ReviewRecent advances on the development of magnesium alloys for biodegradable implants
Graphical abstract
Real/possible applications of biodegradable magnesium implants (a) cardiovascular stents (BIOTRONIK, Berlin, Germany, under clinical trial), (b) MAGNEZIX screw (received CE mark in Europe), (c) microclip for laryngeal microsurgery (pure magnesium), (d) biodegradable orthopedic implants, (e) wound-closing devices (WZ21).
Section snippets
Introduction: smart implants of magnesium-based alloys
Biodegradable implants, acting as “smart” implants, have attracted increasing interest in the last few years. The main driving force to develop biodegradable implants is their degradation properties in the physiological environment (the terms “degradation” and “corrosion” convey similar meanings but are used in the context of in vivo and in vitro, respectively, in this paper). The opportunity afforded by this class of material is that the clinical function of permanent implants can be achieved
The design criteria of the biodegradable materials
Biodegradable materials are designed to provide temporary support during the healing process of a diseased or damaged tissue and to progressively degrade thereafter [44]. This concept requires the materials to provide appropriate mechanical properties for the intended use and suitable corrosion resistance for progressive degradation. It also requires the materials to possess acceptable biocompatibility and bioactivity within the human body, as new-generation biomaterials [45], [46]. Obviously,
Design strategy
Pure magnesium in the as-cast condition has a very low strength, at just under 30 MPa, and a very fast corrosion rate of 2.89 mm year–1 in 0.9% NaCl solution [51]. Generally, alloying elements can directly strengthen the mechanical properties by solid-solution strengthening, precipitation hardening and grain-refinement strengthening [52]. Alloying elements introduced to strengthen the matrix must have high and temperature-dependent solubility in magnesium. The solubility mainly depends on the
Mg–Al-based alloys
Aluminum has a very high maximum solubility in Mg (12.7 wt.%). Al dissolved in the Mg matrix forms γ-Mg17Al12 and α-Mg phases, resulting in obvious solid-solution strengthening. Mg–Al-based alloys exhibit excellent castability, moderate mechanical properties and good corrosion resistance [52]. Increasing the amount of Al can greatly improve the corrosion resistance of Mg–Al-based alloys [68]. Zn or Mn is often used in combination with Al to improve the room-temperature strength and ductility.
Stents
Stent implantation has been proven to be an effective therapy for pulmonary artery branch stenosis, as well as for coarctation and obstruction within the venous system, saving millions of patients [28]. Heublein et al. [25], [26] were the first to investigate the possibility of making biodegradable stents with Mg-based alloys in 2000–2003. Twenty stents (with a length of 10 mm, an unequal strut thickness of 150–200 μm and a mass of 4 mg) fabricated from AE21 Mg-based alloy were implanted into the
Conclusion and future trends
The present review shows that significant progress has been made over the last 15 years in both the development of Mg-based alloys and the characterization of in vitro and in vivo performances of possible “smart implants”. The design criteria for the next-generation implants require the materials to provide appropriate mechanical properties, suitable corrosion and excellent biocompatibility, and to be bioactive in the human body [45], [46]. To achieve these benchmarks, the key is to develop the
Acknowledgements
This study was supported by the National Science Foundation of the USA through the Engineering Research Center for Revolutionizing Metallic Biomaterials (NSF ERC-RMB) at North Carolina A&T State University. The authors are grateful to Dr. Boyce Collins and Dr. Leon White at North Carolina A&T State University for assistance in editing.
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