THE MANY FACES OF REGENERATION: ENGINEERING MULTIFUNCTIONAL SCAFFOLDS FOR CRANIOMAXILLOFACIAL APPLICATIONS

A mandibular bone defect is a part of the lower jaw where bone is missing. People affected by large mandibular defects face consequences that go far beyond esthetics. For example, they often struggle to chew or speak, and this can affect daily life and overall well-being. Mandibular defects can arise from congenital conditions, accidents or, in most cases, from surgical removal of tumors in the mouth. Oral cancers continue to increase worldwide and so does the number of patients who require jaw reconstruction. To date, the gold-standard treatment has been to replace the missing bone with a piece of the patient’s own fibula, taken from the leg. However, the procedure is extremely complex. It requires long surgeries, advanced microsurgical skills, and two surgical interventions, exposing patients to pain and high-risk potential complications. In addition, the fibula does not naturally match the shape, size, or mechanical properties of the jaw. This mismatch can make it difficult to restore facial appearance or place dental implants correctly. These limitations have motivated researchers to explore new solutions based on bone tissue engineering, a field that aims to help the body regenerate its own bone. Instead of taking bone tissue from another part of the body, scientists design 3D porous structures (i.e. scaffolds), made from biodegradable materials that temporarily support cells as they grow new bone tissue. The scaffolds can be made of different materials and can be customized to fit the shape of a patient’s defect. The use of additive manufacturing technologies to fabricate these constructs gives great freedom in terms of shape and materials. Despite many promising results, engineered scaffolds still struggle to break into clinical use, mainly due to failure in repairing large defects and ensuring vascularization. The work described in this thesis focuses on developing new strategies to design scaffolds that can better support both bone formation and the growth of blood vessels, which are essential for the survival of newly formed tissue. We started by an analysis of the state of art (Chapter 2) to understand the main approaches that are currently being investigated and their limitations. We then explored several aspects involved in the design of a scaffold, presenting alternative or complementary solutions. One strategy focused on the shape of the scaffold pores (Chapter 3). Inspired by natural bone structures, we developed two different pore geometries (star and diamond) and showed their influence on stem cells differentiation into bone-forming cells. We found that diamond pores enhanced expression of osteogenic genes, while the star pores promoted stronger mineral deposition. These results highlighted how geometry can guide different aspects of bone formation. Another approach focused on the materials used to fabricate the scaffolds (Chapter 3 & 4). We proposed mixing polymers with bioactive materials containing ceramics or bioactive glasses. These materials resemble the chemical composition of natural bone and can release ions that stimulate cells to differentiate into bone-forming cells and encourage blood vessel growth. Our results showed that these composite scaffolds improved osteogenic and angiogenic marker expression in stem cells and the ability of endothelial cells to form vessel-like structures. To accelerate biological effects even further, we also developed surface coatings to release pro-angiogenic molecules from the scaffold surface (Chapter 5). These molecules not only induced hypoxic response, pushing cells to activate formation of blood vessels, but also enhanced the differentiation of stem cells into bone-forming cells. We demonstrated that this coating triggered activation of both angiogenic and osteogenic pathways, confirming the strong interplay between these two processes. Finally, we explored the possibility of pre-vascularizing scaffolds before implantation (Chapter 6). We created a hybrid scaffold by combining a hard scaffold with a soft hydrogel and were able to grow micro-capillary-like structures in vitro before implantation. This showed that a pre-vascularized construct could support early blood vessel organization, a critical step for improving implant integration in large bone defects. Together, these strategies provided new insights into the design of next-generation mandibular implants. Although challenges remain before such scaffolds can fully replace current surgical approaches, the work of this thesis pushes a step closer to less invasive and more effective treatments for patients who need jaw reconstruction.