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Imagine going through a surgery where the doctor proposes the use of a temporary implant that dissolves by itself with time in the human body, thereby avoiding a painful second surgery. As great as that would sound, the challenges are plenty when it comes to designing an implant that has mechanical properties close to that of the human bone, is biocompatible and degrades at an appreciable rate till the bone heals.

Of the many available temporary implant materials, magnesium is considered a potential candidate, although its fast degradation rate is a serious limitation.

One strategy to slow down their corrosion rate includes alloying it with suitable non-toxic metals. ZK60, a Mg-Zn-Zr alloy, shows positive attributes satisfying the major criteria mentioned, although it degrades within 12 weeks.

In order to further tune the corrosion rate of this alloy, biodegradable polymer coatings seem to be an option, given their ease of application through a simple spin coating procedure.

Of the various biocompatible polymers available, poly (3-hydroxybutyrate), referred to as PHB, a semi-crystalline short chain polyhydroxyalkanoate (PHA), shows promise for use as a coating on Mg alloys. Also, in order to best mimic the human physiological condition of homeostasis, the study of a dynamic electrolyte flow condition using a recirculating flow system would be quite useful.

We thought it would be worthwhile to study the corrosion of a PHB-coated ZK60 and measure quantitatively the extent of degradation using an ensemble of characterization techniques including electrochemical impedance spectroscopy (EIS), grazing-incidence X-ray diffraction (GI-XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), pH and open circuit potential measurements.

The novelty of the work lies in using an electrolyte flow system to study the corrosion of a polymer-coated magnesium alloy. The results of this study are published in .

Our first experiments measured the corrosion of the bare ZK60 alloy in modified simulated body fluid (m-SBF) electrolyte. We found that the dynamic electrolyte condition, in contrast to the static immersion case, was more aggressive in degrading the alloy by measuring a lower polarization resistance for the former using EIS.

We reasoned that the condition necessary for formation of salt precipitates that could offer protection (although quasi or partial viz. due to lots of cracks in the corrosion products) was not met due to the ions being continuously exchanged by the flowing electrolyte. This could be supported by SEM and FTIR analysis which showed evidence of salt precipitates and their functional groups respectively.

In order to delay the onset of corrosion of bare ZK60, we first did a pre-treatment step on the alloy using sodium hydroxide solution, followed by coating it with PHB of different thicknesses, herein referred to PHB_thin and PHB_thick.

We found through SEM that the PHB_thin had lots of pores while PHB_thick was devoid of them. In order to evaluate their capability to retard corrosion of ZK60, we studied the PHB-coated alloy in m-SBF for nine days under dynamic flow conditions.

Using EIS, we measured a much lower "polarization" resistance after nine days for PHB_thin showing that it was much more degraded than PHB_thick. We attributed this to the "through-going" pores which allowed easy ingress of electrolyte and corroded the alloy.

This reasoning could be further strengthened based on GI-XRD which showed the absence of crystalline peaks corresponding to PHB, indicating a degraded polymer. Also, through XPS analysis we could identify the formation of Mg, Ca and phosphate-based corrosion products.

Finally, we could also see a lot of precipitates using the SEM. In contrast, we found that PHB_thick showed much higher protection ability with little degradation due to low pore density. We could reason this from the presence of PHB in GI-XRD and formation of a lower extent of corrosion products using XPS.

In essence, through this work, we could show that by tuning the pore density, one could control the extent to which a Mg alloy degraded. It remains to be seen whether the corrosion products formed within the pores of the polymer coating provide protection over extended time.

We believe this study could open up avenues for using polymer-coated Mg alloy as temporary implants to protect the bone while it heals, but dissolves eventually.

This story is part of , where researchers can report findings from their published research articles. for information about Science X Dialog and how to participate.

Vijayshankar Dandapani is an Associate Professor in the Department of Metallurgical Engineering and Materials Science at the Indian Institute of Technology (IIT), Bombay. He works in the area of electrochemistry and corrosion.