One-step solvent-free process for the fabrication of high loaded PLA/HA composite filament for 3D printing

Calcium phosphate, like hydroxyapatite (HA), is the principal bioactive material for bone grafts substitute, but being ceramics materials, it is not suitable for low-cost 3D printing techniques, such as fused deposition modeling (FDM). To this aim, HA is usually mixed with a 3D-printable thermoplastic polymer, like polylactic acid (PLA), in order to realize customized substitutes by FDM technique. However, in order to enhance the bioactivity of the substitute, it is important to produce high hydroxyapatite-loaded 3D-printable filaments. In this study, an innovative one-step solvent-free process for the production of PLA filaments, loaded with different amounts of HA, up to 50 mass%, was proposed. The filaments have been firstly characterized by rheological and thermal analysis [thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)] in order to assess their suitability for the 3D printing process, in terms of viscosity at melting point and degradation temperature. The filaments were then characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy in order to analyze the dispersion of HA in the polymeric matrix and the influence of the inorganic filler on the amorphous structure of the polymer. All filaments produced evidenced a value of viscosity suitable for the FDM process at the melting point (measured by DSC analysis) and a degradation temperature (measured by TGA analysis) higher than the temperature selected for the 3D printing process, still evidencing a good dispersion of the HA powder in the PLA matrix, even at the highest content of the filler. For this reason, all filaments were used to build objects of simple geometry by FDM. The preliminary high loaded 3D-printed samples present the same thermal and structural properties of the filaments indicating that the printing process does not alter the properties of the composite material.