Formation Mechanism,Biodegradation,and Biocompatibility Of Organic Coating Electrografted On ZE21B Mg Alloy Surface

Magnesium(Mg)alloys have become one of the most promising degradable metal for cardiovascular stent due to their excellent biocompatibility and biodegradability.However,the rapid degradation rate causes the decrease of mechanical property prior to their full-service period,and become the most server obstacle confronting Mg alloys henceforth.Surface modification could reduce the degradation rate of Mg alloys and improve their biocompatibility spontaneously.Polymer coatings have draw widely attention in the surface treatment of Mg alloys for vascular stents for their good flexibility and drug-delivery properties.Nevertheless,traditional polymer coatings are commenly prepared via dipping,spinning and spraying methods,which are just physically or electrostatically adsorbed on Mg alloys,and the coatings are subject to cracking or peeling-off during the crimping and expansion process for the weak adhesion.Electrografting is a method that could form covalent bonding between organic molecules and conductors or semiconductors.The anodic grafting on active metals cause the leaching of metal cations,which weakens the mechanical properties of metals,and the impact is more obvious for cardiovascular stents with smaller size.Cathodic grafting is capable of forming chemical bonding via the electron transfer between the metals and the organic molecules,which could avoid the dissolution of metal ions and improve the adhesion between the organic coatings and the metal substrates.To date,the reported researches regarding on cathodic electrografting are mainly focused on transition metals,and the application of cathodic grafting on active metals such as Zn and Mg are rarely reported.In this paper,vinyl monomers were adopted to research the application of cathodic reduction electrografting on ZE21B Mg alloy for cardiovascular stent.The influences of functional groups on the grafting behavior of vinyl monomers on Mg alloy,and the impact of process parameters and grafting methods on the structure and peoperties of poly ethylacrylate(PEA)were investigated.Time-of-Flight secondary ion mass spectrometry(TOF-SIMS)was used to explore the combine method between PEA and Mg alloy,first-principles and molecular dynamics were apllied to model the bonding mechanism and formation pattern of PEA coating on Mg alloys.Electrochemical polarization,and electrochemical impedance spectroscopy(EIS)were applied to evaluate the effect of experimental process parameters on the degradation rate of PEA coatings,and in vitro immersion tests was used to probe the degradation behavior of PEA coating in simulated body fluid.Hemocompatibility and cytocompatibility tests were employed to explore the biological properties of PEA coating and drug delivery composite coating,which provides references for the design of surface coatings of Mg alloy for cardiovascular stents.The main conclusions of this study are as follows:(1)The appropriate potential range is the predominant factor for the success of the electricgrafting for certain metal and monomer,and it should be limited in the scope of the reduction potential of the monomer and lower than the self-corrosion potential of the alloy,and the overpotential must be adequate to ensure the transfer of electrons.It is noteworthy that vinyl monomers with carboxyl(acrylic acid)or hydroxyl(hydroxyethyl acrylate)were not prone to form complete coatings on the surface of Mg alloys due to the fact that the reduction potentials of carboxyl and hydroxyl groups are higher than that of vinyl group.Hence,these two functional groups might be reduced prior to the vinyl group during the experiments,thus the adsorption of the monomers on the surface of the Mg alloy was influenced.EA molecular,which has only one active froup and side chains with high flexibility,could form highly coverd and protective coating on Mg alloy.The surface roughness of PEA coatings prepared by galvanostatic was relatively large and accompanied by the generation of pittings.The surface of PEA coatings prepared by cyclic voltammetry method were unform and compact without obvious defects,which provides better protective properties for Mg alloy.(2)Micromorphologies indicate that PEA coatings appeared to be regular networks.The appearance of C₂H₃Mg^-in TOF-SIMS spectrum showed that PEA coating is covalently bonded with Mg alloy.Molecular dynamic results showed that EA or PEA formed stable adsorption on Mg surface rather than Mg O surface,indicating that PEA coating can not be obtained via electrografting on passivated Mg surface,and cathodic electrografting is the only way to ensure the bonding between EA and Mg alloy.PEA coating appeared to be networkly distributed on macroscopic,and PEA chains grows spirally from micro view,and PEA chains are mutual combined through hydrogen bonds.The results of first-principles simulation were consistent with the experimental results,and proved that the 3s orbital electrons in Mg atoms and the 2p_zorbital electrons in C atoms participated in the formation of new bonds.The addition of Zn atoms enhanced the the binding energy between EA moleculars and Mg alloy,the addition of Y atoms weakened the affinity of EA and Mg alloy,but the binding between Y and EA was more stable.(3)Orthogonal experiment was used to evaluate the influence of process parameters on the electrochemical behavior of PEA coatings,the results showed that the influencing sequence is scan rate>cycles>monomer concentration.PEA coating with a scan rate of 20 m V/s,monomer concentration of 1.5 mol/L and 7 cycles showed slowest degradation rate,which enhanced0.27 V of the self corrosion rate of Mg alloy,and decreased the current density by 3 orders of magnitude.The results of immersion tests demonstrated that a fully covered PEA coating could effectively retarded the degradation of Mg alloy,the PEA coating remained intact after 10 days of immersion,and no obvious degradation product was observed on the surface.PEA chains occurred reorganization after contacting SBF solution,then swelled and degraded in the form of hydrolysis of esters.(4)PEA coating showed excellent hemocompatibility,but did not promote the proliferation of endothelial cells.The composite everolimus delivered coating could remarkably reduce the adsorption of macrophages,the amounts of endothelial cells increased with prolonged incubation time,and the activity of endothelial cells improved as well.The results demonstrated that PEA coating with everolimus could inhibit the occurrence of inflammation,ensured the growth and proliferation of endothelial cells,and promote the repair of vessels.