Introduction: With the increased usage of magnetic resonance imaging (MRI) as a diagnostic tool in clinical trials, the currently-used metals for vascular stents, such as 316L stainless steel (SS), Co-Cr alloys and Ni-Ti alloys, are challenged by their MRI incompatibility, due to their constituents containing ferromagnetic or paramagnetic elements.
To provide new MRI compatible vascular stents, the Nb-xTa-2Zr (30≤x≤70) series alloys were selected in the current work. Several key properties of these alloys were optimized in terms of stent requirements, including magnetic susceptibility, elastic modulus and tensile properties [1]. Using revised simulated body fluid (r-SBF), the electrochemical corrosion behavior of an Nb-60Ta-2Zr alloy for MRI compatible vascular stents was characterized [2]. To assess the hemocompatibility, platelet adhesion (lactate dehydrogenase activity), platelet activation (P-selectin expression), coagulation and hemolysis of Nb-60Ta-2Zr alloy were investigated, For comparison, parallel assessments for these factors were performed for the Nb, Ta, 316L SS and L605 Co-Cr alloy (L605) [3].
Experimental Methods: The Nb-60Ta-2Zr alloy ingot was fabricated using electron beam melting (EBM), with subsequent forging and annealing. Magnetic susceptibilities of the alloys were measured using a MPMS-7S superconducting quantum interference device (SQUID).
The chemical composition of the sample surface of the alloy was analyzed using X-ray photoelectron spectrometry (XPS). Simulated body plasma solution was used as the electrolyte. The r-SBF was selected to mimic the ion concentrations of human blood plasma. Electrochemical measurements were performed in a standard three-electrode cell filled with approximately 300 mL r-SBF solution.
Platelet adhesion on the material surface is also characterised by lactate dehydrogenase (LDH) activity. Measurements of prothrombin time (PT), activated partial thrombin time (APTT) and thrombin time (TT) were performed with an automated blood coagulation analyser. Haemolysis testing was performed according to ASTM F 756-00
Results and Discussion: In the as-cast state, a single-phase solid solution with bcc structure was formed in the alloys. The volume magnetic susceptibility (χv) and Young’s modulus (E) of the alloys scaled linearly with the Ta content. Increasing the Ta content gave rise to the decreased χv and the increased E, together with the elevated yield strength but less-changed elongation. From multiple requirements for the stents, the Nb-60Ta-2Zr alloy exhibits an optimal properties, including the χv of nearly 20-fold lower than 316L SS, the E of 149 GPa superior to pure niobium, high mass density of 12.74 g/cm3 favored to the X-ray visibility, yield strength of ∼330 MPa comparable to the 316L SS and a elongation of ∼24%.
After immersion in the solution of r-SBF, the surface passive oxide film of the alloy was identified as a mixture of the Nb, Ta and Zr oxides, with a thickness of approximately 1.3 nm, as shown by XPS analysis. In terms of potentiodynamic polarization curves and EIS, the Nb-60Ta-2Zr alloy manifests a low corrosion rate and high polarization resistance. No localized corrosion was detected.
The propensity for platelet adhesion and activation on the Nb-60Ta-2Zr alloy was at nearly the same level as that for Nb and Ta but was slightly less than those of 316L SS and L605. The mitigated platelet adhesion and activation of the Nb-60Ta-2Zr alloy is associated with its decreased adsorption of fibrinogen. The Nb-60Ta-2Zr alloy has a longer clotting time and exhibits significantly superior thromboresistance than 316L SS and L605. Moreover, the haemolysis rate of the Nb-60Ta-2Zr alloy satisfies the bio-safety requirement of the ISO 10993-4 standard.
CONCLUSIONS: Among the Nb-xTa-2Zr (30≤x≤70) series alloys, the Nb-60Ta-2Zr alloy provides optimal properties, considering the multifactorial requirements of the stent. The Nb-60Ta-2Zr alloy possesses a significant advantage in corrosion resistance in the human blood environment, in contrast to current stent metals such as 316L SS and Co-Cr alloys. Regarding blood coagulation, the Nb-60Ta-2Zr alloy has a longer clotting time and exhibits significantly superior thromboresistance compared to 316L SS and the L605 Co-Cr alloy.
References:
[1] H.Z. Li, J. Xu, J. Mech. Beh. Biomed. Mater. 32 (2014)166-176.
[2] H.Z. Li, X. Zhao, J. Xu, Mater. Sci. Eng. C. 56 (2015)205-214.
[3] X.M. Li, et al.,Mater. Sci. Eng. C. 42 (2014)385-395