Study On The Degradation Behavior Of Mg/PLA Composite In Different Physiological Stress Environments

Magnesium alloy reinforced polylactic acid?Mg/PLA?composite has many characteristics like excellent mechanical property,good biocompatibility,controllable degradation,etc.Meanwhile some shortcomings such as the poor strength of PLA and the too fast corrosion rate of magnesium alloy are overcomed.As for the fact that the physiological stress has improtant influences on the degradation of implantable devices,in this paper,the composite with magnesium alloy wire?20vol%?and polylactic acid was prepared by hot pressing process.And the degradation behaviors of this composite in the simulated static,dynamic and step-wise physiological stress environment was studied,and the degradation kinetics was also discussed.The resultes are as follows:At the initial stage of unstress environment,the degradation of PLA matrix and magnesium alloy wire were promoted by improving the temperature of soaking liquid.Moreover,the degradation behavior of PLA matrix at different soaking temperature met the Arrhenius equation,where the hydrolysis activation energy E_a of PLA matrix is 112KJ/mol and the pre-exponential factor k₀ is1.036×10¹⁷day^-1.The bending strength of the composte met the linear relationship with 1/M_v,and the accelerating experiment could indirectly reflect the change of bending strength after a long-term degradation at normal experiment conditions.Static compressive stress would promote tthe degradation of composite material,and the synergy of temperature and static stress had larger influence than a single factor.The degradation behavior of PLA matrix at different soaking temperature also met the Arrhenius equation,where the activation energy and pre-exponential factor were smaller comparing to the unstressed one.The E_a and k₀ under 1.0MPa static compressive stress was 97.5KJ/mol and 5.83×10¹⁴day^-1,respectively.The increase of magnesium alloy wire content enhance the activation energy of PLA hydrolysis and the pre-exponential factor under both unstress condition and static compressive stress.Dynamic compressive stress could accelerate the degradation of the composite.Compared with those of the unstressed composite,the bending strength and PLA viscosity average molecular weight of the composite under a dynamic compressive stress at 0.2MPa and 1.0 Hz after 30 days degradation declined by 8.2%and 26.8%respectively,besides,the weight loss ratio increased by61.3%.Increasing the dynamic stress frequency could also accelerate the degradation.The degradation rate is faster when increasing the soaking temperature.The bending strength and PLA viscosity average molecular weight of the composite under the compressive stress at 1.0Hz and0.2MPa at 60^oC after 7 days degradation were decrease by 89.1%and 89.8%,comparing to those the composite at 37^o C.At the initial stage of degradation,the degradation behavior of PLA matrix at different soaking temperatures and the dynamic compressive stresses met the Arrhenius equation.With the increase of the volume fraction of magnesium alloy wire,the residual ratio of strength,the hydrolysis activation energy and the pre-exponential factor increase,and the higher the frequency and the compressive stress,the more obvious the trend was.The composite with 20vol%magnesium wires had a higher hydrolysis activation energy and pre-exponential factor than that with 10vol%magnesium wires,which were 112KJ/mol and 1.036×10¹⁷day^-11 respectively.Comparing with the composite under static compressive stress,the composite under dynamic compressive stress after 7days degradation had a smaller residual ratio of bending strength,but a larger PLA hydrolysis activation energy and a pre-exponential factor,which were 105KJ/mol and 1.17×10¹⁶day^-1respectively.The degradation behavior of Mg/PLA composites was different in different stages.Throughout the 21-day degradation period,compared to a single load,lower bending strength decay rate,viscous molecular weight of PLA matrix,degradation rate and weight loss ratio were observed from composites under two sets of staged stress.The hydrolysis rate K of PLA was initially low,then increased,which met the change rule of viscous molecular weight of PLA matrix of composite material under staged stress.