Improving the Study of Multiobjective Optimization of a Stent

Abstract This paper is coming with a view to extend and improve the multiobjective optimization of a stent in a fluid structure context studied in the previous works. The stent is assumed to be elastic and is modeled by Euler-Bernouilli equation. To obtain an optimal stent shape, we combine a fluid structure interaction computational method with a -multiobjective evolutionary algorithm

Identification of Hemodynamically Optimal Coronary Stent Designs Based on Vessel Caliber

Coronary stent design influences local patterns of wall shear stress (WSS) that are associated with neointimal growth, restenosis, and the endothelialization of stent struts. The number of circumferentially repeating crowns NC for a given stent de- sign is often modified depending on the target vessel caliber, but the hemodynamic implications of altering NC have not previously been studied. In this investigation, we analyzed the relationship between vessel diameter and the hemodynamically optimal NC using a derivative-free optimization algorithm coupled with computational fluid dynamics. The algorithm computed the optimal vessel diameter, defined as minimizing the area of stent-induced low WSS, for various configurations (i.e., NC) of a generic slotted-tube design and designs that resemble commercially available stents. Stents were modeled in idealized coronary arteries with a vessel diameter that was allowed to vary between 2 and 5 mm. The results indicate that the optimal vessel diameter increases for stent configurations with greater NC, and the designs of current commercial stents incorporate a greater NC than hemodynamically optimal stent designs. This finding suggests that reducing the NC of current stents may improve the hemodynamic environment within stented arteries and reduce the likelihood of excessive neointimal growth and thrombus formation

A Systematic Review on Cardiovascular Stent and Stenting Failure: Coherent Taxonomy, Performance Measures, Motivations, Open Challenges and Recommendations

Cardiovascular stenting is a mature topic but it is still being developed in the research community because of its importance. To provide worthy information about cardiovascular stenting environments and to give support to the researchers, attention must be given to understand the obtainable choices and gaps in this research field. This work aims to examine and examine the literature of each work related to the placement of cardiovascular stents, the failure of the stents and the models of stent designs to provide a good understanding through the investigation of articles published in various contextual aspects, such as motivations, open-challenges and recommendations to improve the field of stent placement. A systematic review is carried-out to map and examine the articles related to cardiovascular stents, the failure of the stents and the models of stent designs through a coherent-taxonomy used in three well-known scientific databases: ScienceDirect, IEEE Explore, and Web of Science. These databases involve literature that highlight arterial stenting. Based-on our inclusion and exception, a total of 90 articles composed the final set that offer various classes and sub-classes. The first class includes the development studies with (42/90) of experimental, computational and combined experimental and computational studies related to stent models performance and stent failure, the second class discussed studies that have been performed on stent design with (32/90), the third class is focused on the framework studies with (10/90), and the fourth class includes problems of stenting long-term with (6/90). The performance of stent designs, which is a research area that requires periodic controls, tools and procedures that could provide a stent design with good mechanical performance, reduce restenosis in the stent and increase fatigue resistance and durability. There have been numerous studies on stent performance that could promise good results in this field. The fields of research in stent designs vary, but all fields are fundamental equally. The expectation of this work could help to emphasize present research chances and, therefore, expand and make further research fields

A reduced-order modeling for efficient design study of artificial valve in enlarged ventricular outflow tracts

A computational approach is proposed for efficient design study of a reducer stent to be percutaneously implanted in enlarged right ventricular outflow tracts (RVOT). The need for such a device is driven by the absence of bovine or artificial valves which could be implanted in these RVOT to replace the absent or incompetent native valve, as is often the case over time after Tetralogy of Fallot repair. Hemodynamics are simulated in the stented RVOT via a reduce order model based on proper orthogonal decomposition (POD), while the artificial valve is modeled as a thin resistive surface. The reduced order model is obtained from the numerical solution on a reference device configuration, then varying the geometrical parameters (diameter) for design purposes. To validate the approach, forces exerted on the valve and on the reducer are monitored, varying with geometrical parameters, and compared with the results of full CFD simulations. Such an approach could also be useful for uncertainty quantification

Computational Design of Structures for Enhanced Failure Resistance

The field of structural design optimization is one with great breadth and depth in many engineering applications. From the perspective of a designer, three distinct numerical methodologies may be employed. These include size, shape, and topology optimization, in which the ordering typically (but not always) corresponds to the order of increasing complexity and computational expense. This, of course, depends on the particular problem of interest and the selected numerical methods. The primary focus of this research employs density-based topology optimization with the goal of improving structural resistance to failure. Beginning with brittle fracture, two topology optimization based formulations are proposed in which low weight designs are achieved with substantially increased fracture resistance. In contrast to the majority of the current relevant literature which favors stress constraints with linear elastic physics, we explicitly simulate brittle fracture using the phase field method during the topology optimization procedure. In the second formulation, a direct comparison is made against results obtained using conventional stress-constrained topology optimization and the improved performance is numerically demonstrated. Multiple enhancements are proposed including a numerical efficiency gain based on the Schur-complement during the analytical sensitivity analysis and a new function which provides additional path information to the optimizer, making the gradient-based optimization problem more tractable in the presence of brittle fracture physics. Subsequently, design for ductile failure and buckling resistance is addressed and a numerically efficient topology optimization formulation is proposed which may provide significant design improvements when ductile materials are used and extreme loading situations are anticipated. The proposed scheme is examined regarding its impact on both the peak load carrying capacity of the structure and the amount of external work required to achieve this peak load, past which the structure may no longer be able to support any increase in the external force. The optimized structures are also subjected to a post-optimization verification step in which a large deformation phase field fracture model is used to numerically compare the performance of each design. Significant gains in structural strength and toughness are demonstrated using the proposed framework. Additionally, the failure behavior of 3D-printed polymer composites is investigated, both numerically and experimentally. A large deformation phase field fracture model is derived under the assumption of plane-stress for numerical efficiency. Experimental results are compared to numerical simulations for a composite system consisting of three stiff circular inclusions embedded into a soft matrix. In particular, we examine how geometric parameters, such as the distances between inclusions and the length of initial notches affect the failure pattern in the soft composites. It is shown that the mechanical performance of the system (e.g. strength and toughness) can be tuned through selection of the inclusion positions which offers useful insight for material design. Finally, a size optimization technique for a cardiovascular stent is proposed with application to a balloon expandable prosthetic heart valve intended for the pediatric population born with Congenital Heart Disease (CHD). Multiple open heart surgical procedures are typically required in order to replace the original diseased valve and subsequent prosthetic valves with those of larger diameter as the patient grows. Most expandable prosthetic heart valves currently in development to resolve this issue do not incorporate a corresponding expandable conduit that is typically required in a neonate without a sufficiently long Right Ventricular Outflow Tract (RVOT). Within the context of a particular design, a numerical methodology is proposed for designing a metallic stent incorporated into the conduit between layers of polymeric glue. A multiobjective optimization problem is solved, not only to resist the retractive forces of the glue layers, but also to ensure the durability of the stent both during expansion and while subject to the anticipated high cycle fatigue loading. It is demonstrated that the surrogate-based optimization strategy is effective for understanding the trade-offs between each performance metric and ultimately efficiently arriving at a single optimized design candidate. Finally, it is shown that the desired expandability of the device from 12mm to 16mm inner diameter is achievable, effectively eliminating at least one open heart surgical procedure for certain children born with CHD

Optimization of Drug Delivery by Drug-Eluting Stents

International audienceDrug-eluting stents (DES), which release anti-proliferative drugs into the arterial wall in a controlled manner, have drastically reduced the rate of in-stent restenosis and revolutionized the treatment of atherosclerosis. However, late stent thrombosis remains a safety concern in DES, mainly due to delayed healing of the endothelial wound inflicted during DES implantation. We present a framework to optimize DES design such that restenosis is inhibited without affecting the endothelial healing process. To this end, we have developed a computational model of fluid flow and drug transport in stented arteries and have used this model to establish a metric for quantifying DES performance. The model takes into account the multi-layered structure of the arterial wall and incorporates a reversible binding model to describe drug interaction with the cells of the arterial wall. The model is coupled to a novel optimization algorithm that allows identification of optimal DES designs. We show that optimizing the period of drug release from DES and the initial drug concentration within the coating has a drastic effect on DES performance. Paclitaxel-eluting stents perform optimally by releasing their drug either very rapidly (within a few hours) or very slowly (over periods of several months up to one year) at concentrations considerably lower than current DES. In contrast, sirolimus-eluting stents perform optimally only when drug release is slow. The results offer explanations for recent trends in the development of DES and demonstrate the potential for large improvements in DES design relative to the current state of commercial devices

A review on the use of finite element simulations for structural analyses of coronary stenting: What can we do nowadays and what do we need to move forward?

In silico studies to perform structural analyses of coronary stenting have attracted the attention of many researchers in the last 25 years. As a consequence, the finite element models used to describe the fundamental elements of stenting simulations, namely the delivery system (consisting of stent and balloon), the diseased artery, and the deployment procedure have had considerable development, paving the way for the application of numerical analyses in both manufacturing and clinical contexts. Indeed, in accordance with the logic of the 3Rs (refine, reduce, and replace), simulations can play a fundamental role in developing new devices and as a support tool for training/education and operational planning activities for clinical personnel. However, the application of such numerical methodologies in the aforementioned contexts of use requires an adequate level of credibility of the models with respect to the risk associated to their use in the decision-making process. Within this framework, this paper proposes a review of the modeling approaches available today for in silico stenting of coronary arteries and a discussion of their actual or potential application areas. In particular, the attention is focused on the different levels of credibility required by the presented contexts of use with respect to the validation activities of numerical models developed up to now

Improving Cardiovascular Stent Design Using Patient-Specific Models and Shape Optimization

Stent geometry influences local hemodynamic alterations (i.e. the forces moving blood through the cardiovascular system) associated with adverse clinical outcomes. Computational fluid dynamics (CFD) is frequently used to quantify stent-induced hemodynamic disturbances, but previous CFD studies have relied on simplified device or vascular representations. Additionally, efforts to minimize stent-induced hemodynamic disturbances using CFD models often only compare a small number of possible stent geometries. This thesis describes methods for modeling commercial stents in patient-specific vessels along with computational techniques for determining optimal stent geometries that address the limitations of previous studies. An efficient and robust method was developed for virtually implanting stent models into patient-specific vascular geometries derived from medical imaging data. Models of commercial stent designs were parameterized to allow easy control over design features. Stent models were then virtually implanted into vessel geometries using a series of Boolean operations. This approach allowed stented vessel models to be automatically regenerated for rapid analysis of the contribution of design features to resulting hemodynamic alterations. The applicability of the method was demonstrated with patient-specific models of a stented coronary artery bifurcation and basilar trunk aneurysm to reveal how it can be used to investigate differences in hemodynamic performance in complex vascular beds for a variety of clinical scenarios. To identify hemodynamically optimal stents designs, a computational framework was constructed to couple CFD with a derivative-free optimization algorithm. The optimization algorithm was fully-automated such that solid model construction, mesh generation, CFD simulation and time-averaged wall shear stress (TAWSS) quantification did not require user intervention. The method was applied to determine the optimal number of circumferentially repeating stent cells (NC) for a slotted-tube stents and various commercial stents. Optimal stent designs were defined as those minimizing the area of low TAWSS. It was determined the optimal value of NC is dependent on the intrastrut angle with respect to the primary flow direction. Additionally, the geometries of current commercial stents were found to generally incorporate a greater NC than is hemodynamically optimal. The application of the virtual stent implantation and optimization methods may lead to stents with superior hemodynamic performance and the potential for improved clinical outcomes. Future in vivo studies are needed to validate the findings of the computational results obtained from the methods developed in this thesis

Biomechanical performance of magnesium stents obtained by ultrasonic-microcasting. Numerical modeling and material experimental characterization

Tese de Doutoramento (Programa Doutoral Em Líderes Para Indústrias Tecnológicas)O desejo constante de superação impulsiona a evolução num mundo moderno onde a mudança ocorre a uma velocidade estonteante. Os desafios tecnológicos e sociais são cada vez mais ambiciosos, o que se traduz na necessidade de soluções extraordinárias, uma situação transversal a todas as áreas do conhecimento. Tomando a área da manufatura e materiais como exemplo, tal torna-se evidente. Aplicações no âmbito das áreas aeroespacial, biomédica e ambiental encabeçam os interesses e prioridades das comunidades científica e industrial, o que é justificado pelo impacto que qualquer avanço nesses domínios poderá significar para a sociedade. Neste sentido, existe uma necessidade de materiais e processos que possibilitem obter o desempenho requerido para aplicação em cenários de grande exigência, como os referidos. O magnésio e as suas ligas são considerados fortes candidatos para aplicação nas áreas em questão, especialmente para o fabrico de stents biodegradáveis, dada a sua excelente biocompatibilidade. Não obstante, a sua baixa conformabilidade à temperatura ambiente, a elevada reatividade e fácil ignição durante o e rápida degradação sob condições fisiológicas dificulta a sua aplicação alargada. O presente estudo focou-se na investigação do potencial da aplicação de tratamento por ultrassons no processo de fundição da liga de magnésio AZ91D-Ca, com o objetivo de melhorar as propriedades do material através da modificação da sua microestrutura. O tratamento foi aplicado em dois momentos distintos: (i) no banho metálico, de forma a promover a sua desgaseificação e (ii) durante o vazamento e solidificação do material, de modo a promover a afinação e modificação da microestrutura. A degradação do material e consequente efeito nas propriedades mecânicas foi investigada através da realização de testes dinâmicos de imersão em Earle’s Balanced Salt Solution. As curvas de tensão-deformação resultantes dos ensaios de tração do material imerso até 7 dias foram utilizadas como dados de entrada do modelo numérico desenvolvido com o objetivo de simular o processo de colocação do stent e otimizar a sua geometria. Os resultados obtidos demonstraram que o tratamento por ultrassom pode modificar significativamente a microestrutura da liga de magnésio AZ91D-Ca, melhorando as suas propriedades mecânicas e de resistência à corrosão, o que, de acordo com os dados obtidos através da simulação numérica, se mostrou determinante no desempenho do stent.In the modern world, change happens at lightning speed, and the eagerness for great deeds drives evolution. The technological and societal goals are increasingly ambitious, translating into the need for outstanding and complex solutions: this is a transversal and unquestionable truth to all the knowledge areas. Considering the manufacturing and materials area example, this becomes crystal clear. The aerospace, biomedical, and environmental-related applications have been leading the interests and priorities of both research and industrial communities, which is motivated by the potential impact any advance in this area can have in society. In this sense, there is a need for materials and processes that can offer the properties required for application in such demanding scenarios. Magnesium and its alloys are considered strong candidates for these applications, especially biodegradable stents manufacturing, because of their excellent biocompatibility. Nonetheless, some issues, such as poor formability at room temperature, high reactivity and easy ignition during the melting process and loss of mechanical strength motivated by fast degradation under physiological conditions, hinder their widespread. This study investigated the potential of applying ultrasound treatment to the casting process of AZ91D-Ca alloys, aiming at improving the material’s properties by tailoring their microstructure. Ultrasonic vibration was applied in two moments: (i) to the magnesium melt to promote its degassing and (ii) during the pouring and subsequent solidification to induce microstructural refinement and modification. Dynamic immersion tests in Earle’s Balanced Salt Solution were carried out to assess the degradation behavior of the material and its effect on the mechanical properties. Uniaxial tensile tests were performed on the material after immersion for up to seven days, and the strain-stress curves were used as input data for a numerical model developed to simulate the stent deployment procedure and optimize the device’s design. It has been demonstrated that ultrasound treatment could significantly modify the microstructure of AZ91D-Ca alloys, improving their mechanical and corrosion properties. Furthermore, the material’s enhanced properties have been demonstrated to play a significant role in the stent performance assessed through numerical simulation, stressing the potential of this technique for manufacturing magnesium-based biomedical devices.Fundação para a Ciência e a Tecnologia (FCT) for funding this work through the PhD grant PD/BD/140094/2018, in the context of MIT Portugal Program

Tensile strength of pine and ash woods – experimental and numerical study

The mechanical properties define the behaviour of the timber under external loads, resulting directly from the timber anisotropic and heterogeneity characteristics. Depending upon the type of applied load the failure can be tensile, shear or torsion. When load enter the plastic regime, the stress-strain relationship passes through a maximum called the tensile strength. The tensile strength of wood being constant above the fibre saturation point, it increases with decreasing moisture content below the fibre saturation. This can be related to where the water is absorbed in the microstructure. Their study is of great interest allowing the rational use of different wood species for structural and building purposes