Reinforcement of experimental composite materials based on amorphous calcium phosphate with inert fillers.

ARTICLE IN PRESS

DENTAL-2402; No. of Pages 9

d e n t a l m a t e r i a l s x x x ( 2 0 1 4 ) xxx–xxx

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema

Reinforcement of experimental composite materials based on amorphous calcium phosphate with inert fillers Danijela Marovic a,∗ , Zrinka Tarle a , Karl-Anton Hiller b , Rainer Müller c , Martin Rosentritt d , Drago Skrtic e , Gottfried Schmalz b a

Department of Endodontics and Restorative Dentistry, School of Dental Medicine, University of Zagreb, Croatia Department of Operative Dentistry and Periodontology, University Hospital Regensburg, University of Regensburg, Germany c Institute of Physical and Theoretical Chemistry, University of Regensburg, Germany d Department of Prosthodontics, University Hospital Regensburg, University of Regensburg, Germany e Dr. Anthony Volpe Research Center, ADA Foundation, Gaithersburg, MD, USA b

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. The aim of this study was to examine the influence of the addition of glass fillers

Received 20 July 2013

with different sizes and degrees of silanization percentages to remineralizing composite

Received in revised form

materials based on amorphous calcium phosphate (ACP).

8 November 2013

Methods. Four different materials were tested in this study. Three ACP based materials: 0-

Accepted 5 June 2014

ACP (40 wt% ACP, 60 wt% resin), Ba-ACP (40 wt% ACP, 50 wt% resin, 10 wt% barium-glass) and

Available online xxx

Sr-ACP (40 wt% ACP, 50 wt% resin, 10 wt% strontium-glass) were compared to the control

Keywords:

ites were characterized using scanning electron microscopy. Flexural strength and modulus

material, resin modified glass ionomer (Fuji II LC capsule, GC, Japan). The fillers and composAmorphous calcium phosphate

were determined using a three-point bending test. Calcium and phosphate ion release from

Flexural strength

ACP based composites was measured using inductively coupled plasma atomic emission

Flexural modulus

spectroscopy.

Ion release

Results. The addition of barium-glass fillers (35.4 (29.1–42.1) MPa) (median (25–75%)) had

Fillers

improved the flexural strength in comparison to the 0-ACP (24.8 (20.8–36.9) MPa) and glass ionomer control (33.1 (29.7–36.2) MPa). The admixture of strontium-glass (20.3 (19.5–22.2) MPa) did not have any effect on flexural strength, but significantly improved its flexural modulus (6.4 (4.8–6.9) GPa) in comparison to 0-ACP (3.9 (3.4–4.1) GPa) and Ba-ACP (4.6 (4.2–6.9) GPa). Ion release kinetics was not affected by the addition of inert fillers to the ACP composites. Significance. Incorporation of barium-glass fillers to the composition of ACP composites contributed to the improvement of flexural strength and modulus, with no adverse influence on ion release profiles. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗ Corresponding author at: Department of Endodontics and Restorative Dentistry, School of Dental Medicine, University of Zagreb, Gunduliceva 5, 10 000 Zagreb, Croatia. Tel.: +385 1 4899 203; fax: +385 1 4802 159. E-mail addresses: [email protected], [email protected] (D. Marovic).

http://dx.doi.org/10.1016/j.dental.2014.06.001 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Marovic D, et al. Reinforcement of experimental composite materials based on amorphous calcium phosphate with inert fillers. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.06.001


2

ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 4 ) xxx–xxx

1.

Introduction

Contemporary restorative dental medicine requires not only esthetic materials which restore tooth structures, but also materials which are able to heal carious-affected hard dental tissue [1]. Over the last few decades, efforts have been increased to produce bioactive materials able to reverse the carious process and to remineralize caries-affected tissue. Amorphous calcium phosphate (ACP) based composite resins are intended as remineralizing/anti-demineralizing agents. ACP is well known as a direct precursor of hydroxyapatite (HA) and has a major role in the biomineralization processes of teeth and bones [2,3]. In an aqueous environment, ACP composite materials release calcium and phosphate ions, providing supersaturating concentrations sufficient to trigger the apatite build-up [4,5] and remineralize demineralized enamel [6,7]. At the same time, those characteristics pose limitations in clinical applications when durability or resistance to crack and deformation upon load are required. Bioactive ACP particles do not have a reinforcing role such as silanized glass or silica fillers, which are used in most of the commercially available dental composite materials. ACP particles are not silanized, as this has an adverse effect on calcium and phosphate ion release without providing any advancement in mechanical properties [8]. Hence, the amount of ACP had to be reduced to the minimal level, which also preserved the remineralizing properties without additionally undermining the mechanical properties. This has lead to the optimization of the composition to 40 wt.% of ACP [4]. Flexural strength of composite materials is mostly dominated by the degree of conversion of the organic matrix [9,10], filler volume [11] and the filler to matrix interfacial relationship [12]. In conventional composites, filler particles increase strength, stiffness and decrease dimensional changes [13], while their silanization ensures better resin to filler interaction and deflects the fracture line. This contributes to higher flexural strength and an overall improvement of mechanical properties [14,15]. The filler load is directly correlated to the flexural strength and modulus, as stated by many authors [11,16–18]. However, fillers without silanization do not provide sufficiently high flexural strength for composite materials [12]. Composites with silanized fillers show higher flexural strength in comparison to those with unsilanized fillers [12,15,16]. A lack of silanized reinforcing fillers in ACP based composite resins have insufficient mechanical properties as a consequence [8]. This group of authors has recently tested the influence of the addition of silanized nanosilica to the ACP composite resin formulation. The study proved that the principle of the admixture of inert fillers is successful in improving flexural strength and increasing the level of calcium and phosphate ion release in comparison to ACP composites without non-releasing fillers [19]. However, it was emphasized that the agglomeration of silica nanoparticles, due to their large surface area, may contribute to the hydrolytic degradation. This in turn enhances ion release, but it also might negatively affect the long-term mechanical properties. Similar conclusions were drawn in another study which examined the effect of various types of

silanized fillers on the degree of conversion of ACP composite resins [20]. Taking into account that the random clustering of ACP particles in resin matrix is also recognized as one of the reasons for diminished strength of ACP composites [5], any additional agglomeration is an undesired property. Different authors agree that the agglomerated particles could act as strength controlling flaws which initiate crack and consequently lead to fracture [21,22]. In contrast, conventional glass microfillers have a lower surface area than nanofillers and do not show the tendency to agglomeration. Thus, it is expected that their addition to ACP composites might provide better interaction to the resin phase and higher strength and modulus. At the same time, it is necessary to examine if the inert glass fillers interfere with the ion release kinetics. The present study was aimed to investigate the effect of glass fillers of various sizes and degrees of silanization on ACP based composite resins. The null-hypothesis was that the addition of fillers does not have an influence on flexural strength, flexural modulus and ion release of ACP based composites.

2.

Materials and methods

2.1.

Materials

2.1.1.

Synthesis of zirconia ACP fillers

The synthesis of Zr-ACP fillers followed the procedure by Skrtic et al. [23]. Zr–ACP precipitated instantaneously in a closed system at 23 ◦ C upon rapidly mixing equal volumes of a 800 mmol/L Ca(NO3 )2 solution, a 536 mmol/L Na2 HPO4 solution that contained a molar fraction of 2% Na4 P2 O7 as a stabilizing component for ACP, and an appropriate volume of a 250 mmol/L ZrOCl2 solution (mole fraction of 10% based on the calcium reactant) [24]. The reaction pH varied between 8.6 and 9.0. The suspension was filtered; the solid phase was washed subsequently with ice-cold ammoniated water and acetone and then lyophilized. ACP fillers were kept in a desiccator to avoid exposure to humidity and premature conversion to apatite until being used in composites.

2.1.2.

Formulation of resin

The experimental resin was the same for all ACP based materials containing 62.8 wt% of ethoxylated bisphenol A dimethacrylate (EBPADMA; Esstech, PA, USA), 23.2 wt% of triethylene glycol dimethacrylate (TEGDMA; Esstech), 10.4 wt% of 2-hydroxyethyl methacrylate (HEMA; Esstech), 2.6 wt% of methacryloxyethyl phthalate (MEP; Esstech), 0.2 wt% of the photo oxidant camphorquinone (CQ; Aldrich, WI, USA) and 0.8 wt% of photo reductant ethyl-4- (dimethylamino) benzoate (4E; Aldrich). Using a magnetic stirrer, the monomers and photo activators were mixed (in the absence of blue light) until a uniform consistency was achieved.

2.1.3.

Tested materials

The compositions of the three ACP composite materials used in this study, as well as the glass-ionomer control are shown in Table 1. The ACP fillers, silanized glass fillers (Table 2) and resin were mixed in lightproof containers in a dual asymmetric centrifugal mixing system (Speed Mixer TM DAC 150 FVZ,


ARTICLE IN PRESS


3


Table 1 – The composition of tested materials. Material

ACP control (0-ACP)

ACP + 10% barium-glass (Ba-ACP)

ACP + 10% strontium-glass (Sr-ACP)

Fuji II LC capsulea (FII; GC, Tokyo, Japan; LOT 0908197)

Resin or liquid

60 wt.% EBPADMA based hydrophilic resin



24 wt.% liquid: PAA, HEMA, proprietary ingredient, 2,2,4-trimethyl hexamethylene dicarbonate, TEGDMA

Fillers

40 wt.% ACP

40 wt.% ACP 10 wt.% Ba fillers

40 wt.% ACP 10 wt.% Sr fillers

76 wt.% (fluoro) alumino silicate glass

a

Composition provided by the manufacturer; EBPADMA, ethoxylated bisphenol A dimethacrylate, PAA, polyacrylic acid, HEMA, 2-hydroxyethyl methacrylate, TEGDMA, triethylene glycol dimethacrylate

Hauschild & Co KG, Hamm, Germany) at 2700 rpm for 135 s, followed by pressing of the composite pastes through a three roller mixer (EXAKT 50, EXAKT, Norderstedt, Germany) three times, to ensure paste homogeneity.

2.2.

Methods

2.2.1.

Characterization of ACP and glass fillers

The micromorphology of all the fillers used in this study was examined by scanning electron microscopy (SEM; Quanta FEG 400; FEI Company, Netherlands). The fillers were fixed by adhesive foil and were not modified or sputtered for imaging. Images were recorded in low vacuum using a large field detector at a working distance of 10 mm and 4 kV electric potential.

2.2.2.

Three-point bending test (3PBT)

Four materials were subjected to the 3PBT: ACP control with no inert fillers (0-ACP), ACP with 10% of barium-glass fillers (Ba-ACP), ACP with 10% of strontium-glass fillers (Sr-ACP) and as control a resin modified glass ionomer FII. Ten samples were made per material. The stainless steel mold with openings 22 mm × 2 mm × 2 mm in size was first coated with silicone spray (Silikonspray, Seidel Medizin GmbH, Buchendorf, Germany). Each side of the mold was then covered with a poly(ethylene terephthalate) film (PET; Frasaco Universal Streifen, Franz Sachs & Co., Tettanang, Germany) and a glass slide. The composite material was dispensed by capsules (KerrHawe Composite Gun; KerrHawe SA, Bioggio, Switzerland). The samples were polymerized with overlapping areas for 200 s from the top and bottom side using a Bluephase C8 LED

curing unit (Ivoclar VIvadent, Schaan, Liechtenstein) in a high power mode with a intensity of 1050 mW/cm2 (mean value from three consecutive light intensity measurements by dental radiometer (CureRite, Dentsply Caulk, Milford, USA)). Glass ionomer samples were immersed in deionized water at room temperature (21 ◦ C) for 15 min after light curing, and then removed from the mold. All samples were polished with 600 grit silicon carbide abrasive paper (Buehler, Lake Bluff, IL, USA) on each side and stored in deionized water for 24 h at 37 ◦ C. The exact dimensions of each sample were measured before subjecting them to the three-point flexural test at the universal testing machine Zwick Z010 (Zwick Roell AG, Ulm, Germany), operated by the testXpert II software system (version 2.1, Zwick Roell AG, Ulm, Germany) at a 20 mm span with a crosshead speed of 0.75 mm/min. The flexural strength (FS) is calculated according to the formula: = 3FL/2bh2 (MPa), where F is the maximum load (force); L is the length of the support span; b is width and h is thickness of the sample. Flexural modulus of elasticity (FM) was determined as: E = FL3 /4bh3 d (GPa), where d represents deflexion of the sample corresponding to the load F.

2.3.

Micromorphology of ACP composites

Composite samples previously used in the 3PBT were polished with silicon carbide paper (Buehler GmbH, Düsseldorf, Germany) with decreasing roughness – 600, 800 and 1200 grit and afterwards with aluminum oxide powder (Buehler) with deceasing particle size – 1.0, 0.3 and 0.005 ␮m with distilled

Table 2 – Specifications of inert fillers added to the ACP test materials, as provided by the manufacturer. Fillers

Composition (approximate values; wt%)

Size (d50/d99 [␮m])

Barium glass (Ba)

SiO2 55.0% BaO 25.0% B2 O3 10.0% Al2 O3 10.0% F 2.00%

0.77/2.28

Strontium glass (Sr)

SiO2 60.0% SrO 15% B2 O3 15.0% Al2 O3 15.0% F 2.00%

0.99/2.95

Silanization (wt%)

Refractive index

Product name/ manufacturer

6

1.52

GM39923 Schott, Germany

3.2

1.50

G018-163 Schott, Germany



4


water as a medium. The SEM imaging of composite samples was performed in low vacuum using large-field (LFD) and solid-state (SSD) detectors operating at an acceleration voltage of 15 kV and a working distance of approximately 10 mm.

2.4.

Ion release test (IR) and ion activity product

Calcium and phosphate IR was measured only for the ACP containing materials. A total of 60 samples were made, 20 samples for each material. Teflon rings (IBG Monoforts Vertriebs GmbH; 5 mm inner diameter, 2 mm high) were filled with ACP composite materials dispensed by capsules and covered from both sides with PET films and glass slides. Each sample was polymerized for 80 s (40 s each side) by Bluephase C8 curing unit in high power polymerization mode. The samples were kept at room temperature in a dark container for 24 h before the immersion of each sample in 8 ml of HEPES-buffered saline solution (pH = 7.4) after which all the samples were kept in closed vials in an incubator at 37 ◦ C. Thus, 8.83 mm2 of sample surface was available for ion release per milliliter of saline solution. Two approaches were used to attain information on IR. In the first, static approach, samples remained in the same vial for 1, 7, 14, or 28 days (five samples/material/time point). In the second, dynamic approach, the same samples that were eluted for one day in static approach were consecutively transferred into a fresh solution after 7, 14 and 28 days. The concentrations of the eluted calcium and phosphate ions were measured in the solutions after removing the samples using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Spectro Flame EOP, Spectro Analytical Instruments, Kleve, Germany). Standard curves were prepared from serial dilutions of Ca and P standard solutions (TraceCert, Fluka, Buchs, Switzerland). The ion activity product (IAP) was computed with respect to stoichiometric apatite Ca10 (OH)2 (PO4 )6 using Chemist Application (version 1,0,1,0; Micromath Research, St. Louis, MO, USA). The thermodynamic stability of immersion solutions containing Ca2+ and PO4 3− ions released from the composite disks was calculated using the Gibbs free energy expression G◦ = −2.303(RT/n)log(IAP/Ksp) (kJ/mole), where R is the ideal gas constant, T is the absolute temperature, n is the number of ions in the IAP (n = 18 for HAP) and Ksp is the thermodynamic solubility product.

2.5.

Statistical analysis

The results of the 3PBT (ten samples/four materials) and IR test data (five samples/three materials/four time points) for static and dynamic conditions were descriptively expressed as medians with 25–75% quantiles. For statistical analysis, Mann–Whitney U-test and Wilcoxon Rank Sum test were applied for the comparison of the independent and the dependent experimental groups, respectively. The level of significance was set to ˛ = 0.05. For multiple comparisons ˛ was adjusted to ˛*(k) = 1−(1 − ˛)1/k applying the Error Rates Method, where k describes the number of pairwise tests to be considered. All analyses were performed in SPSS 19.0 software (SPSS Inc., Chicago, USA).

3.

Results

3.1.

Characterization of ACP and glass fillers

Micromorphology of the filler particles was characterized by SEM. Fig. 1a shows an image of ACP particles, which were widely distributed in size, from submicron to 30–40 ␮m. ACP tends to agglomerate and partially crystallized structures could be observed. Barium-glass fillers (Fig. 1b) were irregular in shape and also contained agglomerates, but the individual particles were clearly distinguishable. This effect could be attributed to the preparation of the sample for SEM examination. In images depicting irregular shaped strontium-glass fillers (Fig. 1c), no agglomeration was microscopically visible.

3.2.

Three-point bending test (3PBT)

Fig. 2a shows the FS of the tested materials. The FS of the BaACP (35.4 (29.1–42.1) MPa), was statistically higher than that of 0-ACP control (24.8 (20.8–36.9) MPa), which amounts to 42.74% increase. No statistical difference was found between FII (33.1 (29.7–36.2) MPa) and the Ba-ACP composite. The FS of Sr-ACP (20.3 (19.5–22.2) MPa) was not statistically different from that of 0-ACP. Fig. 2b demonstrates the significant increase in FM with the addition of inert fillers in comparison to the 0-ACP control (3.9 (3.4–4.1) GPa). Sr-ACP had also higher FM (6.4 (4.8–6.9) GPa) than Ba-ACP (4.6 (4.2–6.9) GPa), but not higher than glass ionomer control FII (9.1 (8.6–11.4) GPa).

3.3.

Micromorphology of ACP composites

Defects due to flexural and compressive forces exerted during 3PBT were visible in all the ACP containing materials, but not in all the parts of the sample. It was noticed that the defects mostly surrounded the ACP filler particles, but rarely went through them, although some of them had visible porous structures. The ACP fillers exhibited two different electron densities, which appear as darker and lighter particles. Some of the darker particles contained defects (Fig. 3a). Both Ba(Fig. 3b) and Sr-glass fillers (Fig. 3c) had highly electron-dense profiles, which corresponds to their composition.

3.4.

Ion release test (IR) and ion activity product

Fig. 4 shows the levels of Ca2+ and PO4 3− ions detected in the saline for the static system. The ion release was generally higher for 0-ACP, but not statistically significant (p > 0.05). One exception was found in the static system at day 14, where a significantly lower amount of Ca2+ was released from Ba-ACP and Sr-ACP than from 0-ACP (p < 0.05). At the same time point, the PO4 3− ion release was also lower for Sr-ACP, but not for Ba-ACP. Generally, the values attained in the dynamic system (Fig. 5) were higher than in the static system for the same time point of measurement (p < 0.05). There was no difference among the tested materials regarding Ca2+ and PO4 3− ion release in the dynamic system, except for Ca2+ release from Sr-ACP at 14th day.




5

Fig. 2 – Flexural strength (a) and modulus (b) for all ACP based composites and glass-ionomer control (median; 25–75% quantiles; different letters indicate statistically different materials).

The IAP and Gibbs free energy calculations (Table 3) showed that in both systems, the attained solution supersaturations were conducive for HAP formation (G◦ < 0) for all tested materials.

4.

Fig. 1 – SEM of: (a) ACP fillers, (b) Ba-glass fillers and (c) Sr-glass fillers (original magnification 10,000×).

Discussion

The present study was aimed to improve mechanical properties of ACP based composite resins by means of the addition of silanized glass fillers to the composition. Additionally, this modification should not interfere with the remineralizing properties, which arise from calcium and phosphate release. The null hypothesis was partially rejected; the results indicate that the flexural strength and modulus of Ba-ACP are superior to the 0-ACP. At the same time, ion release was not inhibited in experimental formulations. It is generally recognized that increased filler level will improve flexural [11,16–18] and compressive strength [25,26], flexural modulus [11] as well as hardness [21,27] of the composite materials. However, studies investigating the influence of complex binary or tertiary filler mixtures on mechanical properties of dental composite resins are rare [12,28,29]. One study has found that the mechanical properties were



6


Fig. 4 – Calcium and phosphate ion concentrations released from ACP-based composites in the static system [median (25–75% quantiles)]. Asterisks denote statistically significant differences (p < 0.05) in comparison to the 0-ACP for a given time point.

Fig. 3 – SEM of tested composite materials: (a) 0-ACP – the arrow points to the defect surrounding the large ACP agglomerate; (b) Ba-ACP – the arrow shows the defect beside the ACP filler particle, smaller Ba-fillers are indicated in the square; (c) Sr-ACP – the arrow shows the defect beside the ACP filler particle, irregular Sr-fillers are indicated in the square.

enhanced in composites with wider granulometric distributions of filler particles, which was explained by a denser packing of the disperse phase and consequent crack deflection as toughening mechanism [30]. Test composites in the present study were comprised of two kinds of fillers, unsilanized ACP fillers which were 6 ␮m in size and irregularly shaped silanized Ba- or Sr-glass fillers (0.77 ␮m and 0.99 ␮m, respectively). Surprisingly, only the introduction of Ba-fillers to the ACP composite formulation increased the flexural strength, whereas the addition of Sr-fillers even lowered the FS, however not statistically significant. This phenomenon could be partially explained by the fact that smaller fillers tend to have a more pronounced effect on the improvement of the material’s strength [29,31]. The smaller Ba-fillers were able to distribute more homogeneously between the larger ACP particles and therefore reinforce the resin matrix. It was evident from the SEM images of the ACP composites that the cracks were mostly formed around larger ACP agglomerates, clearly due to the weak interaction of ACP with the resin. In the present study, the Ba-ACP material containing glass particles with a degree of silanization of 6% achieved higher FS than the corresponding Sr-ACP material with inert fillers, with a degree of silanization of 3.2%. At first, it may seem that




Fig. 5 – Cumulative values of calcium and phosphate ion concentrations released from ACP-based composites in the dynamic system [median (25–75% quantiles)]. Asterisk denotes statistically significant difference (p < 0.05) in comparison to the 0-ACP for a given time point.

the difference in the degree of silanization may be partially responsible for the difference in FS; however, previous studies have found that the amount of the silanizing agent does not influence the strength of the material [32,33]. As the filler size decreases, its surface area increases, as does the amount of silane needed for optimal coverage [34]. Our calculations (based on Ref. [34]) show that the silanization percentages obtained for the inert fillers used in this study are more than sufficient to achieve minimum uniform coverage of the filler surface needed for a strong and durable interphase. Larger Srfillers have a surface area of approximately 8 m2 /g and need less silane than smaller Ba-fillers with a surface area of 13 m2 /g (manufacturer’s data). Therefore, a higher FS in Ba-ACP than in Sr-ACP could only be a consequence of their size and the larger filler/resin contact area, not their degree of silanization. The modulus of elasticity is an important factor which greatly influences the clinical behavior of a composite material. Although both Ba-ACP and Sr-ACP showed a significant improvement of FM in relation to the control ACP without inert fillers, Sr-ACP was also higher than Ba-ACP and achieved values close to those of the conventional hybrid composites [11]. Increase of the filler content accounts for the increase of FM of ACP-based composites after addition of glass-fillers.

7

Additionally, the difference between FM values of Ba-ACP and Sr-ACP could be attributed to the higher degree of conversion and the higher crosslinking density of the polymer network in Sr-ACP than in Ba-ACP, which was demonstrated in previous studies [20,34]. This proof-of-principle study showed that the introduction of inert fillers to the formulation of ACP composites is beneficial for their mechanical properties. The rate of improvement is not high, but the quantity of reinforcing fillers added is also relatively small. In comparison to similar calcium phosphate based materials, FS is approximately the same as for the resin-based Ca–PO4 cement [35], whereas a hydroxyapatitebased composite and an ACP nanocomposite have FS two to three times higher than those reported here [36,37]. In the current form, ACP composites could be used as a base material under restorations in situations where complete caries removal could lead to pulp exposure. Improvement of the mechanical properties of the ACP composite resins and preserving the sufficient ion release at the same time are contradictory goals. Water diffusion may cause deleterious effects on the material’s strength and improve the ion release kinetics. Therefore, it is important to accurately determine the optimal composition and the amount of silanized fillers that can be added to ACP composites. The results of the static and dynamic approach for ion release testing indicate that addition of silanized glass fillers does not inhibit the release of calcium and phosphate ions needed for the remineralization of caries processes in the tooth. The Baand Sr-fillers appear to have no detrimental effect on Ca2+ and PO4 3− ion release from ACP composites. Additionally, dynamic system values were almost two times higher than the static values for all materials and time points after the first day. This might suggest that the potential for ion release is high. Hypothetically, when placed at a demineralized tooth substrate, this group of materials could provide sustained release of ions if the Ca2+ and PO4 3− ion concentrations are constantly decreasing. When compared to other studies investigating the ion release from ACP composites, it can be seen that the concentrations they obtained are higher. This can easily be explained by a larger volume of HEPES-buffered saline solution than in this study (100 ml in previous studies vs. 8 ml in this study) [4]. The constant stirring of the solution has probably also contributed to the higher calcium and phosphate concentrations than in the present study. Also, it is important to emphasize that the ion release test in this study was conducted in neutral pH conditions. Other studies investigating similar ACP-containing materials have found that ion release is even higher in low pH conditions, as found in the oral cavities of high-caries risk individuals [36]. However, mechanical properties deteriorate more rapidly in an acidic environment. Therefore, it seemed to be an interesting idea to reduce the quantity of releasing fillers and increase the reinforcing fillers [36]. This would minimize the unwanted effects low pH has on a material’s strength and maintain sufficient levels of ion release for a time when it is most needed. Further investigation of ACP-based composite materials in various pH conditions could lead to the optimization of their composition by reducing the amount of ACP necessary for remineralization. Variations in the quantity of ACP fillers and reinforcing


ARTICLE IN PRESS


8


Table 3 – Ion activity product (IAP) and thermodynamic stabilitya (G◦ ) of ion release solutions containing Ca2+ and PO4 3− at different time intervals [mean value (SD)]. Material

Immersion time (days)

Static system

Dynamic system

IAP (SD)

G◦ (kJ/mol)(SD)

IAP (SD)

G◦ (kJ/mol)(SD)

0-ACP

1 7 14 28

108.7 (1.3) 104.2 (1.4) 101.1 (0.4) 100.8 (0.9)

−[2.7 (0.5)] −[4.1 (0.4)] −[5.1 (0.1)] −[5.2 (0.3)]

108.7 (1.3) 101.9 (1.0) 98.8 (1.1) 96.8 (0.9)

−[2.7 (0.5)] −[4.8 (0.3)] −[5.9 (0.3)] −[6.5 (0.3)]

Ba-ACP

1 7 14 28

110.3 (2.1) 104.5 (1.0) 102.0 (0.9) 100.7 (0.8)

−[2.2 (0.4)] −[4.0 (0.3)] −[4.8 (0.3)] −[5.2 (0.2)]

110.3 (1.5) 103.2 (1.3) 100.2 (1.2) 97.7 (1.1)

−[2.2 (0.3)] −[4.4 (0.4)] −[5.4 (0.4)] −[6.2 (0.3)]

Sr-ACP

1 7 14 28

109.5 (0.7) 104.6 (1.2) 102.6 (0.9) 100.9 (1.0)

−[2.4 (0.2)] −[4.0 (0.4)] −[4.6 (0.2)] −[5.2 (0.3)]

110.5 (1.7) 103.6 (1.7) 101.2 (1.5) 98.3 (2.0)

−[2.1 (0.3)] −[4.3 (0.5)] −[5.1 (0.5)] −[6.0 (0.6)]

a

Negative G◦ values indicate that the solution is supersaturated with respect to stoichiometric hydroxyapatite Ca10 (OH)2 (PO4 )6 . The thermodynamic solubility product of Ca10 (OH)2 (PO4 )6 used in calculations was pKsp = 117.0.

fillers could enhance mechanical properties and enable these materials to be used in load-bearing areas.

5.

Conclusion

The experimental ACP material with 10% of Ba-glass fillers had the highest flexural strength among the ACP-based materials and there was no statistically significant difference from the commonly used glass ionomer. This effect was attributed to the small size of filler particles, larger resin/filler interface and the resistance of composite resins to crack initiation and propagation upon load. Flexural modulus was also improved by introducing inert fillers to the composition of ACP-based resins, especially 10% of Sr-fillers. Finally, the addition of inert glass fillers, used in small amounts of 10 wt.% have minor influence on ion release profiles of ACP composites, but still sufficient to conduce the formation of hydroxyapatite.

Acknowledgments This study was supported by the Ministry of Science, Education and Sports, Republic of Croatia (Grants nos. 065-0352851-0410 and 098-0982904-2952), Croatian Science Foundation, Forschungsgemeinschaft Dental, University Hospital Regensburg, University of Regensburg and NIDCR (DE13169). Generous contribution of the fillers from Schott (Mainz, Germany) and the monomers from Esstech (Essington, PA, USA) and Sigma–Aldrich (Milwaukee, WI, USA) are gratefully acknowledged.

references

[1] Anusavice KJ. Present and future approaches for the control of caries. J Dent Educ 2005;69:538–54. [2] Dorozhkin SV. Amorphous calcium (ortho)phosphates. Acta Biomater 2010;6:4457–75.

[3] Gajjeraman S, Narayanan K, Hao J, Qin C, George A. Matrix macromolecules in hard tissues control the nucleation and hierarchical assembly of hydroxyapatite. J Biol Chem 2007;282:1193–204. [4] O’Donnell JN, Langhorst SE, Fow MD, Antonucci JM, Skrtic D. Light-cured dimethacrylate-based resins and their composites: comparative study of mechanical strength, water sorption and ion release. J Bioact Compat Polym 2008;23:207–26. [5] Lee SY, Regnault WF, Antonucci JM, Skrtic D. Effect of particle size of an amorphous calcium phosphate filler on the mechanical strength and ion release of polymeric composites. J Biomed Mater Res B: Appl Biomater 2007;80:11–7. [6] Skrtic D, Hailer AW, Takagi S, Antonucci JM, Eanes ED. Quantitative assessment of the efficacy of amorphous calcium phosphate/methacrylate composites in remineralizing caries-like lesions artificially produced in bovine enamel. J Dent Res 1996;75:1679–86. [7] Langhorst SE, O’Donnell JN, Skrtic D. In vitro remineralization of enamel by polymeric amorphous calcium phosphate composite: quantitative microradiographic study. Dent Mater 2009;25:884–91. [8] Skrtic D, Antonucci JM, Eanes ED, Eidelman N. Dental composites based on hybrid and surface-modified amorphous calcium phosphates. Biomaterials 2004;25:1141–50. [9] Calheiros FC, Daronch M, Rueggeberg FA, Braga RR. Degree of conversion and mechanical properties of a BisGMA:TEGDMA composite as a function of the applied radiant exposure. J Biomed Mater Res B: Appl Biomater 2008;84:503–9. [10] Tarle Z, Knezevic A, Matosevic D, Skrtic D, Ristic M, Prskalo K, et al. Degree of vinyl conversion in experimental amorphous calcium phosphate composites. J Mol Struct 2009;924–926:161–5. [11] Ilie N, Hickel R. Investigations on mechanical behaviour of dental composites. Clin Oral Investig 2009;13:427–38. [12] Ikejima I, Nomoto R, McCabe JF. Shear punch strength and flexural strength of model composites with varying filler volume fraction, particle size and silanation. Dent Mater 2003;19:206–11. [13] Ferracane JL. Current trends in dental composites. Crit Rev Oral Biol Med 1995;6:302–18. [14] Drummond JL. Degradation, fatigue, and failure of resin dental composite materials. J Dent Res 2008;87:710–9.




[15] Arikawa H, Kuwahata H, Seki H, Kanie T, Fujii K, Inoue K. Deterioration of mechanical properties of composite resins. Dent Mater J 1995;14:78–83. [16] Karmaker A, Prasad A, Sarkar NK. Characterization of adsorbed silane on fillers used in dental composite restoratives and its effect on composite properties. J Mater Sci Mater Med 2007;18:1157–62. [17] Chung KH, Greener EH. Correlation between degree of conversion, filler concentration and mechanical properties of posterior composite resins. J Oral Rehabil 1990;17:487–94. [18] Ferracane JL, Berge HX, Condon JR. In vitro aging of dental composites in water – effect of degree of conversion, filler volume, and filler/matrix coupling. J Biomed Mater Res 1998;42:465–72. [19] Marovic D, Tarle Z, Hiller K, Müller R, Ristic M, Rosentritt M, et al. Effect of silanized nanosilica addition on remineralizing and mechanical properties of experimental composite materials with amorphous calcium phosphate. Clin Oral Investig 2014;18:783–92. [20] Marovic D, Tarle Z, Ristic M, Music S, Skrtic D, Hiller K, et al. Influence of different types of fillers on the degree of conversion of ACP composite resins. Acta Stomatol Croat 2011;45:231–8. [21] Hosseinalipour M, Javadpour J, Rezaie H, Dadras T, Hayati AN. Investigation of mechanical properties of experimental Bis-GMA/TEGDMA dental composite resins containing various mass fractions of silica nanoparticles. J Prosthodont 2010;19:112–7. [22] Garoushi S, Lassila LV, Vallittu PK. Influence of nanometer scale particulate fillers on some properties of microfilled composite resin. J Mater Sci Mater Med 2011;22:1645–51. [23] Skrtic D, Antonucci JM, Liu DW. Ethoxylated bisphenol dimethacrylate-based amorphous calcium phosphate composites. Acta Biomater 2006;2:85–94. [24] Skrtic D, Antonucci JM, Eanes ED. Amorphous calcium phosphate-based bioactive polymeric composites for mineralized tissue regeneration. J Res Natl Inst Stand Technol 2003;108:167–82. [25] Kim KH, Ong JL, Okuno O. The effect of filler loading and morphology on the mechanical properties of contemporary composites. J Prosthet Dent 2002;87:642–9.

9

[26] Zimmerli B, Strub M, Jeger F, Stadler O, Lussi A. Composite materials: composition, properties and clinical applications. A literature review. Schweiz Monatsschr Zahnmed 2010;120:972–86. [27] Neves AD, Discacciati JA, Orefice RL, Jansen WC. Correlation between degree of conversion, microhardness and inorganic content in composites. Pesqui Odontol Bras 2002;16:349–54. [28] Miyasaka T, Yoshida T. Effect of binary and ternary filler mixtures on the mechanical properties of composite resins. Dent Mater J 2000;19:229–44. [29] Miyasaka T. Effect of shape and size of silanated fillers on mechanical properties of experimental photo cure composite resins. Dent Mater J 1996;15:98–110. [30] Ornaghi BP, Meier MM, Rosa V, Cesar PF, Lohbauer U, Braga RR. Subcritical crack growth and in vitro lifetime prediction of resin composites with different filler distributions. Dent Mater 2012;28:985–95. [31] Suzuki S, Leinfelder KF, Kawai K, Tsuchitani Y. Effect of particle variation on wear rates of posterior composites. Am J Dent 1995;8:173–8. [32] Sideridou ID, Karabela MM. Effect of the amount of 3-methacyloxypropyltrimethoxysilane coupling agent on physical properties of dental resin nanocomposites. Dent Mater 2009;25:1315–24. [33] Debnath S, Ranade R, Wunder SL, McCool J, Boberick K, Baran G. Interface effects on mechanical properties of particle-reinforced composites. Dent Mater 2004;20: 677–86. [34] Karabela MM, Sideridou ID. Synthesis and study of properties of dental resin composites with different nanosilica particles size. Dent Mater 2011;27:825–35. [35] Dickens SH, Flaim GM, Takagi S. Mechanical properties and biochemical activity of remineralizing resin-based Ca–PO4 cements. Dent Mater 2003;19:558–66. [36] Xu HH, Moreau JL, Sun L, Chow LC. Nanocomposite containing amorphous calcium phosphate nanoparticles for caries inhibition. Dent Mater 2011;27:762–9. [37] Santos C, Luklinska ZB, Clarke RL, Davy KW. Hydroxyapatite as a filler for dental composite materials: mechanical properties and in vitro bioactivity of composites. J Mater Sci Mater Med 2001;12:565–73.


Remineralizing amorphous calcium phosphate based composite resins: the influence of inert fillers on monomer conversion, polymerization shrinkage, and microhardness.

Enzyme-assisted calcium phosphate biomineralization on an inert alumina surface.

Toughening Fe-based Amorphous Coatings by Reinforcement of Amorphous Carbon.

Epoxy Resin Composite Based on Functional Hybrid Fillers.

chitosan composite on bone healing in rats: calcium phosphate induces osteon formation.

Color stability of nanocrystallite-treated and silicate-treated fillers of calcium phosphate composite resin: an in vitro study.

Effect of calcium phosphate-based fillers on the structure and bonding strength of novel gelatin-alginate bioadhesives.

Magnetic Sensors Based on Amorphous Ferromagnetic Materials: A Review.

Effect of Fluoride, Casein Phosphopeptide-Amorphous Calcium Phosphate and Casein Phosphopeptide-Amorphous Calcium Phosphate Fluoride on Enamel Surface Microhardness After Microabrasion: An in Vitro Study.

Reinforcement of injectable calcium phosphate cement by gelatinized starches.

Atomic structure of intracellular amorphous calcium phosphate deposits.

Calcium and phosphate release from resin-based materials containing different calcium orthophosphate nanoparticles.

Glycochenodeoxycholic acid inhibits calcium phosphate precipitation in vitro by preventing the transformation of amorphous calcium phosphate to calcium hydroxyapatite.

Amorphous calcium phosphate phase-mediated crystal nucleation kinetics and pathway.

Shear-mediated crystallization from amorphous calcium phosphate to bone apatite.

Comparative investigation of phosphate-based composite cathode materials for lithium-ion batteries.

polylactide composite coated tantalum scaffold: facile preparation, fast biomineralization and subchondral bone defect repair application.

Amorphous Calcium Phosphate-Based Bioactive Polymeric Composites for Mineralized Tissue Regeneration.

Sustained delivery of calcium and orthophosphate ions from amorphous calcium phosphate and poly(L-lactic acid)-based electrospinning nanofibrous scaffold.

On Structure and Properties of Amorphous Materials.

A safer disposal of hazardous phosphate coating sludge by formation of an amorphous calcium phosphate matrix.

Osteoconductive property of a mechanical mixture of octacalcium phosphate and amorphous calcium phosphate.

The Influence of Processing and the Polymorphism of Lignocellulosic Fillers on the Structure and Properties of Composite Materials-A Review.

Effect of phospholipids on the transformation of amorphous calcium phosphate to hydroxapatite in vitro.