A recent article in Scientific Reports proposed enhancing the corrosion resistance and biocompatibility of AZ31b magnesium (Mg) alloy by incorporating carbon ions onto its surface through carbon plasma immersion ion-implantation (C-PIII). Electrochemical and hydrogen-evolution analyses showed improved corrosion resistance, while MTT and cell-adherence assays confirmed enhanced biocompatibility.
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Background
Mg alloys have received considerable attention as biomaterials due to their biocompatibility, biodegradability, and density and elastic modulus comparable to bone. These qualities make them suitable for applications such as temporary implants, orthopedic implants, porous scaffolds, and orthodontic anchors.
However, Mg has the lowest standard reduction potential among commercial metals, making it prone to corrosion. This susceptibility can result in subcutaneous emphysema and compromised mechanical integrity, limiting its clinical use. Consequently, enhancing the corrosion resistance of Mg and its alloys is essential for medical applications.
Carbon, a fundamental element in all life, is highly biocompatible due to its biological and chemical inertness with cells and body fluids, making it suitable for in vitro and in vivo corrosion resistance. Additionally, carbon does not release toxic elements into the body. This study proposed carbon ion implantation as a promising method to enhance the corrosion resistance and biocompatibility of AZ31b Mg.
Methods
Commercially procured AZ31b Mg alloys were cut into different-sized samples, polished, and cleaned before ion implantation. C-PIII was performed using the AZ31b samples at three carbon-ion densities: 1×1018, 1.5×1018, and 2×1018 ions/cm2.
The implanted samples were characterized via scanning electron microscopy (SEM) to compare surface morphologies before and after implantation and via Raman spectroscopy to examine the carbon layer patterns on their surfaces. The chemical compositions of the modified samples were determined using energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.
Cathodic Tafel polarization and electrochemical impedance spectroscopy (EIS) were conducted to evaluate the corrosion resistance of the carbon-ion-implanted alloy samples and to understand the underlying corrosion mechanisms. Additionally, hydrogen-evolution analysis was carried out in simulated body fluid (SBF) at intervals of 24, 48, 72, and 120 hours to assess chemical corrosion resistance over time.
To evaluate biocompatibility, an MTT assay was performed using mouse MC3T3-E1 cells. Extraction media for the assay was prepared by immersing the implanted specimens in Dulbecco’s Modified Eagle Medium (DMEM) for 72 hours. Absorbance was measured with a microplate reader, and cell culture images were captured with an inverted optical microscope for further analysis.
Results and Discussion
SEM images of the pristine AZ31b alloy revealed a smooth, mirror-like surface interrupted only by minor grinding scratches. Following C-PIII, a modification layer was observed on the surfaces, displaying immeasurably small carbon nanoparticles likely resulting from high-energy ion bombardment.
Raman scattering spectra of the C-PIII samples showed patterns characteristic of amorphous carbon. The data indicated a positive correlation between hydrophobicity and amorphous carbon content, with hydrophobicity increasing as more carbon was injected.
Compared to the unmodified AZ31b sample, the carbon-implanted samples demonstrated significantly lower corrosion current density values. This is attributed to the amorphous carbon layer that formed on the magnesium alloy surface, effectively blocking liquid contact with the substrate.
Carbon ions at interstitial sites created precipitates or metastable phases, contributing to the corrosion resistance of AZ31b under physiological conditions.
Hydrogen-evolution analysis in simulated body fluid showed the pristine AZ31b sample had the highest corrosion rate, with a total hydrogen evolution volume of 15.1 mL. By contrast, the implanted samples displayed reduced hydrogen volumes: 12.5 mL for 1×1018 ions/cm2, 10.15 mL for 1.5×1018 ions/cm2, and the lowest for the 2×1018 ion/cm2.
The pristine AZ31b alloy had a consistently higher average hydrogen evolution rate than all C-PIII samples, indicating the carbon layer effectively reduced degradation in the treated samples.
In the MTT assay, MC3T3-E1 pre-osteoblast viability was higher in the C-PIII sample extracts than in the DMEM control after one day of incubation. Viability for C-PIII samples remained over 80 % of the control after seven days, confirming the non-toxicity of the carbon-implanted samples. Certain carbon compounds in the C-PIII sample extracts appeared to promote MC3T3-E1 pre-osteoblast proliferation in vitro.
Conclusion
The study successfully incorporated carbon ions into the surface of AZ31b Mg alloy to enhance its corrosion resistance and biocompatibility. Ion implantation produced an amorphous graphite layer over the alloy surface, leading to a marked decrease in corrosion current density in SBF, indicating improved corrosion resistance.
The EIS curves further confirmed the enhanced electrochemical stability of the implanted Mg alloy samples. Notably, the corrosion resistance increased with loading across the considered range of carbon loadings (1×1018 to 2×1018 ion/cm2). Additionally, no cytotoxic response was observed in MC3T3-E pre-osteoblasts when in contact with extracts from the implanted samples. Alternatively, these extracts promoted cell growth in vitro.
Journal Reference
Zheng, C., et al. (2024). Corrosion resistance and biocompatibility of carbon ion implanted AZ31B magnesium alloy. Scientific Reports. DOI: 10.1038/s41598-024-77543-y, https://www.nature.com/articles/s41598-024-77543-y
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