29 September 2009 ETH Zurich

Metallic Glass for Bone Surgery

Arc melter in which a plasma of up to 3,000 deg C is produced between a tungsten tip (center) and a water-cooled copper plate

Arc melter in which a plasma of up to 3,000°C is produced between a tungsten tip (center) and a
water-cooled copper plate. Image credits: ETH Zurich / LMPT

It is possible that broken bones will in the near future be fixed using metallic glass. Materials researchers at ETH Zurich have developed an alloy that could herald a new generation of biodegradable bone implants. Their results have been published in the online edition of Nature Materials.

When bones break, surgeons need 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. In order to reduce the burden on patients, materials researchers have taken up the task of producing implants from bioabsorbable metals. These implants should stabilize the bones only for as long as they need to heal. The metal dissolves in the body over time, rendering removal surgery unnecessary. 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, all magnesium alloys have one major drawback: when they dissolve they produce hydrogen (H2), which can be harmful to the body. Around the magnesium implants gas bubbles develop which hinder bone growth and thus the healing process, and potentially cause infection.

No side effects thanks to more zinc

Materials researchers working with Jörg Löffler, Professor of Metal Physics and Technology at ETH Zurich, have now eliminated these side-effects. They have succeeded in producing an innovative magnesium-zinc-calcium alloy in the form of a metallic glass which is biocompatible and shows significantly more favourable degradation behaviour. Metallic glasses are produced by rapid cooling of the molten material. The speed of the cooling process prevents the atoms from adopting the crystal structure found in traditional metals. As a result, metallic glasses have an amorphous structure like that of window glass. Thanks to this procedure, the researchers can add much more zinc to the molten magnesium than is possible with conventional alloys.

The glassy alloy developed by the ETH researchers Bruno Zberg, Peter Uggowitzer and Jörg Löffler contains up to 35% zinc and 5% calcium atoms, with the rest made up of magnesium. A crystalline magnesium-zinc alloy can contain a maximum of 2.4% zinc atoms. If the percentage is higher, an undesired crystalline phase precipitates in the magnesium matrix. The magnesium-zinc-calcium glass can be produced in a thickness of up to 5 millimetres. The major advan-tage of a high percentage of zinc is that it changes the corrosion behaviour of the magnesium fundamentally. In fact, clinical tests with small platelets of the new magnesium-zinc-calcium alloy showed no hydrogen evolution! Thus this new alloy, in the form of a metallic glass, has considerable potential as a non-harmful bone implant material. The research work has been published in the online version of Nature Materials.

Further information

Metallic glass
Metallic glass is a hot topic in the field of materials science. Only since the early 1990s, says Jörg Löffler, could metallic glasses be produced in larger dimensions and be used as engineering material in the form of 'bulk metallic glasses'. Since then laboratories have been searching intensively for applications for such glasses, which have several advantages over traditional metals. They are much more elastic than crystalline materials and demonstrate a strength that is two to three times higher. These properties would be valuable in bone implants, as metallic glass parts could carry out the same function as their traditional crystalline counterparts but be much smaller. However, metallic glasses also have some disadvantages: their plasticity is not as high as that of normal metals; and, depending on the composition of the alloy, individual components can often only be produced in thicknesses of a few millimetres to centimetres (otherwise they cannot be cooled quickly enough, and a crystal structure forms). For this reason metallic glasses are mostly used in filigree devices, for example in the field of sensory applications and microtechnology.

Original: Zberg B, Uggowitzer PJ & Löffler JF. MgZnCa glasses without clinically observable hydrogen evolution for biodegradable implants. Nature Materials. Published online 27th September 2009, doi: 10.1038/NMAT2542

Source: ETH Zurich /...

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