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The purpose of this work is to propose a workflow that couples experimental and computational activities aimed at developing a credible digital twin of a commercial coronary bioresorbable vascular scaffold when direct access to data about material mechanical properties is not possible. Such a situation is be faced when the manufacturer is not involved in the study, thus directly investigating the actual device is the only source of information available. The object of the work is the Fantom® Encore polymeric stent (REVA Medical) made of Tyrocore™. Four devices were purchased and used in mechanical tests that are easily reproducible in any mechanical laboratory, i.e. free expansion and uniaxial tension testing, the latter performed with protocols that emphasized the rate-dependent properties of the polymer. Given the complexity of the mechanical behaviour observed experimentally, it was chosen to use the Parallel Rehological Framework material model, already used in the literature to describe the behaviour of other polymers, such as PLLA. Calibration of the material model was based on simulations that replicate the tensile test performed on the device. Given the high number of material parameters, a plan of simulations was done to find the most suitable set, varying each parameter value in a feasible range and considering a single repetitive unit of the stent, neglecting residual stresses generated by crimping and expansion. This strategy resulted in a significant reduction of computational cost. The performance of the set of parameters thus identified was finally evaluated considering the whole delivery system, by comparing the experimental results with the data collected simulating free expansion and uniaxial tension testing. Moreover, radial force testing was numerically performed and compared with literature data. The obtained results demonstrated the effectiveness of the digital twin development pipeline, a path applicable to any commercial device whose geometric structure is based on repetitive units.

The current interest of those dealing with medical research is the preparation of digital twins. In this frame, the first step to accomplish is the preparation of reliable numerical models. This is a challenging task since it is not common to know the exact device geometry and material properties unless in studies performed in collaboration with the manufacturer. The particular case of modeling Ni–Ti stents can be highlighted as a worst-case scenario due to both the complex geometrical features and non-linear material response. Indeed, if the limitations in the description of the geometry can be overcome, many difficulties still exist in the assessment of the material, which can vary according to the manufacturing process and requires many parameters for its description. The purpose of this work is to propose a coupled experimental and computational workflow to identify the set of material properties in the case of commercially-resembling Ni–Ti stents. This has been achieved from non-destructive tensile tests on the devices compared with results from Finite Element Analysis (FEA). A surrogate modeling approach is proposed for the identification of the material parameters, based on a minimization problem on the database of responses of Ni–Ti materials obtained with FEA with a series of different parameters. The reliability of the final result was validated through the comparison with the output of additional experiments.

Coronary stent fracture is still an unresolved issue in the field of minimally invasive cardiovascular interventions due to its high rate of incidence and uncertain clinical consequences. Recent studies, based on clinical data, proved that there are several factors which can be identified as independently responsible of coronary stent fracture. Among these, calcifications, which increase the local stiffness and heterogeneity of atherosclerotic plaques, seem to play a major role. From a mechanical point of view, stent fracture in coronary arteries is triggered by the cyclic loading of pulsatile blood pressure combined with the movement of cardiac wall. In this context, this study aims at simulating the stent expansion in a model of epicardial atherosclerotic coronary artery and correlating the effects of cyclic blood pressure and cardiac wall movement on the stent fatigue resistance. Two ideal cases of atherosclerotic plaques were modelled: the first one included a localised plaque calcification; the latter one did not include such calcification. Results of stress/strain and fatigue analyses confirmed the influence of the plaque calcification on potential fracture of the devices. In addition, the effects of cardiac wall movement were quantified as more dangerous causes of the stent fatigue fracture with respect to the internal blood pressure oscillations. In conclusion, this study demonstrates the increased risk of coronary stent fracture associated to the presence of localised plaque calcifications. This work also suggests the necessity of more realistic biomechanical models which takes into account the heterogeneity of atherosclerotic plaques in order to assess the mechanical performances of coronary stents.

The ID Venous System is an innovative device proposed by ID NEST MEDICAL to treat venous compression disorders that involve bifurcations, such as the May-Thurner syndrome. The system consists of two components, ID Cav and ID Branch, combined through a specific connection that prevents the migration acting locally on the pathological region, thereby preserving the surrounding healthy tissues. Preliminary trials are required to ensure the safety and efficacy of the device, including numerical simulations. In-silico models are intended to corroborate experimental data, providing additional local information not acquirable by other means. The present work outlines the finite element model implementation and illustrates a sequential validation process, involving seven tests of increasing complexity to assess the impact of each numerical uncertainty separately. Following the standard ASME V&V40, the computational results were compared with experimental data in terms of force-displacement curves and deformed configurations, testing the model reliability for the intended context of use (differences < 10%). The deployment in a realistic geometry confirmed the feasibility of the implant procedure, without risk of rupture or plasticity of the components, highlighting the potential of the present technology.

Drug-coated balloons (DCBs) are designed to deliver an anti-proliferative drug to the stenotic vessel to combat restenosis after an angioplasty treatment. However, significant drug loss can occur during device navigation toward the lesion site, thus reducing the delivery efficiency and increasing the off-target drug loss. In this framework, this study aimed to design a novel in vitro setup to estimate the drug loss due to blood flow–coating interaction during tracking. The system consists of a millifluidic chamber, able to host small drug-coated flat patches representative of DCBs, connected at the inlet to a syringe pump able to provide an ad hoc flow and, at the outlet, to a vial collecting the testing fluid with possible drug removed from the specimen. Unlike other studies, the device presented here uniquely evaluates flow-related drug loss from smaller-scale DCB samples, making it a precise, easy-to-use, and efficient assessment tool. In order to define proper boundary conditions for these washing off tests, computational fluid dynamics (CFD) models of a DCB in an idealized vessel were developed to estimate the wall shear stresses (WSSs) experienced in vivo by the device when inserted into leg arteries. From these simulations, different target WSSs were identified as of interest to be replicated in the in vitro setup. A combined analytical–CFD approach was followed to design the testing system and set the flow rates to be imposed to generate the desired WSSs. Finally, a proof-of-concept study was performed by testing eight coated flat specimens and analyzing drug content via high-performance liquid chromatography (HPLC). Results indicated different amounts of drug loss according to the different imposed WSSs and confirmed the suitability of the designed system to assess the washing off resistance of different drug coatings for angioplasty balloons.

Stents are tubular meshed endoprostheses implanted mini-invasively through a transcatheter intervention to guarantee the patency of body conduits, mainly in cardiovascular applications. In pediatric cardiology, stenting has become an accepted procedure in the treatment of congenital heart disease (CHD) as an alternative to open-heart surgery. CHD refers to a range of defects affecting the heart’s structure and function arising from abnormal development during pregnancy. While during fetal life, the presence of additional shunts allows for the establishment of parallel circulation and survival of gestation, CHD is not compatible with extrauterine life, and medical intervention is required soon after birth. This review aims to discuss the state of the art of stenting in CHD. Despite the severity of these pathologies, investment from the industry remains limited due to the restricted number of cases, and dedicated devices are still missing. As a consequence, commercially available adult stents are commonly exploited on an off-label basis in newborns without any optimization for the specific anatomy and required function. In this review, a classification of the available stents is provided, resuming the manufacturing technologies, materials, and geometrical aspects to obtain the target biomechanical performance. After analyzing the fetal circulation, different forms of CHD amenable to stenting are considered, collecting the stents currently adopted and discussing the clinical outcomes to outline the features of an ideal device.

Background/Objective: Paclitaxel-coated balloons (PCBs) can deliver efficacious drug concentrations to treated arterial segments but are known to exhibit high tracking losses. We aimed to define the governing factors impacting tracking loss and to contrast its drug distribution consequences with those of PCB inflation at the treatment site. Methods: Four naïve and four in-stent restenosis (ISR) porcine superficial femoral arteries (SFA) were treated with PCBs, and plasma samples were collected post-tracking and post-inflation. Animals were sacrificed <1 h post-intervention, and local, upstream, and downstream tissues were collected for paclitaxel quantification. Computationally driven quantitative benchtop-tracking and frictional PCB-sliding experiments modeled paclitaxel loss and delivery to upstream tissue. Results: Paclitaxel concentrations in plasma peaked pre-inflation and declined 30-fold immediately post-inflation. Correspondingly, losses of 30% and 1% of nominal PCB load were measured in vitro during, respectively, tracking over single bend and during device insertion. Mean paclitaxel concentrations were equally high at ISR and naïve SFA treatment sites (56,984 vs. 79,837 ng/g, p > 0.99) and ranged from 9 to 89 ng/g in tissues downstream of these treatment sites. Sampling of non-target upstream iliac artery tissues revealed paclitaxel concentration of 4351 ± 4084 ng/g. Benchtop sliding of PCB samples onto ex vivo porcine artery samples exhibited efficient, pressure independent frictional paclitaxel transfer (124 µg at 0.05 atm vs 126 µg at 0.1 atm, p > 0.99). Conclusions: PCB interactions at porcine vessel bends led to premature tracking loss, resulting in peak plasma concentrations exceeding post-inflation concentrations, and delivery to upstream tissue that is plausibly explained as arising from efficient friction-mediated coating transfer.

The objective of this study is to compare the coronary and pulmonary blood flow dynamics resulting from two configurations of systemic-to-pulmonary artery shunts currently utilized during the Norwood procedure: the central (CS) and modified Blalock Taussig (MBTS) shunts. A lumped parameter model of the neonatal cardiovascular circulation and detailed 3-D models of the shunt based on the finite volume method were constructed. Shunt sizes of 3, 3.5 and 4mm were considered. A multiscale approach was adopted to prescribe appropriate and realistic boundary conditions for the 3-D models of the Norwood circulation. Results showed that the average shunt flow rate is higher for the CS option than for the MBTS and that pulmonary flow increases with shunt size for both options. Cardiac output is higher for the CS option for all shunt sizes. Flow distribution between the left and the right pulmonary arteries is not completely balanced, although for the CS option the discrepancy is low (50–51% of the pulmonary flow to the right lung) while for the MBTS it is more pronounced with larger shunt sizes (51–54% to the left lung). The CS option favors perfusion to the right lung while the MBTS favors the left. In the CS option, a smaller percentage of aortic flow is distributed to the coronary circulation, while that percentage rises for the MBTS. These findings may have important implications for coronary blood flow and ventricular function.

Mechanical loads on the polymeric fibers of oxygenating bundles are commonly present due to bundle press-fitting during device assembly and blood pressure load. However, computational fluid dynamics (CFD) simulations for fiber bundle optimization neglect possible changes in microstructure due to such deformations. The aim of this study is to investigate the impact of fiber deformability on bundle microstructure and fluid dynamics mainly in terms of permeability. Fibers from commercial mats typically used for blood oxygenators were mechanically tested and based on these experimental data, a material model was developed to simulate the structural deformations the fibers undergo under press-fitting and blood pressure loads. Then, CFD simulations were performed on deformed bundle repetitive units to investigate permeability under varying loading conditions. The effects of different bundle geometric parameters on the variation of bundle permeability due to press-fitting were evaluated. Bundle press-fitting results in significant changes in microstructure that are reflected in a bundle permeability more than halved for a 15% press-fitting. This impact on permeability is present in all the simulated fiber bundles and becomes more pronounced as the pitch between fibers and thus bundle porosity decreases. Instead, the analyses on pressurized bundle show only small deformations caused by pressure load, with permeability changes below 1%. While blood pressure effects could be neglected, bundle press-fitting turns out to have a significant impact on bundle microstructure and permeability. Neglecting such microstructure variations during CFD simulations could also lead to incorrect assessment of the local fluid dynamics within the bundle.

NiTi (nickel-titanium) stents are nowadays commonly used for the percutaneous treatment of peripheral arterial disease. However, their effectiveness is still debated in the clinical field. In fact a peculiar cyclic biomechanical environment is created before and after stent implantation, with the risk of device fatigue failure. An accurate study of the device fatigue behavior is of primary importance to ensure a successful stenting procedure. Regulatory authorities recognize the possibility of performing computational analyses instead of experimental tests for the assessment of medical devices. However, confidence in numerical methods is only possible after verification and validation of the models used. For the case of NiTi stents, mechanical properties are strongly dependent on the device dimensions and the whole treatments undergone during manufacturing process. Hence, special attention should be paid to the accuracy of the description of the device geometry and the material properties implementation into the numerical code, as well as to the definition of the fatigue limit. In this paper, a path for setting up an effective numerical model for NiTi stent fatigue assessment is proposed and the results of its application in a specific case study are illustrated.