Bioresorbable metal scaffold for cardiovascular application: current knowledge and future perspectives

Recent research advances on corrosion mechanism and protection, and novel coating materials of magnesium alloys: a review

Magnesium alloys have achieved a good balance between biocompatibility and mechanical properties, and have great potential for clinical application, and their performance as implant materials has been continuously improved in recent years. However, a high degradation rate of Mg alloys in a physiological environment remains a major limitation before clinical application. In this review, according to the human body's intake of elements, the current mainstream implanted magnesium alloy system is classified and discussed, and the corrosion mechanism of magnesium alloy in vivo and in vitro is described, including general corrosion, localized corrosion, pitting corrosion, and degradation of body fluid environment impact etc. The introduction of methods to improve the mechanical properties and biocorrosion resistance of magnesium alloys is divided into two parts: the alloying part mainly discusses the strengthening mechanisms of alloying elements, including grain refinement strengthening, solid solution strengthening, dislocation strengthening and precipitation strengthening etc.; the surface modification part introduces the ideas and applications of novel materials with excellent properties such as graphene and biomimetic materials in the development of functional coatings. Finally, the existing problems are summarized, and the future development direction is prospected.

Current Status and Outlook of Temporary Implants (Magnesium/Zinc) in Cardiovascular Applications

Medical application materials must meet multiple requirements, and the designed material must mimic the structure, shape. and support the formation of the replacing tissue. Magnesium (Mg) and Zinc alloys (Zn), as a “smart” biodegradable material and as “the green engineering material in the 21st century”, have become an outstanding implant material due to their natural degradability, smart biocompatibility, and desirable mechanical properties. Magnesium and Zinc are recognized as the next generation of cardiovascular stents and bioresorbable scaffolds. At the same time, improving the properties and corrosion resistance of these alloys is an urgent challenge. particularly to promote the application of magnesium alloys. A relatively fast deterioration rate of magnesium-based materials generally results in premature mechanical integrity compromise and local hydrogen build-up, resulting in restricted applicability. This review article aims to give a comprehensive comparison between Zn-based alloys and Mg-based alloys, focusing primarily on degradation and biocompatibility for cardiovascular applications. The recent clinical trials using these biodegradable metals have also been addressed.

Causes, clinical findings and therapeutic options in chylomicronemia syndrome, a special form of hypertriglyceridemia

The prevalence of hypertriglyceridemia has been increasing worldwide. Attention is drawn to the fact that the frequency of a special hypertriglyceridemia entity, named chylomicronemia syndrome, is variable among its different forms. The monogenic form, termed familial chylomicronemia syndrome, is rare, occuring in 1 in every 1 million persons. On the other hand, the prevalence of the polygenic form of chylomicronemia syndrome is around 1:600. On the basis of the genetical alterations, other factors, such as obesity, alcohol consumption, uncontrolled diabetes mellitus and certain drugs may significantly contribute to the development of the multifactorial form. In this review, we aimed to highlight the recent findings about the clinical and laboratory features, differential diagnosis, as well as the epidemiology of the monogenic and polygenic forms of chylomicronemias. Regarding the therapy, differentiation between the two types of the chylomicronemia syndrome is essential, as well. Thus, proper treatment options of chylomicronemia and hypertriglyceridemia will be also summarized, emphasizing the newest therapeutic approaches, as novel agents may offer solution for the effective treatment of these conditions.

The Development of Design and Manufacture Techniques for Bioresorbable Coronary Artery Stents

Coronary artery disease (CAD) is the leading killer of humans worldwide. Bioresorbable polymeric stents have attracted a great deal of interest because they can treat CAD without producing long-term complications. Bioresorbable polymeric stents (BMSs) have undergone a sustainable revolution in terms of material processing, mechanical performance, biodegradability and manufacture techniques. Biodegradable polymers and copolymers have been widely studied as potential material candidates for bioresorbable stents. It is a great challenge to find a reasonable balance between the mechanical properties and degradation behavior of bioresorbable polymeric stents. Surface modification and drug-coating methods are generally used to improve biocompatibility and drug loading performance, which are decisive factors for the safety and efficacy of bioresorbable stents. Traditional stent manufacture techniques include etching, micro-electro discharge machining, electroforming, die-casting and laser cutting. The rapid development of 3D printing has brought continuous innovation and the wide application of biodegradable materials, which provides a novel technique for the additive manufacture of bioresorbable stents. This review aims to describe the problems regarding and the achievements of biodegradable stents from their birth to the present and discuss potential difficulties and challenges in the future.

Biosafety and efficacy evaluation of a biodegradable magnesium-based drug-eluting stent in porcine coronary artery

Although the drug-eluting stent (DES) has become the standard for percutaneous coronary intervention (PCI)-based revascularization, concerns remain regarding the use of DES, mainly due to its permanent rigid constraint to vessels. A drug-eluting bioresorbable stent (BRS) was thus developed as an alternative to DES, which can be absorbed entirely after its therapeutic period. Magnesium (Mg)-based BRSs have attracted a great deal of attention due to their suitable mechanical properties, innovative chemical features, and well-proven biocompatibility. However, the primary disadvantage of Mg-based BRSs is the rapid degradation rate, resulting in the early loss of structural support long before the recovery of vascular function. Recently, a new type of patented Mg-Nd-Zn-Zr alloy (JDBM) was developed at Shanghai Jiao Tong University to reduce the degradation rate compared to commercial Mg alloys. In the present investigation, a poly(D,L-lactic acid)-coated and rapamycin eluting (PDLLA/RAPA) JDBM BRS was prepared, and its biosafety and efficacy for coronary artery stenosis were evaluated via in vitro and in vivo experiments. The degree of smooth muscle cell adhesion to the PDLLA/RAPA coated alloy and the rapamycin pharmacokinetics of JDBM BRS were first assessed in vitro. JDBM BRS and commercial DES FIREHAWK were then implanted in the coronary arteries of a porcine model. Neointimal hyperplasia was evaluated at 30, 90, and 180 days, and re-endothelialization was evaluated at 30 days. Furthermore, Micro-CT and optical coherence tomography (OCT) analyses were performed 180 days after stent implantation to evaluate the technical feasibility, biocompatibility, and degradation characteristics of JDBM BRS in vivo. The results show the ability of a PDLLA/RAPA coated JDBM to inhibit smooth muscle cell adhesion and moderate the drug release rate of JDBM BRS in vitro. In vivo, low local and systemic risks of JDBM BRS were demonstrated in the porcine model, with preserved mechanical integrity after 6 months of implantation. We also showed that this novel BRS was associated with a similar efficacy profile compared with standard DES and high anti-restenosis performance. These findings may confer long term advantages for using this BRS over a traditional DES.

Implanted Flexible Electronics: Set Device Lifetime with Smart Nanomaterials

Flexible electronics is one of the most attractive and anticipated markets in the internet-of-things era, covering a broad range of practical and industrial applications from displays and energy harvesting to health care devices. The mechanical flexibility, combined with high performance electronics, and integrated on a soft substrate offer unprecedented functionality for biomedical applications. This paper presents a brief snapshot on the materials of choice for niche flexible bio-implanted devices that address the requirements for both biodegradable and long-term operational streams. The paper also discusses potential future research directions in this rapidly growing field.

Degradation and interaction with bone of magnesium alloy WE43 implants: A long-term follow-up in vivo rat tibia study

The aim of this in vivo study was to examine the degradation and biocompatibility of the WE43 magnesium alloy containing magnesium yttrium, rare earth elements, and zirconium over a one-year long-term follow-up period. Additionally, we compared anodized WE43 implants with monolithic ones and clarified the effect of the anodization. WE43 cylindrical implants with and without anodization (length, 10 mm; diameter, 0.3 mm) were transplanted into the rat tibia. In both groups, the development of corrosion and the change in implant volume were evaluated by in vivo micro-computed tomography until 12 months, and the bone tissue reaction was observed histologically. In the monolithic WE43 implants, hydrogen gas was evident until 14 days and the volume loss was 36.3% after 12 months. In the anodized WE43 implants, the development of hydrogen gas was inhibited and the volume loss was 27.7% after 12 months. The anodized WE43 implants showed a significantly slower corrosion process in the early phase. Therefore, these implants may require a prolonged period to degrade completely and may even resist complete degradation. At one year post surgery, bone maturation progressed and lamellar bone structure developed around the implant in both groups. In conclusion, the WE43 implants showed good long-term stability and biocompatibility in bone tissue.

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.

Vascular Intervention: From Angioplasty to Bioresorbable Vascular Scaffold

Coronary artery disease (CAD) is the leading cause of mortality and morbidity worldwide. Clinically, CAD can be potentially managed through surgical artery bypass. However, due to the highly invasive nature, surgical intervention has been gradually replaced by percutaneous transluminal coronary angioplasty and recently by percutaneous coronary revascularization using metallic stents. However, the permanent presence of metallic scaffolds inevitably impairs arterial physiology and may induce a variety of adverse effects, such as inflammation, restenosis, thrombosis, and neoatherosclerosis. To address these limitations, revascularization using bioresorbable vascular scaffolds (BVSs) has emerged as the most promising approach. After angioplasty, BVSs provide temporarily mechanical support and are completely resorbed over defined time. This transient nature allows favorable arterial remodeling and avoids thrombosis and in-stent restenosis. However, the theoretical advantages of BVSs have yet to be demonstrated. In this chapter, we first review the evolution of nonsurgical vascular intervention approaches over the past few decades. Next, we discuss the current status of BVS development and propose potential approaches to addressing the limitations associated with the current BVSs.

Surface Finish and Back-Wall Dross Behavior during the Fiber Laser Cutting of AZ31 Magnesium Alloy

Magnesium alloys are of increasing interest in the medical industry due to their biodegradability properties and better mechanical properties as compared to biodegradable polymers. Fiber laser cutting of AZ31 magnesium alloy tubes was carried out to study the effect of cutting conditions on wall surface roughness and back-wall dross. During the experiments, an argon gas chamber was adapted in order to avoid material reactivity with oxygen and thus better control the part quality. A surface response methodology was applied to identify the significance of pulse overlapping and pulse energy. Our results indicate minimum values of surface roughness (Ra < 0.7 μm) when the spot overlapping is higher than 50%. A back-wall dross range of 0.24% to 0.94% was established. In addition, a reduction in back-wall dross accumulations was obtained after blowing away the dross particles from inside the tube using an argon gas jet, reaching values of 0.21%. Laser cutting experimental models show a quadratic model for back-wall dross related with the interaction of the pulse energy, and a linear model dependent on pulse overlapping factor for surface roughness.

Coronary Stents: History, Design, and Construction

The history of percutaneous coronary intervention (PCI) is marked by rapid technological advancements that have taken place over the past 40 years. After a period of balloon angioplasty, which was marred by risk of abrupt vessel closure and vessel recoil, balloon expandable metal alloy stents were the mainstay of PCI. The introduction of drug eluting stents (DES) targeted in-stent restenosis, a common mode of stent failure, and ushered in the current PCI era. Since the first generation of DES, advances in polymer science and stent design have advanced the field. The current generation of DES has thin struts, are highly deliverable, have biocompatible or absorbable polymers, and outstanding safety and efficacy profiles. In this review, we discuss the technological advancements in stent development, design, and construction, with an emphasis on balloon expandable stents. The aspects of stent properties, metal alloys, bioresorbable vascular scaffolds, drug elution, and polymers will be covered.

Stent-less percutaneous coronary intervention using rotational atherectomy and drug-coated balloon: A case series and a mini review

BACKGROUND

Experiences of rotational atherectomy (RA) followed by drug-coated balloon (DCB) dilation alone (RA/DCB) for de novo coronary artery lesion have been limited.

CASE SERIES

Case 1 (65 year-old male) with silent myocardial ischemia and hemodialysis had old anterior myocardial infarction and intact LM/LCx, and underwent RA/DCB against a diffuse calcified lesion in the proximal LAD and a tandem lesion in the proximal RCA. Case 2 (88 year-old female) with post-infarction unstable angina had severe thrombocytopenia and anemia due to myelodysplastic syndrome (platelet 6000/μL, hemoglobin 8.3 g/dL), and underwent RA/DCB against a severe stenosis in the mid LCx after transfusions. Case 3 (47 year-old male) with silent myocardial ischemia due to possible sequelae of Kawasaki disease underwent RA/DCB against a restenotic lesion at the in-let of the calcified aneurysm in the proximal LAD. In all of the patients, PCI was successfully completed under optical frequency domain imaging (OFDI) without complications. Follow-up CAG performed 6-7 months after the procedure revealed no restenosis in case 1 and case 3, and all of the 3 cases have been free of cardiovascular/hemorrhagic events for 11-37 months since the last stent-less procedures.

CONCLUSIONS

These cases suggest that RA/DCB under OFDI might be an alternative stent-less revascularization therapy of choice for patients who may be unsuitable for drug-eluting stent implantation.

Strontium and bisphosphonate coated iron foam scaffolds for osteoporotic fracture defect healing

The purpose of this work was to investigate new bone formation in macroporous iron foams coated with strontium (FeSr) or bisphosphonate (FeBiP) compared to plain iron foam (Fe) and empty defect in a critical size metaphyseal bone defect model in ovariectomized rats. 60 female rats were subjected to bilateral ovariectomy and multi-deficient diet for 3 months. A 4 mm wedge shaped metaphyseal osteotomy was created, fixed with a mini-plate and subsequently filled with Fe, FeSr, FeBiP or left empty. After 6 weeks, μCt analysis revealed a statistically significant increased bone formation at the implant interface in FeSr compared to FeBiP (p = 0.035) and Fe (p = 0.002), respectively. Increased mineralized tissue was also seen within the pores in FeSr (p = 0.023) compared to Fe. Histomorphometry revealed significantly increased bone formation at the implant interface in FeSr (p < 0.001) and FeBiP (p = 0.006) compared to plain Fe with increased osteoblast and decreased osteoclast activity in combination with increased BMP2 and decreased RANKL/OPG in immunohistochemistry. ToF-SIMS analysis showed overlapping Ca signals with Fe for both FeSr and FeBiP thereby indicating tissue in-growth into the scaffolds. In conclusion, iron foam with strontium or bisphosphonate coating are of further interest in metaphyseal fracture defects in osteopenic bone.

A strain-mediated corrosion model for bioabsorbable metallic stents

This paper presents a strain-mediated phenomenological corrosion model, based on the discrete finite element modelling method which was developed for use with the ANSYS Implicit finite element code. The corrosion model was calibrated from experimental data and used to simulate the corrosion performance of a WE43 magnesium alloy stent. The model was found to be capable of predicting the experimentally observed plastic strain-mediated mass loss profile. The non-linear plastic strain model, extrapolated from the experimental data, was also found to adequately capture the corrosion-induced reduction in the radial stiffness of the stent over time. The model developed will help direct future design efforts towards the minimisation of plastic strain during device manufacture, deployment and in-service, in order to reduce corrosion rates and prolong the mechanical integrity of magnesium devices.

STATEMENT OF SIGNIFICANCE

The need for corrosion models that explore the interaction of strain with corrosion damage has been recognised as one of the current challenges in degradable material modelling (Gastaldi et al., 2011). A finite element based plastic strain-mediated phenomenological corrosion model was developed in this work and was calibrated based on the results of the corrosion experiments. It was found to be capable of predicting the experimentally observed plastic strain-mediated mass loss profile and the corrosion-induced reduction in the radial stiffness of the stent over time. To the author's knowledge, the results presented here represent the first experimental calibration of a plastic strain-mediated corrosion model of a corroding magnesium stent.

Computational Modeling of the Mechanical Performance of a Magnesium Stent Undergoing Uniform and Pitting Corrosion in a Remodeling Artery

Coronary stents made from degradable biomaterials such as magnesium alloy are an emerging technology in the treatment of coronary artery disease. Biodegradable stents provide mechanical support to the artery during the initial scaffolding period after which the artery will have remodeled. The subsequent resorption of the stent biomaterial by the body has potential to reduce the risk associated with long-term placement of these devices, such as in-stent restenosis, late stent thrombosis, and fatigue fracture. Computational modeling such as finite-element analysis has proven to be an extremely useful tool in the continued design and development of these medical devices. What is lacking in computational modeling literature is the representation of the active response of the arterial tissue in the weeks and months following stent implantation, i.e., neointimal remodeling. The phenomenon of neointimal remodeling is particularly interesting and significant in the case of biodegradable stents, when both stent degradation and neointimal remodeling can occur simultaneously, presenting the possibility of a mechanical interaction and transfer of load between the degrading stent and the remodeling artery. In this paper, a computational modeling framework is developed that combines magnesium alloy degradation and neointimal remodeling, which is capable of simulating both uniform (best case) and localized pitting (realistic) stent corrosion in a remodeling artery. The framework is used to evaluate the effects of the neointima on the mechanics of the stent, when the stent is undergoing uniform or pitting corrosion, and to assess the effects of the neointimal formation rate relative to the overall stent degradation rate (for both uniform and pitting conditions).

Bioresorbable Scaffolds: Current Evidences in the Treatment of Coronary Artery Disease

Percutaneous coronary revascularization strategies have gradually progressed over a period of last few decades. The advent of newer generation drug-eluting stents has significantly improved the outcomes of Percutaneous Coronary Intervention (PCI) by substantially reducing in-stent restenosis and stent thrombosis. However, vascular inflammation, restenosis, thrombosis, and neoatherosclerosis due to the permanent presence of a metallic foreign body within the artery limit their usage in complex Coronary Artery Disease (CAD). Bioresorbable Scaffolds (BRS) represent a novel approach in coronary stent technology. Complete resorption of the scaffold liberates the treated vessel from its cage and restores pulsatility, cyclical strain, physiological shear stress, and mechanotransduction. In this review article, we describe the advances in this rapidly evolving technology, present the evidence from the pre-clinical and clinical evaluation of these devices, and provide an overview of the ongoing clinical trials that were designed to examine the effectiveness of BRS in the clinical setting.

Absorb bioresorbable stents for the treatment of coronary artery disease

Bioresorbable stents are considered to be the 'fourth revolution' in percutaneous coronary intervention. The first clinically available Absorb(®) bioresorbable device is made of poly-l-lactic acid polymer and elutes everolimus. The process of bioresorption is completed in 3 years. The introduction of this device into clinical practice went through several logical phases: first-in-man studies, randomized Absorb II study with moderately complex patients and lesions, registries of real life patient population and reports of challenging cases. The procedural results are excellent; many insights have been gained by intracoronary imaging. Intermediate-term outcomes are very encouraging both from imaging and from clinical perspectives. The issue of increased stent thrombosis rate was raised in one study, but other studies have been reassuring. Excellent lesion preparation, sizing and complete expansion of the Absorb device are crucial for optimal procedural and clinical results. Results of ongoing large randomized studies will determine the future role of this technology.

Radiological, histological, and hematological evaluation of hydroxyapatite-coated resorbable magnesium alloy screws placed in rabbit tibia

Titanium (Ti) screw has excellent mechanical property, and osseointegration capacity. However, they require surgery for removal. In contrast, polymer screws are resorbable, but they have poor mechanical properties. In this research, magnesium alloy screws (WE43: Mg-Y-Nd-Zr) that have advantages of titanium and polymer were manufactured. In addition, to increase biocompatibility and control degradation rate, the Mg alloy was coated with hydroxyapatite (HA). Torsion test and corrosion test were performed in vitro. For clinical, radiological and histological evaluation, on the eight rabbits, two HA-coated screws were installed in left tibia, and two noncoated screws were installed in right tibia. Each four rabbits were sacrificed 6 and 12 weeks postoperatively. For hematological evaluation, the same type of screws were installed on both legs. Complete blood count (CBC), Mg²⁺ concentrate were sampled from the ear central artery on the operation day for a control point, and at 1, 2, 4, 6, 8, and 12 weeks. Mg alloy screws have no differences of biocompatibility according to the HA coating. However, resorption of screw was slower in case of the HA coating. The hematological problem related releasing of Mg was not found. The results suggest that Mg alloy screws have feasibility for clinical application. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1636-1644, 2017.

Chylomicronaemia--current diagnosis and future therapies

This Review discusses new developments in understanding the basis of chylomicronaemia--a challenging metabolic disorder for which there is an unmet clinical need. Chylomicronaemia presents in two distinct primary forms. The first form is very rare monogenic early-onset chylomicronaemia, which presents in childhood or adolescence and is often caused by homozygous mutations in the gene encoding lipoprotein lipase (LPL), its cofactors apolipoprotein C-II or apolipoprotein A-V, the LPL chaperone lipase maturation factor 1 or glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1. The second form, polygenic late-onset chylomicronaemia, which is caused by an accumulation of several genetic variants, can be exacerbated by secondary factors, such as poor diet, obesity, alcohol intake and uncontrolled type 1 or type 2 diabetes mellitus, and is more common than early-onset chylomicronaemia. Both forms of chylomicronaemia are associated with an increased risk of life-threatening pancreatitis; the polygenic form might also be associated with an increased risk of cardiovascular disease. Treatment of chylomicronaemia focuses on restriction of dietary fat and control of secondary factors, as available pharmacological therapies are only minimally effective. Emerging therapies that might prove more effective than existing agents include LPL gene therapy, inhibition of microsomal triglyceride transfer protein and diacylglycerol O-acyltransferase 1, and interference with the production and secretion of apoC-III and angiopoietin-like protein 3.

Vascular restoration therapy and bioresorbable vascular scaffold

This article describes the evolution of minimally invasive intervention technologies for vascular restoration therapy from early-stage balloon angioplasty in 1970s, metallic bare metal stent and metallic drug-eluting stent technologies in 1990s and 2000s, to bioresorbable vascular scaffold (BVS) technology in large-scale development in recent years. The history, the current stage, the challenges and the future of BVS development are discussed in detail as the best available approach for vascular restoration therapy. The criteria of materials selection, design and processing principles of BVS, and the corresponding clinical trial results are also summarized in this article.