Processing and characterization of new metastable Fe-Mg(-Zn) bioalloys

Abstract

Traditionally, non-degradable materials like titanium or stainless steel are used in medical fixations and stents. However, their removal necessitates a second surgery, increasing costs and patient recovery time. Consequently, there's growing interest in temporary implants that avoid this. This PhD thesis focused on designing and developing metastable iron-based biodegradable alloys using high-energy milling techniques (planetary and attrition ball milling) for temporary metallic implants. Initially, Fe-Mg alloys were explored due to iron's excellent mechanical properties and biocompatibility, combined with magnesium's high degradation rate. However, their immiscibility posed a challenge. Planetary ball milling was used to integrate Mg into Fe as a solid solution. Results showed a maximum 5 wt.% Mg integration without severe oxidation. Degradation and cytotoxicity were assessed in various simulated physiological media (PBS, HBSS, DMEM), revealing increased degradation for Fe-5 wt.% Mg powder, particularly in modified HBSS. Cytotoxicity was attributed to increased reactivity and potential for insoluble product formation. Attrition ball milling, offering higher energy transfer and atmospheric control, was used to further enhance Mg solid solution formation and mitigate oxidation. Zinc was introduced to modify iron's crystal structure, facilitating Mg incorporation. This technique yielded Fe-Mg and Fe-Zn-Mg powders with improved Mg solid solution and lattice expansion, as confirmed by XRD and TEM. Spark plasma sintering (SPS) was employed for bulk sample compaction, minimizing oxidation and porosity. However, SPS led to partial Mg and Zn loss and secondary phase formation (periclase, zincite). Degradation tests in PBS, HBSS, and modified HBSS revealed phosphate layer formation and higher corrosion rates for Zn-containing materials, attributed to insoluble products. Cytotoxicity assays showed higher toxicity in Zn-containing samples, followed by Fe-Mg and Fe. To improve biocompatibility, hydroxyapatite (HAp) coatings were developed using laser induced single-step coating (LISSC) in DMEM and HBSS with increased Ca and P. A thin HAp layer was successfully produced, particularly on Fe-Mg samples.