Surface microstructuring for controlled drug release in coronary stents [Elektronische Ressource] / Michael Stöver

TechnischeUniversitätMünchen
LehrstuhlfürMedizintechnik

SurfaceMicrostructcouringronaforyrStecontntsrolledDrugReleasein

vStöhaelMicer

VollständigerAbdruckdervonderFakultätfürMaschinenwesenderTechnischen
UniversitätMünchenzurErlangungdesakademischenGradeseines

genehmigtenDissertation

Dr.-Ing.)(enieuroktor-IngD

Vorsitzender:Univ.-Prof.Dr.-Ing.HorstBaier

PrüferderDissertation:

1.Univ.-Prof.Dr.med.,Dr.-Ing.habilErichWintermantel
2.Univ.-Prof.Dr.SannakaisaVirtanen,
Friedrich-Alexander-UniversitätErlangen-Nürnberg

DieDissertationwurdeam13.09.2006beiderTechnischenUniversitätMünchen
eingereichtunddurchdieFakultätfürMaschinenwesenam21.11.2006angenommen.

Abstract

Thisthesisintroducesanoveltypeofelectrochemicalmicrostructuringfor316L
stainlesssteel.Themicrostructuringmethodsweredevelopedasabasisforadrug
elutingcoronarystentsystem.Coronarystentsareusedinordertodilatenarrowed
arteries.Inordertopreventinflammationreactions,whichleadtoanexcessive
proliferationofcellsandthustoarenarrowingoftheartery,thenewgenerationof
stentsarecoatedwithdrugs.Theaimofthisthesiswastocreatethreedimensional
microstructuresonthesurfaceasabasisfordrugcoatings.Themicrostructuresare
supposedtoprovideaprotectionforthedrugduringtheimplantationprocessand,
moreover,controlthereleaseofthedrugbyprovidingmicrodepotsretainingacertain
amountofthedrug.Itwasfoundthatsuitablemicrostructurescanbegenerated
byacombinedetchingmethodcomprisingafirstelectrochemicalgrainboundary
etchingstepfollowedbyanisotropicetchingstep.Thefirststepisperformedwith
nitricacidandcreatesamicrogridofnarrowgrainboundaryfurrows.Thesefurrows
arehollowedoutbyasecondetchingstep,carriedouteitherwithphosphoricacid,
hydrochloricacidoroxalicacid.Thegeneratedstructureswerefoundtobesuperior
orequalintermsofcorrosionpropertiesandcellproliferationbehaviorcompared
toconventionalgritblastedstents.Itwasshownthattheinvitroreleasebehavior
couldbesubstantiallydecelerated,withuptothreetimesmoredrugretentionafter
thefirstweekofreleaseincomparisontogritblastedstents.

I

sungZusammenfas

EswurdeeinneuesVerfahrenzurOberflächenmodifikationvonbiokompatiblem
MikrEdelstahlometerbentwicereickhelt,ausmitdendemintrdurcinsischhenelektroMatchemiscerialstrukheÄturentzungerzeugtMikrwoerstrdenukturkenönnen.im
DasVerfahrendientalsBasisfüreineMedikamentenbeschichtungvonStützenfür
verengteHerzkranzgefässe(Stents).EingroßesProblembeidieserBehandlungist
eineDiesehkoheanndurcWiedervheineerscBeschlussrate,hichtungdiedesdurchStenEntsmitzüntendungsrtzünduneaktgionenshemmendverursacenhtMedikwird.a-
mentenverhindertwerden.MitHilfedesVerfahrenswerdenineinemzweistufigen
ÄtMedikzprozamessentaMikroufzunehkavitätenmenundverzeugt,erzögdieertindfreizuseterzLagene.Essind,wurdeeinenagechwigsseewiesen,Mengedaassn
dieerzeugtenOberflächeninHinblickaufinvitroKorrosions-undZellbesiedelungs-
eigenschaftenbesserodergleichwertigzukonventionellensandgestrahltenOber-
flächensind.DerinvitroMedikamentenReleasekonnteerheblichverzögertwerden,
dieerzeugtenOberflächenwiesennacheinerWocheReleasenocheineimVergleich
zusandgestrahltenStentsbiszudreimalhöhereMengeangespeichertemMedika-
auf.tmen

II

Acknowledgements

IliketothankProf.Dr.Dr.ErichWintermantelforprovidingmetheopportunity
todevelopthisthesis,forhiscordialityandtheproductiveatmosphereinhisinsti-
e.tutMyrefereeProf.Dr.SannakaisaVirtanenalsodeservesmygratidueforherinterest
inthetopic,hersupportandherproductiveinputs.
SpecialthankstoDr.JuliaWillwhoguidedmethroughthefirstandmostdemand-
ingpartofthisthesisandcontributedsubstantiallytothesuccessofthiswork.
IalsowouldliketothankDr.JuliaKunzeforsupervisingtheelectrochemicalanal-
ysis.IamcommittedtogreatthanktoGernotKrobathforthetirelessthoroughand
criticalproofreading.InthiscontextIalsoliketothankDr.MarkusEblenkamp
andMichaelBrosch.
MycolleguesMagdaRenke-GluszkoandDr.TomSchratzenstallerdeservethanks
forthegoodcooperationandsupport.ThanksalsotoDr.BorisBehnischfrom
TransluminaLabsforprovidingthecountlessamountsofstentsandfortheproduc-
ions.discussetivMoreoverIliketothankallotherpersonswhocontributedtothesuccessofthiswork:
SusanneSchnell-Witteczek,RalfRettig,SteffiBlechinger,BarbaraRöhrnbauer,Do-
minikRietzel,FlorianBrunschön,ShadiSabeti,NilsKlink,HannaDressel,Adrian
Frenzel,UliEbnerandJosephHintermair.
NotatleastIwouldliketothankmyparentsandmysisterfortheirconsistent
supportatallmydecisions.

III

tentsCon

1Introduction1
1.1SurfaceModificationofImplants....................1
1.2StentsasaTreatmentofCoronaryHeartDisease...........3
1.2.1PathologyofAtherosclerosis...................3
1.2.2CoronaryStenting........................5
1.2.3PathologyofRestenosisandStrategiesforitsPrevention...6
1.2.4DrugDeliveryStentSystems...................8

2WorkingHypothesisandProceeding

10

3ElectrochemicalTheory16
3.1Basics...................................16
3.2SelectiveEtching.............................18
3.3TheAlloyingElementsof316L.....................19
3.4ElectrochemicalBehaviorofCr-Ni-Steels................21
3.4.1EffectofHydrochloricAcidonStainlessSteel.........22
3.4.2EffectofNitricAcidonStainlessSteel.............25
3.4.3EffectofOxalicAcidonStainlessSteel.............26
3.4.4EffectofPhosphoricAcidonStainlessSteel..........27

4MaterialandMethods29
4.1Material..................................29
4.2TheEtchingAssembly..........................30
4.2.1CalculationofCurrentDensities.................33
4.3TopographicSurfaceAnalysis......................34
4.4CrossSectionalMicrographs.......................36
4.5SoftwareAnalysisoftheDepotStructures...............37
4.6CalculationofDepotProperties.....................37
4.7CoatingProcessandAnalysisofCoatings...............39

IV

5

6

7

8

4.8ChemicalSurfaceAnalysis........................
4.8.1SEM-EDS.............................
4.8.2AugerElectronSpectroscopy...................
4.9ElectrochemicalAnalysis.........................
4.9.1CyclicVoltammetry.......................
4.9.2PotentiostaticTestsforDeterminationofIonRelease.....
4.10CellSeedings...............................
4.10.1SeedingProcess..........................
4.10.2PreparationforSEM-Imaging..................
4.11MechanicalTests.............................

MicrostructuringwithHydrochloricAcid
5.1Results...................................
5.2AnalysisoftheFormationProcess....................

MicrostructuringwithNitricAcid
6.1Results...................................
6.2AnalysisoftheFormationProcess....................
6.3ChemicalAnalysis............................
6.4MathematicalDescriptionoftheEtchingProcess...........

CombinationEtchingsonNitricAcidBasis
7.1PhosphoricAcid..............................
7.1.1ParameterAnalysisonStents..................
7.1.2SoftwareAnalysis.........................
7.2HydrochloricAcid.............................
7.2.1ParameterAnalysisonStents..................
7.2.2SoftwareAnalysis.........................
7.3OxalicAcid................................
7.3.1ParameterAnalysisonStents..................
7.3.2SoftwareAnalysis.........................
7.4EtchingswithoutpreviousElectropolishing...............

PerformanceofthemodifiedStents
8.1ChemicalPerformance..........................
8.1.1CyclicVoltammetry.......................
8.1.2ReleaseTestsofNickel......................
8.1.3AnalysisofthePassivityLayer.................
8.2BiologicalPerformance..........................

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V

8.2.1CellSeeding............................95
8.3MechanicalPerformance.........................100
8.3.1RadialStrengthTests.......................100
8.3.2FatigueTests...........................101
8.4CoatingBehavior.............................102
8.5ReleaseKinetics..............................104

9SummaryandOutlook108
9.1Summary.................................108
9.1.1MicrostructuringwithHydrochloricAcid............108
9.1.2MicrostructuringwithNitricAcid................109
9.1.3CombinationEtchings......................110
9.1.4Performance............................111
9.1.5Conclusion.............................112
9.2Outlook..................................112

114dixenApp1010.1DeterminationofGrainSize.......................114
10.2CalculationofPolygonLine.......................114
10.3CellCultures...............................115
10.4CompositionofRingersSolution.....................117
10.5ApproximationofStrain.........................117

VI

onductintroI1

1.1SurfaceModificationofImplants

Apluralityofsubstanceshasbeenestablishedforthepurposeofsubstitutingor
supportingorganicfunctionsofthehumanbody,rangingfrommetalstoceramics
topolymers.Almostallmaterialscurrentlyusedarestillbeingoptimizedinterms
ofcompositionandprocessing.Inrecenttimes,paralleltothedevelopmentinother
fields,thefocushasbeenaimingmoreandmoreatthesurfacepropertiesofthema-
terial.Sincebodytissuenormallyinteractsonlywiththeupmostfewmicrometers
ofanimplantedmaterial,thechemicalandtopographicpropertiesoftendecideover
thesuccessorfailureofanimplantationprocess.Depe