Abstract
Objectives
To investigate the crystalline phases, morphological features and functional groups on the surface of sintered Y:TZP/TiO 2 composite ceramics before and after the application of a biomimetic bone-like apatite layer. The effect of TiO 2 content on the composite’s characteristics was also evaluated.
Methods
Samples of Y:TZP containing 0–30 mol% TiO 2 were synthesized by co-precipitation, followed by filtration, drying and calcination. The powders were uniaxially pressed and sintered at 1500 °C/1 h. To obtain biomimetic coatings the samples were exposed to sodium silicate solution and then to a concentrated simulated body fluid solution. The surfaces, before and after coating, were characterized by diffuse reflectance infrared Fourier transformed spectroscopy, X-ray diffraction analysis and scanning electron microscopy.
Results
The surfaces of all Y:TZP/TiO 2 samples were covered with a dense and uniform calcium phosphate layer with a globular microstructure. This layer was crystalline for specimens with 30% of TiO 2 and amorphous for specimens with 0 and 10% of TiO 2 . Chemical analysis indicated that this layer was composed of type A carbonate apatite. Among the materials tested, the composite with 10% of TiO 2 showed the best overall chemical and physical features, such as higher density and more cohesive amorphous apatite layer.
Significance
Y-TZP-based materials obtained in the present investigation by means of the successful association of a calcium phosphate biomimetic layer with small amounts TiO 2 should be further explored as an option for ceramic dental implants with improved bioactivity.
1
Introduction
Substituting commercially pure titanium or titanium alloys ( e.g. , Ti–6Al–4V) for dental implants is one of the biggest challenges faced by biomaterials scientists nowadays. Titanium based implants have well recognized advantages like excellent osteointegration demonstrated by the high levels of bone-to-implant contact reported in clinical studies , and the high fracture toughness typically found in metallic alloys which guarantees low fracture rates.
The highly biocompatible behavior of titanium implants is associated to the corrosion resistance of the passivation oxide layer. Although this layer is very homogenous and strongly bonded to the implant surface, allergenic responses have been associated to the use of titanium dental implants and biocompatibility tests have demonstrated that ceramic materials like alumina or zirconia are significantly more biocompatible than titanium alloys .
Due to their high biocompatibility, ceramic materials like yttria stabilized tetragonal zirconia polycrystals (Y-TZP) have been the focus of many studies and are proposed as important candidates to substitute titanium implants. The main drawback related to the use of ceramic materials in structural biomedical applications is their relatively low fracture toughness ( K Ic ), especially when compared to metals. However, because of the its intrinsic martensitic transformation toughening mechanism, and the compressive stresses associated to it, Y-TZP ( K Ic = 9.0 MPa m 1/2 ) is currently commercialized as a substitute for metallic implants since it has sufficient mechanical properties to withstand the stresses found in the oral cavity. In addition, Y-TZP is a white material, and therefore can partially overcome the poor esthetic results associated to metal structures when in contact with gingival tissues.
Two key issues still need to be addressed with regards to the use of Y-TZP for dental implants: low temperature degradation (LTD) and bioinertness. LTD, also known as “aging”, was initially observed on the surface of Y-TZP femoral head implants that underwent water-assisted martensitic transformation with subsequent increase in roughness and nucleation/growth of cracks toward the bulk of the material . Further clinical research is still necessary to understand the implications of LTD in the clinical use of Y-TZP dental implants .
Bionertness can be defined as the lack of interaction between the Y-TZP surface and the surrounding living tissues The poor bioactivity of these ceramics usually leads to problems in the osteointegration process, as it hinders the cellular response in terms of proliferation, migration and adhesion and can ultimately cause implant failure due to formation of a fibrous capsule around the artificial root . Hence, surface modification of Y-TZP dental implants can be an important procedure to improve the final clinical results in terms of osteointegration.
Surface modification of Y-TZP surfaces for biomedical purposes is currently performed by material subtraction or addition of bioactive layers. Removing material from the implant surface can cause leaching of ytria, undesirable phase transformations, and nucleation of critical surface flaws . On the other hand, both good osteoinegration results and faster bone regeneration have been reported after the addition of bioceramic layers to Y-TZP implants, such as calcium phosphate or bioactive glass . These additive approaches also present disadvantages like adhesion problems between the layer and the implant and heterogeneity of the applied film .
Another approach that aims at solving the problem of bioinertness in zirconia-based implants is the development of new composite ceramics by adding bioactive materials to a zirconia matrix. In a previous study , our research group has successfully developed a novel composite for implant applications by combining ZrO 2 and TiO 2. The idea behind the creation of this material was to take advantage of the good mechanical properties of ZrO 2 and the bioactivity of TiO 2 ceramics. Fortunately these two materials can be combined to form a composite due to the high solid solubility of the original oxides.
ZrO 2 –TiO 2 has been successfully used in other applications as catalysts, dielectric materials, and photosensitive cells . However, our initial study on ZrO 2 –TiO 2 sintered ceramic was the first to propose this specific material for biological applications. It was demonstrated that the surfaces of samples containing both zirconia and titania had better proliferation results after cell culture experiments compared to pure zirconia or titania. Moreover, others have showed that the presence of TiO 2 induced in vitro bone-like apatite formation and stimulated osteoconductivity in vivo .
In the present study, our aim was to improve the previously developed ZrO 2 –TiO 2 composite by adding a bioactive/biomimetic bone-like apatite layer. This type of layer can improve osteointegration and has been successfully applied to coat other bioceramics such as silicon nitride and alumina–zirconia composites . However, its use to functionalize ZrO 2 –TiO 2 biocomposites has not been explored yet. Apatite thin films can be produced in acellular simulated body fluids (SBF) with ionic composition similar to that of the inorganic part of human blood plasma . After immersion of the ceramic surface in a concentrated simulated body fluid (1.5 SBF), the sodium silicate solution acts as a nucleating agent . This treatment creates several calcium phosphate precursor sites on the ceramic surface, which eventually create a suitable condition for nucleation and growth of calcium phosphate phases.
The objective of this investigation was to compare the crystalline phases, morphological features and functional groups found on the surface of sintered Y:TZP/TiO 2 composite ceramics before and after the application of a biomimetic bone-like apatite layer. The effect of TiO 2 content (from 0 to 30%) on the aforementioned characteristics of the ceramic composite was also assessed. The main hypothesis of the study was that the addition of biomimetic layer to the Y:TZP/TiO 2 composite would result in a surface layer with morphological and physicochemical properties that are significantly better than those of their non-layered counterparts from the biological standpoint, i.e. , the presence of a TiO 2 and calcium phosphate bioactive layer with globular aspect, which is expected to favor the osteointegration process.
2
Materials and methods
2.1
Powder synthesis
The synthesis of Y:TZP/TiO 2 powders was described in our previous paper . Briefly, zirconium oxchloride, titanium chloride and ytttrium chloride solutions were prepared to obtain different amounts of TiO 2 in the Y:TZP/TiO 2 composites ( Table 1 ). The suspensions were filtered, washed with water, ethanol and n-butanol. After azeotropic distillation, the Y:TZP/TiO 2 ceramic powders were dried at 100 °C calcined (800 °C/1 h, Fornitec), and milled in a high energy attrition mill for 24 h using zirconia ball media in ethyl alcohol.
| Sample | Composition (mol%) | TD (g/cm 3 ) | ||
|---|---|---|---|---|
| ZrO 2 | Y 2 O 3 | TiO 2 | ||
| Z | 97 | 3 | 0 | 6.01 |
| Z T10 | 87.3 | 2.7 | 10 | 5.77 |
| Z T30 | 67.9 | 2.1 | 30 | 5.34 |
2.2
Powder characterization
Ceramic powders were classified with different sieves (60, 150, 270, and 325 Mesh/Tyler). The specific surface area was determined using the N 2 gas adsorption method. N 2 gas molecules were adsorbed on the surfaces of the samples and the surface area of the powder calculated by the BET (Brunauer, Emmet and Telller) method. The mean diameters of the particles were estimated using Eq. (1) .
S BET = 6 ρ m D
where S BET is the specific surface area, ρ m is the theoretical density of the mixtures and D is the mean particle diameter.
The crystalline phases of the Y:TZP/TiO 2 powders were identified from X-ray powder diffraction profiles (XRD, Rigaku DMAX 3000 diffractometer, Cu Kα). The diffraction profiles were compared with files of the International Centre for Diffraction Data (ICDD) for standard phases 037-1484, 081-1545 and 034-0415, corresponding to monoclinic zirconium dioxide (ZrO 2 ), tetragonal zirconium dioxide (ZrO 2 ) and zirconium titanate (ZrTiO 4 ) phases, respectively. The morphological features of the powders were examined in a scanning electron microscope with a field emission gun (SEM-FEG).
2.3
Y:TZP/TiO 2 samples processing
The milled and homogenized powder mixtures were dried at 90 °C and uniaxially pressed at 50 MPa using cylindrical metallic dies (6 mm in diameter), that resulted in pellets ∼4 mm high. Ten samples of each composition were prepared. The samples were then sintered at 1500 °C for 60 min in a furnace (Lindberg Blue). The sample surfaces were rectified using a diamond-encrusted drill (D-91, Winter).
2.4
Samples characterization
Density measurements were performed using the geometric method and the final densities of the samples were expressed in terms of the theoretical density of each mixture. The theoretical densities of all powder mixtures were determined using the rule of mixtures ( Table 1 ), and considering 6.01 g/cm 3 and 4.24 g/cm 3 as the theoretical densities of Y:TZP and rutile TiO 2 , respectively. Ten samples of each composition were prepared. The roughness of the ceramic surfaces was measured with a portable Mitutoyo Surftest 211 roughmeter. Mean roughness (Ra) was estimated using the mean value of three different parallel oriented values. Densities and roughness data were estimated considering the mean and standard deviation of the ten individual measurements for each composition, which were further compared by analysis of variance (ANOVA) with a 5%level of significance ( p ≤ 0.05).
The sintered samples were analyzed using x ray powder diffraction analysis (XRD, Rigaku DMAX 3000 diffractometer, Cu Kα) to identify the crystalline phases. To observe the crystal shapes and grain sizes as well as their distribution, a scanning electron microscope (SEM, XL 30, Philips) was used, considering at least two samples for each composition.
Superficial functional groups of the sintered samples (two samples for each composition) were analyzed by diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy (Thermo Nicolet, Nexus 400) between 400 and 4000 cm −1 .
2.5
Biomimetic experimental method
2.5.1
Coating procedure
The Y:TZP/TiO 2 samples (five for each group) were coated with a calcium phosphate phase using the biomimetic method, as described in our earlier work . Briefly, samples were immersed in sodium silicate solution for seven days, washed in deionized water, dried at room temperature, and immersed in 1.5 SBF for seven days. The solution was changed every two days. After the coating procedure, the Y:TZP/TiO 2 samples were washed and dried. All coating procedures were performed in a shaker at 40 rpm at 36.5 °C (TE-420, Tecnal).
2.5.2
Characterization of coated Y:TZP/TiO 2 samples
After coating, the surfaces of the samples were characterized by X-ray diffraction analysis (Rigaku DMAX 2000 difratometer), scanning electron microscopy (Philips, XL30) and diffuse reflectance infrared Fourier transformed spectroscopy (DRIFT, Thermo Nicolet, Nexus 400). Two specimens were used for each analysis.
2
Materials and methods
2.1
Powder synthesis
The synthesis of Y:TZP/TiO 2 powders was described in our previous paper . Briefly, zirconium oxchloride, titanium chloride and ytttrium chloride solutions were prepared to obtain different amounts of TiO 2 in the Y:TZP/TiO 2 composites ( Table 1 ). The suspensions were filtered, washed with water, ethanol and n-butanol. After azeotropic distillation, the Y:TZP/TiO 2 ceramic powders were dried at 100 °C calcined (800 °C/1 h, Fornitec), and milled in a high energy attrition mill for 24 h using zirconia ball media in ethyl alcohol.
