The Research Of Surface Modification And Degradation Microenvironment Of Magnesium-based Cardiovascular Stent Materials

In the field of cardiovascular stents intervention, magnesium and magnesium alloysstents have became a research focus for scholars both domestic and abroad, as they not onlyhave excellent mechanical properties that could maintain the repair of blood vessels, but alsothe degradable and absorbable properties could avoid being removed by a second surgery.However, the key obstacle of using Mg and Mg alloys apply to the medical materials wastheir rapid degradation rate especially in the physiological system.The Mg and Mg alloyshave a rapid degradation rate in the physiological system, which weaken mechanicalproperties before the blood vessel had sufficiently healed. Even worse, during the degradation,ion concentrations and pH at the material‘s surface must vary and thus may affect markedlythe cells‘milieu. High local alkaline microenvironment arisen by the accumulation of Mg2+may cause inflammation at the implanted site, which weaken the the progress of theendothelialization of cardiovascular stents. Hence, in order to make use of magnesium-basedmaterials feasible for surgical implantation, it is crucial that the degradation rate should beencontrolled in order to provide an optimum microenvironment and accelerate theendothelialization of stent. From the perspective of making a microenvironment that issuitable to the cell growth, this study control the change of the pH value and ion concentrationat the implanted site caused as the degradation of Mg alloys by the preparation of micro-arcoxidation（MAO） coating and piezoelectric polymer（PHB） film on the surface of Mg alloy.In this paper, MAO coating had prepared on the surface of AZ31magnesium alloy bymicro-arc oxidation technique and optimized the process parameters influencing the structureand properties of the coating. The result showed that with the increase of electrolyteconcentration, the pore in the coating became more nonuniform in either dimension ordistribution, and the anti-corrosion phases of the coating increased first then decreased. Theincrease of voltage could promote the formation of coating, but when the voltage got a limitvalue, the coating contained more and more defects even appeared fall off. Meanwhile, thevoltage also affected the quantity of anti-corrosion phases. The additives could enhance theproperties of coating in a certain extent. It found that the number of pore decreased then thepore diameter of coatings increased and the pores distributed orderly on the coating surfacewith the increase of NaF concentration. Nevertheless, when the concentration of fluoride ionswas so high, the defects of coatings increased, including microcracks and some largerdiameter pores. The corrosion resistance of coating varied accordingly as the trend of mainphases composition of coating increased first then decreased. The additive glycerol significantly affected the structure, thickness and the quantity of anti-corrosion phases ofcoating by influencing the electrolyte conductivity. The size of micro-pores in the MAOcoating was in the range of0.8¹.05μm.The main phase composition of coating was consistsof MgAl₂O₄, MgAlSi_1.5O₆and MgSiO₃.During the degradation microenvironment experiment, this paper discussed the optimalconditions for cell growth. The in-vitro studies indicated a rather high pH might exist in themicroenvironment for bone formation since pH8⁸.5was shown to be optimum and theoptimum concentration of magnesium ions was3⁴mM for cell viability. However, MAOcoating which was prepared with the optimal process parameters could not satisfy an ideal pHvalue and ions concentration for cell growth. Meanwhile, in order to strengthen the toughnessof cardiovascular stents and the application of piezoelectric effect promote the cell growth,the polyhydroxybutyrate （PHB） piezoelectric film was prepared on the surface of MAOcoating by spinning technique. The size of micro-pores in the polymer film was in the rangeof2.25μm³.25μm and the average size was2.54μm. The MAO/PHB composite coating wasmainly consists of C, O, Mg and Si elements. The piezoelectric constant （D33） of coating was20.2±0.02pC/N. The results of degradation experiment showed that Mg alloy with MAO/PHBcoating could meet the microenvironment for the cell growth during the degradation.The biocompatibility of the material was evaluated by the protein adsorption and cellexperiments. The results showed that the protein adsorption capacity of the samples wasMg-MAO/PHB> Mg-MAO> Mg. The cytotoxicity of Mg, Mg-MAO and Mg-MAO/PHBwas level2, level1, and level1respectively. The sample Mg-MAO/PHB showed relativelylow degradation rate due to the effective protection of MAO/PHB coating, so which could beable to create more suitable slightly alkaline environment for cell growth and promoted thecell proliferation.