Evaluation of reinforced strength and remineralized potential of resins with nanocrystallites and silica modified filler surfaces

Materials Science and Engineering C 33 (2013) 1143–1151

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Evaluation of reinforced strength and remineralized potential of resins with nanocrystallites and silica modified filler surfaces Wen-Cheng Chen a,⁎, Hui-Yu Wu b, Hong-Sen Chen b, 1 a b

Advanced Medical Devices and Composites Laboratory, Department of Fiber and Composite Materials, College of Engineering, Feng Chia University, Taichung, 40724, Taiwan, ROC Department of Oral Hygiene, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 13 August 2012 Received in revised form 8 October 2012 Accepted 1 December 2012 Available online 9 December 2012 Keywords: Nanocrystallites Resin composites Calcium phosphate Silanization Mineralization

a b s t r a c t Surface-modified dicalcium phosphate anhydrous particles that were treated with an ion-rich solution and a silane-coupling agent were evaluated as fillers for resin composites. The physiochemical properties of these composites were characterized. The properties of the specimens as reinforcements, which were modified using various surface conditions and 30% and 50% filler to composite mass ratios (30% and 50%) were measured before and after they were immersed in water for 24 h. All groups were of the same strength and showed no significant changes after immersion. However, the groups showed a significant increase in the modulus after 24 h of immersion. The filler surfaces with nanocrystallites had the highest modulus, whereas the fillers treated with silanization had the lowest ion concentration in the solution and highest remineralization ability after immersion. The strength and brittleness were increased by the modified fillers with nanocrystallites on the surfaces and by the increased amount of fillers in the resin composites. Filler surfaces that were modified with silica hindered interfacial interactions and consequently had better flexibility and less brittleness during the light-curing process. Surface modifications of reinforced particles using nanocrystallites and silica films have superior potential applications in restorative medicine. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The two major challenges when using dental restorative composite resins are secondary caries and restoration fractures [1]. With the advances in nanotechnology and biomaterials, novel dental composite resins have been developed to prevent secondary caries and, moreover, to regenerate tooth structure [2]. Calcium phosphates (CaPs) have been studied and used as reinforcements in the dental resin matrix for the prevention of caries in composite resin fillers. For example, hydroxyapatite (HA; Ca10(PO4)6(OH)2), is the structural prototype of the major mineral component in the enamel and the cortical parts of teeth and bones. HA is the final stable product in the precipitation of high concentrations of calcium (Ca 2+) and phosphate ions, namely, H2PO4−, HPO42−, and PO43−, existing in solutions from neutral or basic environments (pH of 7 to 9 at 37 °C) [3]. Dental composite resins composed of CaP fillers in a resin matrix have been proposed; these have the potential to remineralize carious enamel and dentin lesions in vitro [4–6]. The fillers have similar

⁎ Corresponding author at: Department of Fiber and Composite Materials, Feng Chia University, 100, Wenhwa Rd., Seatwen, Taichung, 40724, Taiwan, ROC. Tel.: +886 4 24517250x3413; fax: +886 4 24514625. E-mail addresses: [email protected], [email protected] (W.-C. Chen). 1 Equal contribution to correspondence. 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.12.022

functions as those of the re-mineralizing agents in anti-sensitivity toothpaste. These agents deliver supersaturated ions to the surface to promote the restoration of demineralized areas. Restorative materials that contain CaPs as reinforcements in a polymer matrix are referred to as “smart composites” in several studies [7]. Skrtic et al. [8] evaluated the mechanical strength of amorphous CaPs and found that this weak filler could only be used as a preventive sealant. Another group used HA combined with silica whiskers as the filler for dental resin composites and substantially improved the mechanical properties of composites filled with calcium phosphate cement (CPC) particles without whiskers [9]. Recent studies performed by Xu et al. [10–14] demonstrated the mechanical properties and the ion-releasing ability at different pH values of tetracalcium phosphate (TTCP) and anhydrous dicalcium phosphate (DCPA), which are the main ingredients of CPC, when these were mixed separately with silica whisker fillers of varied particle sizes and filler-to-matrix mass ratios. The results suggest that the combination of releasing nano-fillers with stable and strong reinforcing fillers may yield a resin matrix composite with both stress-bearing and caries-inhibiting capacities; however, such combined materials are not yet available in dental restoration to date [15]. Difficulties arise in the long-term remineralization abilities of the materials and their performances when used in a dynamic oral environment. Recent studies have investigated the effects of relatively small amounts of CaP fillers, which were hardly exposed to the top surfaces of the restorative resins [9–15]. Furthermore, their dissolution

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causes rapid ion exhaustion prior to surface layer remineralization. In our previous studies [16,17], we characterized a novel modified surface treatment method using TTCP to reinforce CaP bone cements. The few noteworthy studies on the remineralization potential of composite resin on DCPA ceramics recommended the use of reinforcement and modified resin bonding agents that contain hydroxyl groups and silicates for increased, durable bond strengths. The objectives of the present study were to characterize the physiochemical effects of different amounts of the filler based on the unmodified and modified surface pretreatment of DCPA with nanocrystallites or further silanization and to evaluate the remineralization abilities at the early stages of immersion. These applications are intended to minimize or prevent the cycle of secondary caries and restoration failure.

2.2. Silica modification A solution of 100 mL cyclohexane solvent with 4 (v/v%) (3-mercaptopropyl) trimethoxysilane (MPTMS) and 2 (v/v%) n-propylamine (Alfa Aesar GmbH & Co. KG, Karlsruhe, Germany) was used as the saline-coupling agent for the silica modification of the particle surfaces. DCPA powder (5 g) was added to the colloidal solution with rapid agitation for 30 min at room temperature. The temperature was then raised to 60 °C, and the solvent was further removed by drying the samples in a vacuum. The effects of the deposited organosilane films on the filler surface were evaluated by Fourier-transform infrared spectroscopy (FT-IR) analysis (Nicolet 6700; Thermo, MA, USA). 2.3. Resin composites

2. Materials and methods 2.1. Treatment of nanocrystallite film The particle surfaces of the DCPA crystallites were treated using our previously developed method with some modifications [18]. DCPA powder (CaHPO4, Alfa Aesar GmbH & Co. KG, Karlsruhe, Germany) was used, which had a particle distribution size of (1 to 3) μm and a purity of 98%. Crystallite formation on the filler surfaces was performed using 5 g of DCPA powder mixed in 40 mL saturated solution with a molar calcium-to-phosphate ratio of 2.0 and operated at a stabilized lower pH value between 4.5 and 5.0 for 20 min at room temperature (26 °C). After the solid film of nanocrystallites was deposited on the powders, the particles were vacuum filtered, washed twice with deionized water, rinsed with 95% alcohol, and then dried in an oven. To examine the effects of the surface coating on the particles, a drop of the surface-treated powder that was dispersed in ethanol was placed on a #325 mesh carbon grid (3 mm in diameter) and allowed to dry. The specimen was then coated with a thin carbon film for electrical conductivity during transmission electron microscopy (TEM). A JEOL JEM-3010 electron microscope was used at 200 kV. The observed patterns were indexed, identified in detail, measured, and compared with the d value in the Joint Committee on Powder Diffraction Standards (JCPDS). The observed diffraction patterns were compared with those of known CaP phases and other generated electron diffraction.

The resin matrix was mainly composed of bisphenol-A diglycidyl methacrylate (bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) monomers. The free radical polymerization initiation system in the photocurable composite resin used the light initiator camphoroquinone (CQ, Sigma-Aldrich Co., Buchs, Switzerland) with dimethylaminoethyl methacrylate (DMAEMA) as the accelerator and butylated hydroxyl toluene (BHT) as the photostabilizer. The chemicals used in the present study were all obtained from Sigma-Aldrich, and their molecular formulas are listed in Table 1. The resin matrix was composed of 48.75 wt.% bis-GMA, 48.75 wt.% TEGDMA, 1.0 wt.% CQ, 1.0 wt.% DMAEMA, and 0.50 wt.% BHT. To form a resin composite, the organic matrix and inorganic fillers of the unmodified and surface-treated DCPA featuring the filler-based resin composites with filler-to-composite ratios of 30 wt.% and 50 wt.% were prepared in a dark room and mixed with a magnetic stirrer until the colloid was well formed. The resin composite was uniformly mixed and then loaded into a 10 mL syringe for injection into the mold. The syringe was covered in aluminum foil to prevent any possible reactions induced by room light. The resin composites were used within 24 h after preparation. 2.4. Physiochemical analyses To evaluate the effects on the strength of the particle surfaces that were modified with (w) or without (w/o) nanocrystallites, silica, and

Table 1 Materials used as the matrix in the dental composite resins. Materials, abbreviations and chemical formula Bisphenol A diglycidyl methacrylate, Bis-GMA (2,2-bis[p(2′hydroxy-3′-methacryloxypropoxy)phenyl] propane)

Triethylene glycol dimethacrylate, TEGDMA 2-[2-[2-(2-methylprop-2-enoyloxy)ethoxy]ethoxy]ethyl 2-methylprop-2-enoate

Camphoroquinone, CQ 1.7.7-trimethylbicyclo-[2,2,1]-hepta-2,3-dione

Imethylaminoethyl methacrylate, DMAEMA Methacrylic acid 2-(dimethylamino)ethyl ester

Buthylated hydroxyl toluene, BHT 2,6-Bis(1,1-dimethylethyl)-4-methylphenol

Structural formula

W.-C. Chen et al. / Materials Science and Engineering C 33 (2013) 1143–1151 Table 2 Test groups of different preparation and measurement procedures. Tested sample prepared procedures

Nomenclature of groups

The untreated group without surface modified DCPA particle The group treated with silanized DCPA particle The group treated with nanocrystallites on the DCPA surface The group of further treated with silane coupling agent on the surfaces of w/ nanocrystal samples

w/o mod w/ silica w/ nanocrystal w/ nanocrystal/ silica

combined coatings, with 30 wt.% and 50 wt.% filler to composite ratios. The sample mold for the diametral tensile strength (DTS) test was prepared with an opening that was 6 mm in diameter and 3 mm in depth. The curing procedures were divided into different steps after injecting the samples into the mold to validate the successful curing. The test resin sample was placed into a cylindrical metal mold. The light-cure process was set at the top and bottom sites in the mold for 40 s each; both the left and right sides of the demolded samples were likewise cured for 40 s each using a light-cure machine (Demetron Optilux 401; Kerr, USA). The test samples were divided into two groups: (1) the dried-sample group that was tested immediately after curing, and (2) the wet-sample group that was prepared by further immersing the cured dried sample into deionized water at 37 °C for 24 h. The wet-sample group was used to evaluate the remineralization ability of the composite resins, with the immersion ratio set at 1 g of sample to 10 mL of liquid. The nomenclature of the four test groups is listed in Table 2. The strengths of the different specimens were measured immediately using a desktop mechanical tester (Lloyd Instruments, Tokyo, Japan) at a crosshead speed of 2.0 mm/min. The specimens were diametrically compressed, thereby introducing tensile stress into the material in the plane of the force application until 40% of sample deformation had occurred. The ultimate tensile strength was measured using stress–strain curves. The DTS was calculated using the formula [19–22]: DTS ¼ 2P=πDT

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where P is the load applied, D is the diameter of the cylinder, and T is the thickness of the cylinder. Six duplicate specimens were prepared and analyzed for each group (n = 6) to compute the modulus of elasticity that corresponds to a yield point offset within 2% to 5%. Remineralized topographies and the composition of elements on topographies were examined using field-emission scanning electron microscopy (SEM; Hitachi S-3000N; Hitachi, Tokyo, Japan) combined with energy dispersive X-ray spectrometry (EDS; Horiba EX220, Japan) to study the fractured surfaces. 2.5. Quantitative analysis of ions releasing The samples were immersed in deionized water at 37 °C for 24 h, and the solutions were processed for inductively coupled plasma atomic emission spectroscopy (ICP-AES) using an ICP-AES unit (Optima 2000DV; Perkin-Elmer Instruments, Shelton, CT, USA). The testing principles and practices of validation of analytical procedures were based on the principle of machine commended. The wavelength of the emission lines allows analysis in the (160 to 900) nm range. 2.6. Statistical analyses The statistical analyses of the results used one-way and three-way ANOVA to investigate the effects of different factors. The Student's t-test was used to evaluate the significant group comparisons between different populations with respect to the filler amount, DTS, and modulus value using the JMP 9.0 software (SAS Institute, Inc., Cary, NC, USA). In all cases, the results were considered to be significantly different when p b 0.05. 3. Results and discussion 3.1. Characterized the fillers The bright field (BF) and dark field (DF) TEM micrographs (Fig. 1a and b) show that the DCPA surface was saturated with nano-sized

Fig. 1. Bright-field (a) and dark-field (b) images, as well as diffraction patterns (c) of the initial form of DCPD nanocrystallites on DCPA particle surfaces. All rings as indexed denote the presence of DCPD crystallites in a DCPA matrix.

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crystallites after surface modification of the powder in the ion-rich solution; the calcium-to-phosphate molar ratio of which was 2.0. According to Zhang et al. [23], during the initial stages prior to the nucleation burst for apatite formation, the phase selectivity and stability affects the more acidic phases of DCPD such that the transformation into apatite is delayed. The DCPD nanocrystallites were probably formed in the highly ionic hydrated layers of the DCPA surface. The analyzed selected area diffraction (SAD) pattern confirmed the formation of DCPD nucleation at the saturated state (Fig. 1c). Although DCPA and DCPD have many crystallographic planes with similar d values, the clear presence of a SAD pattern indicates the presence of a DCPD phase that reprecipitated onto the DCPA surfaces. The solid film on the particle surfaces was due to the DCPD phase that was more stable and dissolved more slowly than DCPA in a relatively basic solution [24]. DCPA was then allowed sufficient time to react in an aqueous alkaline solution at higher Ca2+ strength, spotted ring patterns of DCPD were formed and dominated the precipitation phase. This phenomenon indicated that great quantities of DCPD nanocrystal clusters were largely precipitated on the filler (DCPA) surfaces. Similar results wherein DCPD crystallized from the metastable supersaturated solutions after seeding with CaPs have been shown by many studies [25–27]. DCPD and DCPA are relatively highly soluble CaPs that are convertible into octacalcium phosphate (OCP, Ca8(HPO4)2(PO4)4·5H2O) and apatite when soaked in the solutions [26]. A silane-coupling agent is a bifunctional molecule that is capable of reacting with the silanol groups of ceramic fillers to form Si―O―Si bonds with filler surfaces and with the resin phase through graft copolymerization [8,28,29]. MPTMS is a well-recognized coupling agent that is commercially available for the grafting of thin solid silica films onto mineral surfaces, which allows a substantial improvement of material properties. The FT-IR spectra of the original DCPA fillers and the filler surfaces that were further modified with MPTMS are shown in Fig. 2. The C C band at (1638 to 1646) cm −1 in both groups, the Si―O―Si bands at (1000 to 1250) cm −1 of MPTMS, and in the P―O―P stretching mode of DCPA and DCPD were not unique [10,30]. Notably, the filler surfaces that did not go through the silanization process did not show any absorbed spectrum within the (2700 to 2900) cm −1 range, except for the CH and alkoxylate bonds of the MPTMS-modified group within the unique signals at (2850 to 2950) cm −1. These results indicated the initial organosilane adherence to the particle surfaces through hydrogen bonding between the silanol groups and the phosphate group of

Fig. 3. DTS with y-axis labeling on the right and modulus with y-axis labeling on the left of 30 wt.% (a) and 50 wt.% (b) ratios of various treatment fillers in the resin matrix before and after immersion.

DCPA or DCPD. Therefore, the formation of silane molecules on the particle surface film was further confirmed as presented in the FT-IR analyses (Fig. 2).

Fig. 2. FT-IR spectra of DCPA particles that were untreated and treated with nanocrystallites and further treated with the silane coupling agent MPTMS.

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Table 3a Statistical analysis of DTS, modulus, and performance of different surface modification processes before and after 24 h immersion (n = 6). Measurements

Student's t-test statistical analysis: p-value/group comparisons DTS

Modulus

Samples before immersion w/o mod 0.0124c 30% >50% w/ silica 0.183 Nonea w/ nanocrystal 0.122 Nonea w/ nanocrystal/silica 0.0336c 30% >50% Samples after immersion w/o mod 0.0156c 30% >50% w/ silica 0.0212c 30% >50% w/ nanocrystal 0.0058c 30% >50% w/ nanocrystal/silica b0.0001c 30% >50%

0.0003c 50% > 30% b0.0001c 50% > 30% b0.0001c 50% > 30% 0.1428 Nonea 0.0068c 50% > 30% 0.0656 Nonea b0.0001c 50% > 30% 0.0369c 50% > 30%

Tukey's pairwise comparison: p-value/group comparisonsb Measurements

30 wt.%

50 wt.%

Samples before immersion DTS 0.2467 Nonea Modulus b0.0001c S > NS, U > N Samples after immersion DTS 0.0032c U = S, U > N, NS; S = N = NS Modulus b0.0001c N> U = NS = S Abbreviations w/o mod: U; w/ silica: S; w/ nanocrystal: N; w/ nanocrystal/silica: NS

0.0038c S=N=NS; NS>U; N=U b0.0001c N >U = NS, S 0.2512 Nonea b0.0001c N >U = NS >S

Fig. 4. Amount of calcium (a) and phosphate (b) ions released into deionized water after samples were immersed for 24 h. The untreated group has significantly higher amounts than other groups.

3.2. Strengths before and after the immersion

a

This symbol indicates the testing groups shown to not be significantly different (p > 0.05). b Here are the results of all pairwise comparisons (subset for alpha = 0.05). c The groups shown to be significantly different (p b 0.05).

The tendencies of DTS and modulus of elasticity for the 30 wt.% and 50 wt.% fillers in the different surface modified groups are shown in Fig. 3. Statistical analyses results are summarized in Tables 3a and 3b. In general, the amount of filler mainly affected the DTS and modulus with the statistical analysis within groups shown

Table 3b Variation of different surface modification processes in sample conditions before and after 24 h immersion based on the Student's t-test of the DTS and modulus within groups with 30 wt.% and 50 wt.% reinforced filler (n = 6). Groups

Testing

Statistical analysis

30 wt.%

50 wt.%

w/o mod

DTS

p-Value t-Test of p-Value t-Test of p-Value t-Test of p-Value t-Test of p-Value t-Test of p-Value t-Test of p-Value t-Test of p-Value t-Test of

0.0888 Nonea b0.0265b After immersion > before immersion 0.3540 Nonea b0.0001b After immersion > before immersion 0.4672 Nonea 0.0016b After immersion > before immersion 0.0888 Nonea 0.0002b After immersion > before immersion

0.1338 Nonea 0.1774 Nonea 0.3624 Nonea b0.0001b After immersion > before immersion 0.6453 Nonea 0.0002b After immersion > before immersion 0.1338 Nonea b0.0001b After immersion > before immersion

Modulus w/ silica

DTS Modulus

w/ nanocrystal

DTS Modulus

w/ nanocrystal/silica

DTS Modulus

a b

two populations two populations two populations two populations two populations two populations two populations two populations

This symbol indicates the testing groups shown to not be significantly different (p >.05). The groups shown to be significantly different (p b .05).

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as significant (p b 0.05). Only the groups of fillers that were modified with silica did not cause a significant effect on the DTS in the sequences both before and after 24 h immersion (Table 3a). However, the same groups that were modified with silica showed a significant decrease in the modulus. This phenomenon showed that the sample with a thin silica film coating on the filler surfaces would be more ductile, as confirmed by the modulus values that decreased by approximately 30% to 40% (Fig. 3). This change involved the decreased stiffness of the samples caused by the silica, which is a softer material than DCPA as a film coating and has been identified at the interfaces between the reinforcement and resin matrix [28,31,32]. In contrast to the result of the silica-modified fillers, the contact interfaces within the areas were enlarged by the treatment of the DCPD nanocrystallites on the surfaces of the original DCPA fillers. Thus, the modulus obviously increased, and the resin composite became stiffer. In the group comparisons of the two populations in the sequences before and after immersion, the strengths in DTS through varied surface modifications did not show significant differences (p > 0.05). Nonetheless, the modulus values after immersion were significantly higher than the averages before immersion (p b 0.05 except for the w/o mod groups, Table 3b). The highest modulus values were observed in the group of filler surfaces of samples without silanization but with crystallite modification and 50 wt.% reinforcement after

the 24 h immersion. These outcomes were due to the larger amount of reinforcement in the composite that would have the higher ability to disperse the contact forces by isotropic fillers, thereby resulting in increased brittleness [33,34]. After immersion, all the groups of resin composites notably showed higher brittleness than those without immersion. However, their DTS tests remained almost unchanged between the groups before and after immersion (Fig. 3 and Table 3b). The interaction of water in the resin composites after the immersion of the dried samples was shown to have a significantly positive effect on the strength of the modulus. Additionally, the groups with silanization that underwent 24 h of immersion had the largest increased amplitude in modulus, such that their values were nearly doubled. 3.3. Ions releasing, fracture surfaces and micro-structural changes on the topographical surfaces The effects of different filler-surface modified processes on the amounts of calcium and phosphate ions that were released in the solution are shown in Fig. 4. The unmodified group had higher ion strengths in the solutions than the other groups with surface modification. The resin composites with fillers through silanization revealed fewer amounts of ions in the solution than those with filler surfaces without silica after 24 h immersion. These results are mainly caused

Fig. 5. The morphologies of fracture surfaces (a), topographies of the surfaces (b), and respective element-mapping of different ratios of reinforced filler amounts in the resin matrix (c) before and after 24 h immersion. White arrows indicate the dimpled patterns of ductile resin composites. Blue dots: mapped positions of Ca and P elements; red dots: mapped positions of Si element; with their respective morphologies.

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Fig. 5 (continued).

by two reasons: 1) the decrease in the amount of ions through silanization could be due to the hydrophobic silane coating on the particles, thereby slowing the dissolution hypothesis that was also discussed by Xu et al. [10,14]; and 2) the reactions between calcium, phosphate, and silicate compounds that occur in an aqueous environment can be characterized as dissolution/re-precipitation reactions. The re-precipitation of the silicate-containing compounds is easier with lower ions levels in the solution [12]. The ion release was mainly due to the dissolution of the filler surfaces. Calcium silicate- and calcium phosphate-related compounds are the most stable chemicals under natural conditions. Ions in the solution would be re-precipitated on the composite surface, thereby forming remineralized deposits by which the concentration of ions in the solution decreased. Thus, resin composites underwent remineralization with a controllable dissolution and re-precipitation rate [29]. The strength and performance of resin composites depend on the nature of their resin matrix (continuous phases), filler (reinforced phase), and binder (interfacial phases) as well as the efficacy of the polymerization process [2,5]. The synergy that exists between the organic polymer matrix and the inorganic reinforcing filler phase is principally mediated by the interfacial/interphasial phase [35]. In the fracture surfaces of all groups (Fig. 5), no interface was observed between the filler and resin matrix under SEM observation. This result suggested that satisfactory linkages of the filler-reinforced additive and resin matrix in the curing processes. Specifically, the partial dimple patterns of ductile fracture surfaces

were more evident in the groups with silanization, as shown in Fig. 5a. After immersion, the resin composites showed increased brittleness, and the dimpled pattern became less distinct. The topographical surfaces and the mapped relative positions of Ca, P, and Si elements are shown in Fig. 5b and c. The Si mapping had a different and high contrast against the filler zone, further confirming the incorporation of a uniform silica coating on the surfaces of the DCPA particles. The fusion between the particles and the matrix was shown in the groups with silanization, especially in the surfacemodified groups with nanocrystallites and, further silanization. The larger the amount of fillers in composites, the higher the solubility that resulted from the large amounts of exposed composite fillers in the solutions. This observation reflected the higher remineralization ability. Despite their low CaP solubility, DCPA or DCPD is more soluble at neutral pH than apatite, which causes the derived apatite phase to precipitate onto the sample surfaces [3]. Low amounts of ions in the solution were found in the immersed groups with silanization (Fig. 4), especially after the combined procedures of modified filler surfaces with nanocrystallites and silanization. The ions existing in solution were related to the remineralization ability, such that the densest structure of the precipitates was found in the nanocrystal/silica group with the 50 wt.% resin composite, which is due to the presence of aqueous Si that can react in the form of orthosilicic acid (Si(OH)4) [29]. These ions act directly in remineralization and are able to induce the precipitation of apatite from electrolyte solutions.

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Fig. 5 (continued).

4. Conclusions With the development of adding calcium phosphates to dental resin composites, the ability to release ions to remineralize the relative-apatite phases for the restorative shrinkage between the natural teeth and resins required a higher filler mass ratio. In addition to the reduced polymerization shrinkage, the groups of filler surfaces modified with nanocrystallites showed properties of increased hardness and a resultant abrasion resistance, fracture resistance, and remineralization. To retain the strength of composite resins and to accumulate the desirable ion releasing rates, nanocrystallized modification on the filler surfaces was performed for composite dental resins. The application of such treated DCPA granule formulations, which were applied as reinforcements in a resin matrix, is proposed, excluding the traditional treatment by a functional group of silica on fillers. The use of calcium phosphate fillers in dental resins as prosthetic substitution materials has been proven potentially effective for various clinical applications. Acknowledgments The authors acknowledge with appreciation the assistance of Ms Ya-Shun Chen and Chia-Ling Ko with this research. We acknowledge support for this research from the National Science Council of the Executive Yuan, Taiwan, Republic of China, under contract NSC992314-B037-051-MY3.

References [1] D.C. Sarrett, Dent. Mater. 21 (2005) 9–20. [2] H. Wang, M. Zhu, Y. Li, Q. Zhang, H. Wang, Mater. Sci. Eng. C 31 (2011) 600–605. [3] R.Z. LeGeros, in: H. Myers (Ed.), Monographs in Oral Sciences, vol. 15, Karger, Basel, Switzerland, 1991, pp. 1–201. [4] D. Skrtic, A.W. Hailer, S. Takagi, J.M. Antonucci, E.D. Eanes, J. Dent. Res. 75 (1996) 1679–1686. [5] S.H. Dickens, G.M. Flaim, S. Takagi, Dent. Mater. 19 (2003) 558–566. [6] S.E. Langhorst, J.N. O'Donnell, D. Skrtic, Dent. Mater. 25 (2009) 884–891. [7] G.E. Schumacher, J.M. Antonucci, J.N.R. O'Donnell, D. Skrtic, J. Am. Dent. Assoc. 138 (2007) 1476–1484. [8] D. Skrtic, J.M. Antonucci, E.D. Eanes, F.C. Eichmiller, G.E. Schumacher, J. Biomed. Mater. Res. 53 (2000) 381–391. [9] H.H.K. Xu, J.B. Quinn, J. Biomed. Mater. Res. 57 (2001) 165–174. [10] H.H.K. Xu, M.D. Weir, L. Sun, Dent. Mater. 23 (2007) 1482–1491. [11] H.H.K. Xu, L. Sun, M.D. Weir, S. Takagi, L.C. Chow, J. Biomed. Mater. Res. B 81B (2007) 116–125. [12] H.H.K. Xu, M.D. Weir, L. Sun, S. Takagi, L.C. Chow, J. Dent. Res. 86 (2007) 378–383. [13] H.H.K. Xu, M.D. Weir, L. Sun, S. Ngai, S. Takagi, L.C. Chow, J. Mater. Sci. Mater. Med. 20 (2009) 1771–1779. [14] H.H.K. Xu, M.D. Weir, L. Sun, Dent. Mater. 25 (2009) 535–542. [15] H.H.K. Xu, M.D. Weir, L. Sun, J.L. Moreau, S. Takagi, L.C. Chow, J.M. Antonucciz, J. Dent. Res. 89 (2010) 19–28. [16] W.C. Chen, C.P. Ju, J.H. Chern Lin, J. Mater. Sci. Lett. 21 (2002) 1583–1585. [17] J.C. Chen, C.L. Ko, C.J. Shih, Y.C. Tien, W.C. Chen, J. Dent. 40 (2012) 114–122. [18] J.C. Wang, C.L. Ko, C.C. Hung, Y.C. Tyan, C.H. Lai, W.C. Chen, C.K. Wang, J. Dent. 38 (2010) 158–165. [19] D. Xie, I.D. Chung, G. Wang, D. Feng, J. Mays, Eur. Polym. J. 40 (2004) 1723–1731. [20] H. Fong, S.H. Dickens, G.M. Flaim, Dent. Mater. 21 (2005) 520–529. [21] I.S. Medeiros, M.N. Gomes, A.D. Loguercio, L.E.R. Filho, J. Oral Sci. 49 (2007) 61–66. [22] W. Lien, K.S. Vandewalle, Dent. Mater. 26 (2010) 337–344. [23] G. Zhang, R. Huang, Z. Li, X. Yang, X. Chen, W. Xia, X. Sun, G. Yang, C. Gao, Z. Gou, J. Inorg. Biochem. 113 (2012) 1–8.

W.-C. Chen et al. / Materials Science and Engineering C 33 (2013) 1143–1151 [24] D. Knaack, M.E.P. Goad, M. Aiolova, C. Rey, A. Tofighi, P. Chakravarthy, D.D. Lee, J. Biomed. Mater. Res. 43 (1998) 399–409. [25] J.C. Heughebaert, J. Cryst. Growth 78 (1986) 192–198. [26] S. Mandel, A.C. Tas, Mater. Sci. Eng. C 30 (2010) 245–254. [27] L. Sun, L.C. Chow, S.A. Frukhtbeyn, J. Res. Natl. Inst. Stand. Technol. 115 (2010) 243–255. [28] D. Skrtic, J.M. Antonucci, E.D. Eanes, Dent. Mater. 12 (1996) 295–301. [29] A.M. Pietak, J.W. Reid, M.J. Stott, M. Sayer, Biomaterials 28 (2007) 4023–4032. [30] J. Xu, I.S. Butler, F.R. Gilson, Spectrochim. Acta A 55 (1999) 2801–2809.

1151

[31] C.K. Wang, C.Z. Wang, J.C. Wang, C.C. Hung, W.Y. Li, W.C. Chen, J. Non-Cryst. Solids 356 (2010) 927–932. [32] M.H. Chen, J. Dent. Res. 89 (2010) 549–560. [33] J.N. O'Donnell, J.M. Antonucci, D. Skrtic, J. Bioact. Compat. Polym. 21 (2006) 169–184. [34] S.H. Dickens-Venz, S. Takagi, L.C. Chow, R.L. Bowen, A.D. Johnston, B. Dickens, Dent. Mater. 10 (1994) 100–106. [35] J.M. Antonucci, S.H. Dickens, B.O. Fowler, H.H.K. Xu, W.G. McDonough, J. Res. Natl Inst. Stand. Technol. 110 (2005) 541–558.