Fabrication and characterization of Mg matrix nanocomposites for biomedical applications
Biomaterials have for many years been in use in the human body to improve body functions and replace damaged tissues. The market for orthopaedic biomaterials is currently worth over £30 billion globally and is expected to grow at 15% annually over the next decade. Bio-inert metals and ceramics dominate the market. These implants are, however, alien to the human body with limited biocompatibility, causing long-term irritation, chronic inflammation and other biological responses in patients. More critically, secondary surgeries are normally needed after 8-12 years for implant retrieval, which is expensive and painful. Therefore, extensive research has been carried out to find biodegradable implants, which can be dissolved in the human body and replaced slowly by the surrounding tissues, eliminating secondary surgeries and other harmful effects.
Magnesium (Mg) alloys are biodegradable with excellent biocompatibility and favourable elastic modulus, which is similar to that of human bones. They represent a potential new generation of biomaterials to replace currently used metals, ceramics and polymers. Unfortunately, the toughness of pure Mg is not satisfactory for orthopaedic surgeries and current Mg alloys all contain toxic elements such as Al and Mn. More importantly, the available Mg alloys corrode/dissolve too quickly in the human body, which is harmful to implantation surgeries. There have been studies to improve their biomedical performance by developing novel alloys free of toxic elements, novel synthesis and processing techniques, and surface modification, etc. A promising approach is to form an Mg alloy matrix composite using nanocrystalline ceramic particles of natural human bone compositions such as hydroxyapatite (HA) and beta-tricalcium phosphate (BTCP). Recent investigations have shown that both nano HA and BTCP particle reinforced Mg alloy matrix composites exhibit improved mechanical and electrochemical properties, which will lead to improved biological performance. However, the manufacturing routes currently adopted, via powder metallurgy and conventional casting, cause severe particle agglomeration, resulting in heterogeneous microstructures and properties.
This project is supported by EPSRC through MeDe Innovation Centre Fresh Idea Award, aiming to develop a new route for fabricating biodegradable Mg matrix particulate nanocomposites with controllable strength and degradability. The project objectives include:
- The development of a novel technology combining high shear solidification and severe plastic deformation for fabricating magnesium matrix nanocomposites.
- Optimization of matrix alloy compositions and reinforcing particle type, size and volume fraction in terms of achieving satisfactory mechanical and electrochemical/corrosion properties.
- Characterization of microstructures, mechanical and electrochemical properties and in-vitro tests to evaluate the corrosion performance of the material.
- Scientific understanding of a) wetting behaviour and improved mixing under intensive melt shearing; b) defects consolidation and particle dispersion during deformation; c) Mg/particle interface structures and strengthening mechanisms; d) factors that govern the degradation rate.
The research addresses the core issues of degradability control for biodegradable magnesium biomaterials by introducing a new concept of corrosion protection and utilizing an advanced fabrication technology, which combines high shear solidification (HSS) and equal channel angular extrusion (ECAE). The key point is to promote the formation of a uniform and dense surface protection layer of biostable nanoparticles of the natural bone constituent (HA and BTCP). It offers a novel approach to the design and manufacturing of Mg biomaterials and exciting opportunities to other researchers for further scientific advances in metallic biomaterials research. The academic impact on wider scientific research communities will be significant since corrosion control in conjunction with strength enhancement is also one of the fundamental issues in materials development. Our research outcomes on microstructural refinement, particle distribution improvement and corrosion control mechanisms will be directly or indirectly applicable to wider scientific and technological research fields.
The provision of novel Mg matrix nanocomposites with controllable degradability and mechanical properties offers more choices and freedom for orthopaedic surgeries, and has the potential to change the paradigm in the design of medical devices with biodegradability under control, representing a step change in orthodaedic implantation.
The project deliverables have the potential to change the structure of the UK biomaterials market for implants and medical devices, which is currently dominated by foreign products from USA and continental Europe. The commercialization of the fabrication technology and industrial production of the Mg matrix nanocomposites will create opportunities for new businesses and jobs, in addition to the provision of new biomaterials. This will help the UK industry enhance their competitiveness in the UK market and worldwide.