The effect of grain refinement of titanium on its mechanical performance and cell response

Titanium and its alloys have long been in the focus of biomedical research as some of the most suitable materials for implant production. This fact is attributed to an excellent combination of mechanical, biological and physico-chemical properties, such as low density and high mechanical strength, resulting in the highest specific strength among most common implant materials, reduced elastic modulus (compared to stainless steel or CoCrMo alloys), excellent corrosion resistance, and enhanced biocompatibility. Recently, titanium alloy Ti6Al4V, the most common titanium implant material so far, was in the focus of several studies that claimed it has a potential to be toxic to humans due to the release of aluminium and vanadium ions in the surrounding tissues when placed within the host. This problem can be overcome by using titanium with lower alloying content, for example, commercially pure (CP) titanium, such as Grade 2 or Grade 4. Unfortunately, CP titanium shows significant decrease in mechanical properties compared to Ti6Al4V. There are ways, however, to enhance the strength of materials without altering their chemical composition. Particularly, severe plastic deformation (SPD) has been shown to dramatically enhance the strength of various materials while retaining good ductility. It is well known that the surface of medical implants plays a crucial role in the success and outcome of the surgery and determines the longevity of implants. Therefore, it is common to modify the surface of these devices in order to increase the surface area, alter its chemistry or wettability. There are numerous techniques available nowadays that allow modification of implant surface, however, only a handful of them were implemented in industry. One of such processes - SLA (Shot-blasted with Large grit and Acid etching) - is considered one of the most promising since it has been adopted by several different companies around the world. The main aim of the present work was to use SPD to increase the strength of CP titanium of two purity grades - Grade 2 and Grade 4 – in order to develop a material that, as a result of a combination of high strength and good biocompatibility, could be used to replace Ti6Al4V as a major titanium-based material on the market. Moreover, we performed all experiments on samples with two distinctly different types of surfaces – polished and SLA-treated – to highlight the effect of grain refinement on the outcome of subsequent surface modification. To the best of our knowledge, no work examining a combined effect of grain refinement and surface modification on mechanical and biological properties has been previously described in the literature. The results discussed in this thesis suggest that, as a result of a significant refinement of microstructure by SPD processing, mechanical properties of CP titanium can match (fatigue) or even exceed (tensile) those of Ti6Al4V. It was observed that SLA treatment results in the formation of a surface layer with refined structure which aided in enhancing fatigue properties of as-received titanium after surface modification. Although such layer was not found in case of SPD-processed samples, which led to a slight decrease of fatigue life compared to samples with polished surface, fatigue properties of SPD-processed titanium were still superior to those of Ti6Al4V. Fatigue testing in simulated body fluid, designed to test how titanium with refined structure reacts to aggressive environment, indicated no deterioration of fatigue life of titanium, regardless of microstructure. It has been demonstrated that surface properties of titanium were notably influenced by the grain size and the subsequent SLA treatment. Roughness of polished samples was shown to be a function of the grain size of material. At the same time, mechanical properties are believed to be responsible for differences in roughness of SLA-modified surface, as surface roughening primarily occurs by means of grit-blasting with its effect highly dependent on the strength/ductility of the surface. Wettability of titanium has been observed to be determined by the surface texture, being a product of variations in the processing route. This effect retained after severe surface roughening as well, although, overall, surface of SLA-treated titanium samples were found to be much more hydrophobic. Simultaneously, variations in wettability between samples of different conditions became less pronounced. Chemical analysis of CP Ti with varying microstructure and surface quality revealed no effect of microstructure on the chemical state of the polished surface. At the same time, it was noted that SLA treatment of ultrafine-grained titanium may favour the formation of a thicker, more chemically uniform oxide layer. Finally, an assessment of biological properties of CP Ti has been made. The results indicate a positive effect of ultrafine-grained structure and associated surface properties on the attachment, proliferation and differentiation of two types of tissue cells – human osteosarcoma SaOS-2 cells and adipose-derived mesenchymal stem cells (adMSC) on both types of surfaces. At the same time, bacterial adhesion was also significantly enhanced on the surface of SLA-treated titanium. This fact suggests that an utilisation of surface modification techniques in industrial processes of implant production should be most carefully controlled in order to reduce possibility of serious complication that may be caused by bacterial colonisation. Overall, experimental results presented in this thesis indicate that SPD, especially in combination with surface modification techniques, such as SLA treatment, has a great potential to improve both mechanical and biological properties of commercially pure titanium to make it a highly favourable and competitive candidate for the replacement of Ti6Al4V alloy. Our research suggests that there may be only one potential drawback of a combination of SPD and SLA, namely, enhanced adhesion of bacterial cells on the surface of such materials. However, this negative effect is not attributed to SPD processing, but rather to the increased surface roughness inherent to the SLA treatment. We suggest that this issue can be addressed by developing proper handling, storage and pre-implantation preparation protocols, a process that was not covered in current work but could be included as a part of the basis for the future work.