Recurrent Benign Urethral Strictures Treated with Covered Retrievable Self-Expandable Metallic Stents: Long-Term Outcomes over an 18-Year Period

Recent Advances in Manufacturing Innovative Stents

Cardiovascular diseases are the most distributed cause of death worldwide. Stenting of arteries as a percutaneous transluminal angioplasty procedure became a promising minimally invasive therapy based on re-opening narrowed arteries by stent insertion. In order to improve and optimize this method, many research groups are focusing on designing new or improving existent stents. Since the beginning of the stent development in 1986, starting with bare-metal stents (BMS), these devices have been continuously enhanced by applying new materials, developing stent coatings based on inorganic and organic compounds including drugs, nanoparticles or biological components such as genes and cells, as well as adapting stent designs with different fabrication technologies. Drug eluting stents (DES) have been developed to overcome the main shortcomings of BMS or coated stents. Coatings are mainly applied to control biocompatibility, degradation rate, protein adsorption, and allow adequate endothelialization in order to ensure better clinical outcome of BMS, reducing restenosis and thrombosis. As coating materials (i) organic polymers: polyurethanes, poly(ε-caprolactone), styrene-b-isobutylene-b-styrene, polyhydroxybutyrates, poly(lactide-co-glycolide), and phosphoryl choline; (ii) biological components: vascular endothelial growth factor (VEGF) and anti-CD34 antibody and (iii) inorganic coatings: noble metals, wide class of oxides, nitrides, silicide and carbide, hydroxyapatite, diamond-like carbon, and others are used. DES were developed to reduce the tissue hyperplasia and in-stent restenosis utilizing antiproliferative substances like paclitaxel, limus (siro-, zotaro-, evero-, bio-, amphi-, tacro-limus), ABT-578, tyrphostin AGL-2043, genes, etc. The innovative solutions aim at overcoming the main limitations of the stent technology, such as in-stent restenosis and stent thrombosis, while maintaining the prime requirements on biocompatibility, biodegradability, and mechanical behavior. This paper provides an overview of the existing stent types, their functionality, materials, and manufacturing conditions demonstrating the still huge potential for the development of promising stent solutions.

Design and Biomechanical Analysis of a Novel Retrievable Peripheral Vascular Stent

Structurally retrievable drug-eluting stents may have valuable clinical applications because they do not leave any foreign materials inside the patient's body. This article presents a novel design of retrievable peripheral vascular stent and the results from biomechanical analysis of its performance. Using the finite element analysis method, principal parameters of the stent were studied. Moreover, to ensure the practicability of the retrieval process, simulation, and in vitro experiments were performed. The retrieval force reached the maximum value when the whole retrievable part had been retrieved. Furthermore, the force was gradually increased during the retrieval process and remained constant after the main part had been retrieved. When the stent was being compressed, the maximum strain of the stent occurred at the connection between the stent's retrieval part and the main body part, at a value of 4%. The index of nonuniformity of the stent was too small to be counted both at the end of the compression and self-expansion processes. With the increase of moment, the bending stiffness (EI) of the stent decreased gradually. After bending moment was applied, the large strain region was mainly located in the stent's main body part rather than the retrieval part. The results of preliminary stent retrieval experiments demonstrated that the stent could be retrieved successfully. This novel retrievable stent displays promising biomechanical performance. The preliminary experiments demonstrated that the stent could be retrieved smoothly from the blood vessels.