A Dual-task Design Of Corrosion-controlling And Osteo-compatible Hexamethylenediaminetetrakis- (Methylene Phosphonic Acid) Coating On Magnesium

Magnesium as well as its alloys appears increasingly as a revolutionary bio-metal for full biodegradable implants application but the biggest challenges exist in its too fast biocorrosion/degradation. Both corrosion-controllable and biocompatible Mg-based bio-metal is highly desirable in clinic.In present work, hexamethylenediaminetetrakis (methylenephosphonic acid) [HDTMPA, (H2O3P-CH2)2-N-(CH2)6-N-(CH2-PO3H2)2], as a natural and bioactive organic substance, was covalently immobilized and chelating-deposited onto Mg surface by means of chemical conversion process and dip-coating method to full fill dual-task performance of corrosion-protective and osteo-compatible functionalities. The chemical grafting of HDTMPA molecules, by participation of functional groups on pretreated Mg surface, ensured a firmly anchored base layer, and then subsequential chelating reactions of HDTMPA molecules guaranteed a homogenous and dense HDTMPA coating deposition on Mg substrate.Through the optimization of the concentration of HDTMPA, we finally found the Mg-OH@HDTMPA(5 mM) guaranteed the most homogenous and dense morphology. The shifting of phosphate group in the FT-IR spectrum and the P-Mg-O in the high-resolution of XPS spectrum all evidently indicated the existence of covalently immobilization and chelating-deposition in the deposition process. The HDTMPA coated Mg show a more stable and lower open circuit potential (OCP), which indicate a more stabilized thermodynamic state and also slowdown the hydrogen evolution reaction on cathode which determines the corrosion rate step (RDS). Electrochemical impedance tests show that the HDTMPA coating can greatly improve the corrosion resistance and mitigate the localization corrosion degree from the point of kinetics. The long term immersion tests indicated that the Mg-OH@HDTMPA(5 mM) has the outstanding corrosion resistance, because of its self’s lower free corrosion potential(Ecorr) as well as the homogenous and dense coating, whilst the blocking effect of HDTMPA molecule to the Mg2+during the degradation process. Electrochemical corrosion and immersion degradation results reveal that the HDTMPA coated Mg provides a significantly better controlled bio-corrosion/degradation behavior in phosphate buffer saline solution (37±0.5℃) as compared with untreated Mg from perspective of clinic requirement.The primary hemo-compatibility tests show that the HDTMPA modified Mg possesses clinically acceptable hemolysis ratio values of less than 5.0%, merely ca.1.1%, lower than that of pure Mg (ca.33.45%) and stainless steel (ca.1.34%). Moreover, the HDTMPA coated Mg exhibits osteo-compatible in that it induces not only bioactivity of bone-like apatite (HA&OCP) precipitation but also promotes osteoblast cells adhesion and proliferation. Our well-controlled biodegradable and biocompatible HDTMPA modified Mg might bode well for bone implant application.