Key Takeaways & Executive Findings
- •• • MAO/ZIF-8@PD coating reduced corrosion current density by three orders of magnitude (from ~10^-5 to ~10^-8 A/cm²) compared to MAO coating, providing superior corrosion protection for AZ31 Mg alloy implants. • • Under 808 nm NIR laser, both coatings achieved ≥99.5% bactericidal rates against E. coli and S. aureus, combining photothermal (PTT) and photodynamic (PDT) effects with Zn2+ release. • • Photothermal conversion efficiencies were 44.01% (two-step) and 48.57% (one-step), indicating effective NIR-to-heat conversion for localized antibacterial therapy. • • The one-step (OS) method produced a different 2D-co-3D MOF structure with higher photothermal efficiency and distinct degradation behavior, offering tunable performance for biomedical coatings.
Abstract
Magnesium alloys are promising biodegradable bone implant materials due to their biocompatibility and mechanical compatibility, but rapid degradation and postoperative bacterial infection limit clinical application. Here, zeolitic imidazolate framework-8 (ZIF-8) and 3,4,9,10-perylenetetracarboxylic diimide (PD) composite coatings (ZIF-8@PD) were fabricated in situ on micro-arc oxidation (MAO) coated AZ31 alloys via two-step and one-step (OS) methods. The MAO/ZIF-8@PD and MAO/ZIF-8@PD (OS) coatings reduced corrosion current density by three and two orders of magnitude, respectively, compared to MAO coating, due to the physical barrier of the 2D-co-3D MOF structure. Under 808 nm near-infrared laser irradiation, photothermal and photodynamic effects from PD, combined with contact killing by released Zn2+ ions, achieved bactericidal rates ≥99.5% against E. coli and S. aureus. Photothermal conversion efficiencies were 44.01% and 48.57% for the two-step and one-step coatings, respectively. The distinct Zn2+ sources led to different 2D-co-3D MOF structures, influencing degradation and antibacterial behavior. These coatings offer a strategy to enhance corrosion resistance and antibacterial activity of Mg alloys for biomedical applications.
1. Introduction
Biodegradable magnesium alloys hold potential for temporary bone fixation, yet their clinical translation is hindered by two intertwined challenges: uncontrolled corrosion in physiological environments and high susceptibility to postoperative infections. Conventional surface modifications, such as micro-arc oxidation (MAO), provide initial corrosion resistance but lack active antibacterial functionality. Organic coatings may release biocides but often compromise mechanical integrity or biocompatibility. The need for a coating that simultaneously mitigates corrosion and eradicates bacteria without systemic toxicity remains unmet.
This study addresses this bottleneck by engineering a hybrid metal-organic framework (MOF) coating that integrates two-dimensional (2D) perylene diimide (PD) sheets with three-dimensional (3D) ZIF-8 crystals. The 2D-co-3D architecture creates a physical barrier against corrosive media, while PD's photothermal and photodynamic properties, activated by near-infrared (NIR) light, generate localized hyperthermia and reactive oxygen species. Concurrently, the degradation of ZIF-8 releases Zn2+ ions, providing contact-killing antibacterial action. By comparing two-step and one-step fabrication routes, the authors demonstrate tunable performance, offering a rational design for next-generation biodegradable implant coatings.
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Hao Chen, Xiao-Tong Yu, Cheng-Xu Yi, Lan-Yue Cui, Yu Wang, Shuo-Qi Li, Cheng-Bao Liu, Fen Zhang, Wei-Xiang Sun, Rong-Chang Zeng (2026). In vitro degradation and photo-stimulated antibacterial activity of 2D-co-3D MOFs coating on AZ31 magnesium alloy. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3722-3
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Frequently Asked Questions
What is the long-term stability of the MOF coating under physiological conditions, and how does the degradation rate compare to uncoated AZ31?
The study reports corrosion current density reductions of three orders of magnitude for the two-step coating, indicating significantly improved barrier properties. However, long-term in vitro degradation data beyond initial polarization tests are not provided. The MOF coating degrades over time, releasing Zn2+, which may affect local pH and biocompatibility. Further immersion tests in simulated body fluid are needed to quantify degradation rates and ion release profiles.
How does the photothermal conversion efficiency translate to in vivo temperature rise, and what is the risk of thermal damage to surrounding tissue?
With efficiencies of 44.01% and 48.57%, the coatings can generate significant heat under 808 nm laser irradiation. The actual temperature rise depends on laser power density and exposure time. The study does not report in vivo temperature data. Typically, maintaining tissue temperature below 45°C is critical to avoid thermal damage. Optimization of laser parameters is necessary to achieve antibacterial efficacy while ensuring safety.
What is the mechanism of Zn2+ release from the MOF coating, and does it correlate with the degradation of the coating?
Zn2+ is released as ZIF-8 degrades in aqueous environments, particularly under acidic conditions. The study indicates that different fabrication methods lead to different Zn2+ sources and MOF structures, affecting release kinetics. The antibacterial effect is attributed to contact killing by Zn2+, but the exact release rate and its impact on cell viability need further quantification. The corrosion protection is partly due to the physical barrier, but as the coating degrades, protection diminishes.
How does the one-step (OS) coating achieve higher photothermal efficiency (48.57%) compared to the two-step (44.01%)?
The higher efficiency likely results from a more uniform distribution of PD within the MOF matrix, enhancing light absorption and heat conversion. The OS method may produce a more integrated 2D-co-3D structure with better interfacial contact, reducing heat loss. However, the study does not provide detailed structural characterization to explain the difference. Further analysis of morphology and composition is needed.
What are the scalability and cost implications of fabricating these MOF coatings on industrial-scale Mg alloy implants?
The fabrication involves in situ growth on MAO-coated alloys, which is a multi-step process. The one-step method is simpler and may be more scalable. However, the use of organic linkers and solvents could increase cost and require careful handling. The study does not provide cost analysis. For clinical translation, cost-effectiveness and reproducibility must be demonstrated.
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