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Editorial

Development and Application of Biodegradable Metals

School of Mechanical Engineering, University of Applied Sciences Stralsund, 18435 Stralsund, Germany
Metals 2022, 12(8), 1312; https://doi.org/10.3390/met12081312
Submission received: 1 August 2022 / Accepted: 3 August 2022 / Published: 5 August 2022
(This article belongs to the Special Issue Development and Application of Biodegradable Metals)

1. Introduction and Scope

Magnesium-, zinc- and iron-based alloys as biodegradable metals have been a focus of scientific attention for their ability to eliminate the need for a second surgery in order to remove implants made with such materials. In research, they have been assessed for alloy development, new manufacturing processes, innovative testing methods and a variety of applications. Alloy development aims to balance appropriate mechanical properties, moderate degradation rates, and biocompatibility. Strengthening mechanisms might not always promote the best degradation behavior. Magnesium alloys are known to show non-uniform corrosion behavior, and they tend to crack under corrosion stress. Mg-RE alloys and RE-free alloys are currently under investigation, as are Mg-Al alloys and Al-free alloys. Another strategy is to aim for high-impurity alloys. Zinc-based alloys exhibit a lack of mechanical stability and iron-based alloys are known for slow degradation. Molybdenum has previously been rarely investigated, but may also fulfill the requirements of biodegradable metals.
Most of the biodegradable metal alloys studied so far are processed by casting and extrusion; some applications are based on wire drawing, and additive manufacturing is of growing interest due to its unique design capabilities. Many testing methods on mechanical and degradation properties are well-established, whereas others, such as in vitro test procedures for full cytocompatibility assessment, as well as fatigue and stress degradation, are under improvement. The community is deeply engaged in discussing the relationship between in vitro and in vivo properties. Potential applications of biodegradable metal alloys are represented by structural materials for orthopedics, such as pins and screws, and temporary cardiovascular devices, such as stents and wires.

2. Contributions

Most of the papers published in this Special Issue, entitled “Development and Application of Biodegradable Metals”, investigate magnesium alloys as biodegradable metals, and one paper investigates the degradation behavior and biocompatibility of a molybdenum alloy [1]. As seen in the Special Issue “Magnesium Alloys for Biomedical Applications” in 2020 [2], current research on magnesium alloys focuses on Al-free alloys. Accordingly, one paper of this Special Issue is based on a magnesium–manganese alloy [3], one study focuses on magnesium–dysprosium [4], others focus on magnesium–silver [5] or pure magnesium [6], and three papers are based on magnesium–zinc alloys [7,8,9]. This Special Issue shows that alongside alloying and optimizing casting [4] and extrusion parameters, manufacturing based on powder metallurgy [1,7,8], the optimization of surface properties [3,9], techniques to further refine grain size, such as wire drawing [5], and severe plastic deformation techniques, such as equal channel angular pressing (ECAP) and high-pressure torsion (HPT) [6], are key to the improvement of mechanical and corrosion properties.
The first article by Redlich et al. [1] on the degradation behavior and biocompatibility of pure molybdenum, a manufactured and commercially available metallurgical powder, shows that molybdenum gradually dissolves in modified Simulated Body Fluid, releasing molybdate anions. The dissolution rate after 28 days is 10 µm/y for both materials and dissolution accelerates over time. A non-passivating, uniform and slowly soluble degradation product layer was observed. Based on apoptosis and necrosis assays with molybdenum ion extracts and colonization tests with human endothelial and smooth muscle cell lines on molybdenum substrates, it was found that no adverse effects on cell viability were observed for concentrations expected from the dissolution of implants with typical geometries. Substrates were densely colonized by both cell lines. This shows that molybdenum does not trigger thrombogenic or inflammatory responses. To summarize, in combination with its favorable mechanical properties and the renal excretion of bio-available molybdate ions, molybdenum may be an alternative to established bioresorbable metals.
The antibacterial potential of magnesium-based materials was studied by Emelyanenko et al. [3]. Superhydrophobic coatings on a magnesium-based alloy were fabricated and the behavior in bacterial dispersions of Pseudomonas aeruginosa and Klebsiella pneumoniae cells in phosphate-buffered saline was analyzed. It was shown that the immersion of such coatings in bacterial dispersions causes notable changes in the sample morphology, dependent on the bacterial dispersion composition and the type of bacterial strain. The authors found that the interaction of the superhydrophobic coatings with the bacterial dispersion caused the formation of biofilms and sodium polyphosphate films, which provided enhanced barrier properties in magnesium dissolution and hence medium alkalization in dispersion. From the electrochemical data for superhydrophobic samples in continuous contact with corrosive bacterial dispersions for 48 h, a high level of anticorrosion protection was observed.
The study by Ahlers et al. [4] focuses on the influence of cooling conditions on the evolution of long-period stacking-ordered (LPSO) phases and the corrosion behavior of as-cast Resoloy®. Metallographic and corrosive tests are used to monitor the changes in the properties of this material. Heat treatment was performed different ways: solution heat treatment over three different time periods, followed by quenching in water and cooling in air at ambient temperature. The initial homogeneous fine grains of the as-cast microstructure grow with increasing heat treatment time, and slow cooling leads to additional grain growth. Cooling in air leads to faint lamellar LPSO structures, which develop from bulk LPSO structures during the cooling process. With that, this paper provides important information on the corrosion behavior of the different microstructures: the corrosion rate of the cooled samples increases with increasing grain size. The magnesium matrix between the LPSO lamellae starts to corrode first, the LPSO lamellae act as a barrier. Solution heat treatment weakens the galvanic corrosion by reducing the normal potential difference between matrix and secondary phases.
Magnesium–silver alloys, of interest for their antibacterial properties in addition to good biocompatibility and biodegradability, are studied in the form of thin wires (250 μm diameter) for scaffolding by Meyer et al. in [5]. The evaluation of their degradation rate and homogeneity is a challenge by traditional methods. Therefore, 3D imaging using X-ray near-field holotomography with sub-micrometer resolution is applied to study the degradation of thin Mg-2Ag and Mg-6Ag wires in Dulbecco’s modified Eagle’s medium and 10% fetal bovine serum after 1 to 7 days. The authors found that at 3 days of immersion the degradation rates of the alloys in recrystallized and solution-annealed conditions were similar, but at 7 days higher silver content and solution annealing led to decreased degradation rates. This reveals the influence of Ag content and precipitates on the degradation. In addition, with decreasing degradation rates the pitting factor increases. The authors point out that due to the relatively small field of view during imaging and the high degradation inhomogeneity of the samples, the standard deviation of the determined parameters was high. However, the results allowed the authors to conclude that Mg-6Ag in the solution-annealed condition emerges as a potential material for thin wire manufacturing for implants.
The study by Silva [6] reports the compression testing of larger pure magnesium samples in different directions, which are processed by hot rolling, ECAP and HPT. Scaffolds were fabricated from the material with different processing histories and immersed in Hank’s solution for up to 14 days to evaluate the corrosion behavior. The results show that severe plastic deformation through ECAP and HPT reduces anisotropy and increases strength and strain rate sensitivity. The hot-rolled material reveals significant anisotropy and low strain rate sensitivity. The rolled material followed by ECAP shows lower anisotropy and higher strain rate sensitivity. The material processed by HPT shows low anisotropy and the highest strain rate sensitivity. The applied severe plastic deformation techniques in this study result in an improvement in strength. The samples processed by hot rolling, ECAP and HPT display a significantly higher corrosion resistance compared to the as-cast material. The authors also found that severe plastic deformation supports uniform corrosion. The scaffolds produced from the material processed by ECAP and HPT maintained their shape, the scaffold holes were still open after 14 days of immersion.
The aim of the paper by Raducanu et al. [7] was the application of mechanical alloying to obtain a new biodegradable Mg-based alloy powder from the Mg-xZn-0.8Ca-0.5Zr system. High biomechanical and biochemical performance was the general aim of this study, as well as the establishment of an efficient processing route for the production of small biodegradable parts for the medical domain by various processing parameters for mechanical alloying. The authors established that for the same milling parameters, the composition of the powders influenced the powder size and shape. The morphology of the milled powders reveals fluctuations in the particle size due to cold welding, fracturing and grinding. The highest experimental milling speed and length of time obtained finer powder particles, almost round-shaped, without pores or various inclusions. The powder with the most uniform size, with 10 wt.% Zn, was finally processed by selective laser melting to obtain a robust, non-friable and homogeneous experimental sample, without cracking, for use in more systematic trials in the future.
Saberi et al. [8] studied graphene oxide (GO) and GO-Cu nanosystems, homogenously dispersed as a reinforcement in the matrix of Mg-Zn alloy using the semi powder metallurgy method. The composite was manufactured by spark plasma sintering, for enhancing the mechanical and antibacterial properties. The GO- and GO-Cu-reinforced composites displayed a 55% higher compressive strength under grain refining, precipitate strengthening and dispersion strengthening, and GO and GO-Cu dual nanofillers presented a synergistic effect on enhancing the effectiveness of load transfer and crack deflection in the Mg-Zn alloy. The GO-Cu dual nanofillers displayed on the one hand a synergistic influence on antibacterial activity through combining the capturing influences of GO nanosheets with the killing influences of Cu, and on the other a slightly negative effect on the corrosion behavior in electrochemical and in vitro immersion evaluation. The authors concluded that the nanocomposites with a low concentration of GO-Cu showed acceptable cytotoxicity for MG-63 cells and revealed a high potential for use as an orthopedic implant material.
The work by Mukhtar et al. [9] demonstrated the significant effect of the mechanical alloying parameters ball size and milling time on the development of Mg-10Zn-5Co alloys, as well as its microstructure, mechanical properties and biocompatibility. For characterization, the microstructure, hardness and Young’s Modulus were used. In vitro biocompatibility analysis of different alloys was performed with MC3T3-E1 osteoblasts. Grain analysis confirmed the even dispersion of particles and scanning electron microscopy analysis showed the formation of laminate, spherical and fine particles with an increase in time and a varied ball size. The formation of intermetallic compounds was confirmed. The microhardness increased with the increase in milling time. The authors found that after milling, the dispersion of particles reduced the elastic modulus of the developed Mg-Zn-Co alloys, bringing it closer to the modulus of natural bone, and improved their biocompatibility. The in vitro analysis with osteoblasts showed that the developed alloys were non-cytotoxic and biocompatible.

3. Conclusions and Outlook

This Special Issue of Metals provides a comprehensive insight into the development and potential applications of biodegradable metals, mainly Mg alloys, but also by means of an article on molybdenum. As the developments of recent years have shown, Al-free alloys are of interest in alloy development, with Mg-Zn alloys appearing especially attractive. Alloy development today goes hand in hand with the consideration of the manufacturing route, with the aim of optimizing the microstructure. Powder metallurgical production is increasingly becoming an object of focus and offers new potential for composites. In addition, surface modifications can provide even more for uniform corrosion. I am confident that this Special Issue will be valuable for future developments and discussions.
As a guest editor of this Special Issue, I hope that the presented papers will be useful to researchers, designers, and practitioners developing and working towards the application of magnesium alloys as implants. I would like to express my gratitude to all the authors for their contributions, as well as to the anonymous reviewers for their efforts in ensuring high-quality publications. Special thanks also go to the editors of Metals for their help, and to the editorial assistants for their valuable engagement and support during the preparation of this Special Issue.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Maier, P. Development and Application of Biodegradable Metals. Metals 2022, 12, 1312. https://doi.org/10.3390/met12081312

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Maier P. Development and Application of Biodegradable Metals. Metals. 2022; 12(8):1312. https://doi.org/10.3390/met12081312

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Maier, Petra. 2022. "Development and Application of Biodegradable Metals" Metals 12, no. 8: 1312. https://doi.org/10.3390/met12081312

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Maier, P. (2022). Development and Application of Biodegradable Metals. Metals, 12(8), 1312. https://doi.org/10.3390/met12081312

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