Biodegradable polymer composites fabricated by 3D printing may overcome many of the challenges associated with permanent non-degradable metals (Ti, stainless steel, Co-Cr) or with biodegradable polymers (PLA, PLGA, and PCL) and biodegradable metals (Mg, Zn, Fe) by achieving a final material with synergistic properties for orthopaedic implant applications.

This thesis deals with the development of a filament production process for combining biodegrad[1]able polymers and metals through a two-step extrusion process. PLA was combined with Mg or Zn particles and extruded into filaments with constant diameter and a homogeneous dispersion of particles, suitable for 3D printing. This strategy was then used to 3D print biocompatible compos[1]ites comprised of medical grade PLDL with a 4% volume fraction of Mg or Zn metallic particles. The addition of Mg and Zn to the PLDL material was able to increase the degradation rate of PLDL, and reduce the acidity of the PBS environment at 37◦C over a 1-year period. The addition of the particles slightly increased the stiffness, but reduced the strength of the PLDL due to the stress concentrations around the metallic particles. The composite materials exhibited excellent biocompatibility properties in terms of the material-cell interactions and cell proliferation. This study demonstrates that the addition of metallic particles to the polymer matrix can tailor the degradation properties but does not improve the mechanical properties.

To overcome this latter limitation, a customised FFF 3D printer was developed to incorpo[1]rate continuous metallic wires into a polymer matrix. The 3D printer comprised of 4 individual print heads capable of printing with 4 different materials, one of which was a continuous metallic wire, while the others could print thermoplastic polymers/composites. Unidirectionally reinforced PLA/Al wire composites were manufactured with 15% and 25% volume fractions of Al wire. The composite reinforced with 25% volume fraction of Al wires showed a six-fold increase in elastic modulus while the strength improved by 63% with respect to the polymeric matrix. Furthermore, medical grade PLDL composite coupons unidirectionally reinforced with a 15% volume fraction of Mg wires were manufactured by 3D printing. Mg wires with and without a surface treatment by plasma electrolytic oxidation were used. The mechanical properties showed a three-fold increase in the elastic modulus and up to a 80% increase in tensile strength compared to the matrix in air and at ambient temperature. Excellent interface strength was observed in the composites containing the Mg wire modified by plasma electrolytic oxidation. The oxide layer on the Mg wires reduced the degradation rate of the Mg wires and suppressed pitting corrosion. The mechanical proper[1]ties of the PLDL matrix in the composite decreased dramatically when tested in water at 37◦C very likely because of the increased chain mobility induced by the disruption of the intermolecular interactions due to the synergistic contribution of water and temperature.

Finally, a multi-material composite material was fabricated using three print heads on the customised printer. The coupon consisted of various layers of PLA, PLA reinforced with Mg particles, and PLA reinforced with Mg wires. This proof-of-concept demonstrates the possibility to create 3D printed multilayer scaffolds in which the properties of each layer can be tailored to meet specific requirements in terms of mechanical properties, degradation rate, and cytocompatibility, opening the path to manufacturing 3D printed multimaterial biomedical devices.