Development of high performance dental resin composites with outstanding antibacterial activity, high mechanical properties and low polymerization shrinkage based on a SiO₂ hybridized tetrapod-like zinc oxide whisker with C=C bonds

Abstract

Outstanding antibacterial activity, low polymerization shrinkage and high mechanical properties are key requirements for dental resin composites (DRCs), however, they are difficult to be simultaneously obtained, and becomes the bottleneck for developing DRCs. Herein, a new kind of functional zinc oxide whisker (ZnOw) with C=C bonds, coded as SK-ZnOw, was synthesized by grafting SiO₂ on the surface of ZnOw using the Stöber method. Then SK-ZnOw was embeded into commercial dental resin (dimethacrylate resin, UBT) to fabricate a novel kind of DRCs (SK-ZnOw/UBT). The structure and comprehensive performances including degrees of conversion, polymerization shrinkages, antibacterial activity and mechanical properties of SK-ZnOw/UBT composites were systematically investigated. The results show that SK-ZnOw/UBT composites have simultaneously excellent and durable antibacterial activity, higher mechanical properties and lower polymerization shrinkages than both UBT resin and ZnOw/UBT composites. Especially, for the SK-ZnOw/UBT composite containing 20 wt% SK-ZnOw, its antibacterial rate and that of sample after storage in distilled water at 37 °C for 20 days are as high as 98.1% and 93.7%, respectively; moreover, its flexural strength, flexural modulus and Vickers hardness are about 1.3, 1.1 and 1.4 times those of corresponding values of ZnOw/UBT composite with 20 wt% ZnOw. Meanwhile the polymerization shrinkage of the former is only 0.8 times of that of the latter, clearly demonstrating that the bottleneck of traditional DRCs in literature has been solved. The outstanding performances of SK-ZnOw/UBT composites suggest that the method provided herein is facile and effective to develop novel functional fillers and high performance DRCs.

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1. Introduction

It is known that pure dental resins do not have antibacterial effect, but also are able to accumulate more dental biofilm compared with enamel and other restorations,¹ and thus leading to secondary caries and fracture failure.² So far, the most effective solution is adding antibacterial agents for endowing DRCs with excellent antibacterial property.³

Antibacterial agents for DRCs can be classified in four types. The first type is releasable antibacterial agents including fluorinated monomers or fillers⁴ and chlorhexidine,^5,6 and so on, and they have excellent antibacterial properties, but these decrease gradually over time and will leave voids in materials, leading to notable reduction of mechanical properties.² For example, with the addition of only 3 wt% chlorhexidine into dental resin, the flexural strength decreased from 93 MPa to 70 MPa. According to ANSI/ADA specification no. 27-2009 (ISO 4049-2009), the clinically acceptable flexural strength is 80 MPa.⁷

The second type of antibacterial agents is Ag-containing substances, which have the best antibacterial activity,⁸ but Ag ion is easily oxidized into metal silver, bringing discoloration; in addition, excessive Ag tends to result in oral tissue injury or disease.⁹ The third type is quaternary ammonium salts,¹⁰ which can be chemically bound within dental resins via active groups, so this type often shows efficient and durable antibacterial activity.^11,12 However, a large loading of quaternary ammonium salts should be used to get high antibacterial activity, but this also tends to get lower mechanical properties.^11,13,14 Cheng et al. added 7 wt% of quaternary ammonium dimethacrylate into amorphous calcium phosphate/dental resin composite,¹⁵ and found that the flexural strength decreased from 63 MPa to 54 MPa. The fourth type is nanosized ZnO (n-ZnO), which is reported to have significant antibacterial activity against S. mutans ATCC 25175.¹⁶ However, Hojati et al.¹⁷ found that the antibacterial activity of commercial DRCs with n-ZnO disappeared over time; besides, a great antibacterial rate was obtained when more than 2 wt% of n-ZnO was used, but the flexural and compressive strengths of modified DRCs decreased to 78 MPa and 80 MPa, about 7–13% and 40% lower than those of DRCs without n-ZnO, respectively, which are not sufficient for clinical use.^7,18 This result is due to the poor dispersion of n-ZnO in matrix and poor interfacial adhesion between n-ZnO and matrix.

All antibacterial agents mentioned above can endow DRC with appreciative antibacterial activity, but also degrade mechanical properties that are not clinically acceptable, so it is a very significant study to develop antibacterial DRCs with high mechanical properties.

On the other hand, note that polymerization shrinkage is a common side effect of curing for thermosetting resins,¹⁹ but which should be avoided in dental application to prevent dental caries, aesthetics and chewing function of teeth.^20,21 In order to establish DRCs with low polymerization shrinkage, the amount of inorganic fillers generally should be more than 40 wt%,²² however such high loading usually degrades the processing characteristics and mechanical properties.^23,24

Zinc oxide whisker (ZnOw) is the only whisker that has unique three-dimensional tetrapod structure, and therefore shows better isotropic and toughening effects compared with zero-dimensional or one-dimensional fillers.^25,26 A recent study showed that ZnOw has the best antibacterial activity among ZnOw, n-ZnO and micro-sized ZnO particles.²⁷ The DRC with ZnOw was solely reported by Niu group²⁸ up-to-date, who added ZnOw into cis-butenedioic anhydride modified epoxy dimethacrylate resin, and found that the antibacterial rate of the composite containing 5 wt% or 10 wt% ZnOw was 94% or 99%, but decreased to 89% or 90% after stored in distilled water at 37 °C for 3 months. In addition, serious agglomeration was found when the loading of ZnOw reached 10 wt%, this is not beneficial to get high mechanical performance, but also reducing the feasibility of controlling the polymerization shrinkage by adjusting the loading of ZnOw. Therefore, it is significant to explore a DRC with low polymerization shrinkage, high antibacterial activity and mechanical properties.

Note that flexural modulus and Vickers hardness are known to be important properties of DRCs,²⁹ but few literature reported the influence of ZnOw on these properties of DRCs to date.

The aim of the present study is establishing a new kind of next generation DRCs with low polymerization shrinkage, outstanding antibacterial activity and high mechanical properties. Starting from ZnOw, SiO₂ particles were hybridized on the surfaces of ZnOw, and a new ZnOw hybrid (SK-ZnOw) was obtained. The influences of surface properties and content of fillers on antibacterial activity and mechanical properties of DRCs were systemically discussed, and the essence behind was also studied.

2. Experimental

2.1 Materials

Dental monomers used in this study were urethane dimethacrylate (UDMA, Sigma-Aldrich), bisphenol A glycerolate dimethacrylate (Bis-GMA, Sigma-Aldrich) and triethylene glycol dimethacrylate (TEGDMA, Aladdin). The photo initiator system was composed of camphorquinone (CQ, Aladdin) and dimethylaminoethyl methacrylate (DMAEMA, Sinopharm Chemical Reagent Co., Ltd., China). ZnOw with a needle length of 10–50 μm, a root diameter of 0.5–5 μm and a specific surface area of 7.74 m² g⁻¹ was purchased from Chengdu Crystrealm Co., Ltd., China. 3-Methacryloxypropyltrimethoxysilane (γ-MPS) was obtained from Shanghai Yuanye Bio-Technology Co., Ltd., China. Ammonia, tetraethyl orthosilicate (TEOS), ethanol and acetic acid were bought from Sinopharm Chemical Reagent Co., Ltd., China.

2.2 Synthesis of SK-ZnOw

SK-ZnOw was synthesized through two steps; they were γ-MPS modified ZnOw, and the hybridization of SiO₂. Typically, 10 wt% γ-MPS relative to ZnOw was added to aqueous ethanol (90%) of which pH was previously adjusted with acetic acid to 4.0, and then the aqueous ethanol was magnetically stirred at 25 °C for 30 min for entire hydrolysis of γ-MPS. After that, appropriate quantity of ZnOw was added, followed by heating the mixture and maintained at 60 °C with stirring for 2 h. Afterwards, the mixture was vacuum distilled to remove volatile substances and washed with aqueous ethanol (50%) to eliminate residual γ-MPS and attached materials. Finally, the powder was obtained by vacuum-dried at 80 °C for 12 h, and named as K-ZnOw.

As shown in Table 1, 8.0 g of K-ZnOw, ammonia (15, 30 or 45 mL) and distilled water (10, 20 or 30 mL) were added into a 1 L three-necked flask containing 450 mL of ethanol with continuous stirring, and then 8.0 g of TEOS was added according to different dropping time (instant, 4 min or 8 min) and kept the temperature at 30 °C for 4, 6 or 8 h. After that, filtering the mixture, and the obtained powder was washed with distilled water for several times, followed by vacuum-drying at 80 °C for 12 h, the resultant product was coded as nSK-ZnOw, where n means the experiment number.

Table 1 Parameters and results of the orthogonal experiment^a

	Factors				SiO2 loading (%)
	A [ammonia] (mL)	B [H2O] (mL)	C [reaction time] (h)	D [dropping time of TEOS]
a 450 mL of ethanol, 8.0 g of K-ZnOw and 8.0 g of TEOS for each experiment, the mixture was heated to 30 °C.
1	15	10	4	Instant	16.71
2	15	20	6	4 min	15.57
3	15	30	8	8 min	20.99
4	30	10	6	8 min	21.9
5	30	20	8	Instant	25.42
6	30	30	4	4 min	17.81
7	45	10	8	4 min	20.61
8	45	20	4	8 min	17.98
9	45	30	6	Instant	15.60
K1	53.27	59.22	52.5	57.73
K2	65.13	58.97	53.07	53.99
K3	54.19	54.4	67.02	60.87
k1	17.76	19.74	17.5	19.24
k2	21.71	19.66	17.69	18.00
k3	18.06	18.13	22.34	20.29
Range	3.95	1.61	4.84	2.29

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2.3 Preparation of the composites

4 g UDMA, 1 g Bis-GMA and 5 g TEGDMA were blended with vigorous stirring to obtain a co-monomer. Under the protection of yellow light, 0.05 g CQ and 0.05 g DMAEMA were added into the co-monomer with stirring to obtain a transparent liquid, followed by adding appropriate quantity of SK-ZnOw with strong stirring at 25 °C for 1 h to get a homogeneous mixture. Then, the mixture was cast into a polytetrafluoroethylene mould fasten by two glass slides and light-cured on each side for 60 s using a commercial light-curing unit (LY-A180, 420–480 nm, 1500 mW cm⁻², Foshan Liang Ya Dental Equipment Co., Ltd., China). Finally, the obtained composite was named as ySK-ZnOw/UBT after removal from the mould, where y represents the weight loading of SK-ZnOw in the whole system, taking values of 20, 30 and 40.

With above procedure except replacing SK-ZnOw with ZnOw, yZnOw/UBT composite was prepared.

Pure resin without fillers was also prepared as control using the same procedure and marked as UBT.

2.4 Characterizations

The SiO₂ loading of SK-ZnOw was determined by means of titration. In particular, 0.2 g of SK-ZnOw and 2–3 drops of methyl orange indicator were mixed with 10 mL of distilled water to obtain a suspension. Then hydrochloric acid with known concentration was added dropwise slowly into the suspension with continuous stirring until the color changed to red and stabilized for at least 30 min. The SiO₂ loading was calculated using eqn (1), each sample was tested for 3 times and took the average of the results.where a is the concentration, and b is the dosage of hydrochloric acid.

The antibacterial rates of SK-ZnOw, ZnOw and K-ZnOw against S. mutans ATCC 25175 were measured using oscillator method according to Chinese standard GB/T 21510-2008.

The antibacterial rates of the composites against S. mutans ATCC 25175 were measured by the direct contact test.²⁸ For each composite, ten specimens with dimensions of (12 ± 0.5) × (12 ± 0.5) × (3 ± 0.1) mm³ were prepared. Five of them were tested after immersed in distilled water at 37 °C for 24 h and served as the non-aged group, and the others were tested after stored under the same conditions for 20 days and served as the 20 day-aged group. The distilled water was changed every 48 h. UBT was used as a blank control, and polytetrafluoroethylene sheet with dimensions of (12 ± 0.5) × (12 ± 0.5) × (2 ± 0.1) mm³ was served as the positive control. Before tests, all specimens were sterilized at 120 °C for 20 min using an autoclave (YM 30K, Shanghai Sanshen Medical Instrument Co., Ltd., China). In order to simulate oral environment, the contact of the bacteria with composites was carried out in the dark. The acquired data of antibacterial rate were analysed with one-way ANOVA (α = 0.05) of SPSS software (version 19.0), and the significant differences were determined using the Tukey's tests.

The degree of conversion was determined using a Fourier transform infrared (FTIR) spectroscopy (Vertex 70, Bruker, Germany) with an attenuated total reflectance (ATR). Three specimens with a diameter of (6 ± 0.1) mm and a thick of (3 ± 0.1) mm were made for each composite. The FTIR spectra of specimens before and after light-cured were recorded with 32 scans at a resolution of 4 cm⁻¹. The peak of the aliphatic C=C at 1638 cm⁻¹, which was consumed during polymerization, was chosen as the analytical absorption band, whereas the peak of the aromatic C–C bond at 1608 cm⁻¹, which was not affected by the curing process, was selected as the reference absorption. The degree of conversion was calculated using eqn (2),

where R_cured stands for the ratio of the absorbance at 1638 cm⁻¹ and 1608 cm⁻¹ of cured specimen, R_uncured is the ratio of the absorbance at 1638 cm⁻¹ to that at 1608 cm⁻¹ of uncured specimen.

Solid-state NMR spectrometer with 400 MHz (WB/AVANCE III, Bruker, Switzerland) was used to record the ¹³C-NMR spectra of ZnOw and K-ZnOw.

An X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher Scientific, USA) was employed to investigate the elementary compositions of ZnOw and K-ZnOw.

The polymerization shrinkage was calculated from the densities measured according to the Archimedes' principle.³⁰ Three samples with a diameter of (6 ± 0.1) mm and a thick of (3 ± 0.1) mm were made for each composite before and after curing, respectively. The weights of each sample in air and in distilled water were measured with an electronic balance accuracy to ±0.001 g (FA2104B, Shanghai Yueping Scientific Instrument Co., Ltd), and the density of each uncured or cured sample was calculated using eqn (3),

where ρ represents the density of sample (g cm⁻³), m_a is the mass of the sample in air (g), m_w reflects the mass of sample in distilled water (g), ρ_w represents the density of distilled water at the exactly measured temperature (g cm⁻³), and ρ_a is the density of air (0.0012 g cm⁻³). Finally, the polymerization shrinkage of each composite was determined using eqn (4),where ρ_uncured represents the density of the uncured specimen (g cm⁻³), and ρ_cured represents the density of the cured specimen (g cm⁻³).

Flexural properties were measured using a Universal Testing Machine (Instron E10000, USA) at a relative humidity of 60 ± 5% in air at 25 °C. A standard three-point flexural test with a span distance of 20 mm at a crosshead speed of 0.75 mm min⁻¹ was conducted according to ISO 4049-2009. Five specimens with dimensions of (2 ± 0.05) × (2 ± 0.05) × (25 ± 1) mm³ were prepared for each composite, and all cured specimens were immersed in distilled water at 37 °C for 24 h before tests.

The Vickers hardness was tested with a micro-hardness tester (HV-1000, Shanghai Aolong Xingdi Testing Instrument Co., Ltd., Former Shanghai Material Testing Machine Factory, China). Each sample has a diameter of (6 ± 0.1) mm and a thickness of (3 ± 0.1) mm, five specimens were prepared for each composite, and all cured specimens were immersed in distilled water at 37 °C for 24 h before tests. Vickers hardness was determined with the application of a 300 g load for 20 s. After the load (P) was applied, the diagonal indent lengths (d₁ and d₂) were measured. Five indentations were made on each sample and the mean value (d) of five indentations on each sample was estimated. The Vickers hardness was calculated according to the standard ASTM E384-11e1 using eqn (5).

A scanning electron microscope (Hitachi S-4700, Japan) was employed to observe the morphologies of samples.

An X-ray diffractometer (XRD) (X'Pert-Pro MPD, Philips, Netherlands) was used to analyse the chemical structures of K-ZnOw and SK-ZnOw.

3. Results and discussion

3.1 Design and synthesis of SK-ZnOw

Fig. 1 shows FTIR spectra of ZnOw and K-ZnOw. No obvious absorption exists in the spectrum of ZnOw, while there are several typical vibration absorptions of groups including CH₃ (2985 cm⁻¹),²³ CH₂ (2899 cm⁻¹),²³ C=O (1725 cm⁻¹),^31–33 Si–O–Si (1067 cm⁻¹)²³ and Si–OH (964 cm⁻¹)³³ in that of K-ZnOw, indicating that γ-MPS has been successfully grafted onto the surface of ZnOw.

Fig. 1 FTIR spectra of ZnOw and K-ZnOw.

Above statement can be further proved by the ¹³C-NMR spectra and XPS patterns of ZnOw and K-ZnOw. As shown in Fig. 2, no obvious carbon shift is observed in the ¹³C-NMR spectrum of ZnOw, however notable characteristic carbon shifts belonging to γ-MPS are found at 18.3, 66.6, 23.1, 10.1 and 45.9 ppm in the ¹³C-NMR spectrum of K-ZnOw.

Fig. 2 ¹³C-NMR spectra of ZnOw and K-ZnOw.

ZnOw and K-ZnOw have similar XPS survey spectra (Fig. 3a). However, as shown in Fig. 3b, a typical peak of Si element is observed in the Si2p spectrum of K-ZnOw, while no peak exists in that of ZnOw. In addition, the O1s spectra (Fig. 3c and d) demonstrate that the O1s peak of ZnOw consists of the lattice oxygen of ZnO (530.1 eV) and the hydroxyl group (531.6 eV);²⁷ while the O1s peak of K-ZnOw includes additional –COO– (532.4 eV) and –Si–O– (530.7 eV).³⁴

Fig. 3 XPS survey (a) and Si2p (b) spectra of ZnOw and K-ZnOw, O1s spectra of ZnOw (c) and K-ZnOw (d).

Therefore, based on above FTIR, ¹³C-NMR and XPS results, it can be concluded that γ-MPS has been successfully grafted onto the surface of ZnOw.

In order to synthesize SK-ZnOw with different loading of SiO₂ grafted, the effect of different factors should be studied. An orthogonal experiment with four factors and three levels (Table 1) was conducted, and the relationships between level and SiO₂ loading of all factors, which depend on the hybrid mechanism of SiO₂, are shown in Fig. 4. As we've known, the hybrid process of SiO₂ comprises the hydrolysis of TEOS and condensation of orthosilicic acid.³⁵ Concretely, TEOS hydrolyses to orthosilicic acid, which was condensate with –OH on the surface of K-ZnOw to form SiO₂ seeds, from which SiO₂ particles or layer is developed. So sufficient H₂O (factor A) is necessary, but if too much H₂O is added, the excessive H₂O as well as the water produced during the condensation of orthosilicic acid may reduce the concentration of orthosilicic acid and the quantity of orthosilicic acid grafted on K-ZnOw surface, and consequently, leading to decreased SiO₂ loading.

Fig. 4 The relationships between level and SiO₂ loading of various factors.

Increasing ammonia amount (factor B) can promote the hydrolysis of TEOS, hence more orthosilicic acid and thus SiO₂ seeds are produced as soon as TEOS is added, leading to more SiO₂ grafted. However, when the amount of ammonia continues to increase, the quantity of orthosilicic acid becomes much larger than that of the hydroxyl on K-ZnOw surface, so the amount of SiO₂ seeds does not increase any longer; when too more amount of ammonia is added, then the system has too more H₂O content, leading to declined SiO₂ loading.

Reaction time (factor C) has a significant impact on the SiO₂ loading. The longer the reaction time is, the greater the size of SiO₂ particles coated on K-ZnOw surface, and the higher the SiO₂ loading.

Dropping time of TEOS (factor D) has a little effect on SiO₂ loading. A lot of orthosilicic acid is produced at the beginning of reaction if TEOS was poured instantly, followed by generation of lots of orthosilicic acid and SiO₂ seeds, and an elevated SiO₂ loading is obtained at last.

Table 1 summarizes all parameters, results and analyses of the orthogonal experiment. According to the range of each factor, the order of various factors based on influencing degree to the SiO₂ loading is: reaction time > amount of ammonia > dropping time of TEOS > amount of water.

Fig. 5 shows SEM photographs of original and modified ZnOw. ZnOw presents a smooth surface while K-ZnOw has a rough one with many glitches owing to the γ-MPS grafted. The surfaces of SK-ZnOw are all very bumpy with a large number of spherical or hemispherical protrusions. Although the SiO₂ loadings of nine kinds of SK-ZnOw are different to some extent, no significant difference can be found in their surface morphologies.

Fig. 5 SEM photographs of ZnOw (a), K-ZnOw (b) and SK-ZnOw fillers ((c) 1SK-ZnOw; (d) 2SK-ZnOw; (e) 3SK-ZnOw; (f) 4SK-ZnOw; (g) 5SK-ZnOw; (h) 6SK-ZnOw; (i) 7SK-ZnOw; (j) 8SK-ZnOw; (k) 9SK-ZnOw).

The chemical structures of original and modified ZnOw are confirmed by FTIR and XRD. As shown in Fig. 6, no obvious absorption is found in the FTIR spectrum of K-ZnOw. However, in the FTIR spectra of 1SK-ZnOw ∼ 9SK-ZnOw, there are two wide and strong absorptions attributing to the asymmetric (1069 cm⁻¹) and symmetrical (795 cm⁻¹) stretching vibrations of Si–O–Si, respectively.³⁶

Fig. 6 FTIR spectra of modified ZnOw.

Fig. 7 shows XRD patterns of original and modified ZnOw and their partially enlarged ones. Typical diffraction peaks for ZnOw can be observed in all patterns (Fig. 5A), illustrating that the modification does not change the ZnOw crystal. However, a distinct and broad peak assigning to amorphous SiO₂ appears at 15–30° in the pattern of each SK-ZnOw (region within the red box in Fig. 5b).³⁷

Fig. 7 XRD patterns (A) and partially enlarged views (B) of original and modified ZnOw.

Fig. 8 gives antibacterial rates of original and modified ZnOw against S. mutans ATCC 25175. All fillers have extremely similar antibacterial rates, so it can be concluded that γ-MPS treatment and SiO₂ hybridization do not affect the antibacterial activity of ZnOw.

Fig. 8 Antibacterial rates of original and modified ZnOw.

Based on above data, it is confirmed that SiO₂ is successfully coated on K-ZnOw surface, and its loading can be adjusted by varying reaction time, ammonia amount, dropping time of TEOS and the quantity of water; meanwhile SK-ZnOw has similar antibacterial activity as ZnOw. Although nine SK-ZnOw particles have different SiO₂ loading, no significant changes in their surface morphologies and antibacterial activities are found. Therefore, we selected 1SK-ZnOw, which has the simplest preparation process, as the representative to fabricate SK-ZnOw/UBT composites for systematically studying the influence of the SK-ZnOw content on the structure and properties of the composites.

3.2 Phase structures of the composites

Fig. 9 shows SEM photographs of flexural cross-sections of composites and UBT. The fractured surface of UBT resin is smooth and uniform, while each composite has a rough one with numerous folds, indicating that crack deflection occurred during the fracture of composites, and thus consume more energy than UBT during fracturing.^38,39 In addition, SK-ZnOw/UBT composites have much rougher fractured surfaces than ZnOw/UBT composites with the same filler content. For both kinds of composites, the surface roughness increases as the filler content increases.

Fig. 9 SEM micrographs of fractured surfaces of ZnOw/UBT, SK-ZnOw/UBT and UBT.

On the fractured surfaces of all ZnOw/UBT composites, there are many obvious and sharp holes, which are internal defects of composites and are formed by the leave of ZnOw during breaking. In the insert figures, apparent gaps between ZnOw and UBT matrix are visually observed, meaning that ZnOw and resin matrix have much weak interfacial force. However, except numerous rumples, there is little hole on the fractured surfaces of SK-ZnOw/UBT composites. According to the insert figures, no interspace exists between 1SK-ZnOw and resin matrix, and the rumples deflect when encountering 1SK-ZnOw, revealing that the interfacial bonding between them is extremely stable. Additionally, because of the relatively high filler content, fillers agglomeration appears in 40ZnOw/UBT and 40SK-ZnOw/UBT composites (region within the red circle).

Fig. 9 demonstrates that ZnOw and UBT have weak interfacial bonding, while this phenomenon is greatly improved for SK-ZnOw/UBT composites. This difference in interface structure makes an important impact on the mechanical properties of composites.

3.3 Antibacterial activity of the composites

Pure dental resins do not have antibacterial effect, but also are able to accumulate more dental biofilm compared with enamel and other restorations.¹ Hojati et al.¹⁷ found that the antibacterial rate of the DRC with 1 wt% n-ZnO was 75%, which increased to the value higher than 90% when 2–5 wt% n-ZnO was added. Unfortunately, the antibacterial activity almost disappeared after these composites were stored for one week. That is to say, the antibacterial activity of the DRC containing n-ZnO is transitory, so it is necessary to research the antibacterial activity of new composites in the present study.

Fig. 10 gives SEM micrographs of S. mutans cells in direct contact with non-aged composites and UBT for 24 h incubation at 37 °C in the dark. As can be seen, after 24 h incubation at 37 °C, there are a large amount of streptococcal chains on UBT surface, while few S. mutans cells can be found on the surfaces of all composites, indicating that non-aged composites have excellent antibacterial activity against S. mutans ATCC 25175.

Fig. 10 SEM micrographs of S. mutans cells after directly contacting with ZnOw/UBT, SK-ZnOw/UBT and UBT for 24 h incubation at 37 °C.

Table 2 lists the antibacterial rates of the non-aged and 20 day-aged composites against S. mutans ATCC 25175. There is no significant difference between various non-aged composites, so the filler content and modification have no evident influence on the antibacterial activity. After 20 day storage, the antibacterial rate of 20ZnOw/UBT composite decreases to 83% while those of others still remain at 90–94% despite a little decline. In particular, 20SK-ZnOw/UBT composite has the highest antibacterial rate, indicating that the composite with stable antibacterial activity can be obtained at a low level of SK-ZnOw.

Table 2 Antibacterial rates of SK-ZnOw/UBT and ZnOw/UBT (values represent means ± SD, n = 5)^a

Sample	Antibacterial rate (%)
	Non-aged group	20 d aged group
a Different lower-case letters in rows indicate statistically significant difference at 5%.
20ZnOw/UBT	95.8 ± 5.7 b	83.4 ± 3.8 a
30ZnOw/UBT	95.2 ± 6.6 b	92.0 ± 4.3 ab
40ZnOw/UBT	99.6 ± 0.4 b	94.4 ± 4.2 b
20SK-ZnOw/UBT	98.1 ± 2.4 b	93.7 ± 6.7 ab
30SK-ZnOw/UBT	98.0 ± 1.4 b	91.6 ± 5.3 ab
40SK-ZnOw/UBT	99.2 ± 0.9 b	90.1 ± 3.6 ab

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T-ZnOw has the same antibacterial mechanism as ZnO.²⁸ The antibacterial activity of ZnO comes from the reactive oxygen species (ROS) produced by crystal defects in the surface layer of ZnO, and the higher the concentration of ROS is, the stronger the antibacterial activity.^27,40 However, Venz and Dickens's study⁴¹ showed that, because of hydrophilic groups in the polymer network, the cured resins of UDMA, Bis-GMA and TEGDMA have large water absorptions, which were 2.67%, 3.60% and 5.45%, respectively, after stored in a humidor with a relative humidity of 100% for 6 months. So the UBT resin used in this study also has large water absorption. Moreover, water absorption is closely related to the interface bonding between fillers and resin matrix, so the poorer the interface bonding is, the higher the water absorption.^42,43 It is not difficult to understand that the quantity of water absorbed by absorption increases, and thus the diffusion and loss of ROS are accelerated, as a result, the concentration of ROS and the antibacterial rate significantly decrease. In the present study, 20ZnOw/UBT composite has the worst interface bonding and lowest filler content, so it has the least ROS concentration after storage, leading to most significantly dropping of the antibacterial rate. As a result of filler agglomeration, the composites with 40 wt% of fillers absorb large quantity of water, and their concentrations of ROS reduce dramatically, therefore their antibacterial rates obviously fall compared with those of the composites with 20 wt% or 30 wt% of fillers. For 20SK-ZnOw/UBT composite, it has good interface bonding between 1SK-ZnOw and resin matrix, and there are no fillers agglomerations, so 20SK-ZnOw/UBT composite maintains high antibacterial rate.

3.4 Degrees of conversion of the composites

Degree of conversion refers to the percentage of C=C bonds participating in the polymerization reaction of all C=C bonds before curing, or the percentage of monomers participating in the polymerization reaction of all monomers before curing. Degree of conversion has an important impact on the polymerization shrinkage and mechanical properties, a high degree of conversion may result in improved mechanical properties and increased polymerization shrinkage.⁴⁴ In general, dental resin has a degree of conversion of 39–79%,⁴⁵ while dental composites have a lower one,⁴⁶ this result is attributed to two reasons. Firstly, the presence of fillers hinders the movement of molecules and reduces the mobile capacity of monomers and radicals;⁴⁶ secondly, the fillers, including ZnO, barium glass powder, and so on, have higher refractive indexes for blue light than dental resin, and thus disturb the spread and penetration to internal system of blue light, leading to the reduction of the photoinitiation efficiency.⁴⁷ Therefore, it is highly important to study the influence of 1SK-ZnOw on the degree of conversion.

The degree of conversion of UBT prepared herein is 75% and decreases to 69–71% after adding fillers (Fig. 11). These values are similar to those in literature,^29,46,48 this is helpful to preserve the original mechanical properties of UBT.

Fig. 11 Degrees of conversion of ZnOw/UBT, SK-ZnOw/UBT and UBT.

Many researchers had reported that the filler content has a significant influence on the degree of conversion. For instance, Halvorson⁴⁶ and Papadogiannis's groups⁴⁴ found that the presence of fillers could limit the movement of monomers and radicals, and thus decreases the degree of conversion; however, this exertion of this influence requires certain conditions.⁴⁹ Specifically, the effect is significant if fillers have small size or large specific surface area (such as 40 nm, 60 m² g⁻¹), while this effect can be ignored when fillers have large size or small specific surface area (such as 3.4 μm, 3.7 m² g⁻¹). Considering the dimensions of ZnOw, the ZnOw loading doesn't significantly affect the degree of conversion of the composites.

The surface modification of fillers also has an impact on the degree of conversion. According to the study of Halvorson et al.,⁴⁶ silanization could reduce the degree of conversion because the C=C bonds of silane layer are restricted around filler surface, and thus become more difficult to participate in the polymerization compared with dental monomers. But the impact can be neglected if the filler level is as low as less than 40 wt%.⁴⁶ So in this study, owing to low filler level (≤40 wt%), no obvious variation has been observed for the degree of conversion between SK-ZnOw/UBT and ZnOw/UBT composites, yet 1SK-ZnOw has silane layer on it. Besides, curing reaction has a relationship with the refractive index of the filler. Shortall et al.⁴⁷ reported that a complete curing was easy to obtain when the refractive indexes of filler and resin matrix were nearly matched. Herein, though amorphous SiO₂ particles on 1SK-ZnOw surface may have a certain influence on the refractive index of ZnOw (refractive index of ZnO is 2.0, and the refractive index of amorphous SiO₂ is 1.46 which nears to that of dental resin^50,51), the curing reaction hasn't been changed significantly maybe due to the low filler content (the filler content in the ref. 47 is 70 wt%, which is much higher than that in this paper), as a result, there is negligible influence on the degree of conversion.

3.5 Polymerization shrinkages of the composites

Polymerization shrinkage is an important performance of DRCs,^21,29 which is within the range of 2–5.6% for commercial DRCs with 35–66 vol% (or 56–80 wt%) fillers.^52,53 Because high filler content generally sacrifices the processing feature, so it is critical to develop DRCs with low polymerization shrinkage at a low filler content.

Fig. 12 shows the polymerization shrinkages of UBT, ZnOw/UBT and SK-ZnOw/UBT composites. The composites have significantly lower polymerization shrinkages than UBT, and SK-ZnOw/UBT has lower polymerization shrinkage than ZnOw/UBT at the same filler content. Especially, with the addition of only 20 wt% 1SK-ZnOw, the polymerization shrinkage reduces from 8.7% of UBT to 5.1%, the reduction degree is as high as 41%. In literature, reducing the polymerization shrinkage to 5%, the filler content should be as high as 25–48 wt% (Table S1 in the ESI†), and a very high filler content (70 wt%) was essential to obtain a reduction of 41% in polymerization shrinkage (the value of 20SK-ZnOw/UBT composite).²⁹ Therefore, it is reasonable to state that 1SK-ZnOw has significant advantage in reducing polymerization shrinkage, this should be attributed to the unique structure of 1SK-ZnOw. In particular, the silane layer on 1SK-ZnOw surface can crosslink with dental resin matrix to form an abundant of connection points, and the rugged surface resulting from SiO₂ hybridization has improved the quantity of resin molecular bonding to 1SK-ZnOw and the bonding strength, eventually leading to the obstacle effect on shrinkage of resin matrix around 1SK-ZnOw.

Fig. 12 Polymerization shrinkages of ZnOw/UBT, SK-ZnOw/UBT and UBT.

3.6 Mechanical properties of the composites

Excellent mechanical properties (Vickers hardness, flexural strength and modulus) are critical for long-term restoration of DRCs.⁵⁴ Fig. 13 gives the flexural strengths of UBT and composites. Compared with UBT, ZnOw/UBT composites have lower flexural strengths, which decrease with the increase of ZnOw content. Similar phenomenon was also previously reported.^55–57

Fig. 13 Flexural strengths of ZnOw/UBT, SK-ZnOw/UBT and UBT.

Attractively, SK-ZnOw/UBT composites have superior flexural strengths compared with UBT. The flexural strength of 20SK-ZnOw/UBT composite is 83 MPa, which is higher than the standard value (80 MPa) claimed in the ISO 4049-2009. With increasing the loading of 1SK-ZnOw, the flexural strength improves evidently and reaches the maximum at 30 wt%, about 1.2 times of that of UBT, indicating that 1K-ZnOw has unique ability to reinforce dental resin. Generally, it is believed that good dispersion of fillers and outstanding interfacial adhesion are critical to gain excellent mechanical properties for filler reinforced composites.⁵⁸ Specifically, on one hand, the existence of vast tiny SiO₂ particles on whisker surface endows the interface with additional riveting effect and thus better reinforcing effect of whiskers;^38,59 on the other hand, the whiskers are separated from each other by the grafted SiO₂ particles, reducing their agglomeration and improving their dispersion.⁵⁹ In addition, the reaction between UBT and C=C bonds on the surface of 1K-ZnOw is beneficial to improve the dispersion of 1K-ZnOw and the interfacial adhesion between 1K-ZnOw with UBT resin. Compared with 30SK-ZnOw/UBT composite, 40SK-ZnOw/UBT composite has decreased flexural strength due to the agglomeration of 1K-ZnOw.

Fig. 14 gives the flexural moduli of UBT and composites. It is well known that flexural modulus is closely related to the surface properties and content of fillers.^5,60 Compared with UBT, all composites have higher flexural moduli, which increases as the filler content increases. This is a general rule of rigid filler reinforced resins.^23,61 Under the same filler content, SK-ZnOw/UBT composite has significantly greater flexural modulus than ZnOw/UBT, and the flexural modulus of 20SK-ZnOw/UBT composite is about 1.6 times of that of UBT. This is attributed to the good dispersion of 1SK-ZnOw as well as high interfacial adhesion with UBT resin as mentioned above. Similarly, 40SK-ZnOw/UBT composite has lower flexural modulus than 30SK-ZnOw/UBT due to the agglomeration of fillers, but the former still has higher flexural modulus than ZnOw/UBT composites and UBT.

Fig. 14 Flexural moduli of ZnOw/UBT, SK-ZnOw/UBT and UBT.

Vickers hardness is the index representing the surface strength, which is greatly significant for DRCs.²⁹ As shown in Fig. 15, the Vickers hardness increases as the filler content increases, and SK-ZnOw/UBT composites have obviously higher Vickers hardness than ZnOw/UBT composites at the same filler content. Especially when the content of 1SK-ZnOw is 20 wt%, the Vickers hardness is as high as 54 HV, higher than those of the majority of commercial DRCs.⁶²

Fig. 15 Vickers harnesses of ZnOw/UBT, SK-ZnOw/UBT and UBT.

Based on above analyses, it is reasonable to state that SK-ZnOw owns much better reinforcing effect than ZnOw. The DRCs based on SK-ZnOw possess higher mechanical properties than those of ZnOw, and only 20 wt% SK-ZnOw can endow DRCs with outstanding comprehensive performances.

4. Conclusions

A new kind of SiO₂ hybridized ZnOw (SK-ZnOw) with C=C bonds on the surface was developed, which has same antibacterial activity as ZnOw. A low filler content (20 wt%) can endow SK-ZnOw/UBT composite with low polymerization shrinkage, high flexural strength and modulus as well as Vickers hardness. Furthermore, the antibacterial rates of 20SK-ZnOw/UBT composite before and after 20 day storage were as high as 98.1% and 93.7%, respectively, completely overcoming the bottleneck problem existed in conventional DRCs. The attractive performances of SK-ZnOw/UBT composite are resulted from the good dispersion of SK-ZnOw in UBT and the suitably high interfacial adhesion between SK-ZnOw and UBT.

Acknowledgements

We thank National Natural Science Foundation of China (21274104), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financially supporting this project.

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Footnote

† Electronic supplementary information (ESI) available: Table S1 summarizes compositions of typical DRCs since 2013. See DOI: 10.1039/c6ra13498j

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