Resumen de Processing and characterization of novel biodegradable and bioresorbable PLA/Mg composites for osteosynthesis
Sandra Carolina Cifuentes Cuellar
This doctoral thesis deals with the development of radically new composites for biodegradable and biocompatible implants of interest in bone repair, overcoming the disadvantages implicit in the use of permanent implants. The material consists of a polymeric matrix (Poly-L-lactic acid, Poly-L,D-lactic acid) reinforced with Mg or Mg5Zn particles, profiting advantages of both materials. The research of the feasibility of processing these composites by processes used in industry, and their suitability in osteosynthesis applications is the focus of this thesis. The project addresses the optimization of manufacturing techniques, design of PLA/Mg composites and their assessment for medical applications by means of physico-chemical, mechanical characterization and in vitro degradation tests. Processability has been demonstrated by high temperature processing techniques such as extrusion/compression and injection moulding. Thermal stability characterization has demonstrated that Mg decreases the thermal degradation of the composite, being the effect more detrimental when increasing the volume fraction of Mg. In all cases, however, the upper limit of the processing window is well above 200 ºC, which enables the use of industrial processes for manufacturing PLA/Mg composites. The scaling up of the extrusion process using a mid-size extruder enabled the reduction of the thermal degradation of the material and allowed the incorporation of Mg particles up to a 15 wt.% within both polymeric matrices. Mechanical characterization has been performed through compression/tensile tests and microindentation instrumented experiments, obtaining information on Young`s modulus and hardness over a wide range of strain rates. Mg particles have an effect during the elastic and post yield regimes but do not affect the yield point, as they increase the compressive Young´s modulus and the compressive strength at plateau of polymers but do not increase the compressive strength at yield. Instrumented indentation results show that addition of Mg particles increases the resistance of the polymer to plastic flow and hardness. The in vitro studies demonstrate that the degradation rate of these materials depend in great extent on crystalline degree, Mg content and Mg particle shape. All the composites release hydrogen at rates that could be tolerated by human body and, when immersed in PBS, they did not surpass the buffer capacity of the solution. In vitro studies performed on composites with 10 wt.% of particles, show that the material composed by an amorphous PLDA matrix reinforced with spherical particles of Mg exhibits the best degradation behaviour. PLDA10Mg-SPH underwent a loss of compressive resistance of 4% after 7 days, and 40% after 28 days. Novel PLA/Mg composites have a great potential as resorbable and biocompatible materials for osteosynthesis, due to their controllable degradation rate and adequate mechanical properties.
