Three fillers with various functionality tailored an adhesive bioactive.
Viscosity increased due to the bioactive glass agglomerates.
Maintained or improved structural and physico-chemical properties of the adhesive through multifunctional particles.
This study aimed to analyze the effect of infiltrating a commercial adhesive with nanosized bioactive glass (BG-Bi) particles or methacryl-functionalized polyhedral oligomeric silsesquioxanes (POSS) on material properties and bioactivity.
An acetone-based dental adhesive (Solobond Plus adhesive, VOCO GmbH, Cuxhaven, Germany) was infiltrated with nanosized bioactive glass particles (0.1 or 1 wt%), or with monofunctional or multifunctional POSS particles (10 or 20 wt%). Unfilled adhesive served as control. Dispersion and hydrodynamic radius of the nanoparticles were studied by dynamic light scattering. Set specimens were immersed for 28 days in artificial saliva at 37 °C, and surfaces were mapped for the formation of calcium phospate (Ca/P) precipitates (scanning electron microscopy/energy-dispersive X-ray spectroscopy). Viscosity (rheometry) and the structural characteristic of the networks were studied, such as degree of conversion (FTIR spectroscopy), sol fraction and water sorption.
POSS particles showed a good dispersion of the particles for both types of particles being smaller than 3 nm, while the bioactive glass particles had a strong tendency to agglomerate. All nanoparticles induced the formation of Ca/P precipitates. The viscosity of the adhesive was not or only slightly increased by POSS particle addition but strongly increased by the bioactive glass particles. The degree of conversion, water sorption and sol fraction showed a maintained or improved network structure and properties when filled with BG-Bi and multifunctional POSS, however, less polymerization was found when loading a monofunctional POSS.
Multifunctional POSS may be incorporated into dental adhesives to provide a bioactive potential without changing material properties adversely.
Over the past decade, functionalization of dental adhesives to combat degradation of the dentin hybrid layer was intensively studied. The degradation of the hybrid layer is attributed to the hydrolytic and enzymatic degradation of hydrophilic resin components and exposed collagen fibrils lacking the protection of polymerized resin . Strategies to prevent the degradation of the hybrid layer and, thus, to increase the longevity of composite restorations, include the inhibition of endogenous proteases and the stimulation of dentin mineralization .
Nanoparticles of amorphous calcium phosphate , bioactive glass or hydroxyapatite have been incorporated into dental adhesives to induce mineral precipitation within the hybrid layer. The nanosized bioactive glass offers better antimicrobial effects and bioactivity through the higher release of alkali species than the conventional bioactive glass in microparticulate form . However, fillers might increase the viscosity of the adhesive , impeding the wettability of the dentin surface and diffusion into demineralized dentin. The unbounded fillers may be washed out easily and could have potentially adverse effects.
The incorporation of the functionalized Polyhedral oligomeric silsesquioxanes (POSS) particles has been shown to improve mechanical properties, such as flexural strength, toughness and to reduce network solubility and degradation . POSS are nanostructured hybrid molecules with an inorganic framework of silicon and oxygen atoms and an outer shell of organic functional groups allowing polymerization . The hybrid character renders a very good dispersion and particle mobility in an organic substances different from a typical hard/compact particle . The multifunctional POSS particles with highly reactive groups lead to higher cross-link density and, thus, also mechanical properties may be improved. At low concentrations, POSS particles were successfully incorporated into experimental composite decreasing the volumetric shrinkage and improving the mechanical properties, e.g. flexural strength, Young’s modulus or hardness . When applied in a dental adhesive, the fillers acting as a cross-link agent may also enhance the bond to a composite through the increase of the shear strength . In the same time, POSS-containing materials were shown to promote the formation of plate-like hydroxyapatite crystals at their surfaces indicating in vitro bioactive properties .
Moreover, fillers like POSS particles can render an adhesive more hydrophobic and increase the monomer conversion. Residual unreacted monomers and oligomers may be extracted from the restorative material to the organism and act as toxic species . When the areas released from the residuals are filled with water, the mechanical properties of the adhesive are lowered and the process of aging is accelerated . Fillers might potentially reduce these adverse effects.
Therefore, the incorporation of POSS particles into dental adhesives might be of great benefit to stimulate mineralization and improve the mechanical properties of the hybrid layer. The present study aimed to analyze the effect of infiltrating a commercial adhesive with POSS compared to nanosized bioactive glass particles on bioactivity and material properties. The null hypothesis was that the studied particles will not make the adhesive bioactive and that there is no difference between the particle-filled adhesives and the neat adhesive system regarding its properties, such as viscosity, degree of conversion, water sorption and sol fraction.
Materials and methods
The adhesive resin of a commercial acetone-based dental adhesive system (Solobond Plus Adhesive, VOCO GmbH, Cuxhaven, Germany) was used in this study and infiltrated with nanosized bioactive glass particles or POSS (Hybrid Plastics Inc., Hattiesburg, USA). According to the manufacturer, Solobond Plus Adhesive contained acetone (>50%), Bis-GMA (10–25%), TEGDMA (10–25%), HEMA (5–10%) and a catalyst (<2.5%). Nanosized bioactive glass particles equivalent to Bioglass 45S5 with 20 wt% Bi 2 O 3 (BG-Bi) were synthesized by flame spray synthesis as described elsewhere and added at two concentrations: 0.1 or 1 wt%. Monofunctional (POSS-1) and multifunctional (POSS-8) methacryl POSS (MA 0702, Methacrylisobutyl POSS and MA 0735 – Methacryl POSS, respectively) were mixed with the adhesive in the concentrations of 10 and 20 wt% ( Fig. 1 ). The chemical structure of both POSS particles is depicted in Fig. 2 . The mixtures were mixed with a magnetic stirrer for 5 min in dark. For the preparation of cured specimens, the solvent was subsequently removed by keeping the samples in vacuum for 10 min. Unfilled adhesive were prepared likewise and served as control. The analogous mixtures were prepared for viscosity experiments with the Solobond Plus adhesive resin without initiator (VOCO GmbH, Cuxhaven, Germany).
Dispersion and size of the particles
Acetone, as the solvent of the Solobond Plus adhesive, was used for the dynamic light scattering (DLS) experiment to study the particle size and possible tendency for aggregation. The adhesive itself is not applicable due to the contributions from various monomeric molecules (HEMA, Bis-GMA, acetone) that overlap with the typical peaks from particles. Acetone is thus a reasonable alternative, as the particles are expected to behave in a similar manner if dispersed in an adhesive with over 50% acetone (Solobond Plus adhesive) as in the pure acetone.
POSS-8 was measured at 4 concentrations: 0.5, 1, 2 and 5 wt%. Due to the additional contribution from the large particles (agglomerates or impurities), the POSS-1 and BG-Bi particles could not been measured directly. 1 wt% of particles dispersed in acetone were centrifuged for 20 min at 21000 g (Fresco21, Heraus, Hanau, Germany) prior to the light scattering experiment (Zetasizer Nano-ZS, Malvern, Worcestershire, Great Britain). The supernatant was used and diluted to 5 reduced concentrations: X, X/2, X/4, X/8, X/16. The BG-Bi particles could not be analyzed due to remaining agglomerates.
DLS measures the intensity correlation function of light scattered at particles immersed in a solvent . The hydrodynamic radius R of the particles is related to the measured relaxation rate Γ of the correlation function by Γ = Q 2 D 0 and the Stokes–Einstein equation
D 0 = k B T / 6 π η R
Here k B is the Boltzmann constant, T represents temperature in Kelvin, η viscosity of the solvent and Q the scattering vector used for DLS. The measurements were analyzed by the instrument build in “General Purpose” mode, which relates the measured correlation function to a size distribution. Each concentration was measured 3 times for about 2 min (13 × 10 s) at 25 °C and the analyzed results were averaged. All measurements were made at an angle of 173°.
Disk-shaped specimens (diameter: 6 mm, height: 2 mm) of each group (n = 3) were prepared in a custom-made silicon form, covered by a glass plate and light-cured for 120 s (Bluephase, Ivoclar Vivadent, Schaan, Lichtenstein, 1090 mW/cm 2 ). Afterwards, the specimens were removed from the mold and cured from the other side for 120 s. Specimens were stored in 15 ml artificial saliva or in distilled water at 37 °C for 28 days without changing the medium.
After the storage period, all specimens were gently rinsed with water and kept in desiccators at room temperature with silica gel for slow drying. The specimens for morphological analysis were sputtered by Platinum-Palladium and inspected with scanning electron microscopy at 10 kV (Ultra Plus, Carl Zeiss GmbH., Jena, Germany,). The elemental composition was determined by the energy-dispersive X-ray spectroscopy at 20 kV (Cryo-FE-SEM, FEI Quanta 200 FEG with Edax, FEI Company, Hillsboro, USA).
The viscosity of the adhesives was measured in systems without initiators with a rheometer (ARG2, TA Instruments, New Castle, USA) using a cone-plate geometry with 60 mm diameter and 0.5° cone angle. The experiments were performed at 25 °C and in dark. The linearity was checked by strain sweep experiment with the frequency of 10 rad/s and strain ranging from 1 to 20% and 20 to 1%. Afterwards, the steady rate mode from 1 rad/s to 10 rad/s was applied (n = 3). The results were fitted by Cross model, and the viscosity was taken from the region where most of the systems reached a constant value ( γ = 20 1/s).
Degree of conversion
Degree of conversion was estimated using a Fourier transform infrared spectrometer (Bruker, Billerica, USA) equipped with an attenuated total reflectance device. For each measurement, 26 spectra were collected in the range 900–4000 cm −1 with a resolution of 4 cm −1 . A background measurement was performed before each experiment. A thin layer of adhesive was applied on the crystal (6 mm diameter, ∼0.25 mm thickness). A time delay of 3 min was used for acetone to be evaporated . After this time, the sample was cured for 20 s with a LED curing lamp (Bluephase, Ivoclar Vivadent, Schaan, Liechtenstein, 1090 mW/cm 2 ) and spectra were collected for the following 10 min.
The degree of conversion was estimated from the ratio of peak heights of aliphatic C C stretching vibrations at 1638 cm −1 ( H al ) and aromatic C C stretching vibrations at 1608 cm −1 ( H ar ) from the background corrected spectra of cured and uncured samples:
DC = 1 − ( H a l H a r ) c u r e d ( H a l H a r ) u n c u r e d