A novel coating method to reduce membrane infolding through pre-crimping of covered stents - Computational and experimental evaluation.

A covered stent has been used to treat carotid artery stenosis to reduce the chance of embolization, as it offers improved performance over bare-metal stents. However, membrane infolding of covered stents can affect efficiency and functionality for treating occlusive disease of first-order aortic branches. In order to mitigate the degree of infolding of the stent once it was re-expanded, we proposed a new coating method performed on the pre-crimped stent. A systematic study was carried out to evaluate this new coating technique: a) in vivo animal testing to determine the degree of membrane infolding; b) structural finite element modeling and simulation were used to evaluate the mechanical performance of the covered stent; and c) computational fluid dynamics (CFD) to evaluate hemodynamic behavior of the stents and risk of thrombosis after stent deployment. The degree of infolding was substantially reduced as demonstrated by the in vivo deployment of the pre-crimped stent compared to a conventional dip-coated stent. The structural analysis results demonstrated that the membrane of the covered stent manufactured by conventional dip-coating resulted in a large degree of infolding but this could be minimized by our new pre-crimped coating method. CFD studies showed that the new coating method reduced the risk of thrombosis compared to the conventional coating method. In conclusion, both simulation and in vivo testing demonstrate that our new pre-crimped coating method reduces membrane infolding compared with the conventional dip-coating method and may reduce risk of thrombosis.

Non-anatomical placement adversely affects the functional performance of the meniscal implant: a finite element study.

Non-anatomical placement may occur during the surgical implantation of the meniscal implant, and its influence on the resulting biomechanics of the knee joint has not been systematically studied. The purpose of this study was to evaluate the biomechanical effects of non-anatomical placement of the meniscal implant on the knee joint during a complete walking cycle. Three-dimensional finite element (FE) analyses of the knee joint were performed, based on the model developed from magnetic resonance images and the loading conditions derived from the gait pattern of a healthy male subject, for the following physiological conditions: (i) knee joint with intact native meniscus, (ii) medial meniscectomized knee joint, (iii) knee joint with anatomically placed meniscal implant, and (iv) knee joint with the meniscal implant placed in four different in vitro determined non-anatomical locations. While the native menisci were modeled using the nonlinear hyperelastic Holzapfel-Gasser-Ogden (HGO) constitutive model, the meniscal implant was modeled using the isotropic hyperelastic neo-Hookean model. Placement of the meniscal implant in the non-anatomical lateral-posterior and lateral-anterior locations significantly increased the peak contact pressure in the medial compartment. Placement of the meniscal implant in non-anatomical locations significantly altered the tibial rotational kinematics and increased the total force acting at the meniscal horns. Results suggest that placement of the meniscal implant in non-anatomical locations may restrain its ability to be chondroprotective and may initiate or accelerate cartilage degeneration. In conclusion, clinicians should endeavor to place the implant as closest as possible to the anatomical location to restore the normal knee biomechanics.

Nanoglass-based balloon expandable stents.

Here, a prototypical metallic nanoglass is proposed as a new alloy for balloon expandable stents. Traditionally, the stainless steel SS 316L alloy has been used as a preferred material for this application due to its proper combination of mechanical properties, corrosion resistance, and biocompatibility. Recently, metallic glasses (MGs) have been considered as promising materials for biodevice applications. MGs often display outstanding mechanical properties superior to those of conventional metallic alloys and overcome some of the weaknesses of SS 316L, such as radiopacity, stainless steel allergy, and thrombosis-induced restenosis. However, commonly used monolithic MGs, which have an amorphous homogeneous microstructure, suffer from lack of ductility that is necessary for deployment of balloon expandable stents. In contrast, nanoglasses, that is, amorphous alloys with heterogeneous microstructure, exhibit enhanced ductility which makes them promising materials for balloon expandable stents. We evaluate the feasibility of a prototypical Zr64 Cu36 nanoglass with a grain size of 5 nm for balloon expandable stents by performing finite element method modeling of the stent deployment process in a coronary artery. We consider the BX-Velocity stent design and the nanoglass mechanical properties calculated from atomistic simulations. The results suggest that nanoglasses are suitable materials for balloon expandable stent applications. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 108B:73-79, 2020.

Optimization of a Novel Preferential Covered Stent through Bench Experiments and in Vitro Platelet Activation Studies.

Bare metal stenting (BMS) does not adequately address the atheroembolic characteristic of carotid artery stenosis. While simple covered stents (CS) may prevent dislodged fragments of the atherosclerotic plaque from entering the blood stream, they also block blood flow into the major branches of the artery alongside the lesion, which is not desirable. Preferential covered stents (PCS) behave as a covered stent in a tubular part of a vessel but maintain side-branch flow over the bifurcation region by means of slits in the membrane. Stent design, membrane material, and slits configuration are the three main components contributing to stent performance. Optimization of PCS designs was conducted and tested. METHODS: A newly designed BMS was developed and compared to a commercially available peripheral stent. Two materials (expanded poly(tetrafluoroethylene)) and silicone polyurethane co-polymers (Elast-eon E2A) were used as stent coverings with slits applied using various cutting methods to form the PCS. These PCS samples were tested for physical resilience, flexibility, ability to preserve side-branch flow, slit edge roughness, and platelet activation. RESULTS: Fabrication of E2A-coated stents required pretreatment of the stent with poly(ethylene glycol) to achieve firm attachment. The newly designed BMS with nine crowns design and larger cell size showed higher flexibility than commercially available stents. A combination of a larger stent cell size, E2A membrane coating, and three slits per stent cell unit configuration resulted in preserved side-branch flow similar to physiological conditions in the flow experiment. Slit edge roughness changed with different cutting methods and laser machine cutting parameters. In vitro studies showed platelet activation was minimal with lower slit edge roughness samples. CONCLUSION: An optimized PCS prototype was developed consisting of a newly designed stent, E2A membrane, and a three-slit pattern created by specific femtosecond laser cutting.

Simulated Bench Testing to Evaluate the Mechanical Performance of New Carotid Stents.

Our group recently developed a novel covered carotid stent that can prevent emboli while preserving the external carotid artery (ECA) branch blood flow. However, our recent in vitro side-branch ECA flow preservation tests on the covered stents revealed the need for further stent frame design improvements, including the consideration to crimp the stent to a low profile for the delivery of the stent system and having bigger cells. Hence, the current work aims to design new bare metal stents with bigger cell size to improve the crimpability and to accommodate more slits so that the side-branch flow could be further increased. Three new stent designs were analyzed using finite element analysis and benchmarked against two commercially available carotid stents in terms of their mechanical performances such as crimpability, radial strength, and flexibility. Results indicated that the new bare metal stent designs matched well against the commercial stents. Hence our new generation covered stents based on these designs can be expected to perform better in side-branch flow preservation without compromising on their mechanical performances.

An Experimental and Computational Study on the Effect of Caval Valved Stent Oversizing.

Heterotopic implantation of transcatheter tricuspid valve is a new treatment option for tricuspid regurgitation. Transcatheter tricuspid valves are implanted onto the cavoatrial junction in order to avoid the challenging task of anchoring the valve onto the complex tricuspid valve annulus. However, little is known about optimum extent of oversizing of the valved stent in a vena cava. In this study, we implanted valves of the same diameter onto the larger sized inferior vena cava (IVC) and a smaller sized superior vena cava (SVC). The valve in the IVC was oversized by 10.7% while the valve in the SVC was oversized by 21.6%. Finite element analysis was performed (i) to assess the strain on the nitinol stent during manufacturing and deployment; (ii) the stents were deployed in a patient-specific vena cava model and the intramural stress of the vena cava was calculated computationally. These valves were fabricated and placed in a silicone model of a patient-specific right atrium which was part of a mock circulatory system that emulated the patho-physiological flow rate and pressure of a patient with tricuspid regurgitation. Flow measurements were conducted by particle image velocimetry (PIV). It was found that the maximum crimping strain on the nitinol stent was 6.85% which was lower than the critical threshold of 10%. The maximum stress on the vena cava was located at the spot where the hooks met the wall. The maximum stress on the IVC was 0.5098 MPa while the maximum stress on the SVC was 0.7 MPa. The maximum Reynolds shear stress (mRSS) in the vena cava was found to be higher in the IVC than SVC with the highest mRSS being 1741 dynes/cm(2) found in the region of high flow during the peak flow phase. The overtly oversized valve in the SVC did not cause flow disturbances and exhibited mostly laminar flows. The mRSS at the downstream of the vena cava valve and the middle of the atrium remained at low magnitudes. However, velocity fluctuations were high in the IVC in all the time points measured. In conclusion, oversizing the valve may assist anchorage; yet, careful consideration should be taken in choosing the extent of oversizing as it may lead to adverse effects.

Numerical Modeling of Intraventricular Flow during Diastole after Implantation of BMHV.

This work presents a numerical simulation of intraventricular flow after the implantation of a bileaflet mechanical heart valve at the mitral position. The left ventricle was simplified conceptually as a truncated prolate spheroid and its motion was prescribed based on that of a healthy subject. The rigid leaflet rotation was driven by the transmitral flow and hence the leaflet dynamics were solved using fluid-structure interaction approach. The simulation results showed that the bileaflet mechanical heart valve at the mitral position behaved similarly to that at the aortic position. Sudden area expansion near the aortic root initiated a clockwise anterior vortex, and the continuous injection of flow through the orifice resulted in further growth of the anterior vortex during diastole, which dominated the intraventricular flow. This flow feature is beneficial to preserving the flow momentum and redirecting the blood flow towards the aortic valve. To the best of our knowledge, this is the first attempt to numerically model intraventricular flow with the mechanical heart valve incorporated at the mitral position using a fluid-structure interaction approach. This study facilitates future patient-specific studies.

Design considerations and quantitative assessment for the development of percutaneous mitral valve stent.

Percutaneous heart valve replacement is gaining popularity, as more positive reports of satisfactory early clinical experiences are published. However this technique is mostly used for the replacement of pulmonary and aortic valves and less often for the repair and replacement of atrioventricular valves mainly due to their anatomical complexity. While the challenges posed by the complexity of the mitral annulus anatomy cannot be mitigated, it is possible to design mitral stents that could offer good anchorage and support to the valve prosthesis. This paper describes four new Nitinol based mitral valve designs with specific features intended to address migration and paravalvular leaks associated with mitral valve designs. The paper also describes maximum possible crimpability assessment of these mitral stent designs using a crimpability index formulation based on the various stent design parameters. The actual crimpability of the designs was further evaluated using finite element analysis (FEA). Furthermore, fatigue modeling and analysis was also done on these designs. One of the models was then coated with polytetrafluoroethylene (PTFE) with leaflets sutured and put to: (i) leaflet functional tests to check for proper coaptation of the leaflet and regurgitation leakages on a phantom model and (ii) anchorage test where the stented valve was deployed in an explanted pig heart. Simulations results showed that all the stents designs could be crimped to 18F without mechanical failure. Leaflet functional test results showed that the valve leaflets in the fabricated stented valve coapted properly and the regurgitation leakage being within acceptable limits. Deployment of the stented valve in the explanted heart showed that it anchors well in the mitral annulus. Based on these promising results of the one design tested, the other stent models proposed here were also considered to be promising for percutaneous replacement of mitral valves for the treatment of mitral regurgitation, by virtue of their key features as well as effective crimping. These models will be fabricated and put to all the aforementioned tests before being taken for animal trials.

Has percutaneous aortic valve replacement taken center stage in the treatment of aortic valve disease?

Modern biomedical advances have propelled percutaneous valve replacement into an effective and powerful therapy for many heart valve diseases, especially aortic valve stenosis. Experiences so far suggest that outcomes for new percutaneous valve replacement surgery compare favorably with that of traditional valve surgery in selected patients with severe symptomatic aortic stenosis. The inception of percutaneous aortic valve replacement (PAVR) began in 1992 when the potential for treating valve diseases was demonstrated through a modern technique of endoluminal deployment of a catheter-mounted crimped stented heart valve in an animal model. The first successful demonstration of such novel technique of surgical replacement of a heart valve was performed in 2002, when valve implantation in a patient with aortic stenosis was reported. Despite initial stumbles and a perception of being an uphill task, PAVR has emerged as one of the breakthroughs in surgical procedures. More than 1500 citations were found in PubMed, half of which were available after 2011. This is primarily because more than 50,000 procedures are being performed in more than 40 countries worldwide, with encouraging outcomes, and several stented valves have been launched in the market. This review provides a detailed analysis of the current state of the art of PAVR. Moreover, a competitive landscape of various devices available in the market and their design considerations, biomaterial selections, and overall hemodynamic performance are presented.

Self-expanding aortic valve stent-material optimization.

Vascular support structures are important devices for treating valve stenosis. Large population of patients is treated for valvular disease and the principal mode of treatment is the use of percutaneous valvuloplasty. Stent devices are proving to be an improved technology in minimal invasive cardiac surgery. This technology now accounts for 20% of treatments in Europe. This new technology provides highly effective results at minimal cost and short duration of hospitalization. During the development process, a number of specific designs and materials have come and gone, and a few have remained. Many design changes were successful, and many were not. This paper discusses the physical behavior of a hooked percutaneous aortic valve stent design using a finite element analysis. Specifically, the effects of crimping was simulated and analyzed for two types of realistic but different Nitinol materials (NITI-1 and NITI-2). The results show that both NITI-1 and NITI-2 had good crimping performance. The analysis performed in this paper may aid in understanding the stent's displacement ranges when subjected to physiological pressures exerted by the heart and cardiac blood flow during abnormal cardiovascular conditions. It may also help to evaluate the suitability of a Nitinol for fabrication purposes.