Construction Of Anticorrosive Coatings On Biomedical Magnesium Alloys For Biodegradability Regulation

More biomaterial scientists and engineers regard magnesium alloys as important and indispensable materials in the field of medical devices,due to their excellent mechanical properties,such as Young’s modulus close to natural bone,good thermal and electrical conductivity.In addition,the unique advantage of biodegradability makes magnesium alloys superior than traditional metal materials,such as titanium alloys,stainless steels and Co-Ni alloys.Magnesium alloys degrade slowly in the human environment,thus protects patients suffer from secondary surgery.In addition,magnesium is one of the trace elements for human body,and participates in various activities such as metabolism and protein synthesis.However,there are still many problems to be solved in the use of magnesium alloys.The chemical nature of magnesium is relatively active,which is likely to cause anodic dissolution and cathodic reduction of hydrogen production at the metal surface.Once the reactions of primary battery occurs,the degradation rate will increase and the corrosion rate will be uncontrollable.Excessive degradation will produce hydrogen,and the collapse of the local structure will cause the mechanical properties to suffer earlier lose,thus magnesium alloys cannot provide support and protection.The problem of uneven corrosion affects the use and service life of magnesium alloys,and does not favour long-term use.In recent decades,there have been many reports on the construction of anti-corrosion coatings on magnesium alloys.Surface modification is a preferred choice among many solutions,because it is time-saving and labor-saving,the preparation processes are simple for industrial use.Atomic layer deposition is a nanofilm deposition technique.It has no selectivity on the morphology and dimension of the substrate.The thickness of the film can be precisely controlled by the deposition cycles,and the deposited film is dense and uniform without defects.In this paper,Ti O₂ nanofilms are deposited on the surface of AZ31 magnesium alloy by atomic layer deposition technique.The corrosion resistance and biocompatibility of Ti O₂nano-films obtained at different deposition conditions are investigated.Under the condition of low-temperature deposition,the more the deposition cycles,the proportional increase of the film thickness.The film denseness is also increased,the pores and defects of the surface are less.According to the results of the electrochemical corrosion test,when the cycles of deposited Ti O₂ nanofilms increase to 200,the corrosion current density of the magnesium alloy is reduced by 58%compared with the unmodified groups.And the corrosion current density is further decreased 74%,when the deposition cycles increase to 400.It can be concluded that the corrosion rate of the magnesium alloy can be controlled by precisely controlling the thickness of the Ti O₂ nanofilm.Surface silanization is an efficient mean to improve corrosion resistance and biocompatibility.Moderate amount of silicon can promote joint and bone metabolism and avoid osteoporosis.3-aminopropyltriethoxysilane（APTES）is prepared on the Ti O₂ nanofilm coated surface.The corrosion current density of the magnesium alloy is reduced by two orders of magnitude.By observing the surface morphology after corrosion,the corrosion rate of the samples coated with Ti O₂ nanofilm and siloxane are much weaker than those of the Ti O₂ nanofilm and the unmodified groups.The results of in vitro experiments show that the Ti O₂/silane coating is beneficial for the cells adherence,growth and proliferation on the surface of magnesium alloy.Moreover,the corrosion behavior of the magnesium alloy can be regulated by the Ti O₂ deposition cycles and the subsequent silanization treatment,so that magnesium alloy can keep a slow corrosion rate at the initial stage and maintain a controlled and homogeneous corrosion rate at the later stage.This paper also constructs a smart coating on the surface of Mg Sr alloy that can use the near-infrared light（808 nm）to achieve rapid self-healing.Exogenous light stimulation can be used as an accurate and effective mean to control the degradation rate of magnesium alloy during serivce life and to provide longer protection aganist corrosion.The light-controlled healing coating is composed of polypyrrole（PPy）/polycaprolactone（PCL）composite coating.Under near-infrared light（NIR,808 nm）irradiation,the photothermal effect of PPy can heat the scratched portion to the low melting point temperature required by PCL in 6 minutes,which can quickly repair defects.At the same time,it helped to control the corrosion rate of magnesium alloys in a small speed range.Electrochemical measurements show that the healed PCL/PPy coating recovered 76.7%of the resistance value(|Z|_{0.01 Hz})at the beginning of the immersion,and the|Z|_{0.01 Hz} value remained 74.8%after 7 days of immersion,comparing to the intact PCL/PPy coating.Due to the excellent photothermal effect from polypyrrole and the deformation behavior from polycaprolactone,the corrosion resistance and mechanical properties of the scratched area can be restored after near-infrared light irradiation.This self-healing strategy can be adjusted by near-infrared light and is expected to be used in the next generation of self-healing protective coatings.