Abstract
Objectives
To evaluate the effect of different surface treatments on the bond strength between polyetheretherketone (PEEK) composite materials and each of two different luting cements.
Methods
One hundred specimens were randomly divided into five groups ( n = 20/group) as follows: (A) no treatment, (B) 98% sulfuric acid, (C) 9.5% hydrofluoric acid, (D) argon plasma treatment, and (E) sandblast with 50 μm Al 2 O 3 particles. Each group was divided into two subgroups of different cements: RelyX™ Unicem and SE Bond/Clearfil AP-X™. The cements were bonded onto the specimens. All specimens were stored in distilled water at 37° for 24 h. Bond strength was measured in a shear test, and failure modes were assessed by stereomicroscopy. The surfaces were observed by SEM after the different pretreatments.
Results
Etching with 98% sulfuric acid and argon plasma treatment can significantly enforce the bond strength of RelyX™ Unicem or SE Bond/Clearfil AP-X™ to PEEK composite materials in comparison to the group of no treatment, hydrofluoric acid or sandblasting ( p < 0.05). No adhesion was established on the groups of no treatment and hydrofluoric acid when RelyX™ Unicem was used. Applying the SE Bond/Clearfil AP-X™ system, no statistical differences were found whether hydrofluoric acid was applied or not ( p > 0.05). The shear bond strength value of using SE Bond/Clearfil AP-X™ was higher than that of using RelyX™ Unicem with the same surface conditioning method ( p < 0.05).
Significance
The use of SE Bond/Clearfil AP-X™ after the surface of PEEK composite material treated with sulfuric acid or argon plasma can be recommended as an effective bonding method.
1
Introduction
Polyetheretherketone (PEEK) is one of the most popular high-performance engineering plastics currently available. PEEK is highly advantageous for applications in many industries, including aerospace, automotive, electronics, and medical equipment, because of its attractive mechanical properties, such as heat resistance, solvent resistance, excellent electrical insulation, good wear resistance, and high fatigue resistance . Several in vivo and vitro studies have shown the good biocompatibility of PEEK . Some studies have designed PEEK composites with porous scaffold structures and mixed them with bioactive materials to improve the biological activity of PEEK . The material also has good dimensional stability; its elastic modulus is between those of cortical and cancellous bone . In addition, PEEK is naturally radiolucent and compatible to imaging techniques, such as computed tomography (CT), magnetic resonance imaging (MRI), and X-ray. The radiolucency of PEEK allows the examination, diagnosis, and treatment of clinical diseases without need for substructure removal or replacement. Thus, PEEK can replace traditional materials (such as titanium and stainless steel) in metallic orthopedic, spinal, and trauma implants .
Modification techniques, such as blending, filling, and fiber reinforcement, are necessary for the design of new PEEK-based materials with optimum properties needed in various application fields . Polyetherimide (PEI)/Nano-SiO 2 /PEEK ternary composite materials showed superior mechanical properties and proper biocompatibility in comparison to pure PEEK. The tensile strength, flexural strength and other mechanical properties of the composites were at optimum levels when the content of nano-SiO 2 was set to 7%.
In dentistry, PEEK is mainly applied in transitional abutments , healing caps , and orthodontic bite sticks. Some excellent physical and biological properties of PEEK meet basic prosthetic requirements in clinical practice. But, the grayish and opaque color of PEEK limits the application in full-coverage restorations. Therefore, PEEK needs to combine with composite resin or veneering. However, the bond strength of the material is low when combined with composite resin because of the inert chemical performance, low surface energy, and surface modification resistance of PEEK . Thus, improving the surface properties of PEEK has become a research hotspot. Adhesive properties, which are important for the stability of prosthesis in dental applications, are influenced by the surface treatment and luting cement. In present study, the commonly used dual cure resin cement (RelyX™ Unicem) and the dentin adhesive (SE Bond) were chosen. Resin cements have become the most commonly used adhesive in the dental field. The adhesive can provide higher retention, better edge seal, and more durable bond effect than conventional luting cements . The resin cement can penetrate into the porous surface of the adherend to form a resin nail, causing obvious effects on micro-mechanical retention. SE Bond is a two-steps sixth generation self-etching dentin adhesive system that can effectively penetrate into the dentin surface with exposed collagen fibers to form a resin protrusion and maintain a closed-state adhesive surface for good bonding effect.
Therefore, this study aims to provide effective treatment methods and suitable adhesives for PEEK composite materials in dental applications by evaluating the ability of currently available surface conditioning methods and luting cements to establish adhesion to PEEK composite materials. It was hypothesized that the different surface treatment methods may affect the bond strength of PEEK composite materials, irrespective whether a universal composite resin cement (RelyX™ Unicem) or an adhesive/composite resin (SE Bond/Clearfil AP-X™) was used.
2
Materials and methods
Two light-curing adhesive systems, RelyX™ Unicem (3M Co. Espe, Germany) or SE Bond (Kuraray, Japan) and Clearfil AP-X™ (Kuraray, Japan), were selected for this study. The descriptions of the adhesives and the resin material included in this study are summarized in Table 1 .
| Material | LOT | Composition |
|---|---|---|
| RelyX™ Unicem | 496601 | Silane-treated silica, glass fillers, pigments, self-cure initiators, light-cure initiators, acetate, dimethacrylates, stabilizers, methacrylated phosphoric esters |
| SE BOND agent | 071244 | 10-MDP, HEMA, hydrophilic dimethacrylate, dl-Camphorquinone, N,N-diethanol-p-toluidine, water |
| SE BOND adhesive | 071244 | 10-MDP, Bis-GMA, HEMA, hydrophobic dimethacrylate, dl-camphorquinone, N,N-diethanol-p-toluidine, silanated colloidal silica |
| Clearfil AP-X™ | 1159AA | Silanated barium glass, silanated colloidal silica, silanated silica, Bis-GMA, TEGDMA, dl-camphorquinone |
2.1
Specimen preparation and treatment
A total of 100 PEEK composite blocks (PEI/Nano-SiO 2 /PEEK, reinforced with 7 wt% Nano-SiO 2 , Jilin University Super Engineering Plastics Research Co., Ltd., China) each measuring 80 mm × 5 mm × 4 mm were prepared. The bonding surfaces of the blocks were polished with silicon carbide (grit 600, 800). The surfaces were cleaned in an ultrasonic water bath (Euronda, Italy) for 10 min and air-dried carefully before surface treatment. The specimens were randomly divided into five test groups ( n = 20) according to the following surface treatments:
2.1.1
Group A: no treatment
Group B (sulfuric acid etching): Bonding surfaces of the PEEK composite materials were etched with 98% sulfuric acid (Beijing Chemical Works, China) for 60 s. The acid was rinsed off with distilled water for 60 s and then dried with oil-free compressed air.
Group C (hydrofluoric acid etching): Bonding surfaces of the PEEK composite materials were etched with 9.5% hydrofluoric acid (Sino-Dentex Co., Ltd., China) for 60 s. The gel was rinsed off with distilled water for 60 s and then dried with oil-free compressed air.
Group D (argon plasma treatment): The surfaces of the PEEK composite materials were treated using radio frequency (RF) plasma device (Nanjing, China) at 13.56 MHz. Argon plasma gas was introduced into the chamber at a flow rate of 30 sccm. The working gas pressure was 30 Pa, and the operating power of the RF reflected power was 500 V. The PEEK samples were treated for 25 min.
Group E (air abrasion): The PEEK composite material surfaces were sandblasted with 50 μm Al 2 O 3 particles (Masel, USA) for 15 s at a pressure of 0.4 MPa and a distance of 10 mm perpendicular to the treated surface. The specimens were cleaned with distilled water for 60 s and then dried with oil-free compressed air.
To perform micro-morphologic examination of each surface treatment group, an additional specimen was produced and analyzed by scanning electron microscopy (SEM, Quanta600, FEI, Netherlands). Each group was sputter-coated with gold and analyzed at 1600× magnification.
2.2
Bonding procedure
Each group was divided into two subgroups ( n = 10) of different cements: RelyX™ Unicem and SE Bond/Clearfil AP-X™. Each sample was prepared on two bonded specimens. A double-sided adhesive tape with two 3.0 mm-diameter circular holes on the surface was placed on the specimen to define the bonding area. Acrylic plastic tubes with an inner diameter of 3.0 mm and a height of 4.0 mm were filled with RelyX™ Unicem or Clearfil AP-X™ and pressed onto the PEEK composite block, where the tube axis was perpendicular to the sample surface. On the one side, RelyX™ Unicem cements were applied in an acrylic plastic tube. Excess cement was removed from the bonding margin using small disposable brushes. The samples were light-cured under a SmartLite PS LED curing light (Dentsply DeTrey, Konstanz, Germany, 950 mW/cm 2 ) for 40 s and then stored at room temperature for 30 min. Thereafter, the cylinders were carefully removed from the specimens. On the other side, when SE Bond/Clearfil AP-X™ was applied, the primer of SE bond adhesive system was used as the first layer of adhesive with micro-brushes for 20 s and then dried thoroughly in moderately mild oil-free air. A layer of SE Bond adhesive was applied, blown with gentle oil-free air, and then light-cured for 10 s. Clearfil AP-X™ composite resin was applied in the acrylic plastic tube and light-cured for an additional 40 s. Finally, the mold was carefully removed. Following the bonding procedure, all specimens were stored in distilled water at 37 °C for 24 h.
2.3
Shear bond strength measurements
Shear bond strength was measured with a Universal Testing Machine (5869 50 kN, Instron, USA). The specimens were fixed with a special fixture. The loading piston was closed to the bonding surface. The load was applied with a crosshead speed of 1 mm/min with the bonding surface parallel to the loading piston. The maximal load was measured before de-bonding occurred. The shear bond strength values were calculated using the formula τ = F / A , where τ is the shear bond strength (MPa), F is the fracture load ( N ), and A is the bond area (mm 2 ). The de-bonding surface was examined under a stereomicroscope (40× magnification) (Shanghai Pudan Optical Instrument Co., Ltd., China) to evaluate the failure type of the samples. Failure was classified into the following types: (i) adhesive failure (no luting cement remnants on the polished specimen surface), (ii) cohesive failure (failure within the PEEK composite materials or resin cements), and (iii) mixed failure (luting cement remnants and polished specimen surface exposed).
2.4
Statistical analysis
SPSS 17.0 (SPSS Inc. Chicago, Illinois, USA) was used to analyze the values of shear bond strength. Two-way ANOVA testing and t tests were used to compare the impact of different luting cements and type of treatments of PEEK composite materials. Tukey post hoc tests ( α = 0.05) were applied. The level of significance was set for 5%.
2
Materials and methods
Two light-curing adhesive systems, RelyX™ Unicem (3M Co. Espe, Germany) or SE Bond (Kuraray, Japan) and Clearfil AP-X™ (Kuraray, Japan), were selected for this study. The descriptions of the adhesives and the resin material included in this study are summarized in Table 1 .
| Material | LOT | Composition |
|---|---|---|
| RelyX™ Unicem | 496601 | Silane-treated silica, glass fillers, pigments, self-cure initiators, light-cure initiators, acetate, dimethacrylates, stabilizers, methacrylated phosphoric esters |
| SE BOND agent | 071244 | 10-MDP, HEMA, hydrophilic dimethacrylate, dl-Camphorquinone, N,N-diethanol-p-toluidine, water |
| SE BOND adhesive | 071244 | 10-MDP, Bis-GMA, HEMA, hydrophobic dimethacrylate, dl-camphorquinone, N,N-diethanol-p-toluidine, silanated colloidal silica |
| Clearfil AP-X™ | 1159AA | Silanated barium glass, silanated colloidal silica, silanated silica, Bis-GMA, TEGDMA, dl-camphorquinone |
2.1
Specimen preparation and treatment
A total of 100 PEEK composite blocks (PEI/Nano-SiO 2 /PEEK, reinforced with 7 wt% Nano-SiO 2 , Jilin University Super Engineering Plastics Research Co., Ltd., China) each measuring 80 mm × 5 mm × 4 mm were prepared. The bonding surfaces of the blocks were polished with silicon carbide (grit 600, 800). The surfaces were cleaned in an ultrasonic water bath (Euronda, Italy) for 10 min and air-dried carefully before surface treatment. The specimens were randomly divided into five test groups ( n = 20) according to the following surface treatments:
2.1.1
Group A: no treatment
Group B (sulfuric acid etching): Bonding surfaces of the PEEK composite materials were etched with 98% sulfuric acid (Beijing Chemical Works, China) for 60 s. The acid was rinsed off with distilled water for 60 s and then dried with oil-free compressed air.
Group C (hydrofluoric acid etching): Bonding surfaces of the PEEK composite materials were etched with 9.5% hydrofluoric acid (Sino-Dentex Co., Ltd., China) for 60 s. The gel was rinsed off with distilled water for 60 s and then dried with oil-free compressed air.
Group D (argon plasma treatment): The surfaces of the PEEK composite materials were treated using radio frequency (RF) plasma device (Nanjing, China) at 13.56 MHz. Argon plasma gas was introduced into the chamber at a flow rate of 30 sccm. The working gas pressure was 30 Pa, and the operating power of the RF reflected power was 500 V. The PEEK samples were treated for 25 min.
Group E (air abrasion): The PEEK composite material surfaces were sandblasted with 50 μm Al 2 O 3 particles (Masel, USA) for 15 s at a pressure of 0.4 MPa and a distance of 10 mm perpendicular to the treated surface. The specimens were cleaned with distilled water for 60 s and then dried with oil-free compressed air.
To perform micro-morphologic examination of each surface treatment group, an additional specimen was produced and analyzed by scanning electron microscopy (SEM, Quanta600, FEI, Netherlands). Each group was sputter-coated with gold and analyzed at 1600× magnification.
2.2
Bonding procedure
Each group was divided into two subgroups ( n = 10) of different cements: RelyX™ Unicem and SE Bond/Clearfil AP-X™. Each sample was prepared on two bonded specimens. A double-sided adhesive tape with two 3.0 mm-diameter circular holes on the surface was placed on the specimen to define the bonding area. Acrylic plastic tubes with an inner diameter of 3.0 mm and a height of 4.0 mm were filled with RelyX™ Unicem or Clearfil AP-X™ and pressed onto the PEEK composite block, where the tube axis was perpendicular to the sample surface. On the one side, RelyX™ Unicem cements were applied in an acrylic plastic tube. Excess cement was removed from the bonding margin using small disposable brushes. The samples were light-cured under a SmartLite PS LED curing light (Dentsply DeTrey, Konstanz, Germany, 950 mW/cm 2 ) for 40 s and then stored at room temperature for 30 min. Thereafter, the cylinders were carefully removed from the specimens. On the other side, when SE Bond/Clearfil AP-X™ was applied, the primer of SE bond adhesive system was used as the first layer of adhesive with micro-brushes for 20 s and then dried thoroughly in moderately mild oil-free air. A layer of SE Bond adhesive was applied, blown with gentle oil-free air, and then light-cured for 10 s. Clearfil AP-X™ composite resin was applied in the acrylic plastic tube and light-cured for an additional 40 s. Finally, the mold was carefully removed. Following the bonding procedure, all specimens were stored in distilled water at 37 °C for 24 h.
2.3
Shear bond strength measurements
Shear bond strength was measured with a Universal Testing Machine (5869 50 kN, Instron, USA). The specimens were fixed with a special fixture. The loading piston was closed to the bonding surface. The load was applied with a crosshead speed of 1 mm/min with the bonding surface parallel to the loading piston. The maximal load was measured before de-bonding occurred. The shear bond strength values were calculated using the formula τ = F / A , where τ is the shear bond strength (MPa), F is the fracture load ( N ), and A is the bond area (mm 2 ). The de-bonding surface was examined under a stereomicroscope (40× magnification) (Shanghai Pudan Optical Instrument Co., Ltd., China) to evaluate the failure type of the samples. Failure was classified into the following types: (i) adhesive failure (no luting cement remnants on the polished specimen surface), (ii) cohesive failure (failure within the PEEK composite materials or resin cements), and (iii) mixed failure (luting cement remnants and polished specimen surface exposed).
2.4
Statistical analysis
SPSS 17.0 (SPSS Inc. Chicago, Illinois, USA) was used to analyze the values of shear bond strength. Two-way ANOVA testing and t tests were used to compare the impact of different luting cements and type of treatments of PEEK composite materials. Tukey post hoc tests ( α = 0.05) were applied. The level of significance was set for 5%.
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