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3DReconstructionandManufactureofRealAbdominalAorticAneurysmsFrModelB.CentreH20849andUniversityLimerick,L.GalwayGalwayGalwayA.PCentreH20849andUniversityLimerick,D.DepartmentDepartmentMcGowanandUniversityPittsburTCentreH20849andUniversityLimerick,emailAbdominalandconsiderinfrarmanufacturmentalandstructionbeutilizingcomputeraideddesign/computeraidedmanufacturetechniquesandcombinedwiththeinjectionmouldingmethod.SiliconerubberformsthebasisoftheresultingAAAmodel.AsNALmanuscriptconductedJournal2008Downloaded24Nov2008to222.190.117.210.RedistributionsubjecttoASMElicenseorcopyrightseehttp//www.asme.org/terms/Terms_Use.cfmomCTScantoSiliconeJ.DoyleforAppliedBiomedicalEngineeringResearchCABERH20850,MaterialsandSurfaceScienceInstitute,ofLimerick,IrelandG.MorrisMedicalTechnologyCentre,MayoInstituteofTechnology,,IrelandCallanan.KellyforAppliedBiomedicalEngineeringResearchCABERH20850,MaterialsandSurfaceScienceInstitute,ofLimerick,IrelandA.VorpofSurgery,ofBioengineering,InstituteforRegenerativeMedicine,CentreforVascularRemodellingandRegeneration,ofPittsburgh,gh,PA.M.McGloughlin1forAppliedBiomedicalEngineeringResearchCABERH20850,MaterialsandSurfaceScienceInstitute,ofLimerick,Irelandtim.mcgloughlinul.ieaorticaneurysmAAAcanbedefinedasapermanentirreversibledilationoftheinfrarenalaorta.AAAsareoftenedtobeanaortawithadiameter1.5timesthenormalenalaortadiameter.ThispaperdescribesatechniquetoerealisticsiliconeAAAmodelsforusewithexperistudies.ThispaperisconcernedwiththereconstructionmanufacturingprocessofpatientspecificAAAs.3DreconfromcomputedtomographyscandataallowstheAAAtocreated.MouldsetsarethendesignedfortheseAAAmodels1Correspondingauthor.ContributedbytheBioengineeringDivisionofASMEforpublicationintheJOUROFBIOMECHANICALENGINEERING.ManuscriptreceivedFebruary7,2007finalreceivedSeptember11,2007publishedonlineApril28,2008.ReviewbyB.BarryLieber.ofBiomechanicalEngineeringCopyright©sessmentofwallthicknessandoverallpercentagedifferencefromthefinalsiliconemodeltothatofthecomputergeneratedmodelwasperformed.IntheserealisticAAAmodels,wallthicknesswasfoundtovarybyanaverageof9.21.Thepercentagedifferenceinwallthicknessrecordedcanbeattributedtothecontractionofthecastingwaxandtheexpansionofthesiliconeduringmodelmanufacture.ThismethodmaybeusedinconjunctionwithwallstressstudiesusingthephotoelasticmethodorinfluiddynamicstudiesusingalaserDoppleranemometry.Inconclusion,thesepatientspecificrubberAAAmodelscanbeusedinexperimentalinvestigations,butshouldbeassessedforwallthicknessvariabilityoncemanufactured.H20851DOI10.1115/1.2907765H20852KeywordsabdominalaorticaneurysmAAA,3Dreconstruction,siliconeIntroductionAnabdominalaorticaneurysmH20849AAAH20850canbedefinedasapermanentandirreversiblelocalizeddilationoftheinfrarenalaortaH208511H20852.IthasbeenproposedthatanAAAisanaortawithadiameter1.5timesthatofthenormalinfrarenalaortaH208512H20852.Currently,thetimingofsurgicalinterventionisdeterminedbythemaximumdiameteroftheAAA,withanAAAdiametergreaterthan5cmdeemedtobeathighriskofrupture.Muchworkhasbeenaimedattherupturepredictionoftheseaneurysms,inparticular,theuseoffiniteelementanalysistodeterminewallstressH208513–8H20852.AlthoughtheuseofnumericalstudiestogainaninsightintothestressactingontheAAAwallisofobviousbenefittotheparticularAAAcase,validationofthesetechniquesisofequalimportance.TheabilitytomanufacturepatientspecificAAAmodelsforexperimentalstudiescouldextendtheuseofpreoperativewallstresstechniques.Theserealisticsiliconemodelscouldbeemployed,notonlyforstressanalyses,similartothephotoelasticworkofMorrisetal.H208519H20852,butalsoforfluiddynamicsstudiesandpostoperativeexperimentaltesting,suchasstentgraftdistractiontesting.ThemodelsarecreatedbyfirstreconstructingavirtualAAAmodel,leadingtomoulddesign,andthentomanufacturingviatheinjectionmouldingtechnique.PreviousresearchhasexaminedtheuseofrapidprototypingasamethodofproducingelastomericreplicasofarterialvesselsH2085110H20852.Thismethod,althoughquickandeffective,doesnotproducethesurfacefinishthatcanbeachievedusingtheinjectionmouldingprocess.Surfacefinishisofparamountimportancewhenusingarterialmodelsforexperimentaltestingusingthephotoelasticmethod,suchasthatpreviouslyconductedatourlaboratoryH208519H20852.OthertechniqueshavebeenemployedinordertomakemodelsforuseinlaserDoppleranemometryH20849LDAH20850H2085111H20852andparticleimagevelocimetryH20849PIVH20850flowstudies,wheresurfacefinishwasoflesserimportancethanwhenconductingwallstressstudies.Theprincipalobjectiveofthisstudyistodescribethemodelingandmanufacturingprocessusedandtodeterminetheeffectivenessofthetechnique.ThisprocessofconvertingastandardcomputedtomographyH20849CTH20850scantoapatientspecificsiliconemodelisofvaluetomanyresearchersinthisfield.Methods3DReconstruction.FourpatientswerechosenfromourAAAdatabase.TheCTscansofeachpatientwerethenimportedintothesoftwarepackageMIMICSH20849Materialise,BelgiumH20850.Thissoftwareallowsthetransformationof2DCTscansintorealistic3Dmodelsofexactgeometry.ThissoftwareusesamarchingsquaresalgorithmtothresholdandsegmenttheregionsofinterestoftheCTscanaccordingtoapredeterminedgrayscalevalue.Oncesegmented,thesoftwaregeneratespolylinesaroundthesegmentedJUNE2008,Vol.130/0345011byASMEregions,thisFig.controllossinitialworksuchderick,performed,constructionareTTheseapatientspecificfirstAAA,outerofthick0.23assumptionmouldlocationfourtheiliacvolvingportant,arteriestheproximalexperimentalhaveagainFig.showsinterest.theFig.aarteries.0345012Downloaded24Nov2008to222.190.117.210.RedistributionsubjecttoASMElicenseorcopyrightseehttp//www.asme.org/terms/Terms_Use.cfmtoausercontrolledlevelofsmoothing.Anexampleofimagesegmentationandpolylinegenerationcanbeseenin1.Inthisstudy,polylineswerecreatedwithapproximately20pointsperscan,allowingoptimumsmoothingwithouttheofmodelaccuracy.ThesepolylinescanthenbeexportedasgraphicsexchangespecificationH20849IGESH20850format.Previoushasutilizedvariousotherformsofreconstructionsoftware,asSCIONIMAGEH20849ReleaseBeta4.0.2,ScionCorporation,FreMDH20850H2085112H20852.ValidationofMIMICSagainstthisworkhasbeenwithapercentagedifferenceof1.2betweentheremethods.ComputerAidedDesignCAD.PolylinescreatedinMIMICSimportedintoPROENGINEERWILDFIRE2.0H20849PTC,ParametricechnologyH20850.Surfacesarethenrecreatedalongthesepolylines.surfacesarethenexactlysplitintotwohalves,thuscreatingtwopiecemouldsetusedinthemanufacturingtechnique.Eachmoulddesignconsistsoftwosetsofmoulds.Themouldisdesignedtoproducethecastingwaxmodeloftheandthesecondsettoproducetheoutersiliconemodel.Themouldisapproximately2mmlargerinallregionsthanthatthewaxmould,soastoproduceasiliconemodelwitha2mmwall.AsthewallofanAAAcanrangeinthicknessof–4.33mmH2085113H20852,awallthicknessof2mmisareasonableandhasbeenusedinpreviousstudiesH2085114H20852.ExamplemoulddesignscanbeseeninFig.2.Eachouterdesignincludessupportsfortheinnerwaxcasttoensureofthewaxmodelinsidethelargeroutermould.OftheAAAsusedinthisstudy,threeAAAsweremodeledwithoutiliacarteriesH20849PatientsA,B,andCH20850,andoneAAAwiththearteriesincludedH20849PatientDH20850.Forexperimentalstudiesinstressanalyses,theiliacarteriesarebelievedtobeunimwhereasforfluiddynamicandstentgrafttesting,theiliacareofparamountimportance.MouldsdesignedwithoutiliacarterieshavecylindricalsectionsincludedbothintheanddistalregionsoftheAAA,toallowattachmenttotestrigs.ComputerAidedManufactureCAM.OncethemouldsetsbeendesignedinPROENGINEER,thedesignsareexportedinIGESformat.Thesefilesareimportedintothesoftware1SegmentationandpolylinegenerationofCTscan.„athefullCTscan,while„bisacloseupoftheregionofForthedesignofmoulds,theAAAwasregardedtobefullvolumeofthelumenandintraluminalthrombus„ILT.2ExamplemoulddesignsofpatientspecificAAAs.„aismoulddesignincludingiliacarteriesand„bwithoutiliac/Vol.130,JUNE2008packageAlphaCAMinordertogeneratethetoolpathcommandsusedtocontrolthemillingmachine.Eachmouldissetupwiththesamereferencepointssoasthateachmouldpiecefitsexactlytogether,ensuringthattheresultingmodelhasanalmostnegligibleseamline.MachiningisperformedbyathreeaxiscomputernumericalcontrolH20849CNCH20850millingmachine.Mouldsaremachinedfromsolidaluminumblocksandarefinishedbyhandtoremoveanyunwantedburrsthatresultfromthemillingprocess.Figure3showsanexamplemachinedmouldpiece.ThisillustrationshowsregionswhereextensionshavebeenincludedintothedesignintheproximalanddistalregionsoftheAAA,andalsotheinletthroughwhichthewaxispoured.Necessaryholesandventswereaddedtoeachmodelaftermachining.ModelManufacture.Allmouldpieceswerecleanedusingacetonepriortouse.Thewaxmouldswerepreheatedto40°Ctominimizethecontractionofthewaxuponpouring.AcastingwaxH20849CastyleneB581,REMETCorporationH20850wasusedtomakethelumencasts.ThewaxlumencastwasthencoatedwithWackerprotectivefilmSF18H20849WackerChemieGmbHH20850.Thelumencastswerethenplacedintotheoutermoulds,whichwerecoatedwithreleasingagentH20849WackerMouldReleaseH20850andthenclamped.ThesiliconerubberH20849WackerRT601H20850wasthenpreparedandslowlyinjectedintothepreheatedoutermould.SiliconerubberwasemployedasthematerialduetoitsnonlinearbehaviorwhensubjectedtolargestrainsH2085115H20852andisbelievedtobeagoodarterialanalog.Themouldisthenplacedintoanovenatatemperatureof50°Candcuredfor24h.Oncecured,themodelisremovedandthetemperatureisincreasedto100°Cinordertomeltthewaxfromthemould.Theresultingsiliconemodelisthenthoroughlycleaned,dried,andinspectedfordefects.ThefullprocedurecanbeseenintheAppendix.ResultsSectioningtheModel.EachmodelwassectionedatregularintervalstoassessthedimensionalaccuracyoftheresultingsiliconemodelcomparedtotheCADmodel.EachsiliconeAAAmodelwascarefullysplitusingascalpellongitudinallyalongtheleftandrightsides,thusleavingeachmodelintwohalves.Eachhalfmodelwasthenaxiallyslicedat10mmintervalsalongthelengthofthemodel,leavingaseriesofcrosssectionalslicesforeachpatientspecificmodel.WallThicknessMeasurements.Foreachcrosssectionalsliceofthesiliconemodel,thewallthicknesswasmeasuredatfour90degequispacedlocationsaroundtheedge.Therefore,wallthicknesswasmeasuredalongtheleft,right,anterior,andposteriorwallsofthefullAAAmodel.Measurementswereobtainedusingadigitalmicrometer.Measurementreadingsrangedfrom40to60readingsperAAAmodel,dependingonthepatient.MeasurementresultswerethenaveragedforeachpatienttailoredsiliFig.3ExamplemachinedmouldpieceofinnerAAAmodelTransactionsoftheASMEconesiliconedardresultswallrefersandAAAMisessultingAAAofresultstherprediction.AAAsFEATable1AveragedwallthicknessmeasurementsatfourlocationsontheAAAwallAxialpositionAnterior24.73TpatientspecificPatientPatientPatientPatientFig.tientcorrespondingsameJournalDownloaded24Nov2008to222.190.117.210.RedistributionsubjecttoASMElicenseorcopyrightseehttp//www.asme.org/terms/Terms_Use.cfmmodelwiththepercentagedifferencebetweentheactualmodelandthe2mmwallCADmodelnoted.Thestandeviationwasalsoincludedintheresults.ThemeasurementcanbeseeninTable1andaresummarizedasatotalmodelthicknessforeachpatientinTable2.Percentagedifferencetothedifferencebetweenthesiliconemodelwallthicknesstheoriginal2mmwallthicknessinthemoulddesign.ThemodelofPatientDincludedtheiliacarteries.WallStressDistribution.Figure4showstheresultingvonwallstressdistributionforPatientAandtheregionofrepeakstress.Theresultsshowhowthepeakstressinthemodelwas0.533MPaandwaslocatedontheanteriorwalltheAAA.ThecombinationoffiniteelementanalysisH20849FEAH20850withvalidatedexperimentalwallstressstudiescouldfurtheuseofnumericalstudiesinthefieldofAAAruptureAmoredetailedstudyofthewallstressexperiencediniscurrentlyinprogress,andwillexpandonthepreliminaryresultspresentedhere.PatientAAveragewallthicknessH20849mmH20850StandarddeviationPercentagedifferencePatientBAveragewallthicknessH20849mmH20850StandarddeviationPercentagedifferencePatientCAveragewallthicknessH20849mmH20850StandarddeviationPercentagedifferencePatientDAveragewallthicknessH20849mmH20850StandarddeviationPercentagedifferenceable2AveragedwallthicknessmeasurementsforeachAAAmodelAveragewallthicknessH20849mmH20850AveragepercentagedifferenceA2.124.24B2.209.01C2.2912.51D2.2211.094ExampleFEAvonMiseswallstressdistributionforPaAshowingaregionofpeakstressontheanteriorwall.Themouldpieceandresultingsiliconemodelofthepatientcanbeseenontherightofthisfigure.ofBiomechanicalEngineeringDiscussionThisstudydescribesaprocedureformanufacturingpatientspecificrubbermodelsofAAAsbothwithandwithouttheiliacarteries.A3DreconstructiontechniqueusingcommerciallyavailablesoftwareiscoupledtoaCAD/CAMtechniquetoachievethedesiredmoulddesignscapableofformingrealisticAAAsusingtheinjectionmouldingmethod.PreviousstudiesH2085114,16H20852haveusedsimilartechniquestoproducerubbermodelsofvascularvessels.Themodelsdevelopedinthisstudyareofgreatercomplexity.Thesereproduciblesiliconemodelscanbeutilizedforexperimentaltestingofvesselhemodynamics,wallstressanalyses,andstentgraftstudies,allofwhichmaycontributetoexperimentalvalidationofnumericalwork.ThistechniquemayallowotherresearcherstobeginmanufacturingrealisticAAAsiliconemodelsforuseintheirexperimentalwork.Inrecentyears,emphasishasbeenplacedontheuseofnumericalstudiestoattempttopredictAAArupture.Theuseofexperimentalresearchintothisareaisalsoofimportance.NotonlycouldthesesiliconeAAAmodelshelpinvalidationpurposesbutmayalsobecomeanimportantassetinAAAruptureprediction.Foreachofthepatientspecificmodelscreated,wallthicknessisthemostvariablefactor.IthasbeenreportedH2085113H20852howtheAAAwallcanrangeinthicknessfrom0.23mmto4.33mm,withaorticwallthicknessreportedtorangefrom1.1mmto3.4mmH208517,17,18H20852.Averagewallthicknessoverthefourmodelswasnotedtobe2.26H110060.39mm.Inthisstudy,wallthicknesslieswithintherangerecordedbypreviousresearchersH208517,17,18H20852,andsocanbedeemedasacceptable.WallthicknessresultsalsofavorablycomparewiththosefoundforarealisticaortabyOBrienetal.H2085114H20852,whorecordedawallthicknessinrealisticaortasof2.39H110060.32mm.Althoughwallthicknessappearstobewithinanacceptablerange,themouldsweredesignedtohaveawallthicknessof2mm.Theresultingsiliconemodelsdifferfromthemoulddesignsbyanaverageof9.21inwallthickness,whichcanbeattributedtothecontractionofthecastingwaxduringsolidificationandthethermalexpansionofthesiliconeduringthecuringprocess.TheselimitationsofthetechniquewerealsoobservedbyOBrienetal.H2085114H20852.Thispreviousworkrecordedpercentagedifferencesinwallthicknesstomoulddesignrangingfrom20forarealisticstraightsectionofanaortato58forasectionofasaphenousvein.TheresultsobservedinthisstudyaredeemedacceptableinthatpercentagedifferencesareconsiderablylessthanthosepreviouslyreportedH2085114H20852.ItshouldalsobementionedthatthemodelsproducedherearefullAAAmodelsandnotstraightvesselsectionsand,therefore,onewouldexpecttheperPosteriorRightLeft1.871.972.092.550.2760.3140.1730.3276.951.784.2321.472.122.292.092.310.2070.4180.2350.2485.8212.564.2613.422.182.172.532.300.2230.2930.3520.3068.087.8920.9913.092.652.112.381.970.6530.2820.4140.2815.1916.111.64JUNE2008,Vol.130/0345013centagedifferencestobehigherthanthosepreviouslyreportedH2085114H20852.Therefore,confidencehasbeenestablishedinthemethodusedtoproducethesemodels.dressed.anduniformwaxinattributedspecificetidealizedwallfactCAD/CAMconsideredwalldepositsthesefromintroducesvaryingincreasedfortheknowniesmericalingmodelsandelstoelasticAAApeaknonuniformitydure.thrombusactionwallthatothersH20851stressvalidatingourcodesoftware,tainisH20851experimentalbasedConclusionmodelsreproducibilitygeneral,ducedindifingmostalwaysmentalmericalmethodsthroughphotoelasticvalidationH208519H20852oralsothroughexperimentaltestingsuchasLDAorPIV.Inconclusion,3DreconstructionandCAD/CAMtechniques0345014Downloaded24Nov2008to222.190.117.210.RedistributionsubjecttoASMElicenseorcopyrightseehttp//www.asme.org/terms/Terms_Use.cfmTheissueofuniformityinwallthicknessshouldalsobeadInthemoulddesigns,thewallthicknesswassetat2mm,therefore,theresultingsiliconemodelsshouldalsohaveawallthickness.Duetoreasonsmentionedabove,thatis,contractionandsiliconeexpansion,thewallthicknessvarieseachAAAmodel.ThesedifferencesinwallthicknesscanbetothecomplexandtortuousgeometryofthesepatientAAAmodels.ThislimitationwasalsonotedbyOBrienal.H2085114H20852.BothOBrienetal.H2085114H20852andChongetal.H2085116H20852producedvascularmodels,andinthesemuchsimplermodels,theuniformityissuewaseasilyovercome,thushighlightingthethatrealisticgeometriesincreasethedifficultyofnotonlytheprocessbutalsothemodelmanufactureitself.Theuseofauniformwallintheseexperimentalmodelsmaybeinappropriate,sinceitisknownthattheactualAAAtissuecanincludevariousformsofarterialtissueH20849calcifiedandthrombusH20850andthusisusuallynonuniform.WhileregionsofbothcalcifiedtissueandthrombuscanbedetectedtheCTscan,theirincorporationintothewallofthemodeladditionalcomplexity.First,incorporatingregionsofmaterialpropertiesintotheAAAwallresultsingreatlycomputationsasthenumericalequationsusedtosolvewallstressattheselocationsbecomeextremelycomplex.Also,primarypurposeofproducingrealisticsiliconemodelsofuniformwallistoexperimentallyvalidatenumericalstudofthesameAAAmodel.MostpreviousworkregardingnustressanalysesofAAAsH208513–8,19–22H20852hasconductedtestusinguniformwalls.OurearlierworkH208519H20852onidealizedAAAofknownuniformwallthicknessprovedquitesuccessfulhaspavedthewayfortheintroductionofrealisticAAAmodtobetestedusingthesametechnique.ThisexperimentalphoworkwaslaternumericallyvalidatedonanidealizedmodelbyourgroupH2085123H20852,whichconfirmedthelocationsofstressonthemodel.ItisplannedtorevisittheconceptofofthesiliconemodelsusingthisdescribedproceWorkhasalsobeguninthislaboratoryontheinclusionofintheAAAmodels.IthasbeensuggestedH208516H20852thattheuseofafluidstructureinterH20849FSIH20850approachtostressanalysismayyieldmoreaccuratestressresultsthanFEAalone.SomeresearchershaveshownthewallstressisincreasedbyamountsH110211H2085121H20852,whereashavereportedincreasesrangingfrom12.5H208516H20852to20.520H20852.AsthereareconflictingresultsinthebenefitsofFSIinwallstudies,theseuniformwalledAAAmodelscanhelptowardbothmethodsofstressstudies.WorkhasbegunwithinlaboratoryontheuseofFSI,employingmeshbasedparallelcouplinginterfaceH20849MPCCI3.0.6,FraunhoferSCAI,GermanyH20850whichcouplesbothABAQUSandFLUENTtogethertoobrealisticwallstressvalues.Notably,theuseofauniformwallwidespreadamongresearchersconductingFSIstudies5,6,19–22H20852,andsothisfutureworkwillallowthenumericalandvalidationofwallstressinrealisticAAAmodelsontheuniformwallthickness.TheprocedureforthemanufactureofpatientspecificAAAhasbeendescribed.Confidencehasbeenestablishedintheoftherubbermodels,andthelimitationsnoted.Inrubbermodelsofgoodgeometricalaccuracycanbeprobysensiblemoulddesignanduseofcontrolledparametersthesiliconeproduction.Modelsshowedamaximumpercentageferenceof9.21betweenthedesignedmouldsandtheresultsiliconemodels.Uniformityofwallthicknessprovedtobethedifficultparametertocontrol,withfinishedsiliconemodelsinspectedandassessedbeforecommencementofexperitesting.Thistechniquemayaidinthevalidationofnu/Vol.130,JUNE2008provedtobesuccessfulinthereplicationofpatientspecificrubberAAAmodelsandmayhelpcontributetotheuseofpatientspecificAAAmodelsinexperimentaltesting.Therefore,theuseofsiliconeAAAmodelswithuniformlythickwallswillhelptowardthevalidationofnumericalwork,forbothwallstressstudiesandflowdynamics.AcknowledgmentTheauthorswouldliketothankH20849iH20850theIrishResearchCouncilforScience,EngineeringandTechnologyH20849IRCSETH20850GrantNo.RS/2005/340,H20849iiH20850GrantNo.R01HL060670fromtheUSNationalHeartLungandBloodInstitute,H20849iiiH20850Dr.EamonKavanagh,anEndovascularsurgeonatMidwesternRegionalHospital,Limerick,forhishelpincollectingpatientdataandbackgroundinformation,andH20849ivH20850SamarthShahfromtheCentreforVascularRemodellingandRegeneration.AppendixRealisticModelManufactureIncreatingthemalewaxmodels,thefollowingstepsaregiven.1.Thefirstsetsofmouldsareusedtocreatethewaxmodels.2.CleanthesurfaceofthemouldswithacetonemakesureitisfreeofloosedebrisandcoatwithmouldreleaseH20849AmbersilFormula8,ChemcraftIndustriesLtd.Dublin,IrelandH20850.3.Boltthetwomouldpiecestogethertightly.4.MeltthecastingwaxH20849CastyleneB581,REMETCorporationH20850onahotplateatatemperatureofaround150°C.5.Preheatmouldsinovento40°C.6.Positionthemouldatanangleof45degtoaidtheliquidwaxtoflowintothemouldandminimizetheriskoftrappingair,thuscreatingvoidsandbubbles.7.Pourthewaxintothemouldasslowlyaspossibletopreventsplashing,whichcanalsocreatevoids.Asthecavityisfilled,themouldisreturnedtotheverticalpositiontofinishpouringthewax.8.Thewaxisthenlefttosolidifyatroomtemperatureforupto4h.Duringthiscoolingperiod,themouldisgentlytappedwithamallettoallowanytrappedairtorisetothesurface.9.Asthewaxcoolsandsolidifies,additionalwaxisaddedtothemouldtoensureacompletewaxmodel.10.Openthemouldsandcarefullyremovethewaxmodelfromthemould.Increatingthesiliconemodel,thefollowingstepsaregiven.1.MixthesiliconeintherequiredratioofsiliconeandcuringagentH2084991H20850.2.Handmixingthematerialcomponentsforaperiodof2minissufficient.Thisliquidsiliconewillrepresenttheaortawall.Theliquidsiliconecontainsairbubblesoncehandmixedwhichmustberemoved.Theworkingtimeforthemixed,componentsatroomtemperatureisaround90min.3.Inordertoremovethetrappedbubbles,placethecontainerofliquidsiliconeintoafreezeruntilallbubbleshavebeennaturallyremoved.Durationoftimeinthefreezerdependsontheviscosityoftheliquidsiliconeandcanvaryfrom1hto3h.4.Oncealltheairhasbeenremoved,suckalltheliquidsiliconeintoa60mlsyringe.5.Cleanthesecondsetofaluminummouldswithacetone.6.SprayonacoatofsiliconemouldreleaseH20849AmbersilFormula8H20850ontoboththealuminummoulds.7.Carefullyremoveanyexcessmaterialandflashfromthewaxmodel.TransactionsoftheASME
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