Macrophage phagocytosis of biomedical Mg alloy degradation products prepared by electrochemical method

Advances in biodegradable materials: Degradation mechanisms, mechanical properties, and biocompatibility for orthopedic applications

Mg-based and Zn-based biodegradable materials have the potential to become the next-generation implant materials to treat bone diseases, because of their desired degradation and mechanical properties. This article reviews the status of these implant materials. The required properties of biodegradable materials such as biodegradability, mechanical properties, and biocompatibility for performance evaluation were briefly discussed. The influence of fabrication techniques, microstructure, alloying elements, and post-processing techniques on the properties of Mg and Zn-based materials was addressed. The degradation mechanism by dissolution, oxidation, and interaction with human body cells was discussed. The biocompatibility of Mg and Zn-based biodegradable materials was analyzed. The significance of in vitro and in vivo biocompatibility testing was highlighted, emphasizing the superiority of in vivo results over cell line studies. This article identifies the many Mg and Zn-based biodegradable materials and summarizes the key findings.

Synchrotron microtomography reveals insights into the degradation kinetics of bio-degradable coronary magnesium scaffolds

Bioresorbable magnesium scaffolds are a promising future treatment option for coronary artery stenosis, especially for young adults. Due to the degradation of these scaffolds (<1 year), long-term device-related clinical events could be reduced compared to treatments with conventional drug eluting stents. First clinical trials indicate a return of vasomotion after one year, which may be associated with improved long-term clinical outcomes. However, even after decades of development, the degradation process, ideal degradation time and biological response in vivo are still not fully understood. The present study investigates the in vivo degradation of magnesium scaffolds in the coronary arteries of pigs influenced by different strut thicknesses and the presence of antiproliferative drugs. Due to high 3D image contrast of synchrotron-based micro-CT with phase contrast (SR-μCT), a qualitative and quantitative evaluation of the degradation morphology of magnesium scaffolds was obtained. For the segmentation of the μCT images a convolutional network architecture (U-net) was exploited, demonstrating the huge potential of merging high resolution SR-μCT with deep learning (DL) supported data analysis. In total, 30 scaffolds, made of the rare earth alloy Resoloy®, with different strut designs were implanted into the coronary arteries of 10 domestic pigs for 28 days using drug-coated or uncoated angioplasty balloons for post-dilatation. The degradation morphology was analyzed using scanning electron microscopy, energy dispersive x-ray spectroscopy and SR-μCT. The data from these methods were then related to data from angiography, optical coherence tomography and histology. A thinner strut size (95 vs. 130 μm) and the presence of paclitaxel indicated a slower degradation rate at 28 d in vivo, which positively influences the late lumen loss (0.5 and 0.6 mm vs. 1.0 and 1.1 mm) and recoil values (0 and 1.7% vs. 6.1 and 22%).

Interaction pathways of implant metal localized corrosion and macrophage inflammatory reactions

Macrophages play a central role in immunological responses to metallic species associated with the localized corrosion of metallic implants, and mediating in peri-implant inflammations. Herein, the pathways of localized corrosion-macrophage interactions were systematically investigated on 316L stainless steel (SS) implant metals. Electrochemical monitoring under macrophage-mediated inflammatory conditions showed a decreased pitting corrosion resistance of 316L SSs in the presence of RAW264.7 cells as the cells would disrupt biomolecule adsorbed layer on the metal surface. The pitting potentials were furtherly decreased when the RAW264.7 cells were induced to the M1 pro-inflammatory phenotype by the addition of lipopolysaccharide (LPS), and pitting corrosion preferentially initiated at the peripheries of macrophages. The overproduction of aggressive ROS under inflammatory conditions would accelerate the localized corrosion of 316L SS around macrophages. Under pitting corrosion condition, the viability and pro-inflammatory polarization of RAW264.7 cells were region-dependent, lower viability and more remarkable morphology transformation of macrophages in the pitting corrosion region than the pitting-free region. The pitting corrosion of 316L SS induced high expression of CD86, TNF-α, IL-6 and high level of intracellular ROS in macrophages. Uneven release of metallic species (Fe2+, Cr3+, Ni2+, etc) and uneven distribution of surface overpotential stimulated macrophage inflammatory responses near the corrosion pits. A synergetic effect of localized corrosion and macrophages was revealed, which could furtherly promote localized corrosion of 316L SS and macrophage inflammatory reactions. Our results provided direct evidence of corrosion-macrophage interaction in metallic implants and disclosed the pathways of this mutual stimulation effect.

Effect of unsoluble corrosion products of WE43 alloys in vitro on macrophages

Magnesium alloys have been used to manufacture biodegradable implants, bone graft substitutes, and cardiovascular stents. WE43 was the most widely used magnesium alloy. The degradation process begins when the magnesium alloy stent is implanted in the body and comes into contact with body fluid. The degradation products include hydrogen, Mg2+ , local alkaline environment, and unsoluble products. A large number of studies focused on Mg2+ and pH in vitro, and in vivo of magnesium alloys, but few studies on unsoluble corrosion products (UCPs). In this study, UCPs of WE43 alloy were prepared by immersion in vitro, and their effects on macrophages were investigated. The results showed that the unsoluble corrosion products were Mg24Y5, Mg12YNd, and MgCO3 ·3H2 O, which were dose-dependent on the apoptosis and necrosis of macrophages. After phagocytosis of UCPs, macrophages mainly metabolize in lysosome, and autophagy also participates in the metabolism of UCPs. It also decreases mitochondrial membrane potential and increases lysosomes, endoplasmic reticulum stress, and P2X7 receptor activation. These will increase reactive oxygen species (ROS) in cells, activating NLRP3 inflammatory corpuscles, activating the downstream pro-IL18 and pro-IL1β, and converting it to IL-18, and IL-1β. However, its pro-inflammatory effect is far lower than that of the classical Lipopolysaccharide (LPS) pro-inflammatory pathway. This work has increased our understanding of magnesium alloy metabolism and provides new ideas for the clinical application of magnesium alloys.

3D-Printed Double-Helical Biodegradable Iron Suture Anchor: A Rabbit Rotator Cuff Tear Model

Suture anchors are extensively used in rotator cuff tear surgery. With the advancement of three-dimensional printing technology, biodegradable metal has been developed for orthopedic applications. This study adopted three-dimensional-printed biodegradable Fe suture anchors with double-helical threads and commercialized non-vented screw-type Ti suture anchors with a tapered tip in the experimental and control groups, respectively. The in vitro study showed that the Fe and Ti suture anchors exhibited a similar ultimate failure load in 20-pound-per-cubic-foot polyurethane foam blocks and rabbit bone. In static immersion tests, the corrosion rate of Fe suture anchors was 0.049 ± 0.002 mm/year. The in vivo study was performed on New Zealand white rabbits and SAs were employed to reattach the ruptured supraspinatus tendon. The in vivo ultimate failure load of the Fe suture anchors was superior to that of the Ti suture anchors at 6 weeks. Micro-computed tomography showed that the bone volume fraction and bone surface density in the Fe suture anchors group 2 and 6 weeks after surgery were superior, and the histology confirmed that the increased bone volume around the anchor was attributable to mineralized osteocytes. The three-dimensional-printed Fe suture anchors outperformed the currently used Ti suture anchors.

In vitro and in vivo evaluation of a novel bioresorbable magnesium scaffold with different surface modifications

The novel Resoloy® rare earth magnesium alloy was developed for bioresorbable vascular implant application, as an alternative to the WE43 used in Biotronik's Magmaris scaffold, which received CE approval in 2016. Initially, the Magmaris showed very promising preclinical and clinical results, but the formation of an unexpected conversion product and a too fast loss of integrity has proven to be a flaw. The safety and efficacy of Resoloy, which is intended to be bioresorbed without any remnants, has been investigated in an in vitro degradation study and a porcine coronary animal model. Four different groups of scaffolds composed of Resoloy (Res) as the backbone material and additionally equipped with a fluoride passivation layer (Res‐F), a polyester topcoat (Res‐P), or a duplex layer composed of a fluoride passivation layer and a polymeric topcoat (Res‐PF) were compared to a Magmaris scaffold in an in vitro degradation test. Preclinical safety and efficacy of Res‐F and Res‐PF were subsequently evaluated in a coronary porcine model for 12 and 28 days. Scanning electron microscope, quantitative coronary angiography, micro‐computed tomography, histopathology, and histomorphometry analyses were conducted to evaluate preclinical parameters and degradation behavior of the scaffolds. Res‐PF with a duplex layer shows the slowest degradation and the longest supporting force of all test groups. The in vitro data are confirmed by the results of the in vivo study, in which Res‐PF exhibited a longer supporting force than Res‐F, but also caused higher neointima formation. Both studied groups showed excellent biocompatibility. A starter colonization of the strut area with cells during bioresorption was observed. The in vitro degradation test shows that a combination of MgF₂ passivation and a PLLA topcoat on a Resoloy magnesium backbone (Res‐PF) leads to a much slower degradation and a longer support time than a Magmaris control group. In a preclinical study, the safety and efficacy of this duplex layer could be demonstrated. The beginning colonization of the degraded strut area by macrophages can be seen as clear indications that the resorption of the intermediate degradation product takes a different course than that of the Magmaris scaffold.

Effect of degradation products of iron-bioresorbable implants on the physiological behavior of macrophages in vitro

The degradation of bioresorbable metals in vivo changes the physicochemical properties in the environment of an implant, such as a stent in the artery wall, and may induce the alteration of the functions of the surrounding cells. The Fe-degradation, from bioresorbable stents, is a particularly intricate process because it leads to the release of soluble (SDP) and insoluble degradation products (IDP) of varied composition. Macrophages are involved in the resorption of the exogenous agents coming from degradation of these materials. In the present work an Fe⁰ ring, made with a pure Fe wire, in contact with macrophage cell cultures was used to simulate the behaviour of a biodegradable Fe-based implant in a biological environment. Non-invasive time-lapse optical microscopy was applied to obtain images of macrophages exposed to Fe-degradation products, without using staining to avoid distortions and artefacts. It was noticed that as metal degraded, the IDP formed in situ accumulated close to the Fe⁰ ring. In this zone, the macrophages showed a dynamic process of uptake of dark Fe-containing products, confirmed by SEM-EDX. These macrophages showed alterations in the morphology and decrease in the motility and viability. The inability of the macrophages to move and to degrade the engulfed products caused a long persistence of IDP in the zone closest to the metal. The deleterious effects of IDP accumulated close to the ring, were significantly worse than those observed in the experiments made with (1) concentrated salt solutions (Fe³⁺ salt 3 mM), with the same amount of precipitates but uniformly distributed in the well, and (2) diluted salt solutions (Fe³⁺ salt 1 mM) with mainly soluble species. The results were confirmed by standard staining protocols that revealed dead cells close to the Fe⁰ ring and oxidative stress in cells exposed to both soluble and insoluble species.

Interaction of different cell types with magnesium modified by plasma electrolytic oxidation

Magnesium (Mg) is a material widely used in industrial applications due to its low weight, ductility, and excellent mechanical properties. For non-permanent implants, Mg is particularly well-suited because of its biodegradability, while its degradation products are not harmful. However, Mg is chemically reactive, and cytotoxic hydrogen gas is released as part of the degradation. This adverse degradation can be tuned using plasma electrolytic oxidation (PEO). With PEO, a surface layer of MgO/Mg(OH)₂ is deposited on the surface of Mg in a controlled way. The electrolytes used during PEO influence the surface's chemistry and topography and thus expectedly the biological response of adhered cells. In this study, thin samples of commercial pure of Mg (c.p Mg) were modified by PEO guided by different electrolytes, and the biological activity was assessed on vascular cells, immune cells, and repair cells (adipose tissue-derived stromal cells, ASCs). Vascular cells were more vulnerable than ASCs for compounds released by surface-coated Mg. All surface coatings supported the proliferation of adhered ASC. Released compounds from surface-coated Mg delayed but did not block in vitro wound closure of fibroblasts monolayers. Preformed endothelial tubes were vulnerable for released compounds, while their supporting ASC was not. We conclude that PEO-based surface-coating of Mg supports adhesion and future delivery of therapeutic vascular repair cells such as ASC, but that the observed vulnerability of vascular cells for coated Mg components warrants investigations in vivo.

Biodegradable Magnesium-Based Implants in Orthopedics-A General Review and Perspectives

Biodegradable Mg-based metals may be promising orthopedic implants for treating challenging bone diseases, attributed to their desirable mechanical and osteopromotive properties. This Review summarizes the current status and future research trends for Mg-based orthopedic implants. First, the properties between Mg-based implants and traditional orthopedic implants are compared on the following aspects: in vitro and in vivo degradation mechanisms of Mg-based implants, peri-implant bone responses, the fate of the degradation products, and the cellular and molecular mechanisms underlying the beneficial effects of Mg ions on osteogenesis. Then, the preclinical studies conducted at the low weight bearing sites of animals are introduced. The innovative strategies (for example, via designing Mg-containing hybrid systems) are discussed to address the limitations of Mg-based metals prior to their clinical applications at weight-bearing sites. Finally, the available clinical studies are summarized and the challenges and perspectives of Mg-based orthopedic implants are discussed. Taken together, the progress made on the development of Mg-based implants in basic, translational, and clinical research has laid down a foundation for developing a new era in the treatment of challenging and prevalent bone diseases.

The bioeffects of degradable products derived from a biodegradable Mg-based alloy in macrophages via heterophagy

Biodegradable magnesium alloys are promising candidates for use in biomedical applications. However, degradable particles (DPs) derived from Mg-based alloys have been observed in tissue in proximity to sites of implantation, which might result in unexpected effects. Although previous in vitro studies have found that macrophages can take up DPs, little is known about the potential phagocytic pathway and the mechanism that processes DPs in cells. Additionally, it is necessary to estimate the potential bioeffects of DPs on macrophages. Thus, in this study, DPs were generated from a Mg-2.1Nd-0.2Zn-0.5Zr alloy (JDBM) by an electrochemical method, and then macrophages were incubated with the DPs to reveal the potential impact. The results showed that the cell viability of macrophages decreased in a concentration-dependent manner in the presence of DPs due to effects of an apoptotic pathway. However, the DPs were phagocytosed into the cytoplasm of macrophages and further degraded in phagolysosomes, which comprised lysosomes and phagosomes, by heterophagy instead of autophagy. Furthermore, several pro-inflammatory cytokines in macrophages were upregulated by DPs through the induction of reactive oxygen species (ROS) production. To the best of our knowledge, this is the first study to show that DPs derived from a Mg-based alloy are consistently degraded in phagolysosomes after phagocytosis by macrophages via heterophagy, which results in an inflammatory response owing to ROS overproduction. Thus, our research has increased the knowledge of the metabolism of biodegradable Mg metal, which will contribute to an understanding of the health effects of biodegradable magnesium metal implants used for tissue repair. STATEMENT OF SIGNIFICANCE: Biomedical degradable Mg-based alloys have great promise in applied medicine. Although previous studies have found that macrophages can uptake degradable particles (DPs) in vitro and observed in the sites of implantation in vivoin vivo, few studies have been carried out on the potential bioeffects relationship between DPs and macrophages. In this study, we analyzed the bioeffects of DPs derived from a Mg-based alloy on the macrophages. We illustrated that the DPs were size-dependently engulfed by macrophages via heterophagy and further degraded in the phagolysosome rather than autophagosome. Furthermore, DPs were able to induce a slight inflammatory response in macrophages by inducing ROS production. Thus, our research enhances the knowledge of the interaction between DPs of Mg-based alloy and cells, and offers a new perspective regarding the use of biodegradable alloys.

Surface Characterization and Corrosion Resistance of Biomedical AZ31 Mg Alloy Treated by Microarc Fluorination

The application prospect of biodegradable materials is being studied extensively. However, the high corrosion rate and its alloys in body fluids have been major limitations of the application of pure Mg (magnesium). To improve corrosion resistance of biodegradable AZ31 Mg alloy, we adopted microarc fluorination within a voltage range of 100-300 V in 46% hydrofluoric acid. To obtain morphologies, chemical compositions, and structural characteristics, field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were performed, respectively. Results showed that the coating was mainly composed of MgF₂. Electrochemical corrosion and immersion tests proved that the corrosion resistance of MAF-treated AZ31 Mg alloy was significantly improved compared with untreated AZ31 Mg alloy in HBSS (Hank's Balanced Salt Solution). Current densities of AZ31, MAF100, MAF150, MAF200, MAF250, and MAF300 were 342.4, 0.295, 0.228, 0.177, 0.199, and 0.212 μA/cm², respectively. The roughness test indicated that samples under MAF treatment of 200 V, 250 V, and 300 V had large surface roughness. Meanwhile, the contact angle measurement and surface free energy test suggested that those samples had smaller contact angle and higher SFE than Ti. Thus, MAF-treated AZ31 Mg alloy might have promising application in various fields.

Characterization and Biocompatibility of Insoluble Corrosion Products of AZ91 Mg Alloys

Biodegradable Mg alloys have good bioactivity and suitable mechanical properties, which is desirable for application in the clinical fields. However, the biocompatibility of Mg alloys during the degradation process has always been a concern. In this paper, the corrosion behavior of AZ91 alloys in a simulated cell culture environment was studied, especially the insoluble corrosion product during the degradation was characterized, and the biocompatibility of the insoluble corrosion products was evaluated. The results of immersion test showed that the corrosion rate of the AZ91 alloy was low under the condition of high CO2 and humidity, especially in phosphate-buffered saline. Moreover, the insoluble corrosion product was MgCO3·3H2O, which showed a needle-like, circular aggregated, and square-radial feature. Meanwhile, MgCO3·3H2O showed poor cell biocompatibility by increasing the pH and Mg2+ content of the culture solution, which may affect the biocompatibility of Mg alloys.

The degradation and transport mechanism of a Mg-Nd-Zn-Zr stent in rabbit common carotid artery: A 20-month study

Mg-based stent is a promising candidate of the next generation fully degradable vascular stents. The latest progress includes the CE approval of the Magmaris ® WE43 based drug eluting stent. However, so far, the long term (more than 1 year implantation) in vivo degradation and the physiological effects caused by the degradation products were still unclear. In this study, a 20 month observation was carried out after the bare Mg-Nd-Zn-Zr (abbr. JDBM) stent prototype was implanted into the common carotid artery of New Zealand white rabbit in order to evaluate its safety, efficacy and especially degradation behavior. The degradation of the main second phase Mg₁₂Nd was also studied. Results showed that the bare JDBM stent had good safety and efficacy with a complete re-endothelialization within 28 days. The JDBM stent struts were mostly replaced in situ by degradation products in 4 month. The important finding was that the volume and Ca concentration of the degradation products decreased in the long term, eliminating the clinicians' concern of possible vessel calcification. In addition, the alloying elements Mg and Zn in the stent could be safely metabolized as continuous enrichment in any of the main organs were not detected although Nd and Zr showed an abrupt increase in spleen and liver after 1 month implantation. Collectively, the long term in vivo results showed the rapid re-endothelialization of JDBM stent and the long term safety of the degradation products, indicating its great potential as the backbone of the fully degradable vascular stent.

STATEMENT OF SIGNIFICANCE

Mg-based stent is a promising candidate of the next generation fully degradable stents, especially after the recent market launch of one of its kind (Magmaris). However the fundamental question about the long term degradation and metabolic mechanism of Mg-based stent and its degradation products remain unanswered. We implanted our patented Mg-Nd-Zn-Zr bare stent into the common carotid artery of rabbits and conducted a 20 months observation. We found that the Ca containing degradation products could be further degraded in vivo. All the alloying elements showed no continuous enrichment in the main organs of rabbits. These findings eliminate the clinicians' concern of possible vessel calcification and element enrichment after the implantation of Mg alloy based stents to some extent.