A photo taken by an electron microscope sent to a competition for the cover page of a prestigious journal Materials Today published by Elsevier brought success to our Ph.D. student Karel Tesař. In the Institute of Physics of the Czech Academy of Sciences and at the Department of Materials of the Faculty of Nuclear Sciences and Physical Engineering of the Czech Technical University in Prague he develops implants which dissolve in the body. His winning photograph captures the growth of corrosion products on a hydrogen bubble in a simulated body fluid solution.
Your photograph caught attention of editors of a prestigious journal and appeared on its cover page. In the current issue of the journal also a short article of yours explaining the scientific background of the photograph1 was published. What do you think the evaluators found interesting in your photo and could you tell us briefly what story is concealed in it?
First of all, it is important to say that it is the work of the whole team not a single person. The photo is related to my research which I am involved in as part of my Ph.D. studies. The topic of my PhD thesis is Thin Magnesium Wires for Biodegradable Implants. I believe that apart from aesthetic qualities of the photo, the evaluators have appreciated mainly the fact that it points out problems which need to be addressed while developing biodegradable implants. Magnesium dissolves in the body and hydrogen and magnesium hydroxide are produced. If the amount of hydrogen is small, the body can deal with it and it is led away. If there is, however, an excessive amount of it, bubbles start to form on the implant. On the interface between the gas and the body environment magnesium hydroxide forms and subsequently magnesium-substituted hydroxyapatite and other calcium phosphates. And it is these corrosion products that are the reason for magnesium being so promising as a bone healing facilitator. From the chemical point of view, they are really similar to the inorganic component of the human bone. The bone can use the minerals in its healing and growth. Seen from the scientific point of view, it is really useful to examine how corrosion products differ on the interface magnesium-body medium and magnesium-hydrogen.
The photo was taken within collaboration with the Institute of Rock Structure and Mechanics where together with Doc. Ing. K. Balík and his colleagues we perform tests of wires in simulated body fluid solutions and we describe corrosion products which are formed there. Together with Lenka Borecká, using an electron microscope we have created an overview of corrosion products after different times of degradation. The winning photo captures the growth of corrosion products on a bubble which is continuously enlarging. Since corrosion takes place on a wire surface, the bubble can grow only where calcium phosphate nucleation has not occurred yet. In this way, a half-closed tube originates which has drawn attention of the evaluators. It needs to be mentioned that at the majority of sites where local corrosion occurs, hydrogen does not develop so quickly. It results in spherical, fully closed bubbles. When bones heal, cells cannot get into such closed space and a cavity is formed which can cause a number of problems in the body. At present we solve the problem of hydrogen formation by adding a small amount of zinc into a magnesium alloy and by coating wires by biodegradable polymers.
Due to a special way of preparation, a magnesium wire will not break as a result of repeated bending. Could you explain to us the mechanisms which are behind this unusual behaviour?
At the Institute of Physics, we manufacture magnesium wires by a direct extrusion method. It works on a simple principle. A rod of 6 mm in diameter is pushed through a steel die where the output hole is only 0.25 mm. We thus get a final wire in one step. It is, however, very complicated to adapt the die to the very demanding conditions that it has to withstand for this production operation. When compared with a common magnesium wire prepared by drawing, our wire represents an advantage mainly in bending deformation. In 2016 we published a study where we explained this fact by reversible twinning2. For the reason of preferred orientation of the crystals in the wire caused by our production method, when bent the wire is deformed mainly by so called twinning. Parts of the crystals move to form regions of the crystal lattice that are connected to the original planes of symmetry (twinning planes). When straightening the wire, it is advantageous for these areas of the crystal to return to the original state. The damage caused by repeated bending is thus smaller than in a wire which is deformed by slip.
Today, healthcare cannot do without biodegradable materials. Did you manage to find a loophole on the market for the use of a metal wire, which would dissolve in the body after some time?
There are rather a large number of cases where such a biodegradable wire could be of use. In comparison with commonly used biodegradable polymers, metal provides higher dimensional stability under load. We want to take advantage of the fact that magnesium corrosion products can facilitate bone healing. A very common treatment that requires the use of wires is the so-called median sternotomy. In order for a cardiothoracic surgeon to reach the heart, it is necessary to cut the sternum longitudinally, perform the operation and fix the separated halves of the sternum together. For this, predominantly steel wires are used. If the wires do not burden a patient, they are not taken out from the body. There are, however, cases when the presence of wires is a problem. Mainly in smaller children when the fixation wire can grow into their sterna as they grow. Some people can also develop intolerance to the presence of metal ions around a steel wire. A biodegradable wire that holds the two halves of the sternum together long enough and then dissolves would therefore be of great benefit. And this is one of the possible applications of our wire. We have examined the mechanical properties of the wire3 and for the time being we are sure that it will withstand the load in children, where it would be of the highest benefit. For use in adults, the wire alloy would need to be further improved. And that is the subject of our further research.
In vivo tests of magnesium wire on rats are underway. How has the metal wire done and is it already possible to estimate from the results when you will proceed to the next stage of testing?
Pilot tests on several rats have confirmed the need to coat the wires with a biodegradable polymer. At present, in cooperation with the 1st Medical Faculty of Charles University, we are evaluating the samples taken. Thanks to them, we will be able to proceed to other tests, which are very close to the final application. At the end of this year, it is planned to start experiments with tying the sternum of piglets in cooperation with the Institute of Animal Physiology and Genetics of the Czech Academy of Sciences.
K. Tesař, P. Köppl, photo: K. Tesař
References:
[1] K. Tesař, K. Balík: Nucleation of corrosion products on H2 bubbles: A problem for biodegradable magnesium implants? Materials Today, In press., DOI: 10.1016/j.mattod.2020.04.001
[2] A. Jäger, S. Habr, K. Tesař: Twinning-detwinning assisted reversible plasticity in thin magnesium wires prepared by one-step direct extrusion, Materials & Design 110, (2016), 895-902., DOI: 10.1016/j.matdes.2016.08.016
[3] K. Tesař, K. Balík, Z. Sucharda, A. Jäger: Direct extrusion of thin Mg wires for biomedical applications, T. Nonferr. Metal. Soc. 30 (2020), 373-381.,DOI: 10.1016/S1003-6326(20)65219-0