IMX Colloquium - Ultrahigh-purified lean magnesium alloys for bioresorbable implant applications

Temporary medical implants that can be resorbed after fracture healing are beneficial for patients and necessary in certain clinical applications. Magnesium-based implants are among the most frequently studied due to their unique properties. However, they often comprise significant amounts of alloying elements to improve their mechanical properties. One well-known example is WE43, which contains large amounts of rare-earth elements (about 4 wt.% Y and 3 wt.% Nd). In contrast, we developed in recent years rare-earth free, lean Mg alloys with alloying contents below 1 at.%, also in ultrahigh-purified (XHP) versions with less than 5 ppm impurity content [1]. This development generated alloys such as ZX10 (MgZn1.0Ca0.3, in wt.% = MgZn0.37Ca0.18, in at.%) [2] and ZX00 (Mg0.45Zn0.45Ca) [3], and other more recent lean Mg–Ca alloys (XHP X0) with alloying contents of less than 0.2 at.% [4]. Via optimized hot-extrusion processing we are now able to tune their microstructure and related properties, generating high-strength alloys with a yield strength of >400 MPa at extended ductility (strong-X0), or >35% ductile alloys at intermediate strength (ductile-X0). 

 

In this way we produced plate-screw implants for large-animal tests, where the plates were made of XHP ductile-X0 for adjustments to the bone shape and the screws were produced from XHP strong-X0. The X0 implants were inserted onto the pelvic bones of six adult female Swiss alpine sheep and compared with WE43 [5]. CT and histological studies revealed an optimal average degradation rate of 0.3 – 0.4 mm/year for both medical alloys. The bone-implant contact, however, was found to be significantly higher for X0 than for WE43, revealing a much better osseointegration for the X0 implants. An analysis of the degradation products after explantation revealed further that rare-earth containing submicron particles remained embedded within the corrosion products of WE43, while X0 underwent complete biodegradation. This shows that XHP lean Mg–Ca alloys present a new class of absorbable bone implants, combining slow degradation, enhanced biocompatibility, and strong mechanical properties. In fact, implants based on ZX00 and X0 received already FDA approval (Bioretec) or an FDA “Breakthrough Device Designation” (ETH Spinoff, Kairos Medical). This research may also generate a paradigm shift towards lean high-strength, highly ductile (L-HS-HD) alloys, where chemically simple materials can contribute to sustainable and more efficient materials recycling [6]. 

 

[1] C. Wegmann et al., “Simultaneous distillation and alloying”, WO 2021/165139 A1. 

[2] M. Cihova et al., ‘Biocorrosion zoomed in: evidence for dealloying of intermetallic nanoparticles in Mg alloys’, Adv. Mater. 31, 1903080 (2019). doi.org/10.1002/adma.201903080 

[3] T. Akhmetshina et al., ‘Quantitative imaging of magnesium biodegradation by 3D X-ray ptychography and electron microscopy’, Adv. Funct. Mater. 34, 2408869 (2024). doi.org/10.1002/adfm.202408869 

[4] T. Akhmetshina et al., ‘High-performance ultra-lean biodegradable Mg–Ca alloys and guidelines for their processing’, Acta Mater. 278, 120247 (2024). doi.org/10.1016/j.actamat.2024.120247 

[5] L. Berger et al., ‘In vivo performance of lean bioabsorbable Mg–Ca alloy X0 and comparison to WE43’, Bioact. Mater. 44 (2025) 501 – 515. doi.org/10.1016/j.bioactmat.2024.09.036 

[6] J. Plummer, ‘Chemically-simple magnesium alloys for biomedical applications’, Commun. Mater. 5, 175 (2024). doi.org/10.1038/s43246-024-00613-1