LSP/MAO composite bio-coating on AZ80 magnesium alloy for biomedical application

Advances in amelioration of plasma electrolytic oxidation coatings on biodegradable magnesium and alloys

Magnesium and its alloys are considered excellent materials for biodegradable implants because of their good biocompatibility and biodegradability as well as their mechanical properties. However, the rapid degradation rate severely limits their clinical applications. Plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), is an effective surface modification technique. However, there are many pores and cracks on the coating surface under conventional PEO process. The corrosive products tend to penetrate deeply into the substrate, reducing its corrosion resistance and the biocompatibility, which makes PEO-coated Mg difficult to meet the long-term needs of in vivo implants. Hence, it is necessary to modify the PEO coating. This review discusses the formation mechanism and the influential parameters of PEO coatings on Mg. This is followed by a review of the latest research of the pretreatment and typical amelioration of PEO coating on biodegradable Mg alloys in the past 5 years, including calcium phosphate (Ca-P) coating, layered double hydroxide (LDH)-PEO coating, ZrO2 incorporated-PEO coating, antibacterial ingredients-PEO coating, drug-PEO coating, polymer-PEO composite coating, Plasma electrolytic fluorination (PEF) coating and self-healing coating. Meanwhile, the improvements of morphology, corrosion resistance, wear resistance, biocompatibility, antibacterial abilities, and drug loading abilities and the preparation methods of the modified PEO coatings are deeply discussed as well. Finally, the challenges and prospects of PEO coatings are discussed in detail for the purpose of promoting the clinical application of biodegradable Mg alloys.

Progress in bioactive surface coatings on biodegradable Mg alloys: A critical review towards clinical translation

Mg and its alloys evince strong candidature for biodegradable bone implants, cardiovascular stents, and wound closing devices. However, their rapid degradation rate causes premature implant failure, constraining clinical applications. Bio-functional surface coatings have emerged as the most competent strategy to fulfill the diverse clinical requirements, besides yielding effective corrosion resistance. This article reviews the progress of biodegradable and advanced surface coatings on Mg alloys investigated in recent years, aiming to build up a comprehensive knowledge framework of coating techniques, processing parameters, performance measures in terms of corrosion resistance, adhesion strength, and biocompatibility. Recently developed conversion and deposition type surface coatings are thoroughly discussed by reporting their essential therapeutic responses like osteogenesis, angiogenesis, cytocompatibility, hemocompatibility, anti-bacterial, and controlled drug release towards in-vitro and in-vivo study models. The challenges associated with metallic, ceramic and polymeric coatings along with merits and demerits of various coatings have been illustrated. The use of multilayered hybrid coating comprising a unique combination of organic and inorganic components has been emphasized with future perspectives to obtain diverse bio-functionalities in a facile single coating system for orthopedic implant applications.

•

The challenges and current status of coatings are reviewed in light of clinical requirements.

•

Multilayered hybrid coatings have been emphasized to obtain diverse bio-functionalities.

•

The future developments and research directions on coatings for biodegradable implants are highlighted.

The Effect of Sodium Tetrafluoroborate on the Properties of Conversion Coatings Formed on the AZ91D Magnesium Alloy by Plasma Electrolytic Oxidation

Magnesium and its alloys are widely used in many areas because of their light weight, excellent dimensional stability, and high strength-to-weight ratio. However, the material exhibits poor wear and corrosion resistance, which limits its use. Plasma electrolytic oxidation (PEO) is an effective surface modification method for producing ceramic oxide layers on Mg and their alloys. The influence of the additions of sodium tetrafluoroborate (NaBF4) and sodium fluoride (NaF) into alkaline-silicate electrolyte on the properties of the conversion layers formed in the magnesium AZ91D alloy has been investigated. Surface morphology and chemical composition were determined by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The anticorrosive properties of the layers were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods in simulated body fluid (SBF). The presence of NaBF4 or NaF in the electrolyte increases the corrosion resistance of the protective layer. However, the best anticorrosive properties show the layers obtained in the presence of NaBF4. This is probably caused by the incorporation of boron and fluorine in the form of Mg (BF4)2 mainly in the barrier layer.

Promising Methods for Corrosion Protection of Magnesium Alloys in the Case of Mg-Al, Mg-Mn-Ce and Mg-Zn-Zr: A Recent Progress Review

High specific strength characteristics make magnesium alloys widely demanded in many industrial applications such as aviation, astronautics, military, automotive, bio-medicine, energy, etc. However, the high chemical reactivity of magnesium alloys significantly limits their applicability in aggressive environments. Therefore, the development of effective technologies for corrosion protection is an urgent task to ensure the use of magnesium-containing structures in various fields of application. The present paper is aimed to provide a short review of recent achievements in corrosion protection of magnesium alloys, both surface treatments and coatings, with particular focus on Mg-Al-Mn-Ce, Mg-Al-Zn-Mn and Mg-Zn-Zr alloys, because of their wide application in the transport industry. Recent progress was made during the last decade in the development of protective coatings (metals, ceramics, organic/polymer, both single layers and multilayer systems) fabricated by different deposition techniques such as anodization, physical vapour deposition, laser processes and plasma electrolytic oxidation.

Plasma Electrolytic Oxidation (PEO) Process-Processing, Properties, and Applications

Plasma electrolytic oxidation (PEO) is a novel surface treatment process to produce thick, dense metal oxide coatings, especially on light metals, primarily to improve their wear and corrosion resistance. The coating manufactured from the PEO process is relatively superior to normal anodic oxidation. It is widely employed in the fields of mechanical, petrochemical, and biomedical industries, to name a few. Several investigations have been carried out to study the coating performance developed through the PEO process in the past. This review attempts to summarize and explain some of the fundamental aspects of the PEO process, mechanism of coating formation, the processing conditions that impact the process, the main characteristics of the process, the microstructures evolved in the coating, the mechanical and tribological properties of the coating, and the influence of environmental conditions on the coating process. Recently, the PEO process has also been employed to produce nanocomposite coatings by incorporating nanoparticles in the electrolyte. This review also narrates some of the recent developments in the field of nanocomposite coatings with examples and their applications. Additionally, some of the applications of the PEO coatings have been demonstrated. Moreover, the significance of the PEO process, its current trends, and its scope of future work are highlighted.

Influences of aggressive ions in human plasma on the corrosion behavior of AZ80 magnesium alloy

Magnesium alloys can work as biomedical materials due to their Young's modules similar to that of bone. Nevertheless, in a human plasma, one of the major drawbacks of these materials is the low corrosion resistance. Here, AZ80 corrosion in the solutions containing chloride, bicarbonate, sulphate and hydrogen phosphate ions were investigated by a short-term immersion test and electrochemical techniques. The results showed that bicarbonate and hydrogen phosphate could retard corrosion rate, while chloride and sulphate accelerated corrosion rate. During the early immersion stage, the corrosion rate increased with the presence of bicarbonate. It was caused by the reaction of bicarbonate and hydroxide promoting the dissolution of magnesium and accelerating corrosion. In the later stage, the reduced corrosion rate was due to the formation of various protective films. The sample formed a new sparse porous MgSO₄·5H₂O compounds in the sulphate ion solution, which could not effectively prevent chloride ions from entering the matrix and thus accelerated the dissolution of magnesium. With the presence of hydrogen phosphate, magnesium phosphate with a much lower solubility was formed, preferentially precipitated on the surface and was not influenced by the chloride ions. The corrosion mechanisms of magnesium alloys in above ions were proposed.

Synergistic effect of deep ball burnishing and HA coating on surface integrity, corrosion and immune response of biodegradable AZ31B Mg alloys

The fast degradation and consequent loss of mechanical integrity is a major problem of biodegradable Mg alloy, which limits its clinical viability. This paper presents the influence of a synergistic approach combining deep ball burnishing and hydroxyapatite (HA) coating on biomechanical integrity, degradation and immune response of Mg alloy (AZ31B). The burnishing resulted in smooth surface topography, increased hardness from 0.87 to 1.45 GPa and induced microstructural disturbances with deformation twins/twin bands, which enabled formation of a dense and compact platelet-like crystals HA coating of 110 μm thickness. Compared to the untreated and burnished specimens, the burnished + HA coated surface provided remarkably higher corrosion resistance as indicated by lower corrosion current density and smaller mass loss. HA coating and surface integrity enhancement by burnishing were predominantly responsible for improved corrosion resistance. HA coating on the burnished surface exhibited hydrophilic properties and adequate bonding strength. While the modified surfaces promoted cell growth, the burnished + HA surface outperformed in exhibiting less pro-inflammatory and high anti-inflammatory cytokines, demonstrating that the treated surfaces were not posing any threat to immune cells. The findings indicate that the synergistic surface treatment can be a viable means to enhance corrosion resistance and immune response of Mg alloys implants.

Magnesium Alloys With Tunable Interfaces as Bone Implant Materials

Magnesium (Mg) based biodegradable materials are a new generation orthopedic implant materials that are intended to possess same mechanical properties as that of bone. Mg alloys are considered as promising substitutes to permanent implants due to their biodegradability in the physiological environment. However, rapid corrosion rate is one of the major constraints of using Mg alloys in clinical applications in spite of their excellent biocompatibility. Approaches to overcome the limitations include the selection of adequate alloying elements, proper surface treatment, surface modification with coating to control the degradation rate. This review focuses on current advances on surface engineering of Mg based biomaterials for biomedical applications. The review begins with a description of corrosion mechanism of Mg alloy, the requirement for appropriate surface functionalization/coatings, their structure-property-performance relationship, and suitability for biomedical applications. The control of physico-chemical properties such as wettability, surface morphology, surface chemistry, and surface functional groups of the coating tailored by various approaches forms the pivotal part of the review. Chemical surface treatment offers initial protection from corrosion and inorganic coating like hydroxyapatite (HA) improves the biocompatibility of the substrate. Considering the demand of ideal implant materials, multilayer hybrid coatings on Mg alloy in combination with chemical pretreatment or inorganic HA coating, and protein-based polymer coating could be a promising technique to improve corrosion resistance and promote biocompatibility of Mg-based alloys.

Biodegradable Magnesium Alloy (ZK60) with a Poly(l-lactic)-Acid Polymer Coating for Maxillofacial Surgery

The purpose of this study was to evaluate the mechanical strength and biodegradation of a ZK60 plate coated with poly(l-lactic)-acid polymer (PLLA) in a LeFort I osteotomy canine model for maxillofacial applications. The PLLA-coated ZK60 plate and screw were evaluated using a LeFort I osteotomy canine model based on five beagles. The presence of wound dehiscence, plate exposure, gas formation, inflammation, pus formation, occlusion, food intake, and fistula formation were evaluated. After 12 weeks, these dogs were sacrificed, and an X-ray micro-computed tomography (µCT) was conducted. Plate exposure, gas formation, and external fistula were not observed, and the occlusion remained stable. Wound dehiscence did not heal for 12 weeks. CT images did not show plates in all the five dogs. A few screw bodies fixed in the bone remained, and screw heads were completely absorbed after 12 weeks. These findings may be attributed to the inability to optimize the absorption rate with PLLA coating. Rapid biodegradation of the PLLA-coated ZK60 occurred due to the formation of microcracks during the bending process. Further improvement to the plate system with PLLA-coated ZK60 is required using other surface coating methods or alternative Mg alloys.

Fabrication and characterization of silk fibroin coating on APTES pretreated Mg-Zn-Ca alloy

To delay the degradation of magnesium alloys, silk fibroin as a natural organic polymer coating was fabricated on a 3-amino-propyltriethoxysilane (APTES) pretreated Mg-Zn-Ca alloy. APTES pretreatment coated the surface of magnesium alloys with amino groups, which can bond with functional groups in silk fibroin to form a compact coating/substrate interface. The influences of the APTES concentration and drying temperature on the coating adhesion and interface were investigated to explore the optimal parameters in the fabrication process. The nanoporous silk fibroin films completely covered the APTES pretreated Mg-Zn-Ca surface, which reached a thickness of ~7 μm. The chemical states for the coated Mg-Zn-Ca alloy were compared to those of the bare Mg-Zn-Ca alloy and the APTES pretreated Mg-Zn-Ca alloy to illustrate the coating mechanism. During in vitro degradation and electrochemical measurements in simulated body fluid (SBF), the samples with the silk fibroin coating showed remarkably improved corrosion resistance and a slower degradation rate compared to those of the bare samples, suggesting that the silk fibroin coating was an effective protection coating for the substrates and can delay the degradation of magnesium alloys. Moreover, a model for the in vitro degradation was proposed. In vitro cell experiments confirmed the excellent biocompatibility of silk fibroin coated Mg-Zn-Ca structure.

Silk fibroin film-coated MgZnCa alloy with enhanced in vitro and in vivo performance prepared using surface activation

Magnesium and its alloys have generated considerable interest as one of the most promising biodegradable metals for biomedical bone implants. However, the enormous challenges are to improve their rapid corrosion excessively as well as to endow them with biocompatibility and biosafety. Herein, we introduce a natural silk fibroin protein coating to control the corrosion resistance and enhance the biocompatibility of MgZnCa alloy. To obtain a robust and reliable coated structure, different surface-activation processes are employed to increase the available functional groups on MgZnCa surfaces before coating. Compared to oxygen plasma activation, our unique vacuum ultraviolet-ozone (VUV/O₃) activation method is effective in realizing uniform silk fibroin films as a protective barrier on MgZnCa alloy surfaces, and the nanoscratch test verified the superior adhesion strength of the silk fibroin-coated magnesium alloy structure. Long-term immersion results combined with electrochemical tests showed the preferable in vitro anticorrosion behavior and a low degradation rate of coated Mg alloy (1/8 times that of uncoated Mg alloy). Cell adhesion and cytotoxicity tests demonstrated that silk fibroin-coated MgZnCa presented improved biocompatibility with bone marrow mesenchymal stem cells. An animal study involving silk fibroin-coated MgZnCa implanted on one side of a rabbit spine for 180 days showed remarkably improved in vivo corrosion resistance, with 1/18 times the degradation rate of uncoated MgZnCa. These results not only comprehensively confirmed the validity of the VUV/O₃-activation method as a coating strategy but also implied the tremendous potential of the modified Mg alloy for application as a degradable biomedical implant material. STATEMENT OF SIGNIFICANCE: MgZnCa alloy is a promising material in clinical implantation. Silk fibroin (SF) is a natural organic material with biocompatibility and biodegradability. To date, the combination of SF and MgZnCa alloy has exhibited considerable prospects for orthopedic applications. The realization of a direct coating is an enormous challenge because strong chemical bonds cannot be easily formed between organic and inorganic materials. To solve this bottleneck, we proposed a unique vacuum ultraviolet-ozone (VUV/O₃) surface-activation method for the first time to modify the Mg alloy surface before SF coating, which significantly enhanced both in vitro and in vivo performance, such as superior biocompatibility and remarkably improved corrosion resistance of magnesium alloys (∼1/18 the in vivo degradation rate of uncoated MgZnCa).

Manipulating the degradation behavior and biocompatibility of Mg alloy through a two-step treatment combining sliding friction treatment and micro-arc oxidation

Manipulating the degradation rate of biomedical Mg alloys has always been a challenge. In this study, a two-step treatment including sliding friction treatment (SFT) and micro-arc oxidation (MAO) was adopted to acquire a unique Mg-based architecture containing three typical layers comprising a MAO coating/nanocrystalline (NC) layer/coarse-grained (CG) matrix. It was found that the modified topmost MAO coating possessed enhanced corrosion resistance, cytocompatibility and hemocompatibility. The intermediate NC layer sandwiched between the coating and CG matrix was an ideal transition layer capable of avoiding degradation rate upsurge caused by coating breakdown; meanwhile, it provided an effective reinforcing effect on the overall mechanical strength. More importantly, the corrosion resistance of these layers was ranked in the order: MAO coating > NC layer > CG matrix. This kind of gradually increasing corrosion rate of the three layers with depth renders the two-step treatment a promising approach to design Mg-based implants possessing controllable degradation rates.

Assessment of the degradation rates and effectiveness of different coated Mg-Zn-Ca alloy scaffolds for in vivo repair of critical-size bone defects

Surgical repair of bone defects remains challenging, and the search for alternative procedures is ongoing. Devices made of Mg for bone repair have received much attention owing to their good biocompatibility and mechanical properties. We developed a new type of scaffold made of a Mg-Zn-Ca alloy with a shape that mimics cortical bone and can be filled with morselized bone. We evaluated its durability and efficacy in a rabbit ulna-defect model. Three types of scaffold-surface coating were evaluated: group A, no coating; group B, a 10-μm microarc oxidation coating; group C, a hydrothermal duplex composite coating; and group D, an empty-defect control. X-ray and micro-computed tomography(micro-CT) images were acquired over 12 weeks to assess ulnar repair. A mechanical stress test indicated that bone repair within each group improved significantly over time (P < 0.01). The degradation behavior of the different scaffolds was assessed by micro-CT and quantified according to the amount of hydrogen gas generated; these measurements indicated that the group C scaffold better resisted corrosion than did the other scaffold types (P < 0.05). Calcein fluorescence and histology revealed that greater mineral densities and better bone responses were achieved for groups B and C than for group A, with group C providing the best response. In conclusion, our Mg-Zn-Ca-alloy scaffold effectively aided bone repair. The group C scaffold exhibited the best corrosion resistance and osteogenesis properties, making it a candidate scaffold for repair of bone defects.

Microstructure and Corrosion Behavior of Composite Coating on Pure Mg Acquired by Sliding Friction Treatment and Micro-Arc Oxidation

For the purpose of detecting the influence of grain structure of a Mg matrix on the microstructure and corrosion resistance of micro-arc oxidation (MAO) coating, prior to MAO processing, sliding friction treatment (SFT) was adopted to generate a fine-grained (FG) layer on coarse-grained (CG) pure Mg surface. It showed that the FG layer had superior corrosion resistance, as compared to the CG matrix, owing to the grain refinement; furthermore, it successfully survived after MAO treatment. Thus, an excellent FG-MAO coating was gained by combining SFT and MAO. The surface morphology and element composition of FG-MAO and CG-MAO samples did not show significant changes. However, the FG layer favorably facilitated the formation of an excellent MAO coating, which possessed a superior bonding property and greater thickness. Consequently, the modified FG-MAO sample possessed enhanced corrosion resistance, since a lower hydrogen evolution rate, a larger impedance modulus and a lower corrosion current were observed on the FG-MAO sample.