第45章要点双语.docx
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第45章要点双语.docx
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第45章要点双语
Chapter13Biomaterials
13.1Introduction
Biomaterialsarematerialsusedinmedicineanddentistrythatareintendedtocomeincontactwithlivingtissue.
Thefamiliartoothfillingiswheremosthumansfirstencounterbiomaterialsbutincreasinglymanypeoplenowrelyonmorecriticalimplantssuchasjointreplacements,particularlyhips,andcardiovascularrepairs.
Undoubtedly,thesebiomaterialimplantsimprovethequalityoflifeforanincreasingnumberofpeopleeachyear,notjustforanageingpopulationwithgreaterlifeexpectancy,butforyoungerpeoplewithheartproblems,injuriesorinheriteddiseases.
Biomaterialshavenowbeensuccessfullydevelopedandusedformorethanageneration.
First-generationbiomaterialslargelydependedonbeinginert,orrelativelyinert,withminimaltissueresponse.
Allthematerialsprovedrelativelybioinertandgaveanaveragelifetimeof10yearsormore.
13.2Requirementsforbiomaterials
Therequirementsforabiomaterialareextremelydemanding.
Replacementorrepairofabodyfeature,tissue,organorfunctionoftennecessitatesthematerialusedtohavespecializedmechanical,physicalandchemicalproperties.
However,theveryfirstrequirementisbiocompatibilitywiththehumanbody,i.e.theabilityofthematerialtoperformwithanappropriatehostresponse.
Forthesuccessfuluseofthebiomaterial,considerationhastobegiventotheappropriatematerialselection,engineeringdesignandmanufacturingprocess.
Whileproperdesignandmanufactureisessential,itisparticularlyimportanttoselectthecorrectmaterialtoprovidetheappropriatepropertiesaswellasbeingbiocompatible,recognizingthatthecombinedinfluenceofmechanicalandchemicalfactorscanbequiteserious.
Itisalsoimportanttorecognizethatthebiologicalenvironmentisnotconstantandthatoxygenlevels,availabilityoffreeradicalsandcellularactivitywillvary.
Corrosionanddegradationcanleadtolossofintegrityoftheimplantand,ofcourse,releaseionsintothebody,oftensettingupanallergicreaction.
Biomaterialapplicationsmakeuseofallclassesofmaterial,metals,ceramics,polymersandcomposites,dividedroughlyintothreeusertypes.Theseare
(i)inertorrelativelyinertwithminimalhostresponse,
(ii)bioactivewhichactuallystimulatesbondingtothesurroundingtissueand
(iii)biodegradablewhichresorbinthebodyoveraperiodoftime.
Themostcommonmetallicmaterialsusedareausteniticstainlesssteels,cobalt–chromiumalloysortitanium.
Recently,titaniumalloys,particularlyTi–6Al–4V,havebeenintroducedbecauseoftheircorrosionresistance,strengthandelasticmodulusbutpoortribologycanstillbeaproblem.
Oftheceramics,aluminiumoxide,calciumphosphate,apatite,carbon/graphiteandbioglassareinusemainlyfortheirinertness,goodwearcharacteristics,highcompressivestrengthandinsomecasesbioactivity.
Theirpoortensilepropertiesandfracturetoughnessaredesignlimitations.
Polymersarewidelyused,bothaloneandincombinationwithceramicsormetals.
Theseinclude;polymethylmethacrylate(PMMA)forcementandlenses;polyethylenefororthopaedics;polyurethaneasbloodcontactmaterial,e.g.vasculartubing,cardiovasculardevices,catheters;polysiloxanesinplasticsurgery,maxillofacialandcardiovascularsurgery;polyestersandpolyamidesinwoundclosuremanagement.
Compositessuchasultrahigh-molecular-weightpolyethylenereinforcedwitheithercarbonfibresortheceramichydroxyapatiteareincreasinglybeingconsideredforapplicationsinvolvinghighcontactstressandwearresistance.
13.3Dentalmaterials
Dentistryhasalwaysbeenverydependentontheuseofbiomaterialsandparticularlyreceptivetotheapplicationofnewdevelopmentsinmetals,ceramics,polymersandcomposites.
Dentalamalgamshavebeenusedincavityfillingsformorethan100years,initiallywithsilver–mercuryamalgams,latermodifiedbytinadditionstocontroltheamountofexpansion.
Theseamalgamsproducedtheweak,corrodibleintermetallicphase,Sn7Hg,andhencethemoderndentalamalgamnowalsocontainscopper(>12%)inordertosuppressthisphase.
Theamalgamismadebymixingsilver,tin,copperalloypowderwithmercuryandthismixtureispackedintothecavitywhereithardenstoproduceastrong,corrosion-resistant,biocompatiblefilling.
ThereissomeevidencethateventhisfillingmaybesusceptibletocorrosionasaresultoftheCu6Sn5phaseandtheadditionofPdhasbeenadvocated.
AttemptstoreplacetheHgamalgambygallium,indium,silver,tin,copperpasteshavenotyetbeencompletelysuccessful.
13.4Thestructureofboneandbonefractures
Inrepairingbonefracturesitisnecessarytoholdthebonetogetherwhilethenaturalhealingprocesstakesplace,usuallyforafewmonths.
Metallicalloyshavebeenuniversallyusedforplates,pins,screws,etc.
Theyareconsideredtohavetherequiredstrengthandrigiditytogetherwithsufficientcorrosionresistance.
Generally,thesefixturesareconsideredtemporaryuntilthebonehasjoinedafterwhichtheyareremoved.
Inmaterialscienceterms,theapparentcomplexityofbonecanbedescribedasacompositemadeupofamatrixofcollagen(polymer)reinforcedwithapproximately50%volumefractionofhydroxyapatite(ceramic)nanometre-scalecrystals.
Mostbonesaremadeofaporouscellularstructure(cancellousbone)coveredwithadensercompactshell.
Thelargedifferenceinelasticmodulusbetweencompetingbiomaterialsandboneisevident.
Ofthevariousmetals,titaniumanditsalloysareclearlythemostsuitableandarebeingincreasinglyused.
Titaniumdoes,however,haveatribologicalweaknessbuttheapplicationofcoatingsandsurfaceengineeringisbeingincreasinglyadoptedtoovercomethisproblem.
13.5Replacementjoints
Thebasicdesignofanartificialhipjointincludesanalloyfemoralstem,withametallicorceramicfemoralheadmovinginanultra-high-molecularweightpolyethylene(UHMWPE)acetabularcup.
Theaveragelifeofthejointisbetterthan10yearsbutimplantstendtoloosenasaresultofboneresorptionduetomodulusmismatch.
Shoulderjoints,likehipjoints,areincreasinglybeingreplacedbymetalprostheses.
Initially,similarprincipleswereappliedtoshoulderjointsastohipjoints,usingastainlesssteelballandsocketwithpolyethylenecup.
However,becauseitisnotaload-bearingjointthetrendinshoulderreplacementissimplytoresurfacethewornpartonthehumerissidewithastainlesssteelcapratherthanreplacebothsidesofthejoint.
AtypicalKneejointconsistsofatibialbaseplateortray,usuallymadeofstainlesssteel,Co–Cr,ortitaniumalloy,withatibialinsert(UHMWPE)thatactsasthebearingsurface.
13.6Reconstructivesurgery
Themanydifferentbiomaterialsusedinplasticsurgeryincludecollagen,silicone,Teflon,polyethylene,Dacronandpolyglocolicacid.
Polysiloxanesarewidelyusedinreconstructiveplasticsurgerybecauseoftheirlackofanytissuereactivity,mechanicalpropertiesandstructure.
Polysiloxanemaybeeitherheatvulcanizedorvulcanizedatroomtemperatureformoredelicatestructuresandisidealforsofttissuereplacementwhererepeatedflexureoccurs.
Commerciallypuretitaniuminperforatedsheetformhasbeenusedbecauseofitsbiocompatibilityandeaseofmanipulationandfixing.
Polymers,particularlysiliconesandpolyurethanes,maybeusedtoreplaceflexibletissuesofthenose,cheekandearregionsoftheface.
Polysiloxaneshavebeenusedforonlaysintheareaofthemolarboneinthelateralsideofthemandibleorovertheforeheadtosmoothit.
Reinforcedwithstainlesssteelwirestheycanalsoreplacethemandible.
Aporouscompositeofpolytetrafluoroethylene(PTFE)strengthenedwithcarbonfibresmaybeusedtoreplacedamagedbonestructures.
13.7Biomaterialsforheartrepair
Nowadays,advancedheartvalvesaremadewithCo–Crortitaniumbodieswithmetalorgraphitediscs,oroccluders,coatedwithpyrolyticcarbon.
Thesecoatingsaremadebyheatingahydrocarbon,suchasmethane,toabout1500°Cdepositingthecarbonvapouronthegraphitesurfacetoathicknessofabout1mm.
SmallamountsofsilanemixedwiththeCH4addsSitothedeposit,increasingitsstrength.
Pyrolyticcarbonisstrongandwearresistantbut,moreimportantly,resiststheformationofbloodclotsonitssurface.
ThediscsareattachedbyacoatedTimetalarmtoafabricringmadeofpolymer(PTFE)whichissewntothetissueoftheheartvalveopening.
Otherdevelopmentsinimprovingheartperformanceincludetheuseofcardiacpacemakerswhichproducesa5Velectricalimpulsefor1/500secondatregularheartbeatrate.
Thesedeviceshavebeenavailableforsometimebuthaveimprovedsignificantlyoverrecentyears.
Thebasicrequirementistoprovideelectricalsignalstotheheartattheappropriateleveltostimulatethepatient’sownelectricalactivitytoproducetheproperphysiologicalchange,normallylinkingthepacemakerintothecardiacsystemsothatitworkswhenneeded.
Itisnecessarytouseartificialarteriesmadefrompolymers;sucharteriesmustbetoughandflexibleenoughtoavoidkinking,withtheaddedrequirementofavoidingtheformationofbloodclots.
Inmodernsurgery,thebloodclottingtendencycanberemovedbyanticoagulants,suchasheparin,buttheultimategoalistoprovideartificialarterieswithnaturalclotresistance.
13.8Tissuerepairandgrowth
Tissuesincludeskin,tendons,ligamentsandcartilages.
Skinhasthedualpropertyofkeepingthebodyfluidsinwhileallowingtheoutwardmovementofmoisturethroughaporousmembrane,whichisimportantincoolingandmaintainingthebod
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