Innovative biodegradable and bioabsorbable polymer matrix composites reinforced with magnesium-base particles (PLA/Mg) for bone repair

摘要

Introduction: Biodegradable materials for bone repair continue to gain popularity due to the avoidance of a removal operation. Constraints on current bioabsorbable polymers are related to their lack of bioactivity and low mechanical properties. Mg and its alloys are among the most interesting alternative options; however, their use as implants has been limited by their fast degradation rate. A new strategy has been developed using polymer/Mg-base composites, where the polymeric matrix will benefit from the Mg higher strength and modulus and Mg will benefit from the surrounding protective matrix. Here we present our latest results on these newly developed bioabsorbable composites. Materials: Poly-L-lactic and poly-(L-lactic-co-D,L-lactic) acid were used as amorphous and crystalline matrices, and irregular (IRR) and spherical (SPH) Mg and Mg-5Zn particles ＜ 50 μm in size were used as reinforcements. Monolithic and composite cylinders were processed by extrusion and moulded by compression. Methods: In vitro degradation behaviour and kinetics were addressed by hydrogen release assays, pH monitoring, water uptake, mass loss and changes in morphology. Compressive mechanical properties as a function of degradation time were also studied. In vitro biocompatibility was analyzed using human mesenchymal stem cells (MSCs). Bacterial adhesion experiments were carried out with S. epidermidis. Results and discussion: The composite with the semi-crystalline matrix presented faster degradation than its amorphous homologous. Within the amorphous matrices, PLLA/Mg releases hydrogen at a higher rate than PLDA/Mg. Composites reinforced with SPH particles present a slower degradation rate than those reinforced with IRR particles. The larger surface of Mg IRR particles increases the reaction sites and accelerates the appearance of macroscopic cracks in the composite at a faster rate than it is expected. The composite reinforced with SPH Mg-5Zn degrades faster than the one reinforced with pure SPH Mg particles due to the presence of MgZn precipitates. In spite of this, it exhibits better corrosion behaviour than the material reinforced with IRR Mg particles indicating that particle shape plays a more important role in controlling PLA/Mg degradation rate than the nature if particle itself. PLA/Mg composites exhibit higher compressive strength and Young's modulus than the neat polymer, approaching the values of cortical bone. Fig. 1: Accumulated amount of hydrogen released as a function of immersion time in PBS Reinforcement of the polymer with SPH Mg or Mg-5Zn particles improves viability of MSCs cultured on the materials. All bacteria deposited on PLDA are 100% viable, while bacteria attached to PLDA/Mg showed a loss in viability. Fig. 2: Cell viability on PLDA/Mg and PLDA/MgZn composites Conclusions: Mg combined with a polymeric matrix results in a synergistic effect and the disadvantages of one material are mitigated by the advantages of the other.