In vitro and in vivo degradation and mechanical properties of ZEK100 magnesium alloy coated with alginate, chitosan and mechano-growth factor

Biofunctionalisation of porous additively manufactured magnesium-based alloys for Orthopaedic applications: A review

Magnesium (Mg) alloys have gained significant attention as a desirable choice of biodegradable implant for use in bone repair applications, largely owing to their unique material properties. More recently, Mg and Mg-based alloys have been used as load-bearing metallic scaffolds for bone tissue engineering applications, offering promising opportunities in the field. The mechanical properties and relative density of Mg-based alloys closely approximate those of natural human bone tissue, thereby mitigating the risk of stress-shielding effects. Furthermore, the inherent biodegradability of Mg-based alloys eliminates the necessity for a second surgical procedure for the removal of the implant, a frequent requirement with conventional non-degradable implants. However, a notable challenge remains in managing the high corrosion rate of Mg and Mg-based alloys within physiological environments to ensure that they meet the necessary functional requirements. Consequently, a comprehensive analysis and understanding of the corrosion behaviour of Mg and Mg-based alloys, coupled with optimisation of their surface properties, assume pivotal significance to ensure successful clinical application. The personalized 3D printing of Mg and Mg-based alloy implants represents a paradigm shift, offering a plethora of advantages, foremost among them being the enhancement of the bone healing process facilitated by the degradable porous structure conducive to bone ingrowth. Also, the emergence of surface functionalisation techniques for Mg-based implants amalgamates the mechanical and degradation properties inherent to metals with the enhanced biofunctionality offered by these coatings. This synergy presents a highly promising avenue for using Mg-based implants as temporary orthopaedic and dental solutions. This comprehensive review provides a detailed analysis of recent advancements encompassing alloying elements, additive manufacturing processes, lattice structures and biofunctionalised coatings to tailor the corrosion resistance, mechanical properties and biocompatibility of Mg-based orthopaedic implants.

An overview of magnesium-based implants in orthopaedics and a prospect of its application in spine fusion

Due to matching biomechanical properties and significant biological activity, Mg-based implants present great potential in orthopedic applications. In recent years, the biocompatibility and therapeutic effect of magnesium-based implants have been widely investigated in trauma repair. In contrast, the R&D work of Mg-based implants in spinal fusion is still limited. This review firstly introduced the general background for Mg-based implants. Secondly, the mechanical properties and degradation behaviors of Mg and its traditional and novel alloys were reviewed. Then, different surface modification techniques of Mg-based implants were described. Thirdly, this review comprehensively summarized the biological pathways of Mg degradation to promote bone formation in neuro-musculoskeletal circuit, angiogenesis with H-type vessel formation, osteogenesis with osteoblasts activation and chondrocyte ossification as an integrated system. Fourthly, this review followed the translation process of Mg-based implants via updating the preclinical studies in fracture fixation, sports trauma repair and reconstruction, and bone distraction for large bone defect. Furthermore, the pilot clinical studies were involved to demonstrate the reliable clinical safety and satisfactory bioactive effects of Mg-based implants in bone formation. Finally, this review introduced the background of spine fusion surgeryand the challenges of biological matching cage development. At last, this review prospected the translation potential of a hybrid Mg-PEEK spine fusion cage design.

A comprehensive review of properties of the biocompatible thin films on biodegradable Mg alloys

Magnesium (Mg) and its alloys have attracted attention as biodegradable materials for biomedical applications owing to their mechanical properties being comparable to that of bone. Mg is a vital trace element in many enzymes and thus forms one of the essential factors for human metabolism. However, before being used in biomedical applications, the early stage or fast degradation of Mg and its alloys in the physiological environment should be controlled. The degradation of Mg alloys is a critical criterion that can be controlled by a surface modification which is an effective process for conserving their desired properties. Different coating methods have been employed to modify Mg surfaces to provide good corrosion resistance and biocompatibility. This review aims to provide information on different coatings and discuss their physical and biological properties. Finally, the current withstanding challenges have been highlighted and discussed, followed by shedding some light on future perspectives.

Natural Coatings and Surface Modifications on Magnesium Alloys for Biomedical Applications

Magnesium (Mg) alloys have great potential in biomedical applications due to their incomparable properties regarding other metals, such as stainless steels, Co-Cr alloys, and titanium (Ti) alloys. However, when Mg engages with body fluids, its degradation rate increases, inhibiting the complete healing of bone tissue. For this reason, it has been necessary to implement protective coatings to control the rate of degradation. This review focuses on natural biopolymer coatings used on Mg alloys for resorbable biomedical applications, as well as some modification techniques implemented before applying natural polymer coatings to improve their performance. Issues such as improving the corrosion resistance, cell adhesion, proliferation, and biodegradability of natural biopolymers are discussed through their basic comparison with inorganic-type coatings. Emphasis is placed on the expected biological behavior of each natural polymer described, to provide basic information as a reference on this topic.

A Systematic Review and Network Meta-Analysis of Biomedical Mg Alloy and Surface Coatings in Orthopedic Application

Magnesium alloys have great application prospects as ideal bone implant materials. However, their poor corrosion resistance limits their clinical orthopedic application. Surface modification promotes the corrosion resistance of magnesium. Conversion coatings, such as calcium phosphate (Ca-P) coating, microarc oxidation (MAO) treatment, and fluoride (FLU) treatment, have been extensively investigated in in vivo studies. This systematic review and network meta-analysis compared the influence of different conversion coatings on bone repair, material properties, and systemic host response in orthopedic applications. Using the PICOS model, the inclusion criteria for biodegradable magnesium and its alloys were determined for in vivo studies. Four databases were used. The standard and weight mean differences with 95% confidence intervals were used to analyze new bone formation and degradation rate. Network structure and forest plots were created, and ranking probabilities were estimated. The risk of bias and quality of evidence were assessed using SYRCLE, CERQual, and GRADE tools. In the qualitative analysis, 43 studies were selected, and the evaluation of each outcome indicator was not entirely consistent from article to article. In the quantitative analysis, 21 articles were subjected to network meta-analysis, with 16 articles on implant degradation and 8 articles for new bone formation. Additionally, SUCRA indicated that Ca-P coating exhibited the highest corrosion resistance, followed by FLU treatment. MAO demonstrated the best capability for new bone formation, followed by Ca-P coating. Ca-P coating exhibited the highest overall performance. To conclude, coated Mg can promote better new bone formation than bare Mg and has considerable biocompatibility. Ca-P-coated Mg and MAO-coated Mg have the greatest potential to significantly promote corrosion resistance and bone regeneration, respectively. The findings of this study will provide a theoretical basis for the investigation of composite coatings and guidance for the orthopedic application of Mg bone implants.

Recent advances in chitosan-based layer-by-layer biomaterials and their biomedical applications

In recent years, chitosan-based biomaterials have been continually and extensively researched by using layer-by-layer (LBL) assembly, due to their potentials in biomedicine. Various chitosan-based LBL materials have been newly developed and applied in different areas along with the development of technologies. This work reviews the recent advances of chitosan-based biomaterials produced by LBL assembly. Driving forces of LBL, for example electrostatic interactions, hydrogen bond as well as Schiff base linkage have been discussed. Various forms of chitosan-based LBL materials such as films/coatings, capsules and fibers have been reviewed. The applications of these biomaterials in the field of antimicrobial applications, drug delivery, wound dressings and tissue engineering have been comprehensively reviewed.

Mg alloy surface immobilised with caerin peptides acquires enhanced antibacterial ability and putatively improved corrosion resistance

Magnesium (Mg) has mechanical properties similar to human bones and Mg alloy is considered ideal medical implant material. However, the high velocity of degradation inside the human inner environment severely hampers the usage of Mg alloys. In this study, caerin peptide 1.9 (F3) and a modified sequence of caerin 1.1 (F1) with anti-bacterial activity, were covalently immobilised on the surface of Mg alloys by plasma chemical click reaction. The in vitro antibacterial activity and corrosion resistance of these caerin peptide-immobilised Mg alloys were investigated in Dulbecco's Modified Eagle Medium (DMEM) solution. Un-immobilised Mg alloy sample, blank drug-sensitive tablet (BASD) and a commonly used antibiotics Tazocin were used for comparison. Results showed that peptide immobilised Mg samples showed observable improved corrosion resistance and prolonged antibacterial effect compared to non-immobilised Mg alloy and free caerin peptides. These results indicate that coating Mg alloy with caerin peptides obviously increases the alloy's antibacterial ability and putatively improves the corrosion resistance in vitro. The mechanism underlying the prolonged antibacterial effect for annealed Mg alloys immobilised with the peptides (especially F3) remains unclear, which worth further experimental and theoretical investigation.

Layered hydroxide/polydopamine/hyaluronic acid functionalized magnesium alloys for enhanced anticorrosion, biocompatibility and antithrombogenicity in vascular stents

Magnesium alloys are promising cardiovascular stent materials due to the favourable physical properties and complete biodegradability in vivo. However, the rapid degradation, poor cytocompatibility and tendency of thrombogenesis hinder practical clinical applications. In order to solve these problems, a facile and highly efficient strategy of alkali treatment combined with subsequent layer-by-layer assembly was used to fabricate a multifunctional coating. A bottom layer hydroxyl (–OH) with negative charge after alkali treatment first formed a solid bond with magnesium matrix to provide a rough outer surface for the further immobilization of functional biomolecules. Afterwards, polydopamine and hyaluronic acid were successively immobilized on alkali-treated magnesium surface via strong electrostatic adsorption and covalent bonding between carboxyl group of hyaluronic acid and amine or hydroxyl of polydopamine to form magnesium/OH/polydopamine/hyaluronic acid. Hydroxyl significantly improves the corrosion resistance while polydopamine and hyaluronic acid layers act as a further barrier to provide better anticorrosion. A balance between biocompatibility and antithrombogenicity has been achieved by adjusting the content of hyaluronic acid on polydopamine surface. The multifunctional magnesium/OH/polydopamine/hyaluronic acid coating with lower hyaluronic acid concentrations expose more active sites of polydopamine molecules to promote endothelial cell proliferation while retaining the intrinsic antithrombogenic function of hyaluronic acid to offer a potential application for vascular stents.

In vitro corrosion of pure Mg in phosphate buffer solution-Influences of isoelectric point and molecular structure of amino acids

Influences of proteins on degradation of magnesium alloys are of great significance but not well understood. In particular the roles of amino acids, the basic unit of proteins in regulating the progress of biodegradation of magnesium based materials remain unclear. This study aims to investigate the impacts of alanine, glutamic acid and lysine on degradation of pure magnesium in phosphate buffer solution through SEM, XPS, FTIR, potentiodynamic polarisation curves, electrochemical impedance spectroscopy and immersion tests. The changed contents of amino acids in solutions were detected by UV-vis spectrophotometer. Results demonstrate that the charges of the selected amino acids imposed significant contribution to suppressing the degradation of pure magnesium in phosphate buffer solution. The presence of amino acids led to the formation of phosphate-based corrosion products, increasing free corrosion potential, and reduction in corrosion current density and solution pH depending on their isoelectric points and molecular structures. A plausible corrosion mechanism organised by amino acids on pure magnesium was proposed.

Layer-by-layer adsorption: Factors affecting the choice of substrates and polymers

The electrostatic layer-by-layer technique for fabrication of multi-layered structures of various sizes and shapes using flat and colloidal templates coupled with polyelectrolyte layer-forming materials has attracted significant interest among both academic and industrial researchers due to its versatility and relative simplicity of the procedures involved in its execution. Fabrication of the multi-layered structures using the electrostatic layer-by-layer method involves several distinct stages each of which holds great importance when considering the production of a high-quality product. These stages include selection of materials (both template and a pair of construction polyelectrolytes), adsorption of the first polyelectrolyte layer onto the selected templates, formation of the second layer comprised of the oppositely charged polyelectrolyte and guided by the interactions between the two chosen polyelectrolytes, and multi-layering, where a selected number of layers are produced, and which is conditioned by both intrinsic properties of the involved construction materials and external fabrication conditions such as temperature, pH and ionic strength. The current review summarises the most important aspects of each stage mentioned above and gives examples of the materials suitable for utilization of the technique and describes the underlying physics involved.

Layered double hydroxide/poly-dopamine composite coating with surface heparinization on Mg alloys: improved anticorrosion, endothelialization and hemocompatibility

Magnesium (Mg) and its alloys are promising cardiovascular stent materials due to their favourable physical properties and complete degradation in vivo. However, rapid degradation and poor cytocompatibility hinder their clinical applications. To enhance the corrosion resistance and endothelialization of the AZ31 alloy, a layered double hydroxide (LDH)/poly-dopamine (PDA) composite coating (LDH/PDA) was successfully fabricated. Polarization curves and the electrochemical impedance spectroscopy Nyquist spectrum test proved that the corrosion resistance of the LDH/PDA sample was significantly improved in vitro. The LDH/PDA sample greatly improved the adherence process and the proliferation rate of human umbilical vein endothelial cells (HUVECs). After culturing for 10 days, the number of living HUVECs on the LDH/PDA sample was comparable to that on the Ti sample whereas the cells barely survived on the AZ31 or LDH coating. Furthermore, heparin was immobilized on LDH/PDA via a covalent bond (LDH/PDA/HEP). The corrosion resistance and long-term proliferation of HUVECs after the introduction of heparin were mildly decreased compared with the L/P sample, but were still greatly improved compared with AZ31, the LDH coating and the PDA coating. Furthermore, the LDH/PDA/HEP sample greatly improved the HUVEC migration rate compared with the LDH/PDA sample, and inhibited platelet adhesion which was intense on the LDH/PDA sample. Both LDH/PDA and LDH/PDA/HEP samples had a low hemolysis rate (2.52% and 0.65%, respectively) in vitro and eliminated the adverse biocompatible effects of the direct PDA coating on the AZ31 substrate in vivo. Our results suggest that the LDH/PDA composite coating with further heparinization is a promising method to modify the surface of Mg alloys by significantly improving corrosion resistance, endothelialization and hemocompatibility.

Bio-Functional Design, Application and Trends in Metallic Biomaterials

Introduction of metals as biomaterials has been known for a long time. In the early development, sufficient strength and suitable mechanical properties were the main considerations for metal implants. With the development of new generations of biomaterials, the concepts of bioactive and biodegradable materials were proposed. Biological function design is very import for metal implants in biomedical applications. Three crucial design criteria are summarized for developing metal implants: (1) mechanical properties that mimic the host tissues; (2) sufficient bioactivities to form bio-bonding between implants and surrounding tissues; and (3) a degradation rate that matches tissue regeneration and biodegradability. This article reviews the development of metal implants and their applications in biomedical engineering. Development trends and future perspectives of metallic biomaterials are also discussed.

Osteosynthesis of a cranio-osteoplasty with a biodegradable magnesium plate system in miniature pigs

Biodegradable magnesium alloys are a new class of implant material suitable for bone surgery. The aim of this study was to investigate plates and screws made of magnesium for osteosynthesis in comparison to titanium in a cranial fracture model. Implants were used for internal fixation of a cranio-osteoplasty in nine minipigs. Computed tomography was conducted repeatedly after surgery. The implants and the adjacent tissues were harvested 10, 20 and 30weeks after surgery and investigated by micro-computed tomography and histological analysis. The surgical procedure and the inserted osteosynthesis material were well tolerated by the animals, and the bone healing of the osteoplasty was undisturbed at all times. The adjacent bone showed formation of lacunas in the magnesium group, resulting in a lower bone-to-implant contact ratio than that of titanium (72 vs. 94% at week 30), but this did not lead to clinical side effects. Radiological measurements showed no reduction in osteosynthesis material volume, but indicated signs of degradation: distinct volumes within the magnesium osteosynthesis group had lower density in micro-computed tomography, and these volumes increased up to 9% at week 30. The histological preparations showed areas of translucency and porosity inside the magnesium, but the outer shape of the osteosynthesis material remained unchanged. No fracture or loosening of the osteosynthesis devices appeared. Soft tissue probes confirmed sufficient biocompatibility. Given their biodegradable capacity, biocompatibility, mechanical strength and visibility on radiographs, osteosynthesis plates made of magnesium alloys are suitable for internal fixation procedures.

STATEMENT OF SIGNIFICANCE

To the best of our knowledge this is the first study that used biodegradable magnesium implants for osteosynthesis in a cranial fracture model. The cranio-osteoplasty in miniature pigs allowed in vivo application of plate and screw osteosynthesis of standard-sized implants and the implementation of surgical procedures similar to those conducted on human beings. The osteosynthesis configuration, size, and mechanical properties of the magnesium implants within this study were comparable to those of titanium-based osteosynthesis materials. The results clearly show that bone healing was undisturbed in all cases and that the biocompatibility to hard- and soft tissue was sufficient. Magnesium implants might help to avoid long-term complications and secondary removal procedures due to their biodegradable properties.

Corrosion resistance and biocompatibility of magnesium alloy modified by alkali heating treatment followed by the immobilization of poly (ethylene glycol), fibronectin and heparin

In recent years, magnesium alloys are attracting more and more attention as a kind of biodegradable metallic biomaterials, however, their uncontrollable biodegradation speed in vivo and the limited surface biocompatibility hinder their clinical applications. In the present study, with the aim of improving the corrosion resistance and biocompatibility, the magnesium alloy (AZ31B) surface was modified by alkali heating treatment followed by the self-assembly of 3-aminopropyltrimethoxysilane (APTMS). Subsequently, poly (ethylene glycol) (PEG) and fibronectin or fibronectin/heparin complex were sequentially immobilized on the modified surface. The results of attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed that the above molecules were successfully immobilized on the magnesium alloy surface. An excellent hydrophilic surface was obtained after the alkali heating treatment while the hydrophilicity decreased to some degree after the self-assembly of APTMS, the surface hydrophilicity was gradually improved again after the immobilization of PEG, fibronectin or fibronectin/heparin complex. The corrosion resistance of the control magnesium alloy was significantly improved by the alkali heating treatment. The self-assembly of APTMS and the following immobilization of PEG further enhanced the corrosion resistance of the substrates, however, the grafting of fibronectin or fibronectin/heparin complex slightly lowered the corrosion resistance. As compared to the pristine magnesium alloy, the samples modified by the immobilization of PEG and fibronectin/heparin complex presented better blood compatibility according to the results of hemolysis assay and platelet adhesion as well as the activated partial thromboplastin time (APTT). In addition, the modified substrates had better cytocompatibility to endothelial cells due to the improved anticorrosion and the introduction of fibronectin. The substrates modified by fibronectin or fibronectin/heparin complex can significantly promote endothelial cell adhesion and proliferation. Taking all these results into consideration, the method of the present study can be used for the surface modification of the magnesium alloy to simultaneously impart it better corrosion resistance, favorable blood compatibility and good cytocompatibility to endothelial cells.

Tackling Mg alloy corrosion by natural polymer coatings-A review

The field of protective coatings for magnesium and its alloys (e.g., AZ31) using natural polymers is reviewed. Polymers utilized are broadly divided into polysaccharides and proteins. For both polymer classes examples are given focusing on coating processing and characterization. Several analysing methods reported in literature are summarized highlighting the different characterization approaches applied in different studies, which makes difficult a direct comparison of the outcomes. In most cases, the protective behavior of coatings was determined using electrochemical impedance spectroscopy or by assessing hydrogen evolution in different fluids. Mechanical tests and in vitro cell culture studies have been also carried out on selected coating systems. Overall, the results show the possibility of applying protective coatings based on natural polymers on magnesium and its alloys, however, in vivo investigations are scarce so that long-term results in relevant conditions are not yet available. A comparison with the use of synthetic polymers is presented and current challenges and areas for future research are discussed, highlighting the need for further investigations in the field, which should enable broadening the applications of Mg and Mg alloys in medicine. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2628-2641, 2016.