Resumen de Procesado por técnicas coloidales y caracterización de materiales polímero/Mg para aplicaciones en la industria biomédica
During recent years, PLA / Mg composite materials have emerged as new bioabsorbable biomaterials for osteosynthesis applications. It has been proposed that the incorporation of Mg particles into a matrix based on a biodegradable polymer can address, on the one hand, the lack of bioactivity and the low mechanical properties of the polymers and, on the other, the high rate of degradation or corrosion of Mg and its alloys. Many methods for the preparation of polymer/bioinorganic material have been reported, some typical examples are extrusion mixing, compression molding and solvent casting. The manufacture challenge of these materials is to achieve a particle homogeneous dispersion in the final PLA structure.
This work reports on a novel processing route that is based on a colloidal mixture of Mg powder and PLA aimed to prevent the drawbacks associated to the polymer degradation during the mixing step of conventional thermoplastic routes and increase the maximum load achieved (10% by mass) during conventional routes. To fulfill this requirement it was necessary to improve the PLA-Mg interaction, as well as to increase the dispersion of the Mg in the final mixture through the surface modification of particles. Mg was modified by the adsorption of two different stabilizers, a surfactant (CTAB) and a polyelectrolyte (PEI). The dispersed and chemically stable suspension of modified Mg particles was mixed with the PLA solution in THF, and was used as feedstock material to obtain tapes, by tape casting, and granules, by drying the mixture under reduced pressure conditions. Besides, these granules were used as feedstock for further manufacturing of bulk cylinders by compression moulding and 3D printing scaffolds by Fused Deposition Moulding (FDM).
The results show that, in the tapes, particle surface modification is directly associated with good load dispersion through interactions between groups present in the PLA and in the stabilizers either by the formation of hydrogen bonds or by ionic and electrostatic interactions. In the case of granules and compression moulding cylinders it is observed a covalent bond between PEI and PLA, which is produced by the temperature applied during processing.
Mechanical behaviour was evaluated in terms of nanoindentation, mechanodynamics and tensile properties for tapes, and compression for compression moulding cylinders. In the case of tapes, a Mg reinforcement effect in the matrix is observed at the proximity of the particles. Young’s module and strength improve in tapes with up to 10 wt.% of Mg. At higher particle content, tensile mechanical improvement could not be verified due to fragilization of the materials. The thermopressed composites present a more evident improvement in the mechanical properties under compression with respect to the neat polymer, observing an increase of 10% in the elastic modulus for composites with 30 wt.% of Mg.
Regarding in vitro behavior, degradation and viability tests were carried out to verify that the new processing defined does not affect the biocompatibility and viability of PLA/Mg composites. The hydrogen release test shows that no material exceeds the amount tolerable by the human body, although the release is not proportional to the load, which is associated with a process of autocatalytic degradation. In the case of tapes, materials processed with modified Mg particles with PEI release less hydrogen than those modified with CTAB. Concerning biological tests, it is proved that the released components during the degradation of the composite do not produce cytotoxicity at the cellular level with fibroblastic cells (MEF) or stromal cells (ST-2). Finally, the direct tests carried out on the composite tapes allow concluding that for all the materials, except for the sample with 50 wt.% Mg, there is a significant increase in cell viability in comparison with the polymer matrix.
