Study on the corrosion resistance and anti-infection of modified magnesium alloy

Abstract

In this paper, a low-cost and multifunctional hydroxyapatite (HA)/pefloxacin (PFLX) drug eluting layer is coated on magnesium (Mg) alloy through a simple hydrothermal and dip process. The drug PFLX could provide effective prevention for bone infection and inflammation due to its broad-spectrum antibacterial property. And HA would promote the growth of new bone and further improve the biocompatibility of implants. Besides, both PFLX and HA exhibits excellent corrosion protection for Mg alloy substrate. This coating is of great value for improving the application of Mg alloy as biomaterials.

Keywords

Magnesium alloy biomaterials anti-corrosion anti-infection biocompatibility

1. Introduction

As potential materials for orthopedic implants and vascular stents, magnesium (Mg) alloy have aroused enormous interest for their desirable features such as low density, excellent mechanical properties and biocompatibility [1–4]. However, Mg alloy are highly susceptible to micro-galvanic corrosion which may induce the stress corrosion cracking in a physiological environment and greatly limited their application as biomaterials. Except poor anti-corrosion, another challenge for Mg alloy as medical implant is the high risks of bacterial growth on the implant. The formation of bacterial biofilm at the tissue/implant interface may give rise to sustained infections and inflammation [5,6]. So it’s crucial to fabricate multifunctional coating on Mg alloy biological scaffold to improve the property of anti-corrosion, biocompatibility and anti-infection etc.

Various coatings have been constructed based on the aim of improving the corrosion resistance and biocompatibility of Mg alloy [7–14]. For example, hydroxyapatite [HA, Ca₁₀(PO₄)₆(OH)₂] is the major mineral constituent of bone matrix, which possesses good biocompatibility and have been used to induce and promote the formation of new bone for its favorable osteoconductivity and osteoinductivity [10–14]. Besides, calcium phosphate films have been widely studied as a modified membrane layer to increase the corrosion resistance of Mg alloys. Although these coatings can effectively improve the corrosion resistance and biocompatibility of Mg alloy, the anti-infection Mg alloy in the implantation process has not been effectively resolved.

As mentioned above, during the degradation process of coating treated magnesium-based implant materials, infections and inflammation may produce adverse tissue reaction. An ideal technique to combat implant-associated infections is target drug delivery. The method adhering antibiotics directly on the surface of implants has the advantage that the release process is localized and the antibiotics are delivered exactly where they are needed. Pefloxacin (PELX) is a sort of medication belonging to the antibiotic group of fluoroquinolones. It’s low-cost and possesses broad-spectrum antibacterial property [15], which would offer effective prevention for bone infection and inflammation. Furthermore, PFLX also can be used as a corrosion inhibitor of metal implant materials in the corrosive medium.

In summary, the design and development of the Mg alloy medical material with good corrosion resistance, biocompatibility and anti-infection are the main objectives of Mg alloy as a medical material. Based on this goal, multi-functional HA/PFLX drug-eluting coating loaded on Mg alloy surface was designed in this study. The densities of Mg alloys closer to cortical bone provide mechanical properties for bone-bearing. In the drug-eluting coating, PFLX serves as the drug and HA as the drug delivery medium. PFLX releasing from HA could provide effective prevention for bone infection and inflammation. Ca, P elements in HA would promote the growth of new bone and further improve the biocompatibility of implants. What’s more, HA in coordination with PFLX, greatly improve the anti-corrosion of Mg alloy implant.

2. Materials and methods

2.1. Fabrication of HA/PFLX composite coating

The pretreatment of Mg alloy is referred to our previous research [16]. The biomimetic solution (0.05 M) was prepared by dissolving EDTA-Na₂, Ca(NO₃)₂, K₂HPO₄ (Aladdin reagent Co., Ltd.) in sequence, and the pH value was adjusted to 5.5. The AZ91 Mg alloy was soaked in the solution for 6 h at 95°C for forming HA coating on the surface. The specimens were then rinsed by distilled water and air dried.

Meanwhile, 1 mg/mL, 10 mg/mL and 100 mg/mL PFLX aqueous solution were prepared, respectively. The samples coated with HA were dipped into PFLX solution for 50 s, donated as HA/PFLX1, HA/PFLX10 and HA/PFLX100, respectively. Then were taken out and dried at room temperature.

2.2. Characterization and tests

Both X-ray diffractometer (XRD; Purkinje General Instrument XD-3) and Fourier transform infrared spectroscopy (FTIR) were carried out to characterize the chemical composition of the coatings. Microstructure and surface morphology of the samples were also observed by a scanning electronic microscope (SEM; HITACHI S-4800). The corrosion resistance of modified samples was evaluated by potentiodynamic polarization test and electrochemical impedance spectroscope (EIS), which were performed in Hank’s solution at 25°C. The instrument was a CS350 system (CorrTest Co. Ltd., Wuhan, China) with a classical three-electrode cell: a specimen with an exposed area of $1 {cm}^{2}$ as a working electrode, a saturated calomel electrode (SCE) as a reference electrode and platinum as a counter electrode. The antibacterial activity was evaluated by the bacterial growth inhibition zones formed around the sample. Samples (10 mm × 10 mm) were placed on the surface of a TSA plate with agar seeded by Escherichia coil (E. coil BL21). The agar plates were then incubated at 37°C for 48 h.

3. Results and discussion

Two major problems for Mg alloy biomaterials are the poor corrosion resistance and infection associated with implantation. In order to solve the problems above, a multifunctional HA/PFLX drug eluting layer was designed on Mg alloy surface. Figure 1 shows the concrete design concept. When the bare Mg alloy biomaterial was implanted into human body, Mg alloys suffer from the micro-galvanic corrosion and bacterial biofilm at the tissue/implant interface was formed. After applying HA protective coating, the corrosion activity of the bare Mg alloys substrate was delayed while the bacterial still existed. Next, we plan modified these protective coatings with antibiotic PFLX. This kind of composite coating could delay and control the degradation process of the bare alloy and, in addition, offer effective prevention for bone infection and inflammation.

Fig. 1.

Blueprint of fabrication of anti-corrosion and antibacterial HA/PFLX composite coating.

In order to investigate the improvemnt of anti-corrosion of Mg alloys substrate coated wirh HA/PFLX coating, electrochemical test was performed. Figure 2(a) depicts the potentiodynamic polarization curves for the bare Mg substrate and sample HA, HA/PFLX1, HA/PFLX10, HA/PFLX100 in Hank’s solution. The corrosion potential (E _corr) and corrosion current density (i _corr) derived from the potentiodynamic polarization are listed in Table 1. It can be seen that coated samples exhibit more excellent corrosion resistance than bare Mg substrate. Noticeably, among the HA/PFLX composite coatings, sample HA/PFLX10 exhibits lowest corrosion current density, slowest corrosion rate and highest corrosion potential, which may be attributed to the increasing concentration of PFLX, because corrosion inhibition effect of PFLX would provide a certain extent protection for coated sample. But as the concentration further increases, the acidity of PFLX will destroy the integrity of HA coating. So the optimum concentration of PFLX was identified as 10 mg/mL. Besides, the i _corr of sample HA/PFLX10 is approximately 50% of that of the sample HA and approximately 30% of that of the bare substrate, suggesting the good protection for Mg alloys. It is worthwhile to notice that the HA/PFLX provide better protection for Mg alloys than HA coating, which further verifies the corrosion inhibition performance of PFLX.

Table 1

Relative electrochemical parameters of potentiodynamic polarization of samples in SBF

Samples	i _corr (Amp/cm²)	E _corr (Volts)	r (mm/a)
Bare substrate	$9.55 \times 10^{- 6}$	$- 1.58$	$2.06 \times 10^{- 2}$
HA	$5.85 \times 10^{- 6}$	$- 1.39$	$1.26 \times 10^{- 2}$
HA/PFLX1	$4.67 \times 10^{- 6}$	$- 1.35$	$1.01 \times 10^{- 2}$
HA/PFLX10	$2.97 \times 10^{- 6}$	$- 1.36$	$0.64 \times 10^{- 2}$
HA/PFLX100	$1.44 \times 10^{- 5}$	$- 1.44$	$3.11 \times 10^{- 2}$

Fig. 2.

(a) Potentiodynamic polarization curves of the Mg substrate, HA coated sample, HA/PFLX coated samples in SBF; (b) Nyquist diagram of the bare Mg substrate, HA coated sample, HA/PFLX coated samples in SBF.

The Nyquist plots of different samples are presented in Fig. 2(b). It’s commonly believed that the diameter of the capacitive loop represents the polarization resistance of work electrode. As shown in Fig. 2(b), the HA coating exhibited the higher impedance value than bare Mg substrate, suggesting the HA coating has the good corrosion protection. Besides, with increasing PFLX, the impedance value of HA/PFLX increases first and then decreases. In accordance with the result of polarization test, sample HA/PFLX10 shows the highest impedance and can be acted as an effective barrier layer, supplying excellent corrosion protection for Mg alloy implant in physiological environment.

In order to detect the chemical composition of modified coating on Mg alloy, X-ray diffraction (XRD) was performed. Figure 3(a) and (b) display the XRD patterns of sample HA and HA/PFLX10, respectively. It can be clearly seen that the diffraction peaks of HA were detected at $2 θ = 23$ °, 34°, 47° and 53° (PDF No. 32-0176) in Fig. 3(a), demonstrating that HA was coated on the sample surface successfully. For the XRD pattern of sample HA/PFLX10, we cannot find any diffraction peaks of PFLX except HA, which perhaps owing to the little content of PFLX in composite coating.

Fig. 3.

XRD patterns of (a) HA coated sample (b) and HA/PFLX coated sample; (c) FT-IR spectra of PFLX, HA and HA/PFLX coated sample.

For verifying the existence of PFLX, Fourier transform infrared spectra (FTIR) were measured. Figure 3(c) depicts the FTIR spectrum of PFLX, HA and sample HA/PFLX. Compared with pure HA, new bands at $1740 {cm}^{- 1}$ , $1483 {cm}^{- 1}$ , $1263 {cm}^{- 1}$ , appeared in sample HA/PFLX. Refer to the FTIR spectrum of PFLX and their molecular structural formula, these new bands should be ascribed to the stretching vibrations of C=O, C–H and C–N in PFLX, respectively, suggesting PFLX was grafted on the surface successfully.

Figure 4 shows the surface morphologies of HA and HA/PFLX10 coating. It can be seen that there are large amounts of small microspheres located on the HA surface (Fig. 4(a)). The enlarged image indicates that the microspheres are composed of numerous nano-slices (Fig. 4(c)). After dipped with PFLX, it seemed that a layer of fog covered the HA surface (Fig. 4(b)). And the magnified image (Fig. 4(d)) showed that the nano-slices get dull and smaller, which can be explained by the tip dissolution effect of acidic PFLX to HA. Associated with the surface morphologies with electrochemical test results, it can be concluded that the improvement of corrosion resistance for HA/PFLX, not only duo to the corrosion inhibit performance of PFLX, but also for the more compact surface microstructure.

Fig. 4.

SEM images of HA coated sample (a) and HA/PFLX10 coated sample (b), (c) and (d) are the magnified images of (a), (b), respectively.

In order to figure out whether the PFLX exhibited antibacterial effects or not, Mg alloys substrate, HA coated sample and HA/PFLX10 coated sample (10 mm × 10 mm) were placed on the surface of a TSA plate with agar seeded by Escherichia coil (E. coil BL21). The agar plates were then incubated at 37°C for 48 h. Figure 5 shows the representative results of the qualitative evaluation of the antibacterial activity of different samples. It can be found that the obvious inhibition zone was formed surrounding the HA/PFLX coated sample, whereas such inhibition zone was not found around Mg substrate and HA coated sample, indicating an antimicrobial activity of HA/PFLX coated sample and PFLX is responsible for the antibacterial activity in the composite coating.

Fig. 5.

Representative images of antibacterial activity against Escherichia coil for Mg substrate (a), HA coated sample (b) and HA/PFLX10 coated sample (c).

4. Conclusion

In summary, a multi-functional HA/PFLX drug eluting coating was successfully fabricated on Mg alloy by a simple hydrothermal and dip process. The antibiotic PFLX used are relatively cheap and possesses broad-spectrum antibacterial property, providing effective prevention for bone infection and inflammation. The drug delivery, HA, would promote the growth of new bone and further improve the biocompatibility of implants. Meanwhile, both the drug and drug delivery medium exhibit excellent corrosion protection for Mg alloy substrate. This method greatly improves the potential application of Mg alloy as implant biomaterials.

Footnotes

Acknowledgement

This work was supported by the Fundamental Research Funds for the Central Universities (XDJK2014C011).

Conflict of interest

The authors have no conflict of interest to report.

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