Liga superelástica ti-25nb (%at): processamento, caracterização e protótipos de dispositivos implantáveis.

Abstract

Shape Memory Alloys are smart materials that have the ability to recover large deformations when compared to common metallic materials such as steel and titanium. Among these alloys, the most common belong to the Ni-Ti system, such as NiTiNOL and Ti-Ni-Nb alloys, which have a low Young modulus and high mechanical strength and corrosion resistance, and therefore have been used in several areas, including medicine. However, the nickel content in these alloys is considered toxic and, in speciĄc cases, such as wear of the titanium oxide layer on the surface of the material, can expose the patient to nickel toxicity. Thus, an option for use in the medical area is the Ti-Nb alloy, which is composed only of biocompatible elements and has other desirable properties, such as high mechanical conformability, corrosion resistance and low Young modulus. In this regard, this work aims to fabricate and characterize a Ti-25Nb (%at.) alloy, as well as to fabricate and test prototypes of implantable medical devices: bone plates and orthopedic staples. The alloy was fabricated by arc melting in an inert environment and cold rolled to a 90% reduction in ingot thickness, resulting in a Ti-Nb sheet. The outer contour of the bone plate, as well as the cutting of the bone staple, was performed by wire EDM cutting, while the inner holes were drilled using high-speed steel drills. The fabricated Ti-Nb alloy showed a maximum recovery of 1.5% strain, maximum stress of 670 MPa and maximum strain of 12.55% until rupture at 37 °C, the average human body temperature, in addition to presenting a Young modulus of 57.32 GPa in tensile test and 52.7 GPa in Vickers ultramicrohardness test. It also proved to be non-cytotoxic by agar diffusion method and showed predominance of β phase in X-ray diffraction (XRD) test due to the presence of 0.814% (%at.) oxygen, acting as a suppressor of orthorhombic martensite formation during cooling. The Ti-Nb bone plate presented bending stiffness of 35.44 N/mm and bending structural stiffness of 3.06×10-3 N/m2 , 1.54 times lower than the values obtained for the steel bone plate, while the Ti-Nb bone staple fabricated presented stiffness of 20.65 N/mm, 3.25 times lower than that of the staple fabricated in stainless steel. In face of this, it is concluded that the Ti-Nb alloy, by presenting low Young modulus, superelasticity and not being cytotoxic, is suitable to be used as a material of implantable medical devices, a conclusion corroborated by the comparison between the Ti-Nb devices and the fabricated steel devices.