Biodegradable polymer/metal composites for bone repair: what standards apply?

摘要

Introduction: Occasionally bone needs the aid of a temporary implant in order to heal, the removal of which sometimes requires an undesirable second operation. A solution to this problem is nowadays offered by biodegradable polymers. These materials, however, have several drawbacks such as low mechanical resistance and bioactivity. A straightforward way to improve the performance of polymers is to combine them with suitable reinforcements. Several materials have been proposed for this purpose, and magnesium and its alloys are among them'1'. The standards for validating these composites should be carefully selected among those applicable to the individual components. In this work, a set of tests is presented that should be used to check the viability of the proposed novel composites. Materials: Polylactic acid (PLA) was used as the matrix, and Mg and Mg-5Zn particles ＜ 50 μm were used as reinforcements. Monolithic and composite specimens were processed by injection moulding and by extrusion plus compression moulding. Methods: In vitro tests were carried out to study degradation behaviour, biocompatibility and bacterial adhesion. The parameters that were checked were hydrogen release, pH, water uptake, mass loss and morphology, and the response of human mesenchymal stem cells and S. epidermidis bacteria. In addition, two types of mechanical tests were performed: compression and hardness. Results and Discussion: The in vitro tests for studying PLA/Mg-base composites were selected based on the information available for the monolithic components. For example, hydrogen release is a key aspect for Mg- base materials because of the serious problems that a fast production of gas may produce in the human body, but it is irrelevant for a polymer. Measurement of pH is relevant for both components, as PLA tends to lower it and Mg to raise it. Another relevant property is related to the water uptake and mass loss of the composite, which are indication of degradation. In the present study, we found that each specific property of one of the components is modulated by the presence of the other in a way that is not easy to envisage. For example, a high crystallinity degree of the matrix may be favourable for the monolithic polymer, but induces a faster degradation rate of Mg, so as to make the amorphous matrix more suitable for the composite. Also, the shape and size of the Mg particles not only modify the degradation rate of the reinforcement, but also the degradation rate of the polymer by varying the pH locally. Similarly, cell and bacteria responses are affected by a combined set of properties of both composite components. Conclusions: The biodegradable composites formed by a polymeric matrix reinforced with Mg-base material represent a challenge from the point of view of standardization, as foresee.