This study aimed to evaluate the physical-chemical properties of experimental dental adhesives containing boron nitride nanotubes (BNNTs) as inorganic fillers.
An experimental adhesive resin was prepared using HEMA-BisGMA, 66/33 wt% (control). Inorganic BNNT fillers were first analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and then incorporated into the adhesive at different concentration (0.05, 0.075, 0.1, 0.15 wt%). Degree of conversion (DC), ultimate strength, contact angle, surface free energy (SFE) microhardness, softening in solvent and bioactivity were assessed.
Scanning and transmission electron microscopy (SEM and TEM) showed BNNTs with diameter ranging from 5 to 10 nm with close end tips. No changes in DC were observed after incorporating BNNTs up to 0.15 wt%. The contact angles of water and α-bromonaphthalene increased (p < 0.05) and consequently the SFE decreased after incorporating BNNTs to the polymer matrix. Microhardness and solvent degradation strength increased after incorporation of 0.075, 0.1 and 0.15 wt% BNNTs. Mineral deposition was found after 7 days of immersion on adhesive specimens after incorporation of BNNT.
The incorporation of BNNTs up to 0.15 wt% improved the chemical and mechanical properties of dental adhesives and promoted mineral deposition.
Incorporation of boron nitride nanotubes into adhesive resin materials improved physical-chemical properties and increased mineral deposition on its surface allowing enhanced properties of the resin-dentin interface. Thus, the novel adhesive material is promising as a dental adhesive and may contribute to the stability of the dentin-resin bonding.
Degradation of the bonded interface is a long-standing concern in the dental adhesive field because it influences bonding effectiveness and consequently the long-term success rate of restorations . After acid-etching and primer/adhesive procedures, exposed collagen fibrils may remain incompletely infiltrated, and therefore prone to degradation . For this reason, several attempts to regenerate dentinal tissue have been recently reported . The ability of bioactive ion-releasing adhesives promoting mineral deposition is a suitable strategy to backfill the mineral-depleted dentin collagen and improve mechanical strength within the hybrid layer .
Notwithstanding, hydrolytic resin degradation within the hybrid layer due to a breaking process of covalent bonds between polymers is considered one of the main reasons for adhesives’ mass loss and reduction in bond strengths to dentin . To overcome these issues, the development of dental adhesives with improved physical and mechanical properties as a function of time has been proposed . Hydrolytic stability , enzymatic degradation resistance , and the ability to interact with biological structures are the main required characteristics for bonding materials. The increase of mechanical and chemical strength of dental adhesives is directly involved in the clinical longevity of the hybrid layer and related to the presence of inorganic fillers that reduce the proportion of organic matrix .
Nanotechnology has contributed to the improvement of adhesive fillers, especially the resolving of nanoscale failures. A wide variety of nanofillers have been used in the form of capsules, rods, tubes, and fibers. Notably, the size of the scale concomitant to surface area increase of these fillers renders a unique combination of mechanical properties by reinforcing the polymeric matrix . Among these nanofillers, boron nitride nanotubes (BNNT) have drawn attention due to their structures being similar to carbon and graphene nanotubes, and hence to their superior mechanical properties and chemical and thermal stability . The covalent bond between boron and nitrogen (B-N) is highly stable; therefore, BNNTs achieve high modulus of elasticity reaching up to 1.2 TPa . BNNTs’ fibrillar nature and tangled aspect make them superhydrophobic, reaching contact angles (CAs) with water up to 170° , thus serving as the basis of self-cleaning products . A suitable biocompatibility of BNNTs was reported, demonstrating a good interaction with organic molecules such as proteins and osteoblasts . These characteristics make them promising nanovectors in biomedical applications . Although most of the studies carried out have explored BNNTs as reinforcing agents to improve their mechanical properties, some of them have demonstrated bioactive properties .
BNNTs have been also proposed as fillers for polymeric grafts and scaffolds . The addition of BNNTs into dental adhesives could improve the mechanical properties of the material and stimulate mineral deposition. Thus, the purpose of this study was to evaluate the influence of BNNTs’ incorporation in experimental adhesive resin. The null hypothesis is that the addition of BNNTs will not influence the properties of adhesive resin.
Material and methods
Scanning and transmission electron microscopy
BNNTs (BNNT, LCC, Newport News, VA, USA) synthesized via a pressurized vapor/condenser method were examined using scanning electron microscopy (EVO MA10, Zeiss, Oberkochen, Germany) with a voltage of 0.2–30 kV and maximum resolution of 3.0 nm, and transmission electron microscopy (JEM2010, Jeol, Tokyo, Japan) operating in a voltage of 200 kV with a point resolution of 0.25 nm and line resolution of 0.14 nm ( Fig. 1 ).
Formulation of experimental adhesive resin
The polymeric matrix was composed by bisphenol A-glycol dimethacrylate (BisGMA) and hydroxyethyl methacrylate (HEMA) purchased from Sigma-Aldrich (St. Louis, MO, USA) and mixed in mass ratio of 66.6 and 33.3 wt.%, respectively. The photo-initiators camphorquinone (CQ), ethyl 4-dimethylamino benzoate (EDAB) and diphenyliodonium hexafluorophosphate (DPIHFP) (Aldrich Chemical, Milwaukee, MI, USA) were added to 1 mol%, as previously reported .
BNNT were incorporated into the polymeric matrix at concentrations of 0.05, 0.075, 0.1 and 0.15 wt%. A control group was maintained without nanotubes. All components were weighed in an analytical balance (AUW220D, Shimadzu Corp., Kyoto, Japan), mixed, and ultrasonicated for 2 h. For all evaluations performing photo-activation, a light-emission diode curing unit (Radii Cal, SDI, Bayswater, VIC, Australia) with an irradiation value of 1.200 mW/cm 2 , determined with a digital radiometer (RD-7, Ecel Ind., Ribeirão Preto, Brazil) was used.
Degree of conversion
The specimens of each group (n = 3), were analyzed by micro-Raman spectroscopy using Senterra equipment (Bruker Optik GmbH, Ettlingen, Baden-Württemberg, Germany) with 3 coadditions for 5 s of irradiation using a 100 mW diode laser with 785-nm wavelength and 50 × 1000 μm aperture size. Spectra were obtained with a spectral resolution of ∼3.5 cm −1 at 440 to 1800 cm −1 range, the same parameters used in mineral deposition evaluation. Spectra were collected in the adhesive at three individual points both previous and after polymerization for 20 s. The average value of the measurements was used for calculation of the ratio of double bond content of monomer to polymer in the adhesive. The aliphatic and aromatic peaks were 1635 cm −1 and 1608 cm −1 .
Microhardness and softening in solvent
Cylindrical resin specimen with standardized dimensions (thickness: 2 mm; diameter: 5 mm) were created using a polyvinylsiloxane mold. In brief, each tested adhesive was inserted into a mold and the outer surfaces were covered with a polyester strip and pressed with a glass slide to squeeze out any excess of adhesive before the light-curing procedures. The specimens were light cured for 20 s on both sides and polished using #600, #1200, and #2000 grits SiC abrasive papers under water irrigation (30 s each).
Five specimens were used for each experimental adhesive. The specimens were stored for 24 h and submitted to initial Knoop microhardness (KHN 1 ) assessment, as previously described . Then, specimens were immersed in a solution of 50% ethanol and 50% water mixture for 2 h at 37 °C and were again submitted to microhardness (KHN 2 ) assessment. KHN measurements were taken using an indenter HMV-2 (Shimadzu Corp., Kyoto, Japan) under a load of 10 g for 5 s. Three indentations were performed on the upper face of each specimen. The Knoop microhardness values were recorded as the average of the three indentations per specimen. The KHN 1 and the KHN 2 values were processed as KHN% by calculation of the Δ between both.
Ultimate tensile strength
Eleven hourglass-shaped samples of each group measuring 8 mm long, 2 mm wide, 1 mm thick, and cross-sectional area of 1 mm 2 were subjected to ultimate tensile strength test. The adhesive resins were inserted into a metallic matrix and a polyester strip was placed on them before light curing at the bottom and at the top of the specimens. The samples were stored for 24 h at 37 °C and submitted to strength test using a mechanical testing machine (Shimadzu EZ-SX, Shimadzu Corp., Kyoto, Japan) at a crosshead speed of 1 mm/min until rupture of the specimen.
Contact angle and surface free energy measurement
Five specimens were created with same manufacturing process and dimensions, as described in Section 2.7 . These were embedded in acrylic resin and analyzed using an optical tensiometer Theta (Biolin Scientific, Stockholm, Sweden) to measure the static CA (θ) to distilled water and to α-bromonaphthalene droplets and the surface free energy. The drop out size, drop rate, displacement rate, and speed dispersion of water or α-bromonaphthalene were: 3.0 μL, 2.0 μL/s, 20.0 μL/s, and 50 mm/min, respectively. The test period was performed over 20 s and the mean contact angle (θ) between the droplet and the solid surface was registered after 10 s. The surface free energy calculation (mN/m) was performed following the OWRK/Fowkes* equation with OneAttension software (Biolin Scientific, Stockholm, Sweden) as previously described .
* γ l s = σ l + σ s − 2 ( σ l D . σ s D + σ l P . σ s P )