Development of binary and ternary titanium alloys for dental implants

Graphical abstract

Highlights

  • Binary and ternary Ti alloys were developed.

  • Ti–Zr alloys exhibited enhanced hardness and electrochemical stability.

  • Ti–Nb–Zr alloys presented an elastic modulus closer to that of bone.

  • The experimental alloys were not detrimental to albumin adsorption.

  • The experimental alloys may be considered a suitable candidate for dental implants.

Abstract

Objective

The aim of this study was to develop binary and ternary titanium (Ti) alloys containing zirconium (Zr) and niobium (Nb) and to characterize them in terms of microstructural, mechanical, chemical, electrochemical, and biological properties.

Methods

The experimental alloys — (in wt%) Ti–5Zr, Ti–10Zr, Ti–35Nb–5Zr, and Ti–35Nb–10Zr — were fabricated from pure metals. Commercially pure titanium (cpTi) and Ti–6Al–4V were used as controls. Microstructural analysis was performed by means of X-ray diffraction and scanning electron microscopy. Vickers microhardness, elastic modulus, dispersive energy spectroscopy, X-ray excited photoelectron spectroscopy, atomic force microscopy, surface roughness, and surface free energy were evaluated. The electrochemical behavior analysis was conducted in a body fluid solution (pH 7.4). The albumin adsorption was measured by the bicinchoninic acid method. Data were evaluated through one-way ANOVA and the Tukey test ( α = 0.05).

Results

The alloying elements proved to modify the alloy microstructure and to enhance the mechanical properties, improving the hardness and decreasing the elastic modulus of the binary and ternary alloys, respectively. Ti–Zr alloys displayed greater electrochemical stability relative to that of controls, presenting higher polarization resistance and lower capacitance. The experimental alloys were not detrimental to albumin adsorption.

Significance

The experimental alloys are suitable options for dental implant manufacturing, particularly the binary system, which showed a better combination of mechanical and electrochemical properties without the presence of toxic elements.

Introduction

Commercially pure titanium (cpTi) has been widely used as the main biomaterial for the manufacture of dental implants . Nevertheless, like any other material used in physiological conditions, it is exposed to mechanical and biological factors that may impair implant survival and long-term treatment success. In this context, alloys have been considered to be the treatment of choice , due to their improved properties, which allow for the development of materials according to clinical demands .

The Ti–6Al–4V alloy is widely used in the replacement of cpTi in situations where high strength is required because of its excellent mechanical performance . However, this material has been shown to be biomechanically incompatible owing to its higher elastic modulus compared with that of bone. Further, Ti–Al–V has been associated with the release of V into the blood and urine , initiation of the inflammatory cascade leading to osteolysis , neurotoxic effects, negative cell viability response, and, consequently, an undesirable outcome for implant biocompatibility with ion release . In addition, Al has been shown to be present in brain tissue of patients with Alzheimer’s disease .

Metal ions and debris released from implant materials are strongly associated with implant corrosion tendencies in physiological conditions . Besides affecting the implant’s biocompatibility, the corrosion phenomenon changes the implant’s mechanical properties and affects the bone through the abrasion and wear regimes . Thus, implant materials must not only fulfill mechanical requirements but also offer appropriate biological and electrochemical properties.

Experimental Ti alloys without the presence of Al and V are being processed and studied to achieve these properties . Zirconium (Zr) and niobium (Nb) elements have attracted much special attention . Zr acts as a solid-solution strengthening component when alloyed with Ti . Ti–Zr alloys present a predominantly α-crystalline structure, which guarantees increased mechanical resistance and excellent electrochemical behavior . In contrast, Nb is a β -stabilizer that is added to Ti to create α + β and β alloys, which have demonstrated more promising properties for biomedical use , such as an excellent combination of low elastic modulus and high tensile strength . In addition, the Ti–Nb–Zr alloy has shown non-toxicity toward osteoblastic cells, no allergy-related problems, and excellent biocompatibility .

To extend the clinical application of implants, it is necessary to develop new alloys that are sufficiently strong, present low elastic modulus, and are stable in a physiological environment. As mentioned above, Ti–Zr and Ti–Nb–Zr alloys appear to be promising candidates for dental implant applications. Extensive studies have been conducted with cpTi and Ti–6Al–4 V , but studies with Ti alloys containing Nb and Zr are limited. Thus, the aim of the current study was to characterize the microstructure and mechanical, chemical, and electrochemical properties of binary and ternary Ti alloys containing Zr and Nb and to conduct a comparison with the materials that are widely used for dental implants: cpTi and Ti–6Al–4V alloy. The biological aspects of such alloys were investigated by means of a protein adsorption assay.

Materials and methods

The experimental design of this study can be seen in Fig. 1 . Two control groups were considered: cpTi and Ti–6Al–4V alloy discs (Mac-Master Carr, Elmhurst, IL, USA) 10 mm in diameter and 2 mm in thickness. These materials were chosen because they are widely used in the manufacture of dental implants.

Fig. 1
Schematic diagram of the experimental design.

Fabrication of experimental alloys

The experimental alloys (in wt%) (Ti–5Zr, Ti–10Zr, Ti–35Nb–5Zr, and Ti–35Nb–10Zr) were melted from pure metals (Ti, Nb, and Zr presented degrees of purity equal or superior to 99.0%) (Sigma–Aldrich, St. Louis, MO, USA) in an arc-voltaic furnace with a water-cooled copper crucible under an argon atmosphere. The ingots were flipped and re-melted five times to ensure homogeneity of the samples . The Ti–Nb–Zr ingots were encapsulated in quartz tubes, heat-treated at 1000 °C for 8 h, and furnace-cooled . All ingots were heated to 1000 °C and hot-swaged to form bars ≈11 mm in diameter. Then, Ti–Nb–Zr was machined into discs (10 mm in diameter and 2 mm in thickness). After that, the Ti–Nb–Zr discs and the Ti–Zr ingots were heat-treated at 1000 °C for 1 h and air-cooled to improve the alloys’ mechanical behavior and to relieve tensions generated during the machining procedure . Ti–Zr ingots were also machined into discs with the abovementioned dimensions.

All discs were polished with #320-, #400-, and #600-grit SiC abrasive papers (Carbimet 2, Buehler, Lake Bluff, IL, USA) in an automatic polisher (EcoMet 300 Pro with AutoMet 250; Buehler, Lake Bluff, IL, USA) for surface standardization. Then, samples were ultrasonically cleaned with deionized water (10 min) and 70% propanol (10 min) (Sigma–Aldrich, St. Louis, MO, USA) and dried with warm air .

Phase characterization

The microstructural phases of the alloys were determined by X-ray diffractometry (XRD). A diffractometer (XRD; Panalytical, X′Pert3 Powder, Almelo, The Netherlands) was used, with Cu–Kα ( λ = 1.540598 Å) radiation and operating at 45 kV and 40 mA at a continuous speed of 0.02° per second and a scan range from 20° to 90°. The microstructural analysis of the alloys was confirmed by scanning electron microscopy (SEM; JEOL JSM-6010LA, Peabody, MA, USA). For that, the samples were conventionally polished as described above and then polished to a mirror finish with diamond paste (MetaDi 9-micron, Buehler, Lake Bluff, IL, USA), a polishing cloth (TextMet Polishing Cloth, Buehler, Lake Bluff, IL, USA), and lubricant (MetaDi Fluid, Buehler, Lake Bluff, IL, USA). Finally, a colloidal silica polishing suspension (MasterMed, Buehler, Lake Bluff, IL, USA) was used together with a ChemoMet I polishing cloth (Buehler, Lake Bluff, IL, USA) . The samples were subsequently etched for 4–5 s with Kroll’s reagent (5% nitric acid, 10% hydrofluoric acid, and 85% water) (Sigma–Aldrich, St. Louis, MO, USA) .

Mechanical properties

The Vickers hardness was measured by means of an indenter (Shimadzu, HMV-2 Micro Hardness Tester, Shimadzu Corporation, Kyoto, Japan) by the application of a 0.5 Kgf load for 15 s. The test was performed in five samples of each group at four randomly distributed points . The mean was calculated for each sample and then for each group to obtain the hardness (expressed in Vickers hardness units—VHN). The elastic modulus was tested by means of a nano-indenter (TI 950 TriboIndente, Hysitron Inc., Eden Prairie, MN, USA) equipped with a diamond Berkovich-type indenter (100 nm in diameter). Indentations were performed in at least ten positions of each sample, with a trapezoidal load function with 2 mN of maximum force. The loading, unloading, and dwell times were 5, 5, and 2 s, respectively. The results represent the average among the obtained values .

Surface characterization

The chemical composition of the cpTi and Ti alloys (on the order of 1 μm 3 ) was checked by energy-dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) analysis was used to verify the chemical composition and state of the outermost oxide layer by means of a spectrometer (Vacuum Scientific Workshop, VSW HA100) .

Atomic force microscopy (AFM) was used to observe the surface topography of the samples. Images of 50 × 50 μm were obtained by microscope (AFM; 5500 AFM/SPM, Agilent Technologies, Chandler, AZ, USA) from two different areas in the non-contact mode (tapping). Gwyddion software (Gwyddion v 2.37; GNU General Public License; Czech Republic) was used for image processing .

The surface roughness parameters (average roughness, Ra; maximum height of the profile, Rt; average maximum height of the profile, Rz; and root mean square roughness, Rq) of the samples were evaluated by profilometry (Dektak 150-d; Veeco, Plainview, NY, USA). The roughness parameters were obtained with a cut-off of 0.25 mm at 0.05 mm/s over 12 s. Three measurements (right, center, and left of the sample) were obtained from five discs of each group and then averaged .

Surface free energy was analyzed with a goniometer (Ramé-Hart 100–00; Ramé-Hart Instrument Co., Succasunna, NJ, USA) and the sessile drop method. The water contact angle (polar component) and the diiodomethane contact angle (dispersive component) were calculated with Ramé-Hart DROPimage Standard software (Ramé-Hart Instrument Co., Succasunna, NJ, USA). The polar and dispersive components and the surface free energy were calculated .

Electrochemical tests

The corrosion assessment was carried out with a potentiostat (Interface 1000, Gamry Instruments,Warminster, PA, USA) and a standardized method of three-electrode cells as required by ASTM International (formerly the American Society for Testing and Materials (ASTM)) (G61–86 and G31–72). A saturated calomel electrode (SCE) was used as the reference electrode, a graphite rod as the counterelectrode, and the exposed surface of the sample as the working electrode. The electrolyte solution used was simulated body fluid (SBF) at 37 ± 1 °C (pH 7.4) to mimic blood plasma. In total, a 10-mL quantity of electrolyte was used for each corrosion experiment . The chemical composition of the SBF (in kg/m 3 ) was NaCl (12.0045), NaHCO 3 (0.5025), KCl (0.3360), K 2 HPO 4 (0.2610), Na 2 SO 4 (0.1065), 1 M HCl (60 mL), CaCl 2 .2H 2 O (0.5520), and MgCl 2 ·H 2 O (0.4575) . Tris was used to achieve a pH = 7.4. The exposed area (in cm 2 ) of Ti materials was determined by AFM (cpTi = 1.07, Ti-6Al–4V = 0.99, Ti–5Zr = 1.01, Ti–10Zr = 1.31, Ti–35Nb–5Zr = 1.06, and Ti–35Nb–10Zr = 1.03).

Electrochemical testing was conducted according to a specific protocol . A cathodic potential (−0.9 V vs. SCE) was applied for 10 min to standardize the oxide layers of the samples. The open circuit potential was monitored for a period of 3600 s to assess the free corrosion potential of the material in the electrolyte solution. For evaluation of the passive layer, electrochemical impedance spectroscopy (EIS) was measured at a frequency of 100 kHz–5 mHz, with the AC curve at a range of ±10 mV applied to the electrode at its corrosion potential. The values were used to determine the real (Z′) and imaginary (Z″) components of impedance, which were used to construct Nyquist, Bode (|Z|), and phase angle plots. The EIS data were analyzed with Echem Analyst software (Gamry Instruments, Warminster, PA, USA). An equivalent electrical circuit was fitted for quantification of the corrosion process in the passive/oxide film formation (polarization resistance, R p , and constant phase element, CPE).

The samples were then polarized from −0.8 to 1.8 V (scan rate of 2 mV/s). Corrosion parameters such as E corr (corrosion potential), I corr (corrosion current density), and Tafel slopes (b c , b a ) were obtained from the potentiodynamic polarization curves by the Tafel extrapolation method (Echem Analyst Software, Gamry Instruments, Warminster, PA, USA). The passivation current density (I pass ) corresponds to the current value of the passivation region of the polarization plot. The percentage corrosion efficiencies with regard to the values of I corr (Eq. (1) ) and R p (Eq. (2) ) were calculated:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='%CE=Icorr*−IcorrIcorr*×100′>%CE=IcorrIcorrIcorr×100%CE=Icorr*−IcorrIcorr*×100
% CE = I corr * − I corr I corr * × 100
<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='%CE=Rp*−RpRp*×100′>%CE=RpRpRp×100%CE=Rp*−RpRp*×100
% CE = R p * − R p R p * × 100

where I corr * and I corr are the corrosion current density of cpTi and the Ti alloys, respectively, and R p * and R p are the polarization resistance of cpTi and the Ti alloys, respectively.

The electrochemical tests were conducted at least five times (n = 5) to ensure reliability and reproducibility.

Protein adsorption

The protein adsorption assay followed a previous protocol . Briefly, five samples of each group were incubated in 100 mg/mL of albumin (Sigma–Aldrich, St. Louis, MO, USA) under horizontal stirring (7.85 rad/s) at 37 °C for 2 h. Samples were washed in phosphate-buffered saline (PBS) (Gibco, Life Technologies, Gaithersburg, MD, USA) to remove non-adherent proteins, and the protein adsorption was measured by the bicinchoninic acid method (BCA Kit, Sigma–Aldrich, St. Louis, MO, USA).

Statistical analyses

The normality of all response variables was tested by the Shapiro–Wilk method, and data were transformed when necessary. One-way ANOVA was used to test the statistically significant differences among groups with regard to roughness, surface energy, hardness, elastic modulus, electrochemical parameters (R p , CPE, E corr , I corr , and I pass ), and protein adsorption. Tukey’s HSD test was used as a post hoc technique for multiple comparisons. A mean difference significant at the 0.05 level was used for all tests (SPSS v. 20.0, SPSS Inc.).

Materials and methods

The experimental design of this study can be seen in Fig. 1 . Two control groups were considered: cpTi and Ti–6Al–4V alloy discs (Mac-Master Carr, Elmhurst, IL, USA) 10 mm in diameter and 2 mm in thickness. These materials were chosen because they are widely used in the manufacture of dental implants.

Fig. 1
Schematic diagram of the experimental design.

Fabrication of experimental alloys

The experimental alloys (in wt%) (Ti–5Zr, Ti–10Zr, Ti–35Nb–5Zr, and Ti–35Nb–10Zr) were melted from pure metals (Ti, Nb, and Zr presented degrees of purity equal or superior to 99.0%) (Sigma–Aldrich, St. Louis, MO, USA) in an arc-voltaic furnace with a water-cooled copper crucible under an argon atmosphere. The ingots were flipped and re-melted five times to ensure homogeneity of the samples . The Ti–Nb–Zr ingots were encapsulated in quartz tubes, heat-treated at 1000 °C for 8 h, and furnace-cooled . All ingots were heated to 1000 °C and hot-swaged to form bars ≈11 mm in diameter. Then, Ti–Nb–Zr was machined into discs (10 mm in diameter and 2 mm in thickness). After that, the Ti–Nb–Zr discs and the Ti–Zr ingots were heat-treated at 1000 °C for 1 h and air-cooled to improve the alloys’ mechanical behavior and to relieve tensions generated during the machining procedure . Ti–Zr ingots were also machined into discs with the abovementioned dimensions.

All discs were polished with #320-, #400-, and #600-grit SiC abrasive papers (Carbimet 2, Buehler, Lake Bluff, IL, USA) in an automatic polisher (EcoMet 300 Pro with AutoMet 250; Buehler, Lake Bluff, IL, USA) for surface standardization. Then, samples were ultrasonically cleaned with deionized water (10 min) and 70% propanol (10 min) (Sigma–Aldrich, St. Louis, MO, USA) and dried with warm air .

Phase characterization

The microstructural phases of the alloys were determined by X-ray diffractometry (XRD). A diffractometer (XRD; Panalytical, X′Pert3 Powder, Almelo, The Netherlands) was used, with Cu–Kα ( λ = 1.540598 Å) radiation and operating at 45 kV and 40 mA at a continuous speed of 0.02° per second and a scan range from 20° to 90°. The microstructural analysis of the alloys was confirmed by scanning electron microscopy (SEM; JEOL JSM-6010LA, Peabody, MA, USA). For that, the samples were conventionally polished as described above and then polished to a mirror finish with diamond paste (MetaDi 9-micron, Buehler, Lake Bluff, IL, USA), a polishing cloth (TextMet Polishing Cloth, Buehler, Lake Bluff, IL, USA), and lubricant (MetaDi Fluid, Buehler, Lake Bluff, IL, USA). Finally, a colloidal silica polishing suspension (MasterMed, Buehler, Lake Bluff, IL, USA) was used together with a ChemoMet I polishing cloth (Buehler, Lake Bluff, IL, USA) . The samples were subsequently etched for 4–5 s with Kroll’s reagent (5% nitric acid, 10% hydrofluoric acid, and 85% water) (Sigma–Aldrich, St. Louis, MO, USA) .

Mechanical properties

The Vickers hardness was measured by means of an indenter (Shimadzu, HMV-2 Micro Hardness Tester, Shimadzu Corporation, Kyoto, Japan) by the application of a 0.5 Kgf load for 15 s. The test was performed in five samples of each group at four randomly distributed points . The mean was calculated for each sample and then for each group to obtain the hardness (expressed in Vickers hardness units—VHN). The elastic modulus was tested by means of a nano-indenter (TI 950 TriboIndente, Hysitron Inc., Eden Prairie, MN, USA) equipped with a diamond Berkovich-type indenter (100 nm in diameter). Indentations were performed in at least ten positions of each sample, with a trapezoidal load function with 2 mN of maximum force. The loading, unloading, and dwell times were 5, 5, and 2 s, respectively. The results represent the average among the obtained values .

Surface characterization

The chemical composition of the cpTi and Ti alloys (on the order of 1 μm 3 ) was checked by energy-dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) analysis was used to verify the chemical composition and state of the outermost oxide layer by means of a spectrometer (Vacuum Scientific Workshop, VSW HA100) .

Atomic force microscopy (AFM) was used to observe the surface topography of the samples. Images of 50 × 50 μm were obtained by microscope (AFM; 5500 AFM/SPM, Agilent Technologies, Chandler, AZ, USA) from two different areas in the non-contact mode (tapping). Gwyddion software (Gwyddion v 2.37; GNU General Public License; Czech Republic) was used for image processing .

The surface roughness parameters (average roughness, Ra; maximum height of the profile, Rt; average maximum height of the profile, Rz; and root mean square roughness, Rq) of the samples were evaluated by profilometry (Dektak 150-d; Veeco, Plainview, NY, USA). The roughness parameters were obtained with a cut-off of 0.25 mm at 0.05 mm/s over 12 s. Three measurements (right, center, and left of the sample) were obtained from five discs of each group and then averaged .

Surface free energy was analyzed with a goniometer (Ramé-Hart 100–00; Ramé-Hart Instrument Co., Succasunna, NJ, USA) and the sessile drop method. The water contact angle (polar component) and the diiodomethane contact angle (dispersive component) were calculated with Ramé-Hart DROPimage Standard software (Ramé-Hart Instrument Co., Succasunna, NJ, USA). The polar and dispersive components and the surface free energy were calculated .

Electrochemical tests

The corrosion assessment was carried out with a potentiostat (Interface 1000, Gamry Instruments,Warminster, PA, USA) and a standardized method of three-electrode cells as required by ASTM International (formerly the American Society for Testing and Materials (ASTM)) (G61–86 and G31–72). A saturated calomel electrode (SCE) was used as the reference electrode, a graphite rod as the counterelectrode, and the exposed surface of the sample as the working electrode. The electrolyte solution used was simulated body fluid (SBF) at 37 ± 1 °C (pH 7.4) to mimic blood plasma. In total, a 10-mL quantity of electrolyte was used for each corrosion experiment . The chemical composition of the SBF (in kg/m 3 ) was NaCl (12.0045), NaHCO 3 (0.5025), KCl (0.3360), K 2 HPO 4 (0.2610), Na 2 SO 4 (0.1065), 1 M HCl (60 mL), CaCl 2 .2H 2 O (0.5520), and MgCl 2 ·H 2 O (0.4575) . Tris was used to achieve a pH = 7.4. The exposed area (in cm 2 ) of Ti materials was determined by AFM (cpTi = 1.07, Ti-6Al–4V = 0.99, Ti–5Zr = 1.01, Ti–10Zr = 1.31, Ti–35Nb–5Zr = 1.06, and Ti–35Nb–10Zr = 1.03).

Electrochemical testing was conducted according to a specific protocol . A cathodic potential (−0.9 V vs. SCE) was applied for 10 min to standardize the oxide layers of the samples. The open circuit potential was monitored for a period of 3600 s to assess the free corrosion potential of the material in the electrolyte solution. For evaluation of the passive layer, electrochemical impedance spectroscopy (EIS) was measured at a frequency of 100 kHz–5 mHz, with the AC curve at a range of ±10 mV applied to the electrode at its corrosion potential. The values were used to determine the real (Z′) and imaginary (Z″) components of impedance, which were used to construct Nyquist, Bode (|Z|), and phase angle plots. The EIS data were analyzed with Echem Analyst software (Gamry Instruments, Warminster, PA, USA). An equivalent electrical circuit was fitted for quantification of the corrosion process in the passive/oxide film formation (polarization resistance, R p , and constant phase element, CPE).

The samples were then polarized from −0.8 to 1.8 V (scan rate of 2 mV/s). Corrosion parameters such as E corr (corrosion potential), I corr (corrosion current density), and Tafel slopes (b c , b a ) were obtained from the potentiodynamic polarization curves by the Tafel extrapolation method (Echem Analyst Software, Gamry Instruments, Warminster, PA, USA). The passivation current density (I pass ) corresponds to the current value of the passivation region of the polarization plot. The percentage corrosion efficiencies with regard to the values of I corr (Eq. (1) ) and R p (Eq. (2) ) were calculated:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='%CE=Icorr*−IcorrIcorr*×100′>%CE=IcorrIcorrIcorr×100%CE=Icorr*−IcorrIcorr*×100
% CE = I corr * − I corr I corr * × 100
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Nov 22, 2017 | Posted by in Dental Materials | Comments Off on Development of binary and ternary titanium alloys for dental implants
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