Staged Degradation Behavior Of Mg/PLA Composite In The Low Frequency And Dynamic Load Environments

Magnesium alloy and polylactic acid(Mg/PLA)composite can complement each other in mechanical properties,acidity and alkalinity of degradation products,difficulty of degradation and degradation rate.It has good biocompatibility,biodegradability,mechanical compatibility,and has broad application prospects in the field of implant fracture fixation devices.Considering that the fixation devices in human body will be affected by different physiological stresses in different service stages,the study of stage degradation behavior of Mg/PLA composite in simulated physiological stress environment can predict the performance changes of degradable fracture fixator after implanted in human body.In this paper,Mg/PLA composites reinforced by directional alignment of magnesium alloy wires(MAWs)in PLA matrix were prepared by hot pressing.Based on the self-made physiological stress experimental platform,the degradation behavior of Mg/PLA composites with different content of magnesium alloy wires under unstressed,static stressed and dynamic compressive stressed in simulated physiological environment were studied.The different effect of stage dynamic stress and consistent dynamic stress were discussed,which provides a reference for predicting the performance change and degradation rate of fracture implant materials in human body.The main results are as follows:The degradation products of PLA and MAWs have synergistic effects.They neutralize each other to stabilize the solution pH.The acid degradation products of PLA promote the corrosion of MAWs and make them produce more alkaline degradation products.Low content of MAWs has a moderating effect on the degradation of PLA,while higher content of magnesium alloy wire inhibits the moderating effect,and PLA has a higher crystallinity.After 30 days of degradation,the composites with high content of magnesium alloy wires have better flexural strength retention.Stress can accelerate the degradation of pure PLA and its composites,and the dynamic compressive stress is more prominent than the static compressive stress.The flexural strength and molecular weight of 10 vol% Mg/PLA composites degraded under dynamic compressive(1MPa,1Hz)stress for 30 days are 9.2% and 12.5% lower than that under static compressive stress(1MPa),respectively.Under the condition of staged dynamic loading,even if the later loading is the same,the greater the stress in the earlier stage,the greater the acceleration effect on the degradation of composite materials,and the longer the time of the greater pressure effect,the more obvious the acceleration effect.Compared with the staged dynamic compressive stress condition,the graded dynamic load combined with compressive stress and bending stress can effectively promote the rapid degradation of composites,but the single dynamic bending stress has a strong destructive effect on composites.The early degradation behavior of composites can be divided into three stages,which accords with the first-order degradation kinetics.It is found that there is a good linear relationship between the change of bending strength and reciprocal molecular weight of pure PLA and composites.A mathematical model of the relationship between the change of bending strength of composites and the applied load and degradation time under a consistent dynamic compressive stress is established.The model can predict the degradation behavior of composites under dynamic compressive stress of different sizes,and is suitable for graded dynamic compressive stress conditions.