Metallic Glass May Serve as Bioabsorbable Material for Bone Implants

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Magnesium-based metallic glass forms only a thin corrosion layer in tissue (above left) while producing no hydrogen (above right). In contrast, traditional magnesium alloys generate undesirable gas bubbles during degradation. (Image: Laboratory of Metal Physics and Technology, ETH Zürich)

When bones break, surgeons fix them using screws and metal plates that are usually made of stainless steel or titanium. Once the bones have healed, the metal parts must be surgically removed from the body. To reduce the burden on patients, materials researchers are working to develop implants made from bioabsorbable metals that do not need to be surgically removed. After healing has taken place, these implants dissolve in the body, rendering removal unnecessary.Implants made of magnesium-based alloys are proving particularly promising as bioabsorbable materials for bone implants. Magnesium is mechanically stable and degrades completely by releasing ions that are tolerated by the body. However, magnesium alloys produce hydrogen when they dissolve, which can cause gas bubbles to develop, hindering bone growth and potentially causing infection.A team of researchers at the Swiss Federal Institute of Technology (ETH Zürich), led by Jörg Löffler from the Laboratory of Metal Physics and Technology, has discovered a way to eliminate these side-effects by producing a magnesium-zinc-calcium alloy in the form of a degradable biocompatible metallic glass.Metallic glasses are 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, 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. While crystalline magnesium-zinc alloys can contain only a maximum of 2.4% zinc atoms without precipitating a crystalline phase in the magnesium matrix, the ETH Zürich material contains up to 35% zinc, 5% calcium, and 60% magnesium.Capable of being produced in sheets up to 5 mm thick, this novel magnesium-zinc-calcium glass has enough zinc to reduce corrosion and prevent hydrogen evolution in tissue. Hence, it can potentially be used as a bioabsorbable bone implant material.