A revolutionary method to treat cardiac infarcts was developed at the University Hospital Zurich in the late seventies. Surgeons used a balloon to open blocked coronary arteries to alleviate infarcts. Later they succeeded in developing this angioplasty technique further into stent angioplasty. This involves using a catheter to insert a tubular wire mesh, the stent, into the blood circulatory system through the femoral artery, positioning it in the blocked artery and expanding it with a balloon. The inserted small perforated tube then supports the blood vessel to prevent it closing up again. Stents are made of steel or other corrosion resistant metals and remain in the body for the rest of the patient’s life. Occasionally this is harmful to the patient because proliferating scar tissue can narrow the coronary blood vessel again in the area of the stent.

A short life instead of a long one

Recently, the medical technology company, Biotronik, undertook a completely new approach: to have stents degrade in the body rather than persisting for as long as possible. A bio-absorbable metal is to be used instead of corrosion resistant alloys. The strategy is persuasive. The tube remains in the body only until its work is done, after which it is slowly degraded and absorbed by the body. The implant should disappear completely after six to twelve months.

In collaboration with Biotronik, the Laboratory of Metal Physics and Technology (LMPT) of ETH Zurich is now developing new alloys for bio-absorbable stents of this type. The materials scientists imposed a series of requirements on the construction material at the start of the project. Professors Peter Uggowitzer and Jörg Löffler of the LMPT, who are managing the project at ETH Zurich, say “All the alloying elements must be well tolerated by the body, the degradation rate of the material must be adjustable and it must be both easily deformable and very rigid so it can support the blood vessel.“ This meant that the researchers could consider only magnesium.

Correcting magnesium’s congenital defects

According to Uggowitzer, however, magnesium has two congenital defects. Because of its hexagonal crystal structure the metal magnesium is difficult to deform and reacts readily with other substances, corroding rapidly; too rapidly to perform its work as a stent in an optimum way.

Five milligrams of magnesium save a human life: a bio-absorbable stent (Photo: Biotronik). large

Through the skilful choice of specific elements added to the magnesium, Uggowitzer’s doctoral student, Anja Hänzi, succeeded in largely eliminating magnesium’s defects.

Following the concept of micro-alloying, which is based on thermodynamic calculations, only “homeopathic” doses of the alloying elements are added. These elements modify the microstructure in such a way that the metal mixture satisfies the stringent requirements (for the stent). Among other consequences (results?), only very small grains form in the solidified alloy, significantly increasing the ductility of the material, i.e. its ability to deform. Another advantage of the special mixture is that the construction material corrodes more slowly and more controllably than pure magnesium or conventional magnesium alloys.

The composition is already patented

Uggowitzer prefers not to comment on the precise composition of the alloy. A patent application has already been filed but more will not be disclosed at present. The Biotronik Company intends to bring the magnesium stent onto the market as soon as clinical tests are complete. The potential is great. About five million stents are used throughout the world each year, costing around 1000 Swiss francs each.

Collaboration with the industry has been particularly rewarding for ETH Zurich and the researchers taking part in this project. Peter Uggowitzer says that the gap between fundamental research and practical application in this instance is very small, and that transfer into practical use is easier than in the case of large-scale engineering applications in transport and traffic, for example, where the novel magnesium alloys are also arousing great interest. He says “For a researcher it is an enormous motivation and at the same time very satisfying to be able to contribute so directly to human well-being.