In vivo degradation of copolymers prepared from L-lactide, 1,3-trimethylene carbonate and glycolide as coronary stent materials

A series of high molecular weight polymers were prepared by ring opening polymerization of L-lactide (L-LA), 1,3-trimethylene carbonate (TMC) and glycolide using stannous octoate as catalyst. The resulting polymers were characterized by gel permeation chromatography, (1)H nuclear magnetic resonance, differential scanning calorimeter and tensile tests. All the polymers present high molecular weights. Compared with PLLA and PTLA copolymers, the terpolymers exhibit interesting properties such as improved toughness and lowered crystallinity with only slightly reduced mechanical strength. In vivo degradation was performed by subcutaneous implantation in rats to evaluate the potential of the copolymers as bioresorbable coronary stent material. The results show that all the polymers conserved to a large extent their mechanical properties during the first 90 days, except the strain at break which exhibited a strong decrease. Meanwhile, significant molecular weight decrease and weight loss are detected in the case of terpolymers. Therefore, the PTLGA terpolymers present a good potential for the development of totally bioresorbable coronary stents.

Fabrication, biodegradation behavior and cytotoxicity of Mg-nanodiamond composites for implant application

Mg-based biodegradable implants offer several advantages over their non-degradable or degradable polymeric counterparts used today. However, the low corrosion resistance of Mg in physiologic environment remained as concerns. In this research, nanodiamond (ND) was uniformly dispersed in Mg matrix to induce a protective layer on Mg surface during corrosion. Compared with pure Mg, fabricated Mg-ND nanocomposites had lower corrosion rates, higher corrosion potential, and higher corrosion resistance. Specifically, the corrosion rate of Mg was reduced by 4.5 times by adding 5 wt% of ND particles. Corrosion inhibition effect of ND was thus validated. The chemical interaction and physical adsorption of the ions from simulated body fluid on ND might be the main reason for enhanced corrosion resistance. In vitro biocompatibility test results indicated that Mg-ND nanocomposites were biocompatible since cells growing in contact with corrosion products of Mg-ND maintained high cell viability and healthy morphology. Therefore, Mg-ND nanocomposites with homogenous ND dispersion, enhanced corrosion resistance, and good biocompatibility might be an excellent candidate material for biodegradable implant application.

Progress in interventional cardiology: challenges for the future

Cardiovascular disease is the leading cause of death in the western and developing countries. Percutaneous transluminal coronary interventions have become the most prevalent treatment option for coronary artery disease; however, due to serious complications, such as stent thrombosis and in-stent restenosis (ISR), the efficacy and safety of the procedure remain important issues to address. Strategies to overcome these aspects are under extensive investigation. In this review, we summarise relevant milestones during the time to overcome these limitations of coronary stents, such as the development of polymer-free drug-eluting stents (DES) to avoid pro-inflammatory response due to the polymer coating or the developement of stents with cell-directing drugs to, simultaneously, improve re-endothelialisation and inhibit ISR amongst other techniques most recently developed, which have not fully entered the clinical stage. Also the novel concept of fully biodegradable DES featured by the lack of a permanent foreign body promises to be a beneficial and applicable tool to restore a natural vessel with maintained vasomotion and to enable optional subsequent surgical revascularisation.

ST-segment elevation myocardial infarction – ideal scenario for bioresorbable vascular scaffold implantation?

Bioresorbable vascular scaffolds (BVS) represent a breakthrough technology for percutaneous coronary intervention (PCI). In this context, because of the unique properties of bioresorbable devices, ST-segment elevation myocardial infarction (STEMI) may represent the ideal scenario for BVS implantation. Consistently, 57% of physicians declare they currently use BVS in this group of patients. However, continuous and growing evidence on the good performance of these devices has been actually shown only in small studies with short- and mid-term follow-up. For these reasons, we need data from sufficiently large observational studies, with long-term follow-up, to confirm that BVS can deliver the same results as 2nd-generation drug-eluting stents when using an appropriate implantation technique. In this review, we discuss the potential advantages of BVS implantation in STEMI patients, together with the most recent evidence from clinical studies, highlighting safety and procedural concerns.

In vitro biodegradation behavior, mechanical properties, and cytotoxicity of biodegradable Zn-Mg alloy

Zinc-Magnesium (Zn-Mg) alloy as a novel biodegradable metal holds great potential in biodegradable implant applications as it is more corrosion resistant than Magnesium (Mg). However, the mechanical properties, biodegradation uniformity, and cytotoxicity of Zn-Mg alloy remained as concerns. In this study, hot extrusion process was applied to Zn-1 wt % Mg (Zn-1Mg) to refine its microstructure. Effects of hot extrusion on biodegradation behavior and mechanical properties of Zn-1Mg were investigated in comparison with Mg rare earth element alloy WE43. Metallurgical analysis revealed significant grain size reduction, and immersion test found that corrosion rates of WE43 and Zn-1Mg were reduced by 35% and 57%, respectively after extrusion. Moreover, hot extrusion resulted in a much more uniform biodegradation in extruded Zn-1Mg alloy and WE43. In vitro cytotoxicity test results indicated that Zn-1Mg alloy was biocompatible. Therefore, hot extruded Zn-1Mg with homogenous microstructure, uniform as well as slow degradation, improved mechanical properties, and good biocompatibility was believed to be an excellent candidate material for load-bearing biodegradable implant application.

Recent advances on the development of magnesium alloys for biodegradable implants

In recent years, much progress has been made on the development of biodegradable magnesium alloys as "smart" implants in cardiovascular and orthopedic applications. Mg-based alloys as biodegradable implants have outstanding advantages over Fe-based and Zn-based ones. However, the extensive applications of Mg-based alloys are still inhibited mainly by their high degradation rates and consequent loss in mechanical integrity. Consequently, extensive studies have been conducted to develop Mg-based alloys with superior mechanical and corrosion performance. This review focuses on the following topics: (i) the design criteria of biodegradable materials; (ii) alloy development strategy; (iii) in vitro performances of currently developed Mg-based alloys; and (iv) in vivo performances of currently developed Mg-based implants, especially Mg-based alloys under clinical trials.

A nontoxic additive to introduce x-ray contrast into poly(lactic acid). Implications for transient medical implants such as bioresorbable coronary vascular scaffolds

Bioresorbable coronary vascular scaffolds are about to revolutionize the landscape of interventional cardiology. These scaffolds, consisting of a poly(L-lactic acid) interior and a poly(D,L-lactic acid) surface coating, offer a genuine alternative for metallic coronary stents. Perhaps the only remaining drawback is that monitoring during implantation is limited to two X-ray contrast points. Here, a new approach to make the biodegradable scaffolds entirely radiopaque is explored. A new contrast agent is designed and synthesized. This compound is miscible with poly(D,L-lactic acid) matrix, and nontoxic to multiple cell types. Blends of poly(D,L-lactic acid) and the contrast agent are found to be hemocompatible, noncytotoxic, and radiopaque. The data show that it is possible to manufacture fully radiopaque bioresorbable coronary vascular scaffolds. Whole-stent X-ray visibility helps interventionalists ensure that the scaffold deploys completely. This important advantage may translate into improved safety, accuracy, and clinical performance of cardiac stents.

Biocompatibility of Coronary Stents

Cardiovascular disease is the dominant cause of mortality in developed countries, with coronary artery disease (CAD) a predominant contributor. The development of stents to treat CAD was a significant innovation, facilitating effective percutaneous coronary revascularization. Coronary stents have evolved from bare metal compositions, to incorporate advances in pharmacological therapy in what are now known as drug eluting stents (DES). Deployment of a stent overcomes some limitations of balloon angioplasty alone, but provides an acute stimulus for thrombus formation and promotes neointimal hyperplasia. First generation DES effectively reduced in-stent restenosis, but profoundly delay healing and are susceptible to late stent thrombosis, leading to significant clinical complications in the long term. This review characterizes the development of coronary stents, detailing the incremental improvements, which aim to attenuate the major clinical complications of thrombosis and restenosis. Despite these enhancements, coronary stents remain fundamentally incompatible with the vasculature, an issue which has largely gone unaddressed. We highlight the latest modifications and research directions that promise to more holistically design coronary implants that are truly biocompatible.

Materials and manufacturing technologies available for production of a pediatric bioabsorbable stent

Transcatheter treatment of children with congenital heart disease such as coarctation of the aorta and pulmonary artery stenosis currently involves the use of metal stents. While these provide good short term results, there are long term complications with their use. Children outgrow metal stents, obligating them to future transcatheter dilations and eventual surgical removal. A bioabsorbable stent, or a stent that goes away with time, would solve this problem. Bioabsorbable stents are being developed for use in coronary arteries, however these are too small for use in pediatric congenital heart disease. A bioabsorbable stent for use in pediatric congenital heart disease needs to be low profile, expandable to a diameter 8 mm, provide sufficient radial strength, and absorb quickly enough to allow vessel growth. Development of absorbable coronary stents has led to a great understanding of the available production techniques and materials such as bioabsorbable polymers and biocorrodable metals. Children with congenital heart disease will hopefully soon benefit from the current generation of bioabsorbable and biocorrodable materials and devices.

Zinc exhibits ideal physiological corrosion behavior for bioabsorbable stents

Zinc is proposed as an exciting new biomaterial for use in bioabsorbable cardiac stents. Not only is zinc a physiologically relevant metal with behavior that promotes healthy vessels, but it combines the best behaviors of both current bioabsorbable stent materials: iron and magnesium. Shown here is a composite image of zinc degradation in a murine (rat) artery.

Microelectromechanical systems and nanotechnology: a platform for the next stent technological era

Nanotechnology is the design and development of materials, structures, and devices with at least 1 dimension between the 1- and 100-nm-size scale. Manipulating matter at the atomic scale offers unique opportunities in material design particularly in biological interfaces. In this short review, we explore the disruptive technological opportunities that nanotechnology and microelectromechanical systems may offer in possible future stent designs along with safety issues that may surface with the use of nanoparticles in medical devices.

Absorbable stent: focus on clinical applications and benefits

Coronary stents have improved very significantly the immediate and long-term results of percutaneous coronary interventions. However, once the vessel has healed, the scaffolding function of the stent is no longer needed, and the presence of a permanent metallic prosthesis poses important disadvantages. This has led to the idea of creating new devices that are able to provide mechanical support for a determined period and then disappear from the vessel, allowing its natural healing and avoiding the risks associated with having a permanent metallic cage, such as stent thrombosis. Absorbable stents currently appear as one of the most promising fields in interventional cardiology. The present article will review the available clinical evidence regarding these devices at present and their future perspectives.

Modular polymer design to regulate phenotype and oxidative response of human coronary artery cells for potential stent coating applications

Polymer properties can be tailored by copolymerizing subunits with specific physico-chemical characteristics. Vascular stent materials require biocompatibility, mechanical strength, and prevention of restenosis. Here we copolymerized poly(ε-caprolactone) (PCL), poly(ethylene glycol) (PEG), and carboxyl-PCL (cPCL) at varying molar ratios and characterized the resulting material properties. We then performed a short-term evaluation of these polymers for their applicability as potential coronary stent coating materials with two primary human coronary artery cell types: smooth muscle cells (HCASMC) and endothelial cells (HCAEC). Changes in proliferation and phenotype were dependent upon intracellular reactive oxygen species (ROS) levels, and 4%PEG-96%PCL-0%cPCL was identified as the most appropriate coating material for this application. After 3days on this substrate HCASMC maintained a healthy contractile phenotype and HCAEC exhibited a physiologically relevant proliferation rate and a balanced redox state. Other test substrates promoted a pathological, synthetic phenotype of HCASMC and/or hyperproliferation of HCAEC. Phenotypic changes of HCASMC appeared to be modulated by the Young's modulus and surface charge of the test substrates, indicating a structure-function relationship that can be exploited for intricate control over vascular cell functions. These data indicate that tailored copolymer properties can direct vascular cell behavior and provide insights for further development of biologically instructive stent coating materials.