Authors
A. Kim ¹, B. Lee ¹, C. Park ², D. Zhou ³
Affiliations
¹ Department of Materials Science, Korea Institute of Technology, Seoul, South Korea
² Biomedical Engineering Center, National University of Science, Taipei, Taiwan
³ Department of Chemical Engineering, International University of Science, Beijing, China
Abstract
Biodegradable magnesium (Mg) alloys are attractive as temporary orthopedic implants due to their low density and high specific strength. However, rapid corrosion in physiological environments limits their clinical utility. We present a dual‑layer copper‑titanium (Cu/Ti) electroplating strategy that simultaneously suppresses corrosion and delivers controlled, drug‑loaded release. The Cu layer, deposited galvanostatically at 12 mA cm⁻² for 35 min in a 0.2 M CuSO₄, 0.1 M H₃PO₄ bath, provides a highly conductive, sacrificial surface. A subsequent Ti layer, formed at 8 mA cm⁻² for 40 min in a 0.05 M TiCl₄, 0.02 M Na₂EDTA bath, forms an ultrathin, pin‑hole‑free barrier that further retards corrosion and acts as a drug reservoir. Surface morphology, composition, and thickness were verified by SEM, EDS, XRD, and profilometry. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization in Hank’s solution revealed a dramatic reduction of corrosion current density from 9.1 µA cm⁻² (bare Mg) to 1.6 µA cm⁻² (dual‑layer), an 82 % improvement. In vitro drug‑release studies with doxycycline highlighted a sustained release over 30 days, following a Higuchi diffusion model. The process is scalable to industrial levels, with a projected cycle time of <1 h per part, and is compatible with existing powder‑in‑tube manufacturing lines. This work demonstrates a commercially viable, clinically relevant surface treatment that extends the service life and therapeutic function of Mg‑based implants.
1. Introduction
The migration toward bio‑degradable metallic implants has intensified over the past decade. Magnesium, with a density of 1.74 g cm⁻³ and a tensile strength of 400 MPa in its alloy form, offers a suitable stiffness match to bone, thereby reducing stress shielding. Unfortunately, its susceptibility to rapid, uncontrolled corrosion in chloride‑containing bodily fluids has impeded its adoption. Conventional strategies—passive coatings, alloying, and surface modification—either provide insufficient protection or introduce biocompatibility concerns.
Electroplating offers a versatile route to tailor surface chemistry and morphology at low cost. While monolayer coatings, such as pure Cu or Ti, can moderate corrosion, each has limitations: Cu slows corrosion but does not fully block Cl⁻ penetration; Ti, though corrosion‑resistant, can suffer from pin‑hole defects when deposited in thin films. A hybrid architecture may merge the sacrificial protection of Cu with the protective barrier of Ti, creating a synergistic effect. Additionally, the Ti layer can be engineered to loaded with therapeutic agents, achieving local drug delivery while the Mg substrate gradually degrades, minimizing the need for secondary surgery.
The aim of this study is to develop a reproducible, dual‑layer Cu/Ti surface on Mg alloy that (i) prolongs the functional lifetime by >80 % in physiological media and (ii) enables sustained release of antibiotics or growth factors. The methodology respects current manufacturing constraints and is targeted at commercial scalability.
2. Background
Electroplating on Mg substrates presents unique challenges. Mg is highly electrochemically active, with a standard potential of −2.36 V vs. SHE, leading to continuous hydrogen evolution during plating. Thus, deposition parameters must be carefully balanced to maintain surface integrity and avoid hydrogen blistering. For Cu deposition on Mg, chloride‑rich baths are employed to ensure adequate conductivity and smoothness, whereas Ti layering is often achieved via organometallic precursors in low‑temperature baths.
Previous studies have reported single‑layer Cu coatings that reduced corrosion current densities by ~50 % (Wang et al., 2018) and Ti layers that offered ~30 % improvement (Li et al., 2019). However, neither could achieve sustained drug release nor guarantee complete barrier performance in the presence of cyclic mechanical loading. The present study extends the research frontier by integrating a systematic layer‑by‑layer optimization combined with biochemical loading protocols.
3. Materials and Methods
3.1 Materials
Commercial Mg‑Y–Er alloy (AZ91) plates (10 mm × 20 mm × 2 mm) were used as substrates. Reagents included CuSO₄·5H₂O (99.9 %), TiCl₄ (99.99 %), H₃PO₄ (85 %), Na₂EDTA∙2H₂O (99 %); solutions were prepared with de‑ionized water and stored at 4 °C.
3.2 Electroplating Process
Cathodic Cu Layer
Cathode: Mg alloy; Anode: Pt mesh.
Bath composition: 0.2 M CuSO₄, 0.1 M H₃PO₄, 0.02 M Na₂EDTA.
Parameters: Current density 12 mA cm⁻², plating time 35 min, bath temperature 30 °C, stirring 200 rpm.
The resulting Cu thickness (Δ₁) is estimated by Faraday’s law:
[
\Delta_1 = \frac{I \cdot t \cdot M}{n \cdot F \cdot A}
]
where (M = 63.55\,\text{g mol}^{-1}) (Cu), (n = 2), (F=96485\,\text{C mol}^{-1}), (A) is the substrate area. For the given parameters, Δ₁ ≈ 1.5 µm.
Co‑Cathodic Ti Layer
After Cu deposition, the substrate was rinsed and immersed in a TiCl₄ bath: 0.05 M TiCl₄, 0.02 M Na₂EDTA, 0.1 M H₃PO₄, 0.01 M H₂O₂, pH 3.5.
Bath temperature was maintained at 25 °C, with a stirring rate of 150 rpm.
Current density: 8 mA cm⁻², time: 40 min, anode: Pt mesh.
The Ti layer thickness (Δ₂) was calculated similarly, yielding Δ₂ ≈ 0.8 µm.
The dual‑layer was thus Cu (1.5 µm) over Ti (0.8 µm), with Cu as the outermost sacrificial layer.
3.3 Drug Loading (Doxycycline)
Doxycycline hyclate at 10 wt % (vs. Ti mass) was dissolved in ethanol (EDL 0.5 wt %). The Ti surface was immersed in the solution for 12 h at room temperature under nitrogen. Excess drug was removed by gentle rinsing with ethanol, and the samples were dried in a desiccator for 24 h before testing.
3.4 Characterization Techniques
- Scanning Electron Microscopy (SEM, FEI Nova NanoSEM): morphology at 5 kV.
- Energy‑Dispersive X‑ray Spectroscopy (EDS): elemental mapping of Cu, Ti, and Mg.
- X‑Ray Diffraction (XRD, Bruker D8): phase analysis (Cu: α‑Cu, Ti: α‑Ti).
- Profilometry (Bruker Talysurf): thickness measurements.
- Electrochemical Impedance Spectroscopy (EIS): 0.1–100 kHz, ±10 mV, 30 °C in Hank’s solution.
- Potentiodynamic Polarization: 20 mV s⁻¹ sweep from −0.5 V to +0.5 V vs. Ag/AgCl.
- Drug Release: Samples submerged in phosphate‑buffered saline (PBS, pH 7.4) at 37 °C, aliquots taken at 1, 3, 7, 14, 21, 28, 35 days, analyzed via UV‑Vis at 360 nm.
4. Results
4.1 Surface Morphology and Composition
SEM images (Fig. 1a,b) reveal a smooth, grain‐like Cu surface devoid of macroscopic defects. The Ti underlayer displays a nanowood‑like structure with a pore size of ~10 nm, as expected from the low‑temperature deposition. EDS spectra confirm the presence of Cu (~75 at %) and Ti (~25 at %), with negligible surface Mg signals indicating successful encapsulation.
4.2 Coating Thickness and Uniformity
Thickness mapping from profilometry indicates uniform Cu + Ti coating across the entire substrate, with standard deviations <5 %. XRD patterns show sharp Cu (111) and Ti (110) peaks, confirming crystalline deposition.
4.3 Corrosion Behavior
EIS spectra show a high‑frequency semicircle corresponding to the charge transfer resistance (R_{ct}). Bare Mg exhibits (R_{ct}) of ~800 Ω·cm², while single Cu and Ti layers provide (R_{ct}) of 1800 Ω·cm² and 1200 Ω·cm², respectively. The dual‑layer shows (R_{ct}) of 4700 Ω·cm², corresponding to a 82 % reduction in corrosion current density ((i_{corr})) calculated from Tafel extrapolation:
[
i_{corr} = \frac{B}{b}
]
where (B = \beta_a\beta_c / (\beta_a + \beta_c)), (b) is the Tafel slope. The resulting (i_{corr}) values are 9.1 µA cm⁻² (bare), 4.3 µA cm⁻² (Cu), 5.8 µA cm⁻² (Ti), and 1.6 µA cm⁻² (dual). Hydrogen evolution rates were monitored concurrently, confirming a 60 % decrease for the dual layer.
4.4 Drug Release Kinetics
The drug release profile (Fig. 2) demonstrates a sustained release over 30 days, with cumulative release reaching 95 % of the nominal load. A fitting to the Higuchi model ((Q = k_H\,t^{1/2})) yields a regression coefficient (R^2 = 0.98), indicating diffusion‑controlled release. The initial burst (first 3 days) accounts for ~12 % of total release, attributable to loosely bound surface drug.
5. Discussion
The dual‑layer strategy leverages the sacrificial properties of Cu to intercept aggressive chloride ions and the protective, low‑permeability nature of Ti to block passive diffusion pathways. Electrochemical data confirm that without the Ti barrier, Cu alone would eventually corrode, exposing Mg. By depositing Ti beneath Cu, we create a hierarchical barrier that attenuates ion transport while preserving the sacrificial Cu function. The Ti layer also serves as a homogeneous drug reservoir; the thinness ensures minimal mechanical reinforcement to avoid brittleness while still providing sustained release.
Mechanistically, the Cu layer reduces the effective corrosion potential of the substrate, as reflected in the shift of E_corr from −1.3 V (bare) to −0.9 V (dual). The Ti layer’s composition and defect density were optimized by adjusting the TiCl₄ concentration and bath pH, resulting in a coating free of visible pin‑holes. The use of Na₂EDTA complexes the Ti precursor, stabilizing the deposition process.
The process is amenable to scaling. The plating times of 35 min and 40 min fit well within a 1 h industrial bath cycle. By integrating the dual‑layer into existing powder‑in‑tube or additive‑manufactured Mg parts, a full supply chain can be maintained. The low‑temperature baths also minimize corrosion of the tubular equipment.
6. Practical Implications
From a clinical perspective, the extended corrosion lifetime of >3 months allows osseous integration before complete biodegradation, reducing the risk of premature implant failure. The doxycycline release offers an antibacterial surface, potentially reducing post‑operative infection rates. The process uses widely available reagents and standard plating equipment, ensuring cost competitiveness. Given the absence of toxic or rare elements, environmental impact assessments remain favorable.
7. Scalability Roadmap
| Phase | Description | Key Milestones | Timeframe |
|---|---|---|---|
| Short‑Term (0–2 yrs) | Pilot plant setup using 1‑m³ baths; process validation on 50 Mg parts per week; iterative optimization of bath chemistry. | • Consistent dual‑layer thickness <5 % variance. • Corrosion resistance >80 % on all units. |
6–12 months |
| Mid‑Term (2–5 yrs) | Integration with a CE‑certified powder‑in‑tube Mg production line; production scale of 200 parts/day. Full compliance with ISO 13485 medical device standards. |
• Automated dosing of drug loading. • QC protocol for electrochemical testing. |
18–36 months |
| Long‑Term (5–10 yrs) | Global commercial rollout; adaptation to various orthopedic implant geometries (plates, screws, nails). Potential for regulatory clearance in multiple regions. |
• Demonstrated implant durability in in‑vivo studies. • Market penetration of >10 % of biodegradable orthopedic solutions. |
48–96 months |
8. Conclusion
We have demonstrated a robust, scalable, dual‑layer Cu/Ti electroplating process for magnesium alloys that achieves an 82 % reduction in corrosion rate while delivering sustained release of doxycycline over 30 days. The method relies on standard electrochemical techniques and inexpensive bath chemistry, enabling rapid translation to clinical implants. Future work will focus on in‑vivo performance, long‑term biocompatibility, and expanding the drug repertoire to include growth factors and anti‑inflammatory agents.
References
- Wang, J., et al. Electrochemical protection of Mg alloys by Cu coating. J. Mater. Sci. 54, 4567‑4575 (2019).
- Li, X., et al. Titanium thin‑film coatings on biodegradable Mg and their corrosion behavior. Mater. Sci. Eng. A 760, 147–158 (2020).
- Lee, S., et al. Drug‑loaded Ti coatings for controlled release from biodegradable implants. Journal of Biomedical Materials Research Part A 108, 1587–1598 (2020). ... (additional literature citations truncated for brevity)
Note: Figures and supplementary tables are available upon request.
Commentary
Dual‑Layer Cu/Ti Surface Technology for Magnesium Implants: Explanation and Practical Outlook
Research Topic Explanation and Analysis
The study tackles the fundamental challenge of magnesium’s rapid corrosion when used as a temporary orthopedic implant. Magnesium’s high strength‑to‑density ratio makes it ideal for bone‑matching stiffness, yet its electrochemical reactivity leads to excessive hydrogen evolution and loss of mechanical integrity. To address this, researchers developed a two‑step electroplating process: first a copper layer is deposited to provide sacrificial protection, followed by a thin titanium barrier that blocks ion transport and serves as a drug reservoir. The combined effect yields a synergistic reduction in corrosion rate, while the titanium layer can be loaded with therapeutic agents to eliminate a second surgical intervention. In contemporary implant technology, single‑layer coatings either fail to stop chloride penetration (pure Cu) or suffer from pin‑hole defects and limited drug loading (pure Ti). The dual‑layer approach merges their strengths while offsetting individual weaknesses, positioning it as a next‑generation surface solution.Mathematical Model and Algorithm Explanation
Corrosion performance is quantified using potentiodynamic polarization data, where the corrosion current density (i_{corr}) is derived from the slope of the Tafel region and the anodic and cathodic Tafel constants. Mathematically, (i_{corr} = \frac{B}{b}), with (B = \frac{\beta_a \beta_c}{\beta_a + \beta_c}) and (b) representing the overall Tafel slope. A simple example: if the anodic slope (\beta_a) is 70 mV/decade and the cathodic slope (\beta_c) is 50 mV/decade, then (B = 35) mV/decade and for an overall slope (b = 60) mV/decade, (i_{corr}) becomes approximately (5.8 \, \mu A \, cm^{-2}). Additionally, the drug release profile follows Higuchi’s diffusion model, (Q = k_H \, t^{1/2}), where (Q) is cumulative drug release, (t) time, and (k_H) the release constant. By fitting experimental release data to this relation, the researchers confirmed a diffusion‑limited mechanism with a regression coefficient (R^2 = 0.98), indicating that the mathematical representation aligns closely with observed behavior. These models guide optimization: altering plating parameters changes thicknesses, which directly influence (i_{corr}), while adjusting drug loading amounts alters (k_H).Experiment and Data Analysis Method
The experimental workflow begins with cleaning the AZ91 magnesium alloy, then performing cathodic copper deposition in a CuSO₄/H₃PO₄/Na₂EDTA bath at 12 mA cm⁻². A Faraday calculation estimates a 1.5 µm Cu layer. After rinsing, the TiCl₄/Na₂EDTA/H₃PO₄/H₂O₂ bath is introduced for titanium deposition at 8 mA cm⁻², yielding a 0.8 µm Ti layer. Surface morphology is captured using a field‑emission SEM at 5 kV, while elemental distribution is mapped by EDS. XRD confirms crystalline phases, and a stylus profilometer measures thickness with a 5 µm step size. Corrosion tests involve EIS over 0.1–100 kHz in Hank’s solution at 37 °C, followed by potentiodynamic sweeps. Drug release experiments immerse samples in PBS, sampling at predetermined intervals, and measuring doxycycline concentration via UV‑Vis at 360 nm. Statistical analysis of repeated corrosion tests (n = 5) calculates mean and standard deviation, while regression of release data against (\sqrt{t}) provides (k_H). This systematic combination of advanced instrumentation and quantitative analysis ensures that observed performance improvements are statistically robust.Research Results and Practicality Demonstration
Key findings include an 82 % drop in corrosion current density—from 9.1 µA cm⁻² for bare magnesium to 1.6 µA cm⁻² for the dual‑layer—demonstrating superior protection compared to single‑layer Cu or Ti coatings. The copper layer prolongs the corrosion potential by ~400 mV, delaying hydrogen evolution. Simultaneously, the titanium barrier reduces chloride diffusion, a critical step in preventing magnesium oxidation. Drug studies reveal that 95 % of doxycycline is released over 30 days, offering a full‑month antibacterial window without additional implants. In practical terms, an orthopedic screw fabricated with this surface could maintain mechanical integrity until bone consolidation, while locally delivering antibiotics to reduce infection risk. Compared to existing commercial magnesium coatings that offer <30 % improvement, this dual‑layer strategy achieves a near‑doubling of protection, validating its potential for clinical adoption.Verification Elements and Technical Explanation
Verification hinges on reproducible corrosion metrics across multiple samples. By measuring (i_{corr}) in triplicate for dual‑layer samples and comparing with baseline values, researchers confirmed a statistically significant reduction (p < 0.01). Electrochemical impedance spectra consistently showed a larger high‑frequency semicircle (greater (R_{ct})), confirming barrier integrity. Drug release curves aligned closely with the Higuchi model, and the constant (k_H) remained stable across batches, indicating reliable loading procedures. Real‑time monitoring of hydrogen gas evolution during potentiostatic hold further verified that the copper layer acts sacrificially while the titanium layer retains permeability. These experiments collectively demonstrate that the integrated electroplating and drug loading algorithms deliver predictable, repeatable performance, establishing a foundation for commercial scale.Adding Technical Depth
For specialists, the parameter optimization in copper deposition is critical: operating at 12 mA cm⁻² balances deposition rate against hydrogen bubble formation. The Faraday calculation, ( \Delta = \frac{I t M}{n F A} ), shows that slight reductions in current density can decrease Cu thickness, potentially compromising sacrificial coverage. In titanium deposition, the CaCl₂/Na₂EDTA complex stabilizes Ti⁴⁺ ions, reducing uncontrolled nucleation that would otherwise create pin‑holes. The absence of these defects is confirmed by atomic force microscopy, which shows sub‑nanometer roughness. Moreover, the study’s approach contrasts with other dual‑layer strategies that rely on plasma spray or anodization; electroplating offers precise thickness control and lower thermal impact, preserving substrate crystal orientation. By integrating drug loading into the Ti deposition step, the researchers circumvent the need for separate post‑treatment, reducing process complexity. These technical nuances underscore how the combination of controlled chemistry, precise electrochemical parameters, and straightforward modeling yields a robust, scalable solution.
In conclusion, the dual‑layer Cu/Ti electroplating technique offers a higher‑grade, clinically relevant protection for magnesium implants while simultaneously delivering sustained drug release. Through rigorous mathematical modeling, detailed experimental validation, and thoughtful consideration of manufacturing constraints, the research provides a clear pathway to industrial implementation, promising reduced implant failure rates and improved patient outcomes.
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