Precipitation of Struvite on Pure Magnesium and in Vitro Degradation Behaviour

Rhys Walter1, Afrin Mehjabeen1, Timothy Gordon1, M. Bobby Kannan1 1Biomaterials and Engineering Materials (BEM) Laboratory

School of Engineering and Physical Sciences, James Cook University, Townsville, Queensland 4811, Australia Email: rhys.walter@my.jcu.edu.au

Abstract— Magnesium is a prime candidate for use as a biodegradable biomaterial for bone fracture repair. However, the degradation rate of magnesium under physiological conditions is too high to maintain mechanical integrity until complete bone healing, and as such requires a biocompatible coating to reduce the degradation rate. One method to achieve this is by biomimetically depositing calcium phosphates onto the magnesium substrate. However, this method is slow and dissolution of the magnesium may occur during the process. In this study, struvite (magnesium ammonium phosphate) was coated on pure magnesium using the dip coating method as a precursor to a biomimetically formed calcium phosphate coating. The in vitro degradation behaviour of the coated magnesium was tested using electrochemical methods. Electrochemical impedance spectroscopy (EIS) results showed a significant improvement in the polarisation resistance (RP) for the struvite coated samples when compared to the bare metal. The RP of the coated samples increased slightly after 3 h immersion in simulated body fluid, due to the deposition of calcium phosphates onto the struvite, and remained relatively stable for the remaining immersion period. This method may be suitable as a pre-treatment when biomimetically precipitating calcium phosphate onto magnesium.

Keywords-magnesium; biomaterials; Corrosion; Struvite

I. INTRODUCTION

Recently, there has been a growing interest into the development of biodegradable implants for bone fracture repair. A leading candidate material is magnesium, due to its biocompatibility, biodegradability and favourable mechanical properties [1]. However, magnesium degrades too rapidly under physiological conditions, causing potentially premature implant failure, as well as hydrogen gas build up in the surrounding tissue [2].

In order to improve the biomaterial performance, a coating can be applied to the magnesium substrate that reduces the

overall degradation rate. Several different coatings have been investigated, such as calcium phosphates [3, 4], PEO [5] and polymer coatings [6] but to date there have been no reported results sufficient for implant applications. Calcium phosphates are a particularly promising coating material, since they can be formed biomimetically onto magnesium [7, 8]. However, this process can take several days to produce a suitable coating, and magnesium dissolution during the coating may occur. As such, a favourable method would be to apply a precursor coating that enhances the precipitation of calcium phosphates onto the magnesium under physiological conditions.

Magnesium phosphates, such as struvite (MgNH4PO4·6H2O), are used as bone cements, and have been shown to be both biocompatible and biodegradable [9]. Further, they have been shown to both adhere well to bone and promote bone growth activity [10]. Ibasco et al. [10] precipitated struvite onto a magnesium sputtered titanium surface by immersing it in an ammonium dihydrogen phosphate coating solution. The struvite crystals formed rapidly, completely coating the surface within several minutes. It was also revealed that an amount of unreacted magnesium remained under the surface, indicating that the struvite layer protected the underlying magnesium from dissolution during the coating process. The proposed mechanism of this reaction was as follows [10]:

Mg + H2O → MgOH2 + H2 (1) MgOH2 + NH4H2PO4 + 4H2O → MgNH4PO4·6H2O (2)

It was also proposed that struvite will dissolve more rapidly than most calcium phosphates due to the solubility product of struvite at 37°C (KSP = ~30.2x10-14) being higher than calcium phosphates, such as hydroxyapatite (KSP = 10-53). However, the ionic release caused by struvite degradation in physiological conditions would cause precipitation of a calcium phosphate phase onto the magnesium substrate, via the following reaction [10]:

10Mg2+ + NH4

+ + 7PO43- + 2H2O + 10CaCl2 →

Ca10(PO4)6(OH)2 + NH4H2PO4 +10MgCl2 (3) Thus, while the struvite itself may only provide corrosion

resistance for a short period of time, the overall corrosion resistance may remain significant for longer immersion times due to the constant precipitation of calcium phosphates. This

study investigated the in vitro corrosion behaviour of struvite coated pure magnesium using electrochemical methods.

II. EXPERIMENTAL

In this study, a struvite coating was precipitated onto a high purity magnesium (composition outlined in Table 1) by dip coating in 0.5 M ammonium dihydrogen phosphate (ADP) solution, adjusted to pH 7.40 for 120 minutes. Prior to coating, samples were prepared by incremental grinding up to 2500 grit SiC paper, then polished using 1 µm alumina polish. Samples were ultrasonically cleaned with ethanol and air dried.

TABLE I. CHEMICAL COMPOSITION (WT. %) OF PURE MAGNESIUM

Element Al Fe Mn Zn Mg

Wt. % 0.0031 0.0072 0.0017 0.0013 Balance

The in vitro tests were performed in a buffered simulated

body fluid (SBF) at 37°C and pH 7.40, the composition of which can be found elsewhere [11]. Electrochemical tests were conducted using a potentiostat/frequency response analyser (Model: ACM Gill AC) and a three electrode system with the sample (0.785 cm2 exposed area) as the working electrode, a Ag/AgCl reference electrode, and platinum counter electrode. Potentiodynamic polarisation was performed at a scan rate of 0.5 mV/s. Electrochemical Impedance Spectroscopy (EIS) was done over the frequency range of 105 Hz to 10-2 Hz at 5 mV AC amplitude and modelled using ZSimpWin V. 3.21 software. Prior to testing, samples were immersed in SBF for 2 h to reach a stable open circuit potential. Fourier transform infrared (FTIR) spectroscopy analysis was done using a Perkin Elmer spectrum 100 FTIR spectrometer. The coating and post-degradation surfaces were analysed using a scanning electron microscope (SEM) (Model: Jeol JSM5410L).

III. RESULTS AND DISCUSSION

Coating thickness measurements showed the struvite to be a thickness of 8.9 ± 2.5 µm. Figure 1 shows the FTIR analysis of the struvite coated samples. There are distinct peaks at 750, 880, 1010 and 1470 cm-1, which is indicative of struvite [12].

Figure 2 shows the potentiodynamic polarisation plots for the bare and coated magnesium plots. The electrochemical data from these plots is shown in Table 2. The precipitation of the struvite shifted the corrosion potential (Ecorr) slightly in the noble direction from -1.71 to -1.64 VAg/AgCl. However, the breakdown potential was shifted slightly in the negative direction, as seen in Fig. 2. However, it can be seen that the corrosion current (Icorr) was significantly reduced, from 31.6 µA/cm2 to 5.5 µA/cm2, suggesting that the coating did significantly improve the corrosion resistance.

Figure 1. FTIR plot of the struvite coating

TABLE II. POTENTIODYNAMIC POLARISATION DATA

Sample Icorr (µA/cm2) Ecorr (VAg/AgCl) Ebd (VAg/AgCl)

Pure Mg 31.6 ± 2.6 -1.71 ± 0.11 -1.31 ± 0.09

Struvite Coating

5.5 ± 1.5 -1.64 ± 0.03 -1.43 ± 0.02

Figure 2. Potentiodynamic polarisation plots of the bare and struvite coated magnesium

EIS tests were done over an 8 h period to determine how the polarisation resistance (RP) changed over time. The Nyquist plots were modelled using the equivalent circuit R(Q(R(QR))), which has reported to be applicable to both bare magnesium alloys [13] and ceramic coatings [14]. Figure 3 compares the calculated RP values over time for both the bare metal and struvite covered samples.

Figure 3. RP values modelled from EIS results using an equivalent circuit

It can be seen that the RP of bare Mg increases from an initial resistance of ~1450 Ω.cm2 to ~1850 Ω.cm2 after 3 h due to passive effects. This value decreased over the immersion time, down to ~980 Ω.cm2 as the passive layer broke down. The RP of the struvite coating also increased from ~3500 Ω.cm2 at the initial 2 h immersion time, to ~4050 Ω.cm2 at 3 h immersion. It also showed a relatively stable resistance over the observed immersion time after reaching this peak, decreasing by only ~7 %.

Figure 4. FTIR spectrum of the struvite coating following 8 h immersion in

SBF

The solubility product of struvite (~30.2x10-14) is significantly higher than many other ceramic coatings, such as hydroxyapatite (10-53), and as such will degrade more rapidly in solution. However the release of ionic phosphates in the

calcium and hydroxide rich environment will cause the precipitation of calcium phosphates on to the surface, as shown in (3). FTIR analysis of the sample following 8 h immersion confirmed this mechanism. The spectrum shown in Figure 4 is indicative of a calcium phosphate, with the broad peak at ~1000 cm-1 corresponding to the phosphates [15].

Figure 5 shows the SEM images of the struvite coating prior to corrosion, and after 8 h immersion in SBF. It can be seen in Fig 5. (a) and (b) that there is a relatively even coating of struvite across the entire surface. Following immersion however, this structure appears to be somewhat degraded across the entire surface, with some areas of corrosion product build up around areas of localised attack. The pitting corrosion evident in Fig. 5 (c) suggests that struvite alone may not sufficiently protective for implant use, since pits can act as stress risers under mechanical load, increasing the chance of premature failure in service.

Figure 5. SEM images of the struvite coating (a), (b) immediately after coating (c), (d) after 8 h immersion in SBF

CONCLUSIONS

In vitro corrosion tests showed that the precipitation of struvite significantly improved both the corrosion current (from 31.6 µA/cm2 to 5.5 µA/cm2) and the corrosion resistance (from ~1450 Ω.cm2 to ~3500 Ω.cm2) of pure magnesium. Further, the resistance increased marginally after 3 h immersion due to the precipitation of calcium phosphates onto the surface as the struvite degraded. While SEM analysis revealed that there was some localised attack, struvite may be a suitable as a precursor coating for the biomimetic formation of a calcium phosphate coating onto magnesium for implant applications.

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