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Topological Design and Additive Manufacturing of Porous Metals for Bone Scaffolds and Orthopaedic Implants: A Review
Abstract
One of the critical issues in orthopaedic regenerative medicine is the design of bone scaffolds and implants that replicate the biomechanical properties of the host bones. Porous metals have found themselves to be suitable candidates for repairing or replacing the damaged bones since their stiffness and porosity can be adjusted on demands. Another advantage of porous metals lies in their open space for the in-growth of bone tissue, hence accelerating the osseointegration process. The fabrication of porous metals has been extensively explored over decades, however only limited controls over the internal architecture can be achieved by the conventional processes. Recent advances in additive manufacturing have provided unprecedented opportunities for producing complex structures to meet the increasing demands for implants with customized mechanical performance. At the same time, topology optimization techniques have been developed to enable the internal architecture of porous metals to be designed to achieve specified mechanical properties at will. Thus implants designed via the topology optimization approach and produced by additive manufacturing are of great interest. This paper reviews the state-of-the-art of topological design and manufacturing processes of various types of porous metals, in particular for titanium alloys, biodegradable metals and shape memory alloys. This review also identifies the limitations of current techniques and addresses the directions for future investigations.
... DfAM processes entail unique considerations, depending on the process to be used. In direct energy deposition (DED), a process that also uses a laser or electron beam as an energy source, designers must ensure proper cooling of the metal parts during the build process to prevent distortion and minimize the use of support structures; in PBF, thorough selection of powder material and consideration of layer thickness and build orientation is required [3,89]. DfAM to the FFF process is peculiar, takes in the limitations of the process, and considers these constraints like part of the design process, orientation, and overhangs, support structures for steep overhangs, wall thickness, part consolidation, and geometric complexity [78,90,91]. ...
... DfAM guidelines and best practices consider the unique DfAM processes entail unique considerations, depending on the process to be used. In direct energy deposition (DED), a process that also uses a laser or electron beam as an energy source, designers must ensure proper cooling of the metal parts during the build process to prevent distortion and minimize the use of support structures; in PBF, thorough selection of powder material and consideration of layer thickness and build orientation is required [3,89]. DfAM to the FFF process is peculiar, takes in the limitations of the process, and considers these constraints like part of the design process, orientation, and overhangs, support structures for steep overhangs, wall thickness, part consolidation, and geometric complexity [78,90,91]. ...
Fused filament fabrication (FFF) is an extrusion-based additive manufacturing (AM) technology mostly used to produce thermoplastic parts. However, producing metallic or ceramic parts by FFF is also a sintered-based AM process. FFF for metallic parts can be divided into five steps: (1) raw material selection and feedstock mixture (including palletization), (2) filament production (extrusion), (3) production of AM components using the filament extrusion process, (4) debinding, and (5) sintering. These steps are interrelated, where the parameters interact with the others and have a key role in the integrity and quality of the final metallic parts. FFF can produce high-accuracy and complex metallic parts, potentially revolutionizing the manufacturing industry and taking AM components to a new level. In the FFF technology for metallic materials, material compatibility, production quality, and cost-effectiveness are the challenges to overcome to make it more competitive compared to other AM technologies, like the laser processes. This review provides a comprehensive overview of the recent developments in FFF for metallic materials, including the metals and binders used, the challenges faced, potential applications, and the impact of FFF on the manufacturing (prototyping and end parts), design freedom, customization, sustainability, supply chain, among others.
... In knee replacement surgery, artificial arthroplasty aims to relieve pain, improve function, and restore range of motion in the patients [3]. Anyhow, failures still occur after the replacement surgery such as loosening, wear, and stress shielding of the implant [4]- [5]. In a previous study, an assessment of the population was conducted to show the factors that affected the failure of tibial tray performance [6]. ...
... Porosity ∅ = 1 -Volume of scaffold Volume of solid structure (4) ...
The medical industry benefits greatly from the additive manufacturing (AM) technology used on customized products. Total knee arthroplasty (TKA) has been widely used however it has drawbacks of stress shielding and loosening due to the excessive daily routine of patients. The problem could be minimized by applying lattice structures to the implant and mimicking the actual density of human bone. This study aims to investigate the optimal design of a Ti6Al4V alloy tibial tray by applying different types of lattice structure designs. A finite element analysis was used to investigate the mechanical behavior of uniform and non-uniform lattice structures in a walking position. Functional gradation structure was optimized on selected regions of the tibial tray with weight reduction and adaptation to the near actual density of the human bone without compromising its mechanical performance. The results indicated that the Voronoi structure has improved stress behavior and the capability to withstand the loads exerted, based on the Von Mises stress result of the Voronoi structure at 35.83 MPa as compared to the gyroid and diamond structures at 61.65 MPa and 49.74 MPa, respectively. The optimal design of the tibial implant was achieved by functionally graded lattice structures, replacing the solid tibial implant. Nurasyrani et al. 154
... The porous structure provides bone growth channels growth, and the integration between the porous structure and new bone increases local stability while reducing the risk of implant dislocation [10]. In addition, the porous scaffold is closer to the bone structure [11], and the porous structure can effectively reduce the elastic modulus, thus reducing the stress shielding effect [12]. The porous structure facilitates the transport of nutrients and promotes bone growth [13]. ...
... Due to the mismatch of material stiffness and the inert isolation of high-density materials, the contact surface between the fusion cage and the vertebral endplate cannot achieve the effect of close contact and match, which may lead to complications such as micromovement and subsidence of the implant [41,42]. A novel design concept is thus adopted, including redesigning the outer frame of the fuser according to the effect of topology optimization at the macro level and filling the optimized gaps with TPMS lattices or BCC lattice at the micro level, and the three designed porous cages demonstrate the mechanical strength, corrosion resistance and biocompatibility of titanium alloy (Ti-6Al-4 V), as well as a lower elastic modulus than titanium alloy, which is beneficial to improve the stress shielding of the contact surface [12]. Its porous structure, similar to bone, provides favourable conditions for bone ingrowth. ...
Background
Porous cages are considered a promising alternative to high-density cages because their interconnectivity favours bony ingrowth and appropriate stiffness tuning reduces stress shielding and the risk of cage subsidence.
Methods
This study proposes three approaches that combine macroscopic topology optimization and micropore design to establish three new types of porous cages by integrating lattices (gyroid, Schwarz, body-centred cubic) with the optimized cage frame. Using these three porous cages along with traditional high-density cages, four ACDF surgical models were developed to compare the mechanical properties of facet articular cartilage, discs, cortical bone, and cages under specific loads.
Results
The facet joints in the porous cage groups had lower contact forces than those in the high-density cage group. The intervertebral discs in all models experienced maximum stress at the C5/6 segment. The stress distribution on the cortical bone surface was more uniform in the porous cage groups, leading to increased average stress values. The gyroid, Schwarz, and BCC cage groups showed higher average stress on the C5 cortical bone. The average stress on the surface of porous cages was higher than that on the surface of high-density cages, with the greatest difference observed under the lateral bending condition. The BCC cage demonstrated favourable mechanical stability.
Conclusion
The new porous cervical cages satifies requirements of low rigidity and serve as a favourable biological scaffold for bone ingrowth. This study provides valuable insights for the development of next-generation orthopaedic medical devices.
... Podstatnou podmienkou pre vytvorenie skafoldu je jeho porézna štruktúra. Poréznosť imituje prirodzenú štruktúru spongiózneho kostného tkaniva s prepojenou sieťou pórov dostupnou pre bunky, ich rast a transport živín [1]. V roku 2016 Wang et al. v práci publikoval svoje zistenia o vplyve veľkosti pórov na urýchlenie osteointegrácie, ktoré by mali byť v rozmedzí 0,1-0,9 mm [2]. ...
... Titánové biomateriály sú stále vo veľkej miere používané na výrobu implantátov. Za predpokladu správneho spracovania, umožňujú spoľahlivý dlhodobý výkon implantátu aj v záťažových situáciách[1,9]. Wang et al. v práci poukázal na skafoldy vyrobené z titánu a jeho zliatin, ako vhodné na výrobu nosných implantátov a opravu kostných defektov, využívaných predovšetkým v ortopédii, vďaka svojim vynikajúcim mechanickým vlastnostiam, dobrej odolnosti proti korózii a priaznivej osteointegrácii[2]. ...
... Unlike ceramics and polymers, porous metallic materials have the advantage of balanced mechanical properties and a unique skeletal structure, which expands their application possibilities in orthopedics [109]. Metallic matrices can have a homogeneous or irregular pore size [110,111]. Homogeneous pore size allows for controlled porosity, providing predictable mechanical properties and scaffold biocompatibility [112,113]. However, the human trabecular bone does not have a consistent porosity, so homogeneous porous matrices are not optimal for cell adhesion and proliferation. ...
We overview recent findings achieved in the field of model-driven development of additively manufactured porous materials for the development of a new generation of bioactive implants for orthopedic applications. Porous structures produced from biocompatible titanium alloys using selective laser melting can present a promising material to design scaffolds with regulated mechanical properties and with the capacity to be loaded with pharmaceutical products. Adjusting pore geometry, one could control elastic modulus and strength/fatigue properties of the engineered structures to be compatible with bone tissues, thus preventing the stress shield effect when replacing a diseased bone fragment. Adsorption of medicals by internal spaces would make it possible to emit the antibiotic and anti-tumor agents into surrounding tissues. The developed internal porosity and surface roughness can provide the desired vascularization and osteointegration. We critically analyze the recent advances in the field featuring model design approaches, virtual testing of the designed structures, capabilities of additive printing of porous structures, biomedical issues of the engineered scaffolds, and so on. Special attention is paid to highlighting the actual problems in the field and the ways of their solutions.
... [20,53,[55][56][57][58] The NiTi-alloy class was a highly promising multifunctional material with desirable structural and functional properties for various biomedical and engineering applications. [28,59,60] NiTi-alloys show low Young's moduli (E) of 20-90 GPa, [4,5] depending on their phase composition, making them ideal for developing novel low-modulus alloys for loadbearing implant applications. NiTi samples were fabricated using a commercial L-PBF machine (Renishaw RenAM 500E) equipped with a fiber laser with a maximum power of 500 W and beam size of 80 μm, on a NiTi baseplate. ...
The development of cellular materials with adjustable mechanical properties, total porosity (Tp), and elastic modulus (E) similar to those of natural bone are eternal pursuits in the field of bone implants. Designing and manufacturing an implant that fulfills all these requirements is a challenging task. In this study, inspired by the trabecular structure of natural bone and developed a biomimetic structural material. Laser powder bed fusion (L‐PBF) is utilized to create the biomimetic structural material. Comprehensive valuations of both the natural trabecular bone and biomimetic structural material are conducted using micro‐CT scanning, nanoindentation testing, finite element (FE) analysis, and compression testing. The results demonstrate that the mechanical properties of the developed biomimetic structural material have excellent controllability. The rod‐plate‐like trabecular (RPT) biomimetic structural material exhibited significantly superior mechanical load‐bearing performance compared to the natural bone trabeculae while maintaining the natural bone's Tp (83.1%) and E (798.1 MPa). The biomimetic structural material effectively balances the combination of strength and E, providing a design template for the next generation of medical implants. It has great potential as a bone repair material for clinical applications, and its adjustable mechanical properties also make it highly promising in the field of tissue engineering.
... The elastic modulus of these traditional Ti alloys (∼105 GPa for CP-Ti and ∼110 GPa for Ti6Al4V) is about 3 times higher than that of human bone (≤30 GPa), which could result in the "stress-shielding" effect. 4 Moreover, the potential release of Al and V ions in Ti6Al4V caused by corrosion may induce long-term health problems. 5 Therefore, an ideal implant material should have a good combination of adequate mechanical properties and good biocompatibility. ...
... The Young's modulus of HTO-treated scaffolds (3141.17 ± 188.42 MPa) was very close to that of trabecular bone, which is around 0.44 GPa [25]. This feature results in a uniform stress distribution and reduces the possible stress concentration effects at the interface. ...
Reconstruction of subarticular bone defects is an intractable challenge in orthopedics. The simultaneous repair of cancellous defects, fractures, and cartilage damage is an ideal surgical outcome. 3D printed porous anatomical WE43 (magnesium with 4 wt% yttrium and 3 wt% rare earths) scaffolds have many advantages for repairing such bone defects, including good biocompatibility, appropriate mechanical strength, customizable shape and structure, and biodegradability. In a previous investigation, we successfully enhanced the corrosion resistance of WE43 samples via high temperature oxidation (HTO). In the present study, we explored the feasibility and effectiveness of HTO-treated 3D printed porous anatomical WE43 scaffolds for repairing the cancellous bone defects accompanied by split fractures via in vitro and in vivo experiments. After HTO treatment, a dense oxidation layer mainly composed of Y2O3 and Nd2O3 formed on the surface of scaffolds. In addition, the majority of the grains were equiaxed, with an average grain size of 7.4 μm. Cell and rabbit experiments confirmed the non-cytotoxicity and biocompatibility of the HTO-treated WE43 scaffolds. After the implantation of scaffolds inside bone defects, their porous structures could be maintained for more than 12 weeks without penetration and for more than 6 weeks with penetration. During the postoperative follow-up period for up to 48 weeks, radiographic examinations and histological analysis revealed that abundant bone gradually regenerated along with scaffold degradation, and stable osseointegration formed between new bone and scaffold residues. MRI images further demonstrated no evidence of any obvious damage to the cartilage, ligaments, or menisci, confirming the absence of traumatic osteoarthritis. Moreover, finite element analysis and biomechanical tests further verified that the scaffolds was conducive to a uniform mechanical distribution. In conclusion, applying the HTO-treated 3D printed porous anatomical WE43 scaffolds exhibited favorable repairing effects for subarticular cancellous bone defects, possessing great potential for clinical application.
... Several meshes have been employed for the approximation of a planned model, which is extremely resource-intensive, in order to produce correct modeling results. Additionally, merging the entire geometry would require dozens of parametric surfaces, which could also have an adverse effect on the outcome [14,15]. Regarding the design task, several design needs, such as mechanical qualities, printability, and other particular demands, should be taken into consideration. ...
This research is centered on optimizing the mechanical properties of additively manufactured (AM) lattice structures via strain optimization by controlling different design and process parameters such as stress, unit cell size, total height, width, and relative density. In this regard, numerous topologies, including sea urchin (open cell) structure, honeycomb, and Kelvin structures simple, round, and crossbar (2 × 2), were considered that were fabricated using different materials such as plastics (PLA, PA12), metal (316L stainless steel), and polymer (thiol-ene) via numerous AM technologies, including stereolithography (SLA), multijet fusion (MJF), fused deposition modeling (FDM), direct metal laser sintering (DMLS), and selective laser melting (SLM). The developed deep-learning-driven genetic metaheuristic algorithm was able to achieve a particular strain value for a considered topology of the lattice structure by controlling the considered input parameters. For instance, in order to achieve a strain value of 2.8 × 10−6 mm/mm for the sea urchin structure, the developed model suggests the optimal stress (11.9 MPa), unit cell size (11.4 mm), total height (42.5 mm), breadth (8.7 mm), width (17.29 mm), and relative density (6.67%). Similarly, these parameters were controlled to optimize the strain for other investigated lattice structures. This framework can be helpful in designing various AM lattice structures of desired mechanical qualities.
... [20] The size of the pores also influences the transportation of cells, nutrients, and growth factors through blood flow. [24] Recently, additive manufacturing (AM) has been proven able to manufacture such macrostructures. [25,26] As an added advantage, the freedom in design inherent to this technology can be utilized to produce new cell-friendly three-dimensional (3D) structures [25] and patient-specific implants, which improves osseointegration due to optimal design. ...
Osseointegration is highly desirable for implants used for bone replacement. Polyetheretherketone (PEEK) is an attractive material due to its characteristics such as high biocompatibility and Young's modulus similar to human bones. However, PEEK is bioinert, meaning cells do not adhere and proliferate on its surface. This problem is addressed in this study, with the goal of enhancing osseointegration of additively manufactured PEEK. The influences of surface modifications and porous structures on cellular behavior were assessed by wettability and in vitro tests with subclone of the human osteosarcoma cell line‐2 osteoblasts. Overall, the combination of surface modification, type of plasma process used, atmospheric pressure versus vacuum‐based, and surface structuring, especially gyroid structures, improve the cellular proliferation on PEEK. Therefore, its ability to enhance osseointegration is highly promising.
The integration of additive manufacturing and topology optimization makes it possible to fabricate complex configurations, especially for microscale structures, which can guarantee the realization of high-performance structural designs. However, topology results often contain microstructures (several multicellular scales) similar to the characteristic length of local macrostructures, leading to errors in structural performance analysis based on classical theories. Therefore, it is necessary to consider the size effect in topology optimization. In this paper, we establish a novel topology optimization model utilizing the integral nonlocal theory to account for the size effect. The approach consists of an integral constitutive model that incorporates a kernel function, enabling the description of stress at a specific point in relation to strain in a distant field. Topology optimization structures based on nonlocal theory are presented for some benchmark examples, and the results are compared with those based on classical medium theory. The material layout exhibits significant differences between the two approaches, highlighting the necessity of topology optimization based on nonlocal theory and the effectiveness of the proposed method.
Bone scaffolds play a crucial role in bone tissue engineering, providing mechanical support for the growth of new tissue while enduring static and fatigue loads. Although polymers possess favourable characteristics such as adjustable degradation rate, tissue-compatible stiffness, ease of fabrication, and low toxicity, their relatively low mechanical strength has limited their use in load-bearing applications. While numerous studies have focused on assessing the static strength of polymeric scaffolds, little has been conducted on their fatigue properties. The current review presents a comprehensive study on the fatigue behaviour of polymeric bone scaffolds. The fatigue failure in polymeric scaffolds is discussed and the impact of material properties, topological features, loading conditions, and environmental factors are also examined. The present review also provides insight into the fatigue damage evolution within polymeric scaffolds, drawing comparisons to the behaviour observed in natural bone. The effect of polymer microstructure, incorporating reinforcing materials, the introduction of topological features, and hydrodynamic/corrosive impact of body fluids in the fatigue life of scaffolds are also discussed. Understanding these parameters is crucial for enhancing the fatigue resistance of polymeric scaffolds and holds promise for expanding their application in clinical settings as structural biomaterials.
Dental implants have been extensively used in patients with defects or loss of dentition. However, the loss or failure of dental implants is still a critical problem in clinic. Therefore, many methods have been designed to enhance the osseointegration between the implants and native bone. Herein, the challenge and healing process of dental implant operation will be briefly introduced. Then, various surface modification methods and emerging biomaterials used to tune the properties of dental implants will be summarized comprehensively.
Multifunctional materials are of interest for biomedical applications due to the different features they exhibit to improve osseointegration of bone implants. Therefore, this work focuses on the fabrication of a three-layer component by powder metallurgy. Samples are composed of one dense layer, another porous and a composite layer using Ti6Al4V alloy powders as matrix. Fabrication consists in axial pressing and then sintering at 1260 ℃. Microstructure was determined by SEM and computed microtomography. Mechanical properties were evaluated by compression tests. Results showed that the densification of three-layer samples was constrained by the layer with lower densification. In spite of that, no defects were developed during sintering. Microstructure obtained shows different characteristics with good bonding between layers. Remaining porosity is interconnected throughout the layers. Mechanical strength is low, close to the range of compact human bones. In conclusion, powder metallurgy is a good alternative route to process and design multifunctional materials.
The advent of three-dimensional (3D) printing technology has revolutionized the production of customized titanium (Ti) alloy implants. The success rate of implantation and the long-term functionality of these implants depend not only on design and material selection but also on their surface properties. Surface modification techniques play a pivotal role in improving the biocompatibility, osseointegration, and overall performance of 3D-printed Ti alloy implants. Hence, the primary objective of this review is to comprehensively elucidate various strategies employed for surface modification to enhance the performance of 3D-printed Ti alloy implants. This review encompasses both conventional and advanced surface modification techniques, which include physical–mechanical methods, chemical modification methods, bioconvergence modification technology, and the functional composite method. Furthermore, it explores the distinct advantages and limitations associated with each of these methods. In the future, efforts in surface modification will be geared towards achieving precise control over implant surface morphology, enhancing osteogenic capabilities, and augmenting antimicrobial functionality. This will enable the development of surfaces with multifunctional properties and personalized designs. By continuously exploring and developing innovative surface modification techniques, we anticipate that implant performance can be further elevated, paving the way for groundbreaking advancements in the field of biomedical engineering.
Synthetic bone grafting materials play a significant role in various medical applications involving bone regeneration and repair. Their ability to mimic the properties of natural bone and promote the healing process has contributed to their growing relevance. While calcium-phosphates and their composites with various polymers and biopolymers are widely used in clinical and experimental research, the diverse range of available polymer-based materials poses challenges in selecting the most suitable grafts for successful bone repair. This review aims to address the fundamental issues of bone biology and regeneration while providing a clear perspective on the principles guiding the development of synthetic materials. In this study, we delve into the basic principles underlying the creation of synthetic bone composites and explore the mechanisms of formation for biologically important complexes and structures associated with the various constituent parts of these materials. Additionally, we offer comprehensive information on the application of biologically active substances to enhance the properties and bioactivity of synthetic bone grafting materials. By presenting these insights, our review enables a deeper understanding of the regeneration processes facilitated by the application of synthetic bone composites.
Background
Prosthesis subsidence and mechanical failure were considered significant threats after vertebral body replacement during the long-term follow-up. Therefore, improving and optimizing the structure of vertebral substitutes for exceptional performance has become a pivotal challenge in spinal reconstruction.
Methods
The study aimed to develop a novel artificial vertebral implant (AVI) with triply periodic minimal surface Gyroid porous structures to enhance the safety and stability of prostheses. The biomechanical performance of AVIs under different loading conditions was analyzed using the finite element method. These implants were fabricated using selective laser melting technology and evaluated through static compression and subsidence experiments.
Results
The results demonstrated that the peak stress in the Gyroid porous AVI was consistently lower than that in the traditional porous AVI under all loading conditions, with a maximum reduction of 73.4%. Additionally, it effectively reduced peak stress at the bone-implant interface of the vertebrae. Static compression experiments demonstrated that the Gyroid porous AVI was about 1.63 times to traditional porous AVI in terms of the maximum compression load, indicating that Gyroid porous AVI could meet the safety requirement. Furthermore, static subsidence experiments revealed that the subsidence tendency of Gyroid porous AVI in polyurethane foam (simulated cancellous bone) was approximately 15.7% lower than that of traditional porous AVI.
Conclusions
The Gyroid porous AVI exhibited higher compressive strength and lower subsidence tendency than the strut-based traditional porous AVI, indicating it may be a promising substitute for spinal reconstruction.
A new class of alloys called magnesium-based alloys has the unique property of being biodegradable inside the humans and animals. In addition to being biodegradable, Mg-based alloys are suitable materials for creating medical implants for utilization in orthopaedic and traumatology therapies due to their inherent biocompatibility and bone-like density. Due to the combination of bioimplant design and manufacturing techniques appropriate to particular applications, additive manufacturing (AM) and three-dimensional (3D) printing now offer a potential production approach. Magnesium (Mg) use in biomedical field is rising year by year due to rising needs in the biomedical sector. In this biomedical field, additive manufacturing (AM) gives you the freedom to create components with complicated shapes and good dimensional stability. Additionally, it opens up a new opportunity for using unique component architectures, expanding the uses for magnesium alloy. The numerous AM techniques utilised to create biomedical implants from magnesium-based alloys were rigorously examined in current study, along with the materials, microscopic structure, mechanical characteristics, biocompatibility, biodegradability and antibacterial properties. It was observed that powder bed fusion (PBF) is a very good method for manufacturing magnesium implants as topology can be carefully controlled in powder bed fusion process. It was observed that selective laser melting process offer more functionality than selective laser sintering process because Mg is completely melted and penetrated deeply during selective laser melting process. Selective laser melting has advantages such as smaller grains, a homogenous phase distribution, an improved solid solution rapid solidification and considerable cooling rates. In this article the difficulties and problems associated with AM methods were recognised from the viewpoints of bioimplant design, characteristics, and applications. Critical exploration is also done on the difficulties and potential of AM of magnesium alloys.
This paper reports the microstructural characteristics and mechanical properties of yttria-stabilized zirconia prepared via fused deposition modelling and slip casting. X-Ray Diffraction peaks indicated that yttria-stabilized zirconia crystallized in tetragonal structure for both slip casted(SC) and fused deposition modelled(FDM) samples. Further, scanning electron microscopy of slip casted sample showcased closely packed structure with fine grains and an average grain size of ∼65 nm whilst fused deposition modelled samples showcased non-homogeneous pores with ∼20 nm grain size. Average relative density of slip casted samples was ∼99.4 % while that of fused deposition modelled sample exhibited ∼96.2 %. The Vickers Hardness of slip casted (∼15.26 ± 0.4 GPa) was ∼10 % higher than the fused deposition modelled samples (∼13.79 ± 0.3 GPa). Likewise, indentation fracture toughness of slip casted (5.78 ± 0.5 MPa m1/2) was 14 % higher than fused deposition modelled samples which could have been due to the change in grain size as well as porosity of the ceramics. Compressive strength of the fused deposition modelled samples was 32 % less than slip casted samples (∼510 ± 10 MPa) due to its non-homogenous pores which led to weakening van der Waals force of attraction.
Repair of large bone defects caused by trauma or disease poses significant clinical challenges. Extensive research has focused on metallic materials for bone repair because of their favorable mechanical properties, biocompatibility, and manufacturing processes. Traditional metallic materials, such as stainless steel and titanium alloys, are widely used in clinics. Biodegradable metallic materials, such as iron, magnesium, and zinc alloys, are promising candidates for bone repair because of their ability to degrade over time. Emerging metallic materials, such as porous tantalum and bismuth alloys, have gained attention as bone implants owing to their bone affinity and multifunctionality. However, these metallic materials encounter many practical difficulties that require urgent improvement. In this article, we systematically reviewed and analyzed the metallic materials used for bone repair, providing a comprehensive overview of their morphology, mechanical properties, biocompatibility, and in vivo implantation. Furthermore, we summarized the strategies and efforts made to address the short‐comings of metallic materials. Finally, we identified and discussed perspectives for the development of metallic materials to guide future research and advancements in clinical practice.
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Topology optimization (TO) is a structural optimization method achieving the fundamental change of the structure. Although TO can generate innovative design, sometimes curious, the designed products were sometimes still conservative due to manufacturing design limitations. The recent development of the additive manufacturing (AM) technology enabled the straightforward connection between the native optima and the final product. Thus, TO is gathering attention again as part of AM’s key technology after the 30 years old birthday. In this chapter TO technology is explained from the basics to applications focusing on AM technology relationships. The detailed topics of this subsection are the basics of a standard TO, typical material interpolation, and typical problem settings; its extension to AM issues, such as overhang limitation, material density optimization, and thermal distortion suppression; and its application to the design of characteristic AM parts or industrial products.
The integration of the endosseous portion of a dental implant relies heavily on interactions in the implant microenvironment. A complex biological cascade forms the basis for osseointegration, using chemical and physical cues from the implant surface to influence the activity of immune and progenitor cells. This further alters the recruitment, migration, proliferation, and maturation of progenitor cells into osteoblasts and stimulates osteoblasts to produce bone. Understanding the biological interactions at the implant surface is important when developing new materials and treatments to enhance osseointegration. The in vitro assays described in this chapter use multiple cell culture techniques, harvest methods, imaging modalities, and biochemical analyses to characterize individual cell types and co-cultures to predict the interaction of these cells in vivo. Surface designs that enhance micro/meso/nano-roughness and alter the free energy of the surface have been shown to alter protein adsorption profiles, promote a wound healing immune resolution, increase net bone apposition to the implant surface, alter osteoclast activity, and create long-term immune tolerance to create a tightly bonded biologic-material interface.
Architected materials that consist of multiple subelements arranged in particular orders can demonstrate a much broader range of properties than their constituent materials. However, the rational design of these materials generally relies on experts’ prior knowledge and requires painstaking effort. Here, we present a data-efficient method for the high-dimensional multi-property optimization of 3D-printed architected materials utilizing a machine learning (ML) cycle consisting of the finite element method (FEM) and 3D neural networks. Specifically, we apply our method to orthopedic implant design. Compared to uniform designs, our experience-free method designs microscale heterogeneous architectures with a biocompatible elastic modulus and higher strength. Furthermore, inspired by the knowledge learned from the neural networks, we develop machine-human synergy, adapting the ML-designed architecture to fix a macroscale, irregularly shaped animal bone defect. Such adaptation exhibits 20% higher experimental load-bearing capacity than the uniform design. Thus, our method provides a data-efficient paradigm for the fast and intelligent design of architected materials with tailored mechanical, physical, and chemical properties.
Total hip replacement (THR) has been the most frequently performed joint replacement in orthopedical surgeries over the past two decades. The National Joint Registry (NJR) revealed that between 10 to 20% of hip implants required revision between the duration of 1 and 10 years in 2022. Most revisions of hip implants were a result of aseptic loosening, dislocation, implant wear, implant fracture, and joint incompatibility, which are all caused by implant geometry disparity. The primary purpose of this review article is to analyze and evaluate the mechanics and performance factors of advancement in hip implants with novel geometries. The existing hip implants can be categorized based on two parts: the hip stem and the joint of the implant. Recent literature reported that inadequate stress distribution from the implant to the implanted femur bone leads to improper bone remodelling, increased bone resorption, and eventually, implant failure. Researchers continue to develop hip implants with a variety of stem morphology to address the stress shielding limitation while maintaining an adequate bone remodeling rate at the bone-implant interface. Research has continued hip implants with porous lattice and Functionally Graded Material (FGM) stems, femur resurfacing, short-stem, and collared stems to achieve uniform stress distribution and adequate bone remodeling at the bone-implant interface. The existing literature reveals that FGM stem geometry hip implants reduce stress shedding, however, higher micromotion at the bone-implant contact interface displacement is still a key research topic. Mechanical stability is still an issue with hip implants, femur resurfacing, collared stems, and short stems. Hip implants are being developed with a variety of joint geometries to decrease wear, improve angular range of motion, and strengthen mechanical stability at the joint interface. Dual mobility and reverse acetabular head-liner hip implant reduce the hip joint's dislocation limits. In addition, researchers reveal that acetabular headliner joints with unidirectional motion have a lower wear rate than traditional ball-and-socket joints. This analytic evaluation highlighted significant elements in the advancement of hip implants by analyzing respective geometry limits. In addition, this review explores potential solutions to existing obstacles in developing a better hip implant system.
Within the human body, the intricate network of blood vessels plays a pivotal role in transporting nutrients and oxygen and maintaining homeostasis. Bioprinting is an innovative technology with the potential to revolutionize this field by constructing complex multicellular structures. This technique offers the advantage of depositing individual cells, growth factors, and biochemical signals, thereby facilitating the growth of functional blood vessels. Despite the challenges in fabricating vascularized constructs, bioprinting has emerged as an advance in organ engineering. The continuous evolution of bioprinting technology and biomaterial knowledge provides an avenue to overcome the hurdles associated with vascularized tissue fabrication. This article provides an overview of the biofabrication process used to create vascular and vascularized constructs. It delves into the various techniques used in vascular engineering, including extrusion-, droplet-, and laser-based bioprinting methods. Integrating these techniques offers the prospect of crafting artificial blood vessels with remarkable precision and functionality. Therefore, the potential impact of bioprinting in vascular engineering is significant. With technological advances, it holds promise in revolutionizing organ transplantation, tissue engineering, and regenerative medicine. By mimicking the natural complexity of blood vessels, bioprinting brings us one step closer to engineering organs with functional vasculature, ushering in a new era of medical advancement.
Mechanical Metamaterials (MMs) are artificially designed structures with extraordinary properties that are dependent on micro architectures and spatial tessellations of unit cells, rather than constitutive compositions. They have demonstrated promising and attractive application potentials in practical engineering. Recently, how to rationally design novel MMs and discover their multifunctional behaviors has received tremendous discussions with rapid progress, particularly in the last ten years with an enormous increase of publications and citations. Herein, we present a comprehensive overview of considerable advances of MMs, including critical focuses at different scales, forward and inverse design mechanisms with optimization formulations, micro architectures of unit cells, and their spatial tessellations in discovering novel MMs and future prospects. The implications in clarifying the four focuses at levels from the global to the physical in MMs are highlighted, that is, unique structures designed for unique functions, unique micro unit cells placed in unique locations, unique micro unit cells designed for unique properties and unique micro unit cells evaluated by unique mechanisms. We examine the inverse designs of MMs with intrinsic mechanisms of structure-property driven characteristics to achieve unprecedented behaviors, which are involved into material designs and multiscale designs. The former primarily optimizes micro architectures to explore novel MMs, and the latter focuses on micro architectures and spatial tessellations to promote multifunctional applications of MMs in engineering. Finally, we propose several promising research topics with serious challenges in design formulations, micro architectures, spatial tessellations and industrial applications.
Lightweight metallic cellular materials (MCMs) have been seen in numerous engineering applications to date, primarily due to their excellent mechanical characteristics such as low mass, high specific stiffness/strength, good energy absorption capacity as well as other multifunctional properties. Moving forwards the configurations of MCMs and the structures are built up by sophisticated interconnected network comprising of various solid struts or plates. Practical applications of cellular materials mainly make use of their homogenized effective properties, which are strongly relied on the mesoscopic and/or microscopic characteristics of unit-cell materials and structures, stimulating significant research interest in this area. In this present review, typical manufacturing technologies and design optimization strategies are briefly outlined for MCMs at first. Then, the advances in the micro-meso-macro (MMM) multiscale mechanical properties are reviewed. A range of inverse identification approaches for characterizing cellular mechanical properties at microscale are compared. Some mesoscale models that are utilized to define the structural configurations of MCMs are discussed. Characterization of macroscale multiaxial mechanical behaviors of MCMs are reviewed in detail, which involves experimental tests, numerical modeling, inverse calculation, and material models available in commercial codes. Finally, the mechanical responses of representative MCMs-based structures as well as the advanced engineering applications are outlined. This systematic review is expected to highlight the state-of-the-art and depict the potential prospects for researchers and engineers in the field of achieving multiscale mechanical properties and practical applications of MCMs.
In this study, multi‐scale triply periodic minimal surface (TPMS) porous scaffolds with uniform and radial gradient distribution on pore size were printed based on the selective laser melting technology, and the influences of porosity, pore size and radial pore size distribution on compression mechanical properties, cell behavior, and bone regeneration behavior were analyzed. The results showed that the compression performance of the uniform porous scaffolds with high porosity was similar to that of cancellous bone of pig tibia, and the gradient porous scaffolds have higher elastic modulus and compressive toughness. After 4 days of cell culture, cells were distributed on the surface of scaffolds mostly, and the number of adherent cells was higher on the small pore size porous scaffolds; After 7 days, the area and density of cell proliferation on the scaffolds were improved; After 14 days, the cells on the small pore size scaffolds tended to migrate to adjacent pores. Animal implantation experiments showed that collagen fiber osteoid was intermittent on scaffolds with high porosity and large pore size, which was not conducive to bone formation. The appropriate pore size and porosity of bone regeneration were 792 um and 83%, respectively, and the regenerative ability of gradient pore size was better than that of uniform pore size. Our study explains the rules of TPMS gyroid structure parameters on compression performance, cell response and bone regeneration, and provides a reference value for the design of bone repair scaffolds for clinical orthopedics.
The use of additive manufacturing, or 3D printing, has progressed beyond prototyping to produce intricate and valuable finished goods. The potential of additive manufacturing has increased across numerous industries due to the expansion of materials, such as high-performance metals and polymers, and advancements in machine capabilities , such as multi-material printing. By enabling complex designs, reduced waste, and quick production, it has the potential to revolutionize the consumer goods, healthcare, automotive manufacturing, and aerospace industries. The development of specialized scaffolds has been made possible by the precise control that additive manufacturing provides over the internal structure of porous materials. This technology has revolutionized tissue engineering and regenerative medicine. Furthermore, it has made it possible to create individualized implants and prosthetics that improve patient comfort and outcomes in orthopedic and dental applications. Also, it provides flexibility in design and customization for individualized implants and prosthetics in orthopedic and dental applications. Design flexibility, waste reduction, improved biocompatibility, quick prototyping, and cost-effectiveness are benefits of additive manufacturing for implants. For personalized healthcare, regenerative medicine, and better patient outcomes, additive manufacturing holds promise as the technology develops with further advancements in printing speed, resolution, and scalability.
This work presents the application of a topology optimisation method to the hip implant design. A three-dimensional implanted femur is modelled and defined as a design domain. The implant, modelled as type 1, is optimised while other materials i.e. cement (type 2) and bone (type 3), are not being optimised. The domain is subjected to a load case, which corresponds to the loads applied when walking. Loads are employed at the proximal end of the implant and the abductor muscle. Loads from other muscles are not considered. The goal of the study is to minimise the energy of implant compliance subjected to several sets of volume reduction. Reductions are set to be 30%, 40%, 50%, 60%, and 70% from the initial volume (Vo). The result of each set is cut into several sections about x-y plane in z-direction in order to observe the topology inside the stem. It was found that implants with 30% Vo, 40% Vo, and 70% Vo had developed open boundaries whereas 50% Vo and 60% Vo had closed boundary and produced possible shape. Therefore, these designs (50% Vo and 60% Vo) are chosen and refined. Both are analysed using the same boundary conditions as before they were optimised. Results of stresses along medial and lateral line are plotted and compared.
Additive manufacturing (AM) is expanding the range of designable geometries, but to exploit this evolving design space new methods are required to find optimum solutions. Finite element based topology optimisation (TO) is a powerful method of structural optimization, however the results obtained tend to be dependent on the algorithm used, the algorithm parameters and the finite element mesh. This paper will discuss these issues as it relates to the SIMP and BESO algorithms. An example of the application of topological optimization to the design of improved structures is given.
Today, laser additive manufacturing (LAM) is used in more and more industrial applications. Due
to a new freedom in design it offers a high potential for weight saving in lightweight applications,
e.g., in the aerospace industry. However, most design engineers are used to design parts for
conventional manufacturing methods, such as milling and casting, and often only have limited
experience in designing products for additive manufacturing. The absence of comprehensive
design guidelines is therefore limiting the further usage and distribution of LAM. In this paper,
experimental investigations on the influence of part position and orientation on the dimension
accuracy and surface quality are presented. Typical basic shapes used in lightweight design have
been identified and built in LAM. Thin walls, bars, and bore holes with varying diameters were
built in different orientations to determine the process limits. From the results of the experiments,
comprehensive design guidelines for lightweight structures were derived in a catalog according to
DIN 2222 and are presented in detail. For each structure a favorable and an unfavorable example is
shown, the underlying process restrictions are mentioned and further recommendations are given.
This paper introduces a two-scale topology optimization approach by integrating optimized structures
with the design of their materials. The optimization aims to find a multifunctional structure composed
of homogeneous porous material. Driven by the multi-objective functions, macrostructural stiffness
and material thermal conductivity, stiff but lightweight structures composed of thermal insulation materials
can be achieved through optimizing the topologies of the macrostructures and their material microstructure
simultaneously. For such a two-scale optimization problem, the effective properties of
materials derived from the homogenization method are applied to the analysis of macrostructure. Meanwhile,
the displacement field of the macrostructure under given boundary conditions is used for the sensitivity
analysis of the material microstructure. Then, the bi-directional evolutionary structural
optimization (BESO) method is employed to iteratively update the macrostructures and material microstructures
by ranking elemental sensitivity numbers at the both scales. Finally, some 2D and 3D numerical
examples are presented to demonstrate the effectiveness of the proposed optimization algorithm. A
variety of optimal macrostructures and their optimal material microstructures are obtained.
While calcium phosphate–based ceramics are currently the most widely used materials in bone repair, they generally lack tensile strength for initial load bearing. Bulk titanium is the gold standard of metallic implant materials, but does not match the mechanical properties of the surrounding bone, potentially leading to problems of fixation and bone resorption. As an alternative, nickel–titanium alloys possess a unique combination of mechanical properties including a relatively low elastic modulus, pseudoelasticity, and high damping capacity, matching the properties of bone better than any other metallic material. With the ultimate goal of fabricating porous implants for spinal, orthopedic and dental applications, nickel–titanium substrates were fabricated by means of selective laser melting. The response of human mesenchymal stromal cells to the nickel–titanium substrates was compared to mesenchymal stromal cells cultured on clinically used titanium. Selective laser melted titanium as well as surface-treated nickel–titanium and titanium served as controls. Mesenchymal stromal cells had similar proliferation rates when cultured on selective laser melted nickel–titanium, clinically used titanium, or controls. Osteogenic differentiation was similar for mesenchymal stromal cells cultured on the selected materials, as indicated by similar gene expression levels of bone sialoprotein and osteocalcin. Mesenchymal stromal cells seeded and cultured on porous three-dimensional selective laser melted nickel–titanium scaffolds homogeneously colonized the scaffold, and following osteogenic induction, filled the scaffold’s pore volume with extracellular matrix. The combination of bone-related mechanical properties of selective laser melted nickel–titanium with its cytocompatibility and support of osteogenic differentiation of mesenchymal stromal cells highlights its potential as a superior bone substitute as compared to clinically used titanium.
The precipitates formed after suitable thermal treatments in seven Ni-rich Ni–Ti–Hf and Ni–Ti–Zr high-temperature shape memory alloys have been investigated by conventional and high-resolution transmission electron microscopy. In both ternary systems, the pre-cipitate coarsening kinetics become faster as the Ni and ternary element contents (Hf or Zr) of the bulk alloy are increased, in agreement with the precipitate composition measured by energy-dispersive X-ray microanalysis. The precipitate structure has been found to be the same in both Hf-and Zr-containing ternary alloys, and determined to be a superstructure of the B2 austenite phase, which arises from a recombination of the Hf/Zr and Ti atoms in their sublattice. Two different structural models for the precipitate phase were optimized using density functional theory methods. These calculations indicate that the energetics of the structure are not very sensitive to the atomic configuration of the Ti–Hf/Zr planes, thus significant configurational disorder due to entropic effects can be envisaged at high temperatures. The precipitates are fully coherent with the austenite B2 matrix; however, upon martensitic transformation, they lose some coherency with the B19 0 matrix as a result of the transformation shear process in the surrounding matrix. The strain accommodation around the particles is much easier in the Ni–Ti–Zr-containing alloys than in the Ni–Ti–Hf system, which correlates well with the lower transformation strain and stiffness predicted for the Ni–Ti–Zr alloys. The B19 0 martensite twinning modes observed in the studied Ni-rich ternary alloys are not changed by the new precipitated phase, being equivalent to those previously reported in Ni-poor ternary alloys.
Custom implants for the reconstruction of craniofacial defects have gained importance due to better performance over their generic counterparts. This is due to the precise adaptation to the region of implantation, reduced surgical times and better cosmesis. Application of 3D modeling in craniofacial surgery is changing the way surgeons are planning surgeries and graphic designers are designing custom implants. Advances in manufacturing processes and ushering of additive manufacturing for direct production of implants has eliminated the constraints of shape, size and internal structure and mechanical properties making it possible for the fabrication of implants that conform to the physical and mechanical requirements of the region of implantation. This article will review recent trends in 3D modeling and custom implants in craniofacial reconstruction.
Background
The development of novel biomaterials able to control cell activities and direct their fate is warranted for engineering functional bone tissues. Adding bioactive materials can improve new bone formation and better osseointegration. Three types of titanium (Ti) implants were tested for in vitro biocompatibility in this comparative study: Ti6Al7Nb implants with 25% total porosity used as controls, implants infiltrated using a sol–gel method with hydroxyapatite (Ti HA) and silicatitanate (Ti SiO2). The behavior of human osteoblasts was observed in terms of adhesion, cell growth and differentiation.
Results
The two coating methods have provided different morphological and chemical properties (SEM and EDX analysis). Cell attachment in the first hour was slower on the Ti HA scaffolds when compared to Ti SiO2 and porous uncoated Ti implants. The Alamar blue test and the assessment of total protein content uncovered a peak of metabolic activity at day 8–9 with an advantage for Ti SiO2 implants. Osteoblast differentiation and de novo mineralization, evaluated by osteopontin (OP) expression (ELISA and immnocytochemistry), alkaline phosphatase (ALP) activity, calcium deposition (alizarin red), collagen synthesis (SIRCOL test and immnocytochemical staining) and osteocalcin (OC) expression, highlighted the higher osteoconductive ability of Ti HA implants. Higher soluble collagen levels were found for cells cultured in simple osteogenic differentiation medium on control Ti and Ti SiO2 implants. Osteocalcin (OC), a marker of terminal osteoblastic differentiation, was most strongly expressed in osteoblasts cultivated on Ti SiO2 implants.
Conclusions
The behavior of osteoblasts depends on the type of implant and culture conditions. Ti SiO2 scaffolds sustain osteoblast adhesion and promote differentiation with increased collagen and non-collagenic proteins (OP and OC) production. Ti HA implants have a lower ability to induce cell adhesion and proliferation but an increased capacity to induce early mineralization. Addition of growth factors BMP-2 and TGFβ1 in differentiation medium did not improve the mineralization process. Both types of infiltrates have their advantages and limitations, which can be exploited depending on local conditions of bone lesions that have to be repaired. These limitations can also be offset through methods of functionalization with biomolecules involved in osteogenesis.
The area of implant osseointegration is of major importance, given the predicted significant rise in the number of orthopaedic procedures and an increasingly ageing population. Osseointegration is a complex process involving a number of distinct mechanisms affected by the implant bulk properties and surface characteristics. Our understanding and ability to modify these mechanisms through alterations in implant design is continuously expanding. The following review considers the main aspects of material and surface alterations in metal implants, and the extent of their subsequent influence on osseointegration. Clinically, osseointegration results in asymptomatic stable durable fixation of orthopaedic implants. The complexity of achieving this outcome through incorporation and balance of contributory factors is highlighted through a clinical case report.
Customised implants manufacture has always presented difficulties which result in high cost and complex fabrication, mainly due to patients' anatomical differences. The solution has been to produce prostheses with different sizes and use the one that best suits each patient. Additive manufacturing (AM) as a technology from engineering has been providing several advancements in the medical field, particularly as far as fabrication of implants is concerned. The use of additive manufacturing in medicine has added, in an era of development of so many new technologies, the possibility of performing the surgical planning and simulation by using a three-dimensional (3D) physical model, very faithful to the patient's anatomy. AM is a technology that enables the production of models and implants directly from the 3D virtual model (obtained by a Computer-Aided Design (CAD) system, computed tomography or magnetic resonance) facilitating surgical procedures and reducing risks. Furthermore, additive manufacturing has been used to produce implants especially designed for a particular patient, with sizes, shapes and mechanical properties optimised, for areas of medicine such as craniomaxillofacial surgery. This work presents how AM technologies were applied to design and fabricate a biomodel and customised implant for the surgical reconstruction of a large cranial defect. A series of computed tomography data was obtained and software was used to extract the cranial geometry. The protocol presented was used for creation of an anatomic biomodel of the bone defect for the surgical planning and, finally, the design and manufacture of the patient-specific implant.
Titanium and its alloys are successfully used in aerospace through to marine applications. Selective laser melting (SLM) is an additive manufacturing technique, which promises to allow production of novel Ti structures. However, there is still a paucity of accepted methods for quantifying build quality. The viability of using X-ray microtomography (μCT) to quantify and track changes in morphology of SLM Ti porous structures at each stage of the post-laser melting production was tested, quantifying its quality through process. Quantification was achieved using an accessible volume tool to determine pore and strut sizes. Removal of partially sintered struts by cleaning was visualised and quantified. 88% of the struts broken by the cleaning process were found to have connecting neck diameters of less than 180 μm with a mean of 109 μm allowing build criteria to be set. Tracking particles removed during cleaning revealed other methods to improve build design, e.g. avoiding low angle struts that did not sinter well. Partially melted powder particles from strut surfaces were quantified by comparing surface roughness values at each cleaning step. The study demonstrates that μCT provides not only 3D quantification of structure quality, but also a feedback mechanism, such that improvements to the initial design can be made to create more stable and reliable titanium structures for a wide variety of applications.
Highly ordered TiO2 nanotubes (NTs) were synthesized by the electrochemical anodization of Ti foils. We extracted the effect of the Ti surface roughness (applying different pre-treatments prior to the anodization) on the length, growth rate and degree of self-organization of the obtained NT arrays. The mechanisms subjacent to the TiO2 NTs formation and growth were correlated not only with the corresponding anodization curves but also to their appropriate
derivatives (1st order) and suitable integrated and/or obtained param eters, to revel the onset and
end of the different electrochemical regimes. This enable s an in depth interpretation of such
details (and physical- chemical insight), for different levels of surface roughness and
topographic features. We found that pre-treat ments that lead to an extremely small Ti surface
roughness and offer enhanced NT length and also provide a significant improvement of the
template organization quality (highly ordered hexagonal NTs arrays over larger areas) , due to
the optimized surface topography. We present a new statistical approach of evaluate highly
ordered hexagonal NTs arrays areas. Large domains with ideally arranged nanotube structures
represented by a hexagonal closed -packed array were obtained (6.61 µm^2), close to the
smallest grain diameter of the Ti foil and three times larger than those so far reported in the
literature. The use of optimized pre-treatments then allowed avoiding a second anodization
step, ultimately leading to highly hexagonal self ordered samples with large organized domains
at reduced time and cost.
We used selective laser melting (SLM) and hot pressing of mechanically-alloyed beta-type Ti-40Nb powder to fabricate macroporous bulk specimens (solid cylinders). The total porosity, compressive strength, and compressive elastic modulus of the SLM-fabricated material were determined as 17% +/- 1%, 968 +/- 8 MPa, and 33 +/- 2 GPa, respectively. The alloy's elastic modulus is comparable to that of healthy cancellous bone. The comparable results for the hot-pressed material were 3% +/- 2%, 1400 +/- 19 MPa, and 77 +/- 3 GPa. This difference in mechanical properties results from different porosity and phase composition of the two alloys. Both SLM-fabricated and hot-pressed cylinders demonstrated good in vitro biocompatibility. The presented results suggest that the SLM-fabricated alloy may be preferable to the hot-pressed alloy for biomedical applications, such as the manufacture of load-bearing metallic components for total joint replacements.
Open-porous scaffolds made of W4 and WZ21 fibres were evaluated to analyse their potential as an implant material. WZ21 scaffolds without any surface modification or coating, showed promising mechanical properties which were comparable to the W4 scaffolds tested in previous studies. Eudiometric testing results were dependent on the experimental setup, with corrosion rates differing by a factor of 3. Cytotoxicity testing of WZ21 showed sufficient cytocompatibility. The corrosion behavior of the WZ21 scaffolds in different cell culture media are indicating a selective dealloying of elements from the magnesium scaffold by different solutions. Long term in-vivo studies were using 24 W4 scaffolds and 12 WZ21 scaffolds, both implanted in rabbit femoral condyles. The condyles and important inner organs were explanted after 6, 12 and 24 weeks and analyzed. The in-vivo corrosion rate of the WZ21 scaffolds calculated by microCT-based volume loss was up to 49 times slower than the in-vitro corrosion rate based on weight loss. Intramembranous bone formation within the scaffolds of both alloys was revealed, however a low corrosion rate and formation of gas cavities at initial time points were also detected. No systemic or local toxicity could be observed. Investigations by μ-XRF did not reveal accumulation of yttrium in the neighboring tissue. In summary, the magnesium scaffold´s performance is biocompatible, but would benefit from a surface modification, such as a coating to obtain lower the initial corrosion rates, and hereby establish a promising open-porous implant material for load-bearing applications.
Statement of significance
Magnesium is an ideal temporary implant material for non-load bearing applications like bigger bone defects, since it degrades in the body over time. Here we developed and tested in vitro and in a rabbit model in vivo degradable open porous scaffolds made of sintered magnesium W4 and WZ21 short fibres. These scaffolds allow the ingrowth of cells and blood vessels to promote bone healing and regeneration. Both fibre types showed in vitro sufficient cytocompatibility and proliferation rates and in vivo, no systemic toxicity could be detected. At the implantation site, intramembranous bone formation accompanied by ingrowth of supplying blood vessels within the scaffolds of both alloys could be detected.
A simple chemical method was established for inducing bioactivity of Ti and its alloys. When pure Ti, Ti‐6Al‐4V, Ti‐6Al‐2Nb‐Ta, and Ti‐15Mo‐5Zr‐3Al substrates were treated with 10M NaOH aqueous solution and subsequently heat‐treated at 600°C, a thin sodium titanate layer was formed on their surfaces. Thus, treated substrates formed a dense and uniform bonelike apatite layer on their surfaces in simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma. This indicates that the alkali‐ and heat‐treated metals bond to living bone through the bonelike apatite layer formed on their surfaces in the body. The apatite formation on the surfaces of Ti and its alloys was assumed to be induced by a hydrated titania which was formed by an ion exchange of the alkali ion in the alkali titanate layer and the hydronium ion in SBF. The resultant surface structure changed gradually from the outermost apatite layer to the inner Ti and its alloys through a hydrated titania and titanium oxide layers. This provides not only the strong bonding of the apatite layer to the substrates but also a uniform gradient of stress transfer from bone to the implants. The present chemical surface modification is therefore expected to allow the use the bioactive Ti and its alloys as artificial bones even under load‐bearing conditons. © 1996 John Wiley & Sons, Inc.
By slowly removing inefficient material from a structure, the shape of the structure evolves towards an optimum. This is the simple concept of evolutionary structural optimization (ESO). Various design constraints such as stiffness, frequency and buckling load may be imposed upon a structure. Depending on the types of design constraints, different rejection criteria for removing material need to be used. For each type of constraints, the corresponding rejection criteria will be discussed in detail in the subsequent chapters. This chapter describes the simplest rejection criterion based on local stress level. Several examples are included to illustrate how the ESO process works.
Selective laser melting (SLM) is characterized by highly localized heat input and short interaction times, which lead to large thermal gradients. In this research, nine different materials are processed via SLM and compared. The resulting microstructures are characterized by optical and scanning electron microscopy. Residual stresses are measured qualitatively using a novel deflection method and quantitatively using X-ray diffraction. Microcracking, surface oxidation and the anisotropy of the residual stress are discussed. The different phenomena interacting with the buildup of residual stress make it difficult to distinguish the possible correlations between material parameters and the magnitude of residual stresses.
This is a comprehensive and accessible overview of what is known about the structure and mechanics of bone, bones, and teeth. In it, John Currey incorporates critical new concepts and findings from the two decades of research since the publication of his highly regarded The Mechanical Adaptations of Bones. Crucially, Currey shows how bone structure and bone's mechanical properties are intimately bound up with each other and how the mechanical properties of the material interact with the structure of whole bones to produce an adapted structure. For bone tissue, the book discusses stiffness, strength, viscoelasticity, fatigue, and fracture mechanics properties. For whole bones, subjects dealt with include buckling, the optimum hollowness of long bones, impact fracture, and properties of cancellous bone. The effects of mineralization on stiffness and toughness and the role of microcracking in the fracture process receive particular attention. As a zoologist, Currey views bone and bones as solutions to the design problems that vertebrates have faced during their evolution and throughout the book considers what bones have been adapted to do. He covers the full range of bones and bony tissues, as well as dentin and enamel, and uses both human and non-human examples. Copiously illustrated, engagingly written, and assuming little in the way of prior knowledge or mathematical background, Bones is both an ideal introduction to the field and also a reference sure to be frequently consulted by practicing researchers.
Essentials of 3D Biofabrication and Translation discusses the techniques that are making bioprinting a viable alternative in regenerative medicine. The book runs the gamut of topics related to the subject, including hydrogels and polymers, nanotechnology, toxicity testing, and drug screening platforms, also introducing current applications in the cardiac, skeletal, and nervous systems, and organ construction. Leaders in clinical medicine and translational science provide a global perspective of the transformative nature of this field, including the use of cells, biomaterials, and macromolecules to create basic building blocks of tissues and organs, all of which are driving the field of biofabrication to transform regenerative medicine.
This chapter discusses the biomechanics of human vertebral trabecular bone. It also describes aging, disease, and repetitive loading. This chapter addresses the behavior of the whole vertebra, including discussion of the role of the cortical shell, intervertebral disc, and posterior elements. Detailed knowledge of the biomechanical behavior of vertebral bone is necessary in order to design robust implants. Design of orthopedic implants for the spine is particularly challenging because vertebral trabecular bone is so weak and the cortices are so thin. As a result, failure of the bone-implant system often originates in the bone. The problem is compounded because the bone properties vary so much across individuals and over time and with disease. The development of minimally invasive surgical repair techniques for vertebral fractures such as vertebroplasty and kyphoplasty also invites biomechanical analysis of vertebral trabecular bone and the whole vertebral body in order to refine those procedures. Subsequently, computational models are in use to refine and indeed develop designs of new implants for the spine.
For reutilizing the cement sludge as a concrete waterproofer, the sludge was treated with stearic acid by chemical method. And we compared the performances of the new waterproofers with those of two others which have been used in demostic area. The particles of the new waterproofer are finer than those of standard sand but larger than those of cement. And the surfaces of the particles are coated with metallic soaps which have strong water repellency. Also the exess stearic acid and metallic soaps coated on the sludge react with the Ca++ or Al+++ when mixed with cement mortar. Therefore, the watertightness and the physical properties of the new waterproofers in cement mortar were superior to those of the others.
Degradable biomaterials constitute a novel class of bioactive biomaterials which are expected to support healing process of a diseased tissue and to degrade thereafter. Two classes of metals have been proposed: magnesium- and iron-based alloys. Three targeted applications are envisaged: orthopaedic, cardiovascular and pediatric implants. Conceptually, biodegradable metals should provide a temporary support on healing process and should progressively degrade thereafter.
Most AM processes require post-processing after part building to prepare the part for its intended form, fit and/or function. Depending upon the AM technique, the reason for post-processing varies. For purposes of simplicity, this chapter will focus on post-processing techniques which are used to enhance components or overcome AM limitations. These include:
Bone is dynamic, highly vascularised tissue with a unique capacity to heal?and remodel. Its main role is to provide structural support for the body. Furthermore the skeleton also serves as a mineral reservoir, supports muscular contraction resulting in motion, withstands load bearing and protects internal organs [1]. Hence, it is logical to say that major alterations in its structure due to injury or disease can dramatically alter one?s body equilibrium and quality of life. There are roughly 1 million cases of skeletal defects a year that require bone-graft procedures to achieve repair. Socioeconomic consequences in treating these patients with bone fractures is a major concern in both the USA and EU, which are likely to increase due to the ageing of their populations. Bone repair may be treated by grafts taken from either the patient?s own existing bone from other sites (autografts) or donor sources (allografts). The use of synthetic substitutes eliminates the need for further surgery for autografts and the risk of infectious disease transmission from allografts. The significant limitations of current treatments have compelled researchers to develop synthetic alternatives for bone reconstruction. In the early 1950s Swedish orthopaedic surgeon Per Ingvar Branemark began studying the healing process of titanium anchoring screws, which proved to be a seminal point for modern dental and orthopaedic implants [2]. Current bone substitutes using metal, ceramic, polymer and composites, though far from ideal, are commonly implanted materials, second only to blood products. Currently, bone tissue engineering is being researched including scaffolds, growth factors and engineering cells which may provide next-generation bone substitutes. Production of such bone implants and scaffolds requires complex structures that would provide the necessary bone shape and morphology. In general, current orthopaedic prostheses are modular in nature and, although scalable, adhere to a range of basic generic designs. The surgeon, therefore, must select the best size fit based upon preoperative evaluation of radiographs. Current production methods for these devices, which are made from metal/ceramic/ polymers, include casting, compression moulding, sintering, and bar stock milling. These approaches have some inherent restrictions, these include the use of static moulds/tooling which do not allow rapid design changes or one-off patient-specific devices to be produced, or they have geometry restrictions, and high material wastage. One school of thought believes that patient-oriented devices have the potential to enhance the longevity of a device by providing a securer fit, especially in those cases where the devices are not cemented. Closeness of fit aims to aid the distribution and normalization of the stresses incurred in the remaining skeletal system, thereby reducing stress shielding, micromotion, and sinkage. Accurate fits are typically achieved by removing the patient?s bone stock to accommodate the prosthesis, thereby destroying valuable viable bone and making any revision surgery more difficult. It would be preferable to produce custom prostheses for individual patients that required little or no healthy bone stock removal to increase the device stability, especially in young patients, and thus increase the options for a successful revision surgery if required [3]. Today reverse engineering (RE) and medical image-based modelling technologies allow the construction of three-dimensional (3D) models of anatomical structures of human body based on information from imaging data such as computerized tomography (CT), magnetic resonance imaging (MRI), and laser (or structured light) scanning [4]. Based on 3D models, advanced mouldless manufacturing techniques, commonly known as solid free-from fabrication (SFF) or rapid prototyping (RP) have been used to build 3D physical models for surgical training, preoperative planning, surgical simulation andmore recently applied to fabricate customised implants and scaffolds for individual patients. This chapter gives a background on bone structure and properties, biomaterials for bone implants and requirements for bone implant and scaffolds. It then introduces state-of-the-art reverse engineering and rapid prototyping techniques to assist in the manufacture of customised bone implants and then focuses on current research on the techniques to fabricate bone implants and scaffolds directly.
The effects of laser surface remelting (LSR) on the microstructural evolution and surface mechanical properties of Ti-Zr beta titanium alloy were investigated. The surfaces of the Ti-Zr alloy was re-melted using a CO2 laser. X-ray diffraction, Scanning electron microscope, Transmission electron microscope, nanoindentation, and microhardness analyses were performed to evaluate the microstructural and mechanical properties of the alloy. The results showed that the alloy microstructure in the remelting region was greatly refined and homogeneous compared with that in the base material because of the rapid remelting and resolidifying. Meanwhile, the metastable hexagonal x phases with the size of 20-50 nm was found and uniformly distributed throughout the b matrix after LSR. Phase transformation and microstructural refinement were the major microstructural changes in the alloys after LSR. The microhardness and elastic modulus in the remelted region clearly increased by 92.9% and 21.78%, respectively, compared with those in the region without laser processing. The strengthening effect of LSR on the mechanical properties of the Ti-Zr alloy was also addressed. Our results indicated that LSR was an effective method of improving the surface mechanical properties of alloys.
Novel ultrafine lamellar (α + β) microstructures comprising ultrafine (∼200-300 nm) α-laths and retained β phases were created via promoting in situ decomposition of a near α′ martensitic structure in Ti-6Al-4V additively manufactured by selective laser melting (SLM). As a consequence, the total tensile elongation to failure reached 11.4% while maintaining high yield strength above 1100 MPa, superior to both conventional SLM-fabricated Ti-6Al-4V containing non-equilibrium acicular α′ martensite and conventional mill-annealed Ti-6Al-4V. The formation and decomposition of α′ martensite in additively manufactured Ti-6Al-4V was studied via specially designed experiments including single-track deposition, multi-layer deposition and post-SLM heat treatment. The essential SLM additive manufacturing conditions for Ti-6Al-4V including layer thickness, focal offset distance and energy density, under which a near α′ martensitic structure forms in each layer and then in situ transforms into ultrafine lamellar (α + β) structures, were determined. This is the first fundamental effort that has realized complete in situ martensite decomposition in SLM-fabricated Ti-6Al-4V for outstanding mechanical properties.
Unlike conventional materials removal methods, additive manufacturing (AM) is based on a novel materials incremental manufacturing philosophy. Additive manufacturing implies layer by layer shaping and consolidation of powder feedstock to arbitrary configurations, normally using a computer controlled laser. The current development focus of AM is to produce complex shaped functional metallic components, including metals, alloys and metal matrix composites (MMCs), to meet demanding requirements from aerospace, defence, automotive and biomedical industries. Laser sintering (LS), laser melting (LM) and laser metal deposition (LMD) are presently regarded as the three most versatile AM processes. Laser based AM processes generally have a complex non-equilibrium physical and chemical metallurgical nature, which is material and process dependent. The influence of material characteristics and processing conditions on metallurgical mechanisms and resultant microstructural and mechanical properties of AM processed components needs to be clarified. The present review initially defines LS/LM/LMD processes and operative consolidation mechanisms for metallic components. Powder materials used for AM, in the categories of pure metal powder, prealloyed powder and multicomponent metals/alloys/MMCs powder, and associated densification mechanisms during AM are addressed. An in depth review is then presented of material and process aspects of AM, including physical aspects of materials for AM and microstructural and mechanical properties of AM processed components. The overall objective is to establish a relationship between material, process, and metallurgical mechanism for laser based AM of metallic components. © 2012 Institute of Materials, Minerals and Mining and ASM International.
Topology optimisation enables profound insight into the optimal material distribution for a given structural objective, applied loading and boundary conditions. The topologically optimal geometry is often geometrically complex and incompatible with traditional manufacturing methods. Additive manufacture can accommodate significantly more complex geometries than traditional manufacture; however , it is necessary that specific design rules be satisfied to ensure manufacturability. Based on identified design for additive manufacture rules, a novel method is proposed that modifies the theoretically optimal topology as required to ensure manufacturability without requiring additional support material. By assessing the manufacturing time and component mass associated with feasible orientations of the proposed geometry, an optimal orientation can be identified. A case study is presented to demonstrate the usefulness of the proposed method.
Computer-aided tissue engineering (CATE) integrates advances of multi-disciplinary fields of Biology, Biomedical Engineering, Information Technology, and modern Design and Manufacturing. Application of CATE to the design and fabrication of tissue scaffolds can facilitate the exploration of many novel ideas of incorporating biomimetic and biological features into the scaffold design. This paper presents some of the salient applications of CATE, particularly in the modeling and design of scaffolds with controlled internal and external architecture; with vascular channels of different sizes; with modular and interconnecting subunits; with multi-layered heterogeneous dense and compact regions; and the scaffolds with designed artificial chambers for drug delivery, embedded growth factors and other sophisticated features.
Natural structural materials are built at ambient temperature from a fairly limited selection of components. They usually comprise hard and soft phases arranged in complex hierarchical architectures, with characteristic dimensions spanning from the nanoscale to the macroscale. The resulting materials are lightweight and often display unique combinations of strength and toughness, but have proven difficult to mimic synthetically. Here, we review the common design motifs of a range of natural structural materials, and discuss the difficulties associated with the design and fabrication of synthetic structures that mimic the structural and mechanical characteristics of their natural counterparts.
Additive manufacturing provides an attractive processing method for nickel–titanium (NiTi) shape memory and pseudoelastic parts. In this paper, we show how the additive manufacturing process affects structural and functional properties of additively manufactured NiTi and how the process parameter set-up can be optimized to produce high quality NiTi parts and components. Comparisons of shape recovery due to shape memory and pseudoelasticity in additively manufactured and commercial NiTi exhibit promising potential for this innovative processing method.
Selective laser melting (SLM) process allows fabricating strong, lightweight and complex metallic structures. To successfully produce metallic parts by SLM, additional structures are needed to support overhanging surfaces in order to dissipate process heat and to minimize geometrical distortions induced by internal stresses. However, these structures are often massive and require additional post-processing for their removal. A minimization of support structures would therefore significantly reduce manufacturing and finishing efforts and costs. This study investigates the manufacturability of overhanging structures using optimized support parts. An experimental study was performed to identify the optimal self-supporting overhanging structures using Taguchi L-36 design. Experimental results revealed that with optimized supports it is possible to build non-assembly mechanism with overhang surfaces. However, it is necessary to correctly orientate the part in the SLM machine in order to build it with a minimal support structure so to obtain the best trade-off between production time, cost, and accuracy.
New metal alloys and metal fabrication strategies are likely to benefit future skeletal implant strategies. These metals and fabrication strategies form the point of view of standard-of-care implants for the mandible were looked. These implants are used as part of the treatment for segmental resection due to oropharyngeal cancer, injury, or correction of deformity due to pathology or congenital defect. The aim of this two part paper is to review the issues associated with the failure of existing mandibular implants that are due to mismatched material properties. Also potential directions for future research was studied. To mitigate these issues the use of low-stiffness metallic alloys has been highlighted. To this end, development, processing and biocompatibility of superelastic NiTi as well as resorbable Magnesium-based alloys were discussed. Additionally, engineered porosity was reviewed as it can be an effective way of matching the stiffness with the tissue surrounding an implant. These porosities and the overall geometry of the implant can be optimized for strain transduction and with a tailored stiffness profile. Rendering patient-specific, site-specific, morphology-specific, and function-specific implants can now be achieved using these and other metals with bone-like material properties by additive manufacturing. The biocompatibility of implants prepared from superelastic and resorbable alloys was also reviewed.
