Highly deformable titanium for implantology applications

Dr Sylwia Przybysz, Eng. since January 2021 leads the project “Isotropic titanium for biomedical applications after large plastic deformation processes”, which received funding from the National Centre for Research and Development in the competition “Leader XI”.
The innovative method of plastic deformation of pure titanium will be based on a combination of two methods: the process of hydrostatic extrusion HE (Hydrostatic Extrusion) and the process of squeezing through an equiaxial angular channel ECAP (Equal Channel Anqular Pressing), the scheme of conducting such a process Fig. 1. Material deformed in this way will have a fragmented, isotropic structure and will be able to be used for medical implants, including for stabilization of bone fractures.

Among all metallic biomaterials pure titanium is currently the best material for various types of implants. Very good biotolerance of titanium in the environment of a living organism causes the process of osteointegration (bone tissue fusion with the titanium surface of the implant). This element has a high affinity for oxygen, so on its surface a passive layer of TiO2 is easily formed, which protects against corrosion. Due to its low strength properties and high plasticity, pure titanium is rarely used as an implant material. Therefore, titanium alloys are commonly used, which are characterized by higher strength due to alloying additives. Unfortunately, at the same time they deteriorate corrosion resistance and are not indifferent to human health. A popular titanium alloy used in orthopedics is, for example, Ti-6Al-4V. The vanadium in it passes into the surrounding tissues and has cytotoxic properties. In addition, Staphylococcus epidermie bacteria, which are settled on the surface of mechanically polished Ti-6Al-4V alloy, accumulate in large numbers in areas with high vanadium concentration and can cause infections at the implant/tissue interface. Efforts are also being made to eliminate aluminum in titanium alloys because this alloying additive causes muscle pain, softens bone, damages nerve cells, and results in dementia-like brain disorders.
The fabrication of a nanostructure in pure titanium makes it possible to obtain a material with strength properties at the level of titanium alloy, while retaining the advantages of pure titanium. Nanotitanium is therefore an extremely attractive material for use in the manufacture of highly stressed implants.
The characteristic feature of the HE process is the generation of the so-called fibrous structure characterized by elongated grains in the direction of the axis of the extruded bar. This results in anisotropy of mechanical properties of the material after the deformation process in mutually perpendicular directions, Fig.2.

In turn, a characteristic feature of the ECAP process is the non-uniform distribution of plastic deformation and shear stresses in the deformation zone, which promotes structural and mechanical heterogeneity of the deformed materials. The application of the ECAP method, as a preliminary plastic processing, will allow the microstructure of titanium to be fragmented before the hydrostatic extrusion process. The change of the deformation mechanism from pure shear in the ECAP process to the state of complex stresses including biaxial compression and uniaxial tension in the HE process will influence the homogeneity of the microstructure in the final product limiting the phenomenon of morphological texture and related anisotropy of mechanical properties.
Thanks to the technology applied in the project, the developed material for medical devices will be distinguished by its high mechanical strength, isotropic structure, biocompatibility and lack of harmful alloying elements such as aluminum, vanadium or niobium. The increased strength of the material, obtained through a complex plastic processing, will allow to reduce e.g. the diameter of screws used in implantology, and thus contribute to the reduction of the diameter of holes drilled in the bone, which are necessary for fixing implants.