High performance dental resin composites with hydrolytically stable monomers

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Available online at www.sciencedirect.com

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

High performance dental resin composites with hydrolytically stable monomers Xiaohong Wang a , George Huyang a , Sri Vikram Palagummi b , Xiaohui Liu a , Drago Skrtic a , Carlos Beauchamp c , Rafael Bowen a , Jirun Sun a,∗ a

Dr. Anthony Volpe Research Center, American Dental Association Foundation, Gaithersburg, MD 20899, USA Biomaterials Group, Biosystems and Biomaterials Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA c Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA b

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. The objectives of this project were to: 1) develop strong and durable dental resin

Received 31 March 2017

composites by employing new monomers that are hydrolytically stable, and 2) demon-

Received in revised form

strate that resin composites based on these monomers perform superiorly to the traditional

5 October 2017

bisphenol A glycidyl dimethacrylate/triethylene glycol dimethacrylate (Bis-GMA/TEGDMA)

Accepted 18 October 2017

composites under testing conditions relevant to clinical applications.

Available online xxx

Methods. New resins comprising hydrolytically stable, ether-based monomer, i.e., triethylene glycol divinylbenzyl ether (TEG-DVBE), and urethane dimethacrylate (UDMA) were

Keywords:

produced via composition-controlled photo-polymerization. Their composites contained

Dental resin composites

67.5 wt% of micro and 7.5 wt% of nano-sized filler. The performances of both copolymers

Dental resins

and composites were evaluated by a battery of clinically-relevant assessments: degree of

Hydrolytically stable resins

vinyl conversion (DC: FTIR and NIR spectroscopy); refractive index (n: optical microscopy);

Composition controlled

elastic modulus (E), flexural strength (F) and fracture toughness (KIC ) (universal mechan-

polymerization

ical testing); Knoop hardness (HK; indentation); water sorption (Wsp ) and solubility (Wsu )

Polymerization stress

(gravimetry); polymerization shrinkage (Sv ; mercury dilatometry) and polymerization stress (tensometer). The experimental UDMA/TEG-DVBE composites were compared with the BisGMA/TEGDMA composites containing the identical filler contents, and with the commercial micro hybrid flowable composite. Results. UDMA/TEG-DBVE composites exhibited n, E, Wsp , Wsu and Sv equivalent to the controls. They outperformed the controls with respect to F (up to 26.8% increase), KIC (up to 27.7% increase), modulus recovery upon water sorption (full recovery vs. 91.9% recovery), and stress formation (up to 52.7% reduction). In addition, new composites showed up to 27.7% increase in attainable DC compared to the traditional composites. Bis-GMA/TEGDMA controls exceeded the experimental composites with respect to only one property, the composite hardness. Significantly, up to 18.1% lower HK values in the experimental series (0.458 GPa) were still above the clinically required threshold of approx. 0.4 GPa.

∗

Corresponding author at: 100 Bureau Drive, Stop 8546, Gaithersburg, MD 20899-8546, USA. E-mail address: [email protected] (J. Sun). https://doi.org/10.1016/j.dental.2017.10.007 0109-5641/© 2017 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

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Significance. Hydrolytic stability, composition-controlled polymerization and the overall enhancement in clinically-relevant properties of the new resin composites make them viable candidates to replace traditional resin composites as a new generation of strong and durable dental restoratives. © 2017 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

In modern dentistry, amalgam dental restorations are being phased-out due to safety concerns [1,2]. Meanwhile, resin composite restorations continue gaining popularity for their aesthetic advantages and clinical practicality [3,4]. However, the average service life of the contemporary polymeric restorations is less than eight years. These restorations are typically inundated by frequent fracturing and development of secondary caries [5]. This relatively short service life together with the concerns regarding leachability of the unreacted monomers and degradation products (such as the infamous bisphenol A (BPA) [6,7]) from these systems evince a need for new, longer-lasting dental resin composites. Finding adequate substitutes for these materials is seen as an ongoing challenge in efforts to improve oral health of dental patients [8,9]. The traditional dental resin composites generally contain three major components: (1) a mixture of dimethacrylate monomers such as bisphenol A glycidyl dimethacrylate (BisGMA), triethylene glycol dimethacrylate (TEGDMA), and/or urethane dimethacrylate (UDMA), that form the resin network upon polymerization; (2) reinforcing filler particles; and (3) a polymerization initiating system [3,4]. Selection of the monomers and the underlying polymerization mechanism strongly influence the rate of polymerization, control sample handling characteristics and, ultimately, the performance of the restoratives [10]. The traditional dental resins typically employ Bis-GMA as a base monomer and the low viscosity TEGDMA as a diluent monomer to enhance material’s handling properties. Bis-GMA/TEGDMA resins produce a dentin-matching color and yield mechanically strong resin networks. Currently, Bis-GMA/TEGDMA-based composites, invented more than 50 years ago by Dr. Bowen at the American Dental Association remain at the top of the resin-based composite list [11–13]. However, when challenged by acid, enzymes and/or cariogenic bacteria, these composites undergo irreversible hydrolysis. Consequently, resin decomposes and the restoration eventually fails [14,15]. New monomers and polymerization mechanisms have been proposed to replace the hydrolysable methacrylate monomers [16–20]. In other developments, a step-growth thiol-ene reaction was proposed as an alternative to the commonly-used radical polymerization [21–24]. This growth mechanism significantly delayed the gelation process towards higher degrees of conversion (DC) and reduced polymerization stress that often causes micro-leakage and tooth fracture. Very high DCs attainable in these systems minimize the amount of unreacted monomers and significantly reduce the leachability of potentially toxic compounds [14,25–27]. Moreover, silorane based dental composites and

composites comprised of thiourethane oligomer showed improved mechanical properties in comparison to methacrylate-based dental composites [28–30]. Our group reported successful design of hydrolytically- and enzymatically-stable, ether-based monomers and their copolymerization with UDMA via composition-controlled mechanism [17]. In this study, we explored how new dental resin composites based on composition-controlled UDMA/hydrolytically stable, ether-based triethylene glycol divinylbenzyl ether (TEG-DVBE) copolymers perform regarding their overall clinical presentation in comparison with the traditional Bis-GMA/TEGDMA systems. For that purpose, a battery of physicochemical and mechanical tests was performed in parallel on the experimental and the control resins/composites. Critical predictors of the materials’ clinical performance including aesthetics, polymerization kinetics, water sorption, stress development, and mechanical stability of the experimental materials matched and/or significantly exceeded those of the controls. It is, therefore, concluded that newly designed resin composites indeed have a strong potential as a new generation of dental restoratives with improved service life.

2.

Materials and methods

2.1.

Formulation of the resins

The methacrylate monomers, UDMA and a mixture BisGMA/TEGDMA (70/30 by weight) were gifts from Esstech Inc. (Essington, PA, USA). The ether-based monomer, triethylene glycol-divinylbenzyl ether (TEG-DVBE), was synthesized and fully characterized in our laboratory as reported [20]. The chemical structures of these monomers are shown in Fig. 1. Compositions of the resins evaluated in this study are provided in Table 1. The acronyms U3V1 and U1V1 represent 3:1 and 1:1 UDMA:TEG-DVBE molar ratio, respectively. Henceforth, these acronyms will be used throughout the text. All monomer mixtures were activated for photo-polymerization by adding 0.2 wt% of photo-oxidant, camphorquinone (CQ; Aldrich, Saint Louis, MO, USA) and 0.8 wt% of photo-reductant, ethyl 4-N,N-dimethylaminobenzoate (4EDMAB; Aldrich, Saint Louis, MO, USA).

2.2.

Fabrication of composites

Each composite contained (mass fraction) 25 wt% resin, 67.5 wt% silanized BaBFAlSiO4 milled glass (Product code ® #907642, Dentsply, York, PA, USA) and 7.5 wt% AEROSIL OX 50 nanoparticles (Evonik Industries, Essen, Germany). These components were mixed by a speed mixer (DAC 150 FVZ,

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Table 1 – Composition (wt%) of photo-activated resins. Resin

UDMA

TEG-DVBE

Bis-GMA

TEGDMA

CQ

4EDMAB

BT control U3V1 U1V1

– 78.3 55.8

– 20.7 44.2

69.3 – –

29.7 – –

0.2 0.2 0.2

0.8 0.8 0.8

where A1 /A0 and A1  /A0  represent the peak-area-ratio of vinyl (both monomers in each system) and internal standard before and after polymerization, respectively [31,32].

2.3.2.

DC determined by near infrared spectroscopy (NIR)

To determine DC of resin and composite specimens NIR spectra were acquired before photo cure and post cure [33–35]. DC was calculated as the percentage change in the integrated peak area of the 6165 cm−1 vinyl absorption band normalized to the 4623 cm−1 aromatic C H absorption band area between the polymer (value after cure) and monomer (value before cure).

2.3.3.

Refractive index (n)

The n of copolymers and their corresponding composites were measured by matching with the refractive index liquids (interval of n = 0.004, Cargille Labs Inc., NJ, USA) at 22 ◦ C. The value of matched n was based upon OLYMPUS BX50 light microscope (OLYMPUS, Tokyo, Japan) observations when the specimen and the n-liquid were indistinguishable.

2.3.4. Fig. 1 – Chemical structure of the monomers employed in resin formulations.

FlackTek, Landrum, SC, USA) at 366 rad/s for 1 min, and then hand mixed for additional 1 min. The mixing process was repeated 3 times to obtain a uniform clay-like paste.

2.3. Physicochemical and mechanical properties of copolymers and composites 2.3.1.

DC determined by FTIR-ATR

The DC was evaluated immediately after curing using a Thermo Nicolet Nexus 670 FT-IR spectrometer (Thermo Scientific, Madison, Wisconsin, USA) with a KBr beamsplitter, an MCT/A detector and an attenuated total reflectance (ATR) accessory. The aromatic C C absorption band at 1608 cm −1 of Bis-GMA and the amide group band of UDMA at 1537 cm−1 were used as the internal standards for Bis-GMA/TEGDMA and UDMA/TEG-DVBE formulations, respectively. The areas of absorption peaks of the vinyl group of TEG-DVBE at 1629 cm−1 and the methacrylate groups of UDMA, TEGDMA or Bis-GMA at 1638 cm−1 were integrated. Peaks were resolved by employing the curve fitting program Fityk (version 0.9.8) [20]. DC was calculated according to the following equation: DC (%) = (A1 /A0 − A1  /A0  )/(A1 /A0 ) × 100

(1)

Total volumetric shrinkage (Sv )

The Sv was measured by a mercury dilatometer (ADA Foundation, Gaithersburg, MD, USA) [36,37]. Approximately 0.1 g of composite was placed on a sandblasted and silanized glass slide. A glass column was clamped to the glass slide and filled with mercury. A LVDT probe (linear variable differential transducer) was then placed on top of the mercury. The composite was light cured through the glass slide, with a radiant exposure of 18 J/cm2 (340 mW/cm2 × 53 s), using a QTH curing unit (QHL 75 – Dentsply, Konstanz, Germany). The Sv (n = 3) was monitored for 60 min after the photo-activation. Data recorded by the probe were used to calculate Sv based on the specimen mass and its density (Sartorius YDK01 Density Determination Kit; Sartorius AG. Goetingen, Germany).

2.3.5. Flexural modulus (E) and flexural strength (F) by 3-point bending To determine E and F, six rectangular specimens/material/test were made by inserting the composite into a stainless-steel mold (25 mm × 2 mm × 2 mm) and covering the surfaces with a Mylar film to prevent air-inhibited layers. The specimen bars were cured (2 min/each open side of the mold) using a Dentsply Triad 2000 visible light curing unit (Dentsply, York, PA, USA) with a tungsten halogen light bulb (75 W and 120 V, 43 mW/cm2 ). After curing, the specimens were stored at room temperature for 24 h. The E of the resins was determined using the Universal Testing Machine (Instron 5500R, Instron Corp., Canton, MA, USA) at a cross-head speed of 1 mm/min. The specimens were placed on a 3-point bending test device with 20 mm distance between supports and an equally distributed load. The E and F values were calculated following ISO4049: 2009 protocols/equations [34].

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2.3.6.

Fracture toughness (KIC )

The KIC was determined following the ISO6872: 2014 using a single edge V-notched beam (SEVNB) method. Five beams (25 mm × 4 mm × 3 mm)/experimental group were fabricated and notched by sawing with an IsoMet Low Speed Saw (Buehler, Lake Bluff, IL, USA) fitted with NTI Flex Diamond Discs (NTI-Kahla GmbH, Kahla, Germany). The notch was sharpened using a razor blade polished with 3 ␮m diamond paste. The KIC was assessed using the same Instron machine as for the 3-point bending test. The specimens were loaded until critically fractured. Additional details on the KIC measurement and its calculation are provided in our previous publication [19].

2.3.7.

Water sorption (Wsp ) and solubility (Wsu ) [38,39]

To measure Wsp and Wsu , the rectangular beam specimens (four/experimental group) identical to those used for 3-point bending tests were made. The polymerized samples were stored at room temperature for 2 day in dark, and then 1 week in a vacuum oven over freshly dried silica gel at 37 ◦ C. Samples were then stored in a desiccator at 23 ◦ C for 1 h before being weighted with a calibrated electronic balance (a resolution of 0.01 mg). This drying cycle was repeated until a constant mass (m1 ) for each specimen was obtained. After drying, the specimens were immersed in distilled water at 37 ◦ C. At predetermined time intervals. i.e., at 3 h, 5 h, 24 h, 2 day, 4 day, 7 day, 10 day and 16 day, specimens were blotted dry with Kimwipes lab tissues, weighted (m2 ) and then further incubated in water. After the last water immersion period, specimens were blotted dry with Kimwipes lab tissues, and kept in a vacuum oven (freshly dried silica gel; 37 ◦ C) until achieving a constant weight (m3 ). The Wsp and solubility Wsu were calculated as follows: W sp (%) = 100(m2 − m1 )/m1

(2)

W su (%) = 100(m1 − m3 )/m1

(3)

Meanwhile, the restoration of elastic modulus Eso (%) was calculated as the ratio of the elastic modulus of the samples dried till constant weight (m3 ) after water sorption and that before water sorption experiment.

2.3.8.

Knoop hardness (HK)

The microhardness machine (Wilson Tukon 2100; Instron Corp., Canton, MA, USA) with indentation loads of (0.25–5) N was used for HK measurements (ASTM standard E 384) [20,34]. The loading time for an indentation was 15 s with a dwell at peak load of 15 s. Indentation sizes were measured with a 10× or a 50× objective. The HK values were calculated by dividing a test force by the indentation projected surface area according to the equation: HK (MPa/m2 ) = 14.229 P/d2

2.4. Simultaneous assessment of polymerization stress and DC The polymerization stress of the composites was measured using the cantilever-beam based tensometer [40,41]. The tensometer was coupled with an in situ NIR spectrometer, which allows simultaneous monitoring of the real-time double-bond conversion in transmission. Briefly, a disk-shaped (2.5 mm in diameter and 2 mm in height) uncured specimen was placed between two flat methacrylate-silane treated quartz rods (to promote specimen/rod adhesion). The upper rod was clamped to the cantilever beam and the lower one was fixed to the base. As the curing light (LZ1-10DB0, LED Engin High Powder LEDs, Mouser Electronics, Mansfield, TX; 40 s irradiation, 500 mW/cm2 intensity at the top end of the lower rod where the specimen is attached) was transmitted through the lower rod onto the specimen, polymerization shrinkage occurred and the resulting axial shrinkage stress caused a deflection in the beam. This deflection was recorded by a displacement sensor at the free end of the beam and used to deduce the axial stress based on a beam formula. The simultaneous measurement of the double-bond (6165 cm−1 ) conversion was realized by guiding the NIR signal through the sides of the specimen using optical fiber cables (1 mm diameter). The dynamic fractional conversion was calculated by taking the peak area of the sample prior to the start of irradiation (Areamonomer ) and at each time point during the polymerization process (Areapolymer ) based on the following formula:

DC (%) = 100 × (1 − Areapolymer /Areamonomer )

(5)

The synchronized stress/DC data were collected continuously for 30 min at clinically-relevant instrumental compliance of 0.33 ␮m/N. The 2.5 mm sample diameter and 2 mm sample height correspond to a C-factor (ratio of bonded area to unbonded area) of 0.625 [42,43].

2.5.

Statistical analysis

The experimental results were analyzed using one-way analysis of variance (ANOVA) with a 95% confidence interval to indicate significant differences.

3.

Results and discussion

3.1. Enhanced performance of composites comprised of hydrolytically stable resins

(4)

where P is the indenter force and d is the long diagonal length [29]. The HK measurements were performed on specimens that have been evaluated by NIR prior to testing. The reported HK values represent the average of five repetitive measurements. The standard uncertainty associated with the HK measurements was 5%.

The composites formulated with hydrolytically stable resins, i.e., U3V1 and U1V1, generally surpassed the BT control regarding clinically-relevant physicochemical and mechanical properties (Table 2). While performing comparably to BT control with respect to n, Sv and E, new composites significantly outperformed BT counterparts with respect to F, KIC , Eso and polymerization stress.

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Table 2 – Comparison of clinically-relevant properties: new composites vs. BT control. Resin composition

BT control U3V1 U1V1

Equivalent performance

Enhanced performance

Refractive index

E (GPa)

Sv (%)

F (MPa)

KIC (MPa m1/2 )

Eso (%)

Stress (MPa)

1.528 1.524 1.528

9.4 (0.1) 9.4 (0.6) 8.4 (0.3)*

3.22 (0.11) 3.64 (0.59) 3.10 (0.30)

97(11) 123(14)* 116(12)*

1.19 (0.05) 1.52 (0.04)* 1.32 (0.03)*

91.9(3.9) 103.6(4.2)* 102.9(2.3)*

2.20(0.08) 1.72(0.05)* 1.04(0.07)*

Note: indicated are mean values with one standard deviation in parenthesis. Asterisk signifies values that are statistically different (p < 0.05) from the control.

Fig. 2 – Refractive indices (n) of homopolymer, copolymer and composite formulations. The optical microscopy images (upper row) of BT resin debris in n-matching liquids (n values as indicated; black/dark interfaces (marked by blue arrows) indicate an n mismatch; red arrow shows an n match). Ranking of material n values in comparison with dentin (n ≈ 1.540) (schematic—lower panel). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.2. Hydrolytically stable monomer with dentin-matching refractive index (n) The n of resin composites is aesthetically important as it indicates how well the restoration matchs natural tooth tissues (n = 1.631 ± 0.007 for enamel and n = 1.540 ± 0.013 for dentin [44]). Upper panels in Fig. 2 illustrate the resin (U1V1) debris in three different n-matching liquids. When the n of resins and the liquid is the same, interface is undistinguishable (middle image; n = 1.540 for U1V1). The resin-liquid interfaces are black or dark lines (indicated by arrows) when n of resin is either higher or lower than that of the matching liquid. The schematic diagram in Fig. 2 illustrates the rank of n among dentin, homopolymers, copolymers, and composites. In depth comparison of the n values led to the following conclusions: the n of homopolymers decreased in the following order: TEG-DVBE (1.572) > Bis-GMA (1.568) > dentin > UDMA (1.510) > TEGDMA (1.506). All copolymers had the same n that well matches the n of dentin (1.540). In composite series, n generally decreased because the n of fillers is approx. 1.520 and fillers constitute 75 wt% of the composites. Replacing TEGDMA with TEG-DVBE in U3V1 and U1V1 formulations has multiple benefits. Resulting n-matching resins are preferred economically since a need for high refractive index fillers which are typically costly [45–47] is alleviated in these systems. TEG-

DVBE’s viscosity is similar to viscosity of TEGDMA and approx. 300 times lower than that of the base monomer, UDMA [17]. Thus, as a diluent monomer, besides its hydrolytic stability, TEG-DVBE has an additional advantage over TEGDMA, a higher refractive index.

3.3. Stronger and tougher composites and composition-controlled copolymerization The U3V1 and U1V1 composites exhibited higher F and KIC than the control BT composites. Since these measurements were carried out with samples that were prepared differently, they had different sizes and dimensions. We used additional DC measurements to validate these results and better understand the implications of these evaluations. Table 3 compares the DCs of the resins for 3-point bending and composites for SEVNB experiments. It is noteworthy that due to the limitation of our infrared instruments and the sample dimensions, NIR was used to determine the DC of the resins and FTIR-ATR was used for SEVNB composites. The DC values at 1 day and 5 day were statistically the same for all three resins. For U1V1 and BT resins, DC values at 5 day were higher than those attained at 1 h post-irradiation. This simple 3-time point, kinetic study suggests that DC was constant and polymerization was hibernated 1 day after light

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Table 3 – DCs attained at different times after light irradiation: comparison of resins (unfilled copolymers) and composites. DC (%) BT control

U3V1

U1V1

Average

STDEV

Average

STDEV

Average

STDEV

a

Resins (E specimens)

1h 1 day 5 day

76.7 79.3 80.6

0.9 0.4 1.1

76.4 77.1 77.9

2.1 0.7 0.3

81.4 82.2 83.6

0.4 0.8 0.8

b

Composites (KIC specimens)

1 day

62.0

2.6

70.3

1.2

79.2

1.9

a b

NIR data (n = 3/group). FTIR-ATR data (n = 5/group).

irradiation. Thus, DCs of the composites were evaluated at 1 day after light irradiation. The DC of the resins (at 5 day post-irradiation) increased in a following order: U1V1 ≥ BT control ≥ U3V1. The observed differences in DC values were too small to significantly impact resins’ performance including elastic modulus or flexural strength. Contrary to the unfilled resins, the attained DCs of new-composites were significantly higher than that of the BT control counterparts. The DC differentials of over 8% (U3V1 vs. BT composites) and over 17% (U1V1 vs. BT composites) can certainly lead to considerable changes in mechanical performance of these composites. The DC determined by FTIR-ATR is the surface DC of the composites. Although the surface DC may be different from the bulk DC, an approx. 20% difference between BT and U1V1 composites requires explanation. The substantially lower DC seen in BT composites may be explained by the limitation of diffusion-controlled co-polymerization of BisGMA/TEGDMA resin [48–50]. In general, at high DCs, the viscosity of the mixture increases and diffusion of monomers, especially high viscosity Bis-GMA, is impeded. Consequently, Bis-GMA monomer does not get easily into contact with the active radicals to be polymerized. The composition of unreacted monomers then shifts toward more high viscosity monomers, i.e., Bis-GMA. Also, it was reported that adding nano-filler decreased the final DC of Bis-GMA/TEGDM-based composites due to decreased local mobility of monomers [51,52]. In contrast, the U1V1 resin has a lower viscosity than the BT control. In addition, the polymerization of U1V1 resins, unlike the Bis-GMA/TEGDMA resins, follows a unique, composition-controlled polymerization mechanism. In that system, regardless of polymerization rate and DC, the composition of uncured monomers i.e., UDMA and TEG-DVBE, remains the same as the equimolar feeding composition [17]. Moreover, the difference in viscosity of monomers (UDMA is approx. 300 times more viscous than TEG-DVBE) had no impact on resin composition. This polymerization mechanism may also contribute to the same DC attained in both unfilled resin (liquid) and highly filled composites (pastes).

3.4.

Balance in mechanical properties

Fig. 3 – (A) Knoop hardness of the experimental resins and composites. (B) Relationship between the hardness and the fracture toughness of the composites. Vertical and horizontal bars represent standard deviations.

Fig. 3A reveals higher HK values in BT resin and composites compared to U3V1 and U1V1 counterparts. The mean HK values for U1V1 (0.477 ± 0.020 GPa) and U3V1 (0.458 ± 0.025 GPa) Please cite this article in press as: Wang X, et al. High performance dental resin composites with hydrolytically stable monomers. Dent Mater (2017), https://doi.org/10.1016/j.dental.2017.10.007

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composites are moderately (14.7%–18.1%) lower than the HK for BT composite (0.559 ± 0.025 GPa) or the commercial dental composite, Filtek Supreme, based on Bis-GMA/TEGMA resin (0.572 GPa) [53]. However, hardness of our experimental composites exceeded (16.8%–21.7%) the HK of silorane-based composites (0.392 GPa for 3 M Filtek Silorane) [53] thus making our new materials viable candidates for clinical use not only based on significantly enhanced E, F and KIC , but their hardness as well. In our new dental material design, the primary task is to improve the service life of the restorations. The material’s fracture toughness (KIC ) correlates well with its durability and, therefore, high KIC is desirable. Material’s hardness (HK) is more representative of wearing and typically needs to meet the basic requirements but not exceed the HK of teeth. In most materials, the properties of hardness and toughness are mutually exclusive, i.e., as hardness increases, toughness decreases [54,55]. For our materials, such relationship is confirmed in Fig. 3B.

3.5.

Improved hydrolytical stability

Via hydrolysis and swelling, water leads to resin degradation and material fatigue, respectively, and, ultimately, impairs durability of composite restorations and their service life [56]. Water sorption (Wsp ) of the experimental resins and their composites is shown in Fig. 4. Both resins and composites exhibited similar Wsp profiles. Since Wsp is a diffusioncontrolled process [57], its rate decreased with time. The Wsp plateaued after 168 h in resin series, and after 244 h in composite series. The differences in Wsp between resin (approx. 3%) and composites (approx. 1%) generally agree with the literature data and suggests that water uptake mainly takes place with the resin matrix [57]. Comparing with the BT control, no significant differences existed between the Wsp of U3V1 and BT control in both resin series and their corresponding composites; Although the Wsp in U1V1 composites was higher (9%) than the BT control, and it was the same (statistically insignificant) in resin series. Overall, the Wsp of new resins and their composites were equivalent to that of the BT control. In composite series, elastic modulus measurements were performed before and after water sorption experiment. Fig. 5A shows the restoration of elastic modulus after 16 day of storing in water. The control, BT composites, recovered to 91.9 ± 3.9% of the pre-immersion elasticity compared to U3V1 and U1V1 composites which both regain their initial E. This result indicates the extraordinary stability of new-resin composites in moist environment in contrast to the irreversible drop in E of BT controls. Although both U3V1 and U1V1 composites contained a hydrolysable UDMA monomer, the integrity of their resin networks remained virtually intact due to the protective action of ester-free TEG-DVBE monomer. This conclusion is further supported by the resin solubility data (Fig. 5B) indicating significantly lower weight loss in U3V1 and U1V1 resins compared to BT formulation. The measured solubility of BT resin 8.1 ± 0.1 ␮g/mm3 agrees well with the literature data 7.0 ± 2.3 ␮g/mm3 [58,59]. In composite series, measured solubility values were within the experimental error range of the experimental method thus preventing us to make distinction between the three

Fig. 4 – Kinetics of water sorption: comparison of the experimental composites (A) and their corresponding resins (B). Vertical bars represent standard deviations.

experimental groups. Considering the fact that the resin matrix is the component of the composites that undergoes degradation and leaching of the unreacted monomers and degradation products, it is safe to assume that the water solubility of composites should follow the same trend as that of resins. The reduced solubility coupled with a 100% recovery of elastic modulus after water immersion suggests that new resins and their composites in water-rich oral environments are more stable than the BT control.

3.6.

Reduced polymerization stress

Polymerization shrinkage and the ensuing polymerization stress may generate micro-leakage at tooth/restoration interface, eventually lead to secondary caries, and in the worst scenario, to tooth fracture. Thus, restoratives that shrink less and generate less stress are highly desirable. For decades, development of such materials has been pursued by

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Fig. 5 – (A) Restoration of elastic modulus in composites specimens following 16 day of aqueous immersion. (B) Corresponding solubility of the resins after 16 day soaking in water. Significant differences with respect to BT control are indicated by asterisk. Vertical bars represent standard deviations.

researchers all over the world. Shrinkage and stress remain one of the key factors in evaluating new dental resin composites. Volumetric shrinkage upon polymerization and the polymerization stress developed in our experimental systems, and BT control are compiled in Table 4. While polymerization shrinkage was indistinguishable in all composites, U3V1 and U1V1 composites have significantly reduced polymerization stress (22% and 53%, respectively) compared to BT composites. In a simplified model [42,43], when polymerization shrinkage is the same, the polymerization stress is proportional to the samples’ elastic modulus. New composites reduced polymerization stress without losing their rigidity. The U3V1 and BT composites have the identical elastic modulus and U3V1 composites generated 22% less stress than their BT

Fig. 6 – The evolution of stress (A) and DC (B) during polymerization measured by tensometer. The insets show the corresponding initial polymerization stage. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

counterparts. The experimental composites slightly differ in E (U1V1 being almost 11% lower than U3V1) but the polymerization stress developed in U1V1 is 40% lower at the equivalent. The simplified model could not explain stress reduction seen in new resin composites. The real-time stress development and the corresponding DC increase (Fig. 6A and B) suggest that the polymerization kinetics of the new resins plays an important role in reducing the polymerization stress. A systematic future study is warranted to fully understand the mechanism(s) behind the stress reduction in new materials. Based on the current experimental data we propose the following explanation for the observed phenomenon: both U3V1 and U1V1 formulations may have delayed gel point during light irradiation due to relative slow polymerization rate. This delay provided sufficient time to release the stress and prevent its excessive build-up in UDMA/TEG-DVBE systems.

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Table 4 – Polymerization volumetric shrinkage and polymerization stress with the accompanying E and DC values: experimental UDMA/TEG-DVBE systems vs. BT controls. Composition

Shrinkage (%)

E of composite (GPa)

Stress (MPa)

DC (%)

BT U3V1 U1V1

3.2(0.1) 3.6(0.6) 3.1(0.3)

9.4(0.1) 9.4(0.6) 8.4(0.3)*

2.20(0.08) 1.72(0.05)* 1.04(0.07)*

58.9(1.6) 65.8(2.4) 57.7(1.4)

Note: significant differences in comparison with BT control are indicated by asterisk; stress measured at an instrument compliance of 0.33 ␮m/N.

4.

Conclusions

New dental resin composites comprising UDMA and hydrolytically-stable TEG-DVBE were prepared and their performance were evaluated and compared with traditional composites based on Bis-GMA/TEGDMA as a control. The study indicated that materials outperform the control, traditional composites based on Bis-GMA/TEGDMA with respect to mechanical strength, toughness and rigidity following water immersion (fully preserved elastic modulus, E (new materials), vs. significant reduction in E (traditional controls)). At the equivalent levels of vinyl conversion, UDMA/TEGDVBE composites generated significantly less stress than control resin composites. Additionally, the composites have a dentin-matching refractive index. Results of this study not only suggest that the UDMA/TEG-DVBA resin composites are promising replacement for Bis-GMA/TEGDMA based dental materials, but also indicate their potential utility in designing a new generation of strong, tough and more durable dental restoratives.

Disclaimer Certain commercial materials and equipment are identified in this manuscript in order to specify adequately the experimental and analysis procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology (NIST) nor does it imply that they are necessarily the best available for the purpose.

Acknowledgments This work was funded by the National Institute of Dental and Craniofacial Research (U01DE023752). Financial support was also provided through the ADA Foundation. The authors would like to thank American Dental Association for their supports. Dr. SJ thanks Mr. Stanislav Frukhtbeyn and Mr. Anthony Giuseppetti for their technical supports. We also would like to thank the Center for Nanoscale Science and Technology (CNST) at NIST for their technical support.

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