Experimental Mechanical Characterization of 3D-Printed Hydroxyapatyte and Finite Element Model Implementation of High Porous Scaffold for Bone Tissue Engineering

Bone is a self-healing tissue, but some pathologies, aging, non-physiologic conditions and trauma may cause a critical size bone defect that cannot heal spontaneously. The implant of bioceramic scaffolds is a promising approach for the treatment of these pathologic conditions. Among bioceramics, hydroxyapatite (HAP) is a good candidate for its chemical affinity with the native bone’s mineral component, thus ensuring biocompatibility and promoting osteo-integration. As HAP mechanical property is heavily influenced by its brittle nature, a robust experimental characterization of the bulk material is required for the design of mechanically reliable scaffolds.<br/>Furthermore, recent developments in 3D printing technologies allow for a wide variety of micro-structured devices meeting patient-specific needs. In this work, we deal with HAP material obtained through DLP-stereolithography which is a printing technology allowing for very fine spatial resolution and high printing fidelity. However, as the technological process includes a sintering phase at high temperature, the properties of the obtained material are strongly affected by the intrinsic defects which characterize the final product. For this reason, in this study, the mechanical characterization of the material samples having a simple geometrical shape and obtained through the same process as that used for the scaffold production and having the same characteristic size of the microstructural features of the scaffolds is proposed.<br/>For this purpose, we 3D-printed three different kinds of beams (i.e. simple beams, notched beams and cantilevers) in small scale (approximate size of beams 1mmx0.4mmx10mm). The samples have been subjected to confocal laser imaging, micro-Computed Tomography scanning and mechanical tests. The confocal laser scanning provided the size of the samples having the purpose to complement mechanical tests and to assess printing fidelity; the micro-CT analyses had the purpose to quantify the intrinsic porosity of the material resulting from the sintering process. The mechanical tests were micro-bending test and Berkovich nanoindentation.<br/>The micro-bending tests were used to assess the flexural stiffness and strength, while the nano-indentation tests have been used to verify whether elastic property exhibits a scale effect, expected in case micro-defects occur in the material.<br/>Using the same material and 3D-printing technology, high porous scaffolds were manufactured through DLP stereolithography. Micro-Computed Tomography scans were performed on the scaffolds to obtain the real geometry of the scaffolds. After a proper binarization process, a finite element model was implemented using the real geometry and the mechanical parameters found in the above-described experiments. Micro-CT based finite element analyses on the 3D-printed scaffolds were performed with the aim to determine the macroscopic elastic and strength properties of the scaffolds. The obtained macroscopic properties are validated through comparison with macroscopic experimental data already available.<br/>The experimental and numerical framework presented in this work has shown the capability of in-silico models of 3D-printed scaffolds to predict biomechanically relevant properties, provided that intrinsic properties of the materials are available with specific reference to the manufacturing process parameters and geometrical size of the micro-architecture.