Functional Mechanical Behavior and Biocompatible Characteristics of Graphene-Coated Cardiovascular Stents

Abstract

Percutaneous Coronary Intervention (PCI) is a treatment method that involves reopening narrowed arteries with a balloon catheter that delivers a cylindrical, mesh-shaped implant device to the site of the stenosis. Currently, by applying a coating to a bare metal stent (BMS) surface to improve biocompatibility, the main risks after PCI, such as restenosis and thrombosis, are reduced while maintaining the basic requirements for the mechanical behavior of the stent itself. In this work, for the first time, the development and optimization process of the spatial structure of the Co-Cr stent (L-605) with a graphene-based coating using cold-wall chemical vapor deposition (CW-CVD) to ensure uniform coverage of the implant was attempted. The CW-CVD process allows the coating of 3D structures, minimizing thermal stress on the surrounding equipment and allowing the deposition of coatings on temperature-sensitive materials. It produces uniform and high-purity films with control over the thickness and composition. The reduced heating of the chamber walls minimizes unwanted reactions, leading to fewer impurities in the final coating. The graphene layers obtained using Raman spectroscopy at different parameters of the CW-CVD process were verified, their properties were investigated, and the functional mechanical behavior of the studied graphene-covered stent was confirmed. In vitro, graphene-coated stents promoted rapid endothelial cell repopulation, an advantage over gold-standard drug-eluting stents delaying re-endothelialization. Also, full-range biocompatibility studies on potential allergic, irritation, toxicological, and pyrogenic reactions of new material in vivo on small animal models demonstrated excellent biocompatibility of the graphene-coated stents.

Keywords: biocompatibility; cardiovascular stent; cold-wall chemical vapor deposition (CW-CVD); endothelialization; graphene; mechanical behavior.

Figure 1

Raman spectra (λ_ex—514 nm) of the cardiovascular stents before (black line) and after CW-CVD (red, green, and blue line) with different deposition temperatures (700 °C, 900 °C, and 1100 °C, respectively).

Figure 2

SEM images of a cardiovascular stent (a) before and (b) after CW-CVD. (c) The stent fracture after crimping. (d–f) Images of critical areas for properly crimped and expanded stent. The scale bar is presented in the appropriate image.

Figure 3

(a) The radial force measurement device. (b) The values of the obtained radial forces of graphene-coated stent (GC-stent) and uncoated stent (BM-stent). The p-value according to the non-parametric Mann–Whitney U test.

Figure 4

(a) The coronary stents used in the test and the BOSE 9400 MAPS system. (b) The macro-photography of the bare metal stent. (c) The macro-photography of the graphene-coated stent. (d) Time evolution of the stent diameter for the reference and graphene-coated stents and (e) pressure-diameter elastic behavior of the stent in the cyclic load-unload test for the reference and graphene-coated stents. The response to cyclic loading confirms that graphene-coated stents are just as safe as uncoated stents, which have been used clinically for many years. The mechanical properties of graphene-coated stents are similar to those of other coatings. The main advantage of graphene coating is increased biocompatibility.

Figure 5

(a) HUVEC cell proliferation on bare metal (BM) stents and graphene-coated (GC) bare metal stents after 72 h quantified in the WST-1 assay. * p < 0.001. (b) The proliferation of HUVEC cells on bare metal (BM) stent and graphene-coated bare metal (GC) stent after 72 h. Cells were visualized by staining the cell’s actin cytoskeleton with phalloidin-FITC and the cell’s nuclei with DAPI. Magnification 400×.

Figure 6

Photographic documentation of the allergy and skin irritation tests performed, where (a) the method of applying the tested graphene-coated samples on the shaved skin of a guinea pig during the GPMT test is presented. The tested implant was placed on the skin of a rabbit similarly during the Rabbit Skin Primary Irritation Test. (b) The site after applying the graphene-coated stent and after a 14-day break and re-applying of the stent. The site was assessed using the Magnusson and Kligman scale in the GPMT test. (c) The site after 72 h where the graphene-coated stent was applied and subjected to erythema and edema assessments on a scale of 0 to 4 in the Rabbit Skin Primary Irritation Test.

Figure 7

Photographic documentation (a–c) of individual stages of intraperitoneal insertion of the tested graphene-coated stents. (d) The autopsy did not show any symptoms of reaction to the tested material. The implanted material samples were loose in the peritoneal cavity and could be easily removed.

Figure 8

The histopathological microscope images show no organ changes following the introduction of graphene-coated stents: chronic response study. No changes were observed in (a) lungs, (b) heart, (c) kidneys, and (d) liver. The results do not differ from typical images characteristic of healthy organs. Below, histopathological images of the skin after (e) 24 h, (f) 48 h, and (g) 72 h, respectively, are shown in the skin irritation tests. The tests were performed on the White New Zealand rabbit. The scale shown in the images indicates 400 μm.