Nano and ultra fine grained metallic biomaterials by severe plastic deformation techniques

Metallic materials are widely studied for load-bearing applications such as orthopaedic implants. Titanium and its alloys find applications for load-bearing medical implants due to their biocompatibility, good corrosion resistance, high specific strength and good bioadhesion. However, the bioactivity of titanium which can be defined as the ability to form a hydroxyapatite (HA) layer, which is similar to the mineral phase of the bone, on its surface when in contact with the biological environment is poor. On the other hand, magnesium and its alloys are becoming the prime choice for degradable biomaterials targeted for temporary applications in cardiac and orthopaedic fields. However, controlling the degradation rate is the essential issue in developing magnesium-based biomaterials. Synthesis of nano/ultra fine grain materials to enhance the biofunctionalisation of orthopaedic implants is of considerable interest as cells live in a nano-featured environment consisting of a complex mixture of pores and fibres of the extracellular matrix. Recently severe plastic deformation (SPD) processes which can achieve considerable grain refinement, typically to the submicrometre or nanometre level, have gained significant attention in materials research. Therefore, using SPD processes to develop grain-refined titanium and magnesium-based materials for implant applications has become a promising strategy in developing new-generation medical materials. Particularly for titanium, nanostructuring results in improved mechanical properties and increased bioactivity. Whereas for magnesium, grain refinement results in controlled degradation due to higher biomineralisation with enhanced tissue response. The present review aims to provide a comprehensive summary of the progress achieved using SPD processes in developing nano/ultra fine grain structured titanium and magnesium for implant applications. Role of smaller grain size on enhancing bioproperties is also discussed including the challenges involved in processing to achieve the grain refinement up to nano/ultra fine grain level.