Enhanced bioactivity and degradation behavior of zinc via micro-arc anodization for biomedical applications

1 Introduction

Biodegradable metals (BMs) are proposed to provide sufficient mechanical support and unique bio-functions during the healing process of tissues and then to be fully absorbed by the human body [1,2]. In the last few decades, zinc-based alloys have gained increasing attention attributed to their inherent properties, for instance, a suitable degradation rate for clinical demand and superior biocompatibility. Notably, Zn²⁺ is the main corrosion product in the physiological environment; also, zinc is one of the essential trace elements and mostly exists in muscle and bone, playing a significant role in biological functions. Moreover, Zn is necessary for the maintenance of bone health, and an appropriate concentration of Zn can stimulate bone tissue growth [3]. However, Zn²⁺ shows a biphasic effect on cytotoxicity, low dose promotes cell activity, and high dose suppresses the cytocompatibility [4].

Surface modification is a facile approach to fabricate a protective layer and thus restrain the burst release of metallic ions [5]. Micro-arc oxidation (MAO) is a relatively simple, effective, and environmentally friendly process that has been well-established for value metals and also explored for Mg and Zn [6,7]. This electrochemical process is carried out in a suitable electrolyte at high anodic potential and leads to the formation of a double-layer oxide structure on the substrate with a dense bottom layer and a porous top layer. However, the compact bottom layer is absent for zinc after MAO treatment [8]. Moreover, oxygen release (from water oxidation) and discharge lead to the removal of the coating. These factors contribute to a limited protective effect of MAO coatings on zinc substrate [9]. Yuan et al. [8] prepared MAO coating on high pure Zn and investigated the cytotoxicity, nevertheless, MAO coating promoted the corrosion rate as compared with bare substrate. Shi et al. [10] studied MAO-PLA coating on zinc alloy, and the composite coating improved biocompatibility and limited corrosion resistance.

Liu et al. [11] reported the in-site generated phosphate and studied its function in Zn stabilization. Su et al. [12] prepared zinc phosphate coating on zinc and showed significantly enhanced biocompatibility and controlled degradation rate. Chakraborty et al. [13] studied the phosphate layer, and the immersion test in simulated body fluid (SBF) demonstrated excellent osteoconduction capability. Phosphates exhibit superior corrosion resistance for zinc-based alloys; thus, the addition of phosphate in the anodization electrolyte is studied and found to enhance the degradation behavior of MAO coating; hence, the high concentration of Zn²⁺ can be suppressed. Electrochemical tests as well as immersion measurements (21 days) are applied to reveal the protective effect and bioactivity of the MAO coatings in view of orthopedic applications.

2 Materials and characterization

Pure zinc sheet (99.99%) was cut into samples with a size of 15 mm × 15 mm × 5 mm; the samples were mechanically abraded with SiC sandpaper (#2000) before use, then ultrasonically washed with ethanol and finally dried in hot air. An MAO equipment (WHD-30, Harbin, China) was applied for this reaction. Pure zinc was used as an anode, and a stainless steel sheet was used as a cathode. Circulating water was utilized to cool down the reaction cell. The MAO process was carried out in an aqueous electrolyte consisting of Na₂SiO₃ (15 g·dm⁻³), NaOH (4 g·dm⁻³), and Na₃PO₄ (2, 4, 5 g·dm⁻³). The applied duty cycle, pulse frequency, voltage, and oxidation time were 500 Hz, 10%, 250 V, and 3 min, respectively. All samples were cleaned with deionized water three times and dried with hot air.

Surface morphology (top view and cross-section) and composition of the MAO coating were characterized with scanning electron microscopy (SEM, Pro X, Phenom, Netherlands) with energy-dispersive X-ray spectroscopy (EDS). Phase information was determined by X-ray diffraction (XRD, XRD-6000, Shimadzu, Japan). Corrosion behavior was tested in SBF [14] at room temperature by electrochemical impedance spectroscopy (with 10 mV amplitude in a frequency range from 10⁴ to 10⁻² Hz). with an electrochemical workstation (PGSTAT302N, Auto-lab, Netherlands). Long-term immersion measurements were carried out in SBF at 37°C in an incubator for 21 days. The composition of SBF is listed in Table 1.

3 Results and discussion

Figure 1 depicts the top view and cross-section morphology of the MAO layer obtained with different electrolyte concentrations of Na₃PO₄ and the composition of each sample, as obtained with EDS and XRD analysis. Typical discharge channel architecture is observed after the MAO process for all cases [10]. For the MAO coating formed in the electrolyte with no phosphate addition (Figure 1a), the thickness is approximately 2 µm, with the voids in the coating. Notably, the thickness of the coating increases with the addition of Na₃PO₄. For the electrolyte with Na₃PO₄, the thickness of the MAO coatings increased with the addition of Na₃PO₄. However, with the addition of Na₃PO₄ (5 g·L⁻¹ or more), the size of the discharge channel increased; also, the brittleness of the coating was promoted, leading to cracks and partial removal of the coating as observed in Figure 1d.

Figure 1

Surface and cross-section morphology of MAO coating on Zn with different electrolyte concentrations of Na₃PO₄: (a) 0 g·L⁻¹, (b) 2 g·L⁻¹, (c) 4 g·L⁻¹, (d) 5 g·L⁻¹, (e) EDS. and (f) XRD.

Figure 1e shows the EDS data (with mass concentration) after the MAO process and annealing treatment. The chemical constitution of anodized samples mainly consists of Zn, O, and Si, with traces of P detected for coating formed in phosphate-containing electrolytes. Figure 1f shows the XRD patterns for bare Zn and for the MAO-coated samples. The MAO coatings mainly consist of ZnO, in agreement with other works [8,10].

In order to study the influence of phosphates on the protective effect of the obtained coatings, electrochemical measurements were carried out in SBF. Figure 2 shows the results of pure zinc and the MAO coatings obtained with different concentrations of Na₃PO₄. The semicircle radius of the capacitive loop in Nyquist plots (Figure 2a) increases with the addition of phosphates; however, for the highest concentration of 5 g·L⁻¹, the protective effect decreases as compared to 4 g·L⁻¹. This tendency is also reflected in Figure 2b and c: the impedance module shows the highest value for 4 g·L⁻¹, indicating the highest corrosion resistance in SBF. Figure 2d shows the potentiodynamic polarization curves, indicating that the MAO coatings decrease the current density in both cathodic and anodic branches, and the increasing corrosion protection effect is again observed for coatings formed in the presence of phosphate in the anodization electrolyte. This can suppress a rapid release of Zn²⁺ in the biomedical application of Zn, and thus, MAO coatings could decrease the cytotoxicity of pure zinc.

Figure 2

Electrochemical measurements for pure zinc and MAO coating with different concentrations of Na₃PO₄: (a) Nyquist plots, (b) impedance spectroscopy, (c) phase angle plots, and (d) potentiodynamic polarization curves.

As mentioned, the MAO layers on zinc alloys have been previously studied. Typically, these coatings can provide only limited corrosion protective effect. In the present study, a dense and better adhesive coating was fabricated by adding phosphate to the anodizing electrolyte, leading to enhanced corrosion protection of pure zinc.

According to the top view and cross-section morphology, together with electrochemical measurements, the electrolyte containing 4 g·L⁻¹ of Na₃PO₄ leading to the best coating properties was used to prepare MAO-P samples for immersion tests. Figure 3 shows SEM images of bare zinc, MAO, and MAO-P samples after immersion test for 21 days. Aggregated particles are observed on the surface of all three group samples; with the prolongation of the test period, more precipitations are observed. For MAO and MAO-P samples, a more uniform distribution of the precipitations is observed. Specifically, in the presence of phosphates in the MAO electrolyte, more precipitations are formed on the coatings. Table 2 shows the EDS data after the immersion test at different intervals. More Ca–P is detected on MAO-P, as compared to pure zinc and MAO samples, indicating superior bioactivity that is promising in view of orthopedic applications. The increasing amount of Ca–P precipitation on the MAO coatings formed in the presence of phosphate in the electrolyte may result from the P species incorporated in the oxide layers, possibly providing nucleation sited for Ca–P precipitation.

Figure 3

Surface morphology after immersion tests: (a)–(c) pure zinc for 6, 14, and 21 days; (d)–(f) MAO for 6, 14, and 21 days; and (g)–(i) MAO-P for 6, 14, and 21 days.

4 Conclusion

In this work, a phosphate-enhanced MAO layer was fabricated on pure zinc. This dense oxide layer shows fewer defects and better adhesive strength than MAO coatings obtained on Zn in the absence of phosphate in the anodization electrolyte. The thickness of the optimized layer is approximately 6 µm. The MAO-P coating can regulate the degradation behavior and improve the bioactivity of Zn in view of orthopedic applications.