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

Magnesium-based bioresorbable flow diverter for intracranial aneurysms: a pilot study of biocompatibility and bioresorption in a rabbit vascular model

BACKGROUND

Bioresorbable flow diverters (BRFDs) have the potential to solve several problems associated with conventional permanent flow diverters. We have constructed bare and poly-L-lactic acid (PLLA)-coated magnesium BRFDs (MgBRFDs) using a high-strength corrosion-resistant magnesium alloy. This study aimed to compare bioresorption and biocompatibility between the two types in a rabbit vascular model to determine which is more clinically feasible in humans.

METHODS

Bare and PLLA-coated MgBRFDs were fabricated by braiding 48 thin magnesium alloy wires. Mechanical testing was conducted. Bare (n=13) and PLLA-coated (n=13) MgBRFDs were implanted into rabbit aortas and harvested 14, 30, and 90 days after implantation. The physical structure of the resolution process was examined using optical coherence tomography (OCT), micro-computed tomography, and scanning electron microscopy (SEM). The biological response of the vascular tissue was examined using SEM and histopathological analysis.

RESULTS

The porosity and pore density of the bare MgBRFD were 64% and 16 pores/mm2, respectively; corresponding values for the PLLA-coated MgBRFD were 63% and 12 pores/mm2, respectively. The OCT attenuation score was significantly higher for the PLLA-coated MgBRFD at all time points (14 days, P=0.01; 30 days, P=0.02; 90 days, P=0.004). OCT, micro-computed tomography, and SEM demonstrated better stent structure preservation with the PLLA-coated MgBRFD. Neointimal thickness did not significantly change over time in either type of MgBRFD (bare, P=0.93; PLLA-coated, P=0.34); however, the number of inflammatory and proliferative cells peaked at 14 days and then decreased.

CONCLUSIONS

Both bare and PLLA-coated MgBRFDs had excellent biocompatibility. The PLLA-coated MgBRFD has greater clinical feasibility because of its delayed bioresorption.

Semi-quantitative elemental imaging of corrosion products from bioabsorbable Mg vascular implants in vivo

While metal materials historically have served as permanent implants and were designed to avoid degradation, next generation bioabsorbable metals for medical devices such as vascular stents are under development, which would elute metal ions and corrosion byproducts into tissues. The fate of these eluted products and their local distribution in vascular tissue largely under studied. In this study, we employ a high spatial resolution spectrometric imaging modality, laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOF-MS) to map the metal distribution, (herein refered to as laser ablation mapping, or LAM) from Mg alloys within the mouse vascular system and approximate their local concentrations. We used a novel rare earth element bearing Mg alloy (WE22) wire implanted within the abdominal aorta of transgenic hypercholesterolemic mice (APOE-/-) to simulate a bioabsorbable vascular prosthesis for up to 30 days. We describe qualitatively and semi-quantitatively implant-derived corrosion product presence throughout the tissue cross sections, and their approximate concentrations within the various vessel structures. Additionally, we report the spatial changes of corrosion products, which we postulate are mediated by phagocytic inflammatory cells such as macrophages (MΦ's).

Progress in research and development of biodegradable metallic vascular stents

Vascular stents are an essential tool in cardiovascular interventional therapy, and their demand is growing with the increasing incidence of cardiovascular diseases. Compared with permanent stents, which are prone to in-stent restenosis, and drug-eluting stents, which may cause late stent thrombosis, biodegradable stents offer advantages. After providing early radial support to prevent elastic recoil, biodegradable stents gradually degrade, allowing the vessel to regain its natural physiological contractility and undergo positive remodeling. A review of the current mainstream biodegradable metal stents, magnesium-based, iron-based, and zinc-based alloys, shows promising findings in both preclinical and clinical research. Magnesium-based stents exhibit good operability and low thrombosis rates, but their limitations include rapid degradation, hydrogen evolution, and significant pH changes in the microenvironment. Iron-based stents demonstrate excellent mechanical strength, formability, biocompatibility, and hemocompatibility, but their slow corrosion rate hampers broader clinical application; accelerating degradation remains key. Zinc-based alloys have a moderate degradation rate but relatively low mechanical strength; enhancing stent strength by alloying with other elements is the main improvement direction for zinc-based stents.

Safety and efficacy of Mg-Dy membrane with poly-L-lactic acid coating for guided bone regeneration

The aim of this study was to evaluate safety and efficacy of a poly-L-lactic acid (PLLA)-coated magnesium (Mg)-Dysprosium (Dy) membrane in guided bone regeneration (GBR) using a rabbit calvarium model. The microstructure of the Mg-Dy membrane surface and thickness of the PLLA coating were examined. In vitro degradation and cytotoxicity test was conducted. The in vivo study used 24 white male rabbits with two 8 mm-diameter defects created on the calvaria; 12 defects were randomly assigned per group: (1) Negative control, (2) positive control, (3) uncoated Mg, and (4) PLLA-coated Mg group. Specimens were harvested at 4, 8, and 12 weeks postoperatively for radiological, histological, and histomorphometric analyses. The PLLA-coated Mg-Dy membrane showed a low degree of degradation, indicating that the coating exerted a protective effect. In the cytotoxicity test, no deformed or degenerated cells were observed. In the in vivo study, radiographic and histomorphometric analyses indicated favorable new bone formation and maintenance of the graft material for PLLA-coated Mg group. PLLA-coated Mg group, compared to the uncoated counterpart, restored the bony contour more completely, without inducing significant inflammatory response. Our results support the safety and efficacy of PLLA-coated Mg-Dy membranes for GBR both in vitro and in vivo.

Degradation of a novel magnesium alloy-based bioresorbable coronary scaffold in a swine coronary artery model

The objective of the study is to investigate the safety, feasibility, and degradation profile of a novel Mg alloy-based bioresorbable coronary scaffold (JFK-PRODUCT BRS) with thin struts (110 μm). Polymer- or Mg alloy-based BRSs have not replaced nondegradable metal stents because of the higher prevalence of scaffold thrombosis and restenosis in clinical practice; these poor clinical outcomes were due to inadequate scaffold designs, including thick struts (more than 150 μm) and their inappropriate degradation processes. Fourteen healthy pigs received 17 JFK-PRODUCT BRSs in the coronary arteries and were sacrificed at 1, 6, 12, 18, and 26 months after implantation. Angiography, optical coherence tomography, microfocus X-ray computed tomography (µCT), scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDX), and histopathological evaluation were performed. The JFK-PRODUCT had a median percent late recoil of 11.28% at 1 month. The µCT observation confirmed that scaffold discontinuity reached 64.8% at 12 months with increased scaffold inner area thereafter, suggesting artery positive remodeling. The inflammation was mild, peaked at 18 months, and decreased thereafter. The SEM-EDX analysis demonstrated gradual degradation of the scaffold with formation of inorganic deposits, presumed to be calcium phosphates. It also revealed the disappearance of calcium phosphates at 26 months, achieving almost complete replacement of the scaffold by biocomponents. The current study demonstrated the safety and feasibility of JFK-PRODUCT with a lower acute recoil rate despite its thin struts. The scaffolds were almost completely disappeared at 26 months after implantation.

Development and Future Trends of Protective Strategies for Magnesium Alloy Vascular Stents

Magnesium alloy stents have been extensively studied in the field of biodegradable metal stents due to their exceptional biocompatibility, biodegradability and excellent biomechanical properties. Nevertheless, the specific in vivo service environment causes magnesium alloy stents to degrade rapidly and fail to provide sufficient support for a certain time. Compared to previous reviews, this paper focuses on presenting an overview of the development history, the key issues, mechanistic analysis, traditional protection strategies and new directions and protection strategies for magnesium alloy stents. Alloying, optimizing stent design and preparing coatings have improved the corrosion resistance of magnesium alloy stents. Based on the corrosion mechanism of magnesium alloy stents, as well as their deformation during use and environmental characteristics, we present some novel strategies aimed at reducing the degradation rate of magnesium alloys and enhancing the comprehensive performance of magnesium alloy stents. These strategies include adapting coatings for the deformation of the stents, preparing rapid endothelialization coatings to enhance the service environment of the stents, and constructing coatings with self-healing functions. It is hoped that this review can help readers understand the development of magnesium alloy cardiovascular stents and solve the problems related to magnesium alloy stents in clinical applications at the early implantation stage.

Adaptation of Vascular Smooth Muscle Cell to Degradable Metal Stent Implantation

Iron-, magnesium-, or zinc-based metal vessel stents support vessel expansion at the period early after implantation and degrade away after vascular reconstruction, eliminating the side effects due to the long stay of stent implants in the body and the risks of restenosis and neoatherosclerosis. However, emerging evidence has indicated that their degradation alters the vascular microenvironment and induces adaptive responses of surrounding vessel cells, especially vascular smooth muscle cells (VSMCs). VSMCs are highly flexible cells that actively alter their phenotype in response to the stenting, similarly to what they do during all stages of atherosclerosis pathology, which significantly influences stent performance. This Review discusses how biodegradable metal stents modify vascular conditions and how VSMCs respond to various chemical, biological, and physical signals attributable to stent implantation. The focus is placed on the phenotypic adaptation of VSMCs and the clinical complications, which highlight the importance of VSMC transformation in future stent design.

A Novel PLLA/MgF2 Coating on Mg Alloy by Ultrasonic Atomization Spraying for Controlling Degradation and Improving Biocompatibility

Problems of rapid degradation and poor biocompatibility (endothelialization and hemocompatibility) limit magnesium (Mg) alloy's further applications in vascular stents. To solve these problems, a novel composite coating was designed on Mg alloy via a two-step method. First, a Mg alloy sample was immersed in hydrofluoric acid. Then, a poly-l-lactic acid (PLLA) coating was made by ultrasonic atomization spraying with 5 and 10 layers (referred to as PLLA(5)-HF-Mg and PLLA(10)-HF-Mg). Characterizations were analyzed from the microstructure, element distribution, and wettability. The degradation behavior was tested with an electrochemical test and immersion test. Endothelialization was investigated using human umbilical vein endothelial cells (HUVECs). Hemocompatibility was examined with a platelet adhesion test. The results showed that the PLLA coating could not only cover the surface, but also could permeate through and cover the holes on the MgF₂ layer, mechanically locked with the substrate. Thus, the composite coating had higher corrosion resistance. The PLLA/MgF₂ coating, especially on PLLA(10)-HF-Mg, enhanced HUVECs' viability and growth. While incubated with platelets, the PLLA/MgF₂ coating, especially on PLLA(10)-HF-Mg, had the lowest platelet adhesion number and activity. Taken together, the novel PLLA/MgF₂ coating controls Mg alloy's degradation by spraying different layers of PLLA, resulting in better endothelialization and hemocompatibility, providing a promising candidate for cardiovascular stents.

The bioresorbable magnesium scaffold (Magmaris)-State of the art: From basic concept to clinical application

Since its introduction to clinical practice, coronary artery stent implantation has become a crucial part of the therapy of coronary artery disease (CAD). Despite the undeniable evolution of percutaneous coronary revascularization procedures, drug-eluting stent (DES) technology shows some limitations. To overcome these limitations bioresorbable vascular scaffolds (BRS) were designed as a vessel-supporting technology allowing for anatomical and functional restoration of the vessel after the scaffold intended resorption. Various materials have been proposed as the basis of the scaffold backbone. In this narrative review, we present second-generation magnesium-alloy bioresorbable scaffold devices (Magmaris; Biotronik). Additionally, we discuss available preclinical and clinical data regarding this new magnesium BRS.

Drug-Eluting, Bioresorbable Cardiovascular Stents─Challenges and Perspectives

Globally, the leading causes of natural death are attributed to coronary heart disease and type 1 and type 2 diabetes. High blood pressure levels, high cholesterol levels, smoking, and poor eating habits lead to the agglomeration of plaque in the arteries, reducing the blood flow. The implantation of devices used to unclog vessels, known as stents, sometimes results in a lack of irrigation due to the excessive proliferation of endothelial tissue within the blood vessels and is known as restenosis. The use of drug-eluting stents (DESs) to deliver antiproliferative drugs has led to the development of different encapsulation techniques. However, due to the potency of the drugs used in the initial stent designs, a chronic inflammatory reaction of the arterial wall known as thrombosis can cause a myocardial infarction (MI). One of the most promising drugs to reduce this risk is everolimus, which can be encapsulated in lipid systems for controlled release directly into the artery. This review aims to discuss the current status of stent design, fabrication, and functionalization. Variables such as the mechanical properties, metals and their alloys, drug encapsulation and controlled elution, and stent degradation are also addressed. Additionally, this review covers the use of polymeric surface coatings on stents and the recent advances in layer-by-layer coating and drug delivery. The advances in nanoencapsulation techniques such as liposomes and micro- and nanoemulsions and their functionalization in bioresorbable, drug-eluting stents are also highlighted.

Current status and outlook of biodegradable metals in neuroscience and their potential applications as cerebral vascular stent materials

Over the past two decades, biodegradable metals (BMs) have emerged as promising materials to fabricate temporary biomedical devices, with the purpose of avoiding potential side effects of permanent implants. In this review, we first surveyed the current status of BMs in neuroscience, and briefly summarized the representative stents for treating vascular stenosis. Then, inspired by the convincing clinical evidence on the in vivo safety of Mg alloys as cardiovascular stents, we analyzed the possibility of producing biodegradable cerebrovascular Mg alloy stents for treating ischemic stroke. For these novel applications, some key factors should also be considered in designing BM brain stents, including the anatomic features of the cerebral vasculature, hemodynamic influences, neuro-cytocompatibility and selection of alloying elements. This work may provide insights into the future design and fabrication of BM neurological devices, especially for brain stents.

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The current status of the application of biodegradable metals (BM) in neuroscience was presented.

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We analyzed the possibility of producing biodegradable cerebrovascular Mg alloy stents for ischemic stroke treatment.

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Key factors in designing BM brain stents were discussed.

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This work may provide insights into the future design and fabrication of BM neurological devices, especially for brain stents.

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.