Spark Plasma Sintered Hydroxyapatite Graphite Nanosheet Composite Mechanical and Cellular Properties

DOI:10.1002/adem.201000300

Spark Plasma Sintered Hydroxyapatite/Graphite Nanosheet and Hydroxyapatite/Multiwalled Carbon Nanotube Composites:Mechanical and in Vitro Cellular Properties**

By Jiangtao Zhu,Hoi Man Wong,Kelvin Wai Kwok Yeung and Sie Chin Tjong*

Recently,the demand for load-bearing implants is ever increasing due to a large number of patients suffering from bone cancer,traf?c accident,trauma,and ageing globally.The development of advanced biomaterials that mimic the properties of human bones is considered of technological importance.Human bones are mainly composed of hydro-xyapatite (HA)nanocrystals,and collagen.Hydroxyapatite (Ca 10(PO 4)6(OH)2)with a stoichiometric Ca/P ratio of 1.67generally exhibits excellent biocompatibility and bioactivity with human tissues.The inherent brittle nature and poor strength of HA limit its orthopedic applications for replacing defective bones.Accordingly,HA is strengthened with large volumes of ceramic micro?llers (up to 50vol%)such as zirconia,silicon carbide or alumina.[1,2]As recognized,nanomaterials with unique structures exhibit higher mechan-ical strength and superior biocompatibility than their counter-parts of micrometer sizes.In recent years,the development of nanosized HA (nHA)biocomposites ?lled with alumina and titania nanoparticles has received considerable attention.[3,4]Alumina and titania nanoceramics as well as nHA with large surface areas promote adhesion and proliferation of osteo-blasts (bone cells).[5]Consequently,nanoceramics with enhanced bioactivity facilitate implant integration into human tissues.

Carbonaceous materials with high chemical inertness such as carbon ?ber,graphite-like amorphous carbon (a-C)and diamond like carbon ?lm are widely recognized to exhibit good biocompatibility.[6]In the case of carbon nanomaterials,one-dimensional carbon nanotubes (CNTs)and two-dimensional graphite nanosheets (GNs)are particularly attractive due to their large aspect ratios,high mechanical strength and superior electrical conductivity.[7,8]The latter aspect can be used to stimulate osteoblats electrically during tissue formation.[9]The unique properties of carbon nanoma-terials offer a wide range of opportunities for producing novel materials for biomedical engineering 3b454ceaf8c75fbfc77db258Ts have been used as nano?llers to toughen HA very recently.[10,11]However,CNTs have some drawbacks including high production cost and large tendency to form clusters.Thus it

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[*]Prof.S.C.Tjong,Dr.J.Zhu

Department of Physics and Materials Science,City University of Hong Kong,Tat Chee Avenue,Kowloon,(Hong Kong)E-mail:aptjong@3b454ceaf8c75fbfc77db258.hk

H.M.Wong,Dr.K.W.K.Yeung

Department of Orthopedics and Traumatology,The University of Hong Kong,(Hong Kong)[**]This work is fully supported by the GRF grant (CityU 120808),

the Research Grants Council of Hong Kong,Hong Kong Special Administrative Region,China.

Hyroxyapatite (HA)and its nanocomposites reinforced with 0.5,1,1.5,and 2wt%graphite nanosheets (GNs)and multi-walled carbon nanotubes (MWNTs)are fabricated by means of spark plasma sintering (SPS)process.The effects of MWNT and GN additions on the morphology,mechanical behavior,cell adhesion,and biocompatibility of HA were studied.Three-point-bending test shows that the bending strength of MWNT/HA nanocomposites increases with increasing MWNT content.However,the bending strength of GN/HA nanocomposites initially increases by adding 0.5wt%GN,and then decreases markedly as the ?ller content increases.Cell culture and viability test results demonstrate that the GNs with diameters of several micrometers retard osteoblast cell adhesion and proliferation on the GN/HA nanocomposite.In contrast,the addition of 2wt%MWNT to HA is bene?cial to promote osteoblast adhesion and proliferation,thereby enhancing the biocompatibility of MWNT/HA nanocomposite.

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COMMUNICATION is dif?cult to disperse CNTs in ceramics homogeneously.[12]GNs having one or more graphene layers are alternative ?llers for reinforcing nanocomposites.The Young’s modulus of graphene is estimated around 0.5–1.0TPa.[13]GNs offer distinct advantages over CNTs such as low material cost and ease of fabrication.Naturally abundant graphite with a layered structure can be transformed to graphite oxide (GO)

upon oxidation in strong acids.GO has emerged as an attractive precursor for large-scale synthesis of graphene-based materials.[14]Heating GO in a furnace generally produces expanded graphite that can be further exfoliated into thin GNs under sonication.[15]GNs are then incorporated into polymers to produce nanocomposites with high mechan-ical strength and good electrical conductivity.[16–19]Little is known on the effect of GN additions on the structure and mechanical properties of ceramics.From the literature,graphene papers promote adhesion and proliferation of osteoblasts.[20]The graphene/chitosan nanocomposites also exhibit good cell biocompatibility.[21]Similarly,CNTs also exhibit excellent biocompatibility and promote osteoblast adhesion.[22–24]However,metal catalyst particles entrapped inside CNTs may affect biological cell responses.[25]The puri?cation process for removing metal catalysts in CNTs is quite time-consuming.In this regard,high purity GNs can re?ect genuine responses of biological cells.Those studies described above are mainly concerned with the

cell responses of CNTs and GNs.Therefore,proper under-standing of the structural behavior and biocompatibility of nHA composites containing low loading levels of CNTs and GNs is necessary for their future applications in orthopedics.The development of novel GN/HA and CNT/HA nanocom-posites requires the use of spark plasma sintering (SPS)for retaining ?ne grain sizes.SPS is an effective technique for sintering nanoceramics at lower temperatures for short periods of time.[26–29]In a previous study,we reported the formation of preferentially oriented HA grains in multiwalled CNT (MWNT)/HA composites due to the SPS processing.[30]The present study reports the fabrication,property character-ization,bioactivity and biocompatibility of GN/HA compo-

sites designed for load-bearing implant applications.To the best of our knowledge,there is no published work reporting the effect of GN additions on the structure,mechanical behavior,and biocompatibility of SPS-prepared HA compo-sites.The mechanical and biocompatibility characteristics of the MWNT/HA composites fabricated by SPS are also addressed and discussed.Experimental Materials Synthetic HA nanorods with an aspect ratio of %3were purchased from Nanjing Emperor Nano Materials Co.(Nanjing,China).Multiwalled carbon nanotubes (MWNTs)were obtained from Nanostructured &Amorphous Materials Inc.(Texas,USA).To disperse MWNTs uniformly in the HA matrix,the tubes were ?rst immersed in 60%HNO 3boiling solution for 18h,washed with distilled water until the pH value reached 7[29].GO ?akes (Grafguard 220–50N)from GrafTech International (Ohio,USA)were subjected to a simple thermal-shock at 7008C for 10s,and then dispersed in distilled water under sonication for 12h to obtain a black GN solution.

Nanocomposite Preparation

MWNT/HA and GN/HA nanocomposites containing 0.5,1,1.5,and 2wt%nano?llers were prepared.Acid-oxidized MWNTs of a given loading level and nHA were indepen-dently dispersed into water under sonication.Then MWNT dispersion was dropwise added into the HA solution under sonication.Similarly,black GN dispersion was dropwise added into the HA solution under sonication.Mixed MWNT/HA and GN/HA solutions were further sonicated for 2h,?ltered,and dried in an oven (508C)for 24h.The mixed composite mixtures were sintered using a spark plasma sintering system (Syntex model SPS-825)via an initial heating of the chamber to 9008C at a rate of 1008C min à1for 2min,and then increased to 10008C for 1min under a pressure of 40MPa.Finally,the specimens were cooled to room temperature in the SPS chamber.Sintered products of 20mm diameter and 5mm thickness were cut into sections with their surfaces aligned parallel to the pressure direction.Materials Characterization

The morphology of GNs was examined in a ?eld-emission scanning electron microscope (SEM)(Jeol JSM 6335).The structure of sintered HA,MWNT/HA,and GN/HA nano-composites were determined with an automatic X-ray diffractometer (Siemens)equipped with Ni-?ltered Cu K a radiation.The morphology of sintered MWNT/HA and GN/HA nanocomposites was observed in a SEM.

Three-point bending specimens with a dimension of 1?2?15mm 3were loaded in an Instron tester (model 1196)under a crosshead speed of 0.02mm min à1.Three specimens of each composition were tested.

Cell Morphology

Mouse osteoblast (MC3T3-E1)cells were used for cultiva-tion on sintered HA,MWNT/HA 2/98,and GN/HA 2/98specimens.The specimens with a dimension of 0.5?2?4.5mm were cut from sintered discs.The cells were cultured on these specimens (n ?3)at a density of 1?104cells cm à2,in high Dulbecco’s Modi?ed Eagle Medium (DMEM,Thermo

Scienti?c)supplemented with 10%v/v fetal bovine serum

(FBS),antibiotics (100U mL à1of penicillin and 100mg mL à1of

streptomycin).Prior to cultivation,the specimens were

sterilized with 70%ethanol aqueous solution and phospha-te-buffered saline (PBS)solution.The 96-well cell culture plate was incubated at 378C in a humidi?ed atmosphere of 5%CO 2/95%air.After seeding for 1and 3d,the specimens were removed from the well,rinsed in PBS,treated with 10%formaldehyde solution to ?x the attached cells.Finally,they were dehydrated in a series of solutions with increasing

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ethanol concentrations (30,50,70,90,100%v/v)followed by critical point drying (CPD)before coating with gold ?lm for SEM examinations.

Biocompatibility Test

Dimethyl thiazolyl diphenyl tetrazolium (MTT)assay was performed to determine the cell viability or biocompatibility of HA,MWNT/HA,and GN/HA nanocomposites.In the test,100m L DMEM suspension containing 1?104MC3T3-E1cells were seeded on sintered HA,MWNT/HA 2/98,and GN/HA 2/98rectangular specimens (0.5?2?4.5mm 3,n ?3)in a 96-well plate.The specimens were placed in a humidi?ed (5%CO 2/95%air)incubator at 378C for 4and 7d,respectively.Then 10m L sterilized MTT solution was added and the specimens were incubated for further 24h.This was followed by the addition of 100m L 10%sodium dodecyl sulfate (SDS)in 0.01M hydrochloric acid.The optical absorbance was detected by a multimode detector (Beckman Coulter DTX 880)at 570nm wavelength against a reference wavelength of 640nm.Positive controls (cell suspensions in various concentrations)were also cultured on the same plate for comparison purposes.Results and Discussion

Figure 1shows a typical SEM image of the prepared GNs.Apparently,GNs with diameters of several micrometers and thickness of about 20–60nm can be readily seen.This implies that GO ?akes are effectively exfoliated into GNs during thermal shock and subsequent sonication treatment.

The X-ray diffraction (XRD)patterns of sintered HA,MWNT/HA and GN/HA nanocomposites with their surfaces perpendicular to the pressure direction are shown in Figures 2and 3,respectively.All the peaks in Figure 2originate from hydroxyapatite.The (002)and (004)re?ections of HA disappear by exposing such specimen surfaces to X-ray radiation.[30]The a -planes of HA,i.e.,(200)and (300)are dominant re?ections of sintered MWNT/HA and GN/HA nanocomposites.In other words,the c -planes of randomly arranged HA nanocrystals,which are perpendicular to their a -planes,tend to orient along the applied pressure direction

during sintering.The (002)peak of graphite with low intensity can also be seen in Figure 3.The weak graphite peak is due to the presence of strong HA re?ections in the XRD patterns.The bending strengths of the GN/HA and MWNT/HA nanocomposites versus ?ller content are shown in Figure 4.For GN/HA nanocomposites,the bending stress increases from 86to 96MPa by adding 0.5wt%GN,thereafter the strength decreases with increasing ?ller content.In contrast,the bending stress of MWNT/HA nanocomposites increases markedly with increasing nanotube content.The bene?cial effect of CNT addition on improving the mechanical strength and toughness of ceramics is well recognized.[31,32]CNTs with large aspect ratios can bear applied load effectively during mechanical testing.Crack bridging,crack de?ection and nanotube pullout are responsible for toughening of the ceramics.For example,Fan et al .reported that the bending strength of alumina can be increased by adding low loading levels of CNTs.[31]It is noted that the mechanical properties of CNT-ceramic nanocomposites depend strongly on sintering temperatures.Conventional furnace sintering at elevated temperatures can cause structural damages to CNTs.The carbon nanotube structure of CNT/alumina

nanocomposites

Fig.1.SEM image showing morphology of two-dimensional

GNs.

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COMMUNICATION can be preserved at temperatures up to %12508C.Above

12508C,CNTs convert to graphite and carbon nano-onion

structures.[33,34]In the present study,the temperatures

employed for the SPS treatment fall in the range of

900–10008C.Thus the structural integrity of MWNTs is

maintained.

Figure 5(a–h)show respective low and high magni?cation

SEM micrographs of sintered GN/HA nanocomposites with

their fracture sections parallel to the applied pressure

direction.Obviously,GNs with several micrometers in

diameter are dispersed homogeneously in the HA matrix.

No agglomerations of GNs can be observed.GNs are located

between the grains of HA crystals.At high ?ller loading,GNs

are poorly bonded to the HA matrix [Figure 5(h)],thus can be

readily pulled out from the HA matrix during bending.This leads to a decrease of the bending stress of GN/HA

nanocomposites with increasing ?ller content.

As recognized,biomaterials promote new tissue formation

by providing active surface sites to foster and

direct cellular attachment,migration and

proliferation.[35]In this respect,designed

nanocomposites should foster adhesion and

proliferation of osteoblasts for their success-

ful applications in orthopedics.Typical

morphologies of MC3T3-E1mouse osteo-

blasts adhered on the surfaces of sintered

HA,MWNT/HA 2/98and GN/HA 2/98

specimens after culturing for 1and 3d are

shown in Figure 6(a–f),respectively.After 1d

cultivation,osteoblast cells are attached and

then ?attened on the specimen surfaces.This

behavior is more pronounced for sintered

HA and MWNT/HA 2/98nanocomposite.

The density of osteoblasts adhered on the

GN/HA surface is lower compared with

other two specimens.By further increasing

the culture time to 3d,the density of adhered

cells increases markedly.The HA and

MWNT/HA surfaces are nearly covered

with osteoblasts.The cells proliferate and

anchor on the specimen surfaces through ?ne

?lopodia at the leading edges.Figure 6(c)and

(f)show typical examples of the cell migra-

tion by extending ?lopodia on the MWNT/

HA 2/98and GN/HA 2/98nanocomposite

surfaces.

MTT assay is a commonly used practice to

assess the viability of biological cells by

reacting with a chemical reagent.Viable cells

reduce the MTT reagent to form a colored

formazan salt.Thus water-soluble MTT is

converted by mitochondrial dehydrogenases

of living cells into water-insoluble formazan

product.The precipitated formazan is dis-

solved in a solution of SDS in diluted HCl acid to yield a colored solution.The optical

Fig.4.Bending strength versus ?ller content for sintered MWNT/HA and GN/HA

nanocomposites.Fig.5.SEM images of sintered GN/HA nanocomposites:(a,b)0.5wt%,(c,d)1wt%,(e,f)1.5wt%,and (g,h)2wt%.%.GNs are marked with arrows.ADVANCED ENGINEERING MATERIALS 2011,13,No.4?2011WILEY-VCH Verlag GmbH &Co.KGaA,Weinheim 3b454ceaf8c75fbfc77db258 339

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absorbance of colored solution is measured with a detector at 570nm.The intensity of color produced is directly related to the number of viable cells.The MTT assay results of sintered HA,MWNT/HA 2/98,and GN/HA 2/98specimens are shown in Figure 7.Obviously,MWNT/HA 2/98nanocom-posite exhibits the highest optical absorbance after 4d culture.The absorbance values of HA and GN/HA 2/98nanocom-posite are almost the same.These indicate that the total

number of viable cells on the MWNT/HA 2/98nanocomposite is higher than that of pure HA or GN/HA.After 7d,the absorbance values for sintered nHA and MWNT/HA 2/98increase signi?cantly,particularly for the latter specimen.This implies that the MWNT/HA 2/98nanocomposite exhibits excellent biocompatibility.In contrast,the absorbance of the GN/HA 2/98nanocom-posite decreases after 7d culturing.The decrease in biocompatibility of the GN/HA 2/98nanocomposite can be attributed to the morphology of GNs.As aforementioned,the diameters of GNs fall in the range of several micrometers.Despite the fact that the aspect ratio (diameter/thickness)of GNs is very large and comparable to that (length/dia-meter)of CNTs,the GN surfaces of several micrometers are unfavorable sites for osteo-blast adhesion and proliferation.The osteo-blasts can attach only on the thickness-wise regions of GNs having dimensions of few nanometers.And such regions must expose to outer composite surface for interacting with osteoblasts.In general,nanostructured materials particularly in the form of nano?-bers can absorb proteins effectively,thereby giving more binding sites for cell adhesion and proliferation.[35,36]Nano?brous materi-als that mimick the nano?brillar structure of

natural extracellular matrix can mediate protein interaction and cell function effectively.[37]As a result,MWNTs (with diameters of few nanometers)embedded in the HA matrix provide favorable sites for osteoblast attachment and proliferation.

Conclusions

Pure HA and its nanocomposites reinforced with GNs and MWNTs were fabricated by means of spark plasma sintering process.The effects of MWNT and GN additions on the morphology,mechanical behavior,cell adhesion,and bio-compatibility of nHA were studied using SEM,three-point bending,cell culture,and MTT assay techniques.SEM examination showed that GNs with diameters of several micrometers and thickness of few nanometers disperse uniformly in the matrix of HA-based nanocomposites.Three-point-bending test revealed that the bending strength of the MWNT/HA nanocomposites increases with increasing MWNT content.However,the bending strength of the GN/HA nanocomposites increased initially by adding 0.5wt%GN,followed by a decrease with increasing ?ller content.Cell culture and MTT assay results demonstrated that the GN addition impedes osteoblast cell adhesion and proliferation on HA.In contrast,the addition of 2wt%MWNT to HA was found to be very effective to promote osteoblast adhesion

and

Fig.6.SEM morphology of the osteoblasts cultured on the surfaces of sintered (a,b)HA,(c,d)2wt%MWNT/

HA,and (e,f)2wt%GN/HA.Culture time is 1and 3d for left and right images,respectively.Filopodia are marked with arrows in

(f).

Fig.7.Proliferation of osteoblasts on the surfaces of positive control,HA,2wt%MWNT/HA,2%GN/HA specimens after 4and 7d cultivation.

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