Dental Resin Composites Reinforced by Rough Core-Shell

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Dental Resin Composites Reinforced by Rough Core-Shell SiO2 Nanoparticles with Controllable Mesoporous Structure Yazi Wang, Hongfei Hua, Yejia Yu, Guoyin Chen, Meifang Zhu, and Xiao-Xia (Julian) Zhu ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.9b00508 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on September 3, 2019

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ACS Applied Bio Materials

Dental Resin Composites Reinforced by Rough Core-Shell SiO2 Nanoparticles with Controllable Mesoporous Structure Yazi Wang a,b, Hongfei Hua c, Yejia Yu c, Guoyin Chen a, Meifang Zhu a,*, X. X. Zhu b,*

a

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and

Engineering, Donghua University, Shanghai 201620, China b

Département de Chimie, Université de Montréal, C.P. 6128, Succursale Centre-ville, Montréal, Québec, H3C 3J7,

Canada c

Department of Oral Surgery, Ninth People's Hospital, Shanghai Jiao Tong University School of Medicine, Key

Laboratory of Stomatology, National Clinical Research Center of Stomatology, Shanghai 200011, China *Corresponding authors. E-mail: [email protected]; [email protected].

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ABSTRACT ：Porous structure within filler particles may improve interfacial bonding between resin matrix and fillers for the preparation of dental resin composites (DRCs). In this study, rough core-shell SiO2 (rSiO2) nanoparticles with controllable mesoporous structures were synthesized via an oil-water biphase reaction system and characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and N2 adsorption–desorption measurements. The influence of mesoporous shell thickness of rSiO2 and mass ratio between rSiO2 and smooth SiO2 (sSiO2) on the physical and mechanical properties of DRCs was studied. The rSiO2 with thin mesoporous shell could form strong physical interlocking with the resin matrix, which improved the mechanical properties with the exception of flexural modulus. The mechanical properties were further optimized by mixing rSiO2 and sSiO2. The flexural strength and compressive strength of the DRC at a mass ratio of 5 : 5 increased by 24.3 and 16.8%, respectively, compared with the DRC filled with sSiO2 alone. There is no statistically significant difference in the flexural modulus between these two DRCs (p > 0.05). The DRCs in this study showed excellent biocompatibility on the human dental pulp cells (HDPCs) as demonstrated by the cytotoxicity tests. The use of rSiO2 provides a promising approach to develop strong, durable, and biocompatible DRCs. KEYWORDS: mechanical properties, biocompatibility, interfacial bonding, mesoporous structure, silica


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1 INTRODUCTION Dental resin composites (DRCs) are prevalent restorative materials for dental caries. They mainly consist of three components: organic monomers which can be polymerized to form a continuous resin matrix, inorganic fillers (such as SiO2, ZrO2, and Al2O3) which are dispersed in the resin matrix to endow the materials suitable physical and mechanical properties, and a small amount of photoinitiators/accelerators [1, 2]. For composite materials, the interfacial phase to bind the resin matrix with the inorganic fillers has an important influence on the service performance of DRCs. The binding should be strong and durable to allow the transfer of the stress from the resin matrix to fillers [3, 4], which can further improve the mechanical properties and the lifespan of DRCs. Silane coupling agents have been widely used in the development of dental materials to improve the interfacial interaction. These silane coupling agents have a typical structure of bifunctional groups. One end of the molecule is alkoxy groups which can be transformed into hydroxyl groups at proper pH values and then react with the hydroxyl groups on the surface of fillers. The other end is organo-functional groups such as methacrylate, acrylate or epoxy groups which can co-polymerize with the organic monomers [5]. However, these coupling agents may be easily degraded in the humid oral environment, making the inorganic fillers debond from the resin matrix [6]. Compared with conventional chemical modification, physical interlocking may be a more effective way to create strong and durable interfacial binding. Porous fillers can allow organic monomers to infiltrate into the pores to form physical interlocking [7-11]. Previous methods to prepare porous fillers include etching with HF [7, 8] and sintering at high temperature (1300 °C) [11], both require rather harsh conditions with a poor control of the pore structure. Other methods using templates such as glucose [9] and cetyltrimethylammonium bromide (CTAB) [12] were also reported. Recently, with the aid of CTAB, our group developed wrinkled mesoporous SiO2 (WMS) with wrinkles extending radially outward from the core. WMS exhibited better reinforcing effects for DRCs than smooth SiO2 (sSiO2) due to the enhanced interaction between WMS and the resin matrix [12]. However, porous fillers possess a larger specific surface area and need more organic resin to envelop than smooth fillers, resulting in a lower filler loading in the DRCs using the porous fillers alone (below 50 wt%) than in commercial DRCs (70–80 wt%) [9, 12]. The low filler loading can give rise to a series of problems, such as limited mechanical properties and larger polymerization shrinkage. To take advantage of the porous structure of the fillers while minimizing or eliminating their problems, we devised a two-step method under mild conditions to synthesize rough core-shell SiO2 (rSiO2)



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nanoparticles with controllable mesoporous structure. The viscosity, mechanical properties, and cytotoxicity of the resultant DRCs have been evaluated to elucidate the reinforcing mechanisms of rSiO2.

2 MATERIALS AND METHODS 2.1 Materials Tetraethyl orthosilicate (TEOS), cetyltrimethylammonium bromide (CTAB), acetone, cyclohexane, 3-methacryloxypropyltrimethoxysilane, bisphenol A glycerolate dimethacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA), camphorquinone (CQ), and ethyl 4-dimethylamino benzoate (4EDMAB) were purchased from Sigma-Aldrich, USA. Ammonium hydroxide (28–30 %) was purchased from VWR, USA. Anhydrous ethanol and sodium hydroxide were purchased from Fisher Scientific, USA. All chemicals were used without further purification. 2.2 Synthesis of rSiO2 nanoparticles The sSiO2 nanoparticles were synthesized according to the Stöber method. Absolute ethanol (3000 mL), deionized water (240 mL), and ammonium hydroxide (28–30 %, 120 mL) were sequentially added into a round bottom flask, followed by stirring at room temperature for 30 min. TEOS (180 mL) was then added into the flask, and the mixture was stirred at room temperature for 20 h. The products were collected through centrifugation, washed with water and then with absolute ethanol, and finally dried in vacuum at 80 °C for 24 h. The rSiO2 nanoparticles were synthesized through growing mesoporous silica shell on the surface of sSiO2. sSiO2 nanoparticles (1 g) were dispersed into deionized water (200 mL) containing CTAB (4 g). Then NaOH aqueous solution (0.1 M, 3.2 mL) was added into the mixture, and then slowly stirred at 60 °C for 2 h. A solution consisting of TEOS (40 mL) and cyclohexane (160 mL) was slowly added into the mixture. The reaction was performed at 60 °C with slowly stirring for a fixed time period (0.5, 2, and 5 h, respectively). The products were collected through centrifugation, and washed with water and then with absolute ethanol. The rSiO2 nanoparticles were then dispersed into acetone and refluxed at 50 °C for 12 h, and this process was repeated several times to remove the CTAB templates. Finally, the products were washed with absolute ethanol and dried in vacuum at 80 °C for 24 h. 2.3 Preparation of DRCs To improve the dispersibility and wettability, the sSiO2 and rSiO2 nanoparticles were silanized according to a reported procedures [13] before mixing with the monomers. The monomers consisted of Bis-GMA and TEGDMA at a mass ratio of 1:1 were blended before CQ and 4-EDMAB, accounting for 4 / 21 ACS Paragon Plus Environment

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0.2 and 0.8 wt% of the organic monomers, respectively, were added and blended in the dark. At last, a three roll mill (Exakt 50i TRM, Germany) was used to blend the silanized fillers and resin completely. 2.4 Measurements and characterization 2.4.1 SiO2 nanoparticles The morphology and size of sSiO2 and rSiO2 nanoparticles were studied by transmission electron microscopy (TEM, JEOL JEM-2100 F, Japan) operated at 200 kV and scanning electron microscopy (SEM, JEOL JSM-7400F, Japan) operated at 2 kV, respectively. The specific surface area, pore size, and pore volume of the rSiO2 nanoparticles were determined by N2 adsorption-desorption experiment (Autosorb-iQ3, Quantachrome, USA) at 77 K and the samples were degassed under vacuum at 180 °C for 12 h before measurements. 2.4.2 Physical and mechanical properties of DRCs The viscosity of DRCs was measured on a rheometer (AR-2000, TA Instruments, USA) with a 20 mm parallel plate geometry. The experiment was performed at angular frequency range of 0.01–100 rad/s at 23 °C. The mechanical properties of DRCs including flexural strength, flexural modulus, and compressive strength were determined on a universal testing machine (Instron 5565, USA). The uncured resin composite were filled into the molds of a specific size (flexural test: 2 mm × 2 mm × 25 mm; compressive test: Φ 4 mm × 6 mm), then cured for 60 s on both sides of the specimen with a dental lamp (Optilux 500, Demetron/Kerr, USA). Five specimens were prepared for each composite sample and all specimens were stored in distilled water at 37 °C for 24 h before measurements. The morphology of fractured surface of DRCs was observed on a SEM. 2.4.3 Cytotoxicity of DRCs Human dental pulp cells (HDPCs), provided by Shanghai Jiao Tong University School of Medicine, were used to evaluate the cytotoxicity of DRCs. The disinfected disc-shaped samples (Φ10 mm × 1 mm, n = 3) were placed into 12-well plates, followed by the additon of HDPCs (1 mL, 5×105 cells/mL) as seeds. The plates were placed into incubator (37 °C, 5% CO2) for a fixed time period (1, 3, and 5 days). The medium of wells was removed and replaced by fresh medium containing 10 % Cell Counting Kit-8 (CCK8, Dojindo, Japan). After incubation for 2 h in the dark, 100 μL of the medium of each well was transferred to 96-well plates. The absorbance was measured with a UV/Vis spectrophotometer (DU800, Beckman Coulter, USA) at a wavelength of 450 nm. The morphology of HDPCs cultured for 3 days on the surface of DRCs was characterized by a fluorescence microscope (Olympus BX53, Japan). Before observation, the HDPCs were dyed with 5 / 21 ACS Paragon Plus Environment


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TRITC-phalloidin (Sigma-Aldrich) and 4’,6-diamidino-2-phenylindole dihydrochloride (DAPI, SigmaAldrich), respectively. 2.5 Statistical analysis Statistical analysis was performed using one-way analysis of variance (ANOVA) and p < 0.05 was considered statistically significant.

3 RESULTS AND DISCUSSION 3.1 Synthesis and characterization of rSiO2 nanoparticles The TEM images show that the rSiO2 nanoparticles possess a clear core-shell structure. The core sizes of the silica nanoparticles obtained with different reaction times are approximately the same (~330 nm), the main difference is the thickness of the mesoporous shells. When the reaction time of the second step to grow the mesoporous shell increased from 0.5 to 2 h and then 5 h, the thickness of mesoporous shell increased from 3 nm (Figure 1 B1 & B2) to 11 nm (Figure 1 C1 & C2) and then 28 nm (Figure 1 D1 & D2), respectively. The SEM images also show that with a longer reaction time, the size of the silica nanoparticles increased due to a thicker mesoporous shell (Figure 1, images of the bottom row). These nanoparticles were further studied by N2 adsorption-desorption measurements. The isotherms of these rSiO2 nanoparticles are typical type-IV curves, which confirm the presence of mesopores. The rSiO2-5h nanoparticles show clearer hysteresis loop in the isotherms at 0.4 < P/P0 < 0.6 than the other rSiO2 nanoparticles, corresponding to a larger pore volume (Figure 2A). The rSiO2 nanoparticles show narrow pore size distribution and approximate pore diameter (Figure 2B). The specific surface area was calculated using the Brunauer−Emmett−Teller (BET) method and the cumulative pore volume was calculated using the density functional theory (DFT) method. The results listed in Table 1 indicate the successful synthesis of the rSiO2 nanoparticles and a good control of their mesoporous structures.


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Figure 1. TEM (top and middle rows) and SEM (bottom row) images of the rSiO2 with different mesoporous shell thickness prepared at different times: 0 h (A1-A3, denoted as sSiO2); 0.5 h (B1-B3, denoted as rSiO2-0.5h); 2 h (C1-C3, denoted as rSiO2-2h); and 5 h (D1-D3, denoted as rSiO2-5h). Table 1. Particle parameters of sSiO2 and rSiO2 with mesoporous structure Particle

a

Mesoporous shell Pore diameter,

Specific surface

Cumulative pore

Maximum loading

type

thickness (nm)

DFT (nm)

area, BET (m2/g)

volume, DFT (cm3/g)

in DRC (wt%)

sSiO2

0

N/A

8.3a

N/A

70

rSiO2-0.5h

3

3.5

26.6

0.046

70

rSiO2-2h

11

4.9

46.5

0.066

69

rSiO2-5h

28

4.9

116.1

0.154

67

Result of theoretical calculation from a previous study [12]. 7 / 21 ACS Paragon Plus Environment


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Figure 2. (A) N2 adsorption-desorption isotherms and (B) pore size distributions of the rSiO2 with different mesoporous shell thickness.


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Scheme 1. Schematic illustration of the formation mechanism of rSiO2 nanoparticles with mesoporous structures. (A) In the oil-water biphase reaction system, the oil phase contains cyclohexane and TEOS, the water phase contains water, NaOH, CTAB, and sSiO2. Upon agitation, CTAB/cyclohexane micelles are formed. TEOS at the oil-water interface is catalyzed by NaOH to generate silicate oligomers which flow into the water phase. (B) The silicate oligomers combine



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with CTAB/cyclohexane micelles to form oligomer/CTAB/cyclohexane spherical assembly which deposite on the surface of sSiO2 as building blocks. (C) The spherical oligomer/CTAB/cyclohexane assembly undergo transformation into cylinders and then crosslink through a Si-O-Si network. (D) The rSiO2 nanoparticles with mesoporous structures are obtained after the removal of CTAB and cyclohexane.

The formation mechanism of rSiO2 nanoparticles with mesoporous structures is illustrated in Scheme 1. The sSiO2 nanoparticles obtained by the Stöber method are dispersed into the aqueous phase containing CTAB as the surfactant and NaOH as the catalyst. The upper oil phase consists of TEOS as silica precursor and cyclohexane as storage medium to dissolve TEOS. Oil-in-water micelles containing droplet of cyclohexane are formed by CTAB. Meanwhile, the TEOS molecules at the oil-water interface are hydrolyzed and condensate to generate silicate oligomers which can absorb on the surface of the micelles due to electrostatic interaction. The oligomer/CTAB/cyclohexane spherical assemblies as the building blocks deposite on the surface of sSiO2 which contains many hydroxyl groups. As the reaction continues, the spherical assemblies are transformed into cylindrical ones through continued deposition of the spherical assemblies. The cylindrical structures keep growing with sufficient reactants and reaction time and will be finally fixed by cross-linked Si-O-Si network.[14-16] After removing CTAB and cyclohexane by extraction with acetone, the rSiO2 nanoparticles with mesoporous structures are obtained. 3.2 Physical and mechanical properties of DRCs The obtained rSiO2 nanoparticles were used to fill the Bis-GMA/TEGDMA resin and the influence of mesoporous shell thickness of rSiO2 on the physical and mechanical properties of DRCs was studied. The maximum filler loading decreased from 70 to 67 wt% with increasing reaction time in the preparation of the rSiO2. The prolonged reaction time caused an increase in the specific surface area of the nanoparticles (Table 1). rSiO2-0.5h has a very thin mesoporous shell and therefore a lower specific surface area, and thus a similar maximum loading to sSiO2. To compare the reinforcing effects of rSiO2, the DRCs were all prepared at the same filler loading of 67 wt%. Rheological properties of DRCs are important for the clinical use and handling, and may affect the final quality of the dental restoration [17]. In this study, complex viscosity of DRCs was measured on a shear rheometer over a range of frequencies. All the DRCs exhibit pseudoplastic characteristics and the viscosity decreases as the shear frequency increases (Figure 3A). This phenomenon is usually observed in the clinical situation where the DRCs readily flow and adapt into the carious cavity with an exerting force. As expected, the viscosity of the DRCs increases with filler loading. It is known that increased filler loading shortens the distance between the particles and reinforces particle-particle interactions [18]. In this study, we also observed that the morphology of filler plays a more important role in the viscosity of 10 / 21 ACS Paragon Plus Environment

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DRCs. The rSiO2 nanoparticles have a higher specific surface area than sSiO2 and, therefore, they can reach a lower maximum filler loading level in the DRCs than sSiO2. The viscosity at maximum loading levels follows the order of rSiO2-5h ＞ rSiO2-2h ＞ rSiO2-0.5h ＞ sSiO2, corresponding to the order of the specific surface area of the filler particles. The rough structure increases the friction among the filler particles and strengthens the interfacial interaction between the fillers and the matrix, leading to an increased viscosity. The mechanical properties are crucial to determine the success of the new DRCs and they are compared in Figure 3. The DRC filled with rSiO2-0.5h shows the best flexural strength and compressive strength at the maximum filler loading and at identical 67 wt% loading (Figure 3 B & C). The differences of the reinforcing mechanisms between sSiO2 and rSiO2 are illustrated in Figure 4. The structural damage of the composites may be in the form of a crack, which may induce the debonding of sSiO2 from the resin matrix due to a weaker interfacial interaction and finally lead to the fracture of the bulk material (Figure 4A). The extension of the crack can be effectively prevented in the presence of rSiO2 with a thin mesoporous shell (Figure 4B). The mesoporous channel within rSiO2-0.5h allows a good infiltration of the monomers and a good physical interlocking between the fillers and the matrix upon curing, improving the interfacial strength of the DRC. However, the addition of rSiO2 has an adverse effect on the flexural modulus which reflects the ability of a material to resist deformation (Figure 3D). The rSiO2 consists of “soft” mesoporous shell and “rigid” solid core, the “soft” part of rSiO2 might be the cause of a slightly lower flexural modulus. Thicker mesoporous shells lead to poorer mechanical properties of the DRCs. One possible explanation is that the uncured monomers cannot completely fill the deeper mesoporous channels, creating mechanical defects (Figure 4C) [7]. The filler loading also affects the mechanical properties of DRCs. The flexural modulus of DRCs increases with the inorganic filler loading level, due to the greater rigidity of inorganic fillers than the resin matrix. However, the flexural strength and compressive strength of DRCs at maximum loading are lower than those at 67 wt% loading. This may be due to the agglomeration of nanoparticles at the maximum loading [19]. Wei’s group filled DRCs with silica containing interconnected pores and the filler loading reached approximately 40–50 wt% [9]. Our group previously developed wrinkled mesoporous SiO2 as dental fillers, which exhibited excellent reinforcing effect. The maximum loading was only 35 wt% due to the high specific surface area (530 m2/g) of the filler particles [12]. In this study, we prepared rSiO2 using a very simple and mild method, and the specific surface area of rSiO2 can be controlled by adjusting reaction time. The maximum loading of DRC filled with rSiO2-0.5h reached 70 wt%, similar to that of DRC filled 11 / 21 ACS Paragon Plus Environment


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with sSiO2. The flexural strength and compressive strength increased by 36.1 and 20.2%, respectively, while the flexural modulus decreased by 14.5%, compared with DRC filled with sSiO2. According to ISO 4049-2009, the flexural strength of DRCs used to repair occlusal areas must reach 80 MPa and there are no suggested threshold values regarding compressive strength and flexural modulus. Among these mechanical properties, flexural strength is more relevant to practical use of DRCs [20]. We followed the ISO 4049-2009 procedure for the testing method used in this study, and the flexural strength of DRC showed a 39.3% improvement over the required value.

Figure 3. (A) Complex viscosity, (B) flexural strength, (C) compressive strength, and (D) flexural modulus of DRCs reinforced by sSiO2 and rSiO2 with different mesoporous shell thickness.


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Figure 4. Reinforcing mechanism of DRCs by (A) sSiO2, (B) rSiO2 with a thin mesoporous shell, and (C) rSiO2 with a thick mesoporous shell.



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3.3 Mixture of rSiO2 and sSiO2

Figure 5. (A) Complex viscosity, (B) flexural strength, (C) compressive strength, and (D) flexural modulus of DRCs reinforced by mixtures of rSiO2-0.5h and sSiO2 at various mass ratios.

The comprehensive performance of the DRCs with the exception of flexural modulus is improved when using the rSiO2 with a thin mesoporous shell. To further optimize the mechanical properties, the DRCs were prepared through mixtures of rSiO2-0.5h and sSiO2 at various mass ratios while the total filler loading was kept at 70 wt%. The DRC filled with rSiO2-0.5h alone has the highest complex viscosity. As the proportion of sSiO2 increases, the complex viscosity of the DRC decreases (Figure 5A). The introduction of sSiO2 improves the flexural modulus of the DRCs (Figure 5D), since sSiO2 is more rigid than rSiO2 -0.5h with a “soft” mesoporous shell, but the presence of rSiO2-0.5h helps the formation of stronger physical interlocking with the resin matrix, improving the flexural strength and compressive strength of DRCs (Figure 5 B & C). In these formulations, the DRC at rSiO2-0.5h : sSiO2 = 5 : 5 shows the best overall mechanical 14 / 21 ACS Paragon Plus Environment

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performance: the flexural strength and compressive strength increase by 24.3 and 16.8%, respectively, in comparison to the DRC filled with sSiO2 alone, and the flexural modulus shows no statistical difference (p > 0.05). 3.4 Surface analysis of fractured DRCs

Figure 6. SEM images of fractured surface of DRCs reinforced by sSiO2 (70 wt%, A1 and A2), mixture of fillers (rSiO2-0.5h : sSiO2=5 : 5, 70 wt%, B1 and B2), and rSiO2-0.5h (70 wt%, C1 and C2).

Several representative DRCs were selected and their fractured surface was analyzed by SEM (Figure 6). The nanoparticles and the resin matrix can be observed clearly from the images. The nanoparticles in 15 / 21 ACS Paragon Plus Environment


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the DRC filled with sSiO2 alone (70 wt%) are rather bare on the surface (Figure 6 A2) in comparison to the samples containing rSiO2-0.5h (Figure 6 B2 & C2), indicating a better adhesion of the resin matrix on rSiO2-0.5h. Obviously, rSiO2-0.5h can bind more tightly to the resin matrix through the mesoporous channel which provides space for the infiltration of the organic monomers. The strong interfacial interlocking makes the stress transfer from matrix to nanoparticles easier, which is beneficial to the improvement of mechanical performance of the DRCs. 3.5 Cytotoxicity of DRCs

Figure 7. The absorbance measured through CCK-8 method after cell culture for 1, 3, and 5 days, respectively. * p < 0.05, compared with commercial Z250 XT; # p < 0.05, compared with DRC reinforced by sSiO2 alone.

The DRC reinforced by mixed fillers (rSiO2-0.5h : sSiO2=5 : 5, 70 wt%) was selected to study the cytotoxicity on HDPCs, and the DRC reinforced by sSiO2 alone (70 wt%) and commercial Z250 XT served as control groups. The results of cell viability after culture for 1, 3, and 5 days are shown in Figure 7. The value of absorbance is positively correlated with cell numbers. Cell proliferation was not affected much by the DRCs, and the cell numbers increase with the time. However, the experimental DRCs show lower cytotoxicity than commercial Z250 XT, which may be due to the difference of the formulations. The formulation of commercial DRC may be more complex to satisfy the real clinical applications. Compared with DRC containing only sSiO2, the introduction of rSiO2 tend to facilitate cell proliferation and the cell numbers are significantly higher at 1 and 3 days of culture (p < 0.05). Previous study confirmed that the DRCs containing mesoporous SiO2 fillers exhibited good cytocompatibility. The matrix-filler interaction can be improved by the presence of the mesoporous structure, which prevents the 16 / 21 ACS Paragon Plus Environment

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diffusion of water and the release of toxic small molecules from the DRCs [21]. The morphology of the cells grown on the DRCs after 3 days of culture was observed through staining cytoskeletal F-actin fibers with phalloidin (red) and nuclei with DAPI (blue) (Figure 8). The cells attached well on these DRCs, and the outline of the cells was clear on the commercial Z250 XT and DRC reinforced by sSiO2 alone, but the cell numbers were higher on the latter (Figure 8 A & B). The cells on the DRC containing mixed fillers were crowded and it was hard to distinguish the boundaries among these cells (Figure 8 C3), which showed better biocompatibility. The morphology of the cells confirms the results of cell viability.

Figure 8. The morphology of HDPCs grown on (A1-A3) the Z250 XT, (B1-B3) DRC reinforced by sSiO2 (70 wt%), and (C1-C3) DRC reinforced by mixed fillers (rSiO2-0.5h : sSiO2=5 : 5, 70 wt%), respectively, after 3 days of culture. The red fluorescence represents the cytoskeletal F-actin fibers and the blue fluorescence represents the nuclei.



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4 CONCLUSION In this study, rSiO2 nanoparticles with controllable mesoporous structure were synthesized and used as new fillers to reinforce the DRCs. The preparation of rSiO2 is mild and simple, which mainly includes two steps: First, the synthesis of sSiO2 according to a common Stöber method; Second, the growth of mesoporous silica shell on the surface of sSiO2 in an oil-water biphase reaction system. The thickness of mesoporous silica shell can be controlled by varying the reaction time, and hence the specific surface area of the porous filler. The thickness of mesoporous shell has an important influence on the physical and mechanical properties of DRCs. The maximum loading of DRC reinforced by rSiO2-0.5h can reach 70 wt%, approaching that of DRC reinforced by sSiO2. The rSiO2 nanoparticles with a thin mesoporous shell can improve the general mechanical properties of DRC with the exception of flexural modulus. A thicker mesoporous shell leads to a decreased maximum loading and poorer mechanical properties of the DRC. The mesoporous structure strengthens the interfacial interaction between the fillers and the resin matrix. Such benefits are more obvious when the mesoporous shell is kept thin enough to allow good mixing of the fillers with the monomers. The physical and mechanical properties of DRCs can be further adjusted by mixing rSiO2-0.5h with sSiO2. The comprehensive properties of DRC at the mass ratio of 5:5 are superior to that of DRC reinforced by sSiO2 alone. In addition, the DRCs also show excellent biocompatibility. The new DRCs may help to address the issue of bulk fracture, a major cause of dental restoration failure.

5 AUTHOR INFORMATION Corresponding authors *E-mail: [email protected]; *E-mail: [email protected].

ORCID Meifang Zhu: 0000-0003-0359-3633 X. X. Zhu: 0000-0003-0828-299X

Notes The authors declare no competing financial interest.

6 ACKNOWLEDGMENTS 18 / 21 ACS Paragon Plus Environment

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The authors acknowledge the financial support from the National Key Research and Development Program of China (2016YFA0201702/2016YFA0201700). YW thanks a scholarship from China Scholarship Council in support of a research exchange with Université de Montréal. Instrumental analyses were supported in part by FRQNT, which funded the Quebec Center for Advanced Materials, of which XXZ is a member.

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Dental Resin Composites Reinforced by Rough Core-Shell

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