National University of Science and Technology MISIS
Together with colleagues from the Ecole de Technologie Superiore (Montreal, Canada), scientists at the National University of Science and Technology MISIS (Moscow, Russia) have developed a new combination of alloy processing that produces solid and durable implants that are fully compatible with the human body. The authors sought to develop an industrial technology for the production of metal rod stocks that are used for creating modern bone implants, particularly for implants to treat spinal problems such as scoliosis.
The working material of this new generation of alloys is based on Ti-Zr-Nb (titanium-zirconium-niobium), which possesses so-called superelasticity, meaning it can restore its original shape against large and repeated deformation. Ti-Zr-Nb is also noted for its high mechanical strength and resistance to corrosion.
According to the MISIS scientists, these alloys are the most promising class of metallic biomaterials. This is due to the unique combination of their biochemical and biomechanical properties: the composition of Ti-Zr-Nb offers complete biocompatibility and high corrosion resistance, while at the same time exhibiting hyperelastic behavior — very similar to normal bone behavior.
“Our method of combined thermomechanical processing of alloys — in particular, radial-displacement rolling and rotary forging — allows researchers to get the highest quality blanks for biocompatible implants by controlling their structure and properties. Such processing of blanks gives them an outstanding resistance to fatigue and overall functional stability,” says Vadim Sheremetyev, one of research authors and a senior research associate at NUST MISIS.
The research article was published in the Journal of Alloys and Compounds. According to the journal article, the study focused on shape memory alloy Ti-18Zr-14Nb. In the researchers’ preliminary study, “a novel combination of radial shear rolling with thermomechanical treatment including cold rolling and post-deformation annealing at different temperatures was applied to Ti-18Zr-14Nb shape memory alloy with the objective of forming various structures in long-length bars dedicated for bone implants fabrication.”
The researchers studied the static mechanical properties and the functional fatigue behavior of Ti-18Zr-14Nb alloy in different structural states. According to the article, “The dynamically polygonized substructure of β-phase formed as a result of radial shear rolling led to an outstanding combination of high ductility ( δ = 35 percent) and superior functional fatigue resistance. After additional TMT (CR e = 0.3 + PDA at 500 °C), the alloy exhibited a highly dislocated nanosubgrained structure of β -phase with subgrain size below 100 nm and a small quantity of α-phase. PDA at 550 °C led to the formation of the polygonized structure in-phase with nano-to-submicrometer-sized subgrains without α-phase present. The latter TMT improved further the functional properties of the alloy by increasing its strength-to-modulus ratio and ensuring stable functional fatigue behavior with low level of accumulated strain.”
Sheremetyev says the high-quality rod stocks have already found a potential customer. One partner of the project is a large Russian manufacturer of medical products that are made of titanium. Together with this partner, the MISIS scientists are now developing a technology to obtain beams for spinal transpedicular fixation, which they say should improve the therapy quality in severe cases of scoliosis. In addition, the scientists are now focused on further developing the thermomechanical processing and optimizing technology modes to obtain materials of the necessary form and sizes with the best combination of properties.