| Magnesium is one of the most electrochemically active metals,the low corrosion resistance of Mg makes Mg alloys appropriate candidates for degradable biomaterials due to their biocompatibility combined with outstanding physical and mechanical properties.In many cases the body needs only the temporary presence of an implant or device,in which case materials exhibiting biodegradability represent a better approach than stable and inert ones.Therefore,more and more attention has been paid to the new generation of degradable magnesium alloys.In recent years,a lot of work has been done on the application of magnesium and its alloys to cardiovascular stents,bone fixation materials,bone repair porous scaffolds.However,the main problem that limits the application of magnesium alloys as implants is its fast degradation rate.The pitting corrosion of magnesium will lead to local mechanical failure of magnesium implants.In addition,excessive precipitation of hydrogen and local high pH value caused by corrosion may lead to inflammatory of surrounding tissues and relative poor cell compatibility.Increasing the corrosion resistance and slowing the corrosion rate of magnesium alloys has been extensively studied and explored.There are three main ways to increase the corrosion resistance of Mg and Mg alloys,including alloying,mechanical processing and surface modification.Among them,surface modification is the most popular method due to its direct and high efficiency,it can not only reduce and control the degradation behavior but also improve the surface biocompatibility of magnesium alloys.Chemical conversion deposition is a simple method and suitable for the surface modification of various complex device.Therefore,the surface modification of biomedical magnesium alloy by chemical conversion method is popular,however,most research focus on inorganic components to deposit the conversion coating which provide poor biocompatibility.Therefore,the development of a new,green,and biocompatible conversion coating for biomedical magnesium alloy to meet the clinical requirements for the magnesium implants is necessary.Tea polyphenols have the effects of anti-inflammatory,antioxidant and so on.Furthermore,they can complex with metals to form stable complexes and cross-linking mechanism of phenolic amines.Based on all the abilities above,the tea polyphenols have the potential to construct conversion coatings on biodegradable magnesium.The tea polyphenols induced organic conversion coatings were confirmed for its ability to slow down the corrosion rate and improve the biocompatibility of Mg substrate in vitro and in vivo,and the biomolecules immobilized on the conversion coating and the potential for application of cardiovascular stents were explored.The main content of the paper is as follows:(1)The study on the factors influencing the construction of polyphenol conversion coating on magnesium alloys.In this part,the parameters of conversion process were explored.Effects of polyphenols with different molecular weights,different magnesium alloy substrates,conversion time and temperature,the concentration and pH value of conversion solution on the formation and corrosion resistance of polyphenol conversion coatings were compared.The molecular weight of the polyphenols affects the connection between the metal complexes and thus influences the degree of coverage of the conversion coating.The corrosion resistances of the polyphenol conversion coatings on different magnesium substrates are different,indicating the composition of the magnesium alloys affect the formation of the conversion coatings.The conversion time,temperature,concentration and pH also have great influences on the formation of the coatings.In this study,through surface morphologies and electrochemical corrosion tests,it was found that proper control of process parameters of conversion can obtain a polyphenol conversion layer with good corrosion resistance.The optimum process parameters for constructing the EGCG conversion coating on MgZnMn are:2.5 mg/mL,20°C,12 h,pH=8.5.(2)Construction of EGCG conversion coating on the magnesium alloys.EGCG conversion coatings prepared on MgZnMn alloy have good corrosion resistance and EGCG has some medicinal values.The EGCG coating on the MgZnMn was prepared by a simple one-step method.The mechanism of the coating formation,the physical and chemical properties,the corrosion resistance and the biocompatibility in vitro and in vivo were studied.The electrochemical results showed that the icorr of MgZnMn is 64.57 ?A/cm2,and icorr of the EGCG coated samples were significantly reduced.In particular,the icorr of MgZnMn-2.5EGCG is the lowest,0.03 ?A/cm2.The results of the immersion experiment showed that after immersion for 320 h,MgZnMn precipitated about 25 ml/cm2 of hydrogen,which was significantly more than that of the EGCG coated samples.The film formation mechanism and corrosion behavior study found that the EGCG-metal complex has the main contribution of the EGCG conversion layer to the corrosion behavior,and the concentration of the EGCG conversion solution affects the content of the complex in the conversion coatings.The EGCG conversion coatings showed improved corrosion resistance and ECs compatibility in vivo.The EGCG conversion coating maintained its anti-inflammatory effect and had a certain inhibitory effect on the release of macrophage inflammatory factors.Subcutaneous implantation experiments also showed corresponding results.(3)The construction of phenol-amine crosslinked conversion coatings on biomedical magnesium alloys for the application on cardiovascular stents.The polyethylenimine(PEI)was introduced in phenolic conversion solution to form a CA/PEI conversion coating and overcome the defects of a simple phenol conversion coating.In short,catechol CA and PEI are deposited on magnesium alloy concurrently to form a phenol-amine crosslinked conversion coating.The addition of PEI made the binding between the complex units enhanced,thereby reducing the cracks of the coatings and enhancing the corrosion resistance.Most importantly,the phenol-amine crosslinked conversion coatings provide amino,phenolic hydroxyl and thiol groups on the surface.These functional groups can be used to immobilize various biomolecules for anti-inflammatory,anticoagulant and other biomedical functions.The heparin was immobilized on the surface of MgZnMn-CA/PEI and a series of biocompatibility evaluations were performed,demonstrating the efficiency of the surface modification.(4)Alternately deposited phenol amine cross-linked conversion coatings on biomedical magnesium alloys.Referring to the model of layer by layer self-assembly coating,we proposed a new way to prepare conversion coating,that is,the magnesium substrates were immersed into the EGCG and PEI solution respectively and alternately cycled.This new method integrates the key steps of coating formation on the surface of magnesium,including the complex of EGCG and metal ions and the crosslinking of EGCG and PEI.The layer by layer cross-linked coating showed strong controllability,the corrosion resistance,biocompatibility can be controlled by the number of cross-links,and the coating can be used for bio-functionalization according to its specific application.After subcutaneous implantation,the weight loss of the coated samples compared to MgZnMn was reduced by about 60%.In summary,green tea polyphenols are selected to construct organic conversion coatings on magnesium alloys.The parameters of conversion process were optimized by electrochemical tests.The EGCG conversion coating and CA/PEI cross-linked conversion coating were prepared on MgZnMn alloy.Combining the physical and chemical properties,corrosion resistance behaviors and biocompatibility in vitro and in vivo of the EGCG and CA/PEI coatings,a new alternate conversion method was proposed for the construction of a layer by layer cross-linked conversion coating.This paper provides important data support for the tea polyphenols induced conversion coatings on biomedical magnesium alloys,and provides new ideas for the surface modification of biomedical magnesium alloys. |
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