For the first time, researchers at the Swiss Federal Institute of Technology in Zurich have observed corrosion of magnesium alloys for biomedical applications at the nanoscale. This is an important step to better predict the degradation of human implants and develop tailor-made implant materials. The results of the study have been published in the journal Advanced Materials.
Magnesium and its alloys are increasingly used in medicine. It can be used as an implant material in orthopedic surgery, such as screws or steel plates, etc .; it can also be used as a stent material to dilate narrow coronary blood vessels in cardiovascular surgery .
Compared to conventional implants made of stainless steel, titanium or polymers, light metals have the advantage of being bioabsorbable, so the implant can be removed from the patient without a second surgery. Magnesium also promotes bone growth, which is beneficial to fracture healing.
However, pure metal magnesium is too soft, so it is not suitable for such surgical applications. In order to obtain the necessary strength, alloying elements must be added, usually the rare earth elements yttrium and neodymium. Because these substances are foreign bodies to the human body, they may accumulate in the body when the implant is removed, and the consequences are unpredictable. For children, this implant is even more inappropriate.
Professor Jorge Lovell from the Metal Physics and Technology Laboratory of the Federal Institute of Technology in Zurich has developed a new family of alloys that contain less than 1% of zinc and calcium in addition to magnesium. These elements, like magnesium, are biocompatible and can be absorbed by the body.
Depending on the manufacturing process, a precipitate formed by three alloying elements will form in the newly developed alloy. These precipitates vary in size, usually only a few tens of nanometers. However, it is necessary to enhance the good mechanical properties of the implant material, and may affect the corrosion rate of the material.
However, the widespread surgical application of these biocompatible magnesium alloys still faces obstacles, mainly because the mechanism of metal degradation in vivo under so-called physiological conditions is poorly understood. Therefore, a reliable prediction of how long such implants will remain in the body cannot be obtained.
Professor Loveller and his colleagues can analyze the structure and chemical changes of magnesium alloys in detail with unprecedented nanometer resolution under simulated physiological conditions of seconds to hours with the help of analytical transmission electron microscope (TEM). The use of this modern technology demonstrates a previously unavailable dealloying mechanism that can determine the degradation of precipitates in the magnesium matrix.
The researchers were able to observe in near real time how calcium and magnesium ions escape from the excreta when they come in contact with simulated body fluids, while zinc ions remain and accumulate. As a result, the chemical composition of the excreta changes continuously, which also proves the fact that their electrochemical activity changes dynamically in the sediment, that is, accelerates the degradation of the implant material.
The lead author of the study, Professor Lofler's doctoral student Martina Shihova, said, "This understanding reverses the previous dogma. So far it has been assumed that the chemical composition of the precipitated phase in the magnesium alloy remains unchanged during the corrosion process. This assumption has led to most predictions about the duration of human implants being wrong. "
Thanks to new discoveries, magnesium alloys can now be designed so that their degradation behavior in the body can be better predicted and controlled more precisely. This is especially necessary for children's implant surgery, because magnesium implants in children degrade much faster than adults. In addition, the degradation rate of magnesium alloys for stents is significantly slower than that of bone plates or screws. Shihova said: "With an understanding of detailed corrosion behavior, we have taken a key step towards the goal of tailoring alloys for different patients and medical applications."