Topological design, permeability and mechanical behavior of additively manufactured functionally graded porous metallic biomaterials

Complete Removal of Residual Particles and Realization of Mechanical Properties to Improve Osseointegration in Additively Manufactured Ti6Al4 V Scaffolds through Flowing Acid Etching

Massive unmelted Ti6Al4 V (Ti64) particles presented across all surfaces of additively manufactured Ti64 scaffolds significantly impacted the designed surface topography, mechanical properties, and permeability, reducing the osseointegration of the scaffolds. In this study, the proposed flowing acid etching (FAE) method presented high efficiency in eliminating Ti64 particles and enhancing the surface modification capacity across all surfaces of Ti64 scaffolds. The Ti64 particles across all surfaces of the scaffolds were completely removed effectively and evenly. The surface topography of the scaffolds closely resembled the design after the 75 s FAE treatment. The actual elastic modulus of the treated scaffolds (3.206 ± 0.040 GPa) was closer to the designed value (3.110 GPa), and a micrometer-scale structure was constructed on the inner and outer surfaces of the scaffolds after the 90 s FAE treatment. However, the yield strength of scaffolds was reduced to 89.743 ± 0.893 MPa from 118.251 ± 0.982 MPa after the 90 s FAE treatment. The FAE method also showed higher efficiency in decreasing the roughness and enhancing the hydrophilicity and surface energy of all of the surfaces. The FAE treatment improved the permeability of scaffolds efficiently, and the permeability of scaffolds increased to 11.93 ± 0.21 × 10-10 mm2 from 8.57 ± 0.021 × 10-10 mm2 after the 90 s FAE treatment. The treated Ti64 scaffolds after the 90 s FAE treatment exhibited optimized osseointegration effects in vitro and in vivo. In conclusion, the FAE method was an efficient way to eliminate unmelted Ti64 particles and obtain ideal surface topography, mechanical properties, and permeability to promote osseointegration in additively manufactured Ti64 scaffolds.

Gaussian random field-based characterization and reconstruction of cancellous bone microstructure considering the constraint of correlation structure

The macro scale physical properties of cancellous bone materials are governed by the microstructural features, which is of great significance for the multi-scale research of cancellous bone and the inverse design of bone-mimicking materials. Therefore, it is essential to characterize the natural cancellous bone samples, and reconstruct the microstructures with the biomimetic osteointegration and mechanical properties. In this research, a novel approach for the characterization and reconstruction of cancellous bone was proposed, based on the medical image analysis and anisotropic three-dimensional Gaussian random field (GRF). The geometric similarity, i.e. the interface curvature distribution (ISD), was meticulously studied, which is important to the osteointegration ability. And the mechanical properties were validated by the stress-strain curves under the large compressive strain simulated by the smoothed particle hydrodynamic (SPH) method. In addition, the effects of the generation parameters of GRF-based biomimetic microstructures on the apparent properties were analyzed. The ISD results demonstrated that both GRF and micro-CT groups had the similar columnar morphological properties, while the latter had more hyperbolic features. And it was found that the GRF-based biomimetic microstructures and the natural bone samples based on micro-CT (MCT) had the similar failure mode. The concordance correlation coefficient between MCT and GRF pairs was 0.8685, with a Pearson ρ value of 0.8804, and significance level p<0.0001. The Bland-Altman LoA was 0.1647 MPa with 95 % (1.96SD) lower and upper bound value between -0.2892 and 0.6185 MPa. The two groups had almost the same elastic modulus with the mean absolute percentage error (MAPE) of 7.84 %. While the yield stress and total conversion energy of the GRF-based samples were lower than those of the natural bone samples, and the MAPE were 16.99 % and 16.27 %, respectively. Although it meant the lower structural efficiency, the huge design space of this approach and advanced 3D printing technology can provide great potential for the design of orthopedic implants.

Wood-inspired metamaterial catalyst for robust and high-throughput water purification

Continuous industrialization and other human activities have led to severe water quality deterioration by harmful pollutants. Achieving robust and high-throughput water purification is challenging due to the coupling between mechanical strength, mass transportation and catalytic efficiency. Here, a structure-function integrated system is developed by Douglas fir wood-inspired metamaterial catalysts featuring overlapping microlattices with bimodal pores to decouple the mechanical, transport and catalytic performances. The metamaterial catalyst is prepared by metal 3D printing (316 L stainless steel, mainly Fe) and electrochemically decorated with Co to further boost catalytic functionality. Combining the flexibility of 3D printing and theoretical simulation, the metamaterial catalyst demonstrates a wide range of mechanical-transport-catalysis capabilities while a 70% overlap rate has 3X more strength and surface area per unit volume, and 4X normalized reaction kinetics than those of traditional microlattices. This work demonstrates the rational and harmonious integration of structural and functional design in robust and high throughput water purification, and can inspire the development of various flow catalysts, flow batteries, and functional 3D-printed materials.

Experimental Evaluation of the Effects of Discrete-Grading-Induced Discontinuities on the Material Properties of Functionally Graded Ti-6Al-4V Lattices

In this study, we compared the material properties of linearly and sharply graded Ti6Al4V additively manufactured samples to investigate whether the more severe discontinuities caused by sharp grading can reduce performance. We performed compression testing with digital image correlation (DIC) in two loading directions for each grading design to simulate iso-stress and iso-strain conditions. We extracted the elastic stiffness, yield strength, yield strain, and energy absorption capacity of each sample. In addition, we used micro-computed tomography (micro-CT) imaging to examine the printing quality and dimensional accuracy. We found that sharply graded struts have a 12.95% increase in strut cross-sectional areas, whereas linearly graded struts produced an average of 49.24% increase compared to design. However, sharply graded and linearly graded FGL samples do not have statistically significant differences in elastic stiffness and yield strength. For the iso-strain condition, the average DIC-corrected stiffnesses for linearly and sharply graded samples were 6.15 GPa and 5.43 GPa, respectively (p = 0.4466), and the yield stresses were 290.4 MPa and 291.2 MPa, respectively (p = 0.5734). Furthermore, we confirmed different types of printing defects using micro-CT, including defective pores and disconnected struts. These results suggest that the loss of material properties caused by manufacturing defects outweighs the adverse effects of discrete-grading-induced discontinuities.

Application of Artificial Intelligence at All Stages of Bone Tissue Engineering

The development of artificial intelligence (AI) has revolutionized medical care in recent years and plays a vital role in a number of areas, such as diagnostics and forecasting. In this review, we discuss the most promising areas of AI application to the field of bone tissue engineering and prosthetics, which can drastically benefit from AI-assisted optimization and patient personalization of implants and scaffolds in ways ranging from visualization and real-time monitoring to the implantation cases prediction, thereby leveraging the compromise between specific architecture decisions, material choice, and synthesis procedure. With the emphasized crucial role of accuracy and robustness of developed AI algorithms, especially in bone tissue engineering, it was shown that rigorous validation and testing, demanding large datasets and extensive clinical trials, are essential, and we discuss how through developing multidisciplinary cooperation among biology, chemistry with materials science, and AI, these challenges can be addressed.

Understanding of trabecular-cortical transition zone: Numerical and experimental assessment of multi-morphology scaffolds

Applications of additive manufacturing (AM) in tissue engineering develop rapidly. AM offers layer-by-layer creation of complex objects, developed to restore functionality of, or replace, damaged tissues. Porous 3D-printed functional gradient structures are of particular interest: their special architecture makes it possible to simulate the heterogeneity of the replaced tissue and, by continuously changing the mechanical properties, to avoid the concentration of stresses that can be caused by abrupt geometric changes. Such structures also allow combinations of different types of unit cells and a smooth transition between them, making design of personalised scaffolds with optimal parameters for the replacement of damaged host tissue at the interface between tissues possible. This paper presents the results of development of scaffold structures with gradients of porosity and multi-morphology using unit cells based on triply periodic minimal surfaces (TPMS). The mechanical behaviour of additively manufactured scaffold prototypes made of polylactide acid (PLA) was studied under compressive loading. Strain fields on their surface were captured using the Vic-3d Micro-DIC digital image correlation system and compared with those obtained with detailed numerical simulations, employing elastic-plastic properties of PLA, obtained in experiments. The effect of gradient parameters and unit-cell morphology on the stress distribution in scaffolds was analysed. A smooth gradient transition between cells with different morphologies was found to reduce the probability of structural failure under intense compressive loading. A good agreement between numerical results and experimental data was achieved, which justifies application of the developed approach to design of personalised bone scaffolds.

Orthopedic Scaffolds: Evaluation of Structural Strength and Permeability of Fluid Flow via an Open Cell Neovius Structure for Bone Tissue Engineering

The ability of bone to regenerate itself through mechanobiological responses is its dynamic property. Mechanical cues from a neighboring environment produce the structural strain to promote blood flow and bone marrow mobility that in turn aids the bone regeneration process. Occurrences of these phenomena are crucial for the success of metallic scaffolds implanted in the host bone tissue. Thus, permeability and fluid flow-induced wall shear stress (WSS) are two parameters that directly influence cell bioactivities inside a scaffold and are crucial for effective bone tissue regeneration. Given that the scaffolds shall be implanted in the body, permeability assessment was carried out using non-Newtonian fluid. In this work, the triply periodic minimal surface scaffolds with Neovius architectures were fabricated by using selective laser melting technology. The estimation of fluid flow was carried out using computational fluid dynamics (CFD) analysis with a non-Newtonian blood fluid model. Further, the structural strength of various open cell Neovius lattices was evaluated using a static compression test, and in vitro cell culture using Alamar blue assay was evaluated. Results revealed that the values of intrinsic blood flow permeability of the three-dimensional (3D)-printed open cell porous scaffold with Neovius architecture were of the same order of magnitude as those of human bone, ranging from 0.0025 × 10-9 to 0.0152 × 10-9 m2. The structural elastic modulus and compressive strength of NOCL40, NOCL50, and NOCL60 lattices range from 3.27 to 3.71 GPa and 194 to 205 MPa, respectively. All of the values are comparable to the human bone, thus making these lattices a suitable alternative for orthopedic applications.

Design and manufacturing of biomimetic scaffolds for bone repair inspired by bone trabeculae

Porous scaffold (PorS) implants, particularly those that mimic the structural features of natural cancellous bone (NCanB), are increasingly essential for the treatment of large-area bone defects. However, the mechanical properties of NCanB-based bionic bone scaffold (BioS) and its performance as a bone repair material have not been fully explored. This study investigates the effect of bionic structure parameters on the mechanical properties and bone reconstruction performance of BioS. Using laser powder bed fusion (L-PBF) technology, different BioS with various structural parameters were created and evaluated using Micro-CT, compression testing, Finite Element (FE) Simulation, and computational fluid dynamics (CFD), and compared to commonly used clinical PorS. Assess the capacity of the BioS scaffold to support and enhance bone reconstruction following implantation through the evaluation of its mechanical properties, permeability, and fluid shear stress (FSS). BioS-85-90 and BioS-80-50 showed suitable mechanical properties, performed well in FE simulation of implantation, demonstrated outstanding abilities for osteoinductive ingrowth and bone tissue differentiation, and proved to be reliable materials for the reconstruction of bone defects. Therefore, BioS shows significant potential for clinical application as a bone reconstruction material, providing a solid foundation for the integration of tissue engineering and bionic design.

Microstructure and Biomechanical Properties in Selective Laser Melting of Porous Metal Implants

Two kinds of porous structure design strategies, ring-support (RS) and column-support (CS), are proposed for human implants. The accurate design of porosity is realized by adjusting the pore characteristics, such as strut diameter, pore diameter, and unit size. Porous specimens with porosity of 50%, 60%, 70%, and 80% were prepared by selective laser melting. The three-dimensional pore structure is basically consistent with the design characteristics, and the measured porosity is slightly lower than design value. The microstructure, microhardness, and friction and wear properties of the samples were studied. The results show that the performance along the scanning orientation is slightly better than that along the forming orientation. The compression and dynamic elastic modulus of porous specimens with different structures and porosities were analyzed. The CS porous with 60-80% porosity has suitable compressive strength and elastic modulus, which is close to that of human tissue, and effectively avoids the stress shielding phenomenon.

Multimaterial 3D and 4D Bioprinting of Heterogenous Constructs for Tissue Engineering

Additive manufacturing (AM), which is based on the principle of layer-by-layer shaping and stacking of discrete materials, has shown significant benefits in the fabrication of complicated implants for tissue engineering (TE). However, many native tissues exhibit anisotropic heterogenous constructs with diverse components and functions. Consequently, the replication of complicated biomimetic constructs using conventional AM processes based on a single material is challenging. Multimaterial 3D and 4D bioprinting (with time as the fourth dimension) has emerged as a promising solution for constructing multifunctional implants with heterogenous constructs that can mimic the host microenvironment better than single-material alternatives. Notably, 4D-printed multimaterial implants with biomimetic heterogenous architectures can provide a time-dependent programmable dynamic microenvironment that can promote cell activity and tissue regeneration in response to external stimuli. This paper first presents the typical design strategies of biomimetic heterogenous constructs in TE applications. Subsequently, the latest processes in the multimaterial 3D and 4D bioprinting of heterogenous tissue constructs are discussed, along with their advantages and challenges. In particular, the potential of multimaterial 4D bioprinting of smart multifunctional tissue constructs is highlighted. Furthermore, this review provides insights into how multimaterial 3D and 4D bioprinting can facilitate the realization of next-generation TE applications.

Design of Ti64/Ta Hybrid Materials by Powder Metallurgy Mimicking Bone Structure

This work reports on the fabrication of a novel two-layer material composed of a porous tantalum core and a dense Ti6Al4V (Ti64) shell by powder metallurgy. The porous core was obtained by mixing Ta particles and salt space-holders to create large pores, the green compact was obtained by pressing. The sintering behavior of the two-layer sample was studied by dilatometry. The interface bonding between the Ti64 and Ta layers was analyzed by SEM, and the pore characteristics were analyzed by computed microtomography. Images showed that two distinct layers were obtained with a bonding achieved by the solid-state diffusion of Ta particles into Ti64 during sintering. The formation of β-Ti and α' martensitic phases confirmed the diffusion of Ta. The pore size distribution was in the size range of 80 to 500 µm, and a permeability value of 6 × 10^-10 m² was close to the trabecular bones one. The mechanical properties of the component were dominated mainly by the porous layer, and Young's modulus of 16 GPa was in the range of bones. Additionally, the density of this material (6 g/cm³) was much lower than the one of pure Ta, which helps to reduce the weight for the desired applications. These results indicate that structurally hybridized materials, also known as composites, with specific property profiles can improve the response to osseointegration for bone implant applications.

Multiscale Homogenization Techniques for TPMS Foam Material for Biomedical Structural Applications

Multiscale techniques, namely homogenization, result in significant computational time savings in the analysis of complex structures such as lattice structures, as in many cases it is inefficient to model a periodic structure in full detail in its entire domain. The elastic and plastic properties of two TPMS-based cellular structures, the gyroid, and the primitive surface are studied in this work through numerical homogenization. The study enabled the development of material laws for the homogenized Young's modulus and homogenized yield stress, which correlated well with experimental data from the literature. It is possible to use the developed material laws to run optimization analyses and develop optimized functionally graded structures for structural applications or reduced stress shielding in bio-applications. Thus, this work presents a study case of a functionally graded optimized femoral stem where it was shown that the porous femoral stem built with Ti-6Al-4V can minimize stress shielding while maintaining the necessary load-bearing capacity. It was shown that the stiffness of cementless femoral stem implant with a graded gyroid foam presents stiffness that is comparable to that of trabecular bone. Moreover, the maximum stress in the implant is lower than the maximum stress in trabecular bone.

Enhancing Mechanical Properties of 3D Printing Metallic Lattice Structure Inspired by Bambusa Emeiensis

Metallic additive manufacturing process parameters, such as inclination angle and minimum radius, impose constraints on the printable lattice cell configurations in complex components. As a result, their mechanical properties are usually lower than their design values. Meanwhile, due to unavoidable process constraints (e.g., additional support structure), engineering structures filled with various lattice cells usually fail to be printed or cannot achieve the designed mechanical performances. Optimizing the cell configuration and printing process are effective ways to solve these problems, but this is becoming more and more difficult and costly with the increasing demand for properties. Therefore, it is very important to redesign the existing printable lattice structures to improve their mechanical properties. In this paper, inspired by the macro- and meso-structures of bamboo, a bionic lattice structure was partitioned, and the cell rod had a radius gradient, similar to the macro-scale bamboo joint and meso-scale bamboo tube, respectively. Experimental and simulated results showed that this design can significantly enhance the mechanical properties without adding mass and changing the printable cell configuration. Finally, the compression and shear properties of the Bambusa-lattice structure were analyzed. Compared with the original scheme, the bamboo lattice structure design can improve the strength by 1.51 times (β=1.5). This proposed strategy offers an effective pathway to manipulate the mechanical properties of lattice structures simultaneously, which is useful for practical applications.

Improving Biocompatibility for Next Generation of Metallic Implants

The increasing need for joint replacement surgeries, musculoskeletal repairs, and orthodontics worldwide prompts emerging technologies to evolve with healthcare's changing landscape. Metallic orthopaedic materials have a shared application history with the aerospace industry, making them only partly efficient in the biomedical domain. However, suitability of metallic materials in bone tissue replacements and regenerative therapies remains unchallenged due to their superior mechanical properties, eventhough they are not perfectly biocompatible. Therefore, exploring ways to improve biocompatibility is the most critical step toward designing the next generation of metallic biomaterials. This review discusses methods of improving biocompatibility of metals used in biomedical devices using surface modification, bulk modification, and incorporation of biologics. Our investigation spans multiple length scales, from bulk metals to the effect of microporosities, surface nanoarchitecture, and biomolecules such as DNA incorporation for enhanced biological response in metallic materials. We examine recent technologies such as 3D printing in alloy design and storing surface charge on nanoarchitecture surfaces, metal-on-metal, and ceramic-on-metal coatings to present a coherent and comprehensive understanding of the subject. Finally, we consider the advantages and challenges of metallic biomaterials and identify future directions.

The optimal design of 3D-printed lattice bone plate by considering fracture healing mechanism

The biomechanical stimulus is the most important factor for fracture healing and mainly determined by the structural stiffness of bone plate. Currently, the materials commonly used in bone plates are stainless steel and titanium, which often lead to stress shielding effects because of their higher elastic modulus compared with the bone. This article suggests an optimal design method of lattice bone plate based on fracture healing theory. First, the mechanical regulation model with deviatoric strain is established to simulate the tissue differentiation process during fracture healing process. The ratio of the average elastic modulus of callus at the 120th day to the elastic modulus of mature bone is used to characterize the fracture healing rate. Second, the optimal elastic modulus of the design domain is obtained by the optimization mathematical model with the maximum fracture healing rate. Then, the design domain is filled with microstructures, the porosity of which is adjusted to make it possible that the equivalent elastic modulus is equal to the optimized value. And the finite element analysis of the bone plate with microstructure is executed. Finally, the designed lattice bone plates are manufactured through 3D printing, and the mechanical test is carried out. The simulation results indicate that the fracture healing rate is maximum when the elastic modulus of material in design domain is 38 GPa under the constraints of fixation stability. And both the finite element analysis and experiment results show that the designed lattice bone plate meet the strength requirements of fracture internal fixation implants.

Definition, measurement, and function of pore structure dimensions of bioengineered porous bone tissue materials based on additive manufacturing: A review

Bioengineered porous bone tissue materials based on additive manufacturing technology have gradually become a research hotspot in bone tissue-related bioengineering. Research on structural design, preparation and processing processes, and performance optimization has been carried out for this material, and further industrial translation and clinical applications have been implemented. However, based on previous studies, there is controversy in the academic community about characterizing the pore structure dimensions of porous materials, with problems in the definition logic and measurement method for specific parameters. In addition, there are significant differences in the specific morphological and functional concepts for the pore structure due to differences in defining the dimensional characterization parameters of the pore structure, leading to some conflicts in perceptions and discussions among researchers. To further clarify the definitions, measurements, and dimensional parameters of porous structures in bioengineered bone materials, this literature review analyzes different dimensional characterization parameters of pore structures of porous materials to provide a theoretical basis for unified definitions and the standardized use of parameters.

An experimental and numerical study of the microstructural and biomechanical properties of human peripheral nerve endoneurium for the design of tissue scaffolds

Biomimetic design of scaffold architectures represents a promising strategy to enable the repair of tissue defects. Natural endoneurium extracellular matrix (eECM) exhibits a sophisticated microstructure and remarkable microenvironments conducive for guiding neurite regeneration. Therefore, the analysis of eECM is helpful to the design of bionic scaffold. Unfortunately, a fundamental lack of understanding of the microstructural characteristics and biomechanical properties of the human peripheral nerve eECM exists. In this study, we used microscopic computed tomography (micro-CT) to reconstruct a three-dimensional (3D) eECM model sourced from mixed nerves. The tensile strength and effective modulus of human fresh nerve fascicles were characterized experimentally. Permeability was calculated from a computational fluid dynamic (CFD) simulation of the 3D eECM model. Fluid flow of acellular nerve fascicles was tested experimentally to validate the permeability results obtained from CFD simulations. The key microstructural parameters, such as porosity is 35.5 ± 1.7%, tortuosity in endoneurium (X axis is 1.26 ± 0.028, Y axis is 1.26 ± 0.020 and Z axis is 1.17 ± 0.03, respectively), tortuosity in pore (X axis is 1.50 ± 0.09, Y axis is 1.44 ± 0.06 and Z axis is 1.13 ± 0.04, respectively), surface area-to-volume ratio (SAVR) is 0.165 ± 0.007 μm^-1 and pore size is 11.8 ± 2.8 μm, respectively. These were characterized from the 3D eECM model and may exert different effects on the stiffness and permeability. The 3D microstructure of natural peripheral nerve eECM exhibits relatively lower permeability (3.10 m² × 10^-12 m²) than other soft tissues. These key microstructural and biomechanical parameters may play an important role in the design and fabrication of intraluminal guidance scaffolds to replace natural eECM. Our findings can aid the development of regenerative therapies and help improve scaffold design.

Analysis of the topological motifs of the cellular structure of the tri-spine horseshoe crab (Tachypleus tridentatus) and its associated mechanical properties

Topological motifs in pore architecture can profoundly influence the structural properties of that architecture, such as its mass, porosity, modulus, strength, and surface permeability. Taking the irregular cellular structure of the tri-spine horseshoe crab as a research model, we present a new approach to the quantitative description and analysis of structure-property-function relationships. We employ a robust skeletonization method to construct a curve-skeleton that relies on high-resolution 3D tomographic data. The topological motifs and mechanical properties of the long-range cellular structure were investigated using the Grasshopper plugin and uniaxial compression test to identify the variation gradient. Finite element analysis was conducted for the sub-volumes to obtain the variation in effective modulus along the three principal directions. The results show that the branch length and node distribution density varied from the tip to the base of the sharp corner. These node types formed a low-connectivity network, in which the node types 3-N and 4-N tended to follow the motifs of ideal planar triangle and tetrahedral configurations, respectively, with the highest proportion of inter-branch angles in the angle ranges of 115-120° and 105-110°. In addition, mapping the mechanical gradients to topological properties indicated that narrower profiles with a given branch length gradient, preferred branch orientation, and network connectedness degree are the main factors that affect the mechanical properties. These factors suggest significant potential for designing a controllable, irregularly cellular structure in terms of both morphology and function.

Next-generation finely controlled graded porous antibacterial bioceramics for high-efficiency vascularization in orbital reconstruction

Eyeball loss due to severe ocular trauma, intraocular malignancy or infection often requires surgical treatment called orbital implant reconstruction to rehabilitate the orbital volume and restore the aesthetic appearance. However, it remains a challenge to minimize the postoperative exposure and infection complications due to the inert nature of conventional orbital implants. Herein, we developed a novel Ca–Zn-silicate bioceramic implant with multi-functions to achieve the expected outcomes. The porous hardystonite (Ca₂ZnSi₂O₇) scaffolds with triply periodic minimal surfaces (TPMS)-based pore architecture and graded pore size distribution from center to periphery (from 500 to 800 μm or vice versa) were fabricated through the digital light processing (DLP) technique, and the scaffolds with homogeneous pores (500 or 800 μm) were fabricated as control. The graded porous scaffolds exhibited a controlled bio-dissolving behavior and intermediate mechanical strength in comparison with the homogeneous counterparts, although all of porous implants presented significant antibacterial potential against S. aureus and E. coli. Meanwhile, the pore size-increasing scaffolds indicated more substantial cell adhesion, cell viability and angiogenesis-related gene expression in vitro. Furthermore, the gradually increasing pore feature exhibited a stronger blood vessel infiltrating potential in the dorsal muscle embedding model, and the spherical implants with such pore structure could achieve complete vascularization within 4 weeks in the eyeball enucleation rabbit models. Overall, our results suggested that the novel antibacterial hardystonite bioceramic with graded pore design has excellent potential as a next-generation orbital implant, and the pore topological features offer an opportunity for the improvement of biological performances in orbital reconstruction.

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The graded porous bioceramics were fabricated through computer-assisted design and digital light processing technique.

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The graded pore architecture could control the biodegradation and mechanical behavior of porous bioceramics.

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The porous Ca-Zn-silicate bioceramics exhibited significant antibacterial potentials against S. aureus and E. coli.

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The gradually increasing pore size feature of scaffolds contributes to cell activity and vascular infiltration.

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The graded porous bioceramics implants achieved complete vascularization within 4 weeks in the enucleation animal model.

Structural Design and Finite Element Simulation Analysis of Grade 3 Graded Porous Titanium Implant

The metal titanium is often used as a dental implant material, and the elastic modulus of solid titanium implants does not match the biological bone tissue, which can easily produce a stress shielding effect and cause implant failure. In this paper, a three-level gradient porous structure implant was designed, and its mechanical and biological adaptability were studied by finite element simulation analysis. Combined with the comprehensive evaluation of the mechanical and biological properties of implants of various structures, the analysis found that a porous implant with porosity of 59.86% of the gradient was the best structure. The maximum equivalent stress of this structure in the mandible that simulated the oral environment was 154.34 MPa, which was less than half of its theoretical compression yield strength. The strain of the surrounding bone tissue lies in the bone compared with other structures, the proportion of the active state of plastic construction is larger, at 10.51%, and the fretting value of this structure and the bone tissue interface is the smallest, at only 10 μm.

Functionally graded additive manufacturing for orthopedic applications

Background

Additive Manufacturing due to its benefits in developing parts with complex geometries and shapes, has evolved as an alternate manufacturing process to develop implants with desired properties. The structure of human bones being anisotropic in nature is biologically functionally graded i,e. The structure possesses different properties in different directions. Therefore, various orthopedic implants such as knee, hip and other bone plates, if functionally graded can perform better. In this context, the development of functionally graded (FG) parts for orthopedic application with tailored anisotropic properties has become easier through the use of additive manufacturing (AM).

Objectives

and Rationale: The current paper aims to study the various aspects of additively manufactured FG parts for orthopedic applications. It presents the details of various orthopedic implants such as knee, hip and other bone plates in a structured manner. A systematic literature review is conducted to study the various material and functional aspects of functionally graded parts for orthopedic applications. A section is also dedicated to discuss the mechanical properties of functionally graded parts.

Conclusion

The literature revealed that additive manufacturing can provide lot of opportunities for development of functionally graded orthopedic implants with improved properties and durability. Further, the effect of various FG parameters on the mechanical behavior of these implants needs to be studied in detail. Also, with the advent of various AM technologies, the functional grading can be achieved by various means e.g. density, porosity, microstructure, composition, etc. By varying the AM parameters. However, the current limitations of cost and material biocompatibility prevent the widespread exploitation of AM technologies for various orthopedic applications.

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.

Biomechanical Effects of 3D-Printed Bioceramic Scaffolds With Porous Gradient Structures on the Regeneration of Alveolar Bone Defect: A Comprehensive Study

In the repair of alveolar bone defect, the microstructure of bone graft scaffolds is pivotal for their biological and biomechanical properties. However, it is currently controversial whether gradient structures perform better in biology and biomechanics than homogeneous structures when considering microstructural design. In this research, bioactive ceramic scaffolds with different porous gradient structures were designed and fabricated by 3D printing technology. Compression test, finite element analysis (FEA) revealed statistically significant differences in the biomechanical properties of three types of scaffolds. The mechanical properties of scaffolds approached the natural cancellous bone, and scaffolds with pore size decreased from the center to the perimeter (GII) had superior mechanical properties among the three groups. While in the simulation of Computational Fluid Dynamics (CFD), scaffolds with pore size increased from the center to the perimeter (GI) possessed the best permeability and largest flow velocity. Scaffolds were cultured in vitro with rBMSC or implanted in vivo for 4 or 8 weeks. Porous ceramics showed excellent biocompatibility. Results of in vivo were analysed by using micro-CT, concentric rings and VG staining. The GI was superior to the other groups with respect to osteogenicity. The Un (uniformed pore size) was slightly inferior to the GII. The concentric rings analysis demonstrated that the new bone in the GI was distributed in the periphery of defect area, whereas the GII was distributed in the center region. This study offers basic strategies and concepts for future design and development of scaffolds for the clinical restoration of alveolar bone defect.

Additively Manufactured Porous Ti6Al4V for Bone Implants: A Review

Ti-6Al-4V (Ti64) alloy is one of the most widely used orthopedic implant materials due to its mechanical properties, corrosion resistance, and biocompatibility nature. Porous Ti64 structures are gaining more research interest as bone implants as they can help in reducing the stress-shielding effect when compared to their solid counterpart. The literature shows that porous Ti64 implants fabricated using different additive manufacturing (AM) process routes, such as laser powder bed fusion (L-PBF) and electron beam melting (EBM) can be tailored to mimic the mechanical properties of natural bone. This review paper categorizes porous implant designs into non-gradient (uniform) and gradient (non-uniform) porous structures. Gradient porous design appears to be more promising for orthopedic applications due to its closeness towards natural bone morphology and improved mechanical properties. In addition, this paper outlines the details on bone structure and its properties, mechanical properties, fatigue behavior, multifunctional porous implant designs, current challenges, and literature gaps in the research studies on porous Ti64 bone implants.

Fabrication and Characterization of Bioactive Gelatin-Alginate-Bioactive Glass Composite Coatings on Porous Titanium Substrates

In this research work, the fabrication of biphasic composite implants has been investigated. Porous, commercially available pure Ti (50 vol % porosity and pore distributions of 100-200, 250-355, and 355-500 μm) has been used as a cortical bone replacement, while different composites based on a polymer blend (gelatin and alginate) and bioactive glass (BG) 45S5 have been applied as a soft layer for cartilage tissues. The microstructure, degradation rates, biofunctionality, and wear behavior of the different composites were analyzed to find the best possible coating. Experiments demonstrated the best micromechanical balance for the substrate containing 200-355 μm size range distribution. In addition, although the coating prepared from alginate presented a lower mass loss, the composite containing 50% alginate and 50% gelatin showed a higher elastic recovery, which entails that this type of coating could replicate the functions of the soft tissue in areas of the joints. Therefore, results revealed that the combinations of porous commercially pure Ti and composites prepared from alginate/gelatin/45S5 BG are candidates for the fabrication of biphasic implants not only for the treatment of osteochondral defects but also potentially for any other diseases affecting simultaneously hard and soft tissues.

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.

Analysis of Mechanical Properties and Permeability of Trabecular-Like Porous Scaffold by Additive Manufacturing

Human bone cells live in a complex environment, and the biomimetic design of porous structures attached to implants is in high demand. Porous structures based on Voronoi tessellation with biomimetic potential are gradually used in bone repair scaffolds. In this study, the mechanical properties and permeability of trabecular-like porous scaffolds with different porosity levels and average apertures were analyzed. The mechanical properties of bone-implant scaffolds were evaluated using finite element analysis and a mechanical compression experiment, and the permeability was studied by computational fluid dynamics. Finally, the attachment of cells was observed by confocal fluorescence microscope. The results show that the performance of porous structures can be controlled by the initial design of the microstructure and tissue morphology. A good structural design can accurately match the performance of the natural bone. The study of mechanical properties and permeability of the porous structure can help address several problems, including stress shielding and bone ingrowth in existing biomimetic bone structures, and will also promotes cell adhesion, migration, and eventual new bone attachment.

Influence of porous tantalum scaffold pore size on osteogenesis and osteointegration: A comprehensive study based on 3D-printing technology

The emerging role of porous tantalum (Ta) scaffold for bone tissue engineering is noticed due to its outstanding biological properties. However, it is controversial which pore size and porosity are more conducive for bone defect repair. In the present work, porous tantalum scaffolds with pore sizes of 100-200, 200-400, 400-600 and 600-800 μm and corresponding porosities of 25%, 55%, 75%, and 85% were constructed, using computer aided design and 3D printing technologies, then comprehensively studied by in vitro and in vivo studies. We found that Ta scaffold with pore size of 400-600 μm showed stronger ability in facilitating cell adhesion, proliferation, and osteogenic differentiation in vitro. In vivo tests identified that porous tantalum scaffolds with pore size of 400-600 μm showed better performance of bone ingrowth and integration. In mechanism, computational fluid dynamics analysis proved porous tantalum scaffolds with pore size of 400-600 μm hold appropriate permeability and surface area, which facilitated cell adhesion and proliferation. Our results strongly indicate that pore size and porosity are essential for further applications of porous tantalum scaffolds, and porous tantalum scaffolds with pore size 400-600 μm are conducive to osteogenesis and osseointegration. These findings provide new evidence for further application of porous tantalum scaffolds for bone defect repair.

Porous Geometry Guided Micro-mechanical Environment Within Scaffolds for Cell Mechanobiology Study in Bone Tissue Engineering

Mechanobiology research is for understanding the role of mechanics in cell physiology and pathology. It will have implications for studying bone physiology and pathology and to guide the strategy for regenerating both the structural and functional features of bone. Mechanobiological studies in vitro apply a dynamic micro-mechanical environment to cells via bioreactors. Porous scaffolds are commonly used for housing the cells in a three-dimensional (3D) culturing environment. Such scaffolds usually have different pore geometries (e.g. with different pore shapes, pore dimensions and porosities). These pore geometries can affect the internal micro-mechanical environment that the cells experience when loaded in the bioreactor. Therefore, to adjust the applied micro-mechanical environment on cells, researchers can tune either the applied load and/or the design of the scaffold pore geometries. This review will provide information on how the micro-mechanical environment (e.g. fluid-induced wall shear stress and mechanical strain) is affected by various scaffold pore geometries within different bioreactors. It shall allow researchers to estimate/quantify the micro-mechanical environment according to the already known pore geometry information, or to find a suitable pore geometry according to the desirable micro-mechanical environment to be applied. Finally, as future work, artificial intelligent - assisted techniques, which can achieve an automatic design of solid porous scaffold geometry for tuning/optimising the micro-mechanical environment are suggested.

Laser powder bed fusion of porous graded structures: A comparison between computational and experimental analysis

Functionally graded porous structures (FGPSs) are gaining interest in the biomedical sector, specifically for orthopaedic implants. In this study, the compressive behaviour of seven different FGPSs comprised of Face Centred Cubic (FCC) and the Octet truss unit cells (OCT) were analysed. The porosity of the structures were graded in different directions (radially, longitudinally, laterally and longitudinally & radially) by varying the strut diameters or by combining the two types of unit cells. The structures were manufactured by laser power bed fusion and compression tests were performed. Radially and laterally porous graded structures were found to outperform uniform porous structures with an increase in stiffness of 13.7% and 21.1% respectively. The experimental and finite element analysis (FEA) results were in good agreement with differences in elastic modulus of 9.4% and yield strength of 15.8%. A new FEA beam model is proposed in this study to analyse this type of structures with accurate results and the consequent reduction of computational time. The accuracy of the Kelvin-Voight model and the rule of mixtures for predicting the mechanical behaviour of different FGPSs was also investigated. The results demonstrate the adequacy of the analytical models specifically for hybrid structures and for structures with smooth diameter transitions.

Analytical and Numerical Investigation of Free Vibration Behavior for Sandwich Plate with Functionally Graded Porous Metal Core

The current work presents a free vibration analysis of a simply supported rectangular functionally graded sandwich plate using a new analytical model. The core of the sandwich plate is made up of porous metal, and the top and bottom faces are made up of homogenous materials. The core metal properties are assumed to be porosity dependent and graded in the thickness direction according to a simple power-law distribution in terms of the volume fractions of the constituents. The contribution of this paper is to evaluate the performance of functionally graded porous materials (FGPMs) as it is used for many biomedical applications, particularly in tissue engineering. Theoretical formulations are based on the classical plate theory to find the free vibration characteristics of the imperfect FGM sandwich plate and include different parameters. Parameters included are graded distributions of porosity, power-law index, core metal type, and aspect ratios. A numerical investigation using finite element analysis (FEA) and the modal analysis was conducted with the assistance of the commercial ANSYS-2020-R2 software to validate the analytical solution. To detect the various parameters influencing the fundamental frequencies of sandwich plate comprehensive numerical results are presented in dimensionless tabular and graphical forms. The results reveal that the frequency parameter of the sandwich plate increases with the increase of the porosity parameter and number of the constraints in the boundary conditions. Furthermore, the increase in the number of layers leads to an increase in the accuracy of the results for the same FGM core thickness. An accepted agreement can be observed between the proposed analytical solution and numerical results with a maximum error discrepancy of 8%.

Investigation of porous cells interface on elastic property of orthopedic implants: Numerical and experimental studies

Application of porous cells in orthopedic implants makes it possible to better approximation of elastic property of human bones. Although the mechanical and biological properties of orthopedic porous implants are studied in many researches, the interaction between different porous unit cells from geometrical compatibility and also, considering manufacturing conditions for the ultimate goal of bone ingrowth is not thoroughly investigated. In this study, a kelvin cell is designed with 530 to 810 μm pore sizes, which is the appropriate range for bone ingrowth. Due to anatomical position of implants in the human body and the limited range of the elastic modulus of kelvin cell with different geometrical parameters, this unit cell is combined with other cells to extend the range of its elastic modulus. After selecting the appropriate combination of cells to achieve desired properties, they are fabricated with Stainless Steel 316 L using radially gradient porosity in the range of 64% to 80%, and then finite element method (FEM) is performed to evaluate the elastic modulus, stress distribution, and strain energy of the proposed structures. Gradient and uniform structures are fabricated using selective laser melting (SLM) to validate FEM results. The simulation and experimental results are close to each other with an average error of about 4.5%. The elastic modulus derived from FEM for the designed gradient structures are in the range of 7.48 to 10.49 GPa, which can be modified, and present mechanical properties close to trabecular or compact bone based on the position and conditions of the bone defect.

Exploration of possible cell chirality using material techniques of surface patterning

Consistent left-right (LR) asymmetry or chirality is critical for embryonic development and function maintenance. While chirality on either molecular or organism level has been well established, that on the cellular level has remained an open question for a long time. Although it remains unclear whether chirality exists universally on the cellular level, valuable efforts have recently been made to explore this fundamental topic pertinent to both cell biology and biomaterial science. The development of material fabrication techniques, surface patterning, in particular, has afforded a unique platform to study cell-material interactions. By using patterning techniques, chirality on the cellular level has been examined for cell clusters and single cells in vitro in well-designed experiments. In this review, we first introduce typical fabrication techniques of surface patterning suitable for cell studies and then summarize the main aspects of preliminary evidence of cell chirality on patterned surfaces to date. We finally indicate the limitations of the studies conducted thus far and describe the perspectives of future research in this challenging field. STATEMENT OF SIGNIFICANCE: While both biomacromolecules and organisms can exhibit chirality, it is not yet conclusive whether a cell has left-right (LR) asymmetry. It is important yet challenging to study and reveal the possible existence of cell chirality. By using the technique of surface patterning, the recent decade has witnessed progress in the exploration of possible cell chirality within cell clusters and single cells. Herein, some important preliminary evidence of cell chirality is collected and analyzed. The open questions and perspectives are also described to promote further investigations of cell chirality in biomaterials.

Biofunctionalization of Porous Ti Substrates Coated with Ag Nanoparticles for Potential Antibacterial Behavior

Ti prosthesis have shown better biological compatibility, mechanical performance, and resistance to corrosion in cases of bone replacements. Nevertheless, fully dense Ti in connection with bone-host tissues show stress-shielding phenomenon that, together with the development of frequent undesirable microbial infections, may lead to implant failures. To overcome these issues, the present study aimed at the development of a novel combination of a chemically functionalized porous Ti substrate with a potentially therapeutic AgNPs coating. Fully dense and porous Ti substrates (30 and 60 vol.%, 100–200 and 355–500 μm, as spacer particles) were studied. Ti surface was treated with acid or basic medium followed by silanization and deposition of AgNPs by “submerged” and “in situ” methods. In general, for similar porosity, mechanical resistance decreased as pore size increased. Acidic reagent and submerged methodology were the best combination for fully dense Ti substrates. Hence, they were also employed for porous Ti substrates. Depending on the porosity of the substrates, variations can be observed both in the size and degree of agglomeration of the deposited AgNPs, entailing differences in the antibacterial behavior of the samples.

Metal Material, Properties and Design Methods of Porous Biomedical Scaffolds for Additive Manufacturing: A Review

Design an implant similar to the human bone is one of the critical problems in bone tissue engineering. Metal porous scaffolds have good prospects in bone tissue replacement due to their matching elastic modulus, better strength, and biocompatibility. However, traditional processing methods are challenging to fabricate scaffolds with a porous structure, limiting the development of porous scaffolds. With the advancement of additive manufacturing (AM) and computer-aided technologies, the development of porous metal scaffolds also ushers in unprecedented opportunities. In recent years, many new metal materials and innovative design methods are used to fabricate porous scaffolds with excellent mechanical properties and biocompatibility. This article reviews the research progress of porous metal scaffolds, and introduces the AM technologies used in porous metal scaffolds. Then the applications of different metal materials in bone scaffolds are summarized, and the advantages and limitations of various scaffold design methods are discussed. Finally, we look forward to the development prospects of AM in porous metal scaffolds.

Osteogenic and antibacterial surfaces on additively manufactured porous Ti-6Al-4V implants: Combining silver nanoparticles with hydrothermally synthesized HA nanocrystals

The recently developed additively manufacturing techniques have enabled the fabrication of porous biomaterials that mimic the characteristics of the native bone, thereby avoiding stress shielding and facilitating bony ingrowth. However, aseptic loosening and bacterial infection, as the leading causes of implant failure, need to be further addressed through surface biofunctionalization. Here, we used a combination of (1) plasma electrolytic oxidation (PEO) using Ca-, P-, and silver nanoparticle-rich electrolytes and (2) post-PEO hydrothermal treatments (HT) to furnish additively manufactured Ti-6Al-4V porous implants with a multi-functional surface. The applied HT led to the formation of hydroxyapatite (HA) nanocrystals throughout the oxide layer. This process was controlled by the supersaturation of Ca²⁺ and PO₄^3- during the hydrothermal process. Initially, the high local supersaturation resulted in homogenous nucleation of spindle-like nanocrystals throughout the surface. As the process continued, the depletion of reactant ions in the outermost surface layer led to a remarkable decrease in the supersaturation degrees. High aspect-ratio nanorods and hexagonal nanopillars were, therefore, created. The unique hierarchical structure of the microporous PEO layer (pore size < 3 μm) and spindle-like HA nanocrystals (<150 nm) on the surface of macro-porous additively manufactured Ti-6Al-4V implants provided a favorable substrate for the anchorage of cytoplasmic extensions assisting cell attachment and migration on the surface. The results of our in vitro assays clearly showed the important benefits of the HT and the spindle-like HA nanocrystals including a significantly stronger and much more sustained antibacterial activity, significantly higher levels of pre-osteoblasts metabolic activity, and significantly higher levels of alkaline phosphatase activity as compared to similar PEO-treated implants lacking the HT.

Structural and Material Determinants Influencing the Behavior of Porous Ti and Its Alloys Made by Additive Manufacturing Techniques for Biomedical Applications

One of the biggest challenges in tissue engineering is the manufacturing of porous structures that are customized in size and shape and that mimic natural bone structure. Additive manufacturing is known as a sufficient method to produce 3D porous structures used as bone substitutes in large segmental bone defects. The literature indicates that the mechanical and biological properties of scaffolds highly depend on geometrical features of structure (pore size, pore shape, porosity), surface morphology, and chemistry. The objective of this review is to present the latest advances and trends in the development of titanium scaffolds concerning the relationships between applied materials, manufacturing methods, and interior architecture determined by porosity, pore shape, and size, and the mechanical, biological, chemical, and physical properties. Such a review is assumed to show the real achievements and, on the other side, shortages in so far research.

Additively manufactured functionally graded biodegradable porous zinc

Topological design provides additively manufactured (AM) biodegradable porous metallic biomaterials with a unique opportunity to adjust their biodegradation behavior and mechanical properties, thereby satisfying the requirements for ideal bone substitutes. However, no information is available yet concerning the effect of topological design on the performance of AM porous zinc (Zn) that outperforms Mg and Fe in biodegradation behavior. Here, we studied one functionally graded and two uniform AM porous Zn designs with diamond unit cell. Cylindrical specimens were fabricated from pure Zn powder by using a powder bed fusion technique, followed by a comprehensive study on their static and dynamic biodegradation behaviors, mechanical properties, permeability, and biocompatibility. Topological design, indeed, affected the biodegradation behavior of the specimens, as evidenced by 150% variations in biodegradation rate between the three different designs. After in vitro dynamic immersion for 28 days, the AM porous Zn had weight losses of 7-12%, relying on the topological design. The degradation rates satisfied the desired biodegradation time of 1-2 years for bone substitution. The mechanical properties of the biodegraded specimens of all the groups maintained within the range of those of cancellous bone. As opposed to the trends observed for other biodegradable porous metals, after 28 days of in vitro biodegradation, the yield strengths of the specimens of all the groups (σy = 7-14 MPa) increased consistently, as compared to those of the as-built specimens (σy = 4-11 MPa). Moreover, AM porous Zn showed excellent biocompatibility, given that the cellular activities in none of the groups differed from the Ti controls for up to 72 h. Using topological design of AM porous Zn for controlling its mechanical properties and degradation behavior is thus clearly promising, thereby rendering flexibility to the material to meet a variety of clinical requirements.

On the mechanical behaviour of additively manufactured metamaterials under dynamic conditions

High-energy absorption and light-weightiness are two critical properties for impact protection in the aerospace sector. In the past, the use of periodic honeycomb structures or random porous metallic foams were the preferred route to obtain a good specific-energy absorption performance. In recent years, the use of additive manufacturing has increased the design freedom creating a new generation of reticulated and porous materials: the metamaterials or lattice materials. The internal geometries of these lattice structures can be tuned for superior optimal properties, e.g., energyabsorption and density. However, the mechanics of these materials under impact need to be understood with the purpose of mechanical optimisation, and the computational models validated. In this work, we present the experimental compressive behaviour, at room temperature, of two Ti6Al4V lattice structures under static and dynamic conditions. The quasi-static tests were performed by using a universal testing machine while the dynamic tests were conducted at 480s-1 with a split-Hopkinson bar. In all cases, the deformation process was filmed to analyse the failure. Finally, finiteelement simulations were done, employing the Johnson-Cook model, to describe the response of the alloy. The simulations were able to reflect the failure characteristics of each metamaterial but were not able to describe the macroscopic response due to the differences between the experimental and computational volume fraction.

Post-Processing Treatment Impact on Mechanical Properties of SLM Deposited Ti-6Al-4 V Porous Structure for Biomedical Application

Additive manufacturing technologies allow producing a regular three-dimensional mesh of interconnected struts that form an open-cell porous structure. Regular porous structures have been used in the orthopedic industry due to outstanding bone anchoring. The aim of the study was to determine how the postprocessing influences the mechanical properties of porous structures made of titanium alloy CL 41TI ELI. The effect of hot isostatic pressing (HIP) as a method of increasing microstructural integrity was investigated here. The influence of surface etching (SE) technique, which was applied to the porous structure for cleaning unmelted titanium powder particles on the surface of connectors from the inner surfaces of a porous structure, was examined in this study. Mechanical properties were investigated by means of compression tests. The results point out that HIP has a minor effect on the mechanical behavior of considered porous structures. The SE is an effective method to clean the surface of a porous structure, which is very important in the case of biomedical applications when loose powder can cause serious health problems. Another effect of the SE is also the strut thickness reduction. Reducing strut thickness of a porous structure with the surface etching decreases its stiffness to the same extent as predicted by the relative density theoretical model but did not result in structural damage.

Design and performance evaluation of additively manufactured composite lattice structures of commercially pure Ti (CP-Ti)

Ti alloys with lattice structures are garnering more and more attention in the field of bone repair or regeneration due to their superior structural, mechanical, and biological properties. In this study, six types of composite lattice structures with different strut radius that consist of simple cubic (structure A), body-centered cubic (structure B), and edge-centered cubic (structure C) unit cells are designed. The designed structures are firstly simulated and analysed by the finite element (FE) method. Commercially pure Ti (CP–Ti) lattice structures with optimized unit cells and strut radius are then fabricated by selective laser melting (SLM), and the dimensions, microtopography, and mechanical properties are characterised. The results show that among the six types of composite lattice structures, combined BA, CA, and CB structures exhibit smaller maximum von-Mises stress, indicating that these structures have higher strength. Based on the fitting curves of stress/specific surface area versus strut radius, the optimized strut radius of BA, CA, and CB structures is 0.28, 0.23, and 0.30 mm respectively. Their corresponding compressive yield strength and compressive modulus are 42.28, 30.11, and 176.96 MPa, and 4.13, 2.16, and 7.84 GPa, respectively. The CP-Ti with CB unit structure presents a similar strength and compressive modulus to the cortical bone, which makes it a potential candidate for subchondral bone restorations.

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Six types of graded lattice structures with different strut radius are designed and simulated by the FE method.

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BA, CA, and CB structures exhibit smaller maximum Von-Mises stress among six type structures.

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CP-Ti with CB structures exhibits similar mechanical properties to the cortical bone.

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Excellent properties make CP-Ti with CB structures an attractive subchondral bone restoration material.

Exploring Macroporosity of Additively Manufactured Titanium Metamaterials for Bone Regeneration with Quality by Design: A Systematic Literature Review

Additive manufacturing facilitates the design of porous metal implants with detailed internal architecture. A rationally designed porous structure can provide to biocompatible titanium alloys biomimetic mechanical and biological properties for bone regeneration. However, increased porosity results in decreased material strength. The porosity and pore sizes that are ideal for porous implants are still controversial in the literature, complicating the justification of a design decision. Recently, metallic porous biomaterials have been proposed for load-bearing applications beyond surface coatings. This recent science lacks standards, but the Quality by Design (QbD) system can assist the design process in a systematic way. This study used the QbD system to explore the Quality Target Product Profile and Ideal Quality Attributes of additively manufactured titanium porous scaffolds for bone regeneration with a biomimetic approach. For this purpose, a total of 807 experimental results extracted from 50 different studies were benchmarked against proposed target values based on bone properties, governmental regulations, and scientific research relevant to bone implants. The scaffold properties such as unit cell geometry, pore size, porosity, compressive strength, and fatigue strength were studied. The results of this study may help future research to effectively direct the design process under the QbD system.

Mechanical behavior of in-situ alloyed Ti6Al4V(ELI)-3 at.% Cu lattice structures manufactured by laser powder bed fusion and designed for implant applications

In the present study, cellular lattice structures for implant applications are reported for the first-time incorporating copper directly by in-situ alloying in the laser powder bed fusion process. The aim to incorporate 3 at.% Cu into Ti6Al4V(ELI) is selected for improved antibacterial properties while maintaining appropriate mechanical properties. Previously, topologically optimized Ti6Al4V(ELI) lattice structures were successfully designed, manufactured and studied for implant applications. The development of a new alloy produced by in-situ alloying of elemental powder mixture of Ti6Al4V(ELI) and pure Cu powders was used here for the production of identical lattice structures with improved antibacterial properties. One of the same as-designed CAD models was used for the manufacturing of these lattices compared to previous work on pure Ti6Al4V(ELI) lattices, making direct comparison of mechanical properties possible. Similar manufacturability highlights the applicability of this alloying technique to other lattice designs. Microstructural characterization was performed by optical and electron microscopies, as well as microCT. Mechanical characterization was performed by means of compression tests and hardness measurements. Results showed that in-situ alloying with copper leads to the formation of localized Cu-rich regions, refinement of martensitic phase and the formation of CuTi₂ intermetallic precipitates, which increased the hardness and strength of the material. Deviations in wall thickness between the as-designed and as-manufactured lattices led to anisotropy of the mechanical properties of the lattices. Higher compressive strength values were obtained when thicker walls were oriented along the loading direction. Nevertheless, alloying with Cu had a higher impact on the compressive strength of lattice structure than the wall thickness deviations. The direct in-situ alloying of copper in Ti6Al4V(ELI) is a promising route for direct manufacturing of antibacterial implants.

Additively manufactured biodegradable porous metals

Partially due to the unavailability of ideal bone substitutes, the treatment of large bony defects remains one of the most important challenges of orthopedic surgery. Additively manufactured (AM) biodegradable porous metals that have emerged since 2018 provide unprecedented opportunities for fulfilling the requirements of an ideal bone implant. First, the multi-scale geometry of these implants can be customized to mimic the human bone in terms of both micro-architecture and mechanical properties. Second, a porous structure with interconnected pores possesses a large surface area, which is favorable for the adhesion and proliferation of cells and, thus, bony ingrowth. Finally, the freeform geometrical design of such biomaterials could be exploited to adjust their biodegradation behavior so as to maintain the structural integrity of the implant during the healing process while ensuring that the implant disappears afterwards, paving the way for full bone regeneration. While the AM biodegradable porous metals that have been studied so far have shown many unique properties as compared to their solid counterparts, the unprecedented degree of flexibility in their geometrical design has not yet been fully exploited to optimize their properties and performance. In order to develop the ideal bone implants, it is important to take advantage of the full potential of AM biodegradable porous metals through detailed and systematic study on their biodegradation behavior, mechanical properties, biocompatibility, and bone regeneration performance. This review paper presents the state of the art in AM biodegradable porous metals and is focused on the effects of material type, processing, geometrical design, and post-AM treatments on the mechanical properties, biodegradation behavior, in vitro biocompatibility, and in vivo bone regeneration performance of AM porous Mg, Fe, and Zn as well as their alloys. We also identify a number of knowledge gaps and the challenges encountered in adopting AM biodegradable porous metals for orthopedic applications and suggest some promising areas for future research.

Tailored mechanical response and mass transport characteristic of selective laser melted porous metallic biomaterials for bone scaffolds

Porous metallic biomaterials developed from pentamode metamaterials (PMs) were rationally designed to mimic the topological, mechanical, and mass transport properties of human bones. Here, a series of diamond-based PMs with different strut parameters were fabricated from a Ti-6Al-4V powder by selective laser melting (SLM) technique. The morphological features, mechanical properties and permeability of PM samples were then characterized. In terms of morphology, the as-built PMs were well consistent with the as-designed ones, although the slight surface deviations were presented in overhanging areas. The PM scaffolds showed a switchable deformation pattern controlled by the slenderness ratio of struts. The double-cone strut topology increases the tortuosity and thereby accelerates the nutrients supply, waste removal, and cell migration to the whole scaffold region and circumambient bone tissue. The measured mechanical properties (i.e., E: 0.59-2.90 GPa, σ_y: 20.59-112.63 MPa) and computational permeability values (k: 9.87-49.19 × 10^-9 m²) of PM scaffolds were all in accordance with those of trabecular bone. The experimental permeability values were linearly dependent on the results of simulations. This study showed the great potential of PMs as bone scaffolds. Moreover, we demonstrated that PM-based porous biomaterials can decouple the mass transport and mechanical properties of bone scaffolds, so as to achieve an unprecedented level of tailoring their multi-physics properties. STATEMENT OF SIGNIFICANCE: The topological diversity can significantly improve the adaptability of the implant to the primary bone tissue. Previous studies revealed that the mechanical and mass transport properties of porous biomaterials are correlated to the material types, porosities and lattice topologies but neglected effects of strut design. We show here the influence of strut morphology on the mechanical and mass transport properties which are independently tailored, that is, the mass transport properties can be markedly increased while maintaining the mechanical properties of mimicking specific bones, vice versa. This study emphasizes the importance of strut topological design in the development of AM porous biomaterials.

Porous Scaffold Design for Additive Manufacturing in Orthopedics: A Review

With the increasing application of orthopedic scaffolds, a dramatically increasing number of requirements for scaffolds are precise. The porous structure has been a fundamental design in the bone tissue engineering or orthopedic clinics because of its low Young's modulus, high compressive strength, and abundant cell accommodation space. The porous structure manufactured by additive manufacturing (AM) technology has controllable pore size, pore shape, and porosity. The single unit can be designed and arrayed with AM, which brings controllable pore characteristics and mechanical properties. This paper presents the current status of porous designs in AM technology. The porous structures are stated from the cellular structure and the whole structure. In the aspect of the cellular structure, non-parametric design and parametric design are discussed here according to whether the algorithm generates the structure or not. The non-parametric design comprises the diamond, the body-centered cubic, and the polyhedral structure, etc. The Voronoi, the Triply Periodic Minimal Surface, and other parametric designs are mainly discussed in parametric design. In the discussion of cellular structures, we emphasize the design, and the resulting biomechanical and biological effects caused by designs. In the aspect of the whole structure, the recent experimental researches are reviewed on uniform design, layered gradient design, and layered gradient design based on topological optimization, etc. These parts are summarized because of the development of technology and the demand for mechanics or bone growth. Finally, the challenges faced by the porous designs and prospects of porous structure in orthopedics are proposed in this paper.

Multi-material additive manufacturing technologies for Ti-, Mg-, and Fe-based biomaterials for bone substitution

The growing interest in multi-functional metallic biomaterials for bone substitutes challenges the current additive manufacturing (AM, =3D printing) technologies. It is foreseeable that advances in multi-material AM for metallic biomaterials will not only allow for complex geometrical designs, but also improve their multi-functionalities by tuning the types or compositions of the underlying base materials, thereby presenting unprecedented opportunities for advanced orthopedic treatments. AM technologies are yet to be extensively explored for the fabrication of multi-functional metallic biomaterials, especially for bone substitutes. The aim of this review is to present the viable options of the state-of-the-art multi-material AM for Ti-, Mg-, and Fe-based biomaterials to be used as bone substitutes. The review starts with a brief review of bone tissue engineering, the design requirements, and fabrication technologies for metallic biomaterials to highlight the advantages of using AM over conventional fabrication methods. Five AM technologies suitable for metal 3D printing are compared against the requirements for multi-material AM. Of these AM technologies, extrusion-based multi-material AM is shown to have the greatest potential to meet the requirements for the fabrication of multi-functional metallic biomaterials. Finally, recent progress in the fabrication of Ti-, Mg-, and Fe-based biomaterials including the utilization of multi-material AM technologies is reviewed so as to identify the knowledge gaps and propose the directions of further research for the development of multi-material AM technologies that are applicable for the fabrication of multi-functional metallic biomaterials. STATEMENT OF SIGNIFICANCE: Addressing a critical bone defect requires the assistance of multi-functional porous metallic bone substitutes. As one of the most advanced fabrication technology in bone tissue engineering, additive manufacturing is challenged for its viability in multi-material fabrication of metallic biomaterials. This article reviews how the current metal additive manufacturing technologies have been and can be used for multi-material fabrication of Ti-, Mg-, and Fe-based bone substitutes. Progress on the Ti-, Mg-, and Fe-based biomaterials, including the utilization of multi-material additive manufacturing, are discussed to direct future research for advancing the multi-functional additively manufactured metallic bone biomaterials.