Webbing and Delamination of Drug Eluting Stent Coatings

Polymer–Metal Composite Healthcare Materials: From Nano to Device Scale

Metals have been investigated as biomaterials for a wide range of medical applications. At nanoscale, some metals, such as gold nanoparticles, exhibit plasmonics, which have motivated researchers’ focus on biosensor development. At the device level, some metals, such as titanium, exhibit good physical properties, which could allow them to act as biomedical implants for physical support. Despite these attractive features, the non-specific delivery of metallic nanoparticles and poor tissue–device compatibility have greatly limited their performance. This review aims to illustrate the interplay between polymers and metals, and to highlight the pivotal role of polymer–metal composite/nanocomposite healthcare materials in different biomedical applications. Here, we revisit the recent plasmonic engineered platforms for biomolecules detection in cell-free samples and highlight updated nanocomposite design for (1) intracellular RNA detection, (2) photothermal therapy, and (3) nanomedicine for neurodegenerative diseases, as selected significant live cell–interactive biomedical applications. At the device scale, the rational design of polymer–metallic medical devices is of importance for dental and cardiovascular implantation to overcome the poor physical load transfer between tissues and devices, as well as implant compatibility under a dynamic fluidic environment, respectively. Finally, we conclude the treatment of these innovative polymer–metal biomedical composite designs and provide a future perspective on the aforementioned research areas.

Specific and nonspecific binding of drug eluted from a half-embedded stent in presence of atherosclerotic plaque

This study is dealt with the two-phase binding (specific and nonspecific) of drug eluted from a half- embedded drug-eluting stent in presence of atherosclerotic plaque. The specific binding due to the interaction of drug molecules with specific receptors and nonspecific binding caused by the trapping of drug in the extra-cellular matrix have been paid due attention. An idealised wall consisting of a plaque and a healthy tissue region has been considered. Moreover, a Dirichlet release condition is imposed on the strut surface. In this investigation, a two-dimensional model governing drug transport and its two-phase binding in cylindrical polar coordinate system has been solved numerically by a finite-difference method. Our simulation predicts that plaque behaves like a physical barrier in two types of the binding process and there is an inverse relationship between bound drug concentration and plaque thickness. Simulations show that a single peak profile of drug is noted when the struts are situated one-strut radius apart and as the inter-strut distance increases, the peak concentration falls and distinct peak profiles over each strut are visualised. The model also reveals that in the region downstream of a strut, the concentration of both bound drug forms in the plaque and healthy regions increases, and eventually, the saturation length of binding sites increases. Predicted results show for smaller Damköhler number, the rapid saturation of binding sites takes place and the stent having thinner strut may perform well in terms of effectiveness as well as efficacy in the stent-based delivery.

Bioresponsive starPEG-heparin hydrogel coatings on vascular stents for enhanced hemocompatibility

Hydrogel coatings can improve the biocompatibility of medical devices. However, stable surface bonding and homogeneity of hydrogel coatings are often challenging. This study exploits the benefits of biohybrid hydrogels of crosslinked four-armed poly(ethylene glycol) and heparin to enhance the hemocompatibility of cobalt‑chromium (CoCr) vascular stents. A bonding layer of dual silane and poly(ethylene-alt-maleic anhydride) (PEMA) treatment was applied to the stent to provide covalent immobilization and hydrophilicity for the homogeneous spreading of the hydrogel. A spray coating technology was used to distribute the aqueous solution of the reactive hydrogel precursors onto the sub-millimeter struts of the stents, where the solution polymerized to a homogeneous hydrogel film. The coating was mechanically stable on the stent after ethanol dehydration, and the stents could be stored in a dry state. The homogeneity and stability of the coating during stent expansion were verified. Quasistatic and dynamic whole blood incubation experiments showed substantial suppression of the pro-coagulant and inflammatory activity of the bare metal by the coating. Translation of the technology to industrial coating devices and future surface modification of stents with anti-inflammatory hydrogels are discussed.

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.

Numerical analysis of paclitaxel-eluting coronary stents: Mechanics and drug release properties

Since theoretical models provide data that cannot be otherwise gathered, numerical methods applied to medical devices analysis have emerged as fundamental tool in preclinical development. Large efforts were done to study mechanical and drug-eluting properties in stents but often the coating modelling is neglected. This work presents a finite element framework to calculate mechanical loads and drug distribution in three commercial drug-eluting stents (Palmaz-Schatz, Palmaz Genesis and Multi Link Vision), to check coatings strength and drug distribution maps in biological tissues. The promising copolymer poly(methylmethacrylate-co-n-butylmethacrylate), loaded with paclitaxel, is analyzed. Results demonstrated that the coating undergoes localized plastic phenomena, and calculated stresses are lower than the ultimate stress, ensuring coating integrity. Computed drug concentration depends on stent geometry and its values are in all cases lower than the toxicity level for this drug.

Modern Stents: Where Are We Going?

Coronary artery stenting is the treatment of choice for patients requiring coronary angioplasty. We describe the major advancements with this technology. There have been significant developments in the design of stents and adjunctive medical therapies. Newer-generation drug-eluting stents (DES) have almost negligible restenosis rates and, when combined with proper anti-platelet treatment and optimal deployment, a low risk of stent thrombosis. The introduction of newer-generation DES with thinner stent struts, novel durable or biodegradable polymer coatings, and new antiproliferative agents has further improved the safety profile of early-generation DES. In parallel the effectiveness has been kept, with a significant reduction in the risk of target lesion revascularization compared with the early-generation DES. However, to date, the development of completely bioresorbable vascular scaffolds has failed to achieve further clinical benefits and has been associated with increased thrombosis. Newer-generation DES-including both durable polymer as well as biodegradable polymer-have become the standard of care in all patient and lesion subsets, with excellent long-term results.

Development of a Bioactive Polymeric Drug Eluting Coronary Stent Coating Using Electrospraying

Drug-eluting stents are now routinely used in the treatment of acute coronary syndromes caused by coronary artery disease. Whilst the sustained release of anti-proliferative drugs from these devices has greatly reduced the need for repeat revascularisation procedures, this approach is not suitable for all patients and appears to delay regrowth of the endothelium, necessitating the use of prolonged dual anti-platelet therapy. Although the development of more advanced stent platforms and drug coatings has produced modest improvements in performance, these devices have not fully addressed the limitations experienced with their first-generation counterparts. In the present study, we developed a novel stent coating that provides controlled sirolimus release from a bioactive polymer (accelerate™ AT) that has previously been shown to support endothelial cell growth in vitro. A bespoke electrospray deposition process provided control over the coating thickness, surface roughness, drug load, and release kinetics. The resultant optimised coating combines rapid release of an anti-proliferative agent from a bioactive polymer coating that promotes re-endothelialisation, thereby offering potential protection against in-stent restenosis and thrombosis. This novel, dual-action coating therefore has significant therapeutic potential, with the enhanced control of drug load and release kinetics offered by electrospray deposition also opening up opportunities for more personalised treatment approaches. Further development and evaluation of these technologies in vitro and in vivo is therefore warranted.

Stable Titania Nanostructures on Stainless Steel Coronary Stent Surface for Enhanced Corrosion Resistance and Endothelialization

Stainless steel (SS) coronary stents continue to present risk of in-stent restenosis that impact its long term safety and efficacy. The present work focuses on developing a drug-free and polymer-less surface on coronary stents by utilizing a titania (TiO₂ ) nanotexturing approach through hydrothermal processing, that will offer improved stent performance in vivo. Mechanically stable and durable nanotextured coatings are obtained on SS stents that also offer good corrosion resistance. In vitro vascular cell (endothelial and smooth muscle cells) studies on surface modified SS show preferential rapid endothelialization with enhanced nitric oxide production and reduce smooth muscle cell proliferation, in comparison to unmodified SS. In vivo evaluation of the nanotextured stents after subcutaneous implantation in rabbits show reduced irritability and minimal localized inflammatory response. These beneficial effects suggest that the stable, easily scalable titania nanosurface modification strategy on coronary stent surfaces can be a much cheaper alternative to drug eluting stents in addressing in-stent restenosis.