Abstract
Introduction
In recent years, there has been a growing interest in bioresorbable metals. Orthopaedic components made from these materials do not require removal by secondary surgery, and offer superior load bearing capability compared to the existing biodegradable polymers.
Research on bioresorbable metals have largely focused on alloys based on a subset of the Mg-Zn-Ca ternary system [1, 2], which are pre-existing elements inside the human body. Cytocompatibility assessments of these alloys have reported no signs of inflammation or adverse cellular reactions [2-4]. Rather than designing for longevity, bioresorbable metals rely on their tendency to corrode in a controlled manner. Hence, controlling their corrosion rates is of utmost importance. In the present work, we have explored the effect of compositional variation on the properties of the Mg-Zn-Ca amorphous metals. Subsequent characterisations are performed to assess their suitability as a bioresorbable material.
Materials and Methods
A mixture of pure elements and master alloys, namely magnesium, zinc, calcium, and Mg-Ca master alloy, were melted in an induction furnace, followed by injection casting to produce the amorphous metallic samples. Pure magnesium (crystalline) was also used in the subsequent characterisation tests for comparison. The thermophysical properties of the as-cast amorphous metals were characterized using x-ray diffraction (XRD) and differential scanning calorimetry (DSC). The biocorrosion performance was assessed by a combination of immersion, potentiodynamic polarisation (PDP) and hydrogen evolution studies. These tests were conducted in cell media, with a sodium bicarbonate buffer, at 37°C and pH 7.4 in a humidified CO2 atmosphere.
Results and Discussion
A range of amorphous metal compositions, from Mg-rich to Ca-rich, were successfully produced. XRD confirmed that the alloys were amorphous. Subsequent characterisation tests revealed that minor alterations in composition were not detrimental to thermophysical properties; however, the critical casting size and corrosion rates were much more sensitive to alloy chemistry. In comparison, the Mg-rich alloys have superior corrosion resistance, whereas the Ca-rich alloys have improved thermophysical properties, thereby allowing them to undertake more complex thermoplastic forming processes.
Conclusion
We have successfully produced amorphous metals with a range of corrosion resistance and thermophysical properties. The combination of biocompatible elements, superior corrosion resistance and reduced hydrogen evolution, make these amorphous metals more suitable for use as bioresorbable orthopaedic components than their crystalline counterparts.
Acknowledgements
The authors would like to thank the Australian Research Council (ARC) for partial funding of this work via the ARC Centre of Excellence for Design in Light Metals (CE0561574).
