| Orthopedic internal fixation implants mainly include wire,bone screw,bone plate and intramedullary nail,etc.Nowadays the widely used internal fixation implants are mainly made from medical stainless steel(316L),cobalt chromium alloy and titanium alloy(Ti6Al4V)and some other bio-inert metals.These materials are generally characterized by high elastic modulus,which in local generates "stress shielding effect",increasing the risk of fracture delayed healing,local osteoporosis and some other complications.Besides,because of their non-degradable property,most of them need secondary surgery to remove,which increases the number and cost of treatment.Magnesium alloy with its elastic modulus close to the human cortical bone to effectively reduce the "stress shielding effect",the density close to the cortical bone and high mechanical strength and biodegradability to avoid secondary surgery and many other advantages,has aroused widespread concern,and it is possible to become the next generation of orthopedic internal fixation material.The main problem of magnesium alloy for orthopedic use is its fast degradation rate,which also results in a relatively short duration of mechanical strength.Therefore,people have used different methods to reduce degradation rate,prolong mechanical strength holding time andimprove biocompatibility at the same time of magnesium alloy.Polymer blending as an important method to find new materials has not been reported in detail until now in the study of surface modification of magnesium alloys.So in this study,the surface modification of magnesium alloy was carried out by using the blending system of carbon dioxide copolymer(PPC)and polylactic acid(PLA),and its effects on degradation and cell compatibility of magnesium alloy were investigated.In this study,different proportions of PPC/PLA polymer blends were coated on the surface of magnesium alloy AZ31 by solution blending.The morphology and composition of the sample surface were characterized by scanning electron microscopy(SEM)and attenuated total reflection infrared spectroscopy(ATR-FTIR).The surface roughness and the thickness of the coating were measured by a surface profiler.The adhesion of different coatings to the substrate were observed by a cross-cut tape test,and the degradation experiment was carried out in simulated body fluid(SBF)in vitro.In addition,the effect of PPC/PLA coating on the cell compatibility of magnesium alloy was investigated by cell proliferation experiments of mouse preosteoblasts(MC3T3)and co-culture of cells and samples.Through the experiment,we found that the surface of the magnesium alloy with PPC/PLA coating still showed a rough contours like its substrate(though blunter)under the scanning electron microscope.The surface roughness of magnesium alloy after surface modification wasclose to that of the original substrate,though slightly reduced.The absorption peaks of the corresponding chemical groups in PPC/PLA molecules appeared in the infrared spectrum of coated magnesium alloy,which indicated that the polymer was successfully coated on the surface of the magnesium alloy.Moreover,the absorption peak of some groups gradually shifted with the change of the blending ratio,indicating that there was some interaction between the molecular chains of PPC and PLA.The degradation rate of PPC/PLA coated magnesium alloy was significantly slower than that of uncoated magnesium alloy in degradation experiment in vitro.By linear regression of data at different time points,we found that with the increase of PPC content,the degradation rate of the coated magnesium alloy tended to increase linearly.The results of cell proliferation test showed that the degradation products of the magnesium alloy could promote the proliferation of MC3T3 cells,and the number of cells in the uncoated magnesium alloy group was more than that of the other groups in 3rd day.In co-culture experiments,the morphology of cells on each surface of magnesium alloy were good.The adhesion number of MC3T3 cells on the surface of PPC/PLA coated magnesium alloy in 6h had no obvious difference with the uncoated group,but the cell proliferation in coated magnesium alloy group in 1st and 3rd day was more than that in uncoated groups.From the experimental results above we draw the following conclusions:(1)Magnesium alloy was successfully coated by PPC/PLAblends,and the surface roughness of the coated alloy was similar to that of the substrate.Moreover,there existed some interaction between the molecular chains of PPC and PLA.(2)PPC/PLA coating could effectively slow down the degradation rate of magnesium alloy in vitro,and with the increase of PPC content,the degradation rate of the coated magnesium alloy had a linear increasing trend.(3)PPC/PLA coated magnesium alloy had no cytotoxicity,and the adhesion of MC3T3 cells on its surface had no obvious difference with uncoated magnesium alloy.However,cell proliferation in coated magnesium alloy group was more than that in uncoated groups,which showed good cell compatibility. |
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