Papers by Keyword: Stent

Abstract: This study proposes to design and fabricate the peripheral stent using NiTi shape memory alloy wires by braiding technique. The 0.2 mm diameter NiTi wire was attached to the automatic braiding machine and fabricated the braided stent under the different height of circular rod holder. Heat-treatment was carried out at 450°C, 500°C, 550°C and 600°C for 10 and 20 minutes, respectively. Then transformation temperature, the shape of cell area, and mechanical properties were evaluated. The results show that complete superelasticity can be confirmed under body temperature in stent prototypes with heat-treatment temperature over 500°C irrespective of heat-treatment time. The prototype with the height of circular mold of 70 mm shown a good braiding pattern with high kink resistance. The radial force shown no significant difference at any section along the length profile. It is noted that stent with the height of circular mold at 70 mm, carrier speed of 30 rad/sec and take-up speed of 4.86 mm/s which shown radial resistive force (ROF) of 2.44 ± 0.05 N/mm, and chronic outward force (COF) of 0.533 ± 0.03 N/mm which is sufficient for vessel expansion.

Abstract: The Co-20Cr-15W-10Ni (CCWN, mass%) alloy has excellent corrosion resistance and strength-ductility balance and is applied in almost all balloon-expandable stent platforms. To further reduce the invasiveness of stent placement, it is necessary to reduce the diameter of the stent. That is, both high strength and high ductility should be achieved while maintaining a low yield stress. In our previous studies, it was discovered that low-temperature heat-treatment (LTHT) at 873 K improves the elongation of the CCWN alloy. In this study, we focused on the grain refinement by swaging and static recrystallization to improve the strength of the alloy. The as-swaged alloy was recrystallized at 1373–1473 K for 100–300 s, followed by LTHT. A fine grain structure with an average grain size of 3–17 μm was obtained by static recrystallization. The η-phase (M₁₂X-M₆X type precipitates, M: metallic elements, X: C and/or N) formed during the recrystallization at 1373–1448 K. The alloys recrystallized at 1448 and 1473 K had a homogeneous structure with a small variation in the grain size. On the other hand, the alloys recrystallized at 1373 and 1423 K had an inhomogeneous structure in which fine and coarse grains were mixed. Both the strength and ductility of the CCWN alloy were improved by combining high-temperature short-time recrystallization and LTHT.

Abstract: Surgical implantation of metallic stents is today a common procedure for restoring narrowed arteries. However, main complications as in-stent restenosis, partial or total thrombosis, inflammation and devices degradation are still a serious clinical concern. The coating of stents with fluorocarbon (CF_x) ultrathin films represents a valuable strategy to limit these complications. Moreover, an additional step for the modification of some key surface properties of CF_x coatings could further enhance their blood compatibility. Therefore, the objective of this work was to develop an oxidation process specific to ultrathin CF_x coatings based on a methanol plasma treatment to modulate their biological response. Oxidized and non-oxidized coatings were investigated by XPS, ToF-SIMS, water contact angle, SEM and AFM. Tunable oxidation of the surface of CF_x coatings was obtained by methanol plasma treatment, thus producing an increase of surface wettability, without affecting morphology, roughness and adhesion of the coatings. Blood test results showed an increased hemocompatibility of the oxidized samples, confirming the hypothesis that such treatment can succeed in modulating the blood contact behavior of the CF_x oxidized coatings.

Abstract: The treatment of aortic aneurysm varies according to each patient. Stent implantation is one of many methods used to stabilize diseases. The information obtained from hemodynamic parameters in the pathological aorta with stent could help doctors in choosing the treatment before the surgery. In this context, the numerical simulation of fluid dynamics is an essential tool that can give us more detailed information that could be exploited by surgeons to find appropriate solutions to some pathologies. In this work, a study of two models of aortas: with and without stent was presented. A comparison of the results of changes in pressures and shear stresses of aortas in systolic and diastolic phases was discussed. The obtained results show a nearly similar tendency of variation for both models, which justifies the effectiveness of the type of stent chosen and its effect on the improvement of the blood flow. The methodology used in this work has revealed the detailed and necessary information for both cases studied and has shown the interest of the numerical tools for diagnosis and surgery.

Abstract: The main originality of this work consists in investigating low cycle fatigue of AISI 316L cardiovascular stents under hypertensive loading. For this purpose, two geometries of stents are expanded to various diameters and subjected to hypertensive blood pressure. Based on a combination between the fatigue parameter of Jiang-Sehitoglu and the relationship of Coffin-Manson, a numerical model for the prediction of the number of cycles to crack failure is developed. The stent is found to exhibit a fatigue life reduction with the increase of the expansion diameter due to ratchetting strain. In addition, the location of the failure is independent on the design. However, the U-shape strut permits a better distribution of pressure over the stent strut resulting in a longer fatigue life as compared to the Ω-shape.

Abstract: This paper is focused on prediction of nitinol stents lifetime. Stents have been tested before and after mechanical loading. Corrosion tests of mechanically loaded specimens indicated impairment of corrosion resistance and SEM observation confirmed presence of micro cracks in surface oxide layer created during heat treatment. Electrochemical impedance spectroscopy and potentiodynamic polarization in physiological solution was used for these particular experiments. Corrosion rate and other parameters have been used for calculation of released nickel ions weight and prediction of application lifetime.

Abstract: In spite of the fact that the US Food and Drug Administration (FDA) has approved Intracoil and s.m.a.r.t Nitinol stents for superficial femoral artery (SFA), some alternative designs of Nitinol stents are being implanted today, representing the off-label use of the devices. Among the currently stents used for the SFA, s.m.a.r.t and Intracoil stents show the most desirable long-term results but it is not understood why. In the present work, delivery of the s.m.a.r.t and Intracoil Nitinol stents and their release inside a stenotic point of the artery was simulated by FE implemented SMA model using a creative manual controlled method. The influence of the stent design on the stent–vessel interactions and stress state within the stent material after completion of the deployment was revealed. It was found that the Intracoil stent shall be more successful in eliminating the stenosis and less prone to fatigue failure, even though it had less thickness and less mean coil diameter.

Abstract: In this paper investigation results of behavior of promising superelastic alloys of Ti-Nb-Zr system in blood-vessel are presented. The possibility of their use in manufacturing of medical stents is examined. Based on analytical review of present scientific papers, four different alloys of Ti-Nb-Zr system are taken in consideration. A finite element modelling of stent behavior during delivery and opening stages is considered. These processes are done for two typical stent geometries and four alloys possessing different mechanical properties. Finite modelling results are analyzed to show the distribution of internal stresses, mechanical aspects of stent installation and effectiveness of various configurations to widen the narrowed vessel. Modeling has allowed to formulate recommendations for optimal mechanical characteristics of the superelastic alloy used for the manufacture of medical stents.

Abstract: In the present study, ABAQUS finite element modelling was used to explore the durability of cardiovascular stent made of AISI 316L. It was found that the blood pressure loading never causes the fracture of the stent. This result was confirmed by the application of two critical plane approaches and one mean stress criterion. However, when subjected to restenosis compressive loading the stent was found to experience an in-service failure. The last proved to be dependent on the stent diameter reduction, rate and location of restenosis. Since restenosis forms gradually and randomly within the artery, two distributions were considered and investigated. The eccentric restenosis turned out to be more deleterious than the concentric one.

Abstract: Laser cutting is one of the key fabrication technologies applied to coronary stent manufacture. This paper reviews the recent progress in laser-based stent manufacturing, including different type of lasers used, laser interaction with different stent materials, process characteristics and quality/productivity issues.