Mechanical and corrosion properties of highly porous Ta-Nb-Sn alloy for intervertebral disc in spinal applications

Ta, Nb, and Sn powders, having a particle size of 30–40 μm, were employed as a raw material for the manufacturing of highly porous, foam-like Ta-Nb-Sn alloy samples. 5–35 %wt. Nb, 1–5% wt. Sn powders, and balanced Ta powders were mixed (Alfa Aesar, USA). Ta, Nb, and Sn powders and 3-mm ZrO₂ balls (metal/ball ratio of 10/1) were ball-milled for 6 hours. Rotation speed was 600 rpm. Carbamide (urea) was used as a poreforming material. Some PVA (polyvinylalcohol) was added to the powder mixture for green strength and compressibility. Mixtures were pressed at 200 MPa into cylindrical samples (12×20 mm). Samples were immersed into the water and then the carbamide (pore former) was removed. Ta-Nb-Sn alloys were sintered at 1400◦C for 240 min in argon.

Microstructure of the Ta-Nb-Sn alloy samples were investigated using field emission gunscanning electron microscopy (FEG-SEM) (FEI Quanta). Mechanical properties were studied using compression tests (Devotrans, Turkey). Young’s modulus was studied using the ultrasonic method (General Electric, USM-Go). As the wettability is important for orthopaedic implants, the contact angle of the Ta-Nb-Sn alloys was measured using optical tensiometer-contact angle meter (Attension Theta) by the sessile drop method.

Computed cone-beam-based tomography (Dental CT) was employed for imaging the pore structure. Samples were exposed to x-ray at 9 mA, 90 kV. Digital images were evaluated by a software (OnDemand3dDental). Tomography device was captured data by using x-ray beam (cone-beam based) during a 360◦ rotation around the Ta alloy.

In addition, digital radiography (DR) was employed for the investigation of the imaging properties. Samples were exposed to x-rays at 90 kV, 14.3 mA, for 10 seconds (Balteau). Phosphor imaging plates were employed (Flex, Carestream). Scanner was VMI 5100MS. Images were processed using Starrview software.

Simulated body fluid (SBF) solution was prepared with a composition of 8.00 (g/L) NaCl, 0.30 (g/L) CaCl₂, 0.20 (g/L) KCl, 0.30 (g/L) MgCl₂, 0.20 (g/L) K2HPO₄, 0.35 (g/L) NaHCO₃, 0.07 (g/L) Na₂SO₄, and some tris [24–30]. The pH was set to 7.40. Alloys were immersed into the SBF for up to 14 days. ICP-MS (Thermo Scientific) was employed to measure the metal ion release levels. Weight change values were determined using dry weight measurements.

Electrochemical corrosion studies were carried out in the SBF solution using a potentiostat (Interface 1000, Gamry). Saturated calomel electrode (SCE) was the reference electrode, graphite was the counter electrode, and the Ta-Nb-Sn alloy was the working electrode. Open circuit potential (OCP) was measured for 7200 seconds. Linear polarization resistance (LPR) was employed to determine the polarization resistance and corrosion rate. Tafel curves were obtained by polarizing the Ta-Nb-Sn alloys with respect to the OCP.

In the present study, in vitro evaluation of the cytotoxicity of the Ta-Nb-Sn alloys were studied by extraction-based 3T3 neutral red uptake (NRU) assay. The 3T3 NRU assay is based on the ability of living cells to uptake supravital neutral red dye, which can penetrate the cell membrane by nonionic diffusion. Neutral red (NR) is a weak cationic dye, with which iving cells can be distinguished from dead cells. The NRU assay provides a quantitative estimation of the number of viable cells in a culture. Positive (sodium laureth sulfate) and negative (polypropylene) samples were prepared to verify the test system. Ta-Nb-Sn alloy test substances were exposed to an immortalised mouse fibroblast Balb/c 3T3 cell line. Absorbance (optical density) measurements were performed at 540 nm in order to determine the viability/survival. Cytotoxicity can be expressed as a reduction of the uptake of neutral red. Dulbecco’s modification of Eagle’s medium (DMEM) cell culture medium, bovine calf serum (BCS), trypsin/EDTA were the chemicals used. Serum culture medium (SCM) was used as the extraction agent because it supports extraction of polar and nonpolar substances and cell proliferation. The cell culture was removed from culture flasks using enzymatic digestion (trypsin/EDTA), and then the cell suspension was centrifuged. The cells were then resuspended and seeded at 104 cells per well in DMEM in a 96-well plate. After incubation, the culture was aspirated. After incubation, culture medium was removed, and the cells were washed with phosphate-buffered saline (PBS), The test system is considered suitable if the viability for the negative specimen is >70% of the blank.

Highly porous open-cell Ta-Nb-Sn alloy foamlike samples were manufactured by the presssinter-based pore former–water leaching route. Pore morphology of the sintered structures replicated the starting morphology and diameter of the urea (carbamide) particles. Figure 1 illustrates the SEM images of (a) Ta powder, (b) Nb powder, (c) Sn powder, and (d) carbamide. Figure 2 illustrates (a)photograph of the sintered Ta-Nb-Sn alloy foam, (b) SEM image of the macro-pores at the cracked surface of the sintered foam, (c) SEM image of the cell wall of the sample (low magnification), and (d) SEM image of the cell-wall of the sample (high magnification). Micropores in the microstructure due to incomplete sintering are not undesirable. Some micropores are important in the transportation of body fluids.

Fig. 1.

Fig. 2.

Table 1 shows the average sizes of the poreformer carbamide (urea) particles and the average pore diameter of the alloys. Mean pore size of the sintered specimen was 590 μm, while mean particle size of the corresponding irregular carbamide pore former was 860 μm. When poreformer carbamide with a mean particle size of 580 μm was used, a mean pore size of 410 μm was obtained in the sintered specimens. The final pore size value is associated to the pore-former carbamide size. Pore shape was also similar to the original pore-former carbamide (urea) morphology. The average diameter of the pore former carbamide was slightly higher than the pores. The difference was attributed to crushing of the carbamide during pressing [1–5].

Figure 3 illustrates the x-ray diffraction (XRD) results of the powders (a) Ta, (b) Nb, (c) Sn powder and (d) the sintered Ta-Nb-Sn alloy. As seen from the Figure 3(a), as-received Ta powder consists of Ta phase. As seen from Figure 3(d), the sintered sample consisted of Ta phase. Some oxides (Ta₂O₅ and Nb₂O₅) were also observed on the surfaces. Figure 4 illustrates the effect of (a) Nb content and (b) Sn content of the alloy on the elastic modulus of the about 70% porous samples.

Fig. 3.

Fig. 4.

Figure 5 illustrates the effect of the porosity level of the alloy on the (a) elastic modulus and (b) compressive stress-compressive strain curve. There are three regions in the curves: an elastic region, a plateau region, and a densification region. The plateau region is important for the energy/impact absorption properties. Yield strength values of the Ta-Nb-Sn alloys increased and the long plateau region get shortened with increasing Nb content. Porosity content of the sintered specimens increasing from 46% to 74% caused the Young’s modulus to decrease from 4.3 GPa to 1.3 GPa.

Fig. 5.

Figure 6 illustrates the change of OCP level with (a) Nb addition and (b) Sn addition. Raising the Nb content of the sample raised the OCP level, whereas raising Sn content slightly lowered the OCP. Tafel curves were employed to evaluate the corrosion performance. Figure 7 illustrates the effect of (a) Nb content of the Ta-Nb-Sn alloy and (b) Sn content of the Ta-Nb-Sn alloy on the Tafel curves. Increasing the Nb content of the samples increased their corrosion potential and decreased their corrosion current density. Increasing Sn content of the specimens decreased the corrosion resistance.

Fig. 6.

Fig. 7.

Figure 8 illustrates the effect of (a) Nb content of the alloy, (b) Sn content of the alloy, and (c) porosity of the specimen on the polarization resistance and corrosion rates. Polarization resistance of the samples increased with incorporation of Nb, and corrosion rate of the decreased with Nb addition. Sn addition lowered the polarization resistance of samples. Increasing Nb content of the alloy samples from 5% to 35%, decreased the electrochemical corrosion rate of the specimen from 3.2 mm/year to 2.0 mm/year. While increasing Sn content of the alloy from 1% to 5%, the electrochemical corrosion rate of the specimen increased from 2.5 mm/year to 3.0 mm/year. In general, Ta is very inert and resists corrosion in acidic solutions. Only highly acidic solutions containing fluoride ions can corrode the Ta. The inert nature of Ta is ideal for biomedical implant purposes. The inert nature and biocompatibility of Ta is a consequence of the surface oxides. Ta has two types of oxide, Ta₂O₅ (more stable) and TaO₂.

Fig. 8.

Figure 9 illustrates the effect of the immersion time on (a) the weight change and (b) metal (Ta) ion release by Ta-35Nb-5Sn alloy in the simulated body fluid. As seen from Figure 9, weight change (loss) and metal (Ta) ion release values of the samples increased with time. Figure 10 illustrates the effect of the Nb addition on (a) weight change (loss) and (b) Ta ion release. As seen from Figure 10, weight loss and Ta ion release values of the samples decreased with incorporation of Nb. Figure 11 illustrates the effect of Sn content of the alloy on the (a) weight change (loss) and (b) Ta ion release in the simulated body fluid. Weight loss and Ta release levels increased with the Sn addition. Figure 12 shows the effect of the porosity content of the Ta-35Nb-5Sn alloy on the (a) weight loss and (b) Ta ion release in simulated body fluid solution for 14 days. Increasing porosity content increased the weight loss and Ta ion release values of the specimens.

Fig. 9.

Fig. 10.

Fig. 11.

Fig. 12.

Wettability (contact angle) of the Ta-Nb-Sn alloys was studied by using optical tensiometer. The mean droplet (water) contact angle for the Ta-Nb-Sn alloys was about 80◦. Figure 13 displays a photograph of the droplet at the surface. As the contact angle is below 90◦, the wettability of the Ta-Nb-Sn alloy is suitable.

Fig. 13.

Computed tomography (CT) and digital radiography (DR) methods were employed for structure imaging. The tomography method was used for determination of pore structure and pore distribution, while the radiography method was used in order to determine the radiopacity of the alloy. Figure 14 shows the images of the foams with 70% porosity in both (a) radiographic view and (b) tomography images. The Ta-Nb-Sn samples show suitable radiopaque behaviour. Radiopaque materials have a bright, white appearance on the radiographs, compared with the relatively darker appearance of the radiolucent materials. For example, bones or metals look white or light grey (visible or radiopaque), whereas polymers, tissue, and skin look black or dark gray (invisible or radiolucent). Implant materials must have radiopacity for surgical operations. Macro or micro-cracks were not detected according to radiography. Figure 14(b) shows the 3D image of the 70% porous foam. The Ta-Nb-Sn foam consisted of homogeneously distributed open pores according to the tomography results.

Fig. 14.

In this study, evaluation of the cytotoxicity of the Ta-Nb-Sn alloy was performed with the 3T3 NRU assay. The assay is based on the ability of living cells to uptake neutral red dye, which can penetrate the cell membrane. Positive (sodium laureth sulphate) and negative (polypropylene) samples were prepared to verify the test system. Ta-Nb-Sn alloy test substances were exposed to immortalised mouse fibroblast Balb/c 3T3 cells. Ta alloy does not have cytotoxic potential. The viability value of Ta alloy was about 88%, which is not lower than the limit value (70%). This means that the Ta alloy has no cytotoxic potential.