Study on a novel antibacterial light-cured resin composite containing nano-MgO

Journal Pre-proof Study on a novel antibacterial light-cured resin composite containing nano-MgO Zhongyuan Wu (Conceptualization) (Data curation) (Formal analysis) (Investigation) (Methodology) (Software) (Visualization) (Writing - original draft), Haiping Xu (Conceptualization) (Data curation) (Investigation) (Methodology) (Writing - review and editing), Wei Xie (Conceptualization) (Data curation) (Investigation) (Methodology) (Validation), Meimei Wang (Conceptualization) (Data curation) (Investigation) (Methodology) (Validation), Cunjin Wang (Formal analysis) (Methodology) (Software), Cheng Gao (Formal analysis) (Methodology) (Software), Fang Gu (Conceptualization) (Data curation) (Methodology) (Project administration) (Supervision) (Visualization) (Writing - review and editing), Jie Liu (Conceptualization) (Supervision) (Writing - review and editing), Jing Fu (Conceptualization) (Data curation) (Methodology) (Project administration) (Supervision) (Visualization) (Writing - review and editing)

PII:

S0927-7765(20)30004-7

DOI:

https://doi.org/10.1016/j.colsurfb.2020.110774

Reference:

COLSUB 110774

To appear in:

Colloids and Surfaces B: Biointerfaces

Received Date:

23 October 2019

Revised Date:

11 December 2019

Accepted Date:

3 January 2020

Please cite this article as: Wu Z, Xu H, Xie W, Wang M, Wang C, Gao C, Gu F, Liu J, Fu J, Study on a novel antibacterial light-cured resin composite containing nano-MgO, Colloids and Surfaces B: Biointerfaces (2020), doi: https://doi.org/10.1016/j.colsurfb.2020.110774

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Study on a novel antibacterial light-cured resin composite containing nano-MgO Zhongyuan Wua, Haiping Xua, Wei Xieb, Meimei Wangc, Cunjin Wanga, Cheng Gaod, Fang Gue,*, Jie Liua, Jing Fua,* a

Department of Stomatology, The Affiliated Hospital of Qingdao University, College of Stomatology,

Qingdao University, Qingdao, 266003, China b

Department of Stomatology, Huikang Hospital, Qingdao University Medical Group, Qingdao, 266000, China c

Department of Stomatology, Weifang Weien Hospital, Weifang, 261000, China

d

Department of Stomatology, Shanghai Stomatological Hospital, Fudan University, Shanghai, 200001, China Qingdao University Medical College, Qingdao, Shandong, 266021, China

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* Corresponding author.

E-mail address: [email protected] (Jing Fu); [email protected] (Fang Gu).

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Graphical abstract

Highlights

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A new type of photocurable resin composite containing nano-MgO was synthesized The introduction of nano-MgO endowed resin composites with excellent antibacterial properties The wear resistance of the resin composites were improved

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Abstract

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A new type of photocurable resin composite containing nano-MgO was synthesized in order to reduce the occurrence of secondary caries. Different mass ratios (0%, 1%, 2%, 4%, 8%) of nano-MgO were added into resin composites. The antibacterial properties of nano-MgO powder and modified resin composites against Streptococcus mutans (S. mutans) were detected by antibacterial ring test and film contact test, respectively. Compressive strength (CS) and wear resistance were determined by a universal testing machine and an abrasion test machine. The results indicated that antibacterial activity and wear resistance of resin composites containing nano-MgO were superior to the control group (p < 0.05). The antibacterial rate reached as high as 99.4% when the mass ratio of nano-MgO was 8%. However, the CS values tended to decline as the content of nano-MgO increase. Hence, the addition of nano-MgO showed excellent antibacterial property to resin composites and enhanced wear resistance, but was detrimental to their mechanical properties.

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Keywords: Nano-MgO; Resin composite; Antibacterial property; Compressive strength; Wear resistance Introduction Resin composites have been widely used to restore various tooth defects due to their aesthetic performance and acceptable physiochemical properties. However, the surface of this class of material is liable to form more biofilms than other restorative materials such as amalgam and glass-ionomers [1]. Polymerization shrinkage is another disadvantage of resin composites and can produce shrinkage stress between restoration and tooth interface, causing microleakage and secondary caries [2-4]. Secondary caries is a major factor of restoration failure [5, 6]. For this reason, many efforts have been made to endow resin composites with antibacterial properties, such as directly adding antibacterial components to the resin matrix, bonding antibacterial functional groups on the resin matrix, or covalently binding antibacterial ingredients to the surface of inorganic fillers [7].

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Generally, antibacterial agents used in dentistry can be classified into organic or inorganic materials. Organic antibacterial agents include organic acids, quaternary ammonium salts, biguanides, phenols, etc. For example, chlorhexidine is a broad-spectrum biguanide organic antibacterial agent that, when added into resin composite, exerts an antibacterial effect after releasing [8]. However, a burst release often occurs in the early stage, resulting in a poor effectiveness in the long-term [9]. Quaternary ammonium salt monomer is a polymerizable organic antibacterial agent, and can be covalently bonded to the resin matrix to form insoluble resin composite, thereby directly killing bacteria or inhibiting the bacteria attachment on the resin material [10]. But this kind of material needs a high dosing content, which may affect the curing performance of the resin composite, reduce mechanical durability, and increase cytotoxicity [11]. In addition, the adhesion of oral salivary proteins to the surface of the material reduces its contact antimicrobial effect [12]. Compared with organic antibacterial agents, inorganic antibacterial agents such as metal and metal oxides have many advantages including strong antibacterial activity, low drug resistance, and good biocompatibility [13]. Nanoparticles present higher antibacterial potency due to the increased surface-to-volume ratio [14]. In dentistry, nanoparticles provide new ideas for developing restoration materials with better performance and the prevention and treatment of caries [15, 16]. Studies have shown that resin composites incorporating silver or zinc oxide nanoparticles have good antibacterial effects on oral cariogenic bacteria such as Streptococcus mutans (S. mutans) and Lactobacillus [17, 18]. Resin composites containing nano-titanium dioxide have also been proven to have resistance to S. mutans and S. sanguinis [19, 20]. However, the materials based on these metal nanoparticles have some defects such as discoloration and antimicrobial effects dependent on the application of ultraviolet light [21].

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Here, to overcome above shortcomings, nano-MgO, which have broad-spectrum antibacterial effect, color stability and good biocompatibility [22], were used to improve the antibacterial activity of the resin composites. According to previous research results, nano-MgO has antibacterial activity against fungi (Candida albicans), viruses, and gram-positive and gram-negative bacteria, such as Staphylococcus aureus, Enterococcus faecalis, Escherichia coli [23-25]. However, there is no report that nano-MgO can be used as an antibacterial agent in dental resin composites. Wear resistance and mechanical strength are two key indicators in choosing resin composites for clinical use. Resin composites with poor wear resistance could lead to loss of anatomical morphology, especially in large prostheses, while resin composites with weak mechanical properties are easy to fracture. Accordingly, the objective of the present study was to investigate the antibacterial effect of resin composites incorporating different mass fractions of nano-MgO and to characterize their wear resistance and mechanical strength. It was hypothesized that (1) the resin composites containing nano-MgO would display activity against S. mutans; and (2) the addition of nano-MgO would not jeopardize the wear resistance and mechanical properties of resin composites. Material and methods

Materials Conventional matrix resins: Bis-GMA (2, 2’-bis-[4-(2-hydroxy-3-methacryloyloxy propoxy) phenyl] propane) and TEGDMA (triethylene glycol dimethacrylate), the initiation systems: CQ (camphorquinone), DMAEMA (N, N-dimethylaminoethyl methacrylate) and silane coupling agent KH-570 (3-(trimethoxysilyl) propyl methacrylate) were purchased from Sigma-Aldrich (USA). Inorganic fillers SiO2 (silicon dioxide) with an average diameter of about 2 μm and inorganic antibacterial agent MgO with an average diameter of about 130 +/- 18 nm were purchased from 2

Aladdin (China). SiO2 fillers were modified by coupling agent KH-570 before use. Briefly, a mixture of 95% ethanol and KH-570 (95:5 mL) were prepared and poured into the container with SiO2 fine powder. Then the mixture was ultrasonicated and stirred for 60 min. After solvent was removed by suction filtration, the mixture was further dried in an oven at 80 °C for 4 h. Preparation of experimental resin composites The base resin formulated with Bis-GMA and TEGDMA (70:30 by mass ratio) was mixed. Subsequently, 1 wt.% CQ as an initiator and 2 wt.% DMAEMA as an accelerator were incorporated and blended. Finally, surface-modified SiO2 (70 wt.%) and nano-MgO powder of different proportions were added and homogenized. Preparations of the specimen were performed at room temperature and under a yellow light environment avoiding premature photopolymerization. Formulation of the experimental resin composites is shown in Table 1. Group A0 was set as a control group. The microstructure of the specimens was analyzed by scanning electron microscopy (SEM, JSM-840, Japan). Nano-MgO

A0

0%

A1

1%

A2

2%

A3

4%

A4

8%

Resin matrix

Photoinitiator system

70% Bis-GMA + 30% TEGDMA

1% CQ + 2% DMAEMA

Table 1 Formulation of the experimental resin composites (wt.%).

Filler

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Group

70% SiO2

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Antibacterial properties of nano-MgO powder The antibacterial properties of neat nano-MgO powder were investigated by a bacteriostatic ring test. S. mutans UA159 (Microbiology Laboratory of Qingdao University, China) grown in brain heart infusion (BHI; Haibo Biotechnology, China) broth at 37 °C under anaerobic conditions. Then, the bacterial suspension with a concentration of 1×105 colony forming unit (CFU/mL) was prepared and uniformly inoculated on the surface of the BHI solid medium. A perforator with a diameter of 5 mm was used to punch holes on the inoculated plate after 5 min at room temperature. Two mg, 4 mg, 6 mg, 8 mg, 10 mg, and 12 mg of nano-MgO powder were weighed using an electronic analytical balance (R200-D, Storius, Germany) and put into the holes, in turn. The hole without nano-MgO powder served as the control group. After anaerobic culture for 18 h at 37 °C, the diameter of zone of inhibition was measured with a vernier caliper and recorded. The experiment was repeated three times.

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Antibacterial activity of experimental resin composite The antibacterial activity of resin composites containing nano-MgO was measured with the film contact method [26]. Five disk-shaped specimens were fabricated for each resin composite by a silicon rubber mold (10.0 mm in diameter × 2.0 mm in thickness), covered with a matrix strip and a glass slide, and were irradiated with a LED light unit (Elipar S10, 3M-ESPE, USA) for 40 s. After light curing, the specimens were immersed in sterile normal saline and left at 37 °C for 24 h to remove uncured monomer. Before the experiment, the specimens were washed with sterile water and wiped with 70% ethanol solution. After 1 min, the specimens were washed with sterile water again. Then, each set of specimens was placed on a BHI agar plate and 15 μL of the bacterial suspension was dropped on the upper surface of each specimen and covered with an aseptic polyethylene (PE) film. After anaerobic culture for 16-18 h at 37 °C, specimen and PE membrane complexes were put into a test tube containing 10 mL of axenic physiological saline and bacteria were fully eluted by shaking. The eluent was diluted and 100 μL was put on a BHI agar plate. Then, the plates were incubated aerobically at 37 °C for 24 h for CFU counting. The experiment was repeated three times. The antibacterial rate was calculated according to the following equation: 𝑟 = (𝑏 − 𝑐)⁄𝑏 × 100% (1) where, r is the antibacterial rate, b is the CFU of the control group, c is the CFU of the experimental group. Abrasive wear test of experimental resin composite The wear resistance was determined with the DMW-1 wear testing machine (Peking University, China). According to YY/T 0113-2015, five specimens for each group were constructed using a 3

self-made cylindrical plastic mold (diameter × height: 10 mm × 6 mm). After complete curing, the specimens were stored in a 37 °C water bath for 24 h. The specimens were ground with 400-grit sandpaper under a load of 22 N with water 75 times. Then, the abrasive slurry comprised of fluorite powder and distilled water (100 g: 25 g) was added to the tray, and resin composites were ground 150 times under a load of 172 N. After ultrasonic cleaning and drying, the initial mass (M 1) was recorded and the density (ρ) was measured by the drainage method according to GB 4472. The initial height (H1) was determined by gauging the height of 13 points on each worn surface with a digimatic indicator (ID-C112AM, Mitutoyo, Japan) and calculating the average value. After that, at the end of wearing 800 times under the condition of 172 N load, the mass (M2) and height (H2) were obtained, and the abrasive surface morphology was observed by SEM. Three replicates were conducted. The volume loss (∆V) and height loss (∆H) were calculated using the following formula: ∆𝑉 = (𝑀1 − 𝑀2 )⁄𝜌

(2)

(∑13 𝑘=1 𝐻1

(3)

∆𝐻 =

− ∑13 𝑘=1 𝐻2 )/13

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Compressive strength (CS) Five cylindrical specimens were prepared for each material using a silicon rubber mold with dimensions of 4 mm (in diameter) × 8 mm (in height). Resin composite was inserted incrementally and removed after completely cured. The specimens were polished with SiC paper, kept at 37 °C water bath for 24 h and then subjected to a universal testing machine (AGS, Shimadzu, Japan) at a crosshead speed of 0.5 mm/s. CS values were calculated and presented as mean ± standard deviation.

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Statistical analysis The experimental data were analyzed using software SPSS 17.0 (USA) by one-way ANOVA and LSD t-test. The correlation between height loss and volume loss of the specimens in abrasive wear test was subjected to Pearson correlation analysis. A value of p < 0.05 was considered statistically significant.

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Result and discussion Fig. 1 illustrates the compositions of the developed resin composite, including Bis-GMA, TEGDMA, SiO2, and nano-MgO. Bis-GMA is a matrix monomer extensively used in polymeric dental materials, which has the characteristics of low volatility and tissue diffusivity [27]. But its viscosity is high. TEGDMA is a diluent and can improve the viscosity of Bis--GMA for better handling [28]. SiO2 is the most common inorganic filler with the highest proportion (about 60 - 90%), which determines the multiple properties of the resin composites. Nano-MgO was introduced into the inorganic filler SiO2 as an antibacterial agent to improve the antibacterial properties of resin composites and prolong the service life of the composite.

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Fig. 1. The schematic diagrams of the materials developed and areas of application.

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Surface structure analysis of the experimental resin composite In Fig. 2, the SEM results show that the KH-570 modified micro-SiO2 and nano-MgO particles were relatively uniformly dispersed in each group of resin matrix and the particle size were different. However, as can be seen from the macroscale pictures on the upper right, control group and the experimental groups with lower content of nano-MgO have irregular depressions on the surface, which was not smooth and compact enough, while with the increase of the content of nano-MgO, the material surface gradually became dense, and smooth. Nano-MgO had a wide size distribution and an average size of 130 +/- 18 nm. As the dispersing phase and reinforcing body of resin composite, inorganic filler type, particle size, and particle distribution can greatly affect the properties of resin composite [29].

Fig. 2. Representative SEM image (× 5000) of the size and dispersion of micro-SiO2 and nano-MgO 5

particles in the resin matrix. The insets are the photographs of the specimens. Antibacterial activity The inhibition zone method is a classic bacteriostatic test. It is also known as the diffusion method and is a technique to determine the antibacterial potency according to the size of a transparent circle formed by restraining the growth of bacteria around the tested materials in agar plates [30]. In Fig. 3A, a bacteriostatic ring was produced around the aperture aside from the control group, which indicated that nano-MgO had an antibacterial effect of anti-S. mutans. The diameter of inhibition zone measured is shown in Fig. 3B. The diameter of inhibition zone gradually enlarged with the increase of nano-MgO powder weight. Except for comparisons between 2 mg and 4 mg, 8 mg and 10 mg, 10 mg and 12 mg, the differences between the other groups were statistically significant (p < 0.05). The result is supported by the findings of Jin et al. [31], that is, the antibacterial activity of nano-MgO was dose-dependent.

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Fig. 3. Anti- S. mutans activity of nano-MgO powder. (A) the inhibition ring. The edges of the rings were marked in white circles. (B) the diameter of inhibition ring. ns No statistically significant difference between the groups under each horizontal line (p > 0.05).

Number of colonies (×105 CFU/mL) 475.3 ± 29.7 153.7 ± 5.7* 94.3 ± 4.0* 22.7 ± 3.0* 2.7 ± 1.2*

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Group A0 A1 A2 A3 A4

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We then tested the antibacterial property of nano-MgO in resin composite using the film contact method. Table 2 presents that the inhibitory effect of experimental resin composites on the growth of S. mutans. When compared with group A0, the total number of colonies for group A1, A2, A3, and A4 were reduced significantly (p < 0.05). From group A1 to A4, the antibacterial effect became more and more significant, and the antibacterial rate of group A4 was as high as 99.4%. This result demonstrated that the resin composite has excellent antibacterial activity after the introduction of nano-MgO powder. As a result, the nano-MgO containing resin composites could dramatically restrain the metabolic activity of S. mutans covering their surface and so achieve the goal of antimicrobial and caries prevention. Hence, the first null hypothesis of the current study that the resin composites containing nano-MgO would display activity against S. mutans, was accepted. Antibacterial rate (%) 67.7 80.2 95.2 99.4

Table 2 The antibacterial activity of resin composites containing different nano-MgO. * denotes that the difference is statistically significant compared with group A0. Due to the ultra-small size and high surface-to-volume ratio, nanoparticles have strong antibacterial activity. Previous studies suggested that the main reasons for the antibacterial effect were the destruction of cell membrane caused by nanoparticles, such as the production of reactive oxygen species and the interaction between nano-MgO and bacterial surface [32-34]. However, the specific 6

antibacterial mechanism is still unclear. Wear resistance and compressive strength (CS) Wear refers to the progressive loss, transfer or residual deformation of surface materials due to the relative motion of two contacting objects. During mastication, resin composite and teeth or food move relative to each other resulting in the resin composite being continuously worn. This wear, especially in the posterior region which bears high occlusal forces, can result in the reduction of the surface volume and edge destruction of the prosthesis. Resin composite wear can be divided into two-body wear and three-body wear. Three-body wear occurs through indirect action of abrasive particles (food particles, etc.) between resin composite and teeth. The DMW-1 wear testing machine used in this experiment, an improved product of the CW3-1 abrasion tester [35], simulates three-body wear. Its coefficient of variation was no more than 5%, which enhances the comparability of materials and the reliability of results. Past research has shown a high correlation between laboratory data of CW3-1 abrasion tester and clinical data [36].

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Fig. 4 exhibits that the height loss and volume loss of group A0 were statistically higher than those of group A2, A3 and A4 (p < 0.05) after 800 wearing cycles. There were no statistically significant differences between the other groups (p > 0.05). Moreover, Pearson correlation analysis showed that the volume loss of experimental resin composites was significantly positively correlated with their height loss (r = 0.978 ~ 0.999, p < 0.05). This validated part of the second null hypothesis that the addition of nano-MgO would not jeopardize the wear resistance of resin composites. Indeed, nano-MgO actually improved the wear resistance.

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Inorganic fillers are an important factor in determining the wear resistance of resin composites. In this study, the hybrid filler composed of micro-silica and nano-MgO was used. Its particle size distribution was wide, which could narrow the gap between the filler particles, enhance the stability of the resin matrix, increase the filler retention and reduce the falling off of the filler [37]. KH-570 is a type of commonly used silane coupling agent, whose siloxy group can form a Si-O-Si bond with siloxy group on the surface of SiO2 filler in the hydrolyzed state. At the same time, the methacrylate group can produce copolymerization with the resin matrix, which can tightly bonded the inorganic filler and the resin matrix [38]. Therefore, the wear resistance of the experimental resin composites was improved. A

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Fig. 4. The height loss (A) and volume loss (B) of resin composites after 800 wearing cycles. * Significantly different compared with the A0 group (p < 0.05).

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The typical worn surface morphology of the specimens is exhibited in Fig. 5. A relatively smooth surface with a few wear marks was observed from group A0 and a handful of white debris from Group A1 under low magnification. In comparison, the surfaces of group A2, A3, and A4 had more obvious wear marks than A0 and A1, which was in consistent with the height and volume loss.

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Fig. 5. Representative SEM images of worn material surfaces after 800 three-body wear cycles.

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The CS of experimental resin composites is shown in Fig. 6. The results indicate that CS values tend to decrease as the proportion of nano-MgO increase. The difference between any two groups was statistically significant (P < 0.05) except for A2 and A3. This could be ascribed to the agglomeration of nano-MgO. Due to the small particle size, large surface-to-volume ratio and high surface energy, nanoparticles are in an energy unstable state and easy to agglomerate and form secondary particles, making the particle size larger [39]. Aggregated nano-fillers showed porous structure, and might enhance the crack propagation in the specimen under load [40]. Therefore, the second null hypothesis of the present study that the addition of nano-MgO would not jeopardize the wear resistance and mechanical properties of resin composites was partially accepted. Namely, the former was accepted and the latter was rejected.

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Fig. 6. The compressive strength (CS) of the experimental resin composites. significant difference between the groups under each horizontal line (p > 0.05).

ns

No statistically

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Conclusion The application of nanotechnology to combat dental caries is a promising direction for the prevention and treatment of tooth decay. Nano-MgO could endow resin composites with excellent antibacterial properties and improve their wear resistance. This technology is promising for use in various dental composites, adhesives, cements and sealants to inhibit dental caries. The agglomeration of nano-MgO, long-term antibacterial effect, improvement of the mechanical properties and antibacterial mechanism remains to be further studied. In addition, in vivo and in situ studies are particularly important and other avenues of future work of nano-MgO resin composites.

Author Contributions Section 8

Zhongyuan Wu: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Visualization; Haiping Xu: Conceptualization, Methodology, Investigation, Data Curation, Writing - Review & Editing; Wei Xie and Meimei Wang: Conceptualization, Methodology, Investigation, Data Curation, Validation; Cunjin Wang and Cheng Gao: Methodology; Software, Formal analysis; Jie Liu: Conceptualization, Writing - Review & Editing, Supervision; Fang Gu and Jing Fu: Conceptualization, Methodology, Data Curation, Writing - Review & Editing, Visualization, Supervision, Project administration.

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Declaration of competing interest There are no conflicts to declare. Acknowledgements

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This work was supported by the National Natural Science Foundation of China [51703106] and Natural Science Foundation of Shandong Province of China [ZR2016EMQ05]. The authors wish to thank Department of Dental Materials, Peking University School and Hospital of Stomatology for hosting the test with the DMW-1 wear testing machine. Thanks to Nicholas Fischer for the editing of this manuscript. References

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