Lec10-Ceramics and Bioglasses.pdf

7/27/2019 Lec10-Ceramics and Bioglasses.pdf

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Ceramics and Glasses


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Definitions

Ceramic: Inorganiccompoundsthatcontainmetallic

andnonmetallicelements,forwhichinteratomicbondingisionicorcovalent,andwhichare

generallyformedathightemperatures.

Glass:(i)Aninorganicproductoffusionthathas

cooledtoarigidconditionwithoutcrystallization;

(ii)Anamorphoussolid.

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http://www.bioen.utah.edu/faculty/pat/Courses/biomaterials/coursenotes.html


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Definitions

Amorphous:(i)Lackingdetectablecrystallinity;(ii)

possessingonlyshortrangeatomicorder;alsoglassyorvitreous

Glassceramic:Polycrystallinesolidspreparedbythe

controlledcrystallization(devitrification)ofglasses.

Bioactivematerial:Amaterialthatelicitsaspecific

biologicalresponseattheinterfaceofthematerial,resultingintheformationofabondbetweenthetissuesandthematerial.

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CrystalversusGlassyCeramics

Crystallineceramicshavelongrangeorder,with

componentscomposedofmanyindividuallyorientedgrains.

Glassymaterialspossessshortrangeorder,and

generallydonotformindividualgrains. Thedistinctionismadebasedonxraydiffraction

characteristics.

Mostofthestructuralceramicsarecrystalline.

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Materialsthatcanbeclassifiedasbioceramics include:

Alumina Zirconia

Calciumphosphates

Silicabasedglassesorglassceramics, Pyrolytic carbons

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Metal CeramicComparison

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Stiffness is comparable to the metal alloys

The biggest problem is fracture toughness (sensitivityto flaws). Rigid plastics < Ceramics = Metals


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WhyUseBioceramics?

GeneralOptions

Toxic/Imunogenic/Diseasetransmission?

MechanicalProperties?

Bioactive? Degradable?

Autograft

Allograft

Metals

Ceramics

Polymers

Composites

Excellent

LowModerate


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Advantages:

inertinbody(orbioactiveinbody);Chemicallyinertin

manyenvironments

highwearresistance(orthopedic&dentalapplications)

highmodulus(stiffness)&compressivestrength

estheticfordentalapplications

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Disadvantages

brittle(lowfractureresistance,flawtolerance)

lowtensilestrength(fibersareexception)

poorfatigueresistance(relatestoflawtolerance)


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PhysicalFormsBioceramics areavailableas:

Microspheres Thinlayersorcoatingsonametallicimplant

Porousnetworks

Compositeswithapolymercomponent Largewellpolishedsurfaces

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BasicApplications:Orthopedics:

boneplatesandscrews total&partialhipcomponents(femoralhead)

coatings(ofmetalprostheses)forcontrolled

implant/tissueinterfacialresponse spacefillingofdiseasedbone

vertebralprostheses,vertebraspacers,iliaccrest

prostheses

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2/21/2011 13Trends Biomater. Artif. Organs, Vol 18 (1), pp 9-17 (2004)


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MechanicalProperties


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Dentistry: dentalrestorations(crownandbridge)

implantapplications(implants,implantcoatings,ridgemaintenance)

orthodontics(brackets)

glassionomercementsandadhesives

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Veneers

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Beforeandafter

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Other: innerearimplants(cochlearimplants)

drugdeliverydevices ocularimplants

heartvalves

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Osteointegration

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Hip Implant

Fast mineralization of the surface

Surface colonization by the

osteoblasts

Stable binding between the formedmineral phase and the implantsurface

Structural continuity to thesurrounding bone



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TypesofBioceramicTissue

Interactions:Dense,inert,nonporousceramicsattachtobone(ortissue)

growthintosurfaceirregularitiesbypressfittingintoadefect

asatypeofadhesivebond(termedmorphologicalfixation)Al2O3

Porousinertceramicsattachbyboneresultingfromingrowth

(intopores)resultinginmechanicalattachmentofbonetomaterial(termedbiologicalfixation)Al2O3

Dense,nonporoussurfacereactiveceramicsattachdirectlyby

chemicalbondingwithbone(termedbioactivefixation)bioactiveglasses&Hydroxyapatite.

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ProcessingofCeramics1.Compounding Mixandhomogenizeingredientsintoawaterbasedsuspension=slurry

or,intoasolidplasticmaterialcontainingwatercalledaclay

2.Forming Theclayorslurryismadeintopartsbypressingintomold(sintering). Thefine

particulatesareoftenfinegrainedcrystals.

3.Drying Theformedobjectisdried,usuallyatroomtemperaturetothesocalled

"green"orleatherystate.

4.Firing

Heatinfurnacetodriveoffremainingwater.Typicallyproducesshrinkage,soproducingpartsthatmusthavetightmechanicaltolerancerequirescare. Porouspartsareformedbyaddingasecondphasethatdecomposesathigh

temperaturesformingtheporousstructure.

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Alumina (Al2O3) andZirconia (ZrO2)

The two most commonly usedstructural bioceramics.

Primarily used as modularheads on femoral stem hip

components. Wear less than metal

components, and the wear

particles are generally bettertolerated.


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FemoralComponent

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Alumina(Al2O3):

singlecrystalaluminareferredtoas

Sapphire Rubyisaluminawithabout1%of

Al3+replacedbyCr3+;yieldsred

color Bluesapphireisaluminawith

impuritiesofFeandTi;various

shadesofblue

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StructureandProperties: mostwidelyusedformispolycrystalline

unique,complexcrystalstructure strengthincreaseswithdecreasinggrainsize

elasticmodulus(E)=360380GPa

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FabricationofBiomedicaldevicesfrom

Al2O3&(ZrO2):

Devicesareproducedbypressingandsinteringfine

powdersattemperaturesbetween1600to1700C. AdditivessuchasMgOadded(


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DentalPorcelain: Ternarycomposition=mixtureofK2OAl2O3SiO2 made

bymixingclays,

feldspars,

and

quartzCLAY=Hydratedaluminosilicate

FELDSPAR=Anhydrousaluminosilicate

QUARTZ=AnydrousSilicate

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CalciumPhosphates

Calciumphosphatecompoundsareabundantinnatureandinliving

systems. Biologicapatitesconstitutetheprincipalinorganicphaseinnormal

calcifiedtissues(e.g.,enamel,dentin,bone)arecarbonatehydroxyapatite,CHA.

Alsofoundinsomepathologicalcalcifications(e.g.,urinarystones,dentaltartarorcalculus,calcifiedsofttissues heart,lung,jointcartilage)

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2/21/2011 31Table from: R. LeGeros Chem. Rev., 2008, 108 (11), pp 47424753


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CalciumPhosphates Severalcalciumphosphateceramicsareconsideredbiocompatible.Of

these,mostareresorbableandwilldissolvewhenexposedto

physiologicalenvironments.Someofthesematerialsinclude,inorderofsolubility:

TetracalciumPhosphate(Ca4P2O9) > AmorphouscalciumPhosphate >alphaTricalciumPhosphate(Ca3(PO4)2) > betaTricalciumPhosphate

(Ca3(PO4)2) >> Hydroxyapatite(Ca10(PO4)6(OH)2)

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Calciumhydroxyapatite

(Ca10(PO4)6(OH)2):HA

Hydroxyapatiteistheprimarystructuralcomponentofbone.

Asitsformulasuggests,itconsistsofCa2+ionssurroundedbyPO42 andOH ions.

Unliketheothercalciumphosphates,hydroxyapatitedoesnotbreakdownunderphysiologicalconditions.

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BoneMimicryPropertiesofCalcium

Phosphates1) InterconnectingPorosity

Interconnectingmacroporosity isintroducedinsynthetic

calciumphosphatesby: Addingporogens (H2O2,polymericporogens)

Foamingmethods

Microporosity dependsonsinteringtemperature

CaP sinteredat1200Cshowssignificantlylessporositythanthatsinteredat1000C

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(A) Bovine bone-derived HA. (B and C) Biphasic calcium phosphate, BCP. The original interconnecting macroporosity

in bone was preserved in A. Macroporsity in B and C was introduced using porogens before sintering. C shows the

presence of concavities.


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Phosphates1) InterconnectingPorosity

Interconnectingmacroporosity isintroducedinsynthetic

calciumphosphatesby: Addingporogens (H2O2,polymericporogens)

Foamingmethods

Microporosity dependsonsinteringtemperature

CaP sinteredat1200Cshowssignificantlylessporositythanthatsinteredat1000C

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SEM of BOP sintered at (A) 105000 and (B) 1200C. Note the presence of microporosities in A and not in B.

f l


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Phosphates2) Biodegradability

InvitrobiodegradationisdeterminedbysuspendingthematerialinacidicbufferandmonitoringthereleaseofCa2+ionswithtime

Theacidicbuffer,tosomeextent,mimicstheacidicenvironmentduringosteoclastic activity(boneresorption).

InvitroorinvivodegradationofCaPs dependsontheircomposition,particlesize,crystallinity (reflectingcrystalsize),porosity,andpreparationconditions.

Degradationorrateofdissolutionproceedsinthefollowingdecreasingorder: TCPbovineboneAp (unsintered)bovineboneAp (sintered)>corallineHA>HA.

ComparingdifferentsyntheticCaPs (unsintered),thesolubilitydecreasesintheorderACP>DCPD>OCP>CDA.

Incorporationofdifferentionsapatitecanincrease(e.g.,CO32,Mg2+,orSr2+)or

decrease(e.g.,F)thesolubilityoftheapatite.

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B Mi i P i f C l i


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Phosphates3) Bioactivity

Bioactivity(propertythatallowsthematerialtodirectlybondwiththe

newformingbone)wasfirstobservedanddescribedbyHenchet

al.inspecialsilicabasedbioactive

glasses(J.Biomed.Mater.Res.1978

2117)

Incontrast,anunmineralized fibroustissueformsattheinterfaceofthe

newboneandbionert materials

Example:directboneattachmentisobservedonaplasmasprayedHA

coatedTialloysurface,whilefibrous

tissueencapsulatestheuncoated

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Bone growth and attachment on Ti alloy cylinder grit blasted with apatitic abrasive on one side (A) and alumina

abrasive on the other side (B). The side grit blasted with apatitic abrasive showed direct bone attachment (A), while the

side grit blasted with alumina showed indirect bone attachment through a nonmineralized fibrous layer. (Proceedings

of the 25th Annual International Society of Biomaterials; Woodhead Publishing: Cambridge, U.K.

, 1998.)


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BoneMimicryPropertiesofCalciumPhosphates

4) Osteoconductivity Whenreferringtobiomaterials,osteoconductivity istheabilityofthematerialto

serveasascaffoldortemplatetoguideformationofthenewlyformingbonealong

theirsurfaces.

InvivotheCHAlayerthatformsonCaP biomaterialsurfacesadsorbscirculatingproteins(fromthebiologicenvironment)onwhichbonecellsattach,migrate,

proliferate,anddifferentiate,leadingtomatrixproductionandbiomineralization.

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Schematic representation of the dissolution/precipitation process involved in formation of CHA on CaP surfaces in

vivo. Acid environment caused by cellular (macrophages, osteoclasts) activity causes partial dissolution of CaP,

causing increased supersaturation of the biologic or physiological fluid, causing precipitation of CHA incorporating003 and other ions and organic molecules (protein).


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BoneMimicryPropertiesofCalciumPhosphates

4) Osteoinductivity Osteoinductivity istheabilityofthematerialtoinducedenovoboneformation

withoutthepresenceofosteogenic factors(Adesirablepropertyinbonesubstitute

materials)

CaP biomaterialsaregenerallyknowntobeosteoconductive butnotosteoinductive.

Osteoinductive propertiescanbeintroducedtoCaP materialsbytwomethods:

(1)designingtheCaPs withappropriategeometry,topography,combinedappropriatemacroporosity/microporosity andconcavitiesthatwillallowthe

entrapmentandconcentrationofcirculatinggrowthfactorsorosteoprogenitor cells

responsibleforboneformationor (2)combiningCaP withgrowthfactors(BMPs,mesenchymal cells)orbioactive

proteins(collagen,OPs,orpeptides)

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(Ca10(PO4)6(OH)2):HA

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(Ca10(PO4)6(OH)2):HA gainedacceptanceasbonesubstitute

repairofbonydefects,repairofperiodontaldefects,maintenanceoraugmentationofalveolarridge,ear

implant,eyeimplant,spinefusion,adjuvanttouncoated

implants.

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HAis:

Ca10(PO4)6(OH)2 SincecollageniscloselyassociatedwithHAinnormal

bone,itisalogicalcandidateforinductionofahost

response.Insomecasesbonegrowthinornear

implantedHAismorerapidthanwhatisfoundwith

controlimplants.IntheliteratureHAissometimes

referredtoasan"osteoinductivematerial.However,HAdoesnotseemtoinducebonegrowthinthesameway

as,say,BMP.

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BioceramicCoatings Coatingsofhydroxyapatiteareoftenappliedtometallic

implants(mostcommonlytitanium/titaniumalloysand

stainlesssteels)toalterthesurfaceproperties. Inthismannerthebodyseeshydroxyapatitetype

materialwhichitappearsmorewillingtoaccept.

Withoutthecoatingthebodywouldseeaforeignbodyandworkinsuchawayastoisolateitfromsurroundingtissues.

Todate,theonlycommerciallyacceptedmethodofapplyinghydroxyapatitecoatingstometallicimplantsis

plasmaspraying.

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BoneFillers Hydroxyapatitemaybeemployedinformssuchas

powders,porousblocksorbeadstofillbonedefectsorvoids.

Thesemayarisewhenlargesectionsofbonehavehadtoberemoved(e.g.bonecancers)orwhenboneaugmentationsarerequired(e.gmaxillofacialreconstructionsordentalapplications).

Thebonefillerwillprovideascaffoldandencouragetherapidfillingofthevoidbynaturallyformingboneandprovidesanalternativetobonegrafts.

Itwillalsobecomepartofthebonestructureandwillreducehealingtimescomparedtothesituation,ifno

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