DEVELOPMENT OF ORGANIZED POROUS NETWORK STRUCTURED ZINC SCAFFOLD USING 3D PRINTING AND MICROWAVE SINTERING

Abstract

Every year, millions of people across the globe go through bone grafting and prosthesis transplantation to treat various kinds of bone defects and injuries, thereby making it the most widely transplanted tissue in the world. Consequently, there is a rising demand for advancement in bone tissue engineering (BTE) to enrich the lives of those affected. The need for BTE becomes apparent when natural bone healing processes prove insufficient to address critical-size defects. Traditionally, autografts and allografts are often used to address such situations. However, these approaches have some disadvantages, such as stress shielding, postoperative complications, and immune reactions. Hence, it becomes essential to explore other alternatives for such cases. One possibility could be using 3D porous scaffolds in bone tissue engineering. These provide optimal growth conditions, thus encouraging migration and bone regeneration. Bone scaffolds usually act as temporary frames or structures that encourage an environment that is helpful to the development and growth of bone tissue cells. The surface morphology of scaffolds is an important factor for their success since roughness promotes cell proliferation, adhesion, and differentiation. The commonly used metallic biomaterials for scaffold fabrication are titanium, stainless steel, and cobalt-based alloys. These biomaterials offer good biocompatibility, excellent mechanical strength, and corrosion resistance, making them suitable for orthopedic applications. Biodegradable metals, such as iron, magnesium, and zinc, are another class of materials. They spearheaded the new developments in making temporary implants by matching their degradation rates with the tissue healing process. Among iron, magnesium, and zinc, zinc is the best choice for degradable materials due to its preferential biocompatibility, physiological need, and degradation rate. For biodegradable metals, powder metallurgy, replication method, and space holder method are common processes or conventional ways to fabricate the bone scaffold. However, these methods produced irregular porosity distribution in scaffolds and compromised structural integrity. To address this issue of the conventional method, advanced techniques such as additive manufacturing are adopted for scaffold manufacturing. Additive manufacturing allows for the generation of complex porous structures with organized porosity and controls the pore size, shape, and their distribution within the scaffold. Furthermore, despite the benefits of additive manufacturing (AM), these advanced methods contain several drawbacks. Challenges with additive manufacturing include poor surface finish, high equipment cost, and low range of material versatility. To address the issues associated with AM, indirect solid free-form manufacturing or Rapid Tooling methods have come into the picture. The Rapid Tooling method gives superior surface finishes with broader biomaterial choices and is a far more cost-effective technique for the mass production of metal scaffolds. By combining the cost-effective polymer 3D printers with conventional manufacturing methods such as casting, sintering, and injection molding in the Rapid tooling (R.T) approach, good product quality can be achieved economically. The current investigation aims to produce metallic zinc scaffolds using the Rapid Tooling method. The scaffolds developed in this study are specifically fabricated for orthopedic applications. Various biodegradable materials have been studied for bone tissue engineering in the past decade. Despite various endeavors, biomaterials like iron and magnesium have seldom achieved an ideal combination of mechanical strength, biocompatibility, and controlled corrosion behavior. Due to the requirements, zinc-based alloys have become more popular as they have outstanding properties for bone regeneration, such as biocompatibility, corrosion behavior similar to the bone healing rate, minimal toxicity, and corrosion resistance. Hence, the relatively new approach to zinc scaffolds used in the current thesis draws attention from the existing issues with iron and magnesium and opens up new developments concerning metallic scaffold manufacturing. Based on 3D printing and microwave sintering, the scaffold-developing process, as reported in the present work, is the foundation for Rapid Tooling. The Rapid Tooling technique, applied in the present investigation, builds metallic zinc scaffolds with an organized porous network structure (OPNS). The developed R.T method controls the structural characteristics of the zinc scaffolds, such as porosity, interconnectivity, and pore size. Moreover, the R.T. method proves to be a cost-effective process for manufacturing metallic scaffolds. Various characterizations and investigations are performed to confirm the feasibility of the R.T. method in developing metal scaffolds for bone tissue regeneration or load-bearing orthopedic applications. The scaffold characteristics, including mechanical strength, corrosive behavior, and cytocompatibility, are assessed through characterization. The strength of the scaffolds is used to determine the capacity to resist the physiological loads placed on them, and corrosion behavior, i.e., degradability is determined to determine the corrosion behavior of developed zinc scaffolds in the body in the long run. Cytocompatibility and direct cell counting studies are conducted to evaluate the biological behavior of zinc scaffolds. The result of the biological study shows the scaffold's interaction with tissue cells. During the evaluation, cell adhesion, proliferation, and differentiation are tested on the scaffold surface. There are several research objectives that cover many key aspects of scaffold evolution and development in this thesis. The first objective is to design and develop the R.T. method, which is versatile for a wide range of metals. After that, the parameters of the 3D printing and microwave sintering for metallic zinc scaffold fabrication are determined by focusing on the parametric optimization of the Rapid Tooling method. Then, this research aims to determine optimal structural and mechanical properties for orthopedic applications. Finally, the degradation behavior and cytocompatibility of metallic zinc scaffolds are evaluated, particularly focusing on the influence of scaffold porosity in in vitro conditions. In summary, the research presented in the thesis contributes to the field of Bone Tissue Engineering (BTE) by proposing a new fabrication approach for metallic scaffolds for orthopedic applications. Hence, by overcoming existing challenges in fabricating metallic scaffolds and exploring a promising newly developed method, this research may explore the opportunities for the further development of BTE and the improvement of the quality of patients in orthopedic surgery.