Biodegradation behavior of micro-arc oxidation coating on magnesium alloy-from a protein perspective.

Protein exerts a critical influence on the degradation behavior of absorbable magnesium (Mg)-based implants. However, the interaction mechanism between protein and a micro-arc oxidation (MAO) coating on Mg alloys remains unclear. Hereby, a MAO coating was fabricated on AZ31 Mg alloy. And its degradation behavior in phosphate buffer saline (PBS) containing bovine serum albumin (BSA) was investigated and compared with that of the uncoated alloy. Surface morphologies and chemical compositions were studied using Field-emission scanning electron microscope (FE-SEM), Fourier transform infrared spectrophotometer (FT-IR) and X-ray diffraction (XRD). The degradation behavior of the bare Mg alloy and its MAO coating was studied through electrochemical and hydrogen evolution tests. Cytotoxicity assay was applied to evaluate the biocompatibility of Mg alloy substrate and MAO coating. Results indicated that the presence of BSA decreased the degradation rate of Mg alloy substrate because BSA (RCH(NH2)COO‾) molecules combined with Mg2+ ions to form (RCH(NH2)COO)2Mg and thus inhibited the dissolution of Mg(OH)2 by impeding the attack of Cl‾ ions. In the case of MAO coated Mg alloy, the adsorption of BSA on MAO coating and the formation of (RCH(NH2)COO)2Mg exhibited a synergistic effect and enhanced the corrosion resistance of the coated alloy significantly. Furthermore, cell bioactive assay suggested that the MAO coating had good viability for MG63 cells due to its high surface area.

Advances in functionalized polymer coatings on biodegradable magnesium alloys - A review.

Magnesium (Mg) and its alloys have become a research frontier in biodegradable materials owing to their superior biocompatibility and excellent biomechanical compatibility. However, their high degradation rate in the physiological environment should be well tackled prior to clinical applications. This review summarizes the latest progress in the development of polymeric coatings on biodegradable Mg alloys over the last decade, regarding preparation strategies for polylactic acid (PLA), poly (latic-co-glycolic) acid (PLGA), polycaprolactone (PCL), polydopamine (PDA), chitosan (CS), collagen (Col) and their composite, and their performance in terms of corrosion resistance and biocompatibility. Feasible perspectives and developing directions of next generation of polymeric coatings with respect to biomedical Mg alloys are briefly discussed. STATEMENT OF SIGNIFICANCE: Magnesium (Mg) and its alloys have become a research frontier in biodegradable materials owing to their superior biocompatibility and suitable biomechanical compatibility. However, the principal drawback of Mg-based implants is their poor corrosion resistance in physiological environments. Hence, it is vital to mitigate the degradation/corrosion behavior of Mg alloys for safe biomedical deployments. This review summarizes the latest progress in development of polymeric coatings on biomedical Mg alloys regarding preparation strategy, corrosion resistance and biocompatibility, including polylactic acid (PLA), poly (latic-co-glycolic) acid (PLGA), polycaprolactone (PCL), chitosan (CS), polydopamine (PDA), collagen (Col) and their composite. In addition, functionalized polymer coatings with Mg alloys exhibits a promising prospect owing to their ability of degradation along with biocompatibility, self-healing, drug-delivery and osteoinduction.

Calcium-Ion Batteries: Current State-of-the-Art and Future Perspectives.

Recent developments in rechargeable battery technology have seen a shift from the well-established Li-ion technology to new chemistries to achieve the high energy density required for extended range electric vehicles and other portable applications, as well as low-cost alternatives for stationary storage. These chemistries include Li-air, Li-S, and multivalent ion technologies including Mg2+ , Zn2+ , Ca2+ , and Al3+ . While Mg2+ battery systems have been increasingly investigated in the last few years, Ca2+ technology has only recently been recognized as a viable option. In this first comprehensive review of Ca2+ ion technology, the use of Ca metal anodes, alternative alloy anodes, electrolytes suitable for this system, and cathode material development are discussed. The advantages and disadvantages of Ca2+ ion batteries including prospective achievable energy density, cost reduction due to high natural abundance, low ion mobility, the effect of ion size, and the need for elevated temperature operation are reviewed. The use of density functional theory modeling to predict the properties of Ca-ion battery materials is discussed and the extent to which this approach is successful in directing research into areas of promise is evaluated. To conclude, a summary of recent achievements is presented and areas for future research efforts evaluated.

Electrochemical surface engineering of magnesium metal by plasma electrolytic oxidation and calcium phosphate deposition: biocompatibility and in vitro degradation studies.

In this study, the surface of magnesium metal was electrochemically engineered for enhanced biocompatibility and controlled degradation in body fluid. Firstly, a plasma electrolytic oxidation (PEO) coating was formed on magnesium, followed by electrochemical deposition of calcium phosphate (CaP) using an unconventional electrolyte. Cytocompatibility tests using L929 cells revealed that the PEO-CaP coating significantly improved the biocompatibility of magnesium. In vitro electrochemical degradation experiments in simulated body fluid (SBF) showed that the PEO-CaP coating improved the degradation resistance of magnesium significantly. The corrosion current density (i corr) of the PEO-CaP coated magnesium was ∼99% and ∼97% lower than that of bare magnesium and the PEO-only coated magnesium, respectively. Similarly, electrochemical impedance spectroscopy (EIS) results showed that the polarisation resistance (R P) of the PEO-CaP coated magnesium was one-order of magnitude higher as compared to the PEO-only coated magnesium and two-orders of magnitude higher than the bare magnesium, after 72 h immersion in SBF. Scanning electron microscopy (SEM) analysis revealed no localized degradation in the PEO-CaP coated magnesium. The study demonstrated that the PEO-CaP coating is a promising combination for enhancing the biocompatibility and reducing the degradation of magnesium for potential biodegradable implant applications.

Biocompatibility and biodegradation studies of a commercial zinc alloy for temporary mini-implant applications.

In this study, the biocompatibility and in vitro degradation behaviour of a commercial zinc-based alloy (Zn-5 Al-4 Mg) were evaluated and compared with that of pure zinc for temporary orthopaedic implant applications. Biocompatibility tests were conducted using human alveolar lung epithelial cells (A549), which showed that the zinc alloy exhibits similar biocompatibility as compared to pure zinc. In vitro degradation evaluation was performed using weight loss and electrochemical methods in simulated body fluid (SBF) at 37 °C. Weight loss measurements revealed that the degradation of the zinc alloy was slightly lower during the initial immersion period (1-3 days), but marginally increased after 5 and 7 days immersion as compared to pure zinc. Potentiodynamic polarisation experiments showed that the zinc alloy exhibits higher degradation rate than pure zinc. However, electrochemical impedance spectroscopy analysis suggests that pure zinc is susceptible to localized degradation, whereas the zinc alloy exhibited passivation behaviour. Post-degradation analysis revealed localized degradation in both pure zinc and the zinc alloy.

Conversion of biowastes to biomaterial: An innovative waste management approach.

The study suggests that biowastes (eggshells and urine) can be potentially used as precursors to produce hydroxyapatite (HAp) biomaterial in a simple chemical process. A batch reactor was used in this work to produce HAp powder from eggshells and synthetic urine (SU). Fine powder of calcined eggshells was dissolved in water to produce aqueous calcium hydroxide. The solution was then mixed with concentrated SU in stoichiometric amounts corresponding to HAp (Ca/P molar ratio∼1.67). The initial pH of the solution was alkaline (pH∼8.5) and particles formed rapidly with slight mixing. Stirring the turbid solution for a longer period (72h) did not show any visual change, but the particle size decreased slightly. When the pH of the solution was adjusted to 5, the solution was initially clear, but particle formation was apparent after 48h stirring. It was noticed that at a slow stirring speed (100rpm), film formation occurred on the solution, whereas at a higher stirring speed (500rpm) no such film formation was observed. X-ray diffraction (XRD) analysis confirmed that the particles (formed at 500rpm) were an amorphous calcium phosphate (CaP). Alkaline treatment at 80°C for 2h converted the amorphous CaP into HAp. Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the particles (formed at 500rpm) suggested that they are calcium-deficient HAp (Ca/P molar ratio 1.58).

Biocompatibility and Degradation of a Low Elastic Modulus Ti-35Nb-3Zr Alloy: Nanosurface Engineering for Enhanced Degradation Resistance.

In this study, the biocompatibility and degradation behavior of a low elastic modulus Ti-35Nb-3Zr alloy were investigated and compared with that of the conventional orthopedic and dental implant materials, i.e., commercially pure titanium (Cp-Ti) and Ti-6Al-4V alloy. The biocompatibility test results suggested that cells proliferate equally well on Ti-35Nb-3Zr and Cp-Ti. The degradation rates of Cp-Ti and Ti-6Al-4V were ∼68% (p < 0.05) and ∼84% (p < 0.05) lower as compared to Ti-35Nb-3Zr, respectively. Interestingly, the passive current density (ipass (1000mv)) of the Ti-35Nb-3Zr alloy was ∼29% lower than that of Cp-Ti, which suggests that the alloying elements in the Ti-35Nb-3Zr alloy have contributed to its passivation behavior. Nanosurface engineering of the Ti-35Nb-3Zr alloy, i.e., a two-step electrochemical process involving anodization (producing nanoporous layer) and calcium phosphate (CaP) deposition, decreased the degradation rate of the alloy by ∼83% (p < 0.05), and notably, it was similar to the conventional Ti-6Al-4V alloy. Hence, it can be suggested that the nanosurface-engineered low elastic modulus Ti-35Nb-3Zr alloy is a promising material for orthopedic and dental implant applications.

Biocompatibility and in Vitro Degradation Behavior of Magnesium-Calcium Alloy Coated with Calcium Phosphate Using an Unconventional Electrolyte.

Calcium phosphate (CaP) was electrochemically coated on a magnesium-calcium (Mg-Ca) alloy using an unconventional electrolyte and a pulse-potential method. The CaP particles of the coating were relatively large, flat, and irregularly oriented; however, they covered the entire alloy surface with a coating thickness of 5 µm. Cytocompatibility tests using L929 cells inoculated in Eagle minimum essential medium supplemented with 10% (v/v) fetal bovine serum (E-MEM+FBS) revealed that CaP coating improved the cytocompatibility of the alloy. It also showed effective suppression of Mg2+ ion release from the substrate of the coated alloy and consequently reduced the pH increase of the medium. In vitro degradation experiments using electrochemical techniques in simulated body fluid (SBF) also suggested significant enhancement of the alloy degradation resistance by CaP coating. Potentiodynamic polarization results showed that the corrosion current density of the coated alloy was ∼95% lower than that of the bare metal. Electrochemical impedance spectroscopy results revealed that the polarization resistance (RP) of the coated alloy was more than an order of magnitude higher than that of the bare metal after 2 h of immersion in SBF. Interestingly, after 72 h of immersion, the measured RP had decreased by ∼82%, and the coating appeared cracked and damaged. The results suggest that SBF is more aggressive than E-MEM+FBS cell culture medium.

Influence of living cells (L929) on the biodegradation of magnesium-calcium alloy.

In this study, the influence of living cells (L929) on the in vitro degradation behaviour of a magnesium-calcium alloy was investigated using an electrochemical technique in the Eagle's minimum essential medium (EMEM) with 10% foetal bovine serum (FBS) under 5% CO2 atmosphere. The degradation of the alloy increased significantly in the medium containing the cells as compared to that without cells. Post-degradation analysis revealed localized degradation in the vicinity of the cells. It is suggested that the cell metabolic activity has induced local pH drop and as a result increased the alloy degradation.

Low elastic modulus Ti-Ta alloys for load-bearing permanent implants: enhancing the biodegradation resistance by electrochemical surface engineering.

In this study, the in vitro degradation behaviour of titanium-tantalum (Ti-Ta) alloys (10-30 wt.% Ta) was investigated and compared with conventional implant materials, i.e., commercially pure titanium (Cp-Ti) and titanium-aluminium-vanadium (Ti6Al4V) alloy. Among the three Ti-Ta alloys studied, the Ti20Ta (6.3×10(-4) mm/y) exhibited the lowest degradation rate, followed by Ti30Ta (1.2×10(-3) mm/y) and Ti10Ta (1.4×10(-3) mm/y). All the Ti-Ta alloys exhibited lower degradation rate than that of Cp-Ti (1.8×10(-3) mm/y), which suggests that Ta addition to Ti is beneficial. As compared to Ti6Al4V alloy (8.1×10(-4) mm/y), the degradation rate of Ti20Ta alloy was lower by ~22%. However, the Ti30Ta alloy, which has closer elastic modulus to that of natural bone, showed ~48% higher degradation rate than that of Ti6Al4V alloy. Hence, to improve the degradation performance of Ti30Ta alloy, an intermediate thin porous layer was formed electrochemically on the alloy followed by calcium phosphate (CaP) electrodeposition. The coated Ti30Ta alloy (3.8×10(-3) mm/y) showed ~53% lower degradation rate than that of Ti6Al4V alloy. Thus, the study suggests that CaP coated Ti30Ta alloy can be a viable material for load-bearing permanent implants.

A mechanistic in vitro study of the microgalvanic degradation of secondary phase particles in magnesium alloys.

The aim of this work was to understand the effect of microgalvanic degradation on secondary phase particles in magnesium alloys under in vitro condition. Pure magnesium and Mg17 Al12 (ß-phase) were galvanically coupled in simulated body fluid and the degradation behavior was studied using electrochemical impedance spectroscopy. The galvanic coupling produced a phosphate/carbonate layer on the ß-phase, which initially increased the degradation resistance. However, the deposited phosphate/carbonate layer rapidly degraded once the galvanic coupling was removed, and ß-phase exhibited similar degradation resistance to that of pure magnesium. A phenomenological model has been presented, demonstrating the galvanic coupling effect.

Improving the packing density of calcium phosphate coating on a magnesium alloy for enhanced degradation resistance.

In this study, an attempt was made to improve the packing density of calcium phosphate (CaP) coating on a magnesium alloy by tailoring the coating solution for enhanced degradation resistance of the alloy for implant applications. An organic solvent, ethanol, was added to the coating solution to decrease the conductivity of the coating solution so that hydrogen bubble formation/bursting reduces during the CaP coating process. Experimental results confirmed that ethanol addition to the coating solution reduces the conductivity of the solution and also decreases the hydrogen evolution/bubble bursting. In vitro electrochemical experiments, that is, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization showed that CaP coating produced in 30% (v/v) ethanol containing coating solution (3E) exhibits significantly higher degradation resistance (i.e., ~50% higher polarization resistance and ~60% lower corrosion current) than the aqueous solution coating. Scanning electron microscope (SEM) analysis of the coatings revealed that the packing of 3E coating was denser than that of aqueous coating, which can be attributed to the lower hydrogen evolution in the former than in the latter. Further increase in the ethanol content in the coating solution was not beneficial; in fact, the coating produced in 70% (v/v) ethanol containing solution (7E) showed degradation resistance much inferior to that of the aqueous coating, which is due to low thickness of 7E coating.

Potentiostatic pulse-deposition of calcium phosphate on magnesium alloy for temporary implant applications--an in vitro corrosion study.

In this study, a magnesium alloy (AZ91) was coated with calcium phosphate using potentiostatic pulse-potential and constant-potential methods and the in vitro corrosion behaviour of the coated samples was compared with the bare metal. In vitro corrosion studies were carried out using electrochemical impedance spectroscopy and potentiodynamic polarization in simulated body fluid (SBF) at 37 °C. Calcium phosphate coatings enhanced the corrosion resistance of the alloy, however, the pulse-potential coating performed better than the constant-potential coating. The pulse-potential coating exhibited ~3 times higher polarization resistance than that of the constant-potential coating. The corrosion current density obtained from the potentiodynamic polarization curves was significantly less (~60%) for the pulse-deposition coating as compared to the constant-potential coating. Post-corrosion analysis revealed only slight corrosion on the pulse-potential coating, whereas the constant-potential coating exhibited a large number of corrosion particles attached to the coating. The better in vitro corrosion performance of the pulse-potential coating can be attributed to the closely packed calcium phosphate particles.

In vitro mechanical integrity of hydroxyapatite coated magnesium alloy.

The mechanical integrity of resorbable implants during service, especially in load bearing orthopaedic applications, is critical. The high degradation rate of resorbable magnesium and magnesium-based implants in body fluid may potentially cause premature in-service failure. In this study, a magnesium alloy (AZ91) was potentiostatically coated with hydroxyapatite at different cathodic voltages in an attempt to enhance the mechanical integrity. The mechanical integrity of the uncoated and hydroxyapatite coated alloys was evaluated after in vitro testing of the coated samples in simulated body fluid (SBF). The uncoated alloy showed 40% loss in the mechanical strength after five days exposure to SBF. However, the hydroxyapatite coated alloy exposed to SBF showed 20% improvement in the mechanical strength as compared to that of the uncoated alloy. The alloy coated potentiostatically at -2 V performed better than the -3 V coated alloy. The cross-sectional analysis of the coatings revealed relatively uniform coating thickness for the -2 V coated alloy, whereas the -3 V coated alloy exhibited areas of uneven coating. This can be attributed to the increase in hydrogen evolution on the alloy during -3 V coating as compared to -2 V coating. The scanning electron micrographs of the in vitro tested alloy revealed that hydroxyapatite coating significantly reduced the localized corrosion of the alloy, which is critical for better in-service mechanical integrity. Thus, the study suggests that the in vitro mechanical integrity of resorbable magnesium-based alloy can be improved by potentiostatic hydroxyapatite coating.

In vitro degradation and mechanical integrity of calcium-containing magnesium alloys in modified-simulated body fluid.

The successful applications of magnesium-based alloys as degradable orthopaedic implants are mainly inhibited due to their high degradation rates in physiological environment and consequent loss in the mechanical integrity. This study examines the degradation behaviour and the mechanical integrity of calcium-containing magnesium alloys using electrochemical techniques and slow strain rate test (SSRT) method, respectively, in modified-simulated body fluid (m-SBF). Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) results showed that calcium addition enhances the general and pitting corrosion resistances of magnesium alloys significantly. The corrosion current was significantly lower in AZ91Ca alloy than that in AZ91 alloy. Furthermore, AZ91Ca alloy exhibited a five-fold increase in the surface film resistance than AZ91 alloy. The SSRT results showed that the ultimate tensile strength and elongation to fracture of AZ91Ca alloy in m-SBF decreased only marginally (approximately 15% and 20%, respectively) in comparison with these properties in air. The fracture morphologies of the failed samples are discussed in the paper. The in vitro study suggests that calcium-containing magnesium alloys to be a promising candidate for their applications in degradable orthopaedic implants, and it is worthwhile to further investigate the in vivo corrosion behaviour of these alloys.