The objectives of this study are to investigate the properties of high aspect-ratio hydroxyapatite (HAP) nanofibers and the reinforcing effect of such fibers on bisphenol A glycidyl methacrylate (BisGMA)/triethylene glycol dimethacrylate (TEGDMA) dental resins (without silica microparticle filler) and dental composites (with silica microparticle filler) with various mass fractions (loading rates).
HAP nanofibers were synthesized using a wet-chemical method and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and thermal gravimetric analysis (TGA). Biaxial flexural strength (BFS) of the HAP nanofibers reinforced dental resins without any microsized filler and dental composites with silica microparticle filler was tested and analysis of variance (ANOVA) was used for the statistically analysis of acquired data. The morphology of fracture surface of tested dental composite samples was examined by SEM.
The HAP nanofibers with aspect-ratios of 600 to 800 can be successfully fabricated with a simple wet-chemical method in aqueous solution. Impregnation of small mass fractions of the HAP nanofibers (5 wt% or 10 wt%) into the BisGMA/TEGDMA dental resins or impregnation of small mass fractions of the HAP nanofibers (2 wt% or 3 wt%) into the dental composites can substantially improve the biaxial flexural strength of the resulting dental resins and composites. A percolation threshold of HAP nanofibers, beyond which more nanofibers will no longer further increase the mechanical properties of dental composites containing HAP nanofibers, was observed for the dental composites with or without silica microparticle filler. Our mechanical testing and fractographic analysis indicated that the relatively good dispersion of HAP nanofibers at low mass fraction is the key reason for the significantly improved biaxial flexural strength, while higher mass fraction of HAP nanofibers tends to lead to bundles that cannot effectively reinforce the dental resins or composites and may even serve as defects and thus degrade the resulting dental resin and composite mechanical properties.
The incorporation of small mass fraction of HAP nanofibers with good dispersion can improve the mechanical property of dental resins and dental composites.
Dental composite resins have been used as popular materials to restore teeth since their introduction about 50 years ago . Compared to dental amalgams, they have less safety concern and possess better esthetic property. Based on the report in 2005, the composites were used in more than 95% of all anterior tooth direct restorations and about 50% of all posterior tooth direct restorations . Despite the significant improvement of resin-based composite, restorative composites still suffer from two key shortcomings: deficiencies of mechanical strength and high polymerization shrinkage, which are responsible for the shorter median survival lifespan of resin-based composites (5–7 years) in comparison with amalgam (13 years) . Dental composites are typically composed of four major components: organic polymer matrix (BisGMA, TEGDMA, urethane dimethacrylate (UDMA), etc.), inorganic filler particles, coupling agents, and the initiator–accelerator system. During the past decade, more efforts have been focused on dental nanocomposite, with a hope that contemporary nanocomposites with ceramic nanofillers should offer increased esthetics, strength, and durability . However, research to date shows that most nanofillers provide only incremental improvements in the mechanical properties with a few exceptions .
Variety of calcium phosphates (CaPs), such as HAP , amorphous calcium phosphates (ACP) , tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA) have been studied as fillers to make mineral releasing dental composites. Skrtic et al. conducted pioneering research to investigate the physicochemical properties of dental composites containing unhybridized and hybridized amorphous calcium phosphates (ACP) . Their research demonstrated that hybridization of ACP fillers using agents, such as tetraethoxysilane (TEOS) or ZrOCl 2 solution, improved the mechanical properties, e.g. biaxial flexural strength, of the composites containing ACP fillers. However, the addition of both hybridized and unhybridized ACP fillers generally degraded the biaxial flexural strength of the resin materials . It was hypothesized that the strength degradation compared to unfilled resin is attributed to poor dispersion and insufficient interaction between ACP and resin. Such hypothesis has been supported by mechanical testing of dental composites containing particles with different sizes . Both nanosized and microsized HAP particles were also studied as dental fillers and the mechanical tests indicated that microsized instead of nanosized HAP was favored in terms of mechanical properties . Xu et al. reported that 20% DCPA (dicalcium phosphate anhydrous) nanoparticles with silicon carbide (SiC) whiskers of average 0.9 μm in diameter and 14 μm in length improved the flexural strength of associated dental composites from 103 ± 32 MPa to 167 ± 23 MPa . In summary, except for the positive results from Xu’s work where the reinforcement is most likely attributed to silicon carbide whiskers, most of other dental composites using CaP based nanoparticles did not have improved mechanical properties.
From the point of view of composite mechanics, fibers are the preferred reinforced materials compared to particles since fibers can provide larger load transfer and they can also facilitate some well-known toughening mechanisms, such as fiber bridging and fiber pullout. Reinforcement with high-strength inorganic fibers indeed demonstrates significant improvement on the mechanical properties of dental composite . Particularly, Xu et al. showed that the impregnation of extremely strong Si 3 N 4 or SiC whiskers with silica nanobeads could result in two-fold increase in composite strength and toughness . Fong et al. reported that the addition of 5 wt% relative low-strength polymer (Nylon 6) nanofibers could also lead to 36% and 26% increase in flexural strength and modulus, respectively . Beyond the benefits of strengthening effects, it has been reported that fibers can reduce the polymerization shrinkage as well . As a result, we anticipated that the addition of HAP nanofibers could substantially improve the properties, especially mechanical properties, of dental composites, which might eventually extend the lifespan of the restoration.
The aim of this study was to investigate the reinforcement of BisGMA/TEGDMA dental resins (without any silica particles) and composites (with conventional glass fillers) with various mass fractions of HAP nanofibers. The novel HAP nanofibers were synthesized by a wet chemical method and characterized by the X-ray diffraction (XRD), thermogravity analysis (TGA) and scanning electron microscopy (SEM). We hypothesize that HAP nanofibers with diameter of around 100 nm should have a very high tensile strength estimated using a ceramic fiber strength model based on Griffith criterion and proposed by Gao , and the high aspect-ratio HAP nanofibers can also provide high load transfer from dental resin matrix to strong nanofibers and eventually result in dramatic improvement of the dental nanocomposite mechanical properties. To test this hypothesis, photo-cured BisGMA/TEGDMA dental resins/composites filled with different mass fractions of HAP nanofibers were systematically fabricated. The biaxial flexural strength was then tested, and analysis of variance (ANOVA) was used for the statistical analysis of the acquired data. The morphology of HAP nanofibers and the fracture surface of dental resins and composites were examined by scanning electron microscopy (SEM).
Materials and methods
Camphorquinone (CQ), ethyl 4-dimethylamino-benzoate (EDMAB), calcium nitrate, sodium dihydrogen phosphate dehydrate, gelatin, urea, and acetone were all purchased from Sigma–Aldrich Company (St. Louis, MO, USA) without further purification. BisGMA, TEGDMA, and the 0.7 μm size silica particles with 2 wt% silane (V-117-2207) were provided by Esstech, Inc. (Essington, PA, USA).
Synthesis of HAP nanofibers
The HAP nanofibers synthesis procedure is modified from a method reported by Zhan et al. . In a typical synthetic process, calcium nitrate (0.02 mol/L), sodium dihydrogen phosphate dehydrate (0.02 mol/L), gelatin (0.4 g/L), and urea (0.04 mol/L) were dissolved and mixed in an aqueous solution at the room temperature. Then, this mixed solution was heated to 95 °C and kept at such temperature for 72 h. Finally, the product was filtered, washed with deionized water, and dried at the room temperature.
Dental composite preparation
The organic matrix contained 49.5 wt% BisGMA and 49.5 wt% diluent comonomer TEGDMA. CQ (0.5 wt%) and EDMAB (0.5 wt%) were added as the initiator and co-initiator, respectively. Various mass fractions of HAP nanofibers were added into the vial to mix with the dental resins (without silica particles) and composites (with silica microparticles and the total inorganic filler mass fraction was 60 wt%) systems. Several drops of acetone were added to reduce the viscosity of mixture and the resin and filler were mixed with the magnetic stir for about 24 h in order to evaporate the acetone. The initiator and co-initiator (CQ and EDMAB) were added into the monomer solution after covering the whole vial with an aluminum foil to prevent the photo-curing. After mixing thoroughly with the initiator and the solution, the high viscosity solution can be added carefully to the circle shaped Teflon mold (diameter 12 mm, thickness 1.5 mm) covered by glass slide. Then, the samples were photo-cured by Triad 2000 (Dentsply, York, PA, USA) for 2 min on each side.
Power X-ray diffraction (XRD) experiments were performed on a Scintag Pad V X-ray diffractometer with Cu Kα radiation (1.54 Å) and a Ni filter. Scans of bulk powder were run at 40 kV and 35 mA.
Thermal gravimetric analysis (TGA)
The amount of organic layer on HAP nanofiber was determined by thermal gravimetric analysis (TGA) using Perkin-Elmer DSC-7 Differential Scanning Calorimeter (Waltham, MA, USA). TGA measurements were performed from room temperature to 900 °C at a heating rate of 5 °C/min. The amount of organic layer on the HAP surface was determined by comparing the weight loss percentage during heating measured using TGA with previously reported TGA data of HAP nanoparticles synthesized by wet-chemical methods .
The HAP nanofibers were heat treated at 400 °C, 600 °C, 800 °C, 1000 °C, 1200 °C, and 1400 °C in air using a box furnace for about 24 h (Carbolite RHF 16/15, Pacific Combustion, Torrance, CA, USA), respectively.
Scanning electron microscope (Quanta 600 FESEM, FEI, Japan) and digital camera (Canon PowerShot SD780, Canon, Japan) were employed to characterize the morphology and size of the HAP nanofibers and the fracture surface of as-fabricated dental resins and composites.
Biaxial flexural strength (BFS)
Biaxial flexural strength of the dental resin and composite samples was measured using a SMS TA.HDPlus Texture Analyzer (Texture Technologies, Scarsdale, NY, USA) . The Analyzer recorded the applied load as a function of time. The BFS was calculated according to Timoshenko and Woinowsky-Kreiger (Eq. (1) ) .
σ = P h 2 ( 1 + V ) 0.485 ln a h + 0.52 + 0.48
where σ was the maximum tensile strength, P was the measured fracture load, a was the radius of the knife-edge support, h was the mean specimen thickness measured from fragments at the point of fracture with a screw-gage micrometer accurate to 10 μm and <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='v’>vv
was the Poisson’s ratio. In comparison with the three point flexural test, the biaxial flexural test is known to be more clinically relevant. In addition, disc specimens are photo-cured in one-shot, eliminating an overlapping photo-curing technique associated with bar-shaped specimens, which might reduce the reliability of strength data .