Novel PA encapsulated PCL hybrid coating for corrosion inhibition of biodegradable Mg alloys: A triple triggered self-healing response for synergistic multiple protection

Surface coatings have been extensively used to control the degradation rate of Mg alloys for bioimplant applications. However, these coatings only act as passive barriers. In corrosive media, structural damage impairs their barrier properties, resulting in rapid degradation of Mg alloys. The present study incorporates phytic acid (PA) as a healing agent in polycaprolactone (PCL) microcapsules with a unique honeycomb core matrix to obtain a self-healing PA-PCLcaps coating. The contact between simulated body fluid (SBF) and PA-PCLcaps coated ZM21 exhibited Cassie-Baxter interfacial states, resulting in significant hydrophobicity with a contact angle (CA) of 116.3. The corrosion potential (Ecorr) and current density (Icorr) were found to be -0.28 V and 1.1 × 10-9 A/cm2, respectively, for PA-PCLcaps coating, resulting in biosafe corrosion rate of 2.5 × 10-4 mm/year. After mechanical scratching, rapid HA mineralization at scratched regions recovered the hydrogen evolution rate (HER, 0.36 mL/cm2/day) and pH change (pH 7.10) of scratched PA-PCLcaps coated ZM21 samples to corresponding unscratched samples within one day of immersion. The coating's self-healing ability could be attributed to PA released from punctured microcapsules, which facilitates HA chelation. The pH-triggered (pH 10) and Mg (II)-triggered (5 mM) conditions enhanced PA release from PA-PCLcaps coating by 2.5 and 3.1 times, respectively. As a result, dense HA mineralization occurred, which protects the coating from structural defects and ensures its durability in stimulating conditions. The findings of present study provide new insight for design of multiple stimuli-feedback based self-healing coatings on biodegradable Mg alloys.