Abstract
Objectives
To investigate chemo-mechanical effects of incorporating alkaline bioactive glass nanoparticles into a light-curable dental resin matrix.
Methods
An unfilled Bis-GMA/TEGDMA material was infiltrated with up to 20 wt% of ultrafine SiO 2 –Na 2 O–CaO–P 2 O 5 –Bi 2 O 3 particles. The unfilled and filled resins were investigated regarding their viscosity before setting and compared to commercially available materials. Set specimens were immersed for 21 days in phosphate buffered saline at 37 °C. Water uptake, pH, Knoop hardness, and degree of conversion of freshly polymerized and stored samples were investigated. Resin surfaces were viewed and mapped in a scanning electron microscope for the formation of calcium phosphate (Ca/P) precipitates. In addition, Raman spectroscopy was performed. Numeric values were statistically compared ( p < 0.01).
Results
Viscosity increased with particle loading, but remained below that of a flowable dental composite material. Water uptake into and pH induction from the polymerized samples also increased with particle loading ( p < 0.01). The addition of 20 wt% nanoparticles had no significant influence on microhardness, yet it slightly ( p < 0.01) increased the degree of conversion after 21 days. Ca/P precipitates formed on specimens filled with 20 wt% of the particles, while they were scarce on counterparts loaded with 10 wt%, and absent on unfilled resin surfaces.
Significance
The results of the current study show that a Bis-GMA-based resin can be functionalized using alkaline nanoparticles. A material with bioactive properties and similar hardness as the unfilled resin was obtained by incorporating 20 wt% of ultrafine SiO 2 –Na 2 O–CaO–P 2 O 5 –Bi 2 O 3 particles into the resin matrix.
1
Introduction
Among the various agents that have been used to cover exposed dentin, resin-based materials and glass-ionomer cements are the most widely used in restorative dentistry . Originally, it was attempted to render these materials as inert as possible . However, more recent advances have included the functionalization of traditional dental materials for their specific application . For materials with a direct contact to dentin, bioactivity, i.e. the induction of calcium phosphate (Ca/P) precipitates, might be a desirable feature. Calcium and phosphorus species released from such bioactive materials could occlude dentinal tubules and/or remineralize carious dentin . A further distinct effect of alkaline biomaterials such as bioactive glasses of the 45S5 type is their induction of a high-pH environment, which renders these materials antimicrobial .
Alkaline bioactive glasses have been introduced first into glass-ionomer cements . The water-permeability of these cements would make them first choice for the incorporation of hydrophilic particles such as bioactive glass. However, the high alkalinity of bioactive glasses of the 45S5 type, i.e. SiO 2 –Na 2 O–CaO–P 2 O 5 mixture, compromises the mechanical properties of the resulting glass-ionomer cements . Apparently, the alkaline particles interfere with the acid-base reaction between the acidic polyelectrolyte and the aluminosilicate glass. On the other hand, nanometric bioactive glass 45S5 can be successfully embedded into a polyisoprene matrix to obtain a potential root-filling material with bioactive features . It has been shown that the application of nanosized bioactive glass particles in biopolymers is in favor compared to micronsized particles in terms of wettability, pH induction and mechanical properties of the resulting composite . Interestingly, it has never been investigated whether such ultrafine bioactive glass particles could be incorporated into dimethacrylate-based dental resins.
The hypothesis tested here was that resin polymerization is not impacted by alkaline nanometric bioactive glass particles, and that these particles embedded in the matrix would continue to exert bioactivity.
2
Materials and methods
2.1
Material preparation
Nanosized, radio-opaque bioactive glass particles were produced by flame spray synthesis . In brief, corresponding metal precursors were combined and combusted in a flame spray setup to result in radio-opaque bioactive glass particles made up of a mixed oxide: SiO 2 –Na 2 O–CaO–P 2 O 5 –Bi 2 O 3 . The bioactive glass used in this study contained 20 wt% bismuth oxide to render the final composite visible on X-ray images. In the remaining 80 wt%, components were combined as in the classic 45S5 bioactive glass : 45% SiO 2 , 24.5% Na 2 O, 24.5% CaO and 6% P 2 O 5 (all in wt%). The particles were collected on a filter, mounted above the flame, sieved (300 μm) and used as received (30–50 nm particle size). The dentin bonding agent Heliobond (Ivoclar Vivadent, Schaan, Liechtenstein; lot: R02303), a mixture of 60 wt% bisphenol-A-glycidyldimethacrylate (Bis-GMA) and 40 wt% triethylene glycol dimethacrylate (TEGDMA), was used as polymeric matrix and infiltrated with 0, 10 or 20 wt% bioactive glass using a dual asymmetric centrifuge (Speedmixer DAC 150, Hauschild Engineering, Hamm, Germany) at 3500 revolutions per minute for 60 s.
2.2
Viscosity assessment before curing
Combined Heliobond and bioactive glass with a loading of 0, 10 and 20 wt% (prepared as described above) were subjected to rheological analysis (Physica MCR 300, Anton-Paar, Zofingen, Switzerland) using a cone-plate geometry (30 mm diameter, 2° cone angle) at 25 °C to determine the viscosity before curing. The space between cone and plate was filled completely with material and excess was removed. Shear rate ramp tests were performed with shear rates ranging from 0.1 to 100 s −1 ( n = 3). The following commercial materials served as reference samples: a filled dentin bonding agent (Optibond FL, Kerr, Orange, CA, USA; lot: 4689425), a fissure sealant (Helioseal, Ivoclar Vivadent; lot: M69511) and a flowable composite (Filtek Supreme XTE Flowable Restorative, 3M ESPE, St. Paul, MN, USA; lot: N368930).
2.3
Immersion in phosphate buffered saline
For this and all subsequent experiments, disk-shaped resin specimens (diameter: 6 mm, height: 2 mm) with three different loadings of bioactive glass (0, 10 and 20 wt%) were prepared in Teflon molds pressed between two glass plates, and light-cured for 5 min in a Spectramat SP1 curing unit (Ivoclar Vivadent) with a tungsten halogen light bulb (600 W, 220 V). Cured resin specimens were immersed in 1 mL phosphate buffered saline (PBS, GIBCO, Invitrogen, Billings, MT, USA) at pH 7.4 and 37 °C for 21 days without changing the aqueous medium.
2.4
pH measurements
The pH ( n = 3) was monitored using a calibrated pH electrode (Seven Easy, Mettler Toledo, Greifensee, Switzerland) after 0.5, 1, 2, 4, 8 h, 1, 3, 7, 14 and 21 days. Mere PBS served as a reference for pH measurements.
2.5
Water uptake
Additionally, the ability of the resin specimens to take up water was calculated by the wet weight W wet (determined after the specimens were removed from the liquid, dipped thrice in distilled water and carefully blotted dry) and the dry weight W dry (determined after vacuum drying during 7 days at room temperature of 23 °C). The water uptake (WU) was calculated according to the following equation:
WU ( % ) = W wet − W dry W dry ⋅ 100
All measurements were performed in triplicates using a precision balance (XS 205, Mettler-Toledo).
2.6
Determination of microhardness
Knoop hardness was measured 24 h after photoactivation, and after 21 days of immersion in PBS at 37 °C using a digital microhardness tester (model no. 1600-6106, Buehler, Lake Bluff, IL, USA). For each specimen (n=3), three indentations were performed under a load of 100 g applied for 20 s at random positions around the center of the irradiated resin surface, and the average of the three readings was calculated.
2.7
Degree of conversion analysis
Degree of conversion (DC) of the filled and unfilled resins was assessed both before and after immersion in PBS using a Fourier transform infrared spectrometer (Vertex 70, Bruker, Fällanden, Switzerland) equipped with an attenuated total reflectance device with a single platinum crystal. Spectra were recorded from 400 to 4000 cm −1 with 64 scans and a resolution of 4 cm −1 . The DC was calculated from the ratio of absorbance intensities of aliphatic C C stretching vibrations (peak height at 1637 cm −1 ) and aromatic C C stretching vibrations (peak height at 1608 cm −1 , internal standard) between the polymerized and unpolymerized samples . Experiments were performed in triplicates.
2.8
Surface analysis
Specimens were mounted on aluminum stubs (12 mm) with the aid of carbon tape and sputtered with 5 nm platinum. A Nova NanoSEM 450 (FEI, Eindhoven, The Netherlands) scanning electron microscope (SEM) was operated at 3 kV for observational scans to assess surface morphology before and after immersion in PBS. Energy-dispersive X-ray spectroscopy (EDX) was performed at 10 kV to investigate elemental composition of the specimens. Additionally, Raman spectroscopy was carried out with 785 nm excitation wavelength using a NIR laser at a power of 300 mW and 100% intensity (inVia Raman Microscope, Renishaw, New Mills, United Kingdom). Spectra were recorded on dry specimens from 500 to 1500 cm −1 with a spot size of 638 nm, with a spectral resolution of 1 cm −1 and without baseline correction.
2.9
Data presentation analysis
Values related to pH are presented using descriptive statistics. Viscosity, water uptake, Knoop hardness and degree of conversion values were evenly distributed (Shapiro–Wilk test), and thus compared with one-way ANOVA followed by Tukey’s HSD post hoc test. The α-type error was set at 0.01 for all statistical analyses ( p < 0.01).
2
Materials and methods
2.1
Material preparation
Nanosized, radio-opaque bioactive glass particles were produced by flame spray synthesis . In brief, corresponding metal precursors were combined and combusted in a flame spray setup to result in radio-opaque bioactive glass particles made up of a mixed oxide: SiO 2 –Na 2 O–CaO–P 2 O 5 –Bi 2 O 3 . The bioactive glass used in this study contained 20 wt% bismuth oxide to render the final composite visible on X-ray images. In the remaining 80 wt%, components were combined as in the classic 45S5 bioactive glass : 45% SiO 2 , 24.5% Na 2 O, 24.5% CaO and 6% P 2 O 5 (all in wt%). The particles were collected on a filter, mounted above the flame, sieved (300 μm) and used as received (30–50 nm particle size). The dentin bonding agent Heliobond (Ivoclar Vivadent, Schaan, Liechtenstein; lot: R02303), a mixture of 60 wt% bisphenol-A-glycidyldimethacrylate (Bis-GMA) and 40 wt% triethylene glycol dimethacrylate (TEGDMA), was used as polymeric matrix and infiltrated with 0, 10 or 20 wt% bioactive glass using a dual asymmetric centrifuge (Speedmixer DAC 150, Hauschild Engineering, Hamm, Germany) at 3500 revolutions per minute for 60 s.
2.2
Viscosity assessment before curing
Combined Heliobond and bioactive glass with a loading of 0, 10 and 20 wt% (prepared as described above) were subjected to rheological analysis (Physica MCR 300, Anton-Paar, Zofingen, Switzerland) using a cone-plate geometry (30 mm diameter, 2° cone angle) at 25 °C to determine the viscosity before curing. The space between cone and plate was filled completely with material and excess was removed. Shear rate ramp tests were performed with shear rates ranging from 0.1 to 100 s −1 ( n = 3). The following commercial materials served as reference samples: a filled dentin bonding agent (Optibond FL, Kerr, Orange, CA, USA; lot: 4689425), a fissure sealant (Helioseal, Ivoclar Vivadent; lot: M69511) and a flowable composite (Filtek Supreme XTE Flowable Restorative, 3M ESPE, St. Paul, MN, USA; lot: N368930).
2.3
Immersion in phosphate buffered saline
For this and all subsequent experiments, disk-shaped resin specimens (diameter: 6 mm, height: 2 mm) with three different loadings of bioactive glass (0, 10 and 20 wt%) were prepared in Teflon molds pressed between two glass plates, and light-cured for 5 min in a Spectramat SP1 curing unit (Ivoclar Vivadent) with a tungsten halogen light bulb (600 W, 220 V). Cured resin specimens were immersed in 1 mL phosphate buffered saline (PBS, GIBCO, Invitrogen, Billings, MT, USA) at pH 7.4 and 37 °C for 21 days without changing the aqueous medium.
2.4
pH measurements
The pH ( n = 3) was monitored using a calibrated pH electrode (Seven Easy, Mettler Toledo, Greifensee, Switzerland) after 0.5, 1, 2, 4, 8 h, 1, 3, 7, 14 and 21 days. Mere PBS served as a reference for pH measurements.
2.5
Water uptake
Additionally, the ability of the resin specimens to take up water was calculated by the wet weight W wet (determined after the specimens were removed from the liquid, dipped thrice in distilled water and carefully blotted dry) and the dry weight W dry (determined after vacuum drying during 7 days at room temperature of 23 °C). The water uptake (WU) was calculated according to the following equation:
WU ( % ) = W wet − W dry W dry ⋅ 100
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