Originally Published MPMN November/December 2009
NEED TO KNOW
Magnesium Implant Biodegrades without Releasing Hydrogen
A magnesium-based alloy suitable for biodegradable implant applications is melted inductively using a high-frequency current.
Image courtesy of LMPT/ETH Zurich
On the heels of success with biodegradable polymer-based implants, researchers have been venturing into the exploration of biodegradable metals. Boasting mechanical stability and biocompatibility, magnesium has emerged as a prime candidate for biodegradable metal implants. But one major drawback has impeded progress: As magnesium degrades, it produces hydrogen, which can prolong the healing process and can even potentially harm the body. Trying to resolve this issue, a team of scientists from the Swiss Federal Institute of Technology, Zurich (Switzerland, www.ethz.ch) has developed a magnesium-based glass that has not exhibited hydrogen evolution in clinical trials.
The focus of widespread research, biodegradable implants offer the advantage of dissolving upon fulfillment of their intended function in the body. This capability renders explantation surgery unnecessary and minimizes the potential for common problems associated with long-term implantation, such as inflammation or, in the case of stents, restenosis.
Metals such as magnesium offer higher mechanical strength than their polymer-based counterparts and release inert ions as they biocorrode. But as magnesium degrades, hydrogen gas bubbles develop around the implant that can delay bone growth or may even cause infection.
Jörg Löffler, a researcher involved in the project and professor of metal physics and technology at the university, and his colleagues may have found a way to circumvent this issue by adding zinc to magnesium to form a magnesium-based metallic glass. A rapid cooling process permits much more flexibility with the chemistry, he says. Quickly cooling the molten material results in an amorphous structure that Löffler likens to that of window glass, rather than yielding the crystalline structure typically found in traditional metals.
“Thanks to this procedure, we can add much more zinc to magnesium than is possible with conventional alloys,” Löffler explains. “By adding more than 28% of zinc atoms to the alloy, a zinc and oxygen-rich passivating layer forms on the metal’s surface, which fundamentally alters the corrosion behavior of the material. Clinical tests with small platelets of the new material showed no hydrogen evolution.”
Exhibiting no hydrogen evolution, the biocompatible metallic glass could actually serve as the basis for the next-generation of biodegradable implants. Producible in thicknesses up to 5 mm, the alloy could potentially be used for bone implants and possibly even vascular stents.
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