microstructure and characteristics of the metalâ€“ceramic

Microstructure and characteristics of the metal–ceramic composite(MgCa-HA/TCP) fabricated by liquid metal infiltration

X. N. Gu,1 X. Wang,2 N. Li,1 L. Li,2 Y. F. Zheng,1,2 Xigeng Miao3

1State Key Laboratory for Turbulence and Complex System and Department of Advanced Materials and Nanotechnology,

College of Engineering, Peking University, Beijing 100871, China2Center for Biomedical Materials and Engineering, Harbin Engineering University, Harbin 150001, China3Institute of Health and Biomedical Innovation, Queensland University of Technology, Kelvin Grove, QLD 4059, Australia

Received 23 December 2010; revised 13 April 2011; accepted 20 April 2011

Published online 23 August 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.31879

Abstract: In this article, a novel MgCa alloy-hydroxyapatite-

tricalcium phosphate (HA/TCP) composite was fabricated

using the liquid alloy infiltration technique. The feasibility of

the composite for biomedical applications was studied

through mechanical testing, electrochemical testing, immer-

sion testing, and cell culture evaluation. It was shown that

the composite had a strength about 200-fold higher than that

of the original porous HA/TCP scaffold but retained half of

the strength of the bulk MgCa alloy. The corrosion test indi-

cated that the resulting composite exhibited an average cor-

rosion rate of 0.029 mL cm�2 h�1 in the Hank’s solution at

37�C, which was slower than that of the bulk MgCa alloy

alone. The indirect cytotoxicity evaluation revealed that 100%

concentrated (i.e., undiluted or as-collected) extract of the

MgCa-HA/TCP composite showed significant toxicity to L-929

and MG63 cells (p < 0.05). In contrast, the diluted extracts

with 50 and 10% concentrations of the MgCa-HA/TCP com-

posite exhibited a similar degree of cell viability (p > 0.05),

equivalent to the grade I cytotoxicity of the standard ISO

10993-5: 1999. VC 2011 Wiley Periodicals, Inc. J Biomed Mater Res

Part B: Appl Biomater 99B: 127–134, 2011.

Key Words: MgCa-HA/TCP composite, scaffold, mechanical

property, corrosion, cytotoxicity

How to cite this article: Gu XN, Wang X, Li N, Li L, Zheng YF, Miao X. 2011. Microstructure and characteristics of the metal–ceramic composite (MgCa-HA/TCP) fabricated by liquid metal infiltration. J Biomed Mater Res Part B 2011:99B:127–134.

INTRODUCTION

Magnesium alloys are attractive candidates for degradableimplant applications due to their good mechanical proper-ties and biocompatibility, in comparison with biodegradablepolymers.1–5 However, their fast degradation/corrosion rateschallenge the practical use of magnesium alloys due to themismatch of the bone healing rate and the gas cavity gener-ated around the implant during its degradation.1–3 Magne-sium alloys also do not have the bone-bonding abilityshown in calcium phosphate bioceramics.

Recently metal matrix composites (MMC), consisting ofan adequate ceramic reinforcement phase, are found to beeffective for simultaneously increasing the strength, slowingdown the corrosion rate, and improving the bioactivity ofmagnesium alloys.6–8 Several processing techniques, includ-ing powder metallurgy (P/M),6,7 the traditional castingmethod,8 and so forth, have been developed. The MMCs,made of magnesium alloy AZ91D as a matrix and 20 wt %hydroxyapatite (HA) particles as a reinforcement through aP/M route, exhibited higher strength, more stable corrosionrate and improved cytocompatibility than the AZ91D ma-

trix.6 According to Ye et al.,8 the introduction of the 1 wt. %gelatin coated nano-sized HA particles into the Mg-2.9Zn-0.7Zr alloy under magnetic agitation also indicated morefavorable corrosion resistance and cytocompatibility thanthe Mg-Zn-Zr matrix alloy.

Among the techniques currently available for the processingof MMCs, infiltration of a molten metal into a porous ceramicpreform represents a technique more suitable for achieving ahigh volume fraction of reinforcements (>50%) in MMCs.9–12

First, this process shows its superiority in the extreme uniformdistribution of the ceramic reinforcements, eliminating residualporosities and interfacial reactions between the reinforcementsand the matrix.9,10 Second, this process also shows the advanta-geous ability of fabricating the final, or near-final shaped com-posite components, minimizing the generally difficult machiningof MMCs.11 Third, when suitable barriers are used to preventthe infiltration of a molten metal, it is also possible to produceselectively reinforced components, within which the metal is re-inforced only where needed.9,11

This technique inspires us to adopt ceramic scaffolds fortissue engineering as the porous ceramic preforms for metal

Correspondence to: Y. F. Zheng; e-mail: [email protected]

Contract grant sponsor: Research Fund for the Doctoral Program of Higher Education; contract grant number: 20100001110011

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 30670560

Contract grant sponsor: Research Funds for the Central Universities; contract grant number: PKUJC2009001

Contract grant sponsor: Program for New Century Excellent Talents in University; contract grant number: NCET-07-0033

VC 2011 WILEY PERIODICALS, INC. 127

infiltration because of their excellent biocompatibility andmature preparation technologies.13–16 In addition, the result-ing properties of these composites can be adjusted by freeselection of different porous scaffolds as well as the moltenmetals. The research reported here was a preliminary studyabout the composite fabricated by molten metal infiltrationtechnique, adopting the HA/TCP scaffolds as the ceramic pre-form and the Mg-1Ca alloy as the metal phase, both of whichshowed good biocompatibility in our previous studies.2,17

The microstructure, mechanical properties, and in vitro corro-sion behavior in simulated body fluid were investigated inthis study. The cell response to the composite extract wasalso evaluated using the indirect cytotoxicity assay.

MATERIALS AND METHODS

Materials preparationThe Mg-1wt %Ca alloy (expressed as MgCa alloy in the fol-lowing) was casted by using commercial pure Mg (99.98%)and Ca (99.95%) in a crucible under a mixed gas atmos-phere of SF6 and CO2. To prepare the porous HA/TCP plat-forms, polyurethane (PU) foams were dipped into the ce-ramic slurry (a mixture of 160 g HA and 40 g b-TCP) andgently squeezed to allow the slurry penetrating the foamsbefore drying at room temperature for at least 24 h. The ce-ramic slurry-coated PU foams were sintered in a furnaceand the final HA/TCP scaffolds were removed from the fur-nace after it had cooled down. The average pore size of thescaffold was 500 lm and the porosity was 87%. The spe-cific fabrication route of the HA/TCP scaffold was describedin our previous work.17

MgCa-HA/TCP composites were fabricated by infiltratingthe molten MgCa alloy into the porous scaffold using a vac-uum suction casting machine. The porous ceramic was pre-heated at 150�C and positioned in the vacuum suction cast-ing machine, which was pre-pumped to 0.02–0.03 MPa. Thematrix MgCa alloy was re-melted in the furnace under amixed gas atmosphere of SF6 and CO2 and the temperaturewas kept at 700–720�C. Then the infiltration of the moltenMgCa alloy was driven by the vacuum and held for 2 minwhile the melt was solidified. For the corrosion and extract-ing tests, the composite samples (with the size of 10 � 10� 2 mm3) were mechanically polished up to 2,000 grit,ultrasonically cleaned in acetone, absolute ethanol anddistilled water and then dried in air.

Microstructural characterizationThe microstructure of the composite was observed by anoptical microscope (Olympus BX51 M) and an environmen-tal scanning electron microscope (ESEM, Quanta 200FEG),equipped with an energy-dispersive spectroscopy (EDS)attachment. An X-ray diffractometer (XRD, Rigaku DMAX2400) was used to characterize the phases of the compositeusing the Cu Ka radiation at the step size of 0.02� with ascanning speed of 4� min�1.

Mechanical testingUniaxial compression testing was conducted on an Instron8562 testing machine at a constant nominal strain rate of

2 � 10�4 s�1 at room temperature. The test samples withthe size of 3 � 3 � 6 mm3 were prepared according to theASTM-E9-09. Five identical samples were used for the com-pressive tests.

Electrochemical testA conventional three electrode cell with the composite sam-ple (the exposed area of 1 cm2) as a working electrode, thesaturated calomel electrode (SCE) as a reference electrode,and a platinum electrode as the counter electrode was used.The electrochemical tests were carried out in the Hank’s so-lution (NaCl 8.00 g L�1, KCl 0.40 g L�1, CaCl2 0.14 g L�1,NaHCO3 0.35 g L�1, MgSO4.7H2O 0.20 g L�1, Na2H-PO4.12H2O 0.12 g L�1, KH2PO4 0.06 g L�1) at 37�C 6 0.5�Cusing an electrochemical workstation (CHI660C). The opencircuit potential (EOCP) was measured as a function of time.The potentiodynamic polarization test was measured from300 mV below the OCP value at a scanning rate of 1 mVs�1. The electrochemical impedance spectroscopy (EIS)measurements were carried out from 100 mHz to 100 kHzat OCP values.

Immersion testThe immersion test was carried out in Hank’s solution at3760.5�C according to ASTM-G31-72. After 1, 10, and 20days’ immersion, the samples were removed out of the solu-tion, rinsed with distilled water, and dried in air. Microstruc-tural and compositional characterization of the corrodedcomposite sample surface, before and after removing thecorrosion products by CrO3 (Chromic Acid) solution, wasexamined by ESEM, EDS, and XRD. The change of pH valueof the Hank’s solution as well as the amount of hydrogengenerated from the samples was monitored using a cali-brated burette according to the method described in Ref.18

throughout the 20 days’ immersion. An average of threemeasurements was taken for each group.

Indirect cytotoxicity evaluationThe experimental composite samples were sterilized byultraviolet-radiation for at least 2 h. The samples were thenimmersed in DMEM in a humidified atmosphere with 5%CO2 at 37�C, with the surface area/extraction medium ratioof 3 cm2 mL�1. After 72 h of incubation, the supernatantfluid was withdrawn and centrifuged, and then was seriallydiluted to 50 and 10% concentrations. On the other hand,Murine fibroblast (L-929) cells and Human osteosarcomacells (MG63) were cultured in the Dulbecco’s modifiedEagle’s medium (DMEM), with 10% fetal bovine serum(FBS), 100 U mL�1 penicillin and 100 lg mL�1 streptomy-cin at 37�C in a humidified atmosphere of 5% CO2. The cellswere incubated in 96-well cell culture plates at 3 � 103

cells/100 lL of medium in each well for 24 h to allow cellattachment. In order to evaluate the cytotoxicity of theextracts from the composite samples, the medium in thewells was then replaced with 100 lL of 100, 50, and 10%extracts and incubated for 1, 3, and 5 days. For spectropho-tometric measurements, 10 lL MTT was added to each wellfor 4 h of incubation and then 100 lL formazan

128 GU ET AL. MICROSTRUCTURE AND CHARACTERISTICS OF THE METAL–CERAMIC COMPOSITE

solubilization solution was added to each well for overnight.The measurements were carried out at 570 nm with a refer-ence wavelength of 630 nm by a microplate reader (Bio-RAD680). The background of MTT results cultured inextracts without cells was subtracted. Meanwhile, the Mgconcentration of the extracts was measured by inductivelycoupled plasma atomic emission spectrometry (Leeman,Profile ICP-AES). The pH value of the extracts was alsomeasured. Statistical analysis was conducted to evaluate thedifferences in cell viability by the analysis of variance(ANOVA). The statistical significance was defined as 0.05.

RESULTS

Microstructure and mechanical propertyThe infiltration of the molten MgCa alloy into the HA/TCPscaffold, with the original total porosity 87%, was success-fully achieved and a typical optical micrograph of the result-ing composite was shown in Figure 1(a). The magnifiedSEM image and the EDS line-scan of the interface betweenthe HA/TCP scaffold and the MgCa alloy matrix [as shownin Figure 1(b)] indicated sharp transitions of the elementintensity profiles. Figure 2 presents the XRD analysis of the

MgCa-HA/TCP composite. The XRD pattern exhibits only thereflection peaks of the a-Mg, Mg2Ca, HA, and TCP phases.

Figure 3 showed the representative compression stress-strain curves of the MgCa-HA/TCP composites. The averagecompressive strength and the elongation for the resultingcomposites were (128.7 6 15.1) MPa and (13.5 6 0.7)%,which were approximately half of those properties of theMgCa alloy but about 200-fold higher than those of the orig-inal porous HA/TCP scaffolds (inset in Figure 3).

Electrochemical measurementsFigure 4 showed the potentiodynamic polarization curves andthe Nyquist plots obtained for the MgCa-HA/TCP composite inHank’s solution at 37�C. The experimental composite clearlyindicated much more positive corrosion potential value (-1.49V), as well as the decreased kinetics of anodic and cathodicreactions, than the bulk MgCa matrix alloy (-1.88 V). The cor-rosion current density of the experimental composite wasmuch lower (15.23 lA cm�2) than the bulk MgCa matrix alloy(178.58 lA cm�2), indicating the better corrosion resistance

FIGURE 1. (a) An optical image showing the microstructure of the MgCa-HA/TCP alloy composite; (b) the interface between the HA/TCP scaffold

and the infiltrated MgCa matrix alloy and the EDS line scan spectra across the interface. [Color figure can be viewed in the online issue, which

is available at wileyonlinelibrary.com.]

FIGURE 2. XRD pattern of the MgCa-HA/TCP alloy composite. [Color

figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

FIGURE 3. Compressive behaviors of three replicate MgCa-HA/TCP

composite samples (the blue, red and green curves), with MgCa ma-

trix alloy and HA/TCP scaffold as controls. [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]

ORIGINAL RESEARCH REPORT

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | OCT 2011 VOL 99B, ISSUE 1 129

of the experimental composite. From Figure 4(b), the EISspectrum of the composite and the matrix alloy showed onesemi-circle indicating an electrochemical process dominatedby the mechanism of charge transfer across the interfaceinvolved in the dissolution of the MgCa alloy substrate. Thepolarization resistance of the samples can be obtained fromthe diameter of the near semi-circular spectra which wasrelated to the corrosion rate. The MgCa-HA/TCP compositeexhibited a higher polarization resistance (� 1200 X cm�2)than the bulk MgCa matrix alloy (� 400 X cm�2).

Immersion testsFigure 5(a) showed the hydrogen evolution over the immer-sion period for the MgCa-HA/TCP composite and the bulkMgCa matrix alloy in the Hank’s solution at 37�C. TheMgCa-HA/TCP composite revealed an amount of hydrogenwithin the same immersion period that was less than one-third of that of the bulk MgCa matrix alloy. In contrast, thepH value of the Hank’s solution incubating the MgCa-HA/TCP composite and the bulk MgCa matrix alloy revealed asimilar changing trend, as shown in Figure 5(b).

Figure 6 illustrated the morphologies of the MgCa-HA/TCP composite after 1, 10, and 20 days’ immersion inHank’s solution at 37�C. The composite exhibited a flat andsmooth surface with the scaffold structure being visible af-ter 1 day immersion and some shallow pits could be seenafter removing the corrosion product on the sample surface[Figure 6(a,b)]. After 10 days of immersion, a corrosionproduct layer was clearly observed on the surface of theMgCa-HA/TCP composite without any sign of the scaffoldstructure [Figure 6(c)]. However, the scaffold structure andthe polishing scratches on the surface of the infiltratedMgCa alloy could be seen after removing the corrosionproduct [Figure 6(d)]. The corrosion product layer becamecompact after 20 days of immersion with some nano-sizedholes on the surface [inset in Figure 6(e)]. Some deep corro-sion pits were observed on the surface of the infiltratedMgCa alloy, revealing the dissolution of the MgCa alloy andthe scaffold structure was still clear after cleaning the corro-sion product [Figure 6(f)]. The EDS results of the compositebefore and after the CrO3 cleaning were shown in Figure 7.It indicated that the corrosion product contained C, O, Mg, P,

FIGURE 4. (a) Potentiodynamic polarization curves and (b) Nyquist plots of the MgCa-HA/TCP alloy composite in the Hank’s solution, with the

MgCa matrix alloy as a control. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 5. (a) Hydrogen evolution behaviour and (b) pH value variation of the Hank’s solution incubating the MgCa-HA/TCP alloy composite as

a function of the immersion time, with the MgCa matrix alloy as a control. [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]


and Ca elements [Figure 7(a)]. After removing the corrosionproduct, the composition of the MgCa-HA/TCP composite[Figure 7(b)] was similar to the uncorroded one. The muchhigher contents of the elements O, P and Ca on the surfaceof the MgCa-HA/TCP composite immersed for 20 days[Figure 7(a)] suggested the deposition of a compound con-taining Ca and P elements. XRD results revealed that thecorrosion products were mainly Mg(OH)2 and HA (as shownin Figure 8). In addition, the peak from the a-Mg was stillhigh indicating the good corrosion resistance of the infil-trated MgCa matrix alloy.

CytotoxicityThe pH value of the 100% MgCa-HA/TCP composite extractwas 8.87 6 0.34 and the Mg ion concentration was 10.066 0.69 mM. Figure 9 showed the cytotoxicity results of the

different concentrated extracts of the MgCa-HA/TCP com-posite using the L-929 and MG63 cells. It was clearly seenthat the 100% composite extract indicated the Grade IIcytotoxicity to L-929 and MG63 cells. After 50 and 10%degrees of dilution, the cell viability was significantlyimproved (p < 0.05) with Grade I toxicity to L-929 andMG63 cells.

DISCUSSION

The MgCa-HA/TCP composite shows an extreme improve-ment in the strength and the elongation in comparison withthe original porous HA/TCP ceramic scaffold.17 According tothe micro-mechanical model of Gibson and Ashby, the crush-ing of ceramic foams is based on bending of the struts.19

The interpenetration of the MgCa alloy phase stabilizes theceramic struts and partially prevents strut bending, resulting

FIGURE 6. Surface morphologies of the MgCa-HA/TCP alloy composite after (a)–(b) 1 day, (c)–(d) 10 days, and (e)–(f) 20 days of immersion in

the Hank’s solution before [(a), (c), and (e)] and after [(b), (d), and (f)] the CrO3 cleaning.

FIGURE 7. EDS results of the MgCa-HA/TCP alloy composite for the 20-day immersion in the Hank’s solution (a) before and (b) after the CrO3

cleaning. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]



in a major improvement of the mechanical properties of theceramic scaffold. However, the resulting MgCa-HA/TCP com-posite indicates inferior values of the mechanical propertycompared to the bulk MgCa alloy probably due to the poorinterfacial bonding between the HA/TCP scaffold and thealloy matrix. Load transfer from the metal matrix to the ce-ramic skeleton is, therefore, supposed to be reduced and theimproved bonding is needed to increase the stiffness anddelay the onset of ceramic/metal de-cohesion.20 Zeschkyet al.12 also reported the reduced mechanical property ofAZ91 infiltrated SiAOAC ceramic foams compared to theAZ91 bulk alloy due to the crack opening at the ceramic/metal interface, which can be improved by further oxidation.Additionally, the pore size and the porosity of the scaffoldalso influences the resulting mechanical property of thecomposite since the debonding depends on the length ofthe dislocation pile up at the interface,21 and, therefore, onthe pore size of scaffold.

In general, the MgCa-HA/TCP composite exhibits aslower corrosion rate than the matrix MgCa alloy. The

formation of the protective layer on the sample surface isresponsible for the improved corrosion resistance of thecomposite. The HA/TCP scaffold in the composite acts asthe hydroxyapatite nuclei and the hydroxyapatite will growspontaneously consuming the Ca2þ and PO4

3-.22 The multi-ple protection effects offered by corrosion product Mg(OH)2and hydroxyapatite may be the reason for the slower corro-sion rate observed for the composite. Witte et al.6 and Yeet al.8 also reported the protective layers, composed of mul-tiple corrosion products, on the AZ91D/HA and Mg-Zn-Zr/HA composites, exhibited better corrosion resistance thanthe single Mg(OH)2 layer. However, severe local corrosionseems to be irritated by the ceramic scaffold for the com-posite. The ceramic/metal interface has the capability offorming a galvanic couple in the electrolyte and the electri-cally conducting scaffold becomes the noble member.23 Thecorrosion initiated along the interface allowing the electro-lyte deep penetration.7,23 As the corrosion proceeds, theMgCa alloy matrix dissolves gradually with an intact HA/TCP scaffold structure left, as shown in Figure 6(d,f). The

FIGURE 9. (a) L-929 and (b) MG63 cell viability for the differently diluted MgCa-HA/TCP alloy composite extracts after 1, 3, and 5 days of cell cul-

ture. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 8. XRD patterns of the MgCa-HA/TCP alloy composite after

the 20-day immersion in Hank’s solution. [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 10. The reduction in corrosion rate for Mg alloy based com-

posite produced by different techniques deduced from the litera-

ture.6,8,24,25 [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]


reduction percentage in corrosion rate of different reportedMg alloy based composites, in comparison with their corre-sponding Mg alloy matrix, was summarized in Figure 10.(The corrosion rate was calculated from the immersion cor-rosion tests.) The stir casted Mg-Zn-Zr/1 wt % HA compos-ite indicates � 58% reduction of corrosion rate,8 and theMg alloy based composites fabricated through P/M route ex-hibit the reduction ratio of corrosion rate in the range of 13to 75%.6,24,25 The infiltrated MgCa-HA/TCP compositeshows about 68% reduction of corrosion rate, which is closeto the reduction ratio for AZ91D/20 wt % HA producedthrough P/M technique.6 Therefore, it reveals that the metalinfiltration method is effective in improving the corrosionresistance of Mg alloy matrix compared to other MMC prep-aration techniques.

The in vitro cytotoxicity results suggest the cell viabilityrelies on the dilution ratio of the MgCa-HA/TCP compositeextract. It can be explained in terms of the synergetic effectof osmolarity and pH value.26 After dilution, 50 and 10%extracts indicate similar L-929 and MG63 cell viability (p >

0.05) with endurable osmolarity and pH value. This phe-nomenon has also been found in the cytotoxicity test of var-ious Mg material extracts, such as pure Mg,27 Mg-Nd-Zn-Zr28 and Mg-Zn-Y,29 and the cell viability increased afteradjusting the pH value of the extracts.28 Given the bodyfluid exchange in vivo, the concentration of degradationproduct should be much lower than that tested in vitro andis assumed to be tolerated in vivo. Furthermore, as the cor-rosion of the MgCa-HA/TCP composite proceeds, good bio-compatibility can be expected with the combined advan-tages of the HA/TCP scaffold and the MgCa matrix alloy. Onthe one hand, the dissolving magnesium ion will activatebone cells1,2 and thus accelerate the bone restoration pro-cess. On the other hand, as the MgCa alloy matrix dissolves,the HA/TCP scaffold structure will be left which showsgood osteoconductivity17 and allows the invasion of cellsand eventually becomes specific tissue. From the abovefacts, the infiltration method shows promising potential inpreparing Mg alloy based composite for orthopedicapplication.

CONCLUSIONS

The MgCa-HA/TCP composite was fabricated by the moltenmetal infiltration technique for orthopedic applications.Compared to the single MgCa alloy matrix, the MgCa-HA/TCP composite exhibited the inferior mechanical property(decreased by 50%) but with superior corrosion resistance(improved by 68%). The average corrosion rate of theresulted composite was 0.029 mL cm�2 h�1 after theimmersion in Hank’s solution for 20 days. The immersiontest indicated that the MgCa alloy matrix gradually dissolvedand induced the apatite deposition. The bioactive HA/TCPscaffold was left behind due to the much slower degrada-tion rate of the ceramic scaffold. In vitro cytotoxicity tests ofthe composite revealed that the cell viability was related tothe extract concentration, with Grade I cytotoxicity for 50and 10% concentrated extracts.

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