J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 7 ( 2 0 1 2 ) 6 9 – 7 6

Available online at www.sciencedirect.com

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Research paper

Effects of nanocrystalline calcium deficient hydroxyapatiteincorporation in glass ionomer cements

Sumit Goenkaa, Rajkamal Balub, T.S. Sampath Kumarb,∗

aDepartment of Metallurgical & Materials Engineering, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra 440010, IndiabMedical Materials Laboratory, Department of Metallurgical & Materials Engineering, Indian Institute of Technology Madras,Chennai 600036, India

A R T I C L E I N F O

Article history:

Published online 16 August 2011

Keywords:

Glass ionomer cement (GIC)

Calcium deficient hydroxyapatite

(CDHA)

Micro-hardness

Compressive strength

Weight loss

Ionic release

A B S T R A C T

Glass ionomer cements (GICs) are clinically attractive filling materials often employed

in the field of dentistry as restorative and luting materials. The present work aims

to formulate bioactive nanocrystalline calcium deficient hydroxyapatite (nCDHA)-GIC

composite cements with improved mechanical and resorption properties of the set

cement than GICs. The nCDHA was synthesized via an accelerated microwave process

and characterized by X-ray powder diffraction (XRD) and Fourier transform infrared

spectroscopy (FT-IR) methods. The synthesized nCDHA was mixed with GIC in different

compositions (5, 10 and 15 wt%) maintaining the powder to liquid ratio. Cylinders of

dimensions 8 mm height and 4 mm diameter were formed using a Teflon mold following

a conventional cement forming technique. The XRD and FT-IR of the cylinders showed

increased intensity and characteristic bands of CDHA with increase in nCDHA content.

The surface cracks and the elemental composition of the set cements were analyzed by

scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Decreased

surface hardness was observed for composite cements with increase in nCDHA addition.

The cement cylinders were tested for ionic release in Millipore water (pH = 7) via inductive

coupled plasma (ICP) spectroscopy and in demineralization solution of pH = 5 to find out

the weight loss in an acidic environment at 37 ◦C performed periodically for 5 weeks. The

ionic release percentage, weight loss and compressive strength were observed to increase

with an increase in nCDHA addition.c⃝ 2011 Elsevier Ltd. All rights reserved.

d

1. Introduction

The glass ionomer is a generic name given to a group ofmaterials widely used in clinical dentistry as teeth fillersand luting cements. The material acquires its name from itsformulation of a glass powder and an ionomer that contains

∗ Corresponding author. Tel.: +91 4422574772; fax: +91 4422574752.E-mail addresses: tssk@iitm.ac.in, tsskumar@gmail.com (T.S. Sam

1751-6161/$ - see front matter c⃝ 2011 Elsevier Ltd. All rights reservedoi:10.1016/j.jmbbm.2011.08.002

path Kumar).

carboxylic acids (Anusavice, 2004). Generally glass ionomercements (GICs) consist of poly acrylic acid and copolymersof poly acrylic acid as the ionomer and an acid decomposablefluoro-aluminosilicate as the glass powders. The resultingcement has been classified as acid-base reaction cement asthey set initially by the reaction of liquid polyacid ionomer

.

70 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 7 ( 2 0 1 2 ) 6 9 – 7 6

with the calcium released from the glass to form insolublepolysalts. Schematic depiction of the GIC setting reaction isshown in Fig. 1. The advantage of GIC’s over other dentalcements include good adhesion to bone, stability in anaqueous environment, lack of exothermic polymerizationand good biocompatibility (Kovarik et al., 2005). However,GIC’s bear limitations like relatively poor mechanical, wearand resorption properties. Attempts that have been made toovercome these limitations include application of differentfiller materials, such as silver-cermets, stainless steelpowders, carbon and alumino-silicate fibers, incorporationof apatite into glass-polyalkenoate etc. (Lohbauer, 2010;Moshaverinia et al., 2011). Nevertheless, materials that mimicboth the structure as well as mineral composition of teeth aremuch preferred for clinical applications.

Hydroxyapatite (HA) is a calcium phosphate bioceramic(molecular formula Ca10(PO4)6(OH)2) with calcium to phos-phorus ratio (Ca/P) of 1.67. It is the main mineral compo-nent of the enamel, comprising of more than 60% of toothdentine by weight. It has excellent biocompatibility, compo-sition and crystal structure similar to apatite in the humandental structure and skeletal system (Dorozhkin, 2008). Var-ious studies carried out to evaluate the effects of apatitepowder addition to restorative dental materials such as GICsare summarized in Table 1. However, the authors have noknowledge of any previous study on calcium deficient hydrox-yapatite (CDHA) with GICs. CDHA with molecular formulaCa10−x(HPO4)x(PO4)6−x(OH)2−x is a variant of hydroxyapatitehaving a Ca/P ratio less than 1.67 and greater than 1.33. It issimilar to that of hydroxyapatite in terms of composition andstructure but has low thermal stability and higher solubilitythan hydroxyapatite (Siddharthan et al., 2004). The advantageof CDHA over other calcium phosphates includes higher spe-cific surface area and superior seeding efficacy compared tothat of HA and ß-tricalcium phosphate (TCP) (Kasten et al.,2003). Its properties including better resorption compared tothe conventional apatites and lower solubility than brushitecements under physiological conditions has made CDHA apotential candidate in the area of orthodontal and bone tissueengineering applications (Dorozhkin, 2008). The nanocrys-talline CDHA (nCDHA) was used as it is similar in size of themineral phase of the bone and its addition is expected toenhance the resorption and mechanical properties of themodified GIC cements. In the present study, the effects of in-corporating nCDHA to improve the mechanical and resorp-tion properties in GIC cement were carried out. The settingphase, surface morphology, molecular interactions, micro-hardness, compressive strength, weight loss and ionic releaseat air-dried and simulated in vitro conditions of the cementcomposites were evaluated.

2. Materials and methods

2.1. Materials

Commercial grade glass powder and the liquid (Fuji II, GCgold label 2, GC International, Japan) were used for cementpreparation. Analytical grade calcium nitrate tetrahydrate(Ca3(NO4)2·4H2O), orthophosphoric acid (H3PO4) and ammo-nium hydroxide (NH4OH) procured from Merck (Bangalore,India) were used for synthesis of nCDHA.

Fig. 1 – Schematic depiction of the setting reaction of GIC.

2.2. Synthesis of nCDHA

The accelerated microwave synthesis method was adaptedfor synthesis of nCDHA (Siddharthan et al., 2004). Briefly,0.5 M solution of Ca3(NO4)2·4H2O was precipitated withappropriate concentration of H3PO4 to get stoichiometric Ca/Pratio at 1.61. NH4OH was added to the mixing solution tomaintain the reaction pH around 10. The white precipitateformed was washed repeatedly to remove unwanted ions(NH4+ and NO2−

3 ) and subjected to microwave irradiation in adomestic microwave oven (BPL India, 2.45 GHz, 800 W power)for 20 min at 60% power. The resulting precipitate was driedand powdered using an agate mortar and pestle.

2.3. Formulation of cements

The nCDHA-GIC composite cylinders were prepared followingthe conventional cement mixing procedure. The powder toliquid ratio was maintained at 2.7/1.0 (g/g). The glass powderand nCDHA (5, 10 and 15 wt%) was thoroughly mixed inan agate mortar and pestle. The mixed powders were thenreacted for 15 s with the polyacid using a spatula to mix andto form a paste on the Teflon coated pad. The paste was thenimmediately transferred into a Teflon mold of 8 mm heightand 4 mm diameter to form the composite cylinders. Thecylinders formed were then removed from the mold, air-driedfor 24 h in an oven at 37 ◦C and stored in a desiccator at roomtemperature. As it has been reported that the failure loadsof air-stored specimens were higher than those of saliva-stored specimens, the above procedure was adopted (Wangand Darvel, 2009). The GIC (control) along with 5 wt%, 10 wt%and 15 wt% nCDHA forming composite cements were codedas 5CDHA-GIC, 10CDHA-GIC and 15CDHA-GIC respectively.

2.4. Characterization

2.4.1. X-ray diffraction analysisThe X-ray powder diffraction (XRD) studies of the set cementcylinders were carried out using a X-ray diffractometer(D8 DISCOVER, Bruker, USA) with Cu Kα radiation (λ =

1.54 Å). The cement cylinder was placed on the samplestage and the diffraction patterns were recorded usingDIFFRAC.SUITETM software at scanning rate of 1 step/s andstep size of 0.1◦/step. One cement cylinder from each set ofGIC, 5CDHA-GIC, 10CDHA-GIC and 15CDHA-GIC cements wasanalyzed by the XRD method.

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Table 1 – Summary of studies carried out to evaluate the effects of various apatite powder addition to GICs.

Cements Apatite Comments References

MP4 and G200D HA The incorporation of HA into glass-polyalkenoate cementscan be done only at low powder-to-liquid ratios, because ofthe high bulk density of HA powder. The precise effect ofincorporating HA has been found to depend on the type ofglass.

(Nicholson et al., 1993)

Fuji IX GP HA No significant difference in compression and diametraltensile strengths were observed between materials at bothtime intervals (1 day and 1 week). The effect of time onhardness was material dependent.

(Yap et al., 2002)

Fuji IX GP HA Addition of HA into GIC enhances and hastens the rate ofdevelopment of the cement’s fracture toughness, maintainslong-term bond strength to dentin and does not impedesustained fluoride release.

(Lucas et al., 2003)

Fuji IX GP HA The addition of HA hastens the development of early (15min, 1 h) flexural strength of GIC in moist or wet conditions.Compared to the control, specimens with 16%–25% HAwhiskers and as well with 19% HA whiskers. A significantincrease was also noted for those with 8%–25% HA granulesadded.

(Arita et al., 2003)

Fuji IX GP HA/ZrO2 Increase in the mechanical properties was found for 4 and12 vol% HA/ZrO2-GICs and decrease for 28 and 40 vol%HA/ZrO2-GICs. Mechanical properties of HA/ZrO2-GICs werefound to be much better than those of HA-GICs.

(Gu et al., 2005)

Fuji II LC HA As the HA concentration increased, the depth of curedecreased. However flexural strength increased and therewas not much change in the sensitivity to ambient light.

(Chae et al., 2006)

Fuji II LC Nano-HA 8% nano-HAP were incorporated into GIC as composite, andpure GIC as control. Three-point flexural strength andcompressive of nano-HAP-added GIC were increasedcompared with pure GIC.

(Mu et al., 2007)

Fuji II commercial GIC Nano-HA and FA The nano-hydroxyapatite/fluoroapatite added cementsexhibited higher CS (177–179 MPa), higher DTS (19–20 MPa)and much higher biaxial flexural strength (28–30 MPa) ascompared to the control group.

(Moshaverinia et al.,2008a)

Fuji II GC Nano-HA andfluoroapatite (FA)

The addition of nano-HA and FA synthesized into Fuji IIcommercial GIC enhanced the mechanical properties(compressive, diametral tensile and biaxial flexural strength)of the resulting cements and their bond strengths to dentin.

Moshaverinia et al.(2008b)

Fuji IX GP HA200, HA100,HA100f, Nano HA

Flexural strength has increased in all HAp-added groups.Specially, Nano HAp and HAp100f group showed significantincrease compared to control after 24 h.

(Yamamoto et al., 2009)

2.4.2. Fourier transform-infrared spectroscopyThemolecular interactions of the cements were characterizedby Fourier transform infrared spectroscopy (FT-IR) using a FT-IR spectrometer (Spectrum One FT-IR spectrometer, Perkin-Elmer, USA). One cement cylinder from each set of pureGIC and nCDHA added GIC cements were ground with KBrand pelletized in a conventional KBr press with a force of100 kg/cm2 for 2 min. The spectra of sample/KBr discs werecollected in the spectral range of 4000–450 cm−1 and thefunctional groups were characterized. All the 4 sets of (GIC,5CDHA-GIC, 10CDHA-GIC and 15CDHA-GIC) cements wereanalyzed by FT-IR spectroscopy.

2.4.3. Scanning electron microscopy and energy dispersiveX-ray analysisThe surface morphology of as prepared cement cylinderswithout any surface coating was observed by scanningelectron microscopy (SEM) using a ESEM microscope (FEI-Quanta 200, Netherlands) operated in environmental mode at

15 kV with relative humidity of 96%–99%. The SEM was fittedwith an energy dispersive X-ray analyzer (EDX) for elementalanalysis. All the 4 sets (GIC, 5CDHA-GIC, 10CDHA-GIC and15CDHA-GIC) of cement samples were characterized by SEM.

2.4.4. MicrohardnessThe surface microhardness of the cement cylinders wasperformed using a Vicker’s microhardness tester (ShimadzuHMV, Japan). The microhardness test was performed with1.961 N load applied for 10 s using a diamond indenter. Themicro hardness (HV) was calculated using the equation

HV = 1.854L/d2 (1)

where, ‘L’ is the load applied (N), and ‘d’ is the averagelength of the diagonal (mm) (Callister and Rethwisch, 2007).The length of the diagonal was taken as an average fromindentation at three different regions of the each sampleand 3 samples were used for each set of microhardnessmeasurements. Totally 4 sets (GIC, 5CDHA-GIC, 10CDHA-GIC

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Fig. 2 – XRD pattern of nCDHA powder, GIC andnCDHA-GIC cement cylinders.

and 15CDHA-GIC) of samples were tested for microhardness.Error bars were calculated for microhardness values based onthe standard errors.

2.4.5. Weight loss and ionic release study

Three cement cylinders from each set of GIC, 5CDHA-GIC, 10CDHA-GIC and 15CDHA-GIC cements were testedseparately for weight loss and ionic release studies. Forweight loss studies, the cylinders were immersed in acidicsolution (pH = 5) to simulate the pH in the mouth and forionic release study, the cylinders were immersed in deionizedwater (pH = 7). For both the studies, the cylinders werekept in caped glass bottles with 50 ml of the respectivesolution and kept in a constant temperature water bathshaker at 37 ◦C. The volume of the solution was preferredto be 50 ml considering surface area to volume relationshipbased on reported experiments on new glass compositionfor GICs (Nourmohammadi et al., 2007). The acidic solutionwas prepared by mixing 1.5 m M CaCl2, 0.9 mM KH2PO4,5 mM acetic acid, 150 m M KCl such that pH = 5 wasobtained (Wang et al., 2007). To check the weight loss, thecylinders were carefully taken out of the solution, washedgently with distilled water, dried at 80 ◦C to remove moistureand weighed. The weight of the cylinders was checked weeklyand freshly prepared 5 ml of the acidic solution was replacedevery 48 h in an attempt to maintain the initial concentrationof the solution throughout the experiment. The ionic releasewas checked for Al, Na, Si, and Ca ions weekly by inductive

coupled plasma spectroscopy (ICP-OES, Perkin-Elmer Optima5300DV). At regular intervals of time, the samples wereremoved from the solutions and the remaining solutions werethen used for measuring the ions. Both the weight loss andionic release studies were not carried out for all the samples.

2.4.6. Compressive strengthCompression tests were performed for triplicate sampleswith a crosshead speed of 1 mm min−1. The compressivestrength of the cylinders was performed using a screw-drivenmechanical testing machine (Model 4467, Instron Corp., MA).The compressive strength (MPa) was calculated before andafter the degradation study using the equation

CS = 4F/πd2 (2)

where, F is the max load until fracture (N) and d is thediameter (mm) of cylindrical pellet (Channasanon et al., 2010).The maximum load for fracture was taken as an averagefrom results of compression tests performed on three cementcylinders per set and 4 set (GIC, 5CDHA-GIC, 10CDHA-GIC and15CDHA-GIC) of cement samples were tested for compressivestrength. Error bars were calculated for compressive strengthbased on the standard errors.

3. Results and discussion

3.1. XRD analysis

The XRD pattern of as synthesized nCDHA (shown in Fig. 2)was compared with that of HA (JCPDS 9-432). The nCDHAshowed characteristic peaks of HA with the absence of otherphases, confirming the phase formed to be that of apatite. Theaverage crystallite size was found to be 24 nm using Scherrer’sformula

t = Kλ/BCosθ (3)

where, t is the average crystallite size (nm); K is the shapefactor (K = 0.9); λ is the wavelength of the X-rays (λ =

1.54056 Å for Cu Kα radiation); B is the full width at halfmaximum (radian) and θ is Bragg’s diffraction angle (degree)(Cullity, 1977). The diffraction peak at 25.9◦ correspondingto the (002) Miller plane family was chosen for calculationof the crystallite size. The synthesized nCDHA upon heatingto 750 ◦C showed peaks corresponding to β-TCP (JCPDS9-169) confirming the formed phase as calcium deficienthydroxyapatite (Siddharthan et al., 2004).

Also, Fig. 2 shows the XRD pattern of GIC and nCDHA-GIC cylinders. XRD pattern of the GIC cylinders showed thenature of the set cement to be amorphous. Crystalline peaksat 25.9◦ and 31.9◦ corresponding to (211) and (002) planesof hydroxyapatite were observed in nCDHA-GIC compositecements and their intensity was observed to increase withnCDHA addition (Milne et al., 1997; Wilson et al., 2003;Moshaverinia et al., 2008a). This may be viewed as an increasein concentration of the nCDHA in nCDHA-GIC compositecements with nCDHA addition. No additional peaks wereobserved for all the samples.

3.2. FT-IR spectroscopy

The FT-IR spectra of as synthesized nCDHA shown in Fig. 3exhibit clear evidence of calcium deficiency by the presence

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Fig. 3 – FTIR spectrum of (a) as synthesized nCDHApowder, (b) GIC, (c) 5CDHA-GIC, (d) 10CDHA-GIC and(e) 15CDHA-GIC cement cylinders.

of HPO2−

4 ion bands at 875 cm−1 (Mortier et al., 1989;Verges et al., 1998; Koutsopoulos, 2002). The stretching andvibrational modes of OH− groups and the internal modes

corresponding to the PO3−

4 groups were observed similar tothat of HA. The intensity of the of the OH− stretching andvibrational bonds observed in the as synthesized nCDHA waslower than that observed in stoichiometric HA, supporting theloss of OH− ions from the unit cell (Wilson et al., 2003).

The FT-IR spectra of GIC and nCDHA-GIC compositecements are also shown Fig. 3. The characteristic apatitebands at wave number 603 cm−1, 566 cm−1 and 472 cm−1

corresponding to structural OH− and PO3−

4 respectively wereclearly seen in nCDHA-GIC composites. The polyacrylate,aliphatic C=C, carbonyl C=O, P–H and C–H stretching bandswere observed in both pure GIC and nCDHA-GIC composites.Decreased absorbance pattern of amorphous Si band at742 cm−1 was observed with increase in nCDHA contentsuggesting the shift towards semi crystalline nature andmolecular interaction of calcium with aluminosilicate. The,P–NH2 and Si–O bands showed a similar pattern to that ofamorphous Si in composite cements (Nicholson et al., 1988;Kakaboura et al., 1996; Yipa and To, 2005).

3.3. Surface morphology and EDX

The SEM micrographs of the GIC and nCDHA-GIC compositecement cylinder surfaces is shown in Fig. 4. The surfaceof the GIC cylinder showed cracks as reported, which maylikely be due to artifact from desiccation or due to thestresses generated during molding and sample preparation(Moshaverinia et al., 2009). No significant difference or patternwas observed between GIC and nCDHA-GIC composites.However, the presence of cracks on the surface might behelpful in degradation so as to allow the bio-fluid to penetratethrough surface into the cavity providing a large surface area

Fig. 4 – SEM micrograph and EDX pattern of GIC and nCDHA-GIC composite cylinders.

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Fig. 5 – Micrograph of indentations on GIC and nCDHA-GIC composite cylinders showing impressions of diamond indenter.

Fig. 6 – Compression and hardness pattern of GIC andnCDHA-GIC composite cylinders (error bars indicatestandard deviation).

to volume ratio. The EDX spectra (Fig. 4) of the cylindersconfirm the presence of Al, Sr, Ca, Si, Na elements in the setcements as well as an increase in Ca and P concentration withincrease in nCDHA addition.

3.4. Microhardness

Microhardness measurement was carried out based onindentations at three different areas on surface of thetriplicate cement cylinders. Fig. 5 shows typical indentationon GIC and nCDHA-GIC composite cement cylinders followingthe Vickers micro-hardness test. The micrographs indicate an

increase in diagonal length with increase in nCDHA addition.Moreover, no additional cracks from the diagonals of theindentation were observed. The mean hardness value wasfound to decrease with the increase in nCDHA content asshown in Fig. 6. GIC showed the maximum hardness valuewhereas 15CDHA-GIC showed the minimum value amongthe samples. The surface cracks observed on the cementcylinders may seem to influence the observed reduction inmicro-hardness of GIC upon addition of nCDHA.

3.5. Weight loss and ionic release

The weight loss behavior of GIC and nCDHA-GIC compositecements in acidic medium is shown in Fig. 7. It was appar-ent that the weight loss increased with time indicating thedegrading behavior of the cement cylinders. The weight lossalso followed an increased pattern from GIC to n15CDHA-GICcomposite. The nCDHA addition increased the weight loss ofthe composite cement which may be due to the higher degra-dation rate of nCDHA in acidic medium compared to GIC.

The ionic release profile of pure GIC and nCDHA-GICcomposite cement cylinders in deionized water (pH = 7)is shown in Fig. 8. A higher amount of ionic release wasobserved for the composites compared to GIC. However,the amount of ion released depends upon the element andthe period of study. The Al ion release profile showed themaximum value in the 1st week followed by of a decreasingpattern, with low levels of detectable Al in the 4th week.The Na ion release profile was almost similar to that of Alion profile except with a sharp decrease in the Na ion levelafter 1st week. High amounts of Si ions were found in thesolution after the 1st week, which increased further in the

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Fig. 7 – Weight loss pattern of GIC and nCDHA-GICcomposite cylinders (error bars indicate standarddeviation).

Fig. 8 – Ionic release pattern of GIC and nCDHA-GICcylinders.

2nd week followed by a gradual decrease in concentration inthe subsequent weeks. Release of Ca ions was observed in thecomposites only which was found to depend on the amountof nCDHA addition and the period of study. This is due to theresorbing nature of nCDHA which acts as a source for Ca ions.The ion release especially of the Ca ion is anticipated to helpearlier mineralization with stimulated differentiation of cellsto mineralized tissue forming cells (Mizuno and Bansai, 2008).The release profile of n15CDHA-GIC cement is similar to theSi ion release profile with a maximum amount of release inthe 3rd week. Overall the nCDHA addition has resulted in theincreased ionic release leading to higher resorption rate forthe GIC composite cements.

3.6. Compressive strength

The mean compressive strength of GIC and nCDHA-GICcomposite cement cylinders in as set as well as afterweight loss and ionic release conditions are also shown

in Fig. 6. The compressive strength of the GIC cementincreases with nCDHA addition but was found to decreasefollowing immersion in fluids for the weight loss and ionicrelease studies. The immersion in fluid has led to ionicreleases causing decrease in compressive strength. However,increased trend in the compressive strength of GIC withCDHA addition was also observed following the degradationstudy in fluids. This is because the nanoparticles tend toresist the compression and the high density of interfaces ofnanomaterial causes increased compressive strength (Meyerset al., 2006). As bonds are formed between the polycarboxylicacids from the polymer matrix and the aluminum and/orcalcium cations from the glass particles during setting ofthe GICs, the addition of calcium containing nCDHA mayhave also played a role in enhancing the bonding. Overall,the compression test results indicate that addition of nCDHAparticles reinforces GIC cement.

It appears that there is no correlation between the micro-hardness and compressive strength of GIC with the additionof nCDHA, because hardness decreased with the addition ofnanoparticles. Based on SEM micrographs, relationships be-tween the hardness values and microstructures of the GICswere reported (Xie et al., 2000). The denser surface textureswith less and smaller voids resulted in a higher hardness. Thesomewhat less dense surface textures with cracks in nCDHAadded GIC cements may have caused opposite trends in thehardness values. However, it is possible that the cracks inthe microstructures may have been caused by dehydrationduring preparation for SEM analysis and future microstruc-tural studies should employ a replica technique to the removeartifacts in the present microstructures of the GICs. Furtherresearch is necessary to establish the relationships of com-pressive strength and microhardness with nCDHA additionfor the GICs.

4. Conclusions

The addition of nano-CDHA in the commercial GIC enhancesthe compressive strength of the resulting cements. It alsoincreases the resorption potential of the CDHA-GIC cementcomposites with an increase in weight loss and ionicrelease in liquids. Thus nCDHA bioceramics are promisingadditives for glass ionomer restorative dental materials withconstructive properties.

Acknowledgments

This work was partially supported by Department of Scienceand Technology (DST), Ministry of Science and Technology,Govt. of India. The authors would like to acknowledge SAIF,IIT Madras for analytical services.

R E F E R E N C E S

Anusavice, K.J., 2004. Phillip’s Science of Dental Materials.Elsevier, India.

Arita, K., Lucas, M.E., Nishino, M., 2003. The effect of addinghydroxyapatite on the flexural strength of glass ionomercement. Dent. Mater. 22, 126–136.

76 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 7 ( 2 0 1 2 ) 6 9 – 7 6

Callister, W.D., Rethwisch, D.G., 2007. Material Science andEngineering: An Introduction, seventh ed. John Wiley & Sons,Inc., New York, 156.

Chae, M.H., Lee, Y.K., Kim, K.N., Lee, J.H., Choi, B.J., Park, K.T.,2006. The effect of hydroxyapatite on bonding strength inlight curing glass ionomer dental cement. Key Eng. Mater. 881,309–311.

Channasanon, S., Soodsawang, W., Monmaturapoj, N., Tan-odekaew, S., 2010. Factors influencing compressive strength ofglass ionomer cement. J. Metal Mater. Mineral 20, 91–94.

Cullity, B.D., 1977. Elements of X-Ray Diffraction, second ed.Addison-Wesley Publishing, New York, 284.

Dorozhkin, S.V., 2008. Calcium orthophosphate cements forbiomedical application. J. Mater. Sci. 43, 3028–3057.

Gu, Y.W., Yap, A.U.J., Cheang, P., Khor, K.A., 2005. Effects ofincorporation of HA/ZrO2 into glass ionomer cement (GIC).Biomaterials 26, 713–720.

Kakaboura, A., Eliades, G., Palaghias, G., 1996. An FTIR studyon the setting mechanism of resin-modified glass ionomerrestoratives. Dent. Mater. 12, 173–178.

Kasten, P., Luginbuhl, R., Griensven, M.V., Barkhausen, T., Krettek,C., Bohner, M., Bosch, U., 2003. Comparison of humanbone marrow stromal cells seeded on calcium-deficienthydroxyapatite, β-tricalcium phosphate and demineralizedbone matrix. Biomaterials 24, 2593–2603.

Koutsopoulos, S., 2002. Synthesis and characterization ofhydroxyapatite crystals: A review study on the analyticalmethods. J. Biomed. Mater. Res. Part A 62, 600–612.

Kovarik, R.E., Haubenreich, J.E., Gore, D, 2005. Glass ionomer ce-ments: a review of composition, chemistry, and biocompati-bility as a dental and medical implant material. J. Long-TermEffect Med. Imp. 15, 655–671.

Lohbauer, U., 2010. Dental glass ionomer cements as permanentfilling materials? Properties, limitations and future trends.Materials 3, 76–96.

Lucas, M.E., Arita, K., Nishino, M., 2003. Toughness, bondingand fluoride release properties of hydroxyapatite-added glassionomer cement. Biomaterials 24, 3787–3794.

Meyers, M.A., Mishra, A., Benson, D.J., 2006. Mechanical propertiesof nanocrystalline materials. Prog. Mater. Sci. 51, 427–556.

Milne, K.A., Calos, N.J., Donnell, J.H., Kennard, C.H.L., Vega, S.,Marks, D., 1997. Glass-ionomer dental restorative. J. Mater. Sci.:Mater. Med. 8, 349–356.

Mizuno, M., Bansai, Y., 2008. Calcium ion release from calciumhydroxide stimulated fibronectin gene expression in dentalpulp cells and the differentiation of dental pulp cells tomineralized tissue forming cells by fibronectin. Int. Endod. J.41, 933–938.

Mortier, A., Lemaitre, J., Rodrique, L., Rouxhet, P.G., 1989.Synthesis and thermal behavior of well-crystallized calcium-deficient phosphate apatite. J. Solid State Chem. 78, 215–219.

Moshaverinia, A., Ansari, S., Movasaghi, Z., Billington, R.W., Darr,J.A., Rehman, I.U., 2008a. Modification of conventional glass-ionomer cements with N-vinylpyrrolidone containing poly-acids, nano-hydroxy and fluoroapatite to improve mechanicalproperties. Dent. Mater. 24, 1381–1390.

Moshaverinia, A., Ansari, S., Moshaverinia, M., Roohpour, N.,Darr, J.A., Rehman, I.U., 2008b. Effects of incorporation

of hydroxyapatite and fluoroapatite nanobioceramics intoconventional glass ionomer cements (GIC). Acta Biomaterialia4, 432–440.

Moshaverinia, A., Roohpour, N., Ansari, S., Moshaverinia, M.,Schricker, S., Darr, J.A., Rehman, I.U., 2009. Effects of N-vinylpyrrolidone (NVP) containing polyelectrolytes on surfaceproperties of conventional glass-ionomer cements (GIC). Dent.Mater. 25, 1240–1247.

Moshaverinia, A., Roohpour, N., Chee, W.W.L., Schricker, S., 2011.A review of powder modifications in conventional glass-ionomer dental cements. J. Mater. Chem. 21, 1319–1328.

Mu, Y.B., Zang, G.X., Sun, H.C., Wang, C.K., 2007. Effect ofnano-hydroxyapatite to glass ionomer cement. West China J.Stomatol. 25, 544–547.

Nicholson, J.W., Brookman, P.J., Lacy, O.M., Wilson, A.D., 1988.Fourier transform infrared spectroscopic study of the role oftartaric acid in glass-ionomer dental cements. J. Dent. Res. 67,1451–1454.

Nicholson, J.W., Hawkins, S.J., Smith, J.E., 1993. The incorporationof hydroxyapatite into glass-polyalkenote (glass-ionomer)cements: a preliminary study. J. Mater. Sci: Mater Med. 4,418–421.

Nourmohammadi, J., Salarian, R., Solati-Hashjin, M., Moz-tarzadeh, F., 2007. Dissolution behavior and fluoride releasefrom new glass composition used in glass ionomer cements.Ceram. Int. 33, 557–561.

Siddharthan, A., Seshadri, S.K., Sampath Kumar, T.S., 2004.Microwave accelerated synthesis of nanosized calciumdeficient hydroxyapatite. J. Mater. Sci.: Mater. Med. 15,1297–1284.

Verges, M.A., Gonzalez, C.F., Gallego, M.M., 1998. Hydrothermalsynthesis of calcium deficient hydroxyapatites with controlledsize and homogeneous morphology. J. Eur. Ceram. Soc. 18,1245–1250.

Wang, Y., Darvel, B.W., 2009. Hertzian load-bearing capacity ofceramic-reinforced glass ionomer cement stored wet and dry.Dent. Mater. 25, 952–955.

Wang, X.Y., Yap, A.U.J., Ngo, H.C., Chung, S.M., 2007. Environmen-tal degradation of glass-ionomer cements: a depth sensing mi-croindentation study. J. Biomed. Mater. Res., Part B. 82, 1–6.

Wilson, R.M., Elliot, J.C., Dowker, S.E.P., 2003. Formate incorpora-tion in the structure of Ca-deficient apatite: rietveld structurerefinement. J. Solid State Chem. 174, 132–140.

Xie, D., Brantley, W.A., Culbertson, B.M., Wang, G., 2000.Mechanical properties and microstructures of glass-ionomercements. Dent. Mater. 16, 129–138.

Yamamoto, A., Arita, K., Lucas, M., Shinonaga, Y., Harada, K., Abe,Y., 2009. The development of a new hydroxyapatite-ionomercement. In: 9th World Congress on Preventive Dentistry,Phuket.

Yap, U.J., Pek, Y.S, Kumar, R.A., Cheang, P., Khor, K.A., 2002.Experimental studies on a new bioactive material: HAIonomercements. Biomaterials 23, 955–962.

Yipa, H.K., To, W.M., 2005. An FTIR study of the effects of artificialsaliva on the physical characteristics of the glass ionomercements used for art. Dent Mater. 21, 695–703.