Microstructural and mechanical characteristics of porous iron prepared by powder metallurgy

Microstructure and Mechanical Properties of Metal Foams Fabricated via Melt Foaming and Powder Metallurgy Technique: A Review

Metal foams possess remarkable properties, such as lightweight, high compressive strength, lower specific weight, high stiffness, and high energy absorption. These properties make them highly desirable for many engineering applications, including lightweight materials, energy-absorption devices for aerospace and automotive industries, etc. For such potential applications, it is essential to understand the mechanical behaviour of these foams. Producing metal foams is a highly challenging task due to the coexistence of solid, liquid, and gaseous phases at different temperatures. Although numerous techniques are available for producing metal foams, fabricating foamed metal still suffers from imperfections and inconsistencies. Thus, a good understanding of various processing techniques and properties of the resulting foams is essential to improve the foam quality. This review discussed the types of metal foams available in the market and their properties, providing an overview of the production techniques involved and the contribution of metal foams to various applications. This review also discussed the challenges in foam fabrications and proposed several solutions to address these problems.

Progress in manufacturing and processing of degradable Fe-based implants: a review

Biodegradable metals have gained vast attention as befitting candidates for developing degradable metallic implants. Such implants are primarily employed for temporary applications and are expected to degrade or resorbed after the tissue is healed. Fe-based materials have generated considerable interest as one of the possible biodegradable metals. Like other biometals such as Mg and Zn, Fe exhibits good biocompatibility and biodegradability. The versatility in the mechanical behaviour of Fe-based materials makes them a better choice for load-bearing applications. However, the very low degradation rate of Fe in the physiological environment needs to be improved to make it compatible with tissue growth. Several studies on tailoring the degradation behaviour of Fe in the human body are already reported. Majority of these works include studies on the effect of manufacturing and processing techniques on biocompatibility and biodegradability. This article focuses on a comprehensive review and analysis of the various manufacturing and processing techniques so far reported for developing biodegradable iron-based orthopaedic implants. The current status of research in the field is neatly presented, and a summary of the works is included in the article for the benefit of researchers in the field to contextualise their research and effectively find the lacunae in the existing scholarship.

A Critical Review of Additive Manufacturing Techniques and Associated Biomaterials Used in Bone Tissue Engineering

With the ability to fabricate complex structures while meeting individual needs, additive manufacturing (AM) offers unprecedented opportunities for bone tissue engineering in the biomedical field. However, traditional metal implants have many adverse effects due to their poor integration with host tissues, and therefore new material implants with porous structures are gradually being developed that are suitable for clinical medical applications. From the perspectives of additive manufacturing technology and materials, this article discusses a suitable manufacturing process for ideal materials for biological bone tissue engineering. It begins with a review of the methods and applicable materials in existing additive manufacturing technologies and their applications in biomedicine, introducing the advantages and disadvantages of various AM technologies. The properties of materials including metals and polymers, commonly used AM technologies, recent developments, and their applications in bone tissue engineering are discussed in detail and summarized. In addition, the main challenges for different metallic and polymer materials, such as biodegradability, anisotropy, growth factors to promote the osteogenic capacity, and enhancement of mechanical properties are also introduced. Finally, the development prospects for AM technologies and biomaterials in bone tissue engineering are considered.

Bone Scaffolds: An Incorporation of Biomaterials, Cells, and Biofactors

Large injuries to bones are still one of the most challenging musculoskeletal problems. Tissue engineering can combine stem cells, scaffold biomaterials, and biofactors to aid in resolving this complication. Therefore, this review aims to provide information on the recent advances made to utilize the potential of biomaterials for making bone scaffolds and the assisted stem cell therapy and use of biofactors for bone tissue engineering. The requirements and different types of biomaterials used for making scaffolds are reviewed. Furthermore, the importance of stem cells and biofactors (growth factors and extracellular vesicles) in bone regeneration and their use in bone scaffolds and the key findings are discussed. Lastly, some of the main obstacles in bone tissue engineering and future trends are highlighted.

Extrusion-based 3D printed biodegradable porous iron

Extrusion-based 3D printing followed by debinding and sintering is a powerful approach that allows for the fabrication of porous scaffolds from materials (or material combinations) that are otherwise very challenging to process using other additive manufacturing techniques. Iron is one of the materials that have been recently shown to be amenable to processing using this approach. Indeed, a fully interconnected porous design has the potential of resolving the fundamental issue regarding bulk iron, namely a very low rate of biodegradation. However, no extensive evaluation of the biodegradation behavior and properties of porous iron scaffolds made by extrusion-based 3D printing has been reported. Therefore, the in vitro biodegradation behavior, electrochemical response, evolution of mechanical properties along with biodegradation, and responses of an osteoblastic cell line to the 3D printed iron scaffolds were studied. An ink formulation, as well as matching 3D printing, debinding and sintering conditions, was developed to create iron scaffolds with a porosity of 67%, a pore interconnectivity of 96%, and a strut density of 89% after sintering. X-ray diffracometry confirmed the presence of the α-iron phase in the scaffolds without any residuals from the rest of the ink. Owing to the presence of geometrically designed macropores and random micropores in the struts, the in vitro corrosion rate of the scaffolds was much improved as compared to the bulk counterpart, with 7% mass loss after 28 days. The mechanical properties of the scaffolds remained in the range of those of trabecular bone despite 28 days of in vitro biodegradation. The direct culture of MC3T3-E1 preosteoblasts on the scaffolds led to a substantial reduction in living cell count, caused by a high concentration of iron ions, as revealed by the indirect assays. On the other hand, the ability of the cells to spread and form filopodia indicated the cytocompatibility of the corrosion products. Taken together, this study shows the great potential of extrusion-based 3D printed porous iron to be further developed as a biodegradable bone substituting biomaterial.

Surface Modifications of Biodegradable Metallic Foams for Medical Applications

Significant progress was achieved presently in the development of metallic foam-like materials improved by biocompatible coatings. Material properties of the iron, magnesium, zinc, and their alloys are promising for their uses in medical applications, especially for orthopedic and bone tissue purposes. Current processing technologies and a variety of modifications of the surface and composition facilitate the design of adjusted medical devices with desirable mechanical, morphological, and functional properties. This article reviews the recent progress in the design of advanced degradable metallic biomaterials perfected by different coatings: polymer, inorganic ceramic, and metallic. Appropriate coating of metallic foams could improve the biocompatibility, osteogenesis, and bone tissue-bonding properties. In this paper, a comprehensive review of different coating types used for the enhancement of one or several properties of biodegradable porous implants is given. An outline of the conventional preparation methods of metallic foams and a brief overview of different alloys for medical applications are also provided. In addition, current challenges and future research directions of processing and surface modifications of biodegradable metallic foams for medical applications are suggested.

Better Surface Integrity and Tribological Properties of Steel Sintered by Powder Metallurgy

The current research reports the improvement in surface integrity and tribological characteristics of steel prepared using a powder metallurgy (PM) by ultrasonic nanocrystal surface modification (UNSM) at 25 and 300 °C. The surface integrity and tribological properties of three samples, namely, as-PM, UNSM-25 and UNSM-300 were investigated. The average surface roughness (R_a) of the as-PM, UNSM-25 and UNSM-300 samples was measured using a non-contact 3D scanner, where it was found to be 3.21, 1.14 and 0.74 µm, respectively. The top surface hardness was also measured in order to investigate the influence of UNSM treatment temperature on the hardness. The results revealed that the as-PM sample with a hardness of 109 HV was increased up to 165 and 237 HV, corresponding to a 32.1% and 57.2% after both the UNSM treatment at 25 and 300 °C, respectively. XRD analysis was also performed to confirm if any changes in chemistry and crystal size were took place after the UNSM treatment at 25 and 300 °C. In addition, dry tribological properties of the samples were investigated. The friction coefficient of the as-PM sample was 0.284, which was reduced up to 0.225 and 0.068 after UNSM treatment at 25 and 300 °C, respectively. The wear resistance was also enhanced by 33.2 and 52.9% after UNSM treatment at both 25 and 300 °C. Improvements in surface roughness, hardness and tribological properties was attributed to the elimination of big and deep porosities after UNSM treatment. Wear track of the samples and wear scar of the counter surface balls were investigated by SEM to reach a comprehensive discussion on wear mechanisms. Overall, it was confirmed that UNSM treatment at 25 and 300 °C had a beneficial effect on the surface integrity and tribological characteristics of sintered steel by the PM that is used in a shock absorber for a car engine.

In vitro degradation behaviour, cytocompatibility and hemocompatibility of topologically ordered porous iron scaffold prepared using 3D printing and pressureless microwave sintering

Biodegradable porous iron having topologically ordered porosity and tailorable properties as per the required application has been the major requirement in the field of biodegradable biomaterials. Hence, in the present study, iron scaffolds with the topologically ordered porous structure were developed and for the first time, the effect of the variation in the topology on the in vitro degradation behaviour, cytocompatibility and hemocompatibility were investigated. Iron scaffold samples were fabricated using a novel process based on the combination of 3D printing and pressureless microwave sintering. To investigate the effect of topology, two different types of topological structures namely Truncated Octahedron (TO) (with variable strut size) and Cubic (C) were used. From the morphological characterization, it was found that fabricated iron scaffold possessed interconnected porosity varying from 50.70%-80.97% which included the random microporosities in the strut and designed macroporosity. Furthermore, it was inferred that the topology of the iron scaffold significantly affected its degradation properties and cytocompatibility. Increase in the weight loss, corrosion rate and reduction in cell viability with the reduction in porosity were obtained. The maximum corrosion rate and weight loss achieved was 1.64 mmpy and 6.4% respectively. Direct cytotoxicity test results revealed cytotoxicity, while prepared iron scaffold samples exhibited excellent hemocompatibility and anti-platelet adhesion property. A comparative study with relevant literature was performed and it was established that the developed iron scaffold exhibited favorable degradation and biological properties which could be tailored to suit appropriate bone tissue engineering applications.

Investigation of porosity on mechanical properties, degradation and in-vitro cytotoxicity limit of Fe30Mn using space holder technique

Bioresorbable metallic implants are considered to be a new generation of transient fixation devices, which provide strong mechanical support during healing as well as effective integration with the host bone tissues, free of secondary surgery. We evaluated the microstructures and mechanical properties of iron‑manganese alloys (Fe30Mn) with 0-, 5-, 10-, and 60-volume percent porosity, which was produced through ammonium bicarbonate (NH₄HCO₃) decomposition. We also investigated the influence of porosity concentration on the corrosion rate and cytotoxicity of the alloy. The average value of maximum compressive strength was 2-fold greater in the 0-vol% scaffolds than that in 60-vol% scaffolds. Scaffolds with 60-vol% porosity exhibited the highest average value of corrosion rate in a potentiodynamic polarization test among the four groups. However, the group influenced cellular viability negatively in a subsequent cytotoxicity test. Fe30Mn scaffolds with 10-vol% NH₄HCO₃ are considered promising resorbable scaffolds based on the results of compression tests, corrosion experiments and cytotoxicity studies.

Cancellous-Bone-like Porous Iron Scaffold Coated with Strontium Incorporated Octacalcium Phosphate Nanowhiskers for Bone Regeneration

The repair of large bone defects poses a grand challenge in tissue engineering. Thus, developing biocompatible scaffolds with mechanical and structural similarity to human cancellous bone is in great demand. Herein, we fabricated a three-dimensional (3D) porous iron (Fe) scaffold with interconnected pores via a template-assisted electrodeposition method. The porous Fe scaffold with a skeleton diameter of 143 μm had the porosity >90%, an average pore size of 345 μm, and a yield strength of 3.5 MPa. Such structure and mechanical strength were close to those of cancellous bone. In order to enhance the biocompatibility of the scaffold, strontium incorporated octacalcium phosphate (Sr-OCP) was coated on the skeletons of the porous Fe scaffold. The coated Sr-OCP was in the form of nanowhiskers with a mean diameter of 300 nm and length of 30 μm. Such Sr-OCP coating could effectively reduce the release rate of the Fe ions to a level which was safe for the human body. Both in vitro cytotoxicity tests by extraction method and direct contact assay confirmed that the Sr-OCP coating could promote the cell adhesion and substantially enhance the biocompatibility of the porous Fe scaffolds. Thus, the cancellous-bone-like porous structure with compatible mechanical properties and excellent biocompatibility enables the present Sr-OCP coated porous Fe scaffold to be a promising candidate for bone repair and regeneration.

Acoustic emission analysis of the compressive deformation of iron foams and their biocompatibility study

We synthesized Fe foams using water suspensions of micrometric Fe₂O₃ powder by reducing and sintering the sublimated Fe oxide green body to Fe under 5% H₂/Ar gas. The resultant Fe foam showed aligned lamellar macropores replicating the ice dendrites. The compressive behavior and deformation mechanism of the synthesized Fe foam were studied using an acoustic emission (AE) method, with which we detected sudden localized structural changes in the Fe foam material. The evolution of the deformation mechanism was elucidated using the adaptive sequential k-means (ASK) algorithm; specifically, the plastic deformation of the cell struts was followed by localized cell collapse, which eventually led to fracturing of the cell walls. For potential biomedical applications, the corrosion and biocompatibility characteristics of the two synthesized Fe foams with different porosities (50% vs. 44%) were examined and compared. Despite its larger porosity, the superior corrosion behavior of the Fe foam with 50% porosity can be attributed to its larger pore size and smaller microscopic surface area. Based on the cytotoxicity tests for the extracts of the foams, the Fe foam with 44% porosity showed better cytocompatibility than that with 50% porosity.

Advances in Fe-based biodegradable metallic materials

This review systematically summarizes recent studies on Fe-based biodegradable metallic materials and discusses these findings in terms of their processing methods, mechanical properties, degradability and biocompatibility.