Additive Manufacturing of hydroxyapatite scaffolds for bone repair

In the last decades, bone tissue regeneration has been one of the most discussed research topics in the field of regenerative medicine. Bone tissue is able to regenerate itself but, in the presence of extensive bone defects, surgery is required to insert a bone graft that will promote healing. Synthetic bone grafts, also known as scaffolds, are porous 3D structures able to imitate the extracellular matrix (ECM) of natural bone and help natural tissue regeneration. Additive manufacturing (AM) plays an important role in the creation of scaffolds with a highly porous and interconnected architecture. The present experimental work aimed at producing hydroxyapatite (HA) highly-interconnected porous scaffolds able to imitate the trabecular architecture of spongy bone. For this purpose, Digital Light Processing (DLP) technology based on stereolithography was innovatively used in order to reproduce the 3D architecture of commercial polyurethane sponges, basing on CAD models deriving from microtomographic reconstruction. Unlike the classic stereolithographic technique, DLP polymerizes an entire layer at a time, drastically reducing processing times. The ceramic scaffolds were manufactured at the Austrian company Lithoz GmbH using the stereolithographic system CeraFab 7500, and later characterized at the Department of Applied Science and Technology (DISAT) of Politecnico di Torino (Italy). Commercial HA powder supplied by Lithoz was used as starting material for scaffolds manufacturing: synthetic HA is, indeed, highly appreciated in Bone Tissue Engineering (BTE) due to its similarity to natural HA, which constitutes more than 60% of the inorganic bone matrix. During the stay in Lithoz, much management has been dedicated to the definition of the slurry composition (LithaBone 480E) and its characterization. In particular, slurry stability was carefully evaluated over a period of 5 weeks, monitoring rheological properties, relative density, maximum bending strength and agglomeration tendency of suspended ceramic particles. Afterwards, HA scaffolds were produced; in particular, a total of 30 cubic scaffolds and 90 cylindrical scaffolds with a 2:1 aspect ratio were printed. Despite the high porosity and the extremely fine trabeculae of the structure, the printing process was successful and no defects were detected after sintering treatments. Microstructural, compositional and morphological characterization of HA scaffolds was carried out at the DISAT laboratory of the Politecnico di Torino. HA was the only crystalline phase detected by X-ray diffraction (XRD), proving that no phase transitions occurred upon high temperature sintering treatment. Scanning Electron Microscopy (SEM) morphological analysis revealed the presence of highly interconnected macropores in the range 200-800 µm; minimal interstitial porosity was observed, thus indicating that a good sintering level was achieved, fundamental to ensure good mechanical performances. Ca/P atomic ratio was 1.89 ± 0.092, really close to the stoichiometric one. In light of the results obtained, the coupling of DLP technology and polymeric sponge µ-CTs can be considered a promising strategy for the realization of HA-based ceramic scaffolds, very faithful to the natural architecture of the spongy bone even though an industrial template was used. Future analysis will be necessary to evaluate the mechanical and biological properties of the manufactured scaffolds both in vitro and in vivo.