Investigation Of The Fabrication And Properties Of Magnesium Alloy Wires/poly(lactic Acid) Composites

The biodegradable composite rods with Mg-2Zn wires reinforced poly（lactic acid）（PLA）for potential application in orthopedic internal fixation are prepared by hot pressing（HP）and hot drawing（HD）method in this work.The composite rods take advantage of magnesium alloy and PLA on the basis of their complementary mechanical and degradation properties.Further,the influence of processing method and microstructure on the mechanical and degradation properties as well as cytocompatibility is deeply investigated.Different surface modifications on Mg-2Zn wires are performed including micro-arc oxidation（MAO）and chemical modification methods such as hydrofluoric acid（HF）,dopamine（DA）andγ-aminopropyltriethoxysilane（KH550）.The surface roughness of wires after MAO is 192.0±5.0nm,most of pore size is in the range of0.5¹.5μm.This kind of microstructure is beneficial to the formation of mechanical interlocking between Mg-2Zn wire and PLA during the fabrication process.The influence of different interface on the interfacial bonding strength is quantitatively evaluated by single wire pull-out experiments.The interfacial bonding state and mechanism between Mg,MgO,MgF₂ and PLA,respectively,are analyzed according to molecular dynamic simulation results by Materials Studio software.It reveals that the interfacial interaction between Mg and PLA is van der Waals force.Additionally,a strong electrostatic interaction between Mg from MgO or MgF₂ and carbonyl oxygen from PLA is formed.Therefore,the interfacial bonding strength and mechanical strength of MAO-Mg2Zn wires/PLA composite rods is the best owing to the mechanical interlocking and electrostatic interaction.The mechanical strength of MAO-Mg2Zn wires/PLA composite rods is obviously improved on the basis of the strengthen and toughness effect of the directionally distributed wires in PLA matrix,and the improved interfacial bonding strength between MAO treated Mg-2Zn wires and PLA,as well as the self-reinforced PLA matrix after HD.The mechanical strength of composite rods increases with the increase of wire volume content.The bending strength of HP MAO-Mg2Zn wires/PLA composite rod researches 210MPa when the wire volume content is 20vol%,which is 2.3 times of that of pure PLA rod with the value of only 90MPa.After HD three passes for the HP composite rod with the initial diameter of 6 mm,the improvement of bending strength has been over 60%.The relationship between the bending strength and the volume content as well as the distribution of wires is analyzed and compared theoretically.The regularly arrangement of PLA chains after HD is beneficial to increase the orientation,crystallinity,melting temperature of PLA,as well as the mechanical strength of HD composite rod.According to the in vitro degradation results,a mathematic model is established,which not only reveals a well-defined relationship between the bending strength,molecular weight,and immersion time,but also can be used to predict the degradation period and further control the degradation rate by adjusting wire volume content and processing steps.Degradation/corrosion starting from the peripheral outer surface plays a dominant role in the degradation process supported by XR-CT results,rather than from the ends,based on the improved interfacial bonding state.The improved orientation and crystallinity slow down the degradation rate of PLA and corrosion rate of wires in the HD-three passes composite rod.The degradation mechanism and rate of PLA strongly rely on wire volume content.For both HP and HD composite rods,more wires content will result in an increase in the degradation rate of PLA owing to the alkaline catalytic hydrolysis.The cytocompatibility of Mg-2Zn rod,PLA rod,HP and HD composite rods is evaluated by co-culturing MC3T3-E1 cells with the extracts of each group of sample.The results indicate that except the Mg-2Zn rod presenting slight cytotoxicity,the other three groups of sample display good cytocompatibility.