Mechanical performance of additively manufactured meta-biomaterials

A Study on the Compressive Behavior of Additively Manufactured AlSi10Mg Lattice Structures

The mechanical behavior of the metallic components fabricated by additive manufacturing (AM) technologies can be influenced by adjustments in their microstructure or by using specially engineered geometries. Manipulating the topological features of the component, such as incorporating unit cells, enables the production of lighter metamaterials, such as lattice structures. This study investigates the mechanical behavior of lattice structures created from AlSi10Mg, which were produced using the laser beam powder bed fusion (LB-PBF) process. Specifically, their behavior under pure compressive loading has been numerically and experimentally investigated using ten different configurations. Experimental methods and finite element analysis (FEA) were used to investigate the behavior of body-centered cubic (BCC) lattice structures, specifically examining the effects of tapering the struts by varying their diameters at the endpoints (dend) and midpoints (dmid), as well as altering the height of the joint nodes (h). The unit cells were designed with varying parameters in such a way that dend is changed at three levels, while dmid and h are changed at two levels. Significant differences in Young's modulus, yield strength, and ultimate compressive strength between the various specimen configurations were observed both experimentally and numerically. The FEA underestimated the Young's modulus corresponding to the configurations with thinner struts in comparison to the higher values found experimentally. Conversely, the FEA overestimated the Young's modulus of those configurations with larger strut diameters with respect to the experimentally determined values. Additionally, the proposed FE method consistently underestimated the yield strength relative to the experimental values, with notable discrepancies in specific configurations.

Materials for 3D printing in healthcare sector: A review

Additive Manufacturing (AM) encompasses various techniques creating intricate components from digital models. The aim of incorporating 3D printing (3DP) in the healthcare sector is to transform patient care by providing personalized solutions, improving medical procedures, fostering research and development, and ultimately optimizing the efficiency and effectiveness of healthcare delivery. This review delves into the historical beginnings of AM's 9 integration into medical contexts exploring various categories of AM methodologies and their roles within the medical sector. This survey also dives into the issue of material requirements and challenges specific to AM's medical applications. Emphasis is placed on how AM processes directly enhance human well-being. The primary focus of this paper is to highlight the evolution and incentives for cross-disciplinary AM applications, particularly in the realm of healthcare by considering their principle, materials and applications. It is designed for a diverse audience, including manufacturing professionals and researchers, seeking insights into this transformative technology's medical dimensions.

Design of the Elastic Modulus of porous lattice structures composed of cells with continuously variable cross section carrying structures

BACKGROUND

Porous bone implants have a wide range of applications for their low elastic modulus and good connectivity. It is necessary to explore an elastic modulus control method that can significantly regulate the elastic modulus under the condition of maintaining a constant porosity.

METHODS

For achieving continuously changing elastic modulus of porous lattice structure, the simple cubic lattice structures were selected as research object, and the distribution of cross-sectional sizes of its carrying structures were set as variable continuous curves. The prediction model for the elastic modulus was established based on the elasticity mechanics and the equal mass assumption. Then, the prediction model is enhanced through compression simulation of the unit cell structure. Finally, the accuracy of prediction model is validated by compression experiments.

FINDINGS

The results indicate that the distribution of cross-sectional size of the carrying structures has a significant impact on the elastic modulus of unit cell structures under the constraint of equal mass. By adjusting the characteristic parameters of distribution curves, the elastic modulus can be changed within a large range.

INTERPRETATION

Variable cross-section can effectively change the elastic modulus of porous structures while ensuring constant porosity. This method has important value in decoupling the influence of geometric parameters on the elastic modulus of porous structures.

Advances in implants and bone graft types for lumbar spinal fusion surgery

The increasing prevalence of spinal disorders worldwide necessitates advanced treatments, particularly interbody fusion for severe cases that are unresponsive to non-surgical interventions. This procedure, especially 360° lumbar interbody fusion, employs an interbody cage, pedicle screw-and-rod instrumentation, and autologous bone graft (ABG) to enhance spinal stability and promote fusion. Despite significant advancements, a persistent 10% incidence of non-union continues to result in compromised patient outcomes and escalated healthcare costs. Innovations in lumbar stabilisation seek to mimic the properties of natural bone, with evolving implant materials like titanium (Ti) and polyetheretherketone (PEEK) and their composites offering new prospects. Additionally, biomimetic cages featuring precisely engineered porosities and interconnectivity have gained traction, as they enhance osteogenic differentiation, support osteogenesis, and alleviate stress-shielding. However, the limitations of ABG, such as harvesting morbidities and limited fusion capacity, have spurred the exploration of sophisticated solutions involving advanced bone graft substitutes. Currently, demineralised bone matrix and ceramics are in clinical use, forming the basis for future investigations into novel bone graft substitutes. Bioglass, a promising newcomer, is under investigation despite its observed rapid absorption and the potential for foreign body reactions in preclinical studies. Its clinical applicability remains under scrutiny, with ongoing research addressing challenges related to burst release and appropriate dosing. Conversely, the well-documented favourable osteogenic potential of growth factors remains encouraging, with current efforts focused on modulating their release dynamics to minimise complications. In this evidence-based narrative review, we provide a comprehensive overview of the evolving landscape of non-degradable spinal implants and bone graft substitutes, emphasising their applications in lumbar spinal fusion surgery. We highlight the necessity for continued research to improve clinical outcomes and enhance patient well-being.

Finite element analysis and in vitro tests on endurance life and durability of composite bone substitutes

Bone structures facilitate the regeneration and repair of bone tissue in regions where it has been damaged or destroyed, either temporarily or permanently. Therefore, the bone's fatigue strength and durability are crucial to its efficacy and longevity. Several variables, such as the construct's material qualities, design, and production procedure, loading and unloading cycles, and physiological conditions influence the endurance life of bone constructs. Metals, ceramics, and polymers are all routinely utilized to create bone substitutes, and each of these materials has unique features that might affect the fatigue strength and endurance life of the final product. The mechanical performance and capacity to promote bone tissue regeneration may be affected by the scaffold's design, porosity, and pore size. Researchers employ mechanical testing under cyclic loading circumstances as one example of an experimental approach used to assess bone construction endurance. These analyses can give us important information about the stress-strain behavior, resistance to multiple loading cycles, and fatigue strength of the new structure. Predicting the endurance life of the developed construct may also be possible with the use of simulations and numerical analyses. Hence, in order to create reliable and efficient constructs for bone tissue engineering, it is crucial to understand their fatigue strength and durability. The purpose of this study is to analyze the effective parameters for fatigue strength of bone structures and to gather the models and evaluations utilized in endurance life assessments.

Research Progress on Laser Powder Bed Fusion Additive Manufacturing of Zinc Alloys

Zinc, along with magnesium and iron, is considered one of the most promising biodegradable metals. Compared with magnesium and iron, pure Zn exhibits poor mechanical properties, despite its mild biological corrosion behavior and beneficial biocompatibility. Laser powder bed fusion (LPBF), unlike traditional manufacturing techniques, has the capability to rapidly manufacture near-net-shape components. At present, although the combination of LPBF and Zn has made great progress, it is still in its infancy. Element loss and porosity are common processing problems for LPBF Zn, mainly due to evaporation during melting under a high-energy beam. The formation quality and properties of the final material are closely related to the alloy composition, design and processing. This work reviews the state of research and future perspective on LPBF zinc from comprehensive assessments such as powder characteristics, alloy composition, processing, formation quality, microstructure, and properties. The effects of powder characteristics, process parameters and evaporation on formation quality are introduced. The mechanical, corrosion, and biocompatibility properties of LPBF Zn and their test methodologies are introduced. The effects of microstructure on mechanical properties and corrosion properties are analyzed in detail. The practical medical application of Zn is introduced. Finally, current research status is summarized together with suggested directions for advancing knowledge about LPBF Zn.

The Interaction between Oral Bacteria and 3D Titanium Porous Surfaces Produced by Selective Laser Melting-A Narrative Review

The interaction between oral bacteria and dental implant surfaces is a critical factor in the success and longevity of dental implants. With advancements in additive manufacturing technologies, selective laser melting (SLM) has emerged as a prominent method for producing titanium implants with highly controlled microstructures and porosities. These 3D printed titanium surfaces offer significant benefits, such as enhanced osseointegration and improved mechanical properties. However, the same surface features that promote bone cell attachment and proliferation may also provide favorable conditions for bacterial adhesion and biofilm formation. Understanding the dynamics of these interactions is essential for developing implant surfaces that can effectively resist bacterial colonization while promoting tissue integration. This narrative review explores the complex interplay between oral bacteria and SLM-produced titanium porous surfaces, examining current research findings and potential strategies for optimizing implant design to mitigate the risks of infection and ensure successful clinical outcomes.

Effects of MgO nanoparticle addition on the mechanical properties, degradation properties, antibacterial properties and in vitro and in vivo biological properties of 3D-printed Zn scaffolds

Bone tissue engineering is the main method for repairing large segment bone defects. In this study, a layer of bioactive MgO nanoparticles was wrapped on the surface of spherical Zn powders, which allowed the MgO nanoparticles to be incorporated into 3D-printed Zn matrix and improved the biodegradation and biocompatibility of the Zn matrix. The results showed that porous pure Zn scaffolds and Zn/MgO scaffolds with skeletal-gyroid (G) model structure were successfully prepared by selective laser melting (SLM). The average porosity of two porous scaffolds was 59.3 and 60.0%, respectively. The pores were uniformly distributed with an average pore size of 558.6-569.3 μm. MgO nanoparticles regulated the corrosion rate of scaffolds, resulting in a more uniform corrosion degradation behavior of the Zn/MgO scaffolds in simulated body fluid solution. The degradation ratio of Zn/MgO composite scaffolds in vivo was increased compared to pure Zn scaffolds, reaching 15.6% at 12 weeks. The yield strength (10.8 ± 2.4 MPa) of the Zn/MgO composite scaffold was comparable to that of cancellous bone, and the antimicrobial rate were higher than 99%. The Zn/MgO composite scaffolds could better guide bone tissue regeneration in rat cranial bone repair experiments (completely filling the scaffolds at 12 weeks). Therefore, porous Zn/MgO scaffolds with G-model structure prepared with SLM are a promising biodegradable bone tissue engineering scaffold.

The use of customized 3D-printed mandibular prostheses with pressure-reducing device: A clinical trial

BACKGROUND

Segmental bone defects of the mandible result in the complete loss of the affected region. We had incorporated the pressure-reducing device (PRD) designs into the customized mandible prostheses (CMP) and conducted a clinical trial to evaluate this approach.

METHODS

Seven patients were enrolled in this study. We examined the association among the history of radiotherapy, the number of CMP regions, the number of chin regions involved, and CMP exposure.

RESULTS

We included five men and two women with an average age of 55 years. We excised tumors with an average weight of 147.8 g and the average weight of the CMP was 68.5 g. No significant difference between the two weights was noted (p = 0.3882). Three patients received temporary dentures and the CMP remained stable in all patients.

CONCLUSION

The use of PRD in CMP may address the previous challenges associated with CMP, but further research is necessary.

Additive manufacturing of degradable metallic scaffolds for material-structure-driven diabetic maxillofacial bone regeneration

The regeneration of maxillofacial bone defects associated with diabetes mellitus remains challenging due to the occlusal loading and hyperglycemia microenvironment. Herein, we propose a material-structure-driven strategy through the additive manufacturing of degradable Zn-Mg-Cu gradient scaffolds. The in situ alloying of Mg and Cu endows Zn alloy with admirable compressive strength for mechanical support and uniform degradation mode for preventing localized rupture. The scaffolds manifest favorable antibacterial, angiogenic, and osteogenic modulation capacity in mimicked hyperglycemic microenvironment, and Mg and Cu promote osteogenic differentiation in the early and late stages, respectively. In addition, the scaffolds expedite diabetic maxillofacial bone ingrowth and regeneration by combining the metabolic regulation effect of divalent metal cations and the hyperboloid and suitable permeability of the gradient structure. RNA sequencing further reveals that RAC1 might be involved in bone formation by regulating the transport and uptake of glucose related to GLUT1 in osteoblasts, contributing to cell function recovery. Inspired by bone healing and structural cues, this study offers an essential understanding of the designation and underlying mechanisms of the material-structure-driven strategy for diabetic maxillofacial bone regeneration.

Debulking of the Femoral Stem in a Primary Total Hip Joint Replacement: A Novel Method to Reduce Stress Shielding

In current-generation designs of total primary hip joint replacement, the prostheses are fabricated from alloys. The modulus of elasticity of the alloy is substantially higher than that of the surrounding bone. This discrepancy plays a role in a phenomenon known as stress shielding, in which the bone bears a reduced proportion of the applied load. Stress shielding has been implicated in aseptic loosening of the implant which, in turn, results in reduction in the in vivo life of the implant. Rigid implants shield surrounding bone from mechanical loading, and the reduction in skeletal stress necessary to maintain bone mass and density results in accelerated bone loss, the forerunner to implant loosening. Femoral stems of various geometries and surface modifications, materials and material distributions, and porous structures have been investigated to achieve mechanical properties of stems closer to those of bone to mitigate stress shielding. For improved load transfer from implant to femur, the proposed study investigated a strategic debulking effort to impart controlled flexibility while retaining sufficient strength and endurance properties. Using an iterative design process, debulked configurations based on an internal skeletal truss framework were evaluated using finite element analysis. The implant models analyzed were solid; hollow, with a proximal hollowed stem; FB-2A, with thin, curved trusses extending from the central spine; and FB-3B and FB-3C, with thick, flat trusses extending from the central spine in a balanced-truss and a hemi-truss configuration, respectively. As outlined in the International Organization for Standardization (ISO) 7206 standards, implants were offset in natural femur for evaluation of load distribution or potted in testing cylinders for fatigue testing. The commonality across all debulked designs was the minimization of proximal stress shielding compared to conventional solid implants. Stem topography can influence performance, and the truss implants with or without the calcar collar were evaluated. Load sharing was equally effective irrespective of the collar; however, the collar was critical to reducing the stresses in the implant. Whether bonded directly to bone or cemented in the femur, the truss stem was effective at limiting stress shielding. However, a localized increase in maximum principal stress at the proximal lateral junction could adversely affect cement integrity. The controlled accommodation of deformation of the implant wall contributes to the load sharing capability of the truss implant, and for a superior biomechanical performance, the collared stem should be implanted in interference fit. Considering the results of all implant designs, the truss implant model FB-3C was the best model.

Mechanical, bioactive, and long-lasting antibacterial properties of a Ti scaffold with gradient pores releasing iodine ions

The ideal bone implant would effectively prevent aseptic as well as septic loosening by minimizing stress shielding, maximizing bone ingrowth, and preventing implant-associated infections. Here, a novel gradient-pore-size titanium scaffold was designed and manufactured to address these requirements. The scaffold features a larger pore size (900 μm) on the top surface, gradually decreasing to small sizes (600 μm to 300 μm) towards the center, creating a gradient structure. To enhance its functionality, the additively manufactured scaffolds were biofunctionalized using simple chemical and heat treatments so as to incorporate calcium and iodine ions throughout the surface. This unique combination of varying pore sizes with a biofunctional surface provides highly desirable mechanical properties, bioactivity, and notably, long-lasting antibacterial activity. The target mechanical aspects, including low elastic modulus, high compression, compression-shear, and fatigue strength, were effectively achieved. Furthermore, the biofunctional surface exhibits remarkable in vitro bioactivity and potent antibacterial activity, even under conditions specifically altered to be favorable for bacterial growth. More importantly, the integration of small pores alongside larger ones ensures a sustained high release of iodine, resulting in antimicrobial activity that persisted for over three months, with full eradication of the bacteria. Taken together, this gradient structure exhibits obvious superiority in combining most of the desired properties, making it an ideal candidate for orthopedic and dental implant applications.

Application of additively manufactured bone scaffold: a systematic review

The application of additive manufacturing (AM) technology plays a significant role in various fields, incorporating a wide range of cutting-edge technologies such as aerospace, medical treatment, electronic information, and materials. It is currently widely adopted for medical services, national defense, and industrial manufacturing. In recent years, AM has also been extensively employed to produce bone scaffolds and implant materials. Through AM, products can be manufactured without being constrained by complex internal structures. AM is particularly advantageous in the production of macroscopically irregular and microscopically porous biomimetic bone scaffolds, with short production cycles required. In this paper, AM commonly used to produce bone scaffolds and orthopedic implants is overviewed to analyze the different materials and structures adopted for AM. The applications of antibacterial bone scaffolds and bone scaffolds in biologically relevant animal models are discussed. Also, the influence on the comprehensive performance of product mechanics, mass transfer, and biology is explored. By identifying the reasons for the limited application of existing AM in the biomedical field, the solutions are proposed. This study provides an important reference for the future development of AM in the field of orthopedic healthcare. In conclusion, various AM technologies, the requirements of bone scaffolds and the important role of AM in building bridges between biomaterials, additives, and bone tissue engineering scaffolds are described and highlighted. Nevertheless, more caution should be exercised when designing bone scaffolds and conducting in vivo trials, due to the lack of standardized processes, which prevents the accuracy of results and reduces the reliability of information.

Method of computational design for additive manufacturing of hip endoprosthesis based on basic-cell concept

Endoprosthetic hip replacement is the conventional way to treat osteoarthritis or a fracture of a dysfunctional joint. Different manufacturing methods are employed to create reliable patient-specific devices with long-term performance and biocompatibility. Recently, additive manufacturing has become a promising technique for the fabrication of medical devices, because it allows to produce complex samples with various structures of pores. Moreover, the limitations of traditional fabrication methods can be avoided. It is known that a well-designed porous structure provides a better proliferation of cells, leading to improved bone remodeling. Additionally, porosity can be used to adjust the mechanical properties of designed structures. This makes the design and choice of the structure's basic cell a crucial task. This study focuses on a novel computational method, based on the basic-cell concept to design a hip endoprosthesis with an unregularly complex structure. A cube with spheroid pores was utilized as a basic cell, with each cell having its own porosity and mechanical properties. A novelty of the suggested method is in its combination of the topology optimization method and the structural design algorithm. Bending and compression cases were analyzed for a cylinder structure and two hip implants. The ability of basic-cell geometry to influence the structure's stress-strain state was shown. The relative change in the volume of the original structure and the designed cylinder structure was 6.8%. Computational assessments of a stress-strain state using the proposed method and direct modeling were carried out. The volumes of the two types of implants decreased by 9% and 11%, respectively. The maximum von Mises stress was 600 MPa in the initial design. After the algorithm application, it increased to 630 MPa for the first type of implant, while it is not changing in the second type of implant. At the same time, the load-bearing capacity of the hip endoprostheses was retained. The internal structure of the optimized implants was significantly different from the traditional designs, but better structural integrity is likely to be achieved with less material. Additionally, this method leads to time reduction both for the initial design and its variations. Moreover, it enables to produce medical implants with specific functional structures with an additive manufacturing method avoiding the constraints of traditional technologies.

Orthopedic meta-implants

Meta-biomaterials, engineered materials with distinctive combinations of mechanical, physical, and biological properties stemming from their micro-architecture, have emerged as a promising domain within biomedical engineering. Correspondingly, meta-implants, which serve as the device counterparts of meta-biomaterials, offer exceptional functionalities, holding great potential for addressing complex skeletal diseases. This paper presents a comprehensive overview of the various types of meta-implants, including hybrid, shape-morphing, metallic clay, and deployable meta-implants, highlighting their unprecedented properties and recent achievement in the field. This paper also delves into the potential future developments of meta-implants, addressing the exploration of multi-functionalities in meta-biomaterials and their applications in diverse biomedical fields.

Exploring the mechanical properties of 3D-printed multilayer lattice structures for use in accommodative insoles

Full-contact insoles fabricated from multilayer foams are the standard of care (SoC) for offloading and redistributing high plantar pressures in individuals with diabetes at risk of plantar ulceration and subsequent lower limb amputation. These devices have regional variations in total thickness and layer thickness to create conformity with a patient's foot. Recent work has demonstrated that metamaterials can be tuned to match the mechanical properties of SoC insole foams. However, for devices fabricated using a multilayer lattice structure, having regional variations in total thickness and layer thickness may result in regional differences in mechanical properties that have yet to be investigated. Three lattices, two dual-layer and one uniform-layer lattice structure, designed to model the mechanical properties of SoC insoles, were 3D-printed at three structure/puck thicknesses representing typical regions seen in accommodative insoles. The pucks underwent cyclic compression testing, and the stiffness profiles were assessed. Three pucks at three structure/puck thicknesses fabricated from SoC foams were also tested. Initial evaluations suggested that for the latticed pucks, structure thickness and density inversely impacted puck stiffness. Behaving most like the SoC pucks, a dual-layer lattice that increased in density as structure thickness increased demonstrated consistent stiffness profiles across puck thicknesses. Identifying a lattice with constant mechanical properties at various structure thicknesses may be important to produce a conforming insole that emulates the standard of care from which patient-specific/regional lattice modulations can be made.

Additively Manufactured SiO2 and Cu-Added Ti Implants for Synergistic Enhancement of Bone Formation and Antibacterial Efficacy

Commercially pure titanium (CpTi), a bioinert metal, is used as an implant material at low load-bearing sites and as a porous coating on Ti6Al4V at high load-bearing sites. There is an unmet need for metallic biomaterials to improve osseointegration and inherent antimicrobial resistance. In this study, we have added 1 wt % SiO2 and 3 wt % Cu to the CpTi matrix and processed via metal additive manufacturing (AM). Si4+ ions promote angiogenesis and osteogenesis. CpTi-SiO2 composition exhibited 4.5 times higher bone formation at the bone-implant interface over CpTi in an in vivo study with a rat distal femur model. In vitro bacterial studies with Gram-positive Staphylococcus aureus bacterium revealed 85% antibacterial efficacy by CpTi-SiO2-3Cu than CpTi. CpTi-SiO2-3Cu did not show any inflammatory markers in vivo, indicating the absence of cytotoxicity, but displayed delayed osseointegration compared to CpTi-SiO2. CpTi-SiO2-3Cu displayed 3-fold higher mineralized bone formation than CpTi. Our results emphasize the synergistic effect of SiO2 and Cu addition in CpTi, promoting enhanced early stage osseointegration and inherent antibacterial efficacy, contributing toward implant longevity and stability in vivo.

Additively manufactured porous scaffolds by design for treatment of bone defects

There has been increasing attention to produce porous scaffolds that mimic human bone properties for enhancement of tissue ingrowth, regeneration, and integration. Additive manufacturing (AM) technologies, i.e., three dimensional (3D) printing, have played a substantial role in engineering porous scaffolds for clinical applications owing to their high level of design and fabrication flexibility. To this end, this review article attempts to provide a detailed overview on the main design considerations of porous scaffolds such as permeability, adhesion, vascularisation, and interfacial features and their interplay to affect bone regeneration and osseointegration. Physiology of bone regeneration was initially explained that was followed by analysing the impacts of porosity, pore size, permeability and surface chemistry of porous scaffolds on bone regeneration in defects. Importantly, major 3D printing methods employed for fabrication of porous bone substitutes were also discussed. Advancements of MA technologies have allowed for the production of bone scaffolds with complex geometries in polymers, composites and metals with well-tailored architectural, mechanical, and mass transport features. In this way, a particular attention was devoted to reviewing 3D printed scaffolds with triply periodic minimal surface (TPMS) geometries that mimic the hierarchical structure of human bones. In overall, this review enlighten a design pathway to produce patient-specific 3D-printed bone substitutions with high regeneration and osseointegration capacity for repairing large bone defects.

Porosity-permeability tensor relationship of closely and randomly packed fibrous biomaterials and biological tissues: Application to the brain white matter

The constitutive model for the porosity-permeability relationship is a powerful tool to estimate and design the transport properties of porous materials, which has attracted significant attention for the advancement of novel materials. However, in comparison with other materials, biomaterials, especially natural and artificial tissues, have more complex microstructures e.g. high anisotropy, high randomness of cell/fibre dimensions/position and very low porosity. Consequently, a reliable microstructure-permeability relationship of fibrous biomaterials has proven elusive. To fill this gap, we start a mathematical derivation from the fundamental brain white matter (WM) formed by nerve fibres. This is augmented by a numerical characterisation and experimental validations to obtain an anisotropic permeability tensor of the brain WM as a function of the tissue porosity. A versatile microstructure generation software (MicroFiM) for fibrous biomaterial with complex microstructure and low porosity was built accordingly and made freely accessible here. Moreover, we propose an anisotropic poro-hyperelastic model enhanced by the newly defined porosity-permeability tensor relationship which precisely captures the tissues macro-scale permeability changes due to the microstructural deformation in an infusion scenario. The constitutive model, theories and protocols established in this study will both provide improved design strategies to tailor the transport properties of fibrous biomaterials and enable the non-invasive characterisation of the transport properties of biological tissues. This will lead to the provision of better patient-specific medical treatments, such as drug delivery. STATEMENT OF SIGNIFICANCE: Due to the microstructural complexity, a reliable microstructure-permeability relationship of fibrous biomaterials has proven elusive, which hinders our way of tuning the fluid transport property of the biomaterials by directly programming their microstructure. The same problem hinders non-invasive characterisations of fluid transport properties in biological tissues, which can significantly improve the efficiency of treatments e.g. drug delivery, directly from the tissues accessible microstructural information, e.g. porosity. Here, we developed a validated mathematical formulation to link the random microstructure to a fibrous material's macroscale permeability tensor. This will advance our capability to design complex biomaterials and make it possible to non-invasively characterise the permeability of living tissues for precise treatment planning. The newly established theory and protocol can be easily adapted to various types of fibrous biomaterials.

High-Cycle Fatigue Behaviour of Polyetheretherketone (PEEK) Produced by Additive Manufacturing

Polyetheretherketone (PEEK) is the leading high-performance thermoplastic biomaterial that can be processed through material extrusion (ME) additive manufacturing (AM), also known as three-dimensional (3D) printing, for patient-specific load-bearing implant manufacture. Considering the importance of cyclic loading for load-bearing implant design, this work addresses the high-cycle fatigue behaviour of 3D-printed PEEK. In this work, printed PEEK specimens are cyclically loaded under stress-controlled tension-tension using different stress levels between 75% and 95% of printed PEEK's tensile strength. The experimental results are used to document 3D-printed PEEK's fatigue behaviour using Basquin's power law, which was compared with previous fatigue research on bulk PEEK and other 3D-printing materials. As a pioneering study on its fatigue behaviour, the results from this work show that 3D-printed PEEK exhibits an above-average fatigue strength of 65 MPa, corresponding to about 75% of its tensile strength. Fracture surface analysis suggests that a transition can occur from ductile to brittle fracture with maximum stresses between 85% and 95% of the tensile strength. Evidence of crack propagation features on fracture surfaces under scanning electron microscope (SEM) observation suggests crack initiation in void defects created by printing deposition that propagates longitudinally through line bonding interfaces along layers. Considering this, 3D-printed PEEK's fatigue behaviour can be strongly related to printing conditions. Further research on the fatigue behaviour of 3D-printed PEEK is necessary to support its use in load-bearing implant applications.

The Novel Hybrid Lattice Structure Approach Fabricated by Laser Powder Bed Fusion and Mechanical Properties Comparison

Aluminum-based cellular structures are gaining a huge traction in several applications, including lightweight aircraft, military equipment, and heat exchangers. With additive manufacturing, the fabrication of complex periodic cellular structures with any unit cell form, size, and volume fraction has become a lot easier, allowing for more investment, research, and attention from both academia and industry. The aim of the research was to assess the manufacturability and performance of AlSi10Mg periodic cellular structures generated using the laser powder bed fusion process. Re-entrant and triply periodic and minimum surface (TPMS) gyroid cells were hybridized into a single cellular structure having identical volume fraction. Because of distinct mechanical properties of TPMS and re-entrant types, these cells were selected and assembled in various patterns to study their manufacturability, deformation behavior, energy absorption, and compressive strength. This work demonstrates good geometric agreement between the manufactured hybrid lattice structures and computer-aided design models. Hybridized structures with several repeated layers of TPMS gyroid and re-entrant cells can result in superior compressive strength and energy absorption than those with only few large layers.

In vitro fatigue behavior and in vivo osseointegration of the auxetic porous bone screw

The incidence of screw loosening, migration, and pullout caused by the insufficient screw-bone fixation stability is relatively high in clinical practice. To solve this issue, the auxetic unit-based porous bone screw (AS) has been put forward in our previous work. Its favorable auxetic effect can improve the primary screw-bone fixation stability after implantation. However, porous structure affected the fatigue behavior and in vivo longevity of bone screw. In this study, in vitro fatigue behaviors and in vivo osseointegration performance of the re-entrant unit-based titanium auxetic bone screw were studied. The tensile-tensile fatigue behaviors of AS and nonauxetic bone screw (NS) with the same porosity (51%) were compared via fatigue experiments, fracture analysis, and numerical simulation. The in vivo osseointegration of AS and NS were compared via animal experiment and biomechanical analysis. Additionally, the effects of in vivo dynamic tensile loading on the osseointegration of AS and NS were investigated and analyzed. The fatigue strength of AS was approximately 43% lower while its osseointegration performance was better than NS. Under in vivo dynamic tensile loading, the osseointegration of AS and NS both improved significantly, with the maximum increase of approximately 15%. Preferrable osseointegration of AS might compensate for the shortage of fatigue resistance, ensuring its long-term stability in vivo. Adequate auxetic effect and long-term stability of the AS was supposed to provide enough screw-bone fixation stability to overcome the shortages of the solid bone screw, developing the success of surgery and showing significant clinical application prospects in orthopedic surgery. STATEMENT OF SIGNIFICANCE: This research investigated the high-cycle fatigue behavior of re-entrant unit-based auxetic bone screw under tensile-tensile cyclic loading and its osseointegration performance, which has not been focused on in existing studies. The fatigue strength of auxetic bone screw was lower while the osseointegration was better than non-auxetic bone screw, especially under in vivo tensile loading. Favorable osseointegration of auxetic bone screw might compensate for the shortage of fatigue resistance, ensuring its long-term stability and longevity in vivo. This suggested that with adequate auxetic effect and long-term stability, the auxetic bone screw had significant application prospects in orthopedic surgery. Findings of this study will provide a theoretical guidance for design optimization and clinical application of the auxetic bone screw.

Physiomimetic biocompatibility evaluation of directly printed degradable porous iron implants using various cell types

Additively manufactured (AM) degradable porous metallic biomaterials offer unique opportunities for satisfying the design requirements of an ideal bone substitute. Among the currently available biodegradable metals, iron has the highest elastic modulus, meaning that it would benefit the most from porous design. Given the successful preclinical applications of such biomaterials for the treatment of cardiovascular diseases, the moderate compatibility of AM porous iron with osteoblast-like cells, reported in earlier studies, has been surprising. This may be because, as opposed to static in vitro conditions, the biodegradation products of iron in vivo are transported away and excreted. To better mimic the in situ situations of biodegradable biomaterials after implantation, we compared the biodegradation behavior and cytocompatibility of AM porous iron under static conditions to the conditions with dynamic in situ-like fluid flow perfusion in a bioreactor. Furthermore, the compatibility of these scaffolds with four different cell types was evaluated to better understand the implications of these implants for the complex process of natural wound healing. These included endothelial cells, L929 fibroblasts, RAW264.7 macrophage-like cells, and osteoblastic MG-63 cells. The biodegradation rate of the scaffolds was significantly increased in the perfusion bioreactor as compared to static immersion. Under either condition, the compatibility with L929 cells was the best. Moreover, the compatibility with all the cell types was much enhanced under physiomimetic dynamic flow conditions as compared to static biodegradation. Our study highlights the importance of physiomimetic culture conditions and cell type selection when evaluating the cytocompatibility of degradable biomaterials in vitro. STATEMENT OF SIGNIFICANCE: Additively manufactured (AM) degradable porous metals offer unique opportunities for the treatment of large bony defects. Despite the successful preclinical applications of biodegradable iron in the cardiovascular field, the moderate compatibility of AM porous iron with osteoblast-like cells was reported. To better mimic the in vivo condition, we compared the biodegradation behavior and cytocompatibility of AM porous iron under static condition to dynamic perfusion. Furthermore, the compatibility of these scaffolds with various cell types was evaluated to better simulate the process of natural wound healing. Our study suggests that AM porous iron holds great promise for orthopedic applications, while also highlighting the importance of physio-mimetic culture conditions and cell type selection when evaluating the cytocompatibility of degradable biomaterials in vitro.

Recent advances in 3D printing of biodegradable metals for orthopaedic applications

The use of biodegradable polymers for treating bone-related diseases has become a focal point in the field of biomedicine. Recent advancements in material technology have expanded the range of materials suitable for orthopaedic implants. Three-dimensional (3D) printing technology has become prevalent in healthcare, and while organ printing is still in its early stages and faces ethical and technical hurdles, 3D printing is capable of creating 3D structures that are supportive and controllable. The technique has shown promise in fields such as tissue engineering and regenerative medicine, and new innovations in cell and bio-printing and printing materials have expanded its possibilities. In clinical settings, 3D printing of biodegradable metals is mainly used in orthopedics and stomatology. 3D-printed patient-specific osteotomy instruments, orthopedic implants, and dental implants have been approved by the US FDA for clinical use. Metals are often used to provide support for hard tissue and prevent complications. Currently, 70-80% of clinically used implants are made from niobium, tantalum, nitinol, titanium alloys, cobalt-chromium alloys, and stainless steels. However, there has been increasing interest in biodegradable metals such as magnesium, calcium, zinc, and iron, with numerous recent findings. The advantages of 3D printing, such as low manufacturing costs, complex geometry capabilities, and short fabrication periods, have led to widespread adoption in academia and industry. 3D printing of metals with controllable structures represents a cutting-edge technology for developing metallic implants for biomedical applications. This review explores existing biomaterials used in 3D printing-based orthopedics as well as biodegradable metals and their applications in developing metallic medical implants and devices. The challenges and future directions of this technology are also discussed.

Additive Manufacturing Using Agriculturally Derived Biowastes: A Systematic Literature Review

Agriculturally derived biowastes can be transformed into a diverse range of materials, including powders, fibers, and filaments, which can be used in additive manufacturing methods. This review study reports a study that analyzes the existing literature on the development of novel materials from agriculturally derived biowastes for additive manufacturing methods. A review was conducted of 57 selected publications since 2016 covering various agriculturally derived biowastes, different additive manufacturing methods, and potential large-scale applications of additive manufacturing using these materials. Wood, fish, and algal cultivation wastes were also included in the broader category of agriculturally derived biowastes. Further research and development are required to optimize the use of agriculturally derived biowastes for additive manufacturing, particularly with regard to material innovation, improving print quality and mechanical properties, as well as exploring large-scale industrial applications.

Additive manufacturing of vascular stents

With the advancement of additive manufacturing (AM), customized vascular stents can now be fabricated to fit the curvatures and sizes of a narrowed or blocked blood vessel, thereby reducing the possibility of thrombosis and restenosis. More importantly, AM enables the design and fabrication of complex and functional stent unit cells that would otherwise be impossible to realize with conventional manufacturing techniques. Additionally, AM makes fast design iterations possible while also shortening the development time of vascular stents. This has led to the emergence of a new treatment paradigm in which custom and on-demand-fabricated stents will be used for just-in-time treatments. This review is focused on the recent advances in AM vascular stents aimed at meeting the mechanical and biological requirements. First, the biomaterials suitable for AM vascular stents are listed and briefly described. Second, we review the AM technologies that have been so far used to fabricate vascular stents as well as the performances they have achieved. Subsequently, the design criteria for the clinical application of AM vascular stents are discussed considering the currently encountered limitations in materials and AM techniques. Finally, the remaining challenges are highlighted and some future research directions are proposed to realize clinically-viable AM vascular stents. STATEMENT OF SIGNIFICANCE: Vascular stents have been widely used for the treatment of vascular disease. The recent progress in additive manufacturing (AM) has provided unprecedented opportunities for revolutionizing traditional vascular stents. In this manuscript, we review the applications of AM to the design and fabrication of vascular stents. This is an interdisciplinary subject area that has not been previously covered in the published review articles. Our objective is to not only present the state-of-the-art of AM biomaterials and technologies but to also critically assess the limitations and challenges that need to be overcome to speed up the clinical adoption of AM vascular stents with both anatomical superiority and mechanical and biological functionalities that exceed those of the currently available mass-produced devices.

Deformable titanium for acetabular revision surgery: a proof of concept

Custom-made triflange acetabular implants are increasingly used in complex revision surgery where supporting bone stock is diminished. In most cases these triflange cups induce stress-shielding. A new concept for the triflange is introduced that uses deformable porous titanium to redirect forces from the acetabular rim to the bone stock behind the implant and thereby reduces further stress-shielding. This concept is tested for deformability and primary stability.Three different designs of highly porous titanium cylinders were tested under compression to determine their mechanical properties. The most promising design was used to design five acetabular implants either by incorporating a deformable layer at the back of the implant or by adding a separate generic deformable mesh behind the implant. All implants were inserted into sawbones with acetabular defects followed by a cyclic compression test of 1800N for 1000 cycles.The design with a cell size of 4 mm and 0.2 mm strut thickness performed the best and was applied for the design of the acetabular implants. An immediate primary fixation was realized in all three implants with an incorporated deformable layer. One of the two implants with a separate deformable mesh needed fixation with screws. Cyclic tests revealed an average additional implant subsidence of 0.25 mm that occurred in the first 1000 cycles with minimal further subsidence thereafter.It is possible to realize primary implant fixation and stability in simulated large acetabular revision surgery using a deformable titanium layer behind the cup. Additional research is needed for further implementation of such implants in the clinic.

Design Optimization of Additive Manufactured Edgeless Simple Cubic Lattice Structures under Compression

This study proposed an optimization framework and methodologies to design edgeless lattice structures featuring fillet and multipipe functions. Conventional lattice structures typically experience stress concentration at the sharp edges of strut joints, resulting in reduced mechanical performance and premature failure. The proposed approach aimed to improve the compression behavior of lattice structures by introducing edgeless features. Through finite element analysis, the optimized fillet edgeless simple cubic unit cell with a fillet radius to strut radius ratio of 0.753 showed a 12.1% improvement in yield stress and a 144% reduction in stress concentration. To validate the finite element analysis, experimental compressive tests were conducted, confirming that the introduction of edgeless functions improved the compressive strength of lattice structures manufactured through additive manufacturing. The optimized fillet edgeless simple cubic lattice structure exhibited the most effective improvement. This approach has promising potential for lattice structure applications.

Titanium Lattice Structures Produced via Additive Manufacturing for a Bone Scaffold: A Review

The progress in additive manufacturing has remarkably increased the application of lattice materials in the biomedical field for the fabrication of scaffolds used as bone substitutes. Ti6Al4V alloy is widely adopted for bone implant application as it combines both biological and mechanical properties. Recent breakthroughs in biomaterials and tissue engineering have allowed the regeneration of massive bone defects, which require external intervention to be bridged. However, the repair of such critical bone defects remains a challenge. The present review collected the most significant findings in the literature of the last ten years on Ti6Al4V porous scaffolds to provide a comprehensive summary of the mechanical and morphological requirements for the osteointegration process. Particular attention was given on the effects of pore size, surface roughness and the elastic modulus on bone scaffold performances. The application of the Gibson–Ashby model allowed for a comparison of the mechanical performance of the lattice materials with that of human bone. This allows for an evaluation of the suitability of different lattice materials for biomedical applications.

Mechanical behaviour of a novel biomimetic lattice structure for bone scaffold

In this research, a new lattice structure based on the octagonal geometry and produced by Additive Manufacturing (AM) technique was proposed. Eight octagons with the same dimensions are combined to each other forming a ring. To obtain an isotropic lattice structure, cubic symmetry was imposed; thus, the unit cell is made of three rings mutually perpendicular, one ring for each principal direction. The aim of this study is the morphological and mechanical characterization of the novel unit cell to check its suitability to the biomechanical field, along with its comparison with other lattice structures currently used as bone scaffold. Electron Beam Melting (EBM) technique was used to produce Ti6Al4V ELI alloy specimens of the novel unit cell and of the truncated octahedron (Kelvin) cell. Three different unit cell sizes were selected to investigate the effect of cell dimensions on the mechanical properties. Morphological analysis was performed through a scanning electron microscope (SEM), to compare the actual structures to the designed ones. On the whole, the new lattice structure provided adequate mechanical properties to be considered as a bone substitute; further tests will be focused on its osteointegration capability.

A Review of Image-Based Simulation Applications in High-Value Manufacturing

Image-Based Simulation (IBSim) is the process by which a digital representation of a real geometry is generated from image data for the purpose of performing a simulation with greater accuracy than with idealised Computer Aided Design (CAD) based simulations. Whilst IBSim originates in the biomedical field, the wider adoption of imaging for non-destructive testing and evaluation (NDT/NDE) within the High-Value Manufacturing (HVM) sector has allowed wider use of IBSim in recent years. IBSim is invaluable in scenarios where there exists a non-negligible variation between the 'as designed' and 'as manufactured' state of parts. It has also been used for characterisation of geometries too complex to accurately draw with CAD. IBSim simulations are unique to the geometry being imaged, therefore it is possible to perform part-specific virtual testing within batches of manufactured parts. This novel review presents the applications of IBSim within HVM, whereby HVM is the value provided by a manufactured part (or conversely the potential cost should the part fail) rather than the actual cost of manufacturing the part itself. Examples include fibre and aggregate composite materials, additive manufacturing, foams, and interface bonding such as welding. This review is divided into the following sections: Material Characterisation; Characterisation of Manufacturing Techniques; Impact of Deviations from Idealised Design Geometry on Product Design and Performance; Customisation and Personalisation of Products; IBSim in Biomimicry. Finally, conclusions are drawn, and observations made on future trends based on the current state of the literature.

Selective Laser Melting (SLM) Additively Manufactured CoCrFeNiMn High-Entropy Alloy: Process Optimization, Microscale Mechanical Mechanism, and High-Cycle Fatigue Behavior

The equiatomic CoCrFeNiMn high-entropy alloy (HEA) possesses excellent properties including exceptional strength-ductility synergy, high corrosion resistance, and good thermal stability. Selective laser melting (SLM) additive manufacturing facilitates the convenient fabrication of the CoCrFeNiMn HEA parts with complex geometries. Here, the SLM process optimization was conducted to achieve a high relative density of as-built CoCrFeNiMn HEA bulks. The mechanisms of process-induced defects and process control were elucidated. The microscale mechanical behaviors were analyzed through in situ scanning electron microscopy observation during the compression tests on micro-pillars of the as-built HEA. The stress-strain characteristics by repeated slip and mechanism of "dislocation avalanche" during the compression of micro-pillars were discussed. The high-cycle fatigue tests of the as-built HEA were performed. It was found that a large number of nano-twins were induced by the fatigue, causing a non-negligible cycle softening phenomenon. The effects of promoted ductility due to the fatigue-induced nano-twins were illustrated. This work has some significance for the engineering application of the SLM additively manufactured CoCrFeNiMn HEA parts.

Multi-Defect Detection in Additively Manufactured Lattice Structures Using 3D Electrical Resistance Tomography

Cellular lattice structures possess high strength-to-weight ratios suitable for advanced lightweight engineering applications. However, their quality and mechanical performance can degrade because of defects introduced during manufacturing or in-service. Their complexity and small length scale features make defects difficult to detect using conventional nondestructive evaluation methods. Here we propose a current injection-based method, electrical resistance tomography (ERT), that can be used to detect damaged struts in conductive cellular lattice structures with their intrinsic electromechanical properties. The reconstructed conductivity distributions from ERT can reveal the severity and location of damaged struts without having to probe each strut. However, the low central sensitivity of ERT may result in image artifacts and inaccurate localization of damaged struts. To address this issue, this study introduces an absolute, high throughput, conductivity reconstruction algorithm for 3D ERT. The algorithm incorporates a strut-based normalized sensitivity map to compensate for lower interior sensitivity and suppresses reconstruction artifacts. Numerical simulations and experiments on fabricated representative cellular lattice structures were performed to verify the ability of ERT to quantitatively identify single and multiple damaged struts. The improved performance of this method compared with classical ERT was observed, based on greatly decreased imaging and reconstructed value errors.

Additively manufactured lattice structures with controlled transverse isotropy for orthopedic porous implants

Additively manufactured lattice structures enable the design of tissue scaffolds with tailored mechanical properties, which can be implemented in porous biomaterials. The adaptation of bone to physiological loads results in anisotropic bone tissue properties which are optimized for site-specific loads; therefore, some bone sites are stiffer and stronger along the principal load direction compared to other orientations. In this work, a semi-analytical model was developed for the design of transversely isotropic lattice structures that can mimic the anisotropy characteristics of different types of bone tissue. Several design possibilities were explored, and a particular unit cell, which was best suited for additive manufacturing was further analyzed. The design of the unit cell was parameterized and in-silico analysis was performed via Finite Element Analysis. The structures were manufactured additively in metal and tested under compressive loads in different orientations. Finite element analysis showed good correlation with the semi-analytical model, especially for elastic constants with low relative densities. The anisotropy measured experimentally showed a variable accuracy, highlighting the deviations from designs to additively manufactured parts. Overall, the proposed model enables to exploit the anisotropy of lattice structures to design lighter scaffolds with higher porosity and increased permeability by aligning the scaffold with the principal direction of the load.

Strength enhancement and modulus modulation in auxetic meta-biomaterials produced by selective laser melting

Meta-biomaterials are applied to orthopedic implants to avoid stress shielding effects; however, there is no reason for the yield strength to be comparable to that of human bone. In this study, a composite unit cell was designed by combining the positive Poisson's ratio (PPR) and negative Poisson's ratio (NPR) unit cells, inspired by the second-phase strengthening theory. The purpose was to increase the strength while maintaining the elastic modulus. All structures were successfully fabricated from Ti-6Al-4V via selective laser melting. The relative density is between 0.08 and 0.24, which falls within the optimal range for bone growth. Mechanical tests indicated that the center of the inclined rod fractured in a stepwise fracture mode, which was consistent with the predictions of the Johnson-Cook model. The elastic modulus ranged from 0.652 ± 0.016 to 5.172 ± 0.021 GPa, and the yield strength varied from 10.62 ± 0.112 to 87.158 ± 2.215 MPa. An improved Gibson-Ashby law was proposed to facilitate the design of gradient structures. When the re-entrant angle was 40°, a hybrid body-centered cubic NPR structure was formed, resulting in a significant improvement in the mechanical properties. Importantly, the yield strength of the proposed composite structures increased by 43.23%, and the compression strength increased by 44.70% under the same elastic modulus. The strengthening mechanism has been proven to apply to other bending-dominated structures. Overall, this imparts unprecedented mechanical performance to auxetic meta-biomaterials and provides insights into improving the reported porous structures. STATEMENT OF SIGNIFICANCE: : Auxetic meta-biomaterials exhibit auxetic properties that can improve the contact between the bone-implant interface and reduce the risk of aseptic failure. To avoid the stress shielding effect, the elastic modulus has traditionally been decreased by increasing the porosity. However, the strength is simultaneously reduced. Therefore, a composite unit cell was proposed to increase strength rather than modulus by combining the positive and negative Poisson's ratio unit cells, inspired by the second-phase strengthening theory. We observed a 43.23% increase in the yield strength of the composite structure without increasing the elastic modulus. This strengthening mechanism has been proven to apply to other bending-dominated structures. Our approach provides insights into improving other bending-dominated structures and broadening their applications for bone implantation.

Hybrid Ti6Al4V/Silk Fibroin Composite for Load-Bearing Implants: A Hierarchical Multifunctional Cellular Scaffold

Despite the tremendous technological advances that metal additive manufacturing (AM) has made in the last decades, there are still some major concerns guaranteeing its massive industrial application in the biomedical field. Indeed, some main limitations arise in dealing with their biological properties, specifically in terms of osseointegration. Morphological accuracy of sub-unital elements along with the printing resolution are major constraints in the design workspace of a lattice, hindering the possibility of manufacturing structures optimized for proper osteointegration. To overcome these issues, the authors developed a new hybrid multifunctional composite scaffold consisting of an AM Ti6Al4V lattice structure and a silk fibroin/gelatin foam. The composite was realized by combining laser powder bed fusion (L-PBF) of simple cubic lattice structures with foaming techniques. A combined process of foaming and electrodeposition has been also evaluated. The multifunctional scaffolds were characterized to evaluate their pore size, morphology, and distribution as well as their adhesion and behavior at the metal-polymer interface. Pull-out tests in dry and hydrated conditions were employed for the mechanical characterization. Additionally, a cytotoxicity assessment was performed to preliminarily evaluate their potential application in the biomedical field as load-bearing next-generation medical devices.

Optimization of Structural and Processing Parameters for Selective Laser Melting of Porous 316L Bone Scaffolds

In the implantation of porous bone scaffolds, good mechanical properties of the scaffold are a prerequisite for the long-term functionality of the implanted scaffolds, which varies according to the structure and the forming process. In this study, the influence of the forming parameters and structure of the Selective Laser Melting (SLM) process on the mechanical properties of 316L stainless steel bone scaffolds was investigated using finite element simulation combined with experimental methods. The mechanism of the influence of the process parameters and structure on the mechanical properties of bone scaffolds was summarized using static compression finite element numerical simulations, compression experiments, hydrodynamic simulations, forming numerical simulations and SLM forming experiments. The results show that the magnitude of residual stress and the distribution of defects under different process parameters had a strong influence on the microstructure and properties of the scaffold, and the residual stress of the Body-Centered Cube (BCC) structure formed at an energy density of 41.7 J/mm³ was significantly reduced, with less surface spheroidization and fewer cracks on the melt pool surface. The smallest grain size of 321 nm was obtained at an energy density of 77.4 J/mm³, while in terms of mechanical properties, the optimization of the structure resulted in an 8.3% increase in yield strength and a reduction in stress concentration. The predictions of stress, deformation, and forming quality during construction with different process parameters, achieved using finite element analysis, are basically in agreement with the experimental results, indicating that the best process parameters for forming BCC structural supports were determined by using finite element simulation combined with experiments; moreover, the distribution and evolution of residual stresses and defects under different process parameters for constructing BCC structures were obtained.

Additive Manufacturing of Biomaterials-Design Principles and Their Implementation

Additive manufacturing (AM, also known as 3D printing) is an advanced manufacturing technique that has enabled progress in the design and fabrication of customised or patient-specific (meta-)biomaterials and biomedical devices (e.g., implants, prosthetics, and orthotics) with complex internal microstructures and tuneable properties. In the past few decades, several design guidelines have been proposed for creating porous lattice structures, particularly for biomedical applications. Meanwhile, the capabilities of AM to fabricate a wide range of biomaterials, including metals and their alloys, polymers, and ceramics, have been exploited, offering unprecedented benefits to medical professionals and patients alike. In this review article, we provide an overview of the design principles that have been developed and used for the AM of biomaterials as well as those dealing with three major categories of biomaterials, i.e., metals (and their alloys), polymers, and ceramics. The design strategies can be categorised as: library-based design, topology optimisation, bio-inspired design, and meta-biomaterials. Recent developments related to the biomedical applications and fabrication methods of AM aimed at enhancing the quality of final 3D-printed biomaterials and improving their physical, mechanical, and biological characteristics are also highlighted. Finally, examples of 3D-printed biomaterials with tuned properties and functionalities are presented.

Additive manufacturing of functionally graded porous titanium scaffolds for dental applications

Graded porous titanium scaffolds are gaining increasing attention as dental implants due to their ability to mimic the mechanical and biological properties of human bone. In this study, we have developed titanium scaffolds with graded primitive structures with porosities of 50.7 %, 61.0 %, 70.5 %, and 80.3 % (denoted as P50, P60, P70, and P80, respectively) for dental applications. The simulation results in the oral environment showed that the maximum von Mises strains and stress of cortical bone tissue around P50, P60, and P70 were lower than 3000 με and 60 MPa, respectively, which was beneficial for bone regeneration. The elastic modulus and yield strength of P50, P60, and P70 ranged within 5.2-13.8 GPa and 88.6-217.8 MPa, respectively. Among these, P60 exhibited the most favorable mechanical properties with a compression yield strength of 163.2 MPa and an elastic modulus of 9.7 GPa, which are desirable mechanical properties for dental material applications. The tested permeabilities of the fabricated specimens were in the range 0.66-6.88 × 10^-9 m², which is within the range of human bone (0.01-12.10 × 10^-9 m²). In vitro biocompatibility assay results showed that P60 and P70 had better potential for cell viability and osteogenesis than P50. It can be concluded that P60, which has a compatible elastic modulus, high yield strength, high permeability, good cytocompatibility, and osteogenesis properties, is a promising candidate for bone-tissue engineering applications in dentistry.

Laser Sintering Approaches for Bone Tissue Engineering

The adoption of additive manufacturing (AM) techniques into the medical space has revolutionised tissue engineering. Depending upon the tissue type, specific AM approaches are capable of closely matching the physical and biological tissue attributes, to guide tissue regeneration. For hard tissue such as bone, powder bed fusion (PBF) techniques have significant potential, as they are capable of fabricating materials that can match the mechanical requirements necessary to maintain bone functionality and support regeneration. This review focuses on the PBF techniques that utilize laser sintering for creating scaffolds for bone tissue engineering (BTE) applications. Optimal scaffold requirements are explained, ranging from material biocompatibility and bioactivity, to generating specific architectures to recapitulate the porosity, interconnectivity, and mechanical properties of native human bone. The main objective of the review is to outline the most common materials processed using PBF in the context of BTE; initially outlining the most common polymers, including polyamide, polycaprolactone, polyethylene, and polyetheretherketone. Subsequent sections investigate the use of metals and ceramics in similar systems for BTE applications. The last section explores how composite materials can be used. Within each material section, the benefits and shortcomings are outlined, including their mechanical and biological performance, as well as associated printing parameters. The framework provided can be applied to the development of new, novel materials or laser-based approaches to ultimately generate bone tissue analogues or for guiding bone regeneration.

Biodegradable Iron and Porous Iron: Mechanical Properties, Degradation Behaviour, Manufacturing Routes and Biomedical Applications

Biodegradable metals have been extensively studied due to their potential use as temporary biomedical devices, on non-load bearing applications. These types of implants are requested to function for the healing period, and should degrade after the tissue heals. A balance between mechanical properties requested at the initial stage of implantation and the degradation rate is required. The use of temporary biodegradable implants avoids a second surgery for the removal of the device, which brings high benefits to the patients and avoids high societal costs. Among the biodegradable metals, iron as a biodegradable metal has increased attention over the last few years, especially with the incorporation of additive manufacturing processes to obtain tailored geometries of porous structures, which give rise to higher corrosion rates. Withal by mimic natural bone hierarchical porosity, the mechanical properties of obtained structures tend to equalize that of human bone. This review article presents some of the most important works in the field of iron and porous iron. Fabrication techniques for porous iron are tackled, including conventional and new methods highlighting the unparalleled opportunities given by additive manufacturing. A comparison among the several methods is taken. The effects of the design and the alloying elements on the mechanical properties are also revised. Iron alloys with antibacterial properties are analyzed, as well as the biodegradation behavior and biocompatibility of iron. Although is necessary for further in vivo research, iron is presenting satisfactory results for upcoming biomedical applications, as orthopaedic temporary scaffolds and coronary stents.

Design Aspects of Additive Manufacturing at Microscale: A Review

Additive manufacturing (AM) technology has been researched and developed for almost three decades. Microscale AM is one of the fastest-growing fields of research within the AM area. Considerable progress has been made in the development and commercialization of new and innovative microscale AM processes, as well as several practical applications in a variety of fields. However, there are still significant challenges that exist in terms of design, available materials, processes, and the ability to fabricate true three-dimensional structures and systems at a microscale. For instance, microscale AM fabrication technologies are associated with certain limitations and constraints due to the scale aspect, which may require the establishment and use of specialized design methodologies in order to overcome them. The aim of this paper is to review the main processes, materials, and applications of the current microscale AM technology, to present future research needs for this technology, and to discuss the need for the introduction of a design methodology. Thus, one of the primary concerns of the current paper is to present the design aspects describing the comparative advantages and AM limitations at the microscale, as well as the selection of processes and materials.

Static and Fatigue Load Bearing Investigation on Porous Structure Titanium Additively Manufactured Anterior Cervical Cages

This study investigates the static and fatigue behavior of porous and conventional anterior cervical cages. Porous structure titanium anterior cervical cages were manufactured using direct selective laser sintering technique. Four different types of cervical cages were designed and manufactured, among which three designs consist of porous structure (type 1, type 2, and type 3) and manufactured using metal 3D printing. Remaining one design (type 4) was manufactured using conventional machining and did not consist any porous structure. All types of manufactured cages were tested in compression under static and fatigue loading conditions as per ASTM F2077 standard. Static and fatigue subsidence testing was performed using ASTM F2267 standard. Static compression testing results of type 1 and type 4 cages reported higher yield load when compared to the type 2 and type 3 cages. Static subsidence testing results reported almost 11% less subsidence rate for additively manufactured cages than the conventional cages. Fatigue subsidence testing results showed that type 2 and type 3 cages can withstood approximately 21% higher number of cycles before subsidence as compare to the type 1 and type 4 cages. During fatigue testing, all the cages design survived 5 million cycles at the 3000 N loading. For 6000 N and 8000 N, loading rate type 2 and type 3 cages showed lower fatigue life when compared to other cages design. Since fatigue life of type 2 and type 3 cage designs were reported lower than other cages design, it is concluded that the performance of the additively manufactured porous cages can be significantly varied based upon the cage design features.

Experimental and Numerical Investigation of a Lattice Structure for Energy Absorption: Application to the Design of an Automotive Crash Absorber

In this work, an experimental and numerical analysis of a lattice structure for energy absorption was carried out. The goal was to identify the most influencing parameters of the unit cell on the crushing performances of the structure, thus guiding the design of energy absorbers. Two full factorial plans of compression tests on cubic specimens of carbon nylon produced by fused deposition modeling (FDM) were performed. The factors were the beam diameter and the number of unit cells. In the first factorial plan, the specimen volume is constant and the dimensions of the unit cell are varied, while the second factorial plan assumes a constant size of the unit cell and the volume changes in accordance with their number. The results showed that the specific energy absorption increases with the diameter of the beam and decreases with the size of the unit cell. Based on these results, a crash absorber for the segment C vehicle was designed and compared with the standard component of the vehicle made of steel. In addition to a mass reduction of 25%, the improved crushing performances of the lattice structure are shown by the very smooth force-displacement curve with limited peaks and valleys.

Main Applications and Recent Research Progresses of Additive Manufacturing in Dentistry

In recent ten years, with the fast development of digital and engineering manufacturing technology, additive manufacturing has already been more and more widely used in the field of dentistry, from the first personalized surgical guides to the latest personalized restoration crowns and root implants. In particular, the bioprinting of teeth and tissue is of great potential to realize organ regeneration and finally improve the life quality. In this review paper, we firstly presented the workflow of additive manufacturing technology. Then, we summarized the main applications and recent research progresses of additive manufacturing in dentistry. Lastly, we sketched out some challenges and future directions of additive manufacturing technology in dentistry.

Mechanisms of fatigue crack initiation and propagation in auxetic meta-biomaterials

The fatigue performance of additively manufactured auxetic meta-biomaterials made from commercially pure titanium has been studied only recently. While certain assumptions have been made regarding the mechanisms underlying their fatigue failure, the exact mechanisms are not researched yet. Here, we studied the mechanisms of crack formation and propagation in cyclically loaded auxetic meta-biomaterials. Twelve different designs were subjected to compression-compression fatigue testing while performing full-field strain measurement using digital image correlation (DIC). The fatigue tests were stopped at different points before complete specimen failure to study the evolution of damage in the micro-architecture of the specimens using micro-computed tomography (micro-CT). Furthermore, finite element models were made to study the presence of stress concentrations. Structural weak spots were found in the inverted nodes and the vertical struts located along the outer rim of the specimens, matching the maximum principal strain concentrations and fracture sites in the DIC and micro-CT data. Cracks were often found to originate from internal void spaces or from sites susceptible to mode-I cracking. Many specimens maintained their structural integrity and exhibited no signs of rapid strain accumulation despite the presence of substantial crack growth. This observation underlines the importance of such microscale studies to identify accumulated damage that otherwise goes unnoticed. The potential release of powder particles from damaged lattices could elicit a foreign body response, adversely affecting the implant success. Finding the right failure criterion, therefore, requires more data than only those pertaining to macroscopic measurements and should always include damage assessment at the microscale. STATEMENT OF SIGNIFICANCE: The negative Poisson's ratio of auxetic meta-biomaterials makes them expand laterally in response to axial tension. This extraordinary property has great potential in the field of orthopedics, where it could enhance bone-implant contact. The fatigue performance of additively manufactured auxetic meta-biomaterials has only recently been studied and was found to be superior to many other bending- and stretch-dominated micro-architectures. In this study, we go beyond these macroscopic measurements and focus on the crack initiation and propagation. Full-field strain measurements and 3D imaging are used to paint a detailed picture of the mechanisms underlying fatigue. Using these data, specific aspects of the design and/or printing process can be targeted to improve the performance of auxetic meta-biomaterials in load-bearing applications.

Auxetic Structures for Tissue Engineering Scaffolds and Biomedical Devices

An auxetic structure utilizing a negative Poisson's ratio, which can expand transversally when axially expanded under tensional force, has not yet been studied in the tissue engineering and biomedical area. However, the recent advent of new technologies, such as additive manufacturing or 3D printing, has showed prospective results aimed at producing three-dimensional structures. Auxetic structures are fabricated by additive manufacturing, soft lithography, machining technology, compressed foaming, and textile fabrication using various biomaterials, including poly(ethylene glycol diacrylate), polyurethane, poly(lactic-glycolic acid), chitosan, hydroxyapatite, and using a hard material such as a silicon wafer. After fabricating the scaffold with an auxetic effect, researchers have cultured fibroblasts, osteoblasts, chondrocytes, myoblasts, and various stem cells, including mesenchymal stem cells, bone marrow stem cells, and embryonic stem cells. Additionally, they have shown new possibilities as scaffolds through tissue engineering by cell proliferation, migration, alignment, differentiation, and target tissue regeneration. In addition, auxetic structures and their unique deformation characteristics have been explored in several biomedical devices, including implants, stents, and surgical screws. Although still in the early stages, the auxetic structure, which can create mechanical properties tailored to natural tissue by changing the internal architecture of the structure, is expected to show an improved tissue reconstruction ability. In addition, continuous research at the cellular level using the auxetic micro and nano-environment could provide a breakthrough for tissue reconstruction.

Fatigue and dynamic biodegradation behavior of additively manufactured Mg scaffolds

Additive manufacturing (AM) has enabled the fabrication of biodegradable porous metals to satisfy the desired characteristics for orthopedic applications. The geometrical design on AM biodegradable metallic scaffolds has been found to offer a favorable opportunity to regulate their mechanical and degradation performance in previous studies, however mostly confined to static responses. In this study, we presented the effect of the geometrical design on the dynamic responses of AM Mg scaffolds for the first time. Three different types of porous structures, based on various unit cells (i.e., biomimetic, diamond, and sheet-based gyroid), were established and then subjected to selective laser melting (SLM) process using group-developed Mg-Nd-Zn-Zr alloy (JDBM) powders. The topology after dynamic electropolishing, dynamic compressive properties, and dynamic biodegradation behavior of the AM Mg scaffolds were comprehensively evaluated. It was found that dynamic electropolishing effectively removed the excessive adhered powders on the surfaces and resulted in similar geometrical deviations amongst the AM Mg scaffolds, independent of their porous structures. The geometrical design significantly affected the compressive fatigue properties of the AM Mg scaffolds, of which the sheeted-based gyroid structure demonstrated a superior fatigue endurance limit of 0.85 at 10⁶ cycles. Furthermore, in vitro dynamic immersion behaviors of the AM Mg scaffolds revealed a decent dependence on local architectures, where the sheeted-based gyroid scaffold experienced the lowest structural loss with a relatively uniform degradation mode. The obtained results indicate that the geometrical design could provide a promising strategy to develop desirable bone substitutes for the treatment of critical-size load-bearing defects. STATEMENT OF SIGNIFICANCE: Additive manufacturing (AM) has provided unprecedented opportunities to fabricate geometrically complex biodegradable scaffolds where the topological design becomes a key determinant on comprehensive performance. In this paper, we fabricate 3 AM biodegradable Mg scaffolds (i.e., biomimetic, diamond, and sheet-based gyroid) and report the effect of the geometrical design on the dynamic responses of AM Mg scaffolds for the first time. The results revealed that the sheeted-based gyroid scaffold exhibited the best combination of superior compressive fatigue properties and relatively uniform dynamic biodegradation mode, suggesting that the regulation of the porous structures could be an effective approach for the optimization of AM Mg scaffolds as to satisfy clinical requirements in orthopedic applications.

High-strength, porous additively manufactured implants with optimized mechanical osseointegration

Optimization of porous titanium alloy scaffolds designed for orthopedic implants requires balancing mechanical properties and osseointegrative performance. The tradeoff between scaffold porosity and the stiffness/strength must be optimized towards the goal to improve long term load sharing while simultaneously promoting osseointegration. Osseointegration into porous titanium implants covering a wide range of porosity (0%-90%) and manufactured by laser powder bed fusion (LPBF) was evaluated with an established ovine cortical and cancellous defect model. Direct apposition and remodeling of woven bone was observed at the implant surface, as well as bone formation within the interstices of the pores. A linear relationship was observed between the porosity and benchtop mechanical properties of the scaffolds, while a non-linear relationship was observed between porosity and the ex vivo cortical bone-implant interfacial shear strength. Our study supports the hypothesis of porosity dependent performance tradeoffs, and establishes generalized relationships between porosity and performance for design of topological optimized implants for osseointegration. These results are widely applicable for orthopedic implant design for arthroplasty components, arthrodesis devices such as spinal interbody fusion implants, and patient matched implants for treatment of large bone defects.