Tài liệu Master's thesis of engineering topological design of porous titanium alloy scaffolds for additive manufacturing of orthopaedic implant applications

Topological design of porous titanium alloy scaffolds for additive manufacturing of orthopaedic implant applications A thesis submitted in fulfilment of the requirements for the degree of Master of Engineering Li Yuan Bachelor of Engineering, RMIT University School of Engineering College of Science, Technology, Engineering and Maths RMIT University August 2020 Declaration I certify that except where due acknowledgement has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work which has been carried out since the official commencement date of the approved research program; any editorial work, paid or unpaid, carried out by a third party is acknowledged; and, ethics procedures and guidelines have been followed. I acknowledge the support I have received for my research through the provision of an Australian Government Research Training Program Scholarship. Li Yuan 4th August, 2020 1 Acknowledgements I would first like to thank my supervisors, Professor Songlin Ding and Distinguished Professor Cuie Wen for their support, assistance, supervision, and guidance throughout my thesis. Without their help, this would not have been possible. Additionally, I would like to thank Dr Guangxian Li for his kind assistance in the analysis and interpretation of data relating to the project. During the two years, I have learnt a lot under their guidance, and their expertise and advice have been enormously valuable throughout my research program. My particularly thanks go to my parents and my wife for their understanding and support. 2 Table of Contents Declaration ................................................................................................................1 Acknowledgements ....................................................................................................2 List of Figures ...........................................................................................................6 Nomenclature ..........................................................................................................10 Abstract ....................................................................................................................13 Chapter 1 Introduction ...........................................................................................15 1.1 Background ........................................................................................................ 15 1.2 Research Scopes................................................................................................. 18 1.3 Research questions ............................................................................................. 18 1.4 Research objectives, innovations and contributions .......................................... 19 1.5 Thesis outline ..................................................................................................... 21 1.6 Summary ............................................................................................................ 23 Chapter 2 Literature Review ...................................................................................24 2.1 Bio-related properties......................................................................................... 24 2.1.1 Non-toxicity and biocompatibility .......................................................................................................................... 25 3 2.1.2 Biomechanical properties .......................................................................................................................................... 26 2.1.3 Biodegradability for temporary implant materials ............................................................................................ 27 2.2 Porous structure and porosity of implant materials ........................................... 29 2.2.1 Porous structure with appropriate pore size and porosity ............................................................................ 30 2.2.2 Effect of porosity on biocompatibility ................................................................................................................... 30 2.2.3 Effect of porosity on mechanical properties ....................................................................................................... 33 2.2.4 Effect of porosity on biodegradability ................................................................................................................... 35 2.3 An overview of additive manufacturing (AM) .................................................. 36 2.3.1 AM procedures .............................................................................................................................................................. 40 2.3.2 Metallic AM techniques .............................................................................................................................................. 41 2.3.3 Other AM techniques .................................................................................................................................................. 43 2.3.4 SLS and SLM ................................................................................................................................................................... 44 2.3.5 EBM.................................................................................................................................................................................... 51 2.3.6 Metallic biomaterials fabricated by AM for implant applications ................................................................ 55 2.4 Summary ............................................................................................................ 57 Chapter 3 Structure design and modelling ............................................................58 3. 1 Triply periodic minimal surface (TPMS) structures ......................................... 58 3.2 Gyroid structure 3D modelling and procedures ................................................. 67 3.3 Summary ............................................................................................................ 73 Chapter 4 Structure analysis ..................................................................................74 4 4.1 Models preparation for finite element analysis and basic theories .................... 74 4.2 Young’s modulus analysis ................................................................................. 79 4.3 Further analysis of gyroid scaffold and introduced cubic .................................. 85 4.4 Summary ............................................................................................................ 89 Chapter 5 Conclusions ............................................................................................90 References................................................................................................................93 Appendices .............................................................................................................108 5 List of Figures Figure 1.1 Orthopedic implants in a knee replacement surgery .................................................. 16 Figure 1.2 The structure of the thesis ........................................................................................... 23 Figure 2.1 Magnesium scaffold structure for biomedical application ......................................... 26 Figure 2.2 Cross-section of human femur with porous structure (trabecular & cortical bone) ... 29 Figure 2.3 Ti scaffolds with 70% porosity and different ranges of pore sizes [41] ..................... 32 Figure 2.4 Scheme of a SLM machine [84] ................................................................................. 45 Figure 2.5 SEM images of Ti6Al4V gyroid lattice surfaces fabricated by SLM: (a) and (b) asbuilt, (c) and (d) after post treatments (heat treatment and sandblasting) [88]............... 48 Figure 2.6 CP-Ti scaffolds fabricated by different AM methods (a) SLM (b) EBM .................. 50 Figure 2.7 SEM images of Tie6Ale4V gyroid lattices surfaces fabricated by EBM: (a) as-built, (b) after post treatment of ceramic blasting [20]. ........................................................... 52 Figure 2.8 (a) Schematic of an EBM machine and (b) its processing chamber [91,92] .............. 54 6 Figure 3.1 Primitive TPMS unit cell and Primitive TPMS structure ........................................... 59 Figure 3.2 Gyroid unit cell with ±0.6 offset ............................................................................... 60 Figure 3.3 3D CAD gyroid unit cells: (a) 3 mm sheet solid gyroid unit cell with 0.3 mm offset thickness and (b) 3 mm network solid gyroid unit cell at 50% volume fraction ............ 65 Figure 3.4 A block of a 3D CAD gyroid scaffold in different views (constituted by 3 mm network solid gyroid unit cell) ........................................................................................ 66 Figure 3.5 (a) a single unit gyroid surface covered by a cubic block (b) a single cell of network based gyroid .................................................................................................................... 69 Figure 3.6 Gyroid surfaces and network-based on gyroid unit cell with different offset (α) values: (a) a 3 mm network-based gyroid structure in an 3 × 3 × 3 mm cubic; (b-1) gyroid surface without offset, (b-2) network-based gyroid unit cell without offset, (c-1) g ...................................................................................................................................... 71 Figure 3.7 Schematic of a normal pore and a deformed pore ...................................................... 73 Figure 4.1 The scaffold structure in .stl format ind different views (a) top view (b) front view (c) 3-dimensional view (d) righ-side view ........................................................................... 77 Figure 4.2 Cross-section of gyroid scaffold in .stl format ........................................................... 78 7 Figure 4.3 Cross-section of single unit gyroid (fulfilled) ............................................................ 79 Figure 4.4 (a) 3mm gyroid unit cell under compression (b) 3mm gyroid unit cell under under tension (c) the Ti scaffold structure under compression (d) the Ti scaffold structure under tension ................................................................................................................... 81 Figure 4.5 3 mm gyroid unit cell compression ............................................................................ 83 Figure 4.6 1×2×3 units compression ............................................................................................ 84 Figure 4.7 3 mm gyroid unit cell tension ..................................................................................... 84 Figure 4.8 2×2×3 units compression ............................................................................................ 85 Figure 4.9 3 mm unit cubic cell modelling and simulation (b) 1×2×3 units cubic scaffold modelling and (c) meshing (d) simulative results of cubic unit cell (e) simulative results of cubic unit scaffold ...................................................................................................... 87 Figure 4.10 Computational result of 3mm gyroid scaffold under compression .......................... 88 Figure 4.11 Computational result of 3mm cubic scaffold under compression ............................ 89 8 List of Tables Table 1 Summary of different AM methods…………………………………………38 Table 2 Features of SLM and EBM in comparison………...……………………………49 Table 3 Summary of different as-built Ti-6Al-4V TPMS architectures .……………63 Table 4 Basic material mechanical property (Ti6Al4V) setup in NX Nastran………….…79 9 Nomenclature AM Additive manufacturing; TPMS Triply Periodic Minimal Surfaces; SLM Selective laser melting; P Porosity; ρstructure Density of the bulk alloy; ρmaterial Density of the porous structure; σpl Plastic collapse strength; E Elastic modulus of the porous material; ρ Density of the porous material; ρs Density of the solid material; 10 σys Yeild strength of the solid material; Es Elastic modulus of the solid material; r Corrosion ratio; M1 The object mass; M2 The object mass after corrosion; ti Immersion time; FDM Fused deposition modelling; PBF Powder bed fusion; SL Stereolithography; DED Direct energy deposition; LOM Laminated object manufacturing; 11 SLS Selective laser sintering; EBM Electron beam melting; α Offset value; L The variable determining the size of unit cell's edge length; 12 Abstract With the development of material science and biological technology, new biocompatible materials such as titanium alloys are gradually used more and more in the manufacturing of medical implants to replace conventional materials including stainless steel and Co-Cr alloys. These new materials showed excellent performances in the load-bearing implant applications and can be used as a replacement of bones and joints in clinical surgeries. Nowadays, with the assistance of additive manufacturing technology, medical implants with complicated topological structures and shapes which were previously not manufacturable with conventional manufacturing approaches are designed and fabricated. It not only significantly increases the feasibility of bone joint replacement by biomedical materials but also has dramatic influences on the development of modern medicine. However, the design of new implants is a challenging task in order to ensure the high quality of implants to avert both biocompatible and mechanical risks. Firstly, when designing the implants, the properties of the biomaterials should be considered, including the physical properties (e.g. density, elastic modulus) and the chemical properties (e.g. biocompatibility). Secondly, the potential negative effects on bone defect treatment should be examined to avoid issues caused by any inappropriate mechanical behaviour of materials. Finally, and more importantly, due to the mismatch of elastic modulus between natural bone and the implants, stress shielding which is another major mechanical issue of metallic implants has to be minimized. A large mismatch of elastic modulus may lead to bone atrophy and implant loosening. 13 A new porous titanium scaffold which exhibits an architecture mimicking that of human bones was developed in this research for the application of implants. The scaffold will be constructed by multiple unit cell sizes, which enables adjusting the elastic modulus to meet different requirements. The porous structure can provide the necessary support for cells to proliferate and maintain their differentiated functions, and their structure defines the ultimate shape of the new bone created during growth processes. A comprehensive literature review was conducted by summarizing the state-of-the-art technologies and critical issues associated with the results of the biomedical porous material fabricated with different methods. Different triply periodic minimal surface (TPMS) structures and their characteristics were investigated, especially for gyroid structures. Three-dimensional modelling of gyroid structure (unit cell and small scaffold) were conducted for further analysis of the design. To address the issue of the higher elastic modulus of Ti, a 3mm Network-based gyroid unit cell and small scaled scaffolds previous designed were analysed in regard to the mechanical behaviours. Specifically, the models were optimized for evaluating the Young’ modules, in order to meet the certain requirement. In addition, a 3mm cubic open-cellular structure was developed as the comparison group in this simulation. Keywords: Metal implant design; Additive manufacturing; Porosity; Stress shielding; TPMS structures; Gyroid 14 Chapter 1 Introduction 1.1 Background Bone is an anisotropic connective tissue consisting of hydroxyapatite, collagen and water. It provides frames for skeleton structural stability, protects vital organs and specialized tissues, and supports the mechanical actions of soft tissues. Bones have excellent regenerative properties and self-healing abilities for the body to recover from physical injury. However, bone's regenerative ability becomes weaker when people become older; and if suffered from a serious trauma or a systemic bone disease, bones may be found extremely difficult to recover their self-healing functions. The needs for orthopaedic implants have dramatically increased in the last two decades. Patients are expecting treatments that allow them to maintain their daily activities and quality of life. Taking Knee replacement as an example (Figure 1.1), the rates of total knee replacement (TKR) have been significantly increased from 1991 to 2006 [1]. In the US, patients spent over US$9 billion on the total knee arthroplasty (TKA) in 2009 and the major demand of TKA was in group aged 45–64 years [2]. The value of the biomaterials market was US$94.1 billion in 2012, and increased to US$134.3 billion in 2017 worldwide [3]. This rapid increase in the biomaterials market has to a certain extent brought benefits for the development of bone tissue engineering (BTE). 15 Figure 1.1 Orthopedic implants in a knee replacement surgery [4] Advanced biomaterials, fabrication methods, and the structural designs of medical devices have been greatly improved in the last twenty years. Materials for medical applications need to meet several criteria, and designed implants should morphologically mimic bone structure and support bone tissue formation (osteogenesis). Biocompatibility, mechanical properties, and biodegradability are the fundamental elements that must be considered. The structure of bone is almost completely constituted by hydroxyapatite crystal, a mineral form of calcium apatite, within an organic matrix of collagen [4]. Of this collagen, 95% is type I, providing the structural integrity for connective tissues in bones, tendons, and ligaments. The remaining 5% of the bone is a combination of proteoglycans and numerous non-collagenous proteins. An effective implant will be accepted by the human body and function properly. An inferior orthopaedic device can trigger serious issues in patients. In Australia, the Therapeutic Goods Administration (TGA) provides 16 regulations for medical devices, such as Australian regulatory guidelines for medical devices (ARGMD), which provide guidance to assist the manufacturers and sponsors of medical devices in meeting the regulatory requirements for legally supplying a medical device in Australia [5,6]. Metal-AM fabrication has been extensively explored in the last decade and this technology has been successfully used in the biomedical field. 3D printed physical models can provide a detailed visualization of clinic cases before surgery, which is a practical approach for surgery planning and accurate diagnosis. Surgeons and medical doctors can simulate surgery processes on 3D printed models to examine the outcomes of the surgery and find any potential surgical risk and failing factors [29, 30]. More importantly, AM technology can produce customized implants for bone replacement and fixation. Since the current implant structure design becomes increasingly complex to meet different requirements (e.g., interconnected porous TPMS scaffold), it could be challenging to manufacture such a metallic implant via conventional fabrication methods such as casting, forging, milling, and turning. This technology is also capable of fabricating complex porous structures with both micro and macro porosity. Due to the high solidification rate of AM techniques, AM-produced implants may exhibit high strength [31]. The choice of implant materials should meet the requirements of the specific implant to ensure nontoxicity and biocompatibility, a porous structure with appropriate pore size and porosity, suitable biomechanical properties including appropriate elastic modulus and high strength, and also biodegradability for temporary implant materials. 17 1.2 Research Scopes The scope of this research is to investigate the mechanical behaviours of alloyed Ti scaffolds fabricated by selective laser melting (that is, a powder bed fusion technique) with different geometries. The structure of the scaffolds will be designed in 3D models and their mechanical properties will be analysed with finite element method. The size effect of the cells on the mechanical behaviour of different scaffolds is going to be investigated, which will be the principle for the optimization of the structure design of scaffolds. The fabrication process of the scaffolds will be analysed, and the effects of different parameters on the quality and performance of the scaffolds after sintering will be investigated. 1.3 Research questions 1) Which TPMS geometric structure provides acceptable mechanical properties in scaffold design? TPMS describes a periodically infinite structure along three independent directions with zero mean curvature of the surface (the concave and convex curvatures are symmetrical at all points. Porous architectures with TPMS topology are constructed by repeating elements with the minimum possible area (unit cells). TPMS is also defined by implicit functions and can be distinguished by curved surfaces even at the junction of struts. In this thesis, the influence of TPMS structure on the mechanical properties of Ti scaffolds is investigated. 18 2) What are the mechanical property differences among the scaffolds with unit cell size of 2 mm, 2.5 mm, 3 mm and mixed sizes? The main properties of the Ti scaffolds are Young’s modulus and the compressive strength which strongly affected by the structure of the scaffolds. The proper cell sizes for Ti scaffolds are 2 mm, 2.5 mm and 3mm. In this thesis, the size effect of the cells was investigated via FE analysis and experiment. The properties of even-sized scaffolds and mix-sized scaffolds are compared. 3) What are the possible reasons that cause the mechanical property differences between the SLM-built and the designed models? This thesis comprehensively analyses the reasons that affects the mechanical properties including the size of cells, the geometry of the cells and the machining parameters used in laser-based AM. 1.4 Research objectives, innovations and contributions Based on the scopes and questions of the research, the overall objective is to investigate different biomedical Ti scaffolds fabricated by selective laser melting. The detailed objectives of this study are: • To use computational modelling and analysis to design porous scaffold structures. 19