDeviceCorrosionTestingThehumanbodyanditsimmuneandinflammatorymechanismscancreateanaggressiveenvironmentformetallicimplantsresultinginimplantrejectionandfailure(Gilbert,Corrosion,2017;73(12):1478-1495).InvitrotestingofsmallimplantsistypicallyperformedperASTMF2129-17btomitigatetheserisks.Whilethistestinghelpsdetermineimplantsusceptibilitytocorrosion,itdoesnottakeintoaccounttheeffectsofcells,proteins,andotherbiologicaleffectsthatimplantswillexperienceinvivo.Despiteitslimitations,ASTMF2129-17bhasundergoneseveralrevisionssinceitsoriginalpublicationin2001tobetterassistmedicaldevicemanufacturersinassessingtheirsmallimplants’resistancetoputtingandcrevicecorrosion.Thestandardprovidesatestmethodforconductingcyclic(forwardandreverse)potentiodynamicpolarizationmeasurementsonwholedevicesintheirfinalformandfinish.Inthesectionsbelow,wedescribethetestmethods,apparatus,resultsandacceptancecriteriausedinvitrocorrosiontestingofsmallimplants. TestApparatus Atypicalmultiportelectrochemicaltestcellisshownontheright.Ithasinflow/outflowportsforthenitrogengasandheatedwatercirculation,portsforthereference,graphitecounterandworking/sampleelectrodes,andaninnerchamberforthesalinebath.Additionalcomponentsinclude:potentiostat,computer,pHmeter,nitrogentank,waterpump,conductiveepoxywithUVlampandmaskforcuring,andstainlesssteelwireforsampleattachment.Reagentssuchasphosphatebufferedsalineareusedforthebath,andisopropylalcoholandtypeIIreagentgradewaterforsamplerinsing.Opticalandelectronscanningmicroscopesareusedforpre-andpost-testinspections. TestMethod Table1providesabriefterminologysummary;forfurtherdetailsconsulttheASTMF2129-17bstandard.
5929BakerRoad,Suite430·Minnetonka,MN55345·P:952.933.1152·F:952.933.1186·E:[email protected]
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Table1:TerminologyEb Breakdownpotential Potentialatwhichpittingorcrevicecorrosionwillinitiateand
propagate.Er Restpotential Potentialintheopencircuitbeforecyclictestbegins.Ef Finalpotential Presetpotentialwherethetestisstopped.Ei Initialpotential Initialpotentialforthecyclictest;sameasEr.Ep Protectionpotential Potentialatwhichthereversescancrossestheforwardscanlower
thanEb.Ifasurfaceisalreadypitted,furtherpittingcanbeginunderEbbutaboveEp.
Ev Vertexpotential Presetpotentialwherethescanreverses.It Thresholdcurrent
densityPresetcurrentdensityatwhichthescandirectionisreversed.Typically,thescanisreversedwhenacurrentdensitytwodecadeshigherthanthecurrentdensityatthebreakdownpotential,Eb,isreached.
Briefly,theimplantisheldinsalineat37±1°CwithstartingpHat7.4±0.2inthesetupshownontheright.Apotentialisintroducedbetweentheimplantandareferenceelectrodeandrampeduptoabovephysiologicalpotentials.Abreakdownwilltypicallywillresultinarapidincreasecurrentdensity,asevidencedontheoutputplotofpotentialvs.logcurrentdensity.Thetotalsurfaceareaofthesampleexposedtothesolutionneedstobecalculatedsothatthecurrentdensitygeneratedbythesamplecanbedetermined.Samplesarepreparedfortestingandthoroughlyinspected.Conductiveepoxyisusedastheinterfacetothesample(specimenholdersvarydependingonthedevicetype),connectedtoapotentiostatthroughawire,andresistanceischeckedtoensuregoodconnection.Thetestcellisfilledwithsaline(minimumof500mL,typically1L),heatedtothetargettemperature,andnitrogengasflowat150mL/minisinitiated.Nitrogengasflowisusedtodeaeratethesolutionandlowerdissolvedoxygenconcentrationtodecreasethepotentialatwhichoxidationandreductioncurrentsareequal. Temperature,pH,andnitrogengasflowtime(30minminimum)arerecorded,thenthedeviceisrinsedandinsertedinthetestcellwiththewire.Salineisaddedintothereferenceelectrodeholder,andtheelectrodeisinsertedat5mmorlessfromthesample,butnotincontactwiththesample.Thenitrogenpurgeiscontinuedthroughoutthetest.Theopencircuittestisinitiated,andErisrecordedafter1hour.Then,thecyclicpolarizationtestisinitiatedata1mV/sscanratebysettingEiandEftoEr.Thecyclicpolarizationiscloselymonitored;ifacurrentdensityincreaselargerthantwodecadesoccurs,voltageisreversed. Whenthetestiscompleted,thefinaltemperature,pH,Er,Eb,Ef,andEparerecorded.Thesampleisremovedfromthetestandrinsed,thewireisclipped~1inchabovethesample,andthesampleisstoredforfurtherpost-testinspection.Whilethestandarddoesnotindicatehowmanysamplesshouldbetested,wetypicallysee3-8samplespersurfacefinish.
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TestResults Testconditionsarerecordedforeachsample:nitrogenflowtime,initialtemperature,initialpH,finaltemperatureandfinalpH.Sampletestresultsaretypicallyprovidedintabularandgraphicalformats.Pre-andpost-testobservationsandinspectionresultsarealsoprovided. Thegraphaboveshowsresultsfromacorrosiontestof8samplesofanitinoldeviceplottedaspotential(mV)vs.logcurrent(A).Resultsshowconsistencybetweenthe8samples;thepurpledatasetwiththeincreaseincurrentdensityisanexampleofabreakdownpotential.AcceptanceCriteria ASTMF2129-17bdoesnotdefineacceptancecriteriabutidentifiesEbasthecriticaldatapointabovewhichpitsnucleateandgrow.Therefore,thehighertheEbis,themoreresistantthemetalistopittingcorrosion.ItisalsorecommendedthatEbvaluesarecomparedtothoseofpredicatedevicesthathavedemonstratedgoodcorrosionresistanceinvivoasreference.Devicemanufacturersdefinetheacceptancecriteriafortheirimplants.Inourexperience,acceptableEbvaluesaremostcommonlysetabove300mV,andtheyareoftensetatmuchhigherthresholdvalues. ConsultationWeoftenperformcorrosiontestingonmedicaldevicestocharacterizetheircorrosionpotential.Ifyouwouldlikeassistancewithyourdevice’scorrosionand/orfatiguetesting,pleasefilloutourtestquestionnaireorcallusat952-933-1152todiscusshowwecanhelp. ElementMaterialsTechnologyoffersthebroadestscopeofmedicaldevicetesting,soifyouarelookingforservicessuchaspackagetesting,microbiologicaltesting,acceleratedshelflifetestingorEMC/EMItesting,contactustoconnectwiththerightlab.
