Preparation And Characterization Of Electropolymerized Dopamine With Poly(L-lactic Acid)on Mg-Zn-Y-Nd Alloy For Vascular Stent

Magnesium(Mg)and its alloys has become one of the most potential materials in bioimplant applications due to their good biocompatibility and degradability.However,the poor corrosion resistance of Mg alloys is a vital challenge to achieve wide application in bioimplant application.Surface modification has became one of the main methods to improve the corrosion resistance of Mg alloys.Polydopamine(PDA)coating,a mussel inspired coating,has attracted a lot of attention in the field of surface modification because of its excellent adhesion.In addition,PDA coating contains various functional groups such as catechol,which provides a possibility for secondary modification.Nevertheless,the traditional dip PDA(dPDA)coating on Mg alloys appears amount of PDA aggregates and also cause severe corrosion of Mg and cracks.These problems could increase the surface roughness of PDA coating and therefore decrease the quality of subsequent modification.In this paper,to overcome the problem of the high surface roughness in traditional dPDA coating,we prepared a smooth electropolydopamine(ePDA)coating on Mg-Zn-Y-Nd alloy from sodium salicylate(SS)and dopamine(DA)aqueous solution by galvanostat method at the first time.Firstly,the electrochemically behaviors of Mg-Zn-Y-Nd alloys in different solutions were characterized by cyclic voltammetry and the theoretical basis was determined.Using spectroscopic ellipsometry(SE),we measured the thickness of ePDA coating and the results reveal that the thickness of the coating increases with the increase of the total charge passing through the working electrode.Then,the surface topographies and the surface roughness of the ePDA coating were characterized by scanning electron microscope(SEM)and atomic force microscope(AFM).The results suggest ePDA coating has a smooth surface with no cracks and the surface roughness(RMS)is about 13.0 nm,which is far less than 31.1 nm of dPDA coating.Meanwhile,the molecular structure and element components were analyzed by X-ray photoelectron spectrometer(XPS)and attenuated total reflection infrared spectrometer(ATR-FTIR).The coating formation mold was proposed which is "anodic dissolution of Mg substrate → passivation of Mg substrate → ePDA coating formation → salicylate ions participates reaction → coating rupture".To investigate the impact of ePDA layer on the properties of secondary modification,we prepared a Poly(L-lactic acid)(PLLA)coating on ePDA,dPDA and Mg-Zn-Y-Nd alloy samples by immersion method and investigated the impact of different coating on corrosion resistance and coating adhesion of PLLA layer.According to ASTM D3359-09 standard,the adhesion of ePDA/PLLA coating could reach to 5(B),much higher than that of Mg/PLLA coating(0(B)),indicating that the ePDA coating could improve the adhesion of PLLA layer and substrate.The corrosion results show that the corrosion current density of ePDA/PLLA is one order of magnitude lower than that of dPDA/PLLA coating,owing to the structural difference of polydopamine anchoring layer.Although affected by corrosion mechanism,the long-term corrosion resistance of ePDA/PLLA and the Mg/PLLA coating remains at a same level,it is obviously higher than that of dPDA/PLLA coating.Therefore,ePDA coating could improve the quality of secondary modification compared with traditional dPDA coating and provide a new insight into the surface modification of Mg alloys by dopamine.