Abstract
Objectives
To investigate the reinforcing effect of urchin-like hydroxyapatite (UHA) in bisphenol A glycidyl methacrylate (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) dental resin (without silica nanoparticles) and dental composites (with silica nanoparticles), and explore the effect of HA filler morphologies and loadings on the mechanical properties.
Methods
UHA was synthesized by a facile method of microwave irradiation and studied by X-ray diffraction (XRD), scanning electron microscope (SEM), and thermogravimetric analysis (TGA). Mechanical properties of the dental resin composites containing silanized UHA were tested by a universal mechanical testing machine. Analysis of variance was used for the statistical analysis of the acquired data. The fracture morphologies of tested composites were observed by SEM. Composites with silanized irregular particulate hydroxyapatite (IPHA) and hydroxyapatite whisker (HW) were prepared for comparative studies.
Results
Impregnation of lower loadings (5 wt% and 10 wt%) of silanized UHA into dental resin (without silica nanoparticles) substantially improved the mechanical properties; higher UHA loadings (20 wt% and 30 wt%) of impregnation continuously improved the flexural modulus and microhardness, while the strength would no longer be increased. Compared with silanized IPHA and HW, silanized UHA consisting of rods extending radially from center were embedded into the matrix closely and well dispersed in the composite, increasing filler-matrix interfacial contact area and combination. At higher filler loadings, UHA interlaced together tightly without affecting the mobility of monomer inside, which might bear higher loads during fracture of the composite, leading to higher strengths than those of dental resins with IPHA and HW. Besides, impregnation of silanized UHA into dental composites (with silica nanoparticles) significantly improved the strength and modulus.
Significance
UHA could serve as novel reinforcing HA filler to improve the mechanical properties of dental resin and dental composite.
1
Introduction
Dental resin composites have been widely used instead of amalgam alloys and considered as versatile and reliable materials for different types of caries . Most dental resin composites consist of the matrix and filler. The matrix is typically derived from the monomer system that includes a free radical initiating system and the filler is utilized to reinforce the softer organic matrix . Since the introduction of dental resin composites in 1960s, extensive research has been attempted to develop materials with acceptable physical and mechanical properties. Owing to the development of nanotechnology and hybrid filling techniques, wear-resistant property, esthetic quality, and longevity of dental resin composites have been dramatically improved . However, some problems remain, which include the adverse effects of polymerization shrinkage , and inadequate mechanical properties of the composites in posterior restorations during service . Current researches demonstrated that most nanoparticle fillers did not yet provide anticipated reinforcing effects to significantly improve mechanical properties of the dental composites . Therefore, fillers that could transfer and bear larger load have attracted numerous attention recently, such as Si 3 N 4 , SiC whisker, tetrapod whisker, inorganic fiber, halloysite nanotubes, and polymer fibers , and dental composites reinforced by these fillers show remarkably enhanced mechanical properties due to the diverse strengthening and toughening mechanisms of the fillers, which provide an effective and promising approach to fabricate strong dental composites.
Hydroxyapatite (HA) is the main component of the mineral part in bone tissue, tooth enamel and dentin. Synthetic HA has excellent biocompatibility and bioactivity, which is widely used as a material for human hard tissue regeneration. Besides, HA is also a promising material to reinforce biomedical composites, and it has drawn extensive attention in the development of biomaterials. Recently, different types of HA, such as particle, nanorod, whisker, and nanofiber, were employed to prepare bionic dental resin composite, and much basic work has been investigated for mechanical properties, service behaviors, stability, and biological properties. Unfortunately, dental composites using HA particles or nanorods possess lower flexural strength and modulus . Hydroxyapatite whisker (HW) or nanofiber could improve the mechanical properties, while higher filler loading tends to result in aggregations that influence filler dispersion in the composite and serve as mechanical defects, which decrease the mechanical properties of the dental resin composite consequently . Therefore, how to fabricate strong and bionic HA filled dental resin composite has gained more attention.
Inspired by the retention ability of the sea urchin spines on the seafloor, here, we report a novel dental resin composite which was embedded and enhanced by silanized urchin-like hydroxyapatite (UHA). UHA is bioactive filler with hierarchical structure, which has a globular appearance and consists of rods extending radially from the center like urchin. With the unique structure that combines features of globular and whisker, UHA is supposed to disperse easily in the matrix compared with nanorod, whisker or fiber-like HA, meanwhile, the radiate rods of UHA could embed and interlock with the matrix to result in efficient strengthening and toughening mechanisms compared with particulate HA. Moreover, the complicated morphology and large surface area of UHA could increase the contact area between filler and matrix, enhancing the interfacial combination. Thus, it is expected that UHA could enhance the mechanical properties of dental resin composites more efficiently . In the past decade, polymers reinforced by filler with similar structure showed significantly improved mechanical properties . However, to the best of our knowledge, no work has been reported about the preparation of dental resin composite enhanced by urchin-like hydroxyapatite. Herein, to explore the reinforcing effect of UHA, we prepared and characterized dental resin/composite filled with silanized UHA. In this study, UHA was synthesized by microwave irradiation and modified by 3-methacyloxypropyltrimethoxysilane (γ-MPS) to improve the compatibility and interfacial binding between UHA and resin matrix. Meanwhile, silanized IPHA and HW were employed to compare the reinforcing performance with UHA. The effect of UHA loading on mechanical properties of the dental resin/composite was also discussed with a view to developing strong, bioactive and reliable dental restorative resin composites.
2
Materials and methods
2.1
Materials
Bis-GMA and TEGDMA were obtained from Sigma-Aldrich (USA). Camphorquinone (CQ), γ-MPS, and ethyl-4-dimethylaminobenzoate (4-EDMAB) were obtained from J&K Scientific (China). Propylamine, cyclohexane, calcium nitrate, sodium dihydrogen phosphate dehydrate, gelatin, urea, calcium nitrate tetrahydrate (Ca(NO 3 ) 2 ·4H 2 O) and dibasic anhydrous sodium phosphate (Na 2 HPO 4 ), ethylenediaminetetracetic acid disodium salt (EDTA), sodium hydroxide (NaOH) were all obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Irregular particulate HA (IPHA) was obtained from Emperor Nano Material Co., Ltd. (Nanjing, China). Silica nanoparticles (Aerosil OX50, average size 40 nm) were purchased from Shanghai Haiyi Co. Ltd. (China). All materials were of analytical grade and used as received without further purification. HW was prepared as previously reported .
The synthesis procedure of UHA was modified from a method reported by Liu . Briefly, 200 mL of a mixed solution of Ca(NO 3 ) 2 ·4H 2 O (0.4 M) and EDTA (0.4 M) was introduced into 200 mL of Na 2 HPO 4 (0.24 M) solution. The pH of solution was adjusted to 13 by adding NaOH solution. The obtained solution was put into a household type microwave oven of 700 W power with a refluxing system and the reaction was performed at 105 °C under ambient air for 30 min. After cooling to room temperature, the precipitate was filtered, washed with deionized water, and dried in oven under vacuum at 70 °C for 2 h.
2.2
Silanization
UHA, IPHA, HW, and silica were silanized respectively following the method as reported . Filler (5.0 g), cyclohexane (500 mL), propylamine (0.2 g) and γ-MPS (0.5 g) were stirred in a 1 L three-neck flask at room temperature for 30 min and then heated at 65 °C for 90 min. The mixture was evaporated on a rotary evaporator at 60 °C to remove the volatile substances and then heated at 95 °C for 2 h. Finally, the product was dried in oven under vacuum at 85 °C for 18 h.
2.3
Preparation of dental resin composites
As illustrated in Fig. 1 , resin matrix containing monomers (Bis-GMA/TEGDMA, 49.5/49.5, wt%) and initiators (CQ/4-EDMAB, 0.2/0.8, wt%) was uniformly mixed with a magnetic stirrer in a 20 mL flat bottom bottle, and then the silanized UHA was slowly added into the matrix while continuously stirring. After mixing thoroughly, the mixture with different filler mass fractions (5 wt%, 10 wt%, 20 wt%, and 30 wt%) was dispersed by ultrasonic method for 15 min and stirred (1000 rpm) for 12 h subsequently. The unfilled resin (0 wt%) and composites containing silanized IPHA and HW served as controls. Silanized UHA and silica (the weight percent ratio of UHA/silica in the composite: 0/60, 5/55, 10/50, 30/30) were mixed with the resin matrix (40 wt% of the composite) using a three-roller mixer (EXAKT 80E, Exakt Apparatebau GmbH & Co., Germany, for fabrication of viscous composite with a higher filler loading) according to the reported method . All the obtained uncured composites were placed in oven under vacuum at room temperature for 8 h to remove air bubbles. Afterwards, the composite was added carefully to the rectangular (25 mm × 2 mm × 2 mm) and circle shaped (Φ 4 mm × 6 mm, Φ 6 mm × 4 mm) silicon rubber molds covered by glass slides. Then, the samples were light-cured by a curing unit (Blue light, 470 nm, SLC-VIII B Hangzhou Sifang Medical Apparatus Co., Ltd., Zhejiang, China) for 60 s on each side. All specimens were polished using a sand paper with a grit number of 1500 #.
2.4
Characterization and evaluation
2.4.1
X-ray diffraction
X-ray diffraction (XRD) pattern of UHA was collected on a X-ray difractometer (XRD, D/Max-2550 PC, RIGAKU, Japan) using Ni-filtered Cu Kα1 radiation ( λ = 0.154 nm) in the 2 θ range 10–80° at a voltage of 40 kV and a current of 200 mA.
2.4.2
Fourier transform infrared spectroscopy (FTIR) analysis
The FTIR analysis was conducted with a method of attenuated total reflection in a FTIR spectrometer (Nicolet Nexus 670, Thermo Fisher, USA). Spectra were collected over 4000–650 cm −1 region and were acquired with a resolution of 4 cm −1 and a total of 10 scans per spectrum.
2.4.3
Thermogravimetric analysis
The amount of grafted γ-MPS on filler was determined by thermogravimetric analysis (TGA). Weight changes as a function of time and temperature were evaluated during a thermal program from 50 to 600 °C at a heating rate of 10 °C/min in nitrogen atmosphere. The measurement was performed on a thermal gravimetric analyzer (STA409PC, NETZSCH, Germany) using 5–10 mg of each sample.
2.4.4
Morphology
Field emission scanning electron microscope (S-4800, Hitachi, Japan) and back-scattered scanning electron microscope (Quanta-250, FEI Company, Czech Republic) were employed to observe the morphology and size of UHA and the fracture surface of dental resins/composites after three-point bending test.
2.4.5
Mechanical properties
Flexural strength, flexural modulus, compressive strength, and diametral tensile strength of the resin composites were measured using a universal mechanical testing machine (Instron 5900, USA) according to the reported methods . Six rectangular bar specimens (25 mm × 2 mm × 2 mm) were prepared for the three-point bending test (span 20 mm, crosshead speed 0.75 mm/min). In the compressive test, six cylinder specimens (Φ 4 mm × 6 mm) and six disk specimens (Φ 6 mm × 4 mm) were prepared for measuring the compressive strength (loading rate 1 mm/min) and the diametral tensile strength (crosshead speed 1.0 mm/min), respectively. Vickers microhardness was determined on six disk-shaped specimens (Φ 6 mm × 4 mm) under a load of 50 g for 10 s using a micro hardness tester (HXD-1000TMC/LCD, Shanghai Taiming Optical Instrument Co., Ltd., China).
2.4.6
Statistical analysis
The acquired data of mechanical properties were analyzed with one-way analysis of variance using SPSS software (version 17, Chicago, IL). Significant differences were determined using the Tukey’s test. References to significant differences were based on a probability of P < 0.05 unless otherwise stated.
2
Materials and methods
2.1
Materials
Bis-GMA and TEGDMA were obtained from Sigma-Aldrich (USA). Camphorquinone (CQ), γ-MPS, and ethyl-4-dimethylaminobenzoate (4-EDMAB) were obtained from J&K Scientific (China). Propylamine, cyclohexane, calcium nitrate, sodium dihydrogen phosphate dehydrate, gelatin, urea, calcium nitrate tetrahydrate (Ca(NO 3 ) 2 ·4H 2 O) and dibasic anhydrous sodium phosphate (Na 2 HPO 4 ), ethylenediaminetetracetic acid disodium salt (EDTA), sodium hydroxide (NaOH) were all obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Irregular particulate HA (IPHA) was obtained from Emperor Nano Material Co., Ltd. (Nanjing, China). Silica nanoparticles (Aerosil OX50, average size 40 nm) were purchased from Shanghai Haiyi Co. Ltd. (China). All materials were of analytical grade and used as received without further purification. HW was prepared as previously reported .
The synthesis procedure of UHA was modified from a method reported by Liu . Briefly, 200 mL of a mixed solution of Ca(NO 3 ) 2 ·4H 2 O (0.4 M) and EDTA (0.4 M) was introduced into 200 mL of Na 2 HPO 4 (0.24 M) solution. The pH of solution was adjusted to 13 by adding NaOH solution. The obtained solution was put into a household type microwave oven of 700 W power with a refluxing system and the reaction was performed at 105 °C under ambient air for 30 min. After cooling to room temperature, the precipitate was filtered, washed with deionized water, and dried in oven under vacuum at 70 °C for 2 h.
2.2
Silanization
UHA, IPHA, HW, and silica were silanized respectively following the method as reported . Filler (5.0 g), cyclohexane (500 mL), propylamine (0.2 g) and γ-MPS (0.5 g) were stirred in a 1 L three-neck flask at room temperature for 30 min and then heated at 65 °C for 90 min. The mixture was evaporated on a rotary evaporator at 60 °C to remove the volatile substances and then heated at 95 °C for 2 h. Finally, the product was dried in oven under vacuum at 85 °C for 18 h.
2.3
Preparation of dental resin composites
As illustrated in Fig. 1 , resin matrix containing monomers (Bis-GMA/TEGDMA, 49.5/49.5, wt%) and initiators (CQ/4-EDMAB, 0.2/0.8, wt%) was uniformly mixed with a magnetic stirrer in a 20 mL flat bottom bottle, and then the silanized UHA was slowly added into the matrix while continuously stirring. After mixing thoroughly, the mixture with different filler mass fractions (5 wt%, 10 wt%, 20 wt%, and 30 wt%) was dispersed by ultrasonic method for 15 min and stirred (1000 rpm) for 12 h subsequently. The unfilled resin (0 wt%) and composites containing silanized IPHA and HW served as controls. Silanized UHA and silica (the weight percent ratio of UHA/silica in the composite: 0/60, 5/55, 10/50, 30/30) were mixed with the resin matrix (40 wt% of the composite) using a three-roller mixer (EXAKT 80E, Exakt Apparatebau GmbH & Co., Germany, for fabrication of viscous composite with a higher filler loading) according to the reported method . All the obtained uncured composites were placed in oven under vacuum at room temperature for 8 h to remove air bubbles. Afterwards, the composite was added carefully to the rectangular (25 mm × 2 mm × 2 mm) and circle shaped (Φ 4 mm × 6 mm, Φ 6 mm × 4 mm) silicon rubber molds covered by glass slides. Then, the samples were light-cured by a curing unit (Blue light, 470 nm, SLC-VIII B Hangzhou Sifang Medical Apparatus Co., Ltd., Zhejiang, China) for 60 s on each side. All specimens were polished using a sand paper with a grit number of 1500 #.
