The clinical challenge of disappearing stents

Systematical evolution on a Zn-Mg alloy potentially developed for biodegradable cardiovascular stents

To reduce the long-term side effects of permanent metallic stents, a new generation of cardiovascular stents called "biodegradable stents" is being extensively developed. Zinc has been considered as a promising candidate material for biodegradable cardiovascular stents due to its excellent biocompatibility and appropriate biodegradability. However, weak mechanical properties limit its further clinic application. In this study, hot extruded pure Zn and Zn-0.02 Mg alloy were prepared. Compared with pure Zn, Zn-0.02 Mg alloy showed more homogeneous microstructure, much smaller grain size and higher mechanical strength. Zn-0.02 Mg alloy presented uniform corrosion morphologies during the immersion process, and its corrosion rates was higher than that of pure Zn. Hemocompatibility results showed that the Zn-based alloy had extremely low hemolysis rate (0.74 ± 0.15%) and strong inhibitory effect on blood coagulation, platelet adhesion and aggregation. Zn-0.02 Mg alloy also exhibited excellent cytocompatibility. Its extracts could significantly promote the proliferation of endothelial cells. Moreover, the antibacterial activities of the Zn-based alloy were demonstrated by spread plate assay, live/dead viability assay and bacterial morphology observation. These results indicate that the extruded Zn-0.02 Mg alloy has a potential in biodegradable cardiovascular stents.

Long-term in vivo study of biodegradable Zn-Cu stent: A 2-year implantation evaluation in porcine coronary artery

In the present study, a novel biodegradable Zn-0.8Cu coronary artery stent was fabricated and implanted into porcine coronary arteries for up to 24 months. Micro-CT analysis showed that the implanted stent was able to maintain structural integrity after 6 months, while its disintegration occurred after 9 months of implantation. After 24 months of implantation, approximately 28 ± 13 vol% of the stent remained. Optical coherence tomography and histological analysis showed that the endothelialization process could be completed within the first month after implantation, and no inflammation responses or thrombosis formation was observed within 24 months. Cross-section analysis indicated that the subsequent degradation products had been removed in the abluminal direction, guaranteeing that the strut could be replaced by normal tissue without the risk of contaminating the circulatory system, causing neither thrombosis nor inflammation response. The present work demonstrates that the Zn-0.8Cu stent has provided sufficient structural supporting and exhibited an appropriate degradation rate during 24 months of implantation without degradation product accumulation, thrombosis, or inflammation response. The results indicate that the Zn-0.8Cu coronary artery stent is promising for further clinical applications. STATEMENT OF SIGNIFICANCE: Although Zn and its alloys have been considered to be potential candidates of biodegradable metals for vascular stent use, by far, no Zn-based stent with appropriate medical device performance has been reported because of the low mechanical properties of zinc. The present work presents promising results of a Zn-Cu biodegradable vascular stent in porcine coronary arteries. The Zn-Cu stent fabricated in this work demonstrated adequate medical device performance both in vitro and in vivo and degraded at a proper rate without safety problems induced. Furthermore, large animal models have more cardiovascular similarities as humans. Results of this study may provide further information of the Zn-based stents for translational medicine research.

Mechanism of Acceleration of Iron Corrosion by a Polylactide Coating

Strong and biodegradable materials are key to the development of next-generation medical devices for interventional treatment. Biodegradable polymers such as polylactide (PLA) have controllable degradation profiles, but their mechanical strength is much weaker than some metallic materials such as iron; on the other hand, tuning the corrosion rate of iron to a proper time range for biomedical applications has always been a challenge. Very recently, we have achieved a complete corrosion of iron stent in vivo within the clinically required time frame by combining a PLA coating, which provides a new biomaterial type for the next-generation biodegradable coronary stents termed as a metal-polymer composite stent. The underlying mechanism of accelerating iron corrosion by a PLA coating remains an open fundamental topic. Herein, we investigated the corrosion mechanism of an iron sheet under a PLA coating in the biomimetic in vitro condition. The Pourbaix diagram (potential vs pH) was calculated to present the thermodynamic driving force of iron corrosion in the biomimetic aqueous medium. Electrochemical methods were applied to track the dynamic corrosion process and inspect various potential cues influencing iron corrosion. The present work reveals that acceleration of iron corrosion by the PLA coating arises mainly from decreasing the local pH owing to PLA hydrolysis and from alleviating the deposition of the passivation layer by the polymer coating.

Corrosion behavior and biocompatibility evaluation of a novel zinc-based alloy stent in rabbit carotid artery model

Zinc (Zn) and its alloys have been proved to be promising candidate materials for biodegradable cardiovascular stents. In this study, a novel extruded Zn-0.02 Mg-0.02Cu alloy was prepared. Compared with pure Zn, the Zn-based alloy showed higher mechanical properties, and the Zn-based alloy could significantly accelerate Zn²⁺ release, reaching 0.61 ± 0.11 μg/mL at 15 days of immersion. In vitro biocompatibility studies demonstrated that the Zn-based alloy had excellent cytocompatibility and hemocompatibility, including low hemolysis rate (0.63 ± 0.12%) and strong inhibitory effect on platelet adhesion. Subsequently, the Zn-based alloy stent was implanted into the left carotid arteries of New Zealand white rabbits for 12 months. All the rabbits survived without any adverse clinical events, and all the stented arteries were patent during the study period. Rapid endothelialization at 1 week of implantation was observed, suggesting a low cytotoxicity and thrombosis risk. The stent corroded slowly and no obvious intimal hyperplasia was observed for 6 months, after which corrosion accelerated at 12 months. In addition, no obvious thrombosis and systemic toxicity during implantation period were observed, indicating its potential as the backbone of biodegradable cardiovascular stents. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1814-1823, 2019.

Branch ostial optimization treatment and optimized provisional t-stenting with polymeric bioresorbable scaffolds: Ex-vivo morphologic and hemodynamic examination

The optimal side-branch (SB) ostium treatment after provisional side-branch scaffolding remains a subject of debate in bioresorbable vascular scaffold (BVS) era. In this study, we evaluated a novel optimized provisional T-stenting technique (OPT) and assessed its feasibility by comparison with T and small protrusion technique (TAP).Two provisional SB scaffolding techniques (OPT, n = 5; TAP, n = 5) were performed using polymeric BVS in a bifurcated phantom. The sequential intermediate snuggling balloon dilation, also called ostial optimal technique, was added to OPT but not TAP to dilate the side-branch ostium while the final snuggling balloon dilation applied for both procedures. Microcomputed tomography (microCT) and optical coherence tomography (OCT) were performed to assess morphology, and computational fluid dynamics (CFD) was performed to assess hemodynamics in the scaffolded bifurcations. Compared with TAP in microCT analysis, OPT created shorter neo-carina length than TAP (0.34 ± 0.10 mm vs 1.02 ± 0.26 mm, P < .01), longer valgus struts length (2.49 ± 0.27 mm vs 1.78 ± 0.33 mm, P < .01) with larger MB ostial area (9.46 ± 0.04 mm vs 8.34 ± 0.09 mm, P < .01). OCT found that OPT significantly decreased the struts mal-apposition (13.20 ± 0.16% vs 1.94 ± 0.54%, P < .01). CFD revealed that OPT generated more favorable flow pattern than TAP, as indicated by less percent (4.68 ± 1.40% vs 8.88 ± 1.21%, P < .01) of low wall shear stress (<0.4 Pa) along the lateral walls.By using BVSs for bifurcation intervention, the sequential intermediate snuggling balloon dilation is feasible for optimizing ostial SB and facilitating subsequent SB scaffolding. Results show OPT is better than TAP for bifurcated morphology and hemodynamics in this ex-vivo study.

Evolution of the degradation mechanism of pure zinc stent in the one-year study of rabbit abdominal aorta model

In the present study, pure zinc stents were implanted into the abdominal aorta of rabbits for 12 months. Multiscale analysis including micro-CT, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and histological stainings was performed to reveal the fundamental degradation mechanism of the pure zinc stent and its biocompatibility. The pure zinc stent was able to maintain mechanical integrity for 6 months and degraded 41.75 ± 29.72% of stent volume after 12 months implantation. No severe inflammation, platelet aggregation, thrombosis formation or obvious intimal hyperplasia was observed at all time points after implantation. The degradation of the zinc stent played a beneficial role in the artery remodeling and healing process. The evolution of the degradation mechanism of pure zinc stents with time was revealed as follows: Before endothelialization, dynamic blood flow dominated the degradation of pure zinc stent, creating a uniform corrosion mode; After endothelialization, the degradation of pure zinc stent depended on the diffusion of water molecules, hydrophilic solutes and ions which led to localized corrosion. Zinc phosphate generated in blood flow transformed into zinc oxide and small amounts of calcium phosphate during the conversion of degradation microenvironment. The favorable physiological degradation behavior makes zinc a promising candidate for future stent applications.

Contemporary Drug-Eluting Stents and Vascular Response

Cardiovascular disease is a leading cause of death and disability worldwide. Current treatment strategies aimed at treating the consequences of coronary artery disease have embraced both optimal medical therapy and catheter based percutaneous coronary intervention with drug-eluting stents (DES). Current-generation DES elute predominantly mammalian target of rapamycin (mTOR) inhibitors, which act primarily as a cytostatic agent that retards vascular smooth muscle cell proliferation and migration; this occurs in response to injury and thus prevents restenosis. While DES have reduced restenosis, the use of first-generation DES was associated with an increased risk of late stent thrombosis and accelerated neointimal atherosclerosis (i.e. neoatherosclerosis), both major contributors to late stent failure. The underlying substrate of late DES failure is likely related to vascular endothelial dysfunction, which occurs after DES implantation. Initial concerns with first-generation DES have led to improvements in stent design, polymer load and biocompatibility, and pharmacologic agents, all of which have helped to improve healing responses, lessen late stent failure, and result in an overall improved safety profile. The armamentarium of DES has expanded from the current-generation durable polymer DES to bioresorbable polymer DES, polymer-free DES, and lastly totally bioresorbable vascular scaffolds with a goal of improving vascular responses and endothelial function while preserving anti-restenotic efficacy. We will review these contemporary DES in relation to their short and long-term effects on vascular biocompatibility and healing responses.

Design, synthesis and ring-opening polymerization of a new iodinated carbonate monomer: a universal route towards ultrahigh radiopaque aliphatic polycarbonates

A universal route towards ultrahigh radiopaque aliphatic polycarbonates was developed based on a new iodinated carbonate monomer.