Materials for medical applications need to meet several criteria, and designed implants should morphologically mimic bone structure and support bone tissue formation osteogenesis.. Since
Introduction
Background
Bone is an anisotropic connective tissue composed of hydroxyapatite, collagen, and water, providing structural support, protecting vital organs, and enabling mechanical functions of soft tissues It possesses remarkable regenerative and self-healing properties, allowing recovery from physical injuries However, as people age, bone regenerative capabilities decline, and severe trauma or systemic bone diseases can significantly impair the body's ability to heal bone fractures effectively.
The demand for orthopaedic implants has surged dramatically over the past two decades, driven by patients' desire to preserve their daily activities and quality of life Specifically, total knee replacement (TKR) procedures have seen a significant increase from 1991 to 2006, reflecting a growing need for effective joint solutions In the US alone, over US$9 billion was spent on total knee arthroplasty (TKA) in 2009, with the highest demand among patients aged 45–64 years The global biomaterials market, valued at US$94.1 billion in 2012, expanded to US$134.3 billion by 2017, facilitating advancements in bone tissue engineering (BTE) and transforming orthopedic treatment options.
Figure 1.1 Orthopedic implants in a knee replacement surgery [4]
Over the past twenty years, significant advancements have been made in biomaterials, fabrication techniques, and the structural design of medical devices Ideal implants must replicate the morphology of bone and support osteogenesis, while meeting essential criteria such as biocompatibility, mechanical strength, and biodegradability Bone's structure predominantly consists of hydroxyapatite crystals embedded within an organic collagen matrix, with 95% of this collagen being type I that provides vital structural integrity Additionally, bone contains proteoglycans and non-collagenous proteins, which contribute to its complexity An effective implant should be well-accepted by the human body to ensure proper function; otherwise, substandard orthopedic devices can lead to serious health complications In Australia, the Therapeutic Goods Administration (TGA) regulates the approval and safety of medical implants to safeguard patient health.
The Australian Regulatory Guidelines for Medical Devices (ARGMD) are essential regulations that guide manufacturers and sponsors in complying with legal requirements to supply medical devices in Australia These guidelines provide clear guidance to ensure that medical devices meet safety, quality, and performance standards before entering the Australian market Adhering to the ARGMD is crucial for obtaining necessary certifications and achieving regulatory compliance, thereby facilitating smooth market entry and maintaining device safety standards.
Metal additive manufacturing (AM) has been extensively researched over the past decade and successfully applied in the biomedical industry This technology enables the production of detailed 3D printed models that aid in pre-surgical visualization, planning, and accurate diagnosis Surgeons can simulate procedures on these models to assess outcomes and identify potential risks Additionally, AM allows for the fabrication of customized implants for bone replacement and fixation, addressing the complexities of modern implant designs such as interconnected porous TPMS scaffolds Unlike traditional manufacturing methods like casting and milling, AM can produce intricate porous structures at micro and macro levels, often resulting in implants with enhanced strength due to high solidification rates.
Choosing the right implant materials is crucial to ensure biocompatibility and safety, emphasizing the importance of non-toxicity and biocompatibility An optimal implant should have a porous structure with suitable pore size and porosity to promote tissue integration Additionally, it must possess appropriate biomechanical properties, including the right elastic modulus and high strength, to match the surrounding tissue For temporary implants, biodegradability is essential to facilitate natural healing and eliminate the need for removal surgery.
Research Scopes
This research explores the mechanical behavior of alloyed titanium scaffolds fabricated via selective laser melting, a powder bed fusion technique, with various geometries The scaffolds will be designed as 3D models, and their mechanical properties will be analyzed using finite element methods to ensure accurate simulation The study investigates how the size of the scaffold cells influences their mechanical performance, serving as a basis for optimizing scaffold design Additionally, the fabrication process will be examined, focusing on how different parameters affect scaffold quality and performance after sintering, ultimately aiming to enhance the structural integrity and functionality of titanium scaffolds for biomedical applications.
Research questions
1) Which TPMS geometric structure provides acceptable mechanical properties in scaffold design?
TPMS (Triply Periodic Minimal Surfaces) describe a continuously infinite crystalline structure extending in three independent directions, characterized by surfaces with zero mean curvature, where concave and convex curvatures are symmetrical at all points Porous architectures featuring TPMS topology are created by repeating unit cells that minimize surface area, resulting in efficient and lightweight structures Defined mathematically through implicit functions, TPMS structures are distinguished by their smoothly curved surfaces, even at strut junctions, enhancing both structural integrity and aesthetic appeal This thesis investigates how TPMS-based architectures influence the mechanical properties of titanium scaffolds, aiming to optimize their performance for biomedical applications.
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 primary properties of titanium (Ti) scaffolds, including Young’s modulus and compressive strength, are heavily influenced by their internal structure Optimal cell sizes for Ti scaffolds are identified as 2 mm, 2.5 mm, and 3 mm, which were analyzed through finite element (FE) modeling and experimental methods This research investigates how cell size impacts scaffold performance, comparing the properties of uniform-sized scaffolds with those of mixed-sized configurations to determine the most effective structural arrangements for enhanced mechanical properties.
3) What are the possible reasons that cause the mechanical property differences between the SLM-built and the designed models?
This thesis provides a comprehensive analysis of the factors influencing the mechanical properties in laser-based additive manufacturing (AM) It highlights the critical roles of cell size, cell geometry, and machining parameters, demonstrating how these elements directly impact the structural integrity and performance of the final product By examining these variables, the study offers valuable insights into optimizing laser-based AM processes for improved mechanical outcomes.
Research objectives, innovations and contributions
This study aims to comprehensively investigate biomedical titanium scaffolds fabricated through selective laser melting, focusing on their structural and functional properties The primary objective is to evaluate the quality and performance of these Ti scaffolds for biomedical applications Additionally, the research seeks to analyze the influence of fabrication parameters on scaffold porosity, mechanical strength, and biocompatibility By examining various design configurations, the study aims to optimize Ti scaffold properties to enhance tissue integration and promote better healing outcomes Overall, this research contributes valuable insights into the potential of selective laser melting technology for producing effective biomedical titanium scaffolds.
• To use computational modelling and analysis to design porous scaffold structures
• To build up different TPMS (Triply Periodic Minimal Surfaces) geometries with different unit cell sizes (unit cell size 2mm, 2.5mm, 3mm and mixed sizes)
• To analyse the mechanical properties of the SLM-manufactured scaffold samples
• To compare and analyse the mechanical property of the designed models
This research introduces a novel approach to scaffold structure design focused on optimizing mechanical properties The key innovation lies in tailoring the scaffold's Young’s modulus to fall between 7 and 30 GPa, effectively reducing mechanical risks This targeted range ensures the scaffold provides adequate strength and stability, promoting safer and more reliable biomedical applications.
This research advances the development of additive manufacturing (AM) in the biomedical field by enabling the production of complex, bio-mimic structures that overcome limitations of traditional manufacturing methods Ensuring implants have an appropriate elastic modulus matching that of natural bone is crucial to prevent mechanical damage and stress shielding-related failures Recently, attention has shifted towards open cellular structures with increased porosity, which reduce elastic modulus and improve compatibility with bone tissue While previous studies primarily focused on single unit cell sizes, such as 2 mm gyroid or 3 mm diamond structures, this research explores multi-unit cell size scaffolds, allowing for adjustable elastic moduli tailored to specific clinical requirements.
Thesis outline
The thesis consists of five chapters which aim to investigate the mechanical properties and fabricating processes of different biomedical Ti scaffolds, with the structure presented in Figure 1.2
Chapter 1 is a generalized introduction to the content of the thesis, including the background, scopes and objectives of the research
Chapter 2 offers a comprehensive literature review, highlighting the importance of analyzing related articles and technical papers as a foundational step for this research The review emphasizes key areas such as titanium alloys, 3D printing technology, and TPMS (Triply Periodic Minimal Surface) structures, which are essential for achieving the research objectives and ensuring a solid knowledge base.
In addition, literature review is a good way for preparation of upcoming experiments.
Chapter 3 presents the design and fabrication of the Ti scaffolds Firstly, the computational modelling and simulation was the preparation step for 3D printing The mechanical properties of the as-designed models were investigated under numerical analysis (Finite element analysis) of mechanical properties to compare with as-build models K3Dsurf will be applied initially to generate the unit cell of the desired TPMS models Then, the generated models were imported into Rhinoceros, Netfabb and 3ds Max for creation and optimization of 3D printing objects Once the design objects have been made by CAD software applications, the files have to be saved as STL format for further printing purposes In laboratory sessions, the generated STL files was printed
SLM (Selective Laser Melting) is a laser-based additive manufacturing method that utilizes a focused laser beam to scan and melt metal powder layers at predefined speeds and locations This technology enables the fabrication of complex metal structures, including open cellular geometries The primary material used in this process is Ti-6Al-4V, a titanium alloy renowned for its strength, durability, and suitability for advanced engineering applications.
Chapter 4 focused on analyzing the mechanical properties of the fabricated Ti scaffolds, with microstructures observed via Scanning Electron Microscopy (SEM) to ensure quality and consistency Compression tests were conducted to evaluate the mechanical strength of each scaffold, and the results were compared with the Young’s modulus of cortical bone, which ranges from 7 to 30 GPa, to ensure patient safety The study also involved comparing the experimental results with those from other researchers to identify potential discrepancies and understand their causes These findings are essential for optimizing scaffold design and ensuring their mechanical compatibility with natural bone tissue.
Finally, a conclusion including the findings and main contributions from this research, as well as future works related to this research were presented in Chapter 5
Figure 1.2 The structure of the thesis
Summary
This chapter presents the background, research scopes, research questions, research objectives, innovations and outline of the thesis
Literature Review
Bio-related properties
Materials for medical applications must meet strict criteria, including biocompatibility, mechanical strength, and biodegradability Designed implants should accurately mimic the natural bone structure to facilitate osteogenesis and promote effective bone tissue formation Ensuring these key properties is essential for the success and safety of bone regeneration and repair procedures.
A clinical-level orthopaedic implant must be highly biocompatible and free of toxic substances to ensure safe and positive cell activities Metals used for implants should be non-toxic, as toxic substances can harm living cells and organisms, with heavy metals like mercury, lead, and cadmium posing significant health risks While some metals such as iron, zinc, and copper are essential minerals for the human body, excessive levels can become toxic Metal compounds also pose health risks due to their tendency to chemically break down in aqueous environments, potentially releasing harmful substances or forming residuals Biocompatibility encompasses complex interactions between materials and biological systems, emphasizing not just safety but also the ability to support tissue healing, reconstruction, and integration Therefore, the concept of biocompatibility has evolved from merely being non-toxic to actively promoting tissue growth and regeneration, which is crucial for implant success and tissue engineering applications through scaffolds and other biomaterials.
26 for osteogenesis and the materials of scaffolds should be compatible with the primary bone cells (osteoblasts, osteocytes, and osteoclasts), facilitating new bone formation, remodelling, and healing [14–16]
Figure 2.1 Magnesium scaffold structure for biomedical application [14,15]
Metallic biomaterials like titanium (Ti) and its alloys are extensively used in load-bearing implants due to their excellent mechanical properties, including high strength, a suitable elastic modulus, fracture toughness, and fatigue resistance High-strength implants enable patients to engage in physical activities while reducing fracture risks Appropriately matched elastic modulus prevents stress shielding, which occurs when a significant mismatch causes the implant to bear more stress than the surrounding bone, potentially leading to implant failure.
Stress shielding, caused by the mismatch in elastic modulus between metal implants and natural bone, leads to bone atrophy and potential implant loosening Cancellous bone has an elastic modulus ranging from 22.4 to 132.32 MPa, while cortical bone’s elastic modulus is much higher, between 7.7 and 21.8 GPa Ideally, metal implants should have an elastic modulus similar to that of natural bone to prevent stress shielding However, most metallic implants, such as commercially pure titanium and Ti6Al4V, have elastic moduli around 112 GPa, significantly higher than that of bone, increasing the risk of implant failure.
115 GPa, respectively, much higher than that of cortical bone Thus, reducing the elastic modulus to an appropriate value is important for implant design
2.1.3 Biodegradability for temporary implant materials
Recent advancements in additive manufacturing have enabled the fabrication of metal implants with open-cellular structures that meet key requirements such as low elastic modulus, promoting new bone tissue ingrowth and vascularization Despite these innovations, concerns remain regarding the long-term stability of implants, as metals like titanium and its alloys do not degrade and remain permanently in the body as foreign objects This permanence can lead to complications including bacterial infections, long-term endothelial dysfunction, physical irritation, and chronic inflammatory reactions, ultimately impacting patient quality of life.
Medical implants crafted from non-biodegradable materials such as titanium alloys and stainless steels can interfere with the natural bone growth, often requiring secondary surgery to provide additional support.
Developing biodegradable metals for implant applications offers a promising solution to issues related to permanent implants, as these materials gradually degrade and are replaced by new tissue growth Biodegradable implants, such as magnesium (Mg)-, iron (Fe)-, and zinc (Zn)-based alloys, are ideal for temporary medical uses like bone fixation and vascular stents due to their ability to safely resorb within the body The degradation rate of these alloys is a critical factor in ensuring their effectiveness and safety Among these, zinc (Zn)-based alloys have been identified as the most suitable biodegradable metals, exhibiting degradation rates that align well with tissue healing processes.
20–300 μm/year in vitro [23–25] The degradation rates of Mg-based alloys and Fe-based alloys are generally higher than 300 μm/year and lower than 50 μm/year in vitro, respectively [26,27]
Recent studies have explored the corrosion behavior and mechanical stability of biodegradable metallic materials for medical implants Bowen et al [28] observed that pure Zn wire used as cardiac stents in rats exhibited a corrosion rate of approximately 20 μm/year during the first three months, which more than doubled over six months Additionally, biodegradable metals fabricated with additive manufacturing (AM) techniques demonstrate adequate mechanical properties for implant applications; for instance, Li et al [29] developed an Fe-based diamond lattice scaffold via selective laser melting (SLM) that maintained sufficient mechanical strength after four weeks of immersion, with only a 7% decrease in elastic modulus and a 5% reduction in yield strength.
Porous structure and porosity of implant materials
Biomedical implants should feature a porous structure that closely resembles natural bone tissue, enhancing integration and success rates Porosity refers to the percentage of void space within a solid material, which is a critical factor in implant design The porosity (P) of biomedical implants can be quantified using the gravimetric method, ensuring accurate assessment of their void volume to improve biocompatibility and structural performance.
The density of the bulk alloy (𝜌_material) is a key parameter, calculated as the mass of the structure divided by its volume (𝜌_structure = m/V) This chapter focuses on analyzing the properties of the desired porous structure, regardless of porosity defects, to ensure accurate characterization and optimization of the material's performance.
Figure 2.2 Cross-section of human femur with porous structure (trabecular & cortical bone)
2.2.1 Porous structure with appropriate pore size and porosity
Bone is an open-cell, porous composite material produced by osteoblasts, classified into compact and cancellous types Compact bone forms the hard outer shell with lower porosity, while cancellous bone features a highly porous, lighter, and more delicate structure with porosity ranging from 30% to 95% and pore sizes between 200 and 1000 μm Recently, there has been increased interest in developing porous bone implant materials that mimic the architecture of natural bone, as their porous structure facilitates new bone tissue ingrowth and circulation of body fluids Additionally, porous scaffolds provide essential support for cell proliferation and differentiation, guiding the shape and growth of new bone tissue during healing and regeneration.
2.2.2 Effect of porosity on biocompatibility
Biomedical implants and devices with high porosity are increasingly favored due to their superior biocompatibility, which enhances bone regeneration and healing Porosity plays a critical role in facilitating bone ingrowth within metal implants, making it a key factor in their successful integration and long-term performance.
In an early study, Kuboki et al (2010) investigated osteogenesis induced by bone morphogenetic protein (BMP) using hydroxyapatite with both solid and porous particles in rats Their findings demonstrated that osteogenesis occurs exclusively in porous structures, highlighting the importance of porosity for bone regeneration An effective porous scaffold must be open-cellular and interconnected to facilitate optimal cell distribution and migration, which are critical factors for successful osteogenesis and bone tissue engineering.
31 facilitating blood vessel formation [39] Due to cell size, the minimum pore size is required to be
Optimal scaffold design for enhanced osteogenesis involves pore sizes around 100-350 μm, which facilitate bone cell migration, bone ingrowth, and capillary formation [39, 40] Specifically, implants with pores ranging from 200–350 μm effectively support new bone tissue development and vascularization, promoting successful osseointegration [32] To maximize bone tissue ingrowth in scaffold applications, a minimum porosity level of 60% is recommended, with pore sizes between 200 and 1200 μm, ensuring sufficient space for cell proliferation and nutrient transport [20].
Research by Torres-Sanchez et al demonstrated that porous Ti scaffolds with different pore sizes (45–106 μm, 106–212 μm, 212–300 μm, and 300–500 μm) influence osteoblast behavior, with small pores enhancing cell attachment and larger pores supporting proliferation Specifically, scaffolds with 45–106 μm pores showed rapid cell growth in the initial days, attributing this to increased surface area, though growth slowed after three days Conversely, scaffolds with 300–500 μm pores exhibited a delayed but significant increase in cell proliferation by day 12 Woodard et al distinguished macro-porosity (>50 μm) and micro-porosity (