发布时间 : 星期日 文章Graphitic Tribological Layers in Metal-on-Metal Hip Replacements - 图文更新完毕开始阅读5b0166b7f242336c1eb95ee8
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Fig.3BandTable1,theemissionlifetimeandquantumyieldofthe3MLCTexcitedstateof[Co(pyacac)3{Re(bpy)(CO)3}3]3+areidenticaltothatoftheGaRe3modelcomplex,anobservationthatindicatesacompleteabsenceofreactivitybetweenthecharge-transferexcitedstateoftheRe-bpyfragmentandtheCoIIIcore.
Ananalysisofthespin-coupledpathwaysfordipolarenergytransferavailableinthesetwosystemsprovidesasurprisinglysimpleexplanationforthismarkeddifferenceinphotophysicalbe-havior(Fig.4).Inbothcompounds,the3MLCTexcitedstatehasaspinmultiplicityof|SD*|=1;energytransferfromthisstatetotheM(pyacac)3coreresultsinreformationofthesingletgroundstateoftheRe-bpymoiety(|SD|=0).InthecaseofM=CrIII,the4A2groundstate(|SA|=3/2)createsaspinmanifoldinthereactantangularmomentumspacespanning|SR|=?,3/2,and5/2;thisrequirescouplingtoanexcitedstateoftheacceptorchar-acterizedby|SA|=?,3/2,or5/2inordertorealizeaspin-allowedpathway.Angularmomentumcon-servationisclearlysatisfiedwiththe4T2excitedstateoftheCrIIIcore(|SA*|=3/2),asarethermo-dynamicconsiderationsbyvirtueoftheresonantconditionthatexistsbetweentheRe-bpyemissionandthe4A2→4T2absorption.Thus,dipolarenergytransfercanproceedthroughthecommonalityofS=3/2statesinboththereactantsandproducts,andexcited-statequenchingofthe3MLCTemis-sionisobserved.UponreplacementofCrIIIbyCoIII,thethermodynamicsofenergytransferareessentiallyunchanged;however,thelow-spind6configurationoftheCo(pyacac)3corefundamen-tallyaltersthemomentumconservationcondition.Specifically,thephosphorescentnatureofthe3MLCT→1A1emissionrequirescouplingtoanexcitedstateoftheCoIIIhaving|SA*|=1,not|SA*|=0,whichdefinesthe1A1→1T1absorption.Di-polarenergytransferisthereforespin-forbiddenfortheCoRe3assembly,thusgivingrisetoemission
fromtheRe-bpyluminophorethatisindistinguish-ablefromthatoftheGaIIImodelcomplex.
Althoughthechemicalsystemsjustdescribedweredesignedspecificallytoillustratetheprin-cipleofangularmomentumconservationindi-polarenergytransfer,itdoesnotappeartousthatthisformalismshouldbelimitedtoenergytrans-fer.Inprinciple,aparallelsetofexpressionsforanychemicalreactioncouldbedraftedinwhichconsiderationofreactantandproductangularmomentaservestodifferentiatevariousthermo-dynamicallyviablepathways.Itseemslikelythattheissuesraisedhereinwillmanifestmoreread-ilyininorganicratherthanorganicsystemsbe-causeofthebroaderarrayofspinstatesgenerallyaccessibleinsuchcompounds;however,webe-lievethattheunderlyingconceptsreflectedinthissimpleformalismandexperimentallyverifiedinourstudyshouldbegeneralizableacrossawidearrayofchemicalprocesses.
ReferencesandNotes
1.L.D.Landau,E.M.Lifshitz,TheClassicalTheoryofFields(Butterworth-Heinemann,Oxford,1995).
2.K.K.Rohatgi-Mukherjee,FundamentalsofPhotochemistry(NewAge,NewDelhi,1978).
3.J.R.Lakowicz,PrinciplesofFluorescenceSpectroscopy(Springer,NewYork,ed.3,2006).
4.M.A.Baldoetal.,Nature395,151(1998).
5.D.Gust,T.A.Moore,A.L.Moore,Acc.Chem.Res.42,1890(2009).
6.E.Wigner,Nachr.Akad.Wiss.GoettingenMath.Phys.K1-2A,375(1927).
7.O.Kahn,MolecularMagnetism(Wiley-VCH,NewYork,1993).8.J.H.MooreJr.,Phys.Rev.A8,2359(1973).
9.C.S.Enos,A.G.Brenton,A.R.Lee,Int.J.MassSpectrom.IonProcess.122,361(1992).
10.R.Burgertetal.,Science319,438(2008).
11.B.T.Weldon,D.E.Wheeler,J.P.Kirby,J.K.McCusker,
Inorg.Chem.40,6802(2001).
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R.H.Holm,Inorg.Chem.40,4389(2001).
13.V.W.Manner,A.D.Lindsay,E.A.Mader,J.N.Harvey,
J.M.Mayer,Chem.Sci.10.1039/c1sc00387a(2011).14.J.N.Harvey,Phys.Chem.Chem.Phys.9,331(2007).15.A.A.Martíetal.,J.Am.Chem.Soc.129,8680(2007).
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18292(1996).
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Inorg.Chem.47,7249(2008).
20.GaIIIhasaclosed-shell,d10valenceelectronicconfiguration.
Assuch,itisneitherredox-activenordoesitpossesselectronicexcitedstatesthatareenergeticallyavailableforenergytransferinthevisibleregion.21.SeesupportingmaterialatScienceOnline.
22.J.N.Demas,J.W.Addington,J.Am.Chem.Soc.98,
5800(1976).
23.D.Rehm,A.Weller,Isr.J.Chem.8,259(1970).24.J.P.Claude,T.J.Meyer,J.Phys.Chem.99,51(1995).25.Y.B.Lei,T.Buranda,J.F.Endicott,J.Am.Chem.Soc.
112,8820(1990).
26.L.S.Forster,Chem.Rev.90,331(1990).
27.Theextinctioncoefficientsfor[Cr(pyacac)3{Re(bpy)(CO)3}3]3+andtheCrIIIreferencecompoundCr(phacac)3revealedthat~9%oftheincidentphotonsatlpump=375nmdirectlyexcitetheCrIIIcore;theremaining~91%areabsorbedbytheRe-bpymoiety.Whenthedataarescaledaccordingly,theobservedemissionintensityat80KislargerthancanbeaccountedforviadirectexcitationofCrIIIbynearlyafactorof10.Further
detailscanbefoundinfig.S2andtheaccompanyingtext.28.Estimatedfromthex-raystructureof
[Ga(pyacac)3{Re(bpy)(CO)3}3](OTf)3(21).29.D.L.Dexter,J.Chem.Phys.21,836(1953).
Acknowledgments:WethankG.Blanchardforassistancewiththetime-correlatedsingle-photoncountingemissionmeasurements,andA.BrownandC.McCuskerforpreliminaryspectroscopicmeasurementsontheCoRe3assembly.SupportedbyNSFgrantCHE-0911592.Metricalparametersforthestructureofcompound3canbeobtainedfreeofchargefromtheCambridgeCrystallographicDataCentreviawww.ccdc.cam.ac.uk/data_request/cifunderreferencenumberCCDC831982.
SupportingOnlineMaterial
www.sciencemag.org/cgi/content/full/334/6063/1684/DC1MaterialsandMethodsFigs.S1toS7TablesS1toS5References(30–39)
20July2011;accepted10November201110.1126/science.1211459
GraphiticTribologicalLayersinMetal-on-MetalHipReplacements
Y.Liao,1R.Pourzal,2M.A.Wimmer,3J.J.Jacobs,1,3A.Fischer,2,3L.D.Marks1*
Arthritisisaleadingcauseofdisability,andwhennonoperativemethodshavefailed,aprostheticimplantisacost-effectiveandclinicallysuccessfultreatment.Metal-on-metalreplacementsareanattractiveimplanttechnology,alower-wearalternativetometal-on-polyethylenedevices.Relativelylittleisknownabouthowslidingoccursintheseimplants,exceptthatproteinsplayacriticalroleandthatthereisatribologicallayeronthemetalsurface.Wereportevidence
forgraphiticmaterialinthetribologicallayerinmetal-on-metalhipreplacementsretrievedfrompatients.Asgraphiteisasolidlubricant,itspresencehelpstoexplainwhythesecomponentsexhibitlowwearandsuggestsmethodsofimprovingtheirperformance;simultaneously,thisraisestheissueofthephysiologicaleffectsofgraphiticweardebris.rthritis,orrheumatism,istheleadingcauseofdisability,affectinganestimated8.6millionpeopleintheUnitedStates
asof2005(1),withcomparableestimateselse-where.By2030,thenumberofAmericanadults
clinicallysuccessfultreatment.Currently,uptothreetotalhipreplacementproceduresper1000inhabitantsareperformedincountriesbelongingtotheOrganisationforEconomicCo-operationandDevelopment(2);Germanyleadswiththelargestnumberofreplacementspercapita,andtheU.S.performsthemostproceduresoverall.Asof2003,~202,500primarytotalhipreplace-mentswereperformedannuallyintheU.S.By2030,thisnumberisexpectedtoincreasebymorethan174%to~572,000hipreplacements.Arthritisofthehiphasanincreasinglylargepublichealthimpactintermsofmorbidity,disability,andthecostofdisabilityandtreatment.Tominimizetheimpactontheindividualpatientandsociety,it
DepartmentofMaterialsScienceandEngineering,North-westernUniversity,Evanston,IL60201,USA.2MaterialsScienceandEngineering,UniversityofDuisburg-Essen,Germany.3DepartmentofOrthopedicSurgery,RushUniversityMedicalCenter,Chicago,IL60612,USA.
*Towhomcorrespondenceshouldbeaddressed.E-mail:l-marks@northwestern.edu
1A
aged65andolder(thesegmentofthepopulationwiththehighestprevalenceofarthritis-relateddisability)willdoubleto~71million(1).Forindividualsafflictedwithend-stagearthritisofthehip,arthroplastyisthemostcost-effectiveand
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isimperativethathipreplacementsaredurable,ideallylastingfortheextentofthepatient’slife.Unfortunately,thematerialscurrentlyusedinhipreplacementshavelimiteddurability,inlargepartduetodegradationinservice.Thedegradativeprocessesincludewearandcorrosion,andthepro-ductsarebioreactivespecies(micrometer-andnanometer-sizedparticulatedebrisandmetalions)thatinteractwithlocaltissuesproducinganad-verseresponsethatmayleadtoimplantfailureandrequirerevisionsurgery.Anotherleadingcauseoftotalhipreplacementfailureispostoperativeinstability(dislocation)(3),whichcanbeminimizedbytheuseoflarge-diameterfemoralheads.Thisexplainsthepopularityofmetal-on-metal(MoM)bearings;forinstance,asusedinupto35%oftheprimarytotalhipreplacementproceduresper-formedintheU.S.MoMbearingsareattractive,astheyhavelesswearthanmetal-on-polyethylenebearings,therebyminimizingdebris-associatedfailures.Inaddition,theirmechanicalpropertiesallowthin-walledacetabularcomponentssothatlargefemoralheadscanbeused.Furthermore,MoMbearingsallowhipresurfacingproceduresthathelptopreservefemoralbonestock,whichishighlydesirableforyoungpatients.
However,thereareincreasingreportsthatMoMtotalhipreplacementsandsurfacereplace-mentsarenotimmunetotheadverselocaltissueresponsesduetodegradationproductsgeneratedbywearandcorrosionthatwereobservedwithearliermetal-on-polyethylenebearingcouples(4).InApril2010andFebruary2011,theregu-latoryagenciesoftheUKandtheU.S.,respec-tively,issuedalertsforMoMhipreplacements.Theseproblemsseemtoberelatedtospecificim-plantdesignsaswellasnonoptimalpositioning,andthereareprosthesesthathavebeenworkingsuccessfullyinpatientsfordecades(5).
ForcurrentMoMbearinghipreplacements,whichalmostalwaysinvolveausteniticcobalt-chrome-molybdenum(CoCrMo)alloys,afairamountisknownaboutthemetallurgyfarbelowtheslidingcontactregion(micronsaway)(6).AlthoughthemetalplaysanimportantroleinFig.1.(A)Thetribolog-icallayer(darkregion)wasscrapedoffwiththeuseofatungstenprobeintheFIB.(B)ThefilmwasattachedtoacopperTEMgrid.
loadbearing,theremustalsobesomelubricationmechanismwithinthehumanbody,asotherwisethewearrateswouldbeprohibitiveandseveremetaltoxicitywouldbecommon.ProteininthesurroundingfluidisknowntoplayacriticalroleforMoMreplacements(7–10).Todate,relative-lylittleisknownabouttheregionwhereactiveslidingtakesplace(boundarylubrication),thetriboactiveregion—onlythatthereisacarbon-richlayer.Wimmeretal.examined42retrievedMoMMcKee-Farrarprosthesesandreportedthatmorethan80%ofthemexhibitedtribologicallayersadheredfirmlytothesurfaceswithvaryingthick-nesses(11).Thetribologicallayerwasalsoformedonmetalcomponentsinsimulatortestsinbovineserum.Itwassuggestedthatthislayerwasgen-eratedduringoperationinhumansynovialfluidorbovineserumenvironments(6,12).Analysesshowedthatthistribologicallayercontainednanoparticles(presumablydetachedfromthesubstrate)ofthemetalandasubstantialamountofcarbonalongwithpossiblecalcium,oxygen,phosphorus,magnesium,nitrogen,sodium,andchlorine(11,13,14).However,relativelyharshmethods,whichcouldeasilydamagecarbon-containingcompounds,wereusedtopreparethesamples.Otherexperiments,focusingmoreonthedegradationproductswithsomesurfacechar-acterization(15,16),alsopointtowardatribo-logicallayerwithdifferentchemistry/propertiesplayingacriticalrole,plussomechemicalchangesaslower–molecularweightfragmentswereob-served.Ithasbeencommonlyassumed(withoutproof)thatthetribologicallayerismadeupofdenaturedproteins(6,11,15–18);thatis,proteinsthathavelosttheirhigher-orderstructure,withperhapssomeminorchemicalchanges.Assuch,itwouldbeintrinsicallybiologicalincharacter,similartolubricationinnaturaljoints.Wefind,however,thatthislayerisverydifferentbothinformandfunction,havingmoreincommonwithlubricationofacombustionenginethanlubri-cationofanaturaljoint.
Ourinitialaimwastounderstandthetribo-logicallayer,mainlytheembeddedmetallicnano-particles,andtolookforprocessessuchascarbonsegregationtograinboundariesinthenear-surfaceregionofthemetal.Intheprocessofroutineini-tialcharacterizationbytransmissionelectronmi-croscopy(TEM)ofsamplesthinnedwithafocusedionbeam(FIB),wenoticedthatelectronenergy-lossspectra(EELS)showedastrongp*prepeak,awell-knownfingerprintofgraphiticcarbon(19,20);high-resolutionelectronmicroscope(HREM)im-agingexperimentssuggestedasimilarconclu-sion(seefigs.S1andS2).
Byitself,thisisnotsufficientproofofgra-phiticcarbon,astheelectronbeaminthemi-croscopecanchangeorganicmaterials;onealsoneedstobeconcernedaboutthesampleprepa-rationmethods.Foradefinitivetest,weneedtoruleoutartifactsfrombothofthesetechniques.Wethereforeusedananomanipulatorinsideadual-beamscanningelectronmicroscopy/FIBsystemtoscrapethetribologicallayeroffofsamplesretrievedfrompatients,asshowninFig.1.Wethengeneratedtwocontrolsamples(21):(i)driedbovinecalfserum,scrapedoffasubstratemadeofCoCrMoalloyusingFIBunderthesameconditions(control1),and(ii)free-standingdriedbovinecalfserumfilmoneitheracoppergridorasiliconmembrane(control2).
200000
serum dried on copper grid
Intensity (counts)150000
49 e/?2
10000050000
3.8x104 e/?2
0
250300350400450
Energy Loss (eV)
Fig.2.EELSspectrumoftheserumdriedonacoppergridfordifferentelectrondoses,withthe49e/?2spectrumoffsetby100,000counts.Thechangewithadoseof3.8×104e/?2wasminimal.
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Comparingcontrol1withcontrol2,thecar-bonpeakofthebovinecalfserumscrapedoffusingthemicroprobeintheFIBisessentiallythesameasthatofserumdrieddirectlyonacoppergridorasiliconmembrane,indicatingthatspec-imenpreparationintheFIBdidnotgenerateanygraphiticcarbon.
Fig.3.EELSspectrumofthetribologicallayer,driedbovineserum,andgraphite.Thep*peakisabsentinthedriedserumbutclearlypresentinthetribologicallayer,withansp2bondingfractionof82%.
Fromthecontrolexperiments(Fig.2),weobservethatatotaldoseof~3.8×104electrons(e)per?2doesnotseverelydisturbthecontrolsamples.Itisworthnotingthatthecarbonp*peakdidformunderaveryhighelectrondoseof5×108e/?2(fig.S3).Afinalcalibrationexperi-mentwastolookatthechangeoftheEELS
spectrumasafunctionofdoseforthetribologicallayers.Supplementalfig.S4showsthatEELSspectraofthetribologicallayerdidnotdisplayanydiscerniblechangeuptoadoseof1.1×105e/?2.Thus,theelectrondosedidnotaffectthep*intensityduringinitiallow-magnificationTEMimagingandEELSspectrumcollection,aswouldbeexpectedbasedonworkontribo-inducedgra-phiticmaterialinnearlyfrictionlesscarbon(22).ThemainexperimentalresultsareshowninFig.3.Atotalofthreesamples[fromthefirstre-trieval(21)]analyzedwithadoseof~58e/?2allshowedaclear,strongpre-edgep*peakaswellaslowoxygensignals,asdidadditionalsamplesretrievedfromtwootherpatients,onefromeach(21).Insomeareascalciumwasalsopresent,whichwebelievecamefrombonefragments.Usinghighlyorderedpyrolyticgraphiteasacalibrant,wedeterminedtheamountofgraphiticcarbonas~82osedontheintensitiesofthep*ands*peaks(23,24).Thetribologicallayerdidnothaveanynitrogen,whereasdriedbovinecalfserumexhibitedastrongnitrogenpeak.BovinecalfserumdriedonacoppergridhasexperimentalC:N:OmolarratiosofC:N:O=1:0.17:0.46intheEELSdata(25),whereasforbovineserumalbuminthetrueratiosareC:N:O=1:0.27:0.30.TheC:OmolarratiointhetribologicallayerisC:O=1:0.07.Usingtheoxygen-integratededgesignalfromcon-trol2asacalibrant,weestimatetheatomicpercentofoxygeninthetribologicalmaterialtobe5%.Figure4showsatypicalHREMmicrographofthetribologicallayeralongwithapowerspec-trum;everyareathinenoughforgoodimagingthatwasexaminedshowedsimilarresults.Short-rangeorderedregionsarepresentwithatypicalsizeofseveralnanometers.Thespacingofthedominantfringesintheimagewasmeasuredtobe~0.34nm,confirmingthatthetribologicallay-erwasprimarilypartiallygraphitizedcarbon.Asasecondaryconfirmation,aRamanspec-troscopyresultforthetribologicallayerfromthefirstretrievalisshowninFig.5.AbroadGlinecorrespondingtothestretchvibrationofsp2bondingwaspresent,togetherwithastrongDlinearound1383cm?1correspondingtothebreathingvibrationindisorderedsp2carbon(26).Forcomparison,boththeGandDlineswereabsentinthedriedserumonaCoCrMosub-strate(fig.S5).TheG-linepositionofthetribo-logicallayerwas1567cm?1,slightlyshiftedfrom1580cm?1ofgraphitecrystalduetotheforma-tionofnanocrystallites(27).UsingtheanalysisofcarbonstructuresbyFerrariandRobertson(27),thetribologicallayerwasinbetweennanocrystal-linegraphiteandamorphoussp2carbon[stage2in(27)]andwasclosertothenanocrystallineside.Thefractionofsp2bondingwasdeterminedfromtheG-linepositiontobe>80osedontheplotsin(27,28).NotethatthepresenceofabroadDlinealsoindicatedtheexistenceofdisorderedgraphiticcarbon.Thegraphiticdomainsize(29),La,canbecalculatedfromtherelativeintensities(21,29,30)as4.5nm,consistentwiththeHREMobservations.
NIntensity (counts)C:N:O=1:0.17:0.46O82% sp2dried bovine serumC:O=1:0.07tribological layergraphite250300350400450500550Energy Loss (eV)
Fig.4.HREMimageofthetribologicallayer.Short-rangeorderedgra-phiticmaterialwithagrainsizeofafewnano-metersispresent.Thein-setpowerspectrumshowsthatthedominantspacingis~0.34nm.
Raman Intensity (counts)Fig.5.RamanspectrumofthetribologicallayerforaCoCrMohipimplantafterbackgroundsub-tractionshowingtheG(red)andD(blue)lines.ThedatahasfitusingtwoGaussianpeaks(purple).
1250010000750050002500
01200
G
D
130014001500160017001800
Raman Shift (cm-1)
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SimilarRamanspectratoFig.5indicatingagraphitictribologicallayerwerefoundatdif-ferentlocationsofthefirstretrievalandfromfourotherretrievals[bothwroughtandcastalloys(21)],aswellasinpin-on-balltests.SpatiallyresolvedRamanimaging(fig.S6)indicatedadefinitivecorrelationbetweenwherethegra-phiticRamansignalcamefromandwherethetribologicallayerwaspresent.Itshouldbenotedthatthereisextensiveliteratureevidenceforacomparabletribologicallayerinmanyotherre-trievalsandinsimulatortests.
Allofthedataindicatethatthetribologicalmaterialhasagraphiticcontentsimilartopar-tiallygraphiticcarbon.Althoughwecannotruleoutasmallimpurityofproteininthetribologicallayer,thefactthatthereisminimalnitrogen,thatahighsp2fractionprecludesahighhydrogencontent,plusthecorrelationoftheRamanandHREMdataastothestructureallowustoinferthatthefractionofproteinislessthan5%,andanyproteinpresentshouldbeconsideredaspartiallydegradedratherthandenaturedprotein.Therearethreeimmediateconsequencesofthisresult—thefirsttwoareimportantforortho-pedicapplications,whereastheotherisanissueforconcern.Thefirstisthatgraphiteisastandardsolidlubricantthatisknowntoperformwellinthepresenceofwater(31,32)andshouldoperatesimilarlyinvivo.Inthetribologicalregion,itwillservethispurpose,reducingfrictionaswellascorrosionandwear,similartosolidlubricantsinanengine.Morethanjustflash-heatingorfric-tionalshearofproteinsisleadingtothislayer;insteadmajorchemicalchangesaretakingplacethathavebeenhintedatinthedegradationproductanalysesmentionedearlier.Althoughwedonotknowtheprecisemechanismwherebythegraphiticmaterialisformed,itisknownthatmanytransitionmetals(here,probablycobalt)willactascatalyststoeliminatewaterorammoniafromorganicma-terials(33,34),similartothewell-knowncokingofheterogeneouscatalysts;freshmetalsurfacesexposedbywearshouldbegoodcatalysts.Asacrudeestimate,thecompositionofalbuminisC3076H4833N821O919S42,andeliminatingwater,ammonia(andhydrogensulfide)wouldgiveanominalcompositionofC3076H448,whichiscom-parabletotypicalhydrogenatedcarbonfilmsusedtoreducefriction.Thereisextensiveevi-dencefortheformationofgraphiticmaterialfromothercarbonallotropesduringsliding,soacon-versionfrommoredisorderedoramorphouscar-bontoagraphiticmaterialisexpected.
Thesecondconsequenceisthatthereisnowadesigntargetforimprovingimplants.Forinstance,onecoulddesigntoimproveadhesionofthegra-phiticlayertothemetalortopromoteitsformationbychangingthealloycompositionandthusreduc-ingwear,friction,andcorrosion.Biocompatibilityofanyadditiveswillbeanissue,sotheproblemisnotassimpleasdesigningabetteralloy.
Lastly,asacaution,wearofthisgraphiticmaterialisgoingtoleadtotheformationofgra-phiticfragmentsinthepseudosynovialfluids,
andthesecanbetransportedtocellsinthenearbyregions.Therecentfinding(4)ofalackofcor-relationbetweentissuedamageandvolumetricwearratesuggeststhatfactorsotherthancobalt-alloyweardebrismaybecontributingtoadversetissuereactions.Graphiticcarbonmaybeonesuchfactor;furtherresearchisneeded.
Insummary,wehavepresentedclearevi-dencethatthetribologicallayerinMoMhipreplacementsisprimarilygraphiticcarbon.Thismaterialformsalayerthatreducesfrictionaswellaswearandcorrosionandsuggestsarouteforthedesignofimprovedimplants,althoughtheremightbephysiologicalimplicationsasso-ciatedwithgraphiticweardebrisinpatients.
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Acknowledgments:ThisworkwasfundedbytheNIHongrantnumber1RC2AR058993-01.NorthwesternUniversityAtomicandNanoscaleExperimentalCenterandBiologicalImagingFacilityareacknowledgedfortheuseoftheirfacilities.WethankK.Shull,K.Wahl,andR.LeapmanforinvaluablecommentsandM.Morlock,Hamburg,Germany,forretrievals.Theauthorshavenocompetingfinancialinterests.Asfulldisclosureofrealorpotentialconflictsofinterest,J.J.J.hasfundingfromZimmerHoldings,Medtronics,andSpinalMotion;consultsforZimmer,Medtronics,JohnsonandJohnson,SpinalMotion,andImplantProtection;andhasstockoptionsinImplantProtection.A.F.hasfundingfromZimmerWinterthurandconsultsforZimmer,AbbotVascular,andCeramtec.
SupportingOnlineMaterial
www.sciencemag.org/cgi/content/full/334/6063/1687/DC1MaterialsandMethodsFigs.S1toS6
12September2011;accepted1November201110.1126/science.1213902
EvidencefromNumericalExperimentsforaFeedbackDynamoGeneratingMercury’sMagneticField
DanielHeyner,1*JohannesWicht,2?NataliaGómez-Pérez,3?DieterSchmitt,2Hans-UlrichAuster,1Karl-HeinzGlassmeier1,2TheobservedweaknessofMercury’smagneticfieldposesalong-standingpuzzletodynamotheory.Usingnumericaldynamosimulations,weshowthatitcouldbeexplainedbyanegativefeedbackbetweenthemagnetosphericandtheinternalmagneticfields.Withoutfeedback,asmallinternalfieldwasamplifiedbythedynamoprocessuptoEarth-likevalues.Withfeedback,thefieldstrengthsaturatedatamuchlowerlevel,compatiblewiththeobservationsatMercury.TheclassicalsaturationmechanismviatheLorentzforcewasreplacedbytheexternalfieldimpact.Theresultingsurfacefieldwasdominatedbyunevenharmoniccomponents.ThiswillallowthefeedbackmodeltobedistinguishedfromothermodelsonceamoreaccuratefieldmodelisconstructedfromMErcurySurface,SpaceENvironment,GEochemistry,andRanging(MESSENGER)andBepiColombodata.
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nMarch2011,theMErcurySurface,SpaceENvironment,GEochemistry,andRanging(MESSENGER)mission(1)enteredorbit
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aroundMercury.Oneofitsobjectivesistoex-ploretheplanet’smagneticfield,whichispre-sumedtobegeneratedbyadynamooperating
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