Additive manufacturing of novel biocompatible metal scaffold for biomedical applications
Creators: name: ATAEE, Arash
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401607
subject: Metals and alloy materials

description: Applications of titanium and its alloys have increased in the biomedical industry over the past few decades. To date, commercially pure titanium (CP-Ti) and Ti-6Al-4V (Ti64) have been widely used due to their superior biocompatibility, specific strength, and corrosion resistance. While Ti64 has been the material of choice for load-bearing applications, CP-Ti, on the other hand, shows lower strength and wear resistance, which limits its application where combined high strength and wear resistance are required. Currently, the mismatch of elastic modulus between dense metal implants (including Ti64 and CP-Ti) and the surrounding bone is one of the main challenges to achieving a good biofixation of implantation. While implants need to have a low modulus to match that of bone, the implant materials must be strong and durable enough to endure applied physiological loads and also are expected to serve for an extended period or even until a lifetime without failure or revision surgery.<br><br>
To overcome this issue, open cellular foams or scaffolds were introduced in biomedical applications which can have significantly reduced stiffness or elastic moduli whilst providing the potential for complete bone ingrowth and ensure a long-term biological fixation. Over the last few decades, achievements have been made to design and manufacture of porous scaffolds that mimic the mechanical behavior of natural bone, however, most of these studies are focused on regular lattice-based geometries with straight-edged struts and pores and sharp turns which might not be ideal for cell attachment, migration, and proliferation.<br><br> 
Recently, a class of periodic minimal surfaces belonged to implicit functions are introduced as attractive candidates for the design of biomorphic scaffold architectures that are known as triply periodic minimal surfaces (TPMS). TPMS have smooth boundless surfaces with mean surface curvature of zero that mimics the topology of trabecular bone.<br><br>
Currently, design and manufacturing of biocompatible metallic scaffolds that simultaneously mimic the topological and biomechanical properties of the natural bone are highly demanded and rarely found in the literature. In this research, Schoen's gyroid (G) structure (a type of TPMS) has been developed and manufactured from Ti64 and CP-Ti by electron beam manufacturing (EBM) and selective laser melting (SLM) methods, respectively. The microstructure, mechanical properties, failure mode, wear resistance, and in vitro biocompatibility of the scaffolds with different unit cell sizes and sample orientations were characterized and evaluated for their suitability for bone implant applications.<br><br>
Ti64 gyroid scaffolds with ~80% porosity at various unit cell sizes were successfully manufactured by EBM and discussed in chapter 4. As-built Ti64 scaffolds exhibited comparable elastic modulus and yield strength to those of trabecular bone and ranged from 637 to 1084 MPa and from 13.1 to 19.2 MPa, respectively. However, the manufactured scaffolds presented several manufacturing irregularities and material agglomerations that lead to considerable mechanical anisotropy in scaffolds within smaller unit cell sizes. All scaffolds showed higher elastic modulus and yield strength in transverse directions and the scaffolds with the largest gyroid unit cell (3 mm) exhibited the lowest mechanical anisotropy. This issue suggested that the smallest manufacturable gyroid unit cell with 70% porosity and lowest manufacturing irregularities is about 3 mm by EBM under the condition of the studied processing parameters. Moreover, it was observed that the relationship between the elastic modulus anisotropy and structural anisotropy ratio is consistent and displays a linear relation with high correlation. These anisotropic mechanical properties are comparable to those of trabecular bone which may assist to decrease the stress shielding effect further. However, as-built EBM manufactured Ti64 scaffolds exhibited completely unsuitable brittle behavior under compression, and the dominant failure mode was found to be by formation of crush bands at peak load with the angle of ~45° to the loading axis. These properties highlighted the necessity of post heat treatment to decomposite the dominant acicular martensitic microstructure which is responsible for the brittleness of the scaffold. However, post heat treatment is not generally preferred by the medical industry as this would require additional time and incur an extra cost. Therefore an attempt has been made to manufacture gyroid scaffolds with higher accuracy, enhanced mechanical properties, higher biocompatibility and also no need for post heat treatment.<br><br> 
Gyroid scaffolds from CP-Ti grade 1 powder were selected to be manufactured by SLM. The microstructure, mechanical properties, failure mode of CP-Ti gyroid scaffolds with different unit cell sizes and sample orientations are characterized and discussed in chapter 5. The SLM manufactured scaffolds exhibited high geometrical accuracy and much lower manufacturing irregularities in comparison to the same scaffolds from Ti64 by EBM. The scaffolds exhibited ultrahigh strength properties and high hardness of 2.27GPa which is 50% higher than that of dense CP-Ti manufactured by traditional cast method. The elastic modulus and yield strength of the as-built scaffolds ranged from 1465 to 2676 MPa and from 44.9 to 56.5MPa, respectively, values which are close to the elastic modulus of trabecular bone and strong enough to bear the physiological loading of implants. Moreover, unlike the as-built Ti64 scaffolds manufactured by EBM, these scaffolds exhibited excellent ductility achieving up to 50% strain and no sign of fracture up to 20-30% strain under compression. The dominant compressive response of the scaffolds was found to be by formation of a plastic hinge which led to the rotation of the struts about the plastic hinges followed by development of local shear bands in struts in the long plateau region.<br><br> 
In the next step, the wear resistance and in vitro biocompatibility of the EBM manufactured Ti64 and SLM manufactured CP-Ti scaffolds were studied and discussed in chapter 6. Nanoindentation was used to assess the nanowear properties of the tiny struts of gyroid scaffolds. It was found that all 3D printed gyroid scaffolds exhibited higher reduced modulus, nanohardness and wear resistance in the transverse direction than those in building direction. Moreover, SLM manufactured CP-Ti scaffolds showed superior wear resistance over cast dense CP-Ti with 75% lower mean worn height during a nanowear test. During the in vitro cell culture study, SLM manufactured CP-Ti scaffolds showed higher cell viability, and cell adhesion density in comparison with EBM manufactured Ti64 scaffolds. In addition, among scaffolds studied in this study, gyroid scaffolds with larger unit cell sizes (G3) showed higher cell adhesion and proliferation of SaOS2 cells.<br><br>
In conclusion, this study demonstrated that SLM is a promising manufacturing method for the fabrication of CP-Ti scaffolds with enhanced mechanical properties for biomedical applications. SLM manufactured CP-Ti gyroid scaffolds with ~70% porosity exhibited boosted mechanical properties including a high nanohardness of ~3.4 GPa, excellent wear resistance and biocompatibility. The results of this research will contribute to lay the foundation for the design, characterization, and development of novel titanium alloys and scaffolds via additive manufacturing for both biomedical and various advanced engineering applications.