dental composites reinforced with hydroxyapatite ...hera.ugr.es/doi/15010363.pdf · dental...

Dental composites reinforced with hydroxyapatite:Mechanical behavior and absorption/elution characteristics

C. Domingo,1 R. W. Arcı́s,1 A. López-Macipe,1 R. Osorio,2 R. Rodrı́guez-Clemente,1 J. Murtra,3

M. A. Fanovich,1 M. Toledano21Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, 08193, Bellaterra, Spain2Facultad de Odontologı́a, Universidad de Granada, Granada, Spain3Facultad de Odontologı́a, Universidad de Barcelona, Barcelona, Spain

Received 21 September 2000; revised 14 February 2001; accepted 5 March 2001

Abstract: The purpose of this study was to analyze thebehavior in water as well as the mechanical and surfaceproperties of experimental composites designed for dentalrestoration. Studied materials were composed of a visible-light-cured monomer mixture as a matrix (bisphenol-a-glycidyl methacrylate with triethyleneglycol dimethacrylateor hydroxyethyl methacrylate) and either micrometric ornanometric hydroxyapatite (HA) particles as a reinforcingfiller. The surface of the filler particles was modified byusing different coupling agents (citric, hydroxysuccinic,acrylic, or methacrylic acid). The hydrolytic stability of theevaluated materials was studied through elution-in-waterand water-uptake tests. Mechanical and surface propertieswere examined through the results of flexural, hardness,

and surface roughness tests. Means and standard deviationswere calculated for each variable. Analysis of variance andmultiple comparison tests were performed. Materials con-taining bisphenol-a-glycidyl methacrylate:triethyleneglycoldimethacrylate and micrometric-HA coated with citrate, ac-rylate, or methacrylate displayed the most favorable results.Improvements should be obtained by increasing the totalfiller amount, and by the introduction of nanometric-HAfiller into a micrometric-HA reinforced composite resin sys-tem. © 2001 John Wiley & Sons, Inc. J Biomed Mater Res 56:297–305, 2001

Key words: Bis-GMA; hydroxyapatite; coupling agent; wa-ter aging; mechanical properties

INTRODUCTION

In dentistry, composite biomaterials are used to re-store teeth. The resin-based composites are composedof inert glass filler particles blended into a matrix of achemically or photo-polymerizable acrylic resin. In thedevelopment of composite materials, the main focushas been on the filler phase. A wide variety of sila-nated inorganic fillers are commercially available.

1–3

An acceptable substitute for human tooth tissues willbe a composite material with similar chemical andphysico-mechanical characteristics to that of the natu-ral substance it must replace. The mineral phase ofbone and teeth is mainly hydroxyapatite (HA). There-fore, synthetic HA would seem a good choice as theinorganic filler to be used in dental restoration or boneimplants.4–7 The use of HA in restorative dentistry

offers several promising advantages, including intrin-sic radio-opaque response, enhanced polishability,and improved wear performance, because syntheticHA has a hardness similar to that of natural teeth.Finally, this material is less expensive than most of theinorganic fillers in use today.

Most commercial dental restorative materials arebased on the bisphenol-a-glycidyl methacrylate (Bis-GMA) monomer.8,9 To achieve a final consistency suit-able for the incorporation of particulate fillers as wellas to increase the degree of conversion, a low molecu-lar weight methacrylate diluent is added to the highlyviscose Bis-GMA monomer. In this work, triethyl-eneglycol dimethacrylate (TEGDMA) was chosen be-cause it is the most commonly used diluent.10 Hy-droxyethyl methacrylate (HEMA) is widely used asadhesive, because it facilitates self-adhesion to themineral phase of dental tissues.11 HEMA was also se-lected as a diluent, because its tendency to interactwith HA was expected to improve the matrix/fillercoherence. Some authors have reported the use ofdifferent coupling agents developed to enhance theinteraction between the filler and the organic ma-trix.12–15

Correspondence to: C. Domingo Pascual; e-mail: [email protected]

Contract grant sponsor: Research Projects CICYT; contractgrant numbers: MAT98-0976-CO2, MAT98-0937-C02.

© 2001 John Wiley & Sons, Inc.

The aim of this study was to gain more insight intothe use of HA (nanometric or micrometric particles) asthe reinforcing filler of visible-light-cured resins de-veloped for dental restoration. The strength of the in-teraction between HA particles and the polymericresin at the interface was analyzed as a function of thecoupling agent. The hydrolytic stability of the com-posites was analyzed through their elution-in-waterand water-uptake characteristics. The internal bulk co-herence was estimated by means of their flexuralproperties (Young’s modulus and flexural strength).Surface roughness and Vickers hardness were evalu-ated to characterize the surface of the materials. Incomparison with the scanning electron microscopy(SEM) micrographs, a more accurate description ofthese properties was given.

MATERIALS AND METHODS

Specimen preparation

The organic matrices consisted of 60 wt % Bis-GMA (Free-man Chemical Corporation, Cheshire, UK) and 40 wt % ofeither TEGDMA (Fluka, Zurich, Switzerland) or HEMA(Fluka, Switzerland) viscosity-modifying comonomer.

Two types of HA particles were precipitated and used asa reinforcing material: 1. nanometric-HA (nano-HA) ob-tained by a refluxing method,16 and 2. micrometric-HA(micro-HA) obtained by a continuous method,17 sintered at1100°C and sieved to separate the fraction between 1 and 5mm. The precipitated nano-HA needle-like particles (50–100nm length) retained citrate molecules (coupling agent) ad-sorbed on the surface. Surface modification of the micro-HAparticles was achieved by shaking samples during 48 h with

either diluted solutions of citric or hydroxysuccinic acid(0.05M, pH = 6.0) or concentrated solutions of acrylic ormethacrylic acid. The presence of the coupling agent on theHA surface was confirmed by infrared spectroscopy.

Table I lists the composition of the 12 experimental restor-ative materials developed for evaluation in this work. Thevisible-light-curing composite pastes were formulated bymixing the liquid monomers, containing the photo-initiatoradditives, and HA particles. Experiments with six materialsof each different monomer composition were performed.Two of the 12 investigated experimental materials werefilled with 50 wt % of nano-HA particles. The remaining 10materials were filled with 60 wt % of micro-HA particles,either coated or uncoated. Finally, specimens of a commer-cial dental composite (Herculitet; Kerr Co., Orange, CA)were also evaluated at identical conditions as the experi-mental composites.

Five specimens of each particular material described inTable I were used for each different measurement. To shapethe specimens, Teflont molds covered with a clear glassplate were used. The samples were irradiated with a visiblelight-curing unit (Translux CL; Hereaus Kulzer, Wehrheim,Germany). After removing the specimens from the molds,they were automatically polished wet on both sides (Exakt-Apparatebau, Norderstdt, Germany) using silicon carbidepaper (up to 1200 grit).

Behavior in water

For the elution-in-water and water-uptake evaluation,disc-shaped specimens (thickness = 0.85 mm and diameter =15 mm) were molded.18 Discs were first conditioned by be-ing placed in a desiccator (containing calcium sulfate) at37°C until a constant weight was achieved (initial weight:m0). Specimens were then placed into separated vials con-taining 20 mL of distilled water at 37°C, and after 1, 6, 24 h,

TABLE IComposition of the Evaluated Composites and Results of the Different Measurements

Mat. HA wt % Coupling Agent

Young’sModulusE (GPa)

FlexuralStrengths (MPa)

VickersHardness

No.

Surface RoughnessElution-in-Water(wt %)

Water-Uptake(wt %)

BeforeRa (mm)

AfterRa (mm)

Series 1: Bis-GMA:TEGDMA (60:40 wt %)A Nano. 50 Sodium citrate 3.9 (0.8)* 26 (6) 38 (6) 3.9 (1.4) 1.7 (0.9) 5.2 (2.1) 7.4 (1.6)B Micro. 60 — 4.5 (0.6) 38 (6) 34 (9) 1.5 (0.8) 0.9 (0.5) 1.4 (0.8) 2.1 (1.0)C Micro. 60 Hydrosuccinic a. 5.2 (0.8) 25 (3) 55 (10) 1.6 (1.0) 1.5 (0.8) >10 6.3 (2.1)D Micro. 60 Citric a. 5.9 (0.7) 48 (4) 56 (10) 1.0 (0.6) 0.7 (0.4) 0.7 (0.1) 0.9 (1.1)E Micro. 60 Acrylic a. 7.3 (1.1) 67 (7) 48 (6) 1.0 (0.5) 0.8 (0.4) 0.3 (0.1) 2.8 (1.5)F Micro. 60 Methacrylic a. 5.9 (0.6) 46 (8) 53 (6) 1.8 (0.8) 0.7 (0.3) 0.5 (0.4) 0.9 (0.6)

Series 2: Bis-GMA:HEMA (60:40 wt %)A* Nano. 5 Sodium citrate 2.8 (0.5) 17 (3) 41 (6) 2.0 (0.9) 1.2 (0.4) >10 >10B* Micro. 60 — 4.3 (0.5) 40 (3) 43 (7) 0.7 (0.5) 1.3 (0.7) 2.1 (0.6) 3.1 (1.2)C* Micro. 60 Hydrosuccinic a. 3.9 (0.2) 24 (2) 41 (5) 1.1 (0.5) 6.3 (3.1) >10 7.2 (1.3)D* Micro. 60 Citric a. 6.9 (1.2) 52 (7) 39 (8) 0.9 (0.5) 2.4 (0.8) 2.5 (1.1) 2.5 (1.0)E* Micro. 60 Acrylic a. 6.2 (0.8) 56 (9) 46 (7) 0.7 (0.4) 3.9 (1.6) 1.7 (1.2) 5.0 (0.3)F* Micro. 60 Methacrylic a. 6.2 (1.0) 52 (8) 49 (12) 0.7 (0.2) 0.9 (0.5) 0.6 (0.2) 1.1 (0.7)

Herculite XRVMR: Bis-GMA-based compositeFiller: 0.6 mm parts., 59% volume 7.9 (1.0) 80 (17) 87 (16) 1.1 (0.6) 1.5 (0.5) 1.6 (0.7) 0.3 (0.1)

*mean value (standard deviations).

298 DOMINGO ET AL.

and, subsequently, at 2-day intervals, they were removed,blot dried, and weighed, and then returned to water. Thisprocedure was continued until a constant weight wasachieved (wet weight: m1). The discs were then replaced inthe desiccator at 37°C until a constant weight was achieved.To ensure total dryness of the materials, specimens wereplaced in a vacuum oven (25 in. of mercury) at 60°C for 24h and then weighed for the last time (dry weight: m2). Varia-tions in weight after the 24-h evacuation period were mini-mal, and no post-curing effect was observed. Water-uptakeand elution-in-water were evaluated according to themethod of Oysaed and Ruyter.19

Percent elution-in-water was evaluated by using Equa-tion (1):

Elution-in-water ~wt%! =m0 − m2

m0× 100 (1)

Percent water-uptake was determined by using Equation (2):

Water-uptake ~wt%! =m1 − m2

m0× 100 (2)

Flexural properties

Flexural properties were evaluated using rectangular-shaped specimens (length = 25 mm, height = 2 mm, andwidth = 2 mm). Preceding the tests, each specimen wasplaced for 1 week in 20 mL of distilled water at 37°C. Uponremoval, specimens were desiccated for 48 h at 37°C. Theflexural properties were quantified by a three-point loadingtest (Instron Test Instrument 4411; Instron Ltd., High Wy-combe, UK) at a crosshead speed of 0.75 mm min−

1. Elastic-

ity or Young’s modulus (E) was calculated from the re-corded load-deflection curve of each specimen. Flexuralstrength (s) was obtained by measuring the load at fracture.

Surface characteristics

Vickers hardness number was measured using disc-shaped specimens (thickness = 2 mm and diameter = 8 mm).Preceding the test, each disc was placed for 1 week in 20 mLof distilled water at 37°C and subsequently desiccated for 48h at 37°C. Specimens were utterly polished with silicon car-bide paper up to 4000 grit. Micro-hardness was quantifiedby applying 0.01 kg load with a pyramidal diamond point(Indenter V-testor 402; Instron Ltd.), and measuring thelength of the diagonal of five square indentations producedin the surface.

Surface roughness (Ra) was measured using disc-shapedspecimens (thickness = 0.85 mm and diameter = 15 mm). Tenspecimens of each material were automatically polishedwith silicon carbide paper up to 4000 grit. In one group offive specimens, the surface roughness was measured di-rectly after polishing. The remaining five specimens wereplaced for 2 weeks at 37°C in 20 mL of distilled water each.Upon removal, specimens were dried for 10 days in a des-iccator at 37°C and the roughness was measured. The aver-age surface roughness of each specimen was determinedwith a profilometer tester (Mitutoyo 201, Tokyo, Japan) infive different directions.

Surface microstructure of several specimens (before andafter 2-week water storing) was examined under an SEM(ZEISS DSM-950; Carl Zeiss, Germany). Specimens weremounted on aluminum stubs with carbon cement, and sput-ter-coated with gold. An accelerating voltage of 20 KV wasapplied at a working distance of 13–16 mm.

Statistical analysis

Mean value and standard deviation were calculated foreach group of specimens. Analysis of variance (ANOVA)tests were performed for each dependent variable (elution-in-water, water-uptake, elasticity, flexural strength, micro-hardness, and surface roughness). The following variableswere considered as main effects: matrix composition, cou-pling agents, and filler particle size. Interactions were alsoincluded in the analysis. Student-Newman-Keuls range testswere used for multiple comparisons and statistical signifi-cance was considered at the 95% confidence level.

RESULTS

Means and standard deviations of measurements ineach material are shown in Table I. ANOVA results foreach dependent variable are displayed in Table II. Sta-tistical differences are schematically represented inFigure 1.

Behavior in water

Water-uptake was affected by the three main effectsand interactions were considered as significant. Elu-tion-in-water was also influenced by the main effects,except for the matrix composition, but interactionswere significant. Water-uptake was high when HEMAwas used as comonomer. The water-uptake and elu-tion values of materials filled with nano-HA (A andA8) were higher than those of materials filled withmicro-HA. Materials involving hydroxysuccinate as acoupling agent (C and C8) also showed high elutionand water-uptake values. The rest of the compositesdisplayed relatively low elution and water-uptakemean values, with no significant differences betweenthem.

Flexural properties

ANOVA showed significant influence of filler par-ticle size and coupling agent on flexural values.Chemical differences on matrix composition neitheraffected flexural properties of these materials, norwere interactions significant. Materials filled with ci-trate-coated nano-HA (A and A8) exhibited signifi-cantly lower values of flexural properties than didsimilar materials filled with citrate-coated micro-HA

299DENTAL COMPOSITES REINFORCED WITH HYDROXYAPITATE

(D and D8). Materials containing uncoated micro-HA(B and B8) also showed low values of Young’s modu-lus. The pretreatment of the micro-HA with citric (Dand D8), acrylic (E and E8), or methacrylic acid (F andF8) had a significant enhancing effect on the Young’smodulus, and, in most cases, on the flexural strengthof the final composite. The incorporation of hydroxy-succinic acid as a coupling agent (C and C8) led to areduction in the flexural strength, and no improve-ment in the elasticity modulus was observed.

Surface properties

Micro-hardness was neither affected by the chemi-cal composition of the matrix nor by the filler particlesize. Significant influence was encountered with re-spect to the coupling agent and interactions. The sixmaterials involving HEMA (series 2) displayed statis-tically comparable values of micro-hardness, whereasthe micro-hardness of materials involving TEGDMA

(series 1) was influenced by the filler particle size (Dwas significantly harder than A), and whether or notthe filler was surface modified (C, D, E, and F wereharder than B).

ANOVA test showed that surface roughness wasaffected by the chemical composition of the matrixand by the filler particle size, whereas the couplingagent had no effect, although interactions were signifi-cant. Nano-HA filled materials (A and A8) were sta-tistically rougher than the materials comprising micro-HA, which were all similar and displayed low rough-ness values. In general, materials involving HEMA(series 2) displayed slightly less roughness values thanthose composed of TEGDMA (series 1). The surfaceroughness of composites in series 1 slightly decreased(significantly for samples A and F) after 2-week waterimmersion. Nevertheless, surface roughness of mate-rials in series 2 significantly increased after 2-weekwater aging (with the exception of samples A8, B8,and F8).

SEM micrographs of the most representativesamples are shown in Figures 2 and 3.

DISCUSSION

Experimental composite materials

The mixture Bis-GMA:HEMA was selected foranalysis, together with the mixture Bis-GMA:TEGDMA,on the basis of the reported strong affinity of HEMA

TABLE IIANOVA Results (Variables Considered as Main Effects:

Matrix Composition, Coupling Agent, and FillerParticle Size)

DependentVariable Main Effects F

Signification(P)

Water-uptake Main effects 12.1 0.000Multiple R = 0.76 Matrix composition 9.33 0.004

Coupling agent 5.1 0.001Filler particle size 29.7 0.000Interactions 2.32 0.070

Elution-in-water Main effects 23.9 0.000Multiple R = 0.85 Matrix composition 1.56 0.216


Flexural strength Main effects 18.7 0.000Multiple R = 0.82 Matrix composition 0.17 0.679


Young’s modulus Main effects 9.69 0.000Multiple R = 0.72 Matrix composition 1.15 0.289


Surface roughness Main effects 21.7 0.000Multiple R = 0.55 Matrix composition 22.1 0.000


Surface hardness Main effects 4.1 0.001Multiple R = 0.28 Matrix composition 1.81 0.180


Figure 1. Schematic representation of the physico-mechanical properties of the evaluated experimental com-posites. A, B, C, D, E, and F are materials from series 1 andA8, B8, C8, D8, E8, and F8 from series 2. Material compositionsand data in the Figure are from Table I. Materials connectedwith horizontal lines are not significantly different (p > 0.05).

300 DOMINGO ET AL.

monomer toward teeth hard tissues, which predomi-nantly contain HA.20 The physicochemical basis of thestrong affinity of HEMA toward HA is the Lewis acid-base interaction between the electron donor HEMAand the electron acceptor HA.

The actual tendency in dental composites research isto reduce the size of the used filler particles from mi-crometric to nanometric sizes.21 Nanometric particleshave a large interface for interaction with the organicmatrix, but they also have a high surface-excess en-ergy. When nano-HA was used as reinforcing filler,the high repulsion forces generated in the interfacecould only be brought down partially for the couplingcitrate, and the formation of agglomerates was un-avoidable. The maximum amount of nano-HA admit-ted by the matrix, independent of matrix composition,was relatively low, ca. 50 wt %. Even in this propor-tion, it was extremely difficult to obtain homogeneousdispersions, and a heterogeneous microstructure wasdetected by SEM examination [Fig. 2(a,b)]. Moreover,the mesoporous agglomerates formed by the HAnanoparticles had a highly hygroscopic nature and re-

tained adsorbed water.14,22 The fraction 1–5 mm HAparticles was selected as a micrometric filler after apilot study, because this fraction was easily dispersedin the matrix up to percent weights of ca. 80 wt %. Inthe present work, the packed amount was fixed at 60wt % in order to have similar loaded values for bothnanometric and micrometric fillers and, hence, com-parable results.

The four tested coupling agents contain the carbox-ylate functionality with a tendency for adsorptiononto the HA surface.6,12 The acrylic and methacrylicacid coupling agents possess the additional acrylateand methacrylate functional group, respectively, witha tendency for interaction with the polymeric net-work.

Behavior in water

The retention of good mechanical properties of den-tal composites in a wet environment is essential to

Figure 2. SEM micrographs of composites filled with nano-HA including: (a) Bis-GMA:TEGDMA (A) before water aging,(b) Bis-GMA:HEMA (A8) before water aging, (c) Bis-GMA:TEGDMA (A) after water aging, and (d) Bis-GMA:HEMA (A8) afterwater aging. Bar = 20 mm.


ensure viability of these materials, because they areproposed for oral use.19,23,24 Water sorption by com-posite materials is a diffusion-controlled process, andthe water-uptake occurs mainly in the organic phasematrix.25

A wide range of elution-in-water and water-uptakevalues were obtained as a function of composite com-position (Table I and Fig. 1). Nevertheless, materialscould be classified in two groups: I. materials with anelution-in-water and water-uptake higher than 3 wt %(A, A8, C, and C8), and II. materials with an elution-in-water and water-uptake similar to or lower than 3wt % (B, B8, D, D8, E, F, and F8). Material E8 could notbe included in any of the two groups.

Differences between the elution-in-water and water-uptake for materials comprising TEGDMA andHEMA were smaller than expected on the basis of thehigher degree of hydrophilicity of HEMA monomerwith respect to TEGDMA. Although monomers simi-lar to HEMA, such as 3-hydroxypropil methacrylate,21

have been used as diluents for Bis-GMA, HEMA hasnot been examined to date—the main reason probably

being the high degree of hydrophilicity of this mono-mer. For the studied composites in this work, the goodinteraction with the HA filler particles would reduceHEMA leaching in the wet environment. However,the mobility of Bis-GMA is expected to be higher inHEMA than in TEGDMA, because HEMA is a muchsmaller molecule and led to a less viscous mixture.The viscosity of TEGDMA is approximately twofoldthe viscosity of HEMA. The degree of monomer con-version in the Bis-GMA + HEMA mixture was prob-ably enhanced by increasing the mobility. A higherdegree of conversion would be reflected in a lowerelution-in-water.

Materials in group I are not adequate for dentalrestoration. Clinically acceptable composites are thosein which the elution-in-water and water-uptake haverelatively low values (2–3 wt % maximum) which arecounterbalanced. In group I are included materialsfilled with nano-HA (A and A8) and materials involv-ing hydroxysuccinate as the coupling agent (C andC8). For these materials, water had an important del-eterious effect. Composites A and A8 absorbed a large

Figure 3. SEM micrographs, taken after water aging, of composites filled with micro-HA including: (a) Bis-GMA:TEGDMA+ hydrosuccinic acid (C), (b) Bis-GMA:HEMA + hydrosuccinic acid (C8), (c) Bis-GMA:TEGDMA + methacrylic acid (F), and(d) Bis-GMA:HEMA + methacrylic acid (F8). Bar = 20 mm.

302 DOMINGO ET AL.

amount of water, because of swelling of the polymermatrix along with water sorption by the highly hygro-scopic nano-HA agglomerates. After 2 weeks of waterimmersion, the percent elution-in-water of materials Aand A8 was also important. This fact was explained onthe basis of a poor photo-conversion of the matrixesbecause of the presence of the adsorbed water in theagglomerates before polymerization, which would re-sult in an increased amount of residual monomers.26,27

Moreover, weak adhesion between agglomeratednano-HA particles and matrix will lead to easy pluck-ing-out of filler particles in the wet environment. Afterwater aging of samples A and A8, defects and voidswere easily observed on the surface [Fig. 2(c,d)] cor-responding to debonded filler particles.

For composites C and C8, the higher values of elu-tion-in-water and water-uptake indicated an inad-equate interface filler/matrix. The hydroxysucciniccoupling agent has neither a polymerizable function-ality nor a chain to provide sufficient dispersion inter-actions with the crosslinked polymeric chains. Forthese materials, voids on the surface were also ob-served because of filler debonding [Fig. 3(a,b)]. Addi-tionally, the high values of elution-in-water could alsobe attributed to the leaching of residual hydrosucci-nate.

Materials in group II presented relatively low val-ues of elution-in-water and water-uptake. This groupcomprises materials filled with uncoated micro-HA (Band B8) and materials containing micro-HA and citrate(D and D8), acrylate (E), or methacrylate (F and F8) asa coupling agent. In Figure 3(c,d), SEM micrographs ofthe water-aged F and F8 composite materials areshown as a representative example of the surface ap-pearance of materials in group II. For these compos-ites, the porosity formation attributed to filler debond-ing was not observed, indicating a favorable adhesionbetween filler and matrix. The Herculite had a similarbehavior in water (Table I) to the one observed formaterials in group II.

Flexural properties

Flexural properties would give an indication of thedurability of the restorative materials.28 In the presentwork, both types of used resin showed similar flexuralvalues. The lack of differences may be attributed to thelarge amount of Bis-GMA used (60 wt % with respectto the comonomer). For both kinds of matrices, mostof the crosslinking, i.e., the internal networking, wasattributed to the more rigid Bis-GMA monomer,which mainly determined the value of the internalflexural properties.10

The lowest strength and elasticity modulus valueswere observed in the nano-HA-filled composites (A

and A8) and the highest values of those propertieswere noticed in the composites containing citrate (Dand D8), acrylate (E and E8), or methacrylate (F and F8)as a coupling agent. For the nano-HA-filled compos-ites (A and A8), sorbed water together with agglom-erate formation resulted in poor flexural properties.The low degree of curing of these materials wouldresult in the development of an inadequate polymernetwork structure. It is expected that mechanicalproperties of composites filled with nano-HA can beimproved by reaching a more homogeneous disper-sion of the nanoparticles in the organic matrix. Addi-tionally, stronger interactions between filler and ma-trix could probably be obtained by optimizing theamount and/or characteristics of the coupling agent.

When micro-HA was used as filler, improved flex-ural results were obtained than when using nano-HA.Only the use of hydroxysuccinic acid as a couplingagent (C and C8) had an adverse effect in the flexuralstrength with respect to the uncoated materials (B andB8). Alternatively, the addition of citric (D and D8),acrylic (E and E8), or methacrylic acid (F and F8) had apositive effect on the flexural mechanical properties.The most appropriate modulus of elasticity for a com-posite resin would be one comparable to that of dentin(14.7 GPa).29 Our composites had values between 6and 7 GPa, but it is important to consider that theweight percent filler value used in this work was rela-tively low (60 wt %), compared with that of the com-mercial dental composites. Most of the dental compos-ites currently used are reinforced with up to 80–90%by weight of silanated inorganic fillers2,30 and the val-ues of the obtained modulus of elasticity ranged from5 to 25 GPa.31 Herculite led to high values of bothstudied flexural properties (Table I) but contained 59%in volume of filler material (∼ 80 wt %). As an expo-nential dependence of Young’s modulus with the per-cent filler fraction has been found for several materi-als,31 it might also be expected, for our materials, thatan improved elasticity modulus would be reached byincreasing the total filler amount.

Surface properties

Surface characteristics are important parameters indetermining the polishability and abrasive-wear rateof restorative materials. Because specimens were pol-ished before testing, the polymeric phase suffered apreferential abrasion, leaving the filler phase exposed.Hence, surface properties were more related to thecharacteristics of the HA inorganic filler. The averageVickers hardness number of human dentin is ∼60.29Values obtained in this work were very close to thatone, especially when Bis-GMA:TEGDMA was used.The highly filled commercial Herculite was harderthan the HA-reinforced composites.


The surface roughness was dramatically affected bythe filler particle size. Nano-HA-filled materials (Aand A8) displayed higher values of roughness than theremainder of studied materials. In general, the surfaceroughness could be related with the filler particlesize.3 The higher surface roughness obtained for ma-terials A and A8 was caused by the protuberant ag-glomerated particles, with sizes >20 mm [Fig. 2(a,b)].The surface roughness of these materials was substan-tially reduced after water aging, probably because ofthe massive filler plucking-out [Fig. 2(c,d)]. Porosityattributed to the loss of particles on the surface seemsto contribute less to surface roughness than the ag-glomerated protuberant particles did. After water ag-ing, the surface roughness of most composites contain-ing HEMA increased, whereas the surface roughnessof composites containing TEGDMA was mainly unaf-fected. Hydrophilic composites involving HEMA hadsuffered more from the deleterious effect of water thanthose involving TEGDMA. The surface roughness ofthe composite resins should be as similar as possible tothe average roughness of enamel-to-enamel occlusalcontacts, 0.64 ± 0.25 mm, which is regarded as biologi-cally smooth.32 Nevertheless, this property is difficultto compare with other published results becauseroughness depends largely on the polishing proce-dure.33,34 Materials labeled D, F, E, B, and F8, dis-played surface roughness values below 1 mm (afterwater aging). The remaining composite materialsshowed a significantly rougher surface than that ofenamel contact areas. After an identical polishing pro-cedure, Herculite displayed roughness values higherthan 1 mm. In general, although flexural propertieswere largely affected by the composite composition,the surface properties (hardness and roughness) wereless influenced.

CONCLUSION

The twelve experimental composite materials stud-ied in this work were manufactured combining twomonomer mixtures, two HA-filler sizes, and four cou-pling agents. Materials containing nanometric par-ticles of HA are unsuitable for clinical performance,because of their high solubility in water. More inves-tigation is needed to further improve the interactionbetween nanometric-HA particles and the organicphase. Several of the midway-filled composite resinsloaded with micrometric-HA particles displayed lowpercent elution and water-uptake values, using indis-tinctly TEGDMA or HEMA as a comonomer for Bis-GMA. In general, filler/matrix coherence was im-proved by using citrate, acrylate, or methacrylate as acoupling agent. For some of the studied dental com-posites, the values of the mechanical properties are

closed to those of commercial materials. A reductionin water-uptake and an increase in flexural propertiesare expected by increasing the total filler amount,which is easy in the materials containing micro-HA,because they have been formed under very differentconditions from the maximum filler load. Because ofits bicompatibility, the use of HA in both skeletal anddental restorations is a fundamental area of research.Probably the introduction of nano-HA fillers into amicro-HA reinforced composite resin system wouldovercome this problem. Nevertheless, the main disad-vantage of using HA as a filler for light-activated poly-mers is its high refractive index and, hence, light scat-tering. By modifying the composition of the light-cured organic phase and/or by varying the crystal sizeand shape characteristics of the inorganic filler, theoptical and mechanical properties of HA-based com-posite could be improved.

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