How hydrogen affects titanium implants

Whether used to treat bone fractures, replace teeth or act as pacemakers, metallic implants have become an irreplaceable part of modern medicine. They must remain stable while integrating with surrounding tissue. A new avenue being explored in the field of biological implants is designing titanium-magnesium hybrid implants that combine the advantages of both materials to create superior implants. Titanium is used for its high strength, while the magnesium component dissolves over time and helps stimulate bone growth. Such materials have the potential to revolutionise metallic implants.

Dr. Richi Kumar, instrument scientist at the Helmholtz-Zentrum Hereon at the MLZ and first author of the publication. © Bernhard Ludewig, FRM II / TUM

Dr. Richi Kumar, instrument scientist at the Helmholtz-Zentrum Hereon at the MLZ and first author of the publication. © Bernhard Ludewig, FRM II / TUM

In collaboration with the Institute for Metallic Biomaterials at the Helmholtz-Zentrum Hereon , a research team involving scientists from the MLZ produced hybrid samples of titanium and magnesium alloys. The researchers then exposed these to a saline solution designed to mimic body fluids for up to several days. The magnesium began to degrade, releasing hydrogen. The researchers observed how the hydrogen penetrated the titanium. Using neutron tomography at the Paul Scherrer Institute (PSI) in Switzerland, the increase in neutron attenuation proved that hydrogen penetration increased over time.

Neutrons “see” hydrogen
„We used neutron tomography because neutrons are very sensitive to hydrogen due to its large scattering cross-section, while X-ray techniques typically fail to detect it. With 3D imaging, it is possible to map the presence of hydrogen inside the volume, providing insights into the extent of hydrogenation of samples,” explains Dr Richi Kumar, instrument scientist at the Helmholtz-Zentrum Hereon at the MLZ.

(Left) Photo illustrating corrosion of Hybrid Ti-Mg samples. (Right) Vertical slice extracted from the 3D neutron tomography volume of the samples before and after corrosion (The colours represent amount of hydrogen). © Dr. Richi Kumar

(Left) Photo illustrating corrosion of Hybrid Ti-Mg samples. (Right) Vertical slice extracted from the 3D neutron tomography volume of the samples before and after corrosion (The colours represent amount of hydrogen). © Dr. Richi Kumar

Hydrogen leaks into the titanium
In aqueous solutions, water is electrochemically split by magnesium, producing hydrogen gas that can be absorbed by titanium. The fact that hydrogen is absorbed by the titanium alloy at room temperature indicates that the natural thin protective layer on the titanium surface, the oxide layer, has been damaged. The higher reactivity of magnesium compared to titanium causes the oxide layer to reduce. This allows hydrogen to “leak” into the titanium more easily.

The hydrogen distributes itself evenly in the titanium from the centre to the outside (radially) but is more concentrated at the edges, along the height. The reason for this is that the centre was initially still covered by magnesium and could therefore absorb less hydrogen.

Additional analyses using X-ray diffraction and electron microscopy confirmed that no new compounds formed from titanium and hydrogen. However, the metal crystal expanded slightly in volume, indicating hydrogen incorporation.

Important findings for developing implants
These findings are important for implant development: when hydrogen penetrates titanium, the material can become brittle, thus impairing the stability of the titanium component.

Richi Kumar summarises: “Improvements in hybrid implant design are essential to ensure that the presence of one material does not affect the properties of the second material. This is not only relevant for hybrid implants but also for any situation where titanium and magnesium are used in close proximity. It highlights the need for thoughtful design and consideration of material interactions.”