Structure-Property Relationships of 3D Printed Thermoplastics for Implantable Orthopedic Devices

3D printing is a promising new technology with the potential to create rapid, high-quality, durable custom implants that will provide physicians and patients with currently unattainable clinical solutions. However, it is falling short on its promise due to its inability achieve adequate mechanical properties compared to conventional manufacturing techniques. This work explores the structure and properties of 3D printed thermoplastics with relevant medical applications through fused filament fabrication (FFF) processing. Four FFF materials (PLA, PEEK, PEKK, and PPSU) are investigated with an emphasis on characterizing printed weld formation and the corresponding mechanical properties. Each material has properties that are suitable for varying applications, but a pervasive requirement for success is strong weld formation between printed layers. Weld formation and strength was tested through tensile testing of bulk samples with layers oriented perpendicular to loading direction and tear testing of individual welds. While PLA, PEEK, and PEKK derive strength from their crystalline structures, crystallinity that developed during printing decreased the strength of materials as it inhibited polymer diffusion across the weld and prevented strong bonds from forming. Increasing crystallinity of as-printed amorphous samples through annealing did not result in higher strength for PLA or PEEK, due to the inability to co-crystallize across the weld interface. However, annealing amorphous PEKK samples resulted in a 26-40% increase in strength. While PEKK achieved the highest strength of 105 MPa, PPSU, a completely amorphous material, was closest to achieving conventional processing properties with a strength of 61 MPa.