Bone is the second most transplanted tissue in the world, with a total of over four million operations using bone grafts or bone replacement materials. 5% of people who receive an implant, such as hip replacement, may require a revision surgery within 10 years, and 15% may need a revision surgery within 20 years; as a result, numerous patients will require at least one implant replacement in their lifetime. In the United States of America alone, over 635000 hip replacements and 72000 hip revision surgeries per year are expected by 2030. Common causes of such surgeries include bone fracture near the implant, implant wear, fracture, and loosening.
From a structural point of view, a limitation of most currently used metallic materials for implants (e.g. titanium alloy Ti-6Al-4V) is their Young’s modulus poorly matching that of the surrounding bones generating stress-shielding. In this context, Additive Manufacturing (AM) recently emerged as a breakthrough technology, enabling patient-customized implants based on complex scaffold materials. AM metallic implants based on titanium (Ti) alloy scaffolds can closely match the bone geometry, local properties and functionalities, allowing to reduce stress-shielding, promote angiogenesis, improve osteoconduction; leading to a remarkable increase in the patients’ quality of life. As a counterpart, AM produces parts with non-equilibrium microstructures, for instance martensitic phase in Ti-6Al-4V, and microvoids that reduce the fatigue performance. One way of tuning mechanical properties of Ti alloys is by means of post-processing heat treatments, which results in phase transformations, microstructure coarsening, and microvoid elimination. While common across a broad range of applications, heat treatments to tune the properties of metallic alloys have been relatively less explored for biomedical implants. In the meantime, theory, modelling, and simulation methods have emerged, which now afford quantitative and predictive simulations of microstructural evolution, but also the effect of different microstructures on the material properties and lifetime.
The M3TiAM project (Multiscale-Multiphysics Modelling of Ti alloy medical implants based on Additive Manufacturing technology) specifically aims at developing computational tools to predict the influence of post-processing on the microstructure and mechanical properties of scaffolds structures, in order to guide and accelerate the design of novel Ti-based implants, down to the level of microstructural design. On the long term, such tools will contribute to the making of a robust closed-loop quality control system, which could be seemingly integrated with in-process monitoring and feedback control systems.
In order to achieve this goal, the underlying specific objectives are proposed:
- Develop an efficient microscale Phase Field-Fast Fourier Transform model (PF-FFT) to predict the microstructure evolution of Ti alloys during post-processing (process-sensitive-model).
- Develop an efficient microscale Crystal Plasticity-Fast Fourier Transform model (CP-FFT) to predict the macroscopic elasto-plastic behaviours of α+β Ti alloys taking into account microstructural features (structure-sensitive-model).
- Experimental characterization of microstructure features and mechanical testing of representative additively manufactured samples, in order to validate and calibrate PF-FFT and CP-FFT models.
