組織再生用壓電材料.ppt

Piezoelectricmaterialsfortissueregeneration:Areview,Speaker：ZhangquanTeammembers:SiheweiandFengtiantian,CONTENTS,1.Introduction2.Piezoelectricityinbiologicaltissues3.Piezoelectricityandstreamingpotential4.Cellularresponsetoelectricalstimulation5.Piezoelectricmaterialsintissueregenerationapplications6.Conclusion,1.Introduction,Piezoelectricmaterialsaresmartmaterialsthatcangenerateelectricalactivityinresponsetominutedeformations.FirstdiscoveredbyPierreandJacquesCuriein1880,deformationresultsintheasymmetricshiftofionsorchargesinpiezoelectricmaterials,whichinducesachangeintheelectricpolarization,andthuselectricityisgenerated.,Piezoelectricmaterialsarewidelyusedinvariouselectronicapplicationssuchastransducers,sensorsandactuators.Forbiomedicalapplications,piezoelectricmaterialsallowforthedeliveryofanelectricalstimuluswithouttheneedforanexternalpowersource.Asascaffoldfortissueengineering,thereisgrowinginterestinpiezoelectricmaterialsduetotheirpotentialofprovidingelectricalstimulationtocellstopromotetissueformation.,Inthisreview,theauthorcoverthediscoveryofpiezoelectricityinbiologicaltissues,andsummarizestheirpotentialasapromisingscaffoldinthetissueengineeringfield.,2.Piezoelectricityinbiologicaltissues,In1940,Martinnoticedthefirstdemonstrationofbiologicalpiezoelectricity,whenhedetectedelectricpotentialsfromabundleofwoolencapsulatedinshellacwhilecompressedbytwobrassplates.Themainconstituentofmammalianhair,wool,hornandhoofis-keratin,whichhasaspiral-helixstructure.Thepiezoelectricityofsuchtissuesisattributedtothecompactalignmentofthesehighlyordered-helicesandtheirinherentpolarization.,-Helixisarighthandedcoilstabilizedbythehydrogenbondsbetweenthehydrogenofoneaminegroupwiththeoxygenofaconsecutivecarbonylgroup.AsdemonstratedinFig.1,thehelicalstructurerepeatedlyalignsthedipolesofthebackboneaminoacidsandcausesasignificantpermanentpolarization.,Fig.1.Schematicillustrationofpermanentpolarizationina-helix.Redarrowsdemonstratethedirectionofthedipolemoment.,Yasudareportedthepiezoelectricityofbonein1954.Later,YasudaandFukadaobservedpiezoelectricityinboiledboneandconsequentlyconcludedthatlivingcellswerenotresponsibleforthepiezoelectricresponse.Theyattributedthepiezoelectricbehaviorofbonetotheapplicationofshearoncollagen.Boneisacompositeofdenselypackedalignedcollagenfibrilscontaininghydroxyapatiteparticles.,Piezoforcemicroscopy(PFM)isamodificationofatomicforcemicroscopy(AFM),whichhasbeenrecentlyusedtostudythepiezoelectricityofnanomaterials.AnACbiasbetweentheconductiveAFMtipandthesubstratebeneaththesampleappliesanelectricfieldthroughthesample,causingadeformationinthepiezoelectricmaterial.TheontactingAFMtipdetectsthedeformation,whichissubsequentlytranslatedtotheamplitudeofthepiezoresponse.PFMcanbeperformedinverticalorlateralmodes;verticaldeflectionoftheAFMtipmanifestsnormaldeformationofthematerial.Inlateralmode,torsionoftheAFMtiprevealssheardeformationofthedomain.,Fig.2showsthetopographyofasinglecollagenfibrilimagedbyAFM,anditscorrespondingshearpiezoelectricityimagedbylateralPFM,demonstratingtheperiodicityofthepiezoforceamplitudeattributedtothegapsandoverlapsinthequarter-staggeredstructureofcollagen.,Fig.2.Topographyofasinglecollagenfibrilimagedbyatomicforcemicroscopy(a),andtheamplitudeofitscorrespondingshearpiezoelectricityacquiredbypiezoforcemicroscopyinthelateralmode(b),Foryears,itwasbelievedthatsincehydroxyapatitecrystalizesinacentrosymmetricspacegroupinthehexagonalsystem,itcouldnotbepiezoelectric.However,computationalstudiesreportedalackofaninversioncenterinhydroxyapatitethatcouldtheoreticallysuggestpossiblepiezoelectricityofthiscrystal.Tofailetal.havedemonstratedthepiezoelectricityofsinteredhydroxyapatiteusingPFM,whichsuggeststhatalongsidecollagen,hydroxyapatitemayalsocontributetothepiezoelectricityofbone.Piezoelectricityofothercollagenoustissuessuchastendon,dentin,cementumandcartilagehavealsobeenreported.Polysaccharidessuchaswoodandchitinaswellaspolynucleotidessuchasdeoxyribonucleicacid(DNA)havealsorevealedpiezoelectricresponse.,3.Piezoelectricityandstreamingpotential,In1892,JuliusWolffsuggestedthatboneremodelsitsarchitectureinresponsetostress.WolffsLawmanifestsitselfinthedenserboneintennisplayersracket-holdingarmsorbonelossinastronauts.Afterthediscoveryofpiezoresponseindrybone,theproposedmechanismtodescribebonegrowthandresorptioninresponsetostresswaspiezoelectricity.,Bassettobservedthatunlikeundeformedsamples,periodicallydeformedcultivatedchickembryonictibiaeproducedlargeperiostealchondroidmassesafter7days,anddescribedWolffslawasanegativefeedbackloop:appliedloadonbonecausesstraininlessdenseregions;whiledenserandconsequentlystifferregionsremainunstrained.Thestrainistransformedintoanlectricfieldthataggregatesandalignsmacromoleculesandionsintheextracellularmatrix,whichstimulatescellstoremodelthebonearchitectureuntilthesignalisswitchedoff.,4.Cellularresponsetoelectricalstimulation,Inadditiontopiezoelectricityandstreamingpotentialsinboneandotherbroustissues,endogenouselectriceldsupto500mV/mmhavebeenreportedinlivingtissues.Thetransportofionicspeciesandmacromoleculesassociatedwithendogenouselectriceldsplaycrucialrolesinembryonicdevelopment,woundhealingandneuralregeneration.Thisvitalroleofelectricityinlivingsystemshaveinspirednumerousinvestigationstoeithermimicbiologicalpiezoelectricityandendogenouselectriceldsormanipulatetransmembranepotentialsbyexternalelectricalstimulationtoenhancecellulargrowthanddifferentiation.,Theattentioninneuralregenerationhasbeendrawntorepairingperipheralnerveinjuriesthroughimprovedneuraldifferentiationanddirectionaloutgrowthofneurites.However,promisingresultsarenotlimitedtoneurons;anearlystudyontheeffectofelectricalstimulationonboneformationshowedthatimplantinginsulatedbatteriesinthemedullarycanalofcaninefemoracausedsubstantialformationofendosteumnearthecathodeina1421dayperiod.Evenintheabsenceofexternalelectricalstimulation,implantingpoledsinteredhydroxyapatitedisksincaninecorticalboneresultedinthellingofa0.2mmgapbetweenthenegativelychargedhydroxyapatitesurfaceandthecorticalbonein14days,whilenoboneformationoccurredusingtheunpoledhydroxyapatitebeforeday28.,Electricalstimulationcanbeappliedthroughthesubstrateorthemedium.Alternatingelectriceldsalsohavebeeninvestigated.AlternatingelectriceldshaveshowntocausemorphologicalchangesandsignicantlyincreasednumberofprocessesinSchwanncells,butdidnotleadtodirectionaloutgrowth.Sinceendogenouselectriceldsandtransmembranepotentialsaredirectratherthanalternating,manyneuralstudiesusedirectelectricalstimulation.Onthecontrary,theperiodicnatureofthestressappliedtobonehasinspiredinvestigationsinbonerepairtofocusonalternatingelectriceldstoenhanceosteoblastproliferationandactivity.Moreover,inordertoavoidelectrodeimplantationandconsequentlyelectrolyticbyproducts,noninvasivestimulationforbonefracturehealingdrewwidespreadattention.,Themechanismsthroughwhichelectricalstimulationcausescellularmigrationandaltersproliferationanddifferentiationarenotyetfullyunderstood.Itisspeculatedthattheelectriceldeffectiseitherdirectbyintracellularcomponentssuchasions,growthfactorsandreceptors,orindirectbyagglomerationorconformationalchangeofextracellularionsandproteins.Freecalciumcations(Ca2+)areconsideredamajorfactorinbothdirectandindirectmechanismsofelectricalstimulation.ElectriceldsredistributeCa2+intheextracellularmatrixoronthesubstrate.IntracellularCa2+concentrationsarealsoreportedtoincreaseduetoelectricalstimulation.,Fig.6illustratesanadaptationofthegalvanotaxisofcellsasreviewedbyMycielskaetal.Directelectricelddepolarizesthecathodalsideofthecellandhyperpolarizestheanodalside.ThisleadstothediffusionofextracellularCa2+throughtheanodalsideintothecell.IncreaseintheCa2+maycauseactindepolymerizationandconsequentlycontractionontheanodalside,whichpushesthecellforwardandthusmakesthecathodalsideofthecellprotrude.ThiscouldexplainthephenomenonobservedbyPatelandPooi.e.theoutgrowthoftheneuritesonthecathodalsideofthecellandthediminishingoftheneuritesontheanodalside.,Theeffectofelectricalstimulationonenhancedboneformationwasinitiallybasedontheindirecthypothesis;piezoelectricityofbonegeneratedelectriceldsthataggregatedchargedionsandmacromoleculesintheboneinterstitialuid,whichresultedinenhancedosteoblastactivity.DirectelectriceldsareassumedtomobilizeCa2+andMg2+towardsthecathodeornegativelychargedsurfaceandcauseapatiteformation,whichcanbecomeascaffoldforboneformationbyosteoblasts.Increasedlevelsofgeneexpressionforbonemorphogenicproteins(BMP-2and-4)asaresultofelectromagneticstimulationwerenoted.Zhuangetal.alsofoundthatcapacitivelycoupledelectricalstimulationincreasedTGF-1geneexpressionaswellastheproliferationofosteoblasts.TGF-1expressionwasmodulatedbythecalciumcalmodulinpathway.,5.Piezoelectricmaterialsintissueregenerationapplications,5.1.PiezoceramicsUsingpiezoelectricmaterialsastissueengineeringscaffoldsenableselectricalstimulationwithouttheneedforelectrodes,externalsourceofelectricityorimplantingbatteries,whichalsoeliminatesthechanceofaccumulatingproductsofelectrolysis.,Piezoelectricscaffoldscangenerateelectricpulsesasaresultoftransientdeformations,whichcanbeimposedbyattachmentandmigrationofcellsorbodymovements.Themostcommonlyusedpiezoelectricmaterialinelectronicsisleadzirconatetitanate(PZT)owingtoitsnotablepiezoelectricandelectromechanicalcouplingcoefficients.Nevertheless,60wt.%ofPZTislead,whicheveninlowdosescausesserioushealthproblemssuchasneurotoxicity,pregnancycomplications,attentiondeficithyperactivityandslowgrowthrateinchildren.TherehavebeeneffortstoreplacePZTwithlead-freepiezoceramics.,Amongpopularlead-freepiezoceramics,zincoxide(ZnO),bariumtitanate(BT),potassiumsodiumniobate(KNN),lithiumsodiumpotassiumniobate(LNKN),andboronnitridenanotubes(BNNT)havesubstantialpiezo-electriccoefficients.Onemajorconcernaboutusingpiezoceramicsintissueengineeringisthecytotoxicityofthesematerials.Mostpiezoelectricceramics,exhibitiondissolutioninbiologicalfluids.WhilesomeofthereleasedionsforinstancePb2+couldbetoxic,otherscouldberelativelysafeorevenfavorableatlowdosesandcytotoxicathigherconcentrations.,CompositesofZnOnanoparticlesinpolyurethanehavebeenemployedtoengineerneuraltissues,whichresultedinlowerdensityofastrocytesasZnOincreased,particularlyoncompositeswithmorethan10%ZnO.Initialresultswerepromising:BTcylindersimplantedincaninefemoraformedastronginterfacialbondwithbone.However,nosignificantdifferencewasobservedbetweenpoled/piezoelectricBTimplantsandtheunpoledones.,Bariumtitanate(BT)wasthefirstlead-freepiezoceramicthatwasinvestigatedinbonerepair.Initialresultswerepromising:BTcylindersimplantedincaninefemoraformedastronginterfacialbondwithbone.However,nosignificantdifferencewasobservedbetweenpoled/piezoelectricBTimplantsandtheunpoledones.,Potassiumsodiumniobate(KNN)andlithiumsodiumpotassiumniobate(LNKN)aretwootherlead-freepiezoceramicswithpiezoelectriccoefficientsofalmosttwoordersofmagnitudelargerthannaturalbone,whosecytotoxicitieshavebeenevaluated.TheviabilityofmousefibroblastsculturedonKNNandLNKNpowdersfor24hwererespectively84%and58%,althoughthevalueforLNKNcouldbeimprovedto72%byadjustingthepH.TheslightlyhighertoxicityofLNKNcouldbeduetothereleaseofLi+inthemediaafterimmersionofthepiezoceramic.Inanotherstudy,ratosteoblastsshowedbetterattachmentandproliferationonthenegativelychargedsurfaceofthepoledLNKNporousscaffoldsthanunpoledscaffolds.,Inanovelapproach,Cernalizedboronnitridenanotubes(BNNT)insidethecytoplasmicvesiclesofPC-12cellsandwirelesslyagitatedthepiezoelectricnanotubesbyexternalultrasound.TheirresultsshowedthatneuritelengthsaswellasthenumberofneuritesperdifferentiatedcellweresignificantlygreaterincultureswithBNNTsfollowedbyultrasoundagitation.Interestingly,theyalsoobservedthatblockingcalciumchannelsdiminishedtheresponsetostimulation,suggestingthatelectricalstimulationcontributestodifferentiationthroughcalciumsignalingpathways.,5.2.PiezopolymersDiscoveryofthepiezoelectricityofpolyvinylidenefluoride(PVDF)byKawaimadethispolymeracandidatesubstituteforPZT.Duetoitsflexibilityandnontoxicity,PVDFhasbeenusedforavarietyofbiomedicalpplications,fromtissueengineeringscaffoldstoimplantableself-powereddevices.,PVDFcrystalizesinvariousphases.-PVDFhasatrans-gaucheconformationwithoutanetdipolemomentandisnonpiezo-eletric(Fig.7(a).-PVDF,however,hasall-transconformation,whichexhibitsanetdipolemomentandisthuspiezoelectric(Fig.7(b).Alternatively,poly(vinylidenefluoride-co-trifluoroethylene(PVDF-TrFE)isacopolymerwithall-transconformation,piezoelectricityandhighelectromechanicalcouplingcoefficient(Fig.7(c).,Fig.7.Chemicalstructuresofa-PVDFwithtrans-Gaucheconformationandnonetdipolemoment(a);b-PVDFwithall-transconformationhavinganetdipolemoment(b);andPVDF-TrFEcopolymerwithaninherentnetdipolemoment(c).,Valentinietal.usedPVDFtobuildguidancechannelsforneuralregeneration.Theyemployedmechanicalstretchingtoorientthedipolesandtransforma-PVDFtob-PVDF,followedbyelectricpolinginordertofixthedipolesandachievepiezoelectrictubestobeusedassciaticnerveguidancechannelstotreatratsciaticnerveswith4mmgaps.After4weeksofimplantation,thenumbersofmyelinatedaxonsinthemidpointofthepiezoelectrictubesweresignificantlyhigherthanthecontroltubes.,Valentinietal.repeatedtheexperimentwithpiezoelectricPVDF-TrFEtoavoidstretching.Theypoledtwogroupstohavenegativelychargedandpositivelychargednerveguidancechannelsaswellasanunpoledgroup.Thepositivelypoledtubeledtothemostmyelinatedaxonsintheregeneratednervesbridgingthe10mmgapsafter4weeks,whilenegativelypoledandunpoledtubesresultedintheintermediateandthefewestnumberofmyelinatedaxons,respectively.Inaseriesofstudies,Arinzehetal.usedelectrospinningtoconstructfibrouspiezoelectricscaffoldsforneuralandbonetissueengineering.ElectrospunPVDF-TrFEfibrousscaffoldsshowedhighercrystallinityandb-phasecontentascomparedtothestartingpowdermaterial.,Inasubsequentstudy,electrospunPVDFandPVDF-TrFEfibrousscaffoldswereusedtoexamineneuriteoutgrowthusingdorsalrootganglions(DRGs).ResultsshowedthatwhileneuritesattachedandextendedonPVDFandPVDF-TrFEfibrousscaffolds,theirattachmentandextensiononfilmscastedfromthesamematerialswerepoor,whichsuggestedthe3-Dfibrousenvironmentfacilitatedneuriteoutgrowth.MeasurementofaverageneuritelengthsandtheaspectratiooftheDRGsafter4daysofcellcultureshowedtherewasalinearrelationshipbetweenfiberdiameterandfiberalignmentwithneuriteextensionanddirectionalityoftheDRGs.,Arinzehetal.investigatedthedifferentiationofhumanneuralstem/precursorcells(hNSC/NPC)onPVDF-TrFEfibrousscaffoldsandfilms.Resultsshowedthatcellsonelectrospunpiezoelectricfibrousscaffoldsweremostlyneuron-likeb-IIItubulins,whileonnonpiezoelectriclaminin-coatedplates,mainlynestinwasexpressed.Guoetal.electrospunblendsofpolyurethane(PU)andPVDFtostudytheeffectofpiezoelectricityonwoundhealingandfibroblastactivity.Formechanicaldeformation,theyusedcultureplateswithflexiblebottomsthatcouldbebiaxiallystretched.HavingobservedanincreaseinthepiezoelectriccoefficientaswellasadecreaseinthemechanicalpropertiesofthecompositescaffoldsasaresultofincreasingPVDF/PUratio,theyusedPVDF/PUof1astheoptimumcompositionforculturingmouseembryofibroblasts.,Similartothepreviouslydiscussedfibronectin(FN)adsorptiontoconductivepolymerswithdirectelectriccurrents,onepossibleapproachtoexplicatefavorablecellularresponsetopiezoelectricmaterialsisenhancedproteinadsorptionduetostaticordynamicelectricchargesonthesurface.PolinghasshowntoresultinhigherFNadsorptionfollowedbyincreasedcellnumberson-PVDFfilms,possiblyduetoorientationofdipolesonthesurface,whichalsomanifestsitselfinhigherhydrophilicityofpoled-PVDFsamplescomparedtounpoledones.,UnlikePVDF,Poly-L-lacticacid(PLLA)isabiodegradablepolymerwithhelicalstructureandaccordinglypermanentlyorienteddipoles.ThediscoveryofthepiezoelectricityofPLLAbyFukada,inspiredattemptstousethisbiocompatibleandbiodegradablepolymerasabonesubstitutewiththecapabilitytomimicthepiezoelectricityofnaturalbone.Currently,PLLAisapprovedbytheU.S.FoodandDrugAdministration(FDA)asaboneimplantdevice.MostoftheresearchonPL