Magnesium for biomedical applications as degradable implants: thermomechanical processing and surface functionalization of a Mg-Ca alloy

Abstract

Degradable implants for bone fixation have been of significant interest since the last decade. Among different materials, magnesium appears as a promising candidate due to its unique combination of properties. Magnesium is very well tolerated by the body and has a natural tendency for degradation. In addition, its low elastic modulus helps to reduce stress-shielding effect during bone healing. Mg- Ca alloys are particularly of interest for the additional processing and property benefits that Ca addition provides. The potential use of these alloys necessitates multi-faceted studies so that microstructures with an optimal compromise between mechanical properties and degradability kinetics are achieved. This work focuses on Mg-2wt.%Ca alloy and aims to provide a path for future optimization of the alloy for implant applications. In this work a new bulk/surface processing approach is proposed: i.e. tailoring the bulk microstructure by thermomechanical treatments and surface functionalization by additive manufacturing. Hot rolling, extrusion and equal channel angular pressing (ECAP) have been used for bulk processing. The characterization results show that while different microstructural features (dislocations, twins, grain size) can account for the improvement in the mechanical strength, the improvement in the corrosion resistance appears as primarily affected by grain size and second phase microstructure. It is found that the severe plastic deformation induced by the ECAP process produces the finest grain structure and second phase particle distribution. This influence results from the dispersion of the second phase Mg2Ca and possibly a more stable oxide layer. The ECAP process also appears as the most effective method to improve the mechanical strength. Surface modification is achieved by designing a surface patterning method that uses silver nanoparticle microdeposition to functionalize the material for antibacterial properties. The deposition is followed by a laser sintering process. A series of depositions are performed to achieve the desired deposition conditions and a reproducible deposition line of 20 μm width and between few hundreds of nanometres and one micrometre thick. Profilommetry, SEM and TEM are used to characterize the silver deposition and the substrate microstructure. A finite element simulation has been conducted to describe the thermal effect of the laser treatment process. The modelling results show that the thermal impact from the laser sintering process extends deep into the substrate and thus needs to be controlled in order to avoid any evolution of the previously designed bulk microstructure. This model can then provide a basis to investigate the impact of different input parameters for further process optimization in future applications.