Bioabsorbable metallic glass
Bioresorbable (or bioabsorbable) metallic glass is a type of amorphous metal, which is based on the Mg-Zn-Ca ternary system. Containing only elements which already exist inside the human body, namely Mg, Zn and Ca, these amorphous alloys are a special type of biodegradable metal .
The first reported metallic glass was an alloy (Au75Si25) produced at Caltech by W. Klement (Jr.), Willens and Duwez in 1960. This and other early glass-forming alloys had to be cooled extremely rapidly (in the order of one mega-kelvin per second, 106 K/s) to avoid crystallization. An important consequence of this was that metallic glasses could only be produced in a limited number of forms (typically ribbons, foils, or wires) in which one or more dimensions were small so that heat could be extracted quickly enough to achieve the necessary cooling rates. As a result, metallic glass specimens (with a few exceptions) were limited to thicknesses of less than one hundred micrometers.
Mg-Zn-Ca based metallic glasses are a relatively new group of amorphous metals, possessing commercial and technical advantages over early compositions. Gu and co-workers produced the first Mg-Zn-Ca BMG in 2005, reporting high glass forming ability, high strength and most importantly exceptional plasticity. This lanthanide-free, Mg-based glass attracted immediate interest due to its low density and cost, and particularly because of its uncharacteristically high ductility. This property was unexpected for such compositions, as the constituent elements are found to be of relatively low Poisson ratio, and hence contribute little to the inherent plasticity of the glass. This unlikely asset was seized upon by Li in 2008, who made use of the Poisson ratio principle and increased Mg content at the expense of Zn to further enhance plasticity. Further improvements were achieved by incremental addition of Ca to the Mg72Zn28 binary composition, producing numerous ternary alloys along the 350 °C isotherm of the Mg-Zn-Ca system.
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