Predictable decomposition

If a plaster cast isn't enough to stabilize a broken leg, screw implants can help it to heal. Magnesium implants even dissolve over time - and make a second operation unnecessary.

It's hardly breaking news that metallic magnesium breaks down in the body. It was used as much as a hundred years ago in medicine - but with limited success. Research has advanced since then, however: a screw and a stent made from magnesium alloys are already approved for use in Germany. "Magnesium occurs naturally in the body, so it's biocompatible and has an anti-inflammatory effect. Plus, it promotes bone growth and the breakdown products are quite similar to our bones," said Dr Berit Zeller-Plumhoff, describing the benefits of such implants. At the Institute of Metallic Biomaterials run by Professor Regine Willumeit-Römer - professor at both Kiel University and Helmholtz Zentrum Hereon in Geesthacht - the applied mathematician researches the decomposition processes of magnesium alloys. Many influencing factors have not yet been fully investigated, such as the components of the magnesium alloys, the mechanical forces placed on the implants in the body, or the conditions of the solution in which they break down. Zeller-Plumhoff wants to develop a model that predicts how magnesium is broken down under different conditions in the body.

Mathematical model to predict decomposition process

We already know that the decomposition of metallic magnesium in aqueous solutions such as blood plasma produces magnesium ions as well as hydrogen gas when the water absorbs the released electrons. The decomposition processes can be studied in more detail with intense X-rays at the Deutsches Elektronen Synchrotron (DESY) in Hamburg, where X-ray tomographic examinations allow a look inside materials. Zeller-Plumhoff uses "nanotomography" here, with which the smallest components and changes in the material become visible at a resolution of 100 nanometres (equivalent to 1/1,000 of the diameter of a human hair).

However, to develop her mathematical model for predicting the decomposition processes, Zeller-Plumhoff needs even more information. That is why, in autumn 2021, she travelled to meet Professor Dmytro Orlov at Lund University in Sweden - financed by a grant from the priority research area KiNSIS at Kiel University. In Orlov's laboratory, it is possible to measure the heat that is released when magnesium breaks down. This enables conclusions to be drawn about the reactions that have taken place before.

Heat measurements in Lund, Sweden, provide help

"Such isothermal calorimetry measurements are very fine. The amount of heat is so small that it can't be measured in degrees. Instead, we measure the electrical signal in the range of microvolts," explained Zeller-Plumhoff. This procedure also makes it possible to determine how much pressure is generated when hydrogen gas is released as magnesium breaks down, which provides further indications about the breakdown processes.

During her seven-week research trip, Zeller-Plumhoff used this method to dissolve magnesium for the first time in complex media that resemble blood plasma in aspects including protein composition. Although she is still evaluating her experiments, she has already made one surprising discovery: although the various solutions show differences in the pressure generated, there is barely any difference in the amount of heat generated. She now needs to interpret what this says about the processes in magnesium decomposition. "Such research trips are great, especially at the start of your career, for trying out new methods and gaining other perspectives on your own work," said Zeller-Plumhoff.

Author: Julia Siekmann