Biodegradable bone implants

Broken bones may be fixed with metallic glass in the future following the development of a new bio-absorbable alloy.

The material, developed by researchers at ETH Zurich, is made of a magnesium-zinc-calcium alloy in the form of a metallic glass. The research team believes the alloy could herald a new generation of biodegradable bone implants.

Some bone breakages require surgeons to insert screws and metal plates to fix the 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 through another surgical procedure.

In order to reduce the burden on patients, the ETH Zurich researchers produced implants from bio-absorbable metals. The team believes these implants will stabilise the bones only for as long as they need to heal. The metal will dissolve in the body over time, rendering removal surgery unnecessary.

The research team decided to create an alloy based around magnesium because the chemical element is mechanically stable and degrades completely by releasing ions.

The problem with magnesium alloys, however, is they produce hydrogen when they dissolve and this can be harmful to the body. Around the magnesium implants gas bubbles develop, which hinder both bone growth and the healing process, and potentially cause infection.

The ETH Zurich researchers believe they have eliminated these side effects with their magnesium-zinc-calcium alloy. Their metallic glass is produced by rapidly cooling the molten material. The speed of the cooling process prevents the atoms from adopting the crystal structure found in traditional metals.

As a result, the metallic glass has an amorphous structure like that of window glass. With this procedure, the researchers claim they can add much more zinc to the molten magnesium than is possible with conventional alloys.

The glassy alloy contains up to 35 per cent zinc and five per cent calcium atoms, with the rest made up of magnesium.

A crystalline magnesium-zinc alloy can contain a maximum of 2.4 per cent 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 5mm.

ETH Zurich researchers claim the major advantage of a high percentage of zinc is that it changes the corrosion behaviour of the magnesium fundamentally.

The team reported that clinical tests with small platelets of the magnesium-zinc-calcium alloy showed no hydrogen evolution. Therefore the researchers believe the new alloy, in the form of a metallic glass, has considerable potential use as a non-harmful bone implant material.