Degradation Behaviors Of Biomedical Magnesium Alloy And Its Composites In The Simulated Physiological Stress Environments

Physiological stress plays a significant role on human metabolism, which also influences the degradation behaviors of the biodegradable implants. Since magnesium alloy (Mg)presents widely biomedical applications to biodegradable implants for bone fixation or cardiovascular stents, the degradation behaviors of magnesium alloy with/without surface coatings in the simulated physiological stress environments (SPSEs) were investigated. Then,the novel biodegradable Mg wires/poly-lactic acid (Mg/PLA or MAWs/PLA) composites are designed and prepared. Thereafter, the degradation behaviors of Mg/PLA composite in the static and dynamic SPSEs are systematically investigated.The degradation behaviors of pure magnesium and its alloys in the static SPSE with the combined interaction of stress and chloride ions (Cl-) are investigated. The results show the degradation rate of Mg increases with the increasing of applied stress and Cl- concentration.For pure Mg,the degradation rate of the sample at 75 MPa in simulated body fluid (SBF) is about 2.4 times that of the unstressed sample. A quantitative relationship between the degradation rate of Mg and mechanical stress is proposed based on the electrochemical results.Further studies indicate the degradation behaviors of Mg are related to load mode and magnitude, which is more sensitive to the bending load comparing with the tensile load. The effect mechanism of stress in SPSEs with different concentrations of Cl- is proposed finally.The degradation behaviors of pure Mg with/without surface coatings in the dynamic SPSEs with different load parameters (including load magnitude and frequency) are studied.The results show that the dynamic load at a low frequency would slow down the degradation rate of pure Mg. After 24 hours immersion, the corrosion current density of Mg in the dynamic SPSE with a stress peak of 16 MPa and the frequency of 0.5 Hz is about 28% of that at the unstressed condition. It seems that dynamic load would promote the precipitation of Ca-P phase. For pure Mg with MgF2 or MgO coating, the dynamic load mainly affects the degradation behaviors of the samples by intensifying the damage of the coating and accelerating the deposition of Ca-P phase. When the load frequency is large (2.5Hz), the damage behavior of the coating plays a dominant role in the dynamic SPSE.Magnesium alloy and polylactic acid (PLA) are complementary to each other in mechanical and degradable behaviors. In this paper, a new degradable Mg/PLA composite with polylactic acid reinforced by magnesium alloy wires (Mg wires) was designed. Mg/PLA composites were prepared to meet the different performance requirements (isotropic or anisotropic) by directional strengthening of Mg wires, self-strengthening of matrix or the strengthening of two-dimensional braided Mg wires. A numerical model based on the equivalent section method (ESM) and finite element method (FEM) is proposed for tailoring the bending strength of the composite unidirectionally reinforced with Mg wires. The composite has excellent impact properties. The impact strength and the failure time for the composite with 20 vol% Mg wires are respectively 15.8 and 0.58 times those of pure PLA.The macroscopic and macroscopic mechanism for the impact behaviors is proposed with the corporation of FEM.In vitro degradation behaviors of Mg/PLA composite in the static SPSEs are investigated.The degradation kinetics of pure-PLA and the PLA matrix in the composite exhibit an Arrhenius-type behavior. For the composite, the synergic degradation of micro arc oxidized Mg wires keeps the steady pH value of immersion fluids and mitigates the degradation of PLA matrix during immersion. However, the static compression stress decreases the activation energy (Ea) and pre-exponential factor (k0) consequently increasing the degradation rate of PLA. Under a compression stress of 1 MPa,Ea and k0 of pure PLA are 57.54 kJ/mol and 9.74×107 day-1, respectively, but 65.5 kJ/mol and 9.81×108 day-1 for the PLA matrix in the composite. Accelerated tests are conducted by rising the immersion temperature in order to shorten the experimental time. Our analysis indicates there are well-defined relationships between the bending strength of the specimens and the PLA molecular weight during immersion,which are independent of the degradation temperature and external compression stress. Finally, a numerical model is established to elucidate the relationship of bending strength, the PLA molecular weight, activation energy, immersion time and temperature.In vitro degradation behaviors of Mg/PLA composite in the dynamic SPSEs are investigated. The results show that dynamic compression stress would accelerate the degradation of Mg/PLA composite and pure PLA. The degradation rate increases with the increasing of stress level and frequency. The influence of dynamic stress on the degradation behaviors of Mg/PLA composite is more significant than that under the static compression stress. The initial degradation behaviors of pure PLA and PLA matrix in the composite follow the first-order degradation kinetics and the synestic interaction of the two components in the composite is experimentally clarified. Finnaly,a numerical model for pure PLA and the composite is proposed to describe the relationship of bending strength, frequency, stress level and degradation time in the dynamic SPSEs.