Enhanced corrosion resistance and reduced cytotoxicity of the AZ91 Mg alloy by plasma nitriding and a hierarchical structure composed of ciprofloxacin-loaded polymeric multilayers and calcium phosphate coating.

Much effort has made to lessen the cytotoxicity and enhance the corrosion resistance of biodegradable magnesium alloys, for example, by depositing multilayered polymeric coatings containing hydroxyapatite. In this work, a hierarchical structure composed of ciprofloxacin (Cip)-loaded on polyacrylic acid (PAA) and poly (ethyleneimine) (PEI) as biocompatible polymeric multilayers and calcium phosphate coating as the top layer is formed by the sol-gel method on the AZ91 Mg alloy with an intermediate layer formed by nitrogen plasma immersion ion implantation. The thicknesses of the multilayered coating and nitrided layer (Mg3 N2 ) are 10 µm and 140 nm, respectively. The corrosion current density decreases by 95.6% and the corrosion potential in the polarization curve shifts to the positive direction by 23%. The passivation process which occurs at defects by deposition of corrosion products mitigates both galvanic and localized corrosion. Slight increase in the contact angle and surface free energy, enhanced corrosion resistance, and reduced cytotoxicity are observed from the multilayered structure. The better corrosion resistance enables better control of release of Cip. Biological assessment indicates that the antibacterial activity against Escherichia coli is improved by 100% after culturing for 24 hr and the cell viability and noncytotoxic behavior of the coated AZ91 are enhanced as well. The corrosion behavior and biological results suggest that the strategy of using a hierarchical structure composed of Cip-loaded polymeric multilayers in conjunction with an intermediate plasma nitrided layer has large potential in the development of biodegradable orthopedic implants made of Mg alloys.

Corrosion resistance, nano-mechanical properties, and biocompatibility of Mg-plasma-implanted and plasma-etched Ta/TaN hierarchical multilayered coatings on the nitrided AZ91 Mg alloy.

A tantalum/tantalum nitride (Ta/TaN) multilayered coating is deposited on plasma-nitridedAZ91 Mg alloy. The top TaN layer undergoes O2+ Ar plasma etching to improve the antibacterial properties and Mg plasma immersion ion implantation (MgPIII) is performed to enhance the biocompatibility and wound healing capability. A uniform, compact, homogeneous, and columnar nanostructured MgPIII and plasma-etched TaN layer with a cluster size of about 17 nm, surface roughness of 0.28 nm, and needle morphology is observed. Although, plasma etching increases the corrosion current density (icorr) from 0.02 to 0.19 µA cm-2due to larger surface roughness and different potentials between sharp points and smooth points, MgPIII decreasesicorrfrom 0.19 to 0.02 µA cm-2besides a more positive corrosion potential. The amounts of Mg+2released to the simulated body fluid (SBF) diminishes from 89.63 ± 0.54 to 60.30 ± 0.47 mg l-1cm-2indicating improved corrosion resistance. Under fever conditions (40 °C),icorrdecreases by 63%, but the open circuit potential does not change due to the constant chemical composition of the surface as well as thicker double layer and less defects, as confirmed by the larger amount of Mg+2of 71.49 ± 0.22 mg l-1cm-2leached to the SBF. In the self-healing process which occurs via the reactions between the tantalum intermediate layer and electrolytes and penetrating ions through the defects as well as formation of oxide compounds, creation and propagation of defects are deterred as shown the 24 h destructive polarization test in SBF. The combination of plasma etching and MgPIII enhance not only the bacterial resistance and biocompatibility of the super-hard TaN layer by providing the rougher surface on TaN-P-Mg, but also the nano-mechanical properties and anticorrosion properties. As a result, the hardness increases by 7%, elastic modulus decreases by 19%, and the stiffness increases by 21%.

Enhanced corrosion resistance, antibacterial properties, and biocompatibility by hierarchical hydroxyapatite/ciprofloxacin-calcium phosphate coating on nitrided NiTi alloy.

Multi-functional hierarchical coatings are deposited on the nitrided NiTi alloy. The nitrided layer is first deposited by nitrogen plasma immersion ion implantation and a middle layer containing porous hydroxyapatite and ciprofloxacin (Cip) is produced before the top calcium phosphate coating is deposited by the sol-gel method. The thicknesses of the coating and nitrided intermediate layer are about 1.54 µm and 160 nm, respectively and Cip penetrates to a depth of about 530 nm. Calcium phosphate reduces surface defects resulting in a surface roughness of 17 ± 2 nm compared to 34 ± 5 nm of the porous hydroxyapatite coating. The corrosion resistance is improved due to reduced defects and localized corrosion as manifested by the decrease in the Ni2+ release rate by 11.6% from 0.0198 to 0.0175 mg L-1 cm-2. The bacterial resistance against E. coli is also improved by about 88 times on account of Cip release and good biocompatibility is confirmed by proliferation of MC3T3 cells. This multi-functional hierarchical coating has large potential in orthopedic and dental applications.

Nanopatterned silk-coated AZ31 magnesium alloy with enhanced antibacterial and corrosion properties.

Because of unique properties such as the lightweight, natural biodegradability, and biocompatibility, magnesium alloys are promising in biomedical implants. However, inadequate corrosion resistance in the physiological environment remains a technical hurdle and application of coatings is a viable means to overcome the deficiency. Also, the antibacterial properties are very important in order to mitigate post-implantation complications arising from bacterial infection. In this study, a biocompatible silk film is deposited on AZ31 Mg alloy to enhance the corrosion resistance and by means of oxygen plasma etching, nature-inspired nanopatterns are fabricated on the surface of the silk film to improve the inherent antibacterial properties. The biocompatibility and antibacterial properties are determined with MC3T3-E1 osteoblast cells and E. coli and S. aureus, respectively. The antimicrobial properties of the silk coated AZ31 are better than those of the bare alloy probably due to the combined effects of the nanopatterns and alkalinity associated with leaching of Mg ions. The ß-sheets formed on the silk film is found to result in 104 times reduction in the corrosion current density and 50% reduction in Mg leaching after 1 day. Although degradation of the ß-sheets is observed to begin after 1 day, the amount of Mg ions leached to the medium from silk-coated AZ31 is still 17% lower than that from the bare one. The biomimicking nanopatterns on the natural silk film improve the corrosion resistance, biocompatibility, and antibacterial properties simultaneously and have large clinical potential.