Surface treatments for controlling corrosion rate of biodegradable Mg and Mg-based alloy implants

Abstract

Due to their excellent biodegradability characteristics, Mg and Mg-based alloys have become an emerging material in biomedical implants, notably for repair of bone as well as coronary arterial stents. However, the main problem with Mg-based alloys is their rapid corrosion in aggressive environments such as human bodily fluids. Previously, many approaches such as control of alloying materials, composition and surface treatments, have been attempted to regulate the corrosion rate. This article presents a comprehensive review of recent research focusing on surface treatment techniques utilised to control the corrosion rate and surface integrity of Mg-based alloys in both in vitro and in vivo environments. Surface treatments generally involve the controlled deposition of thin film coatings using various coating processes, and mechanical surfacing such as machining, deep rolling or low plasticity burnishing. The aim is to either make a protective thin layer of a material or to change the micro-structure and mechanical properties at the surface and sub-surface levels, which will prevent rapid corrosion and thus delay the degradation of the alloys. We have organised the review of past works on coatings by categorising the coatings into two classes—conversion and deposition coatings—while works on mechanical treatments are reviewed based on the tool-based processes which affect the sub-surface microstructure and mechanical properties of the material. Various types of coatings and their processing techniques under two classes of coating and mechanical treatment approaches have been analysed and discussed to investigate their impact on the corrosion performance, biomechanical integrity, biocompatibility and cell viability. Potential challenges and future directions in designing and developing the improved biodegradable Mg/Mg-based alloy implants were addressed and discussed. The literature reveals that no solutions are yet complete and hence new and innovative approaches are required to leverage the benefit of Mg-based alloys. Hybrid treatments combining innovative biomimetic coating and mechanical processing would be regarded as a potentially promising way to tackle the corrosion problem. Synergetic cutting-burnishing integrated with cryogenic cooling may be another encouraging approach in this regard. More studies focusing on rigorous testing, evaluation and characterisation are needed to assess the efficacy of the methods.

The influence of alloying and fabrication techniques on the mechanical properties, biodegradability and biocompatibility of zinc: A comprehensive review

“…On the other hand, nano-scale alterations (e.g., hierarchical design) of metal surfaces are proven effective in enhancing cell attachment and proliferation on implants [205]. In biodegradable metals, nanoporous surfaces have been produced via dealloying or micro-arc oxidation [206].…”

Reduction in the Corrosion Rate of Magnesium and Magnesium Alloy Specimens and Implications for Plain Fully Bioresorbable Coronary Artery Stents: A Review

“…Among the methods are chemical conversion treatment, implantation of ions of a gas, metal, or mixture of gas and metal, micro-arc oxidation (also called plasma electrolytic anodization or anodic spark deposition), electrophoretic deposition, sol gel deposition, physical vapor deposition, and radiofrequency magnetron sputtering. Examples of single-layer coatings are MgF 2 , nanohydroxyapatite, akernanite, and silver doped tricalcium phosphate and examples of multi-layer coatings are Ni-P-ZrO 2 , TiO-poly (lactic acid), and MgF 2 -HA [35] Figure 4. Comparison of hydrogen evolution obtained using ground and cryogenically burnished and ground specimens (Constructed from results given by Pu et al [70]).…”

“…In a plain FBRS, the scaffold is made of Mg or a Mg alloy, whereas, in a coated FBRS, the scaffold, made of either a Mg alloy or a bioresorbable polymer, is coated with a bioresorbable polymer in which an anti-proliferative drug is embedded. There is a growing body of reviews of the literature on bioresorbable stents exclusively [33]- [43], but only very few cover Mg/Mg alloy FBRSs and, of these, fewer still include any coverage of what, arguably, is the most important shortcoming/challenge of this type of stent, which is high corrosion rate (resulting in rapid resorption) [33] [34] [35] [36]. However, 1) in the review by Boland et al [33], the subject is degradation models for both metallic and polymeric components and their application to both Mg-based and polymer-based bioresorbable stents;…”

“…2) in the review by Ma et al [34], the subject is coatings for both Mg-based bioresorbable stents and orthopaedic implants and only a minimal reference to corrosion is made; and 3) the review by Uddin et al [35] focused on surface treatments for controlling the corrosion of Mg and Mg-based alloys for applications in orthopaedics and cardiovascular stenting; and 4) the review by Hornberger et al [36] is on coatings for biomedical applications and not on stents per se and the coverage on corrosion is short.…”

Finite Element Analysis of Surface Integrity in Deep Ball-Burnishing of a Biodegradable AZ31B Mg Alloy

“…Once implanted, they gradually dissolve and are absorbed by the human body. As mechanical properties of Mg alloy are close to that of human bone, they minimize stress shielding, while eliminating the secondary interventional surgery [1]. However, the fast corrosion in human body fluid is a major problem of Mg alloys-i.e., the implant degrades quickly and loses mechanical integrity before the complete healing of bone.…”