Synthesis, mechanical properties and corrosion behavior of powder metallurgy processed Fe/Mg2Si composites for biodegradable implant applications

Surface Modification of an Absorbable Bimodal Fe-Mn-Ag Alloy by Nitrogen Plasma Immersion Ion Implantation

Recently, Fe-Mn-based alloys have been increasingly catching the attention of the scientific community, because of their tunable and outstanding mechanical properties, and suitable degradation behavior for biomedical applications. In spite of these assets, their corrosion rate (CR) is, in general, too low to satisfy the requirements that need to be met for cardiovascular device applications, such as stents. In fact, the CR is not always the same for all of the degradation stages of the material, and in addition, a finely tuned release rate, especially during the first steps of the corrosion pattern, is often demanded. In this work, a resorbable bimodal multi-phase alloy Fe-3Mn-1Ag was designed by mechanical alloying and spark plasma sintering (SPS) to accelerate the corrosion rate. The presence of several phases, for example α-Fe, α-Mn, γ-FeMn and Ag, provided the material with excellent mechanical properties (tensile strength UTS = 722 MPa, tensile strain A = 38%) and a higher corrosion rate (CR = 3.2 ± 0.2 mm/year). However, higher corrosion rates, associated with an increased release of degradation elements, could also raise toxicity concerns, especially at the beginning of the corrosion pattern. In this study, The focus of the present work was the control of the CR by surface modification, with nitrogen plasma immersion ion implantation (N-PIII) treatment that was applied to mechanically polished (MP) samples. This plasma treatment (PT) improved the corrosion resistance of the material, assessed by static degradation immersion tests (SDITs), especially during the first degradation stages. Twenty-eight days later, the degradation rate reached the same value of the MP condition. Nitrogen compounds on the surface of the substrate played an important role in the corrosion mechanism and corrosion product formation. The degradation analysis was carried out also by potentiodynamic tests in modified Hanks' balanced salt solution (MHBSS), and Dulbecco's phosphate buffered saline solution (DPBSS). The corrosion rate was higher in MHBSS for both conditions. However, there was no significant difference between the corrosion rate of the PT in DPBSS (CR = 1.9 ± 0.6 mm/year) and in MHBSS (CR = 2 ± 1.4 mm/year). The cell viability was assessed with human vein endothelial cells (HUVECs) via an indirect metabolic activity test (MTT assay). Due to the lower ion release of the PT condition, the cell viability increased significantly. Thus, nitrogen implantation can control the in vitro corrosion rate starting from the very first stage of the implantation, improving cell viability.

Finite Element Analysis of the Non-Uniform Degradation of Biodegradable Vascular Stents

Most of the studies on the finite element analysis (FEA) of biodegradable vascular stents (BVSs) during the degradation process have limited the accuracy of the simulation results due to the application of the uniform degradation model. This paper aims to establish an FEA model for the non-uniform degradation of BVSs by considering factors such as the dynamic changes of the corrosion properties and material properties of the element, as well as the pitting corrosion and stress corrosion. The results revealed that adjusting the corrosion rate according to the number of exposed surfaces of the element and reducing the stress threshold according to the corrosion status accelerates the degradation time of BVSs by 26% and 25%, respectively, compared with the uniform degradation model. The addition of the pitting model reduces the service life of the BVSs by up to 12%. The effective support of the stent to the vessel could reach at least 60% of the treatment effect before the vessel collapsed. These data indicate that the proposed non-uniform degradation model of BVSs with multiple factors produces different phenomena compared with the commonly used models and make the numerical simulation results more consistent with the real degradation scenario.

Additive manufacturing of bioactive and biodegradable porous iron-akermanite composites for bone regeneration

Advanced additive manufacturing techniques have been recently used to tackle the two fundamental challenges of biodegradable Fe-based bone-substituting materials, namely low rate of biodegradation and insufficient bioactivity. While additively manufactured porous iron has been somewhat successful in addressing the first challenge, the limited bioactivity of these biomaterials hinder their progress towards clinical application. Herein, we used extrusion-based 3D printing for additive manufacturing of iron-matrix composites containing silicate-based bioceramic particles (akermanite), thereby addressing both of the abovementioned challenges. We developed inks that carried iron and 5, 10, 15, or 20 vol% of akermanite powder mixtures for the 3D printing process and optimized the debinding and sintering steps to produce geometrically-ordered iron-akermanite composites with an open porosity of 69-71%. The composite scaffolds preserved the designed geometry and the original α-Fe and akermanite phases. The in vitro biodegradation rates of the composites were improved as much as 2.6 times the biodegradation rate of geometrically identical pure iron. The yield strengths and elastic moduli of the scaffolds remained within the range of the mechanical properties of the cancellous bone, even after 28 days of biodegradation. The composite scaffolds (10-20 vol% akermanite) demonstrated improved MC3T3-E1 cell adhesion and higher levels of cell proliferation. The cellular secretion of collagen type-1 and the alkaline phosphatase activity on the composite scaffolds (10-20 vol% akermanite) were, respectively higher than and comparable to Ti6Al4V in osteogenic medium. Taken together, these results clearly show the potential of 3D printed porous iron-akermanite composites for further development as promising bone substitutes. STATEMENT OF SIGNIFICANCE: : Porous iron matrix composites containing akermanite particles were produced by means of multi-material additive manufacturing to address the two fundamental challenges associated with biodegradable iron-based biomaterials, namely very low rate of biodegradation and insufficient bioactivity. Our porous iron-akermanite composites exhibited enhanced biodegradability and superior bioactivity compared to porous monolithic iron scaffolds. The murine bone cells proliferated on the composite scaffolds, and secreted the collagen type-1 matrix that stimulated bony-like mineralization. The results show the exceptional potential of the developed porous iron-based composite scaffolds for application as bone substitutes.

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.

Dry Sliding Wear and Corrosion Performance of Mg-Sn-Ti Alloys Produced by Casting and Extrusion

The aim of the present study is to investigate the role of Ti on corrosion and the wear properties of Mg-5Sn-xTi (x = 0, 0.15, 0.75, 1.5 wt.%) alloys. The samples were fabricated by conventional casting followed by hot extrusion, and the studies were examined by means of a pin-on-disc tribometer at various loads of 6, 10, and 20 N with constant sliding velocities of 0.04 m/s at ambient temperature. The corrosion performance, using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), was studied in a basic solution containing 3.5 wt.% NaCl. The observation indicated a drop in the wear rate with an increase in Ti, while the average coefficient of friction was raised in higher Ti contents compared to the base material. The sample with 0.75 wt.% Ti exhibited superior wear properties at 6 and 10 N of normal force, while the sample with 0.15 wt.% Ti presented better wear resistance for 20 N. Electrochemical test observations demonstrated that the Ti deteriorated the corrosion features of the Mg-5Sn alloy, owing to the galvanic effects of Ti. The Mg-5Sn alloy exhibited excellent corrosion behavior (corrosion potential (E_corr) = −1.45V and current density (I_corr) = 43.92 A/cm²). The results indicated the significant role of Ti content in modulating wear and corrosion resistance of the Mg-5Sn alloy.

Effect of Magnesium Addition and High Energy Processing on the Degradation Behavior of Iron Powder in Modified Hanks’ Solution for Bioabsorbable Implant Applications

This paper shows the results of applying a combination of high energy processing and magnesium (Mg) as an alloying element in a strategy for enhancing the degradation rate of iron (Fe) for applications in the field of non-permanent medical implants. For this purpose, Fe powder was milled with 5 wt% of Mg (Fe5Mg) and its microstructure and characterized degradation behavior. As-received Fe powder was also milled in order to distinguish between the effects due to high energy processing from those due to the presence of Mg. The powders were prepared by high energy planetary ball milling for 16 h. The results show that the initial crystallite size diminishes from >150 nm to 16 nm for Fe and 46 nm for Fe5Mg. Static degradation tests of loose powder particles were performed in Hanks’ solution. Visual inspection of the immersed powders and the X-ray diffraction (XRD) phase quantification indicate that Fe5Mg exhibited the highest degradation rate followed by milled Fe and as received Fe, in this order. The analysis of degradation products of Fe5Mg showed that they consist on magnesium ferrite and pyroaurite, which are known to present good biocompatibility and low toxicity. Differences in structural features and degradation behaviors of milled Fe and milled Fe5Mg suggest the effective dissolution of Mg in the Fe lattice. Based on the obtained results, it can be said that Fe5Mg powder would be a suitable candidate for non-permanent medical implants with a higher degradation rate than Fe.

Rapid, Energy-Efficient and Pseudomorphic Microwave-Induced-Metal-Plasma (MIMP) Synthesis of Mg2Si and Mg2Ge

Polycrystalline magnesium silicide, Mg2Si and magnesium germanide, Mg2Ge were synthesised from the elemental powders via the microwave-induced-metal-plasma (MIMP) approach at 200 W within 1 min in vacuo for the first...

Production and Characterization of a 316L Stainless Steel/β-TCP Biocomposite Using the Functionally Graded Materials (FGMs) Technique for Dental and Orthopedic Applications

Metallic biomaterials are widely used for implants and dental and orthopedic applications due to their good mechanical properties. Among all these materials, 316L stainless steel has gained special attention, because of its good characteristics as an implantable biomaterial. However, the Young’s modulus of this metal is much higher than that of human bone (~193 GPa compared to 5–30 GPa). Thus, a stress shielding effect can occur, leading the implant to fail. In addition, due to this difference, the bond between implant and surrounding tissue is weak. Already, calcium phosphate ceramics, such as beta-tricalcium phosphate, have shown excellent osteoconductive and osteoinductive properties. However, they present low mechanical strength. For this reason, this study aimed to combine 316L stainless steel with the beta-tricalcium phosphate ceramic (β-TCP), with the objective of improving the steel’s biological performance and the ceramic’s mechanical strength. The 316L stainless steel/β-TCP biocomposites were produced using powder metallurgy and functionally graded materials (FGMs) techniques. Initially, β-TCP was obtained by solid-state reaction using powders of calcium carbonate and calcium phosphate. The forerunner materials were analyzed microstructurally. Pure 316L stainless steel and β-TCP were individually submitted to temperature tests (1000 and 1100 °C) to determine the best condition. Blended compositions used to obtain the FGMs were defined as 20% to 20%. They were homogenized in a high-energy ball mill, uniaxially pressed, sintered and analyzed microstructurally and mechanically. The results indicated that 1100 °C/2 h was the best sintering condition, for both 316L stainless steel and β-TCP. For all individual compositions and the FGM composite, the parameters used for pressing and sintering were appropriate to produce samples with good microstructural and mechanical properties. Wettability and hemocompatibility were also achieved efficiently, with no presence of contaminants. All results indicated that the production of 316L stainless steel/β-TCP FGMs through PM is viable for dental and orthopedic purposes.

Tailoring grain sizes of the biodegradable iron-based alloys by pre-additive manufacturing microalloying

We demonstrated the design of pre-additive manufacturing microalloying elements in tuning the microstructure of iron (Fe)-based alloys for their tunable mechanical properties. We tailored the microalloying stoichiometry of the feedstock to control the grain sizes of the metallic alloy systems. Two specific microalloying stoichiometries were reported, namely biodegradable iron powder with 99.5% purity (BDFe) and that with 98.5% (BDFe-Mo). Compared with the BDFe, the BDFe-Mo powder was found to have lower coefficient of thermal expansion (CTE) value and better oxidation resistance during consecutive heating and cooling cycles. The selective laser melting (SLM)-built BDFe-Mo exhibited high ultimate tensile strength (UTS) of 1200 MPa and fair elongation of 13.5%, while the SLM-built BDFe alloy revealed a much lower UTS of 495 MPa and a relatively better elongation of 17.5%, indicating the strength enhancement compared with the other biodegradable systems. Such an enhanced mechanical behavior in the BDFe-Mo was assigned to the dominant mechanism of ferrite grain refinement coupled with precipitate strengthening. Our findings suggest the tunability of outstanding strength-ductility combination by tailoring the pre-additive manufacturing microalloying elements with their proper concentrations.

Microstructure, Mechanical Properties, and in Vitro Corrosion Behavior of Biodegradable Zn-1Fe-xMg Alloy

Zinc (Zn), one of the promising candidates for biodegradable implant materials, has excellent biocompatibility and biodegradability. In this study, as-cast Zn1FexMg (x ≤ 1.5 wt %) alloys were prepared to systematically explore the effects of magnesium (Mg) alloying on their microstructures, mechanical properties, and biodegradability. The microstructure of Zn1FexMg alloy consisted of Zn matrix, Zn + Mg₂Zn₁₁ eutectic structure, and FeZn₁₃ phase. The addition of Mg not only promoted grain refinement of the alloy, but also improved its mechanical properties. The results of immersion tests showed that the addition of Mg accelerated microcell corrosion between different phases, and the modeling of the corrosion mechanism of alloys in simulated body fluid (SBF) solution was discussed to describe the interaction between different phases in the corrosion process. Zn1Fe1Mg possessed superior comprehensive mechanical properties and appropriate corrosion rate, and the values for hardness, tensile strength, yield strength, elongation, and corrosion rate were 105 HB, 157 MPa, 146 MPa, 2.3%, and 0.027 mm/a, respectively, thus revealing that Zn1Fe1Mg is a preferred candidate for biodegradable implant material.

Hydrolytic Expansion Induces Corrosion Propagation for Increased Fe Biodegradation

Fe is regarded as a promising bone implant material due to inherent degradability and high mechanical strength, but its degradation rate is too slow to match the healing rate of bone. In this work, hydrolytic expansion was cleverly exploited to accelerate Fe degradation. Concretely, hydrolyzable Mg2Si was incorporated into Fe matrix through selective laser melting and readily hydrolyzed in a physiological environment, thereby exposing more surface area of Fe matrix to the solution. Moreover, the gaseous hydrolytic products of Mg2Si acted as an expanding agent and cracked the dense degradation product layers of Fe matrix, which offered rapid access for solution invasion and corrosion propagation toward the interior of Fe matrix. This resulted in the breakdown of protective degradation product layers and even the direct peeling off of Fe matrix. Consequently, the degradation rate for Fe/Mg2Si composites (0.33 mm/y) was significantly improved in comparison with that of Fe (0.12 mm/y). Meanwhile, Fe/Mg2Si composites were found to enable the growth and proliferation of MG-63 cells, showing good cytocompatibility. This study indicated that hydrolytic expansion may be an effective strategy to accelerate the degradation of Fe-based implants.

Highly biodegradable and bioactive Fe-Pd-bredigite biocomposites prepared by selective laser melting

Iron (Fe) has been highly anticipated as a bone implant material owing to the biodegradability and excellent mechanical properties, but limited by the slow degradation and poor bioactivity. In this study, novel Fe-palladium (Pd)-bredigite biocomposites were developed by selective laser melting aiming to improve both the degradation behavior and bioactivity of Fe. The results showed that most Pd formed Pd-rich intermetallic phases (IMPs) with a nearly continuous network while the bredigite phase was distributed at the grain boundaries. In addition, a large amount of much nobler IMPs formed micro-galvanic pairs with the Fe matrix, inducing tremendous micro-galvanic corrosion. The IMPs contained a high amount of Pd2+ with a high reduction potential, which further promoted the efficiency of micro-galvanic corrosion. Moreover, the rapid degradation of bredigite also facilitated the penetration of the corrosion medium. As a result, the Fe-4Pd-5bredigite biocomposite showed a uniform degradation with a rate that is 6 times that of Fe. Furthermore, the developed Fe-Pd-bredigite biocomposites also featured excellent bioactivity, cytocompatibility, and suitable mechanical properties as characterized by the rapid apatite deposition, normal proliferation of human osteoblast-like cells (MG-63), and comparable strength and microhardness with the native bone. Overall, this study opens a new avenue for improving both the degradation and bioactivity of Fe-based composites and may facilitate their applications as biodegradable implants for tissue/organ repair.

Zinc-based alloys for degradable vascular stent applications

The search for biodegradable metals with mechanical properties equal or higher to those of currently used permanent biomaterials, such as stainless steels, cobalt chromium and titanium alloys, desirable in vivo degradation rate and uniform corrosion is still an open challenge. Magnesium (Mg), iron (Fe) and zinc (Zn)-based alloys have been proposed as biodegradable metals for medical applications. Over the last two decades, extensive research has been done on Mg and Fe. Fe-based alloys show appropriate mechanical properties, but their degradation rate is an order of magnitude below the benchmark value. In comparison, alongside the insufficient mechanical performance of most of its alloys, Mg degradation rate has proven to be too high in a physiological environment and corrosion is rarely uniform. During the last few years, Zn alloys have been explored by the biomedical community as potential materials for bioabsorbable vascular stents due to their tolerable corrosion rates and tunable mechanical properties. This review summarizes recent progress made in developing Zn alloys for vascular stenting application. Novel Zn alloys are discussed regarding their microstructural characteristics, mechanical properties, corrosion behavior and in vivo performance.

STATEMENT OF SIGNIFICANCE

Numerous studies on magnesium and iron materials have been reported to date, in an effort to formulate bioabsorbable stents with tailorable mechanical characteristics and corrosion behavior. Crucial concerns regarding poor ductility and remarkably rapid corrosion of magnesium, and very slow degradation of iron, seem to be still not desirably fulfilled. Zinc was introduced as a potential implant material in 2013 due to its promising biodegradability and biocompatibility. Since then, extensive investigations have been made toward development of zinc alloys that meet clinical benchmarks for vascular scaffolding. This review critically surveys the zinc alloys developed since 2013 from metallurgical and biodegradation points of view. Microstructural features, mechanical, corrosion and in vivo performances of these new alloys are thoroughly reviewed and evaluated.

Long-term in vitro degradation behaviour of Fe and Fe/Mg2Si composites for biodegradable implant applications

The major drawback of Fe-based materials for biodegradable implant applications is their slow degradation rate. Addition of second phase particles into the Fe matrix can increase the degradation rate at the beginning of the corrosion process. However, so far, there is neither quantitative data on in vitro degradation nor direct experimental evidence for long-term dissolution of Fe-based biodegradable composites. Here, a series of immersion tests at different exposure intervals (20, 50 and 100 days) to modified Hanks' solution were performed to study the degradation behavior of Fe and Fe/Mg₂Si composites prepared by different powder metallurgy techniques. The results revealed the role of Mg₂Si in the composition and stability of the protective films formed during the static corrosion experiments. Fe/Mg₂Si composites showed higher degradation rates than those of pure Fe at all stages of immersion. Degradation rates at distinct exposure intervals strongly depended on the composition and stability of formed oxide, hydroxide, carbonate and phosphate protective films on the degraded surfaces. The release of Fe ions into the solution at later stages of the experiment was limited due to the barrier effect of the insoluble deposit. This fundamental study provided a basis for the processes of protective film formation in modified Hanks' solution, which enables a detailed identification of its characteristic features.

This fundamental study provides a basis for the processes of protective film formation on degradable Fe-based biomaterials.