Biodegradable metallic glass could yield bone surgery breakthrough

Materials researchers in Zurich have developed a glassy alloy that could herald a new generation of biodegradable bone implants.

When bones break, surgeons often use screws and metal plates to fix the broken bones in place. These supports are usually made of stainless steel or titanium. Once the bones have healed, the metal parts have to be removed from the body via further surgery.

Materials researchers are therefore seeking bio-absorbable materials that will reduce the burden on patients, stabilising the bones only for as long as they need to heal, and dissolving in the body over time.

Implants made of magnesium-based alloys are proving particularly promising. Magnesium is mechanically stable and degrades completely by releasing ions which are tolerated by the body. However, magnesium alloys have one major drawback: when they dissolve they produce bubbles of hydrogen gas, which in the body can hinder bone growth and thus the healing process, and potentially cause infection.

Now, researchers working with Jörg Löffler, Professor of Metal Physics and Technology at ETH Zurich, believe they have eliminated these side effects. They have produced an innovative magnesium-zinc-calcium alloy in the form of a metallic glass which is bio-compatible and shows significantly more favourable degradation behaviour.

Metallic glasses are produced by rapid cooling of the molten material. Fast cooling prevents the atoms from adopting a crystalline metallic structure; instead, they have an amorphous structure like that of window glass. This enabled the researchers to add much more zinc to the molten magnesium than is possible with conventional alloys.

A crystalline magnesium-zinc alloy can contain a maximum of 2.4 per cent zinc. However, the glassy alloy developed by the ETH researchers Bruno Zberg, Peter Uggowitzer and Jörg Löffler contains up to 35 per cent zinc and 5 per cent calcium, with the rest made up of magnesium. The magnesium-zinc-calcium glass can be produced up to 5mm thick.

The high percentage of zinc fundamentally changes the material’s corrosion behaviour: clinical tests with small platelets of the new magnesium-zinc-calcium alloy showed no hydrogen evolution at all, meaning that it has considerable potential as a non-harmful bone implant material.

The research work has been published in the online version of Nature Materials.

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