Direct Ink Writing (DIW) 3D Printing of Biosilicate-Based Glass-Ceramic Scaffolds for Bone Tissue Engineering

Bone tissue has a complex and compact structure that accomplishes multiple functions in the human body: it sustains and protects internal organs, it participates and makes locomotion possible in the muscle-skeletal apparatus, it is also a mineral reserve and the site of mesenchymal stem cells. However, tissue abnormalities can arise from several reasons, including simple fractures, infections, tumor excisions, reconstructive surgeries, congenital deformities, and degenerative diseases. Bones have a great capacity to self-regenerate but only with small defects. For larger damages, this ability is insufficient and to overcome these conditions bone tissue engineering is becoming a solution studied by modern technology. This approach aims to restore the original healthy tissue with the help of biomaterials that mimic the ECM in terms of biomechanics and functionality. In particular, scaffolds are used as cell support, allowing them to populate the structures and construct new normal tissue. Bioactive glasses play a central role in this research field because of their osteoconductive and osteoinductive capabilities. Hence, they are able to not only integrate with the host tissue without rejection but even stimulate it to self-repair. This class of bioceramics can be silicates, borates, or phosphates when Si, B, or P oxides constitute their glass formers respectively. Because of their low solubility, silica-based bioglasses are the perfect choice for bone tissue regeneration. Among them, L. Hench discovered the composition of Bioglass® that today is considered the “gold standard”: it can bond with bones, inducing the formation of a hydroxyapatite layer on the biomaterial surface. Despite this advantageous biocompatibility, it has been noted that the bioactivity of Bioglass® reduces during the crystallization phase. So, the optimal densification characteristics cannot be fully achieved when crystallizes and even porous scaffolds are difficult to obtain. Given these limitations, glass-ceramics have been developed through a nucleation and growth treatment which makes possible the formation of crystals in an amorphous glass matrix. In this way, bioactivity is still present and the hydroxyapatite layer is not delayed. In particular, E. Zanotto by little modifying the traditional recipe, discovered Biosilicate® (23.75 Na2O, 23.75 CaO, 48.5 SiO2, 4 P2O5 wt. %). In the current thesis, the synthesis of Biosilicate-based scaffolds supported by preceramic polymer emulsions is proposed. Polymer precursors are used through a controlled heat treatment, transforming them into ceramics, but maintaining a high resolution and the possibility to be easily shaped like polymers. From the perspective of patient-specific medicine and increasingly customized models, 3D printing seems the perfect candidate. Specifically in this work, scaffolds have been printed with Direct Ink Writing (DIW) technique. With this additive manufacturing methodology, 800-1600 samples were printed using a 0.84mm nozzle starting from commercial silicon resin H44 as a precursor. After being subjected to a heat treatment until 1100°C, they were immersed in Acrylate Soybean Oil (ASO) and subsequently polymerized by UV lamp exposure. This infiltration was an attempt to toughen Biosilicate-based scaffolds which, in precedent works, were extremely porous and weak. In conclusion, the samples were characterized through assessment of morphology, composition, density, porosity, and compression tests in order to understand if these polymer-ceramic composite scaffolds could improve Biosilicate® mechanical properties.