Corrosion-controlling And Biocompatible Zr Ion-chelated 1-hydroxyethylidene-1,1-diphosphonic Acid Coating On Magnesium

Magnesium and its alloys have drawn great attention of investigators for biodegradable implant applications due to the biosafety,ease of biodegradation(corrosion),and excellent mechanical properties.Magnesium is the most promising candidate material for biodegradable implants particularly.However,the biggest challenge for its clinical application lies in that Mg corrodes rapidly.Too fast corrosion of Mg is likely to cause premature mechanical failure of implants;moreover,it may lead to quick accumulation of byproducts,namely evolved hydrogen and increased local pH,eliciting harmful side effects such as necrosis to the surrounding tissues.Studies have shown that one of the most effective strategies to control the corrosion rate of Mg is to introduce a layer/coating on its surface by setting up a barrier between the electrolyte and Mg substrate.The organic coatings have shown great potential both in corrosion controlling and bio-compatible /-functional modification.Phosphonic acids are promising candidate for building such an organic coating on Mg.Organic phosphonic acid not only render them biomimetic and naturally bio-safe,but also can be firmly fixed on the substrate surface by chemical combination.More importantly,this type of molecule typically possesses a powerful chelating ability with metallic ions to form a metal-organic complex.Such complex demonstrated not only can improve coating quality but also offering the possibility to bio-functionalize the bio-metals such as by drugs loading.Herein,we propose to construct the tetravalent metal ions chelated organophosphonic acid metal-organic complex coating on Mg to obtain an enhanced corrosion controlling and potentially biocompatible surface.The phosphonic acid we selected was 1-hydroxyethylidene-1,1-diphosphonic acid(HEDP)which is a small molecule diphosphate(molecular weight 206.3 g/mol).HEDP has been widely used as water treatment agent,cosmetics and corrosion inhibitor etc.and is considered bio-safe.The literature reported that HEDP can be chemically immobilized on alkaline pretreated Mg surface and then deposited on the base;an ultrathin such HEDP film showed good corrosion protection for Mg.One of the reasons that contribute to the effectiveness on the corrosion protection is credited to the HEDP molecules could chelate with Mg ions released spontaneously by Mg corrosion during HEDP deposition.It intrigued us to introduce extraneous metallic ions that may improve further the HEDP coating quality and structure a surface with both corrosion controlling and outstanding bone compatibility.The metallic ion selected was Zirconium(Zr)in this study because it has four binding sites denoting stronger chelating ability than bivalent metallic ions such as Mg;and more importantly Zr and its compounds have been verified good biocompatible prospect for orthopedic surgery.The FTIR and XPS confirmed that the uniform,smooth and compact zirconium ions integrated HEDP compound coating are chemically immobilized to the surface of the magnesium and the chelation between Zr ion with HEDP molecular.The Zr integrated HEDP composite coating is more effective in improving the corrosion resistance of magnesium.The open circuit potential(OCP)show that it is more stable thermodynamic state,potentiodynamic polarization(PDP)showed that it is lower corrosion current density,the electrochemical impedance spectroscopy(EIS)get more impedance;Long term immersion results showed that it released less hydrogen and had a smaller change in pH value.In addition,Zr ion integrated HEDP molecular composite coating also showed excellent bone compatibility.Compared with other sample groups,composite coating modified samples showed that it was more hydrophilic,and promoted the deposition of active Ca-P(hydroxyapatite and calcium phosphate),and more conducive to the adhesion and increment of osteoblasts.The complex coating endowed Mg with not only significantly reduced corrosion current density but also remarkably suppressed long-term in vitro degradation in phosphate buffer saline(PBS).Among them,when the concentration of zirconium ions is 1.0mM,the properties of the samples are the best.Therefore,this kind of Zr ion chelating HEDP molecular composite coating modified magnesium material is expected to advance the progress of magnesium as a degradable bone implant materials.