Cell-Material Interactions in Direct Contact Culture of Endothelial Cells on Biodegradable Iron-Based Stents Fabricated by Laser Powder Bed Fusion and Impact of Ion Release

Remotely Controlled Electrochemical Degradation of Metallic Implants

Biodegradable medical implants promise to benefit patients by eliminating risks and discomfort associated with permanent implantation or surgical removal. The time until full resorption is largely determined by the implant's material composition, geometric design, and surface properties. Implants with a fixed residence time, however, cannot account for the needs of individual patients, thereby imposing limits on personalization. Here, an active Fe-based implant system is reported whose biodegradation is controlled remotely and in situ. This is achieved by incorporating a galvanic cell within the implant. An external and wireless signal is used to activate the on-board electronic circuit that controls the corrosion current between the implant body and an integrated counter electrode. This configuration leads to the accelerated degradation of the implant and allows to harvest electrochemical energy that is naturally released by corrosion. In this study, the electrochemical properties of the Fe-30Mn-1C/Pt galvanic cell model system is first investigated and high-resolution X-ray microcomputed tomography is used to evaluate the galvanic degradation of stent structures. Subsequently, a centimeter-sized active implant prototype is assembled with conventional electronic components and the remotely controlled corrosion is tested in vitro. Furthermore, strategies toward the miniaturization and full biodegradability of this system are presented.

Inherent Antibacterial Properties of Biodegradable FeMnC(Cu) Alloys for Implant Application

Implant-related infections or inflammation are one of the main reasons for implant failure. Therefore, different concepts for prevention are needed, which strongly promote the development and validation of improved material designs. Besides modifying the implant surface by, for example, antibacterial coatings (also implying drugs) for deterring or eliminating harmful bacteria, it is a highly promising strategy to prevent such implant infections by antibacterial substrate materials. In this work, the inherent antibacterial behavior of the as-cast biodegradable Fe69Mn30C1 (FeMnC) alloy against Gram-negative Pseudomonas aeruginosa and Escherichia coli as well as Gram-positive Staphylococcus aureus is presented for the first time in comparison to the clinically applied, corrosion-resistant AISI 316L stainless steel. In the second step, 3.5 wt % Cu was added to the FeMnC reference alloy, and the microbial corrosion as well as the proliferation of the investigated bacterial strains is further strongly influenced. This leads for instance to enhanced antibacterial activity of the Cu-modified FeMnC-based alloy against the very aggressive, wild-type bacteria P. aeruginosa. For clarification of the bacterial test results, additional analyses were applied regarding the microstructure and elemental distribution as well as the initial corrosion behavior of the alloys. This was electrochemically investigated by a potentiodynamic polarization test. The initial degraded surface after immersion were analyzed by glow discharge optical emission spectrometry and transmission electron microscopy combined with energy-dispersive X-ray analysis, revealing an increase of degradation due to Cu alloying. Due to their antibacterial behavior, both investigated FeMnC-based alloys in this study are attractive as a temporary implant material.

Recent advances in Fe-based bioresorbable stents: Materials design and biosafety

Fe-based materials have received more and more interests in recent years as candidates to fabricate bioresorbable stents due to their appropriate mechanical properties and biocompatibility. However, the low degradation rate of Fe is a serious limitation for such application. To overcome this critical issue, many efforts have been devoted to accelerate the corrosion rate of Fe-based stents, through the structural and surface modification of Fe matrix. As stents are implantable devices, the released corrosion products (Fe2+ ions) in vessels may alter the metabolism, by generating reactive oxygen species (ROS), which might in turn impact the biosafety of Fe-based stents. These considerations emphasize the importance of combining knowledge in both materials and biological science for the development of efficient and safe Fe-based stents, although there are still only limited numbers of reviews regarding this interdisciplinary field. This review aims to provide a concise overview of the main strategies developed so far to design Fe-based stents with accelerated degradation, highlighting the fundamental mechanisms of corrosion and the methods to study them as well as the reported approaches to accelerate the corrosion rates. These approaches will be divided into four main sections, focusing on (i) increased active surface areas, (ii) tailored microstructures, (iii) creation of galvanic reactions (by alloying, ion implantation or surface coating of noble metals) and (iv) decreased local pH induced by degradable surface organic layers. Recent advances in the evaluation of the in vitro biocompatibility of the final materials and ongoing in vivo tests are also provided.

Recent Developments in Metallic Degradable Micromotors for Biomedical and Environmental Remediation Applications

Synthetic micromotor has gained substantial attention in biomedicine and environmental remediation. Metal-based degradable micromotor composed of magnesium (Mg), zinc (Zn), and iron (Fe) have promise due to their nontoxic fuel-free propulsion, favorable biocompatibility, and safe excretion of degradation products Recent advances in degradable metallic micromotor have shown their fast movement in complex biological media, efficient cargo delivery and favorable biocompatibility. A noteworthy number of degradable metal-based micromotors employ bubble propulsion, utilizing water as fuel to generate hydrogen bubbles. This novel feature has projected degradable metallic micromotors for active in vivo drug delivery applications. In addition, understanding the degradation mechanism of these micromotors is also a key parameter for their design and performance. Its propulsion efficiency and life span govern the overall performance of a degradable metallic micromotor. Here we review the design and recent advancements of metallic degradable micromotors. Furthermore, we describe the controlled degradation, efficient in vivo drug delivery, and built-in acid neutralization capabilities of degradable micromotors with versatile biomedical applications. Moreover, we discuss micromotors' efficacy in detecting and destroying environmental pollutants. Finally, we address the limitations and future research directions of degradable metallic micromotors.

Additive manufacturing of vascular stents

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

FeMn and FeMnAg biodegradable alloys: An in vitro and in vivo investigation

Iron-based biodegradable metal bone graft substitutes are in their infancy but promise to fill bone defects that arise after incidents such as trauma and revision arthroplasty surgery. Before clinical use however, a better understanding of their in vivo biodegradability, potential cytotoxicity and biocompatibility is required. In addition, these implants must ideally be able to resist infection, a complication of any implant surgery. In this study there was significant in vitro cytotoxicity caused by pure Fe, FeMn, FeMn1Ag and FeMn5Ag on both human foetal osteoblast (hFOB) and mouse pre-osteoblast (MC3T3-E1) cell lines. In vivo experiments on the other hand showed no signs of ill-effect on GAERS rats with the implanted FeMn, FeMn1Ag and FeMn5Ag pins being removed largely uncorroded. All Fe-alloys showed anti-bacterial performance but most markedly so in the Ag-containing alloys, there is significant bacterial resistance in vitro.

FeMn with Phases of a Degradable Ag Alloy for Residue-Free and Adapted Bioresorbability

The development of bioresorbable materials for temporary implantation enables progress in medical technology. Iron (Fe)-based degradable materials are biocompatible and exhibit good mechanical properties, but their degradation rate is low. Aside from alloying with Manganese (Mn), the creation of phases with high electrochemical potential such as silver (Ag) phases to cause the anodic dissolution of FeMn is promising. However, to enable residue-free dissolution, the Ag needs to be modified. This concern is addressed, as FeMn modified with a degradable Ag-Calcium-Lanthanum (AgCaLa) alloy is investigated. The electrochemical properties and the degradation behavior are determined via a static immersion test. The local differences in electrochemical potential increase the degradation rate (low pH values), and the formation of gaps around the Ag phases (neutral pH values) demonstrates the benefit of the strategy. Nevertheless, the formation of corrosion-inhibiting layers avoids an increased degradation rate under a neutral pH value. The complete bioresorption of the material is possible since the phases of the degradable AgCaLa alloy dissolve after the FeMn matrix. Cell viability tests reveal biocompatibility, and the antibacterial activity of the degradation supernatant is observed. Thus, FeMn modified with degradable AgCaLa phases is promising as a bioresorbable material if corrosion-inhibiting layers can be diminished.

The Study on Resolution Factors of LPBF Technology for Manufacturing Superelastic NiTi Endodontic Files

Laser Powder Bed Fusion (LPBF) technology is a new trend in manufacturing complex geometric structures from metals. This technology allows producing topologically optimized parts for aerospace, medical and industrial sectors where a high performance-to-weight ratio is required. Commonly the feature size for such applications is higher than 300-400 microns. However, for several possible applications of LPBF technology, for example, microfluidic devices, stents for coronary vessels, porous filters, dentistry, etc., a significant increase in the resolution is required. This work is aimed to study the resolution factors of LPBF technology for the manufacturing of superelastic instruments for endodontic treatment, namely Self-Adjusting Files (SAF). Samples of thin walls with different incline angles and SAF samples were manufactured from Nickel-Titanium pre-alloyed powder with a 15-45 μm fraction. The printing procedure was done using an LPBF set-up equipped with a conventional ytterbium fiber laser with a nominal laser spot diameter of 55 microns. The results reveal physical, apparatus, and software factors limiting the resolution of the LPBF technology. Additionally, XRD and DSC tests were done to study the effect of single track based scanning mode manufacturing on the phase composition and phase transformation temperatures. Found combination of optimal process parameters including laser power of 100 W, scanning speed of 850 mm/s, and layer thickness of 20 μm was suitable for manufacturing SAF files with the required resolution. The results will be helpful for the production of NiTi micro objects based on periodic structures both by the LPBF and μLPBF methods.

Preparation and Properties of Iron Nanoparticle-Based Macroporous Scaffolds for Biodegradable Implants

Fe-based scaffolds are of particular interest in the technology of biodegradable implants due to their high mechanical properties and biocompatibility. In the present work, using an electroexplosive Fe nanopowder and NaCl particles 100–200 µm in size as a porogen, scaffolds with a porosity of about 70 ± 0.8% were obtained. The effect of the sintering temperature on the structure, composition, and mechanical characteristics of the scaffolds was considered. The optimum parameters of the sintering process were determined, allowing us to obtain samples characterized by plastic deformation and a yield strength of up to 16.2 MPa. The degradation of the scaffolds sintered at 1000 and 1100 °C in 0.9 wt.% NaCl solution for 28 days resulted in a decrease in their strength by 23% and 17%, respectively.

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.