Original papers

194 Ceramics – Silikáty 59 (3) 194-201 (2015)

CHARACTERIZATION OF HIGHLY POROUS 63S BIOACTIVE GLASS

SCAFFOLDS FABRICATED BY TWO FOAMING METHODSSEYED MEHDI MIRHADI*, #HAMED GHOMI**, RAHMATOLLAH EMADI***

*Department of Materials Engineering, Shahreza Branch,Islamic Azad University, 86145-311, Shahreza, Isfahan, Iran

**Young Researchers and Elite Club, Najafabad Branch,Islamic Azad University, Najafabad, Iran

***Biomaterials Group, Department of Materials Engineering,Isfahan University of Technology, Isfahan 84156-83111, Iran

#E-mail: hamed.ghomi@pmt.iaun.ac.ir

Submitted April 16, 2015; accepted June 24, 2015

Keywords: Gelcasting, Porosity, Mechanical properties, Bioactive glass, Biomedical applications

Resorbable 3D macroporous nanostructure 63S bioactive glass scaffolds were fabricated using the two methods of direct foaming of bioactive glass sol and foaming glass slurry for tissue engineering applications. The scaffolds contained an interconnected pore network with macropore sizes in the range of 100 - 400 μm, which provided the potential for tissue ingrowth and vascularization in the human body.The mean values of compressive strength were in the ranges of 0.53 - 0.68 MPa and 0.8 - 0.92 MPa, respectively, for the scaffolds prepared by the first and second methods. The mean values of total and interconnected porosities were in the ranges of 88 - 93 % and 76 - 86 %, respectively. The highly porous and nanosized structure gave rise to a high specific surface area in the scaffolds which stimulated mineralization in the surrounding bones by enhancing bioreactions and leaching of ions from the surface, which facilitate bone repair and fixation. Finally, it was observed that the prepared scaffolds could satisfy the criteria of an ideal scaffold for tissue engineering applications.

INTRODUCTION

Scaffoldsimplantedintoadefectsitearemeanttohelp in situregenerationoftissues.Intissueengineeringapplications,scaffoldsareseededwithcellsandgrowthfactors in vitro toproduce thebasis fora tissuebeforeimplantation [1].This requires scaffoldsof appropriatepore size with interconnected pores to promote cellproliferation, vascular ingrowth and nutrient transpor-tation [2]. Certain compositions of bioactive glasses con-taining SiO2–CaO–P2O5 bond to both soft and hardtissueswithoutformingscartissue[3-4].Thedissolutionproducts of these bioactive glasses (soluble siliconandcalcium)leadtotherapidexpressionofgenesthatregulate both osteogenesis and production of growthfactors.Thesecharacteristicshavestimulatedextensiveinvestigations into bioactive glass materials used as scaffoldsintissueengineering[1,3,5]. Sol-gel derived bioactive glasses, compared to their melt-derived counterparts, reportedly exhibit enhanced resorbability and bioactivity in vitrowithimprovedbonebonding in vivo [3-7]. This has been attributed to gel glasses exhibiting a mesoporous texture (pores in the range of 2 - 50 nm in diameter), which is inherent tothesol-gelprocessandincreasesthespecificsurfacearea [1, 3, 8-12].

An ideal scaffold should combine the beneficialproperties of bioactive glasses with a structure con-taining an interconnected network with macropores(greaterthan100μm)toenabletissueingrowth[13]andnutrientdelivery to thecenterof theregeneratedtissueandmesopores(2nm≤poresize≤50nm)inorder topromote cell adhesion [3, 13]. Another requirement is thatthescaffoldshouldberesorbedatcontrolledratestomatchthatoftissuerepair.Furthermore,theprocessingmethod used should be capable of producing irregularandcomplexshapestomatchthoseofthedefectintheboneof thepatient[1-3,14].Foamingsol-gel-derivedbioactiveglasses,orgelcasting,providesthepotentialformakingscaffoldswiththeseproperties[14].Gelcastingis a well-established method for making high-quality,complex-shaped ceramic pieces by means of in situ solidificationthroughwhichamacromolecularnetworkiscreatedfromthein situpolymerizationofanorganicgelling agent [15-17]. Theobjectiveofthepresentworkwastofabricateand characterize bioactive glass scaffolds with inter-connectedporesofdiametersinexcessof100μmusingthefoamingprocess.Forthispurpose,differentscaffoldswereproducedbybothdirectfoamingofbioactiveglasssol and foaming glass slurry. Finally, the morphologyandmechanicalpropertiesofthetwotypesofscaffoldsthuspreparedwerecomparedtoensuretheachievementofthedesiredproperties.

Characterization of highly porous 63S bioactive glass scaffolds fabricated by two foaming methods

Ceramics – Silikáty 59 (3) 194-201 (2015) 195

ExPERIMENTAL

Starting materials

In this study, calcium nitrate tetrahydrate (Ca(NO3)2 –4H2O, Merck), triethyl phosphate (TEP, Aldrich), tetraethyl orthosilicate (TEOS, Merck), hydrochloric acid (HCl, Merck), and absolute ethanol (C2H5OH, Merck) were used as bioactive glass (BG)precursors. The gelcasting components used included hydrofluoric acid (HF, Merck) and agarose powder(Merck) as the gelling agents, Tergitol (Aldrich) as a surfactant, and tripolyphosphate sodium (TPP) as a dispersant. The gelling agent used for the directfoamingofbioactiveglasssolwashydrofluoricacidandthatforfoamingglassslurrywasagarosepowder.

Preparationofscaffoldsbyfoamingglassslurry

Followingtheproceduresreportedelsewhere[18],63SBGpowderwith65%SiO2,31%CaO,and4%P2O5inmolarpercentageswereprepared.Briefly,TEOSwasdissolved inabsoluteethanolanddeionizedwater,using2MHClas the catalyst, and stirred for30min.TheTEPwasthendissolvedintothepreparedsolution,towhichCa(NO3)2–4H2Owasaddedafter20min.Theclearsolutionwasthenagedat60°Cfor48handdriedat120°Cfor48h.Thepowderthusobtainedwascalcinedat 600°C for 2 h,whichwas then dispersed in doubledistilledwaterbyusing1wt.%TPPasthedispersant.Agarosepowder(7wt.%)wassimultaneouslydissolvedasthegellingagentindoubledistilledwaterbyheatingupto130°C.Thetwosolutionswerethenmixedat80°Cto obtain a slurry which was then foamed by adding 3volumepercent(vol.%)ofTergitolwhilevigorouslyagitatedusingatriple-blademixerbeforeitwaspouredinto themold. The gelling reaction was conducted bycooling the samples to 0°C. The samples were thendemolded, dried at ambient temperature, and sintered at 800and900°Cfor4h.

Preparationofscaffoldsbydirectfoamingofbioactiveglasssol

The glass sol was prepared as described above.Before aging and drying, simultaneous hydrolysisand polycondensation reactionswere allowed to occurduring and after sol preparation. Briefly, aliquots of30 ml of the sol were foamed by vigorous agitationwhile1mlof thesurfactant (Tergitol),doubledistilledwater,and3mlofhydrofluoricacid(HF,acatalystforpolycondensation) were added.As viscosity increasedrapidlyandthegellingpointwasapproached,thesolutionwas poured intomolds.The gelation process providedpermanent stabilization for thebubbles that formedbyairentrapmentduringtheearlystagesoffoamingastheresultofreducedsurfacetensionandvigorousagitation

ofthesolution.Thescaffoldswerethenagedat60°Cfor48h,driedat120°C for48h, andsinteredat800and900°Cfor4h.

Materials characterization

X-ray diffraction analysis

Phase structure analysis of the powder and thescaffolds thus preparedwas performed using anX-raydiffractometer(XRD,PhilipsXpert)withNifilteredCukα (λ cu kα = 0.154186 nm, radiation at 30mA, and 40kV)intherangeof20≤2theta≤70(timeperstep: 1sandstepsize:0.05°).

Specific surface area

ThespecificsurfaceareaofthepreparedBGpowderwas determined by physical adsorption of nitrogengas at -196°C on the surface of the powder and bycalculating the amount of adsorbate gas correspondingtoamonomolecularlayeronthesurface(MicromeriticsInstrument Corp., Gemini). Assuming a hexagonal close packing, the adsorptive gas (N2) has a molecular cross sectionalareaof0.162nm2at-196°C[19]. The specific surface area of the powderwas esti-mated using the multipoint Brunauer-Emmett-Teller (BET)adsorptionisothermequationinitslinearform:

(1)

where,P and Po are the equilibrium and saturation pres-sures of the adsorbates at -196°C,Va is the volume ofgas adsorbed at standard temperature and pressure (STP), Vmisthevolumeofthemonolayer adsorbed gas, and C is the BET constant that is related to the enthalpy ofadsorptionoftheadsorbategasonthepowder. The BET value [P/(Va (Po-P))]wasplottedagainstP/Po according to Equation 1 (i.e., the BET plot). This plot should yield a straight line usually in the appro-ximaterelativepressurerangeof0.05to0.3.Thevalueoftheslopeandthey-interceptofthelinewereusedtocalculate Vm and C.Thefollowingequationswereused:

(2)

(3)

Thespecificsurfacearea,SBET, in m2·g–1,wascal-culated using Equation 4 [20]:

(4)

Average particle size of the prepared powderwascalculated using Equation 5 based on the assumption that thesynthesizedparticleswerespheroids:

[P/(Va (Po – P))] = × +C – 1Vm C

1Vm C

PPo

Vm = 1(slope + intercept)

C = + 1slope

intercept

SBET =Vm × N × A

22 400

Mirhadi S. M., Ghomi H., Emadi R.

196 Ceramics – Silikáty 59 (3) 194-201 (2015)

(5)

where, d is the average particle size (nm), SBET is the specificsurfacearea(m2∙g-1), and 2.87 is the theoretical densityof63SBG.

Scanning electron microscopy analysis

Scanning electron microscopy (SEM, Philips xL 30) was used to study the morphology and size of thepores in the BG scaffolds. Pore size distribution wasdetermined based on the results of image analysis ofSEM micrographs. The scaffolds were sputter-coatedwith a thin layer of gold (about several nanometersthick) using a physical vapor deposition apparatus forimproved resolution.

Transmission electronmicroscopy analysis

Transmission electron microscopy (TEM, Philips CM-200)wasusedtostudytheparticlesizeoftheBGpowders.

Porosity measurement

Theporosityof thescaffoldswhichmaybeeitherinterconnectedorclosedwasmeasuredaccordingtotheArchimedes method [21]. Apparent porosity or inter-connectedporositywasdeterminedusingEquation6:

Apparentporosity=[(Ww – Wd)/(Ww – Ws)] × 100 (6)

where,Wd is theweight of the dry scaffold,Ws is the weightofthescaffoldsuspendedinwater,andWw is the weightofthescaffoldafteritisremovedfromwater.True porosity expressed by Equation 7 includes both interconnectedandclosedporesofthescaffold:

Trueporosity=[(ρ(Ww – Ws) – Wd)/(ρ(Ww – Ws))] × 100 (7)where, ρ is the true density or specific gravity of theglass. Inordertocheckthevalidityoftheresultsobtained,thediameterand theheightof thescaffoldsweremea-sured using a digital caliper rule and the mass of thesampleswasmeasuredusingadigitalbalance.Theva-lue of green density (ρg) was determined as shown inEquation 8:

ρg=Wd/(πr2 h) (8)

where,r is the radius (cm) and histheheight(cm)ofthesample.Thepercentageofporositywasmeasuredwithrespecttodensity,asshowninEquation9:

Trueporosity=[1– (ρg/2.87)] × 100 (9)

where,2.87isthetheoreticaldensityofBG.

Mechanical testing

Thecompressiontesthasbeenwidelyusedforcha-racterizingthemechanicalpropertiesofporousscaffolds.Inthisstudy,parallelplatecompressiontestswerecarriedoutoncylindricalscaffolds(20mminheightand10mmin diameter) using a universal testingmachine (zwick,materialprufung,1,446–60)withacrosshead speedof0.5 mm·min-1.Fivesampleswereusedforeachsinteringtemperatureandfabricationmethodandtheresultswerereported as average values. Compressive strength wasevaluated from themaximumpoint of the stress/straingraph,whichoccurswhenthefirstcrackappearsonthescaffold.Elasticmoduluswascalculatedastheslopeoftheinitiallinearportionofthestress/straingraph.

RESULTS AND DISCUSSION

Figure1a shows the adsorption isothermofnitro-genat-196°ContheBGpowderandFigure1bshowstherelatedBETplot.Thevaluesof BETspecificsurfacearea and C calculatedfromthelinearpartoftheBET plot were determined to be 189.7 m2·g-1 and 57.8, respec-tively.Assuming powder particles to be spherical, theaverage particle size calculated using Equation 5 wasfoundtobe11nm.Thehighspecificsurfaceareacanbeattributed to both the sol-gel preparation method and the nano-sizedparticles,whichleadstoexcellentbioactivity,osteoconductivity, and biodegradability providing fastboneingrowthandimprovedbonebondingin vivo [3-7, 14].Thevaluesofparticlesizeobtainedfromtheresultsof imageanalysisofTEMmicrographsalsoconfirmedthe above results. The TEM micrograph of the BGnanopowder (Figure2) indicates a particle sizeof lessthan 30 nm. TheXRDpatternsof theBGnanopowderand thescaffolds prepared by foaming glass slurry at differentsintering temperatures are shown in Figure 3. Clearly,the observed pattern of BG nanopowder (Figure 3a)confirmstheformationofBGwithanamorphousstruc-ture.Inaddition,nopeakofdiffractionisobservedinthespectrumoftheBGscaffoldsinteredat800°C(Figure3b),indicating that the scaffold cannot have a crystallinestructure and that it, therefore, has a glass structure.However, the spectrum of the BG scaffold sintered at900°C (Figure 3c) exhibits peaks indicative ofLarnite(Ca2SiO4) [22]. It should be noted that the crystallinity ofBGincreaseduponheat treatmentat900°C.This isdue to the formation of the crystalline phase (Larnite)atapproximately900°C,whichaffectedtheresorbabilityand bioactivity of the amorphous BG [23]. The XRDpatternsof the scaffoldspreparedbydirect foamingofbioactiveglasssolatdifferentsinteringtemperaturesareshowninFigure4.Inagreementwiththeaboveresults,the scaffold sintered at 800°C exhibits an amorphousstructurebuttheonesinteredat900°Cexhibitspartialcrystallization of BG to the Larnite (Ca2SiO4) phase.

d = 6 × 103

2.87 × SBET

Characterization of highly porous 63S bioactive glass scaffolds fabricated by two foaming methods

Ceramics – Silikáty 59 (3) 194-201 (2015) 197

Themeanvalues of compressive strength and theelasticmodulusofthescaffoldspreparedbyfoamingglassslurryatdifferentsinteringtemperaturesmeasuredintherangesof0.8-0.92MPaand50-57MPa,respectively.Thevaluesrecordedforthesameparametersinthecaseofthescaffoldspreparedbydirectfoamingofbioactiveglasssolatdifferentsinteringtemperatureswereintherangesof0.53-0.68MPaand49-59MPa(Table1).Totalporosityinthescaffoldspreparedbyfoamingglassslurryandsinteredatdifferenttemperaturesrangedfrom88 to90%,whileopenporosity in the same scaffoldsranged from 76 to 80%.On the other hand, the totalporosity and open porosity values recorded for thescaffolds fabricated by direct foaming of bioactiveglasssolatdifferentsinteringtemperaturesrangedover90-93and81-86%,respectively(Table1). Thedifferencesobservedinthemechanicalproper-ties of the two types of scaffolds were attributed todifferences in porosity, pore size, pore structure, andporemorphology.Inotherwords,thescaffoldspreparedbydirectfoamingofbioactiveglasssolexhibitedhigherporosity, interconnectivity, and pore size distribution. Sinteringat800°Cfor4hprovidesanoptimalcombina-tion of amorphous structure and compressive strengthtogetherwithmacroscopicstructural features favorabletobone ingrowthandangiogenesis [13].This iswhile,duetothebioactivityoftheLarnitephase,thescaffoldssintered at 900°C are appropriate for applications inwhich higher strengths are need to withstand greaterloadings. Table 1 shows the porosity and mechanical

a)

b)

Figure 1. Adsorption isotherm of nitrogen at -196°C (b)BETplotfornitrogenadsorbedat-196°ContheBGpowderprepared.

20

Volu

me

(cm

3 gr

1 )

40

60

80

30

50

70

90

0.80.60 0.2P/P0

0.4 0.7 1.00.90.50.1 0.3

0

[P(Va(P

0–P

))]

0.002

0.004

0.006

0.001

0.003

0.005

0.008

0.007

0.300 0.10P/P0

0.20 0.250.05 0.15

Multipoint BETLinear fit

Slope = 0.022552Y intercept = 0.000397C (BET constatnt) = 57.826996Correlation coefficient = 1.0000

Figure3.XRDpatternsofthea)BGnanopowder,andb),c)scaffoldspreparedbyfoamingglassslurrysinteredat800°and900°Cfor4hour,respectively.

Inte

nsity

(a.u

.)

605020 302θ (°)

Ca2SiO4

a)

b)

c)

40 55 70654525 35

Figure2.TEMmicrographofthepreparedBGnanopowders.

Figure 4. XRD patterns of the scaffolds prepared by directfoaming of bioactive glass sol sintered at a) 800°C and b)900°Cfor4hr.

Inte

nsity

(a.u

.)

605020 302θ (°)

Ca2SiO4

a)

b)

40 55 70654525 35

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198 Ceramics – Silikáty 59 (3) 194-201 (2015)

propertiesofthescaffoldsasafunctionofthepreparationmethod employed and the sintering temperature applied. The increase in mechanical properties with sinteringmaybe attributed to thedensificationof the struts andthereducedporosityofthefoams. Themechanicalpropertiesofbioceramicscaffoldshighly depend on their composition, porosity, pore size, and pore geometry. Increased porosity and pore size lead to significant reductions in mechanical properties. Inaddition,theshapesanddimensionsofthesamplesusedaffect the measured values of mechanical properties.Clearly,compressivestrengthdecreaseswithincreasingsurface area of the test sample. This is because thematerialvolumeisproportionaltotheendsurfacearea;the higher the volume, the higher the density of thedefectscontainedinthescaffold. Comparisons of themechanical and physical pro-perties of different scaffolds including those preparedin this study are presented in Table 2. Obviously, better valuesofcompressivestrengthwereobtainedforbioce-ramic scaffolds fabricated in this study than those for

similar scaffolds reported in the literature. Chen et al. [24-26] reported far lower values of compressivestrength for their highly porous (~ 90 %) Bioglass® scaffolds. Foams obtained by the replication methodexhibit hollow centers in the struts thatwould lead todegraded mechanical properties, which is in contrastto the scaffolds fabricated in the present study whichshowmoreuniformanddenserwalls.Thecompressivestrengthofspongybones(notthestrut)isintherangeof0.2-4MPawhiletheirrelativedensityisabout0.1[26].The compressive strength (0.53 - 0.92 MPa) measured in thescaffoldspreparedinthisstudyfallswithinthisrange,which is sufficient for suchconditionsasmanipulationduring SBF tests. In addition, the compressive strength ofhydroxyapatitescaffoldhasbeenreportedtoincreasesignificantly in vivo due to tissue ingrowth. There is,therefore,noneedforfabricatingscaffoldswithmecha-nicalstrengthssimilartothoseofthebonesincethecellsgrowingon the scaffoldand thenew tissue forming in vitrowillmakeabiocompositethatenhancesthestrengthofthescaffoldtothelevelsrequired[29].

Table1.Porosityandmechanicalpropertiesofthescaffoldsasafunctionofthepreparationmethodemployedandthesinteringtemperature applied.

Preparation method Sintering True porosity Apparent porosity Compressive strength Elastic modulus temperature (%)(S.D.)* (%)(S.D.) (MPa)(S.D.) (MPa)(S.D.)

Foamingglassslurry 800°C 90(±1) 80(±2) 0.8(±0.07) 50(±5)Foamingglassslurry 900°C 88(±2) 76(±2) 0.92(±0.09) 57(±11)Directfoamingofbioactiveglasssol 800°C 93(±1.5) 86(±2) 0.53(±0.12) 49(±9)Directfoamingofbioactiveglasssol 900°C 90(±2) 81(±1) 0.68(±0.05) 59(±7)* Standard deviation

Table2.Overviewofthemechanicalandphysicalpropertiesforvariousscaffoldspreparedbydifferentproceduresincludingtheone employed this study.

Material Process Compressive Pore sizes Porosity

Compressiontestsamples Ref. strength(MPa) (μm) (%)

70S30C Foaming sol-gel derived

1.6-2.5 200-600 81-90 Cylindricalfoams

5 bioactiveglasses (D=27mm;L=9mm)

70S30C Foaming sol-gel derived

0.34-2.26 – 82-88 Cylindricalfoams

8 bioactiveglasses (D=27mm;L=9mm) Rectangular in shape:45S5 BG Replication technique 0.1-0.15 - 92-94 10 mm in height and 24 3×3 mm in cross-section45S5 BG Replication/slurry-

0.42 – 90 – 25 -dipcoating technique Rectangular in shape:45S5 BG Replication technique 0.27-0.42 510-720 89-92 10 mm in height and 26 5×5 mm in cross-section

45S5 BG Mixwithanaqueoussolution,

5.4-7.2 420 in length

5.4-7.2 – 27 compressed, then calcined 100 in breadth

45S5 BG Powdermetallurgy-

1.7-5.5 335-530 64-79 Cylindricalfoams

28 -polymerfoamtechnologies (D=9-12mm;L=3-8mm)

63S BG Foaming glass slurry 0.8-0.92 100-400 88-90 Cylindricalscaffolds This

(D=10mm;L=20mm) study

63S BG Directfoamingof

0.53-0.68 100-400 90-93 Cylindricalscaffolds This

bioactiveglasssol (D=10mm;L=20mm) study

Characterization of highly porous 63S bioactive glass scaffolds fabricated by two foaming methods

Ceramics – Silikáty 59 (3) 194-201 (2015) 199

The morphology, size distribution, and pore inter-connectivityofthescaffoldsfabricatedbybothmethodsinthisstudyandsinteredat900°Cfor4hareshowninthe SEM micrographs presented in Figure 5. Basedonthesemicrographs,thescaffoldstructurecontains a highly interconnected spherical porous networkwithaporesizerangingbetween100and400μm. It is also seen that, in addition to themacroporeswhich provide the potential for tissue ingrowth, thescaffolds exhibit a large amount of micropores which

enhanceboththereleaseofionicproductsandbioactivity.Interconnectedporesarehighlyimportantastheyallowfor body fluid circulation and replacement, nutritionalsupply, ion diffusion, osteoblast cell penetration, andvascularization[8,14,30-31].Poreswithinterconnecteddiametersoflessthan100μmdonotallowforefficientin vivo tissue ingrowth and vascularization so thatthe damaged tissue will not be fully restored [3, 32].Changes inmacroporosity and textural porosity of thescaffoldsduetochangesinthefabricationprocessmay

a)

c)

e)

b)

d)

f)

Figure5.SEMmicrographsofthescaffoldspreparedby:a,c,e)foamingglassslurry,andb,d,f)directfoamingofbioactiveglasssolsinteredat900°Cfor4hatdifferentmagnifications.

Mirhadi S. M., Ghomi H., Emadi R.

200 Ceramics – Silikáty 59 (3) 194-201 (2015)

lead to changes in the dissolution rate and, thereby, the bioactivity of the scaffold. Moreover, changes in theglassdissolution ratewouldaffect thenumberof ionicproducts released by the glass, products that act as geneticstimuli.Animportantfactorinsupplyingtheiondosagerequiredforgeneticstimulationis theabilitytocontrolthedissolutionrateofascaffoldtomatchthatoftissue regeneration [32]. Bioactiveglasseshavebeenofgreatinterestduetotheir excellent osteoconductivity, bioactivity, biocompa-tibility, ability to deliver cells [26], and controllable biodegradability, whichmake them promising scaffoldmaterialsfortissueengineeringapplications[29,33]. Porous bioceramics have been used for bone re-pair and reconstruction to promote the regeneration of damaged tissues as a template for cell interactionand new tissue ingrowth. In fact, porous bioceramicshavebeenused for implantfixationviabone ingrowth(i.e., biological fixation), bone defect filling, boneregeneration via tissue engineering, cell loading, drug delivery, and ocular implant [14, 30-31]. Over the past few decades, important research efforts have beendirectedatdevelopingporousscaffoldswithoptimizedstructure, properties, and composition. An ideal bone tissue scaffold should contain aninterconnected porous structure; that is, it should be highlypermeablewithaporosity>90%andaporesizeintherangeof10-500μmwhichmakeitnotonlysuitablefor cell seeding, tissue ingrowth, and vascularizationbut also for nutrient delivery andwaste removal [34].Furthermore, it should have desirable mechanical strength, biocompatibility, osteoconductivity, and biode-gradability [8, 14]. Themanufacturingmethodemployedinthisstudyisawell-establishedoneformakingceramicsgreenbodywithshortformingtime,highyields,highgreenstrength,and low-costmachining. It has, indeed, been used forpreparing high-quality and complex-shaped dense/po-rous ceramic parts [15]. The scaffolds manufacturedby the foaming process in this study have a sphericalinterconnected pore network with sufficient pore sizeand compressive strength that could be a promising candidateforuseintissueengineering.

CONCLUSIONS

Twofoamingmethodswereusedtofabricatenano-structure63Sbioactiveglassscaffoldswith65%SiO2, 31% CaO, and 4% P2O5 (in mol.%). The scaffoldsprepared by direct foaming of bioactive glass sol andfoaming glass slurry exhibited compressive strengthsof 0.53 - 0.68MPa and 0.8 - 0.92MPa, respectively.The scaffolds showed an interconnected pore networkwithmacropores(100-400μm),atotalporosityintherangeof88-93%,andaninterconnectedporosityintherangeof76-86.Thesepropertiesmakeitpossibletouse

thesescaffoldsintissueingrowthandvascularizationinthe human body. Due to their nanosized structure and highporosity,thescaffoldshaveahighspecificsurfacearea that enhances their bioreactions and ionic leaching from the surface that stimulate mineralization of thesurroundingbones, therebyfacilitatingbonerepairandfixation.Thecombinationofthesepropertiesandthoseintrinsic to the sol-gel derived bioactive glass (such as bone bonding, osteoproduction, and bioresorbability) as wellasthedissolutionproductsstimulateosteogenesisatthegeneticlevel.Hence,thesescaffoldsmaybeclaimedto be capable of meeting the stringent criteria for anidealscaffoldandtohaveahighpotentialforuseinbonetissue engineering applications.

Acknowledgement

The authors are grateful for support of this research by Isfahan University of Technology.

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