Fig 4 - uploaded by Alissa Zaccaria
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(a-b-c) Braided texture in the longitudinal-radial plane (zr, top) and in the longitudinal-circumferential plane (zθ, bottom): (a) undeformed configuration; (b) deformation at which the lateral contact among the wires is first recorded; (c) maximum deformation. (d) Detection of the pitch angle corresponding with the first contact deformation (β 1c ), based on the radial coordinate trend.
Source publication
Braiding technology is nowadays commonly adopted to build stent-like devices. Indeed, these endoprostheses, thanks to their typical great flexibility and kinking resistance, find several applications in mini-invasive treatments, involving but not limiting to the cardiovascular field. The design process usually involves many efforts and long trial a...
Contexts in source publication
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... or compressing the stent, under external radial loads. However, each device presents a specific limit, both considering crimping and expansion deformations, after which, to further deform it, significant forces would be required. This stiffening is due to the activation of contacts among the wires in points distant from the overlapping area (Fig. 4b). If higher forces are applied, the stent may deform further but never exceed the physical limit defined by the wires' diameter (Fig. ...
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... expansion deformations, after which, to further deform it, significant forces would be required. This stiffening is due to the activation of contacts among the wires in points distant from the overlapping area (Fig. 4b). If higher forces are applied, the stent may deform further but never exceed the physical limit defined by the wires' diameter (Fig. ...
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... pitch angle related to the maximum compressed configuration (β min in Fig. 4c) is obtained assuming that between two crossing points (circumferential distance ≈ πD/N), the longitudinal displacement should be equal to the diameter of the ...
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... regards the pitch angle at which the first contact among the wires appeared (β 1c in Fig. 4b), the intertwined geometry plays the main role. It is possible to assume that the contact occurs on the mean plane (cylindrical surface with diameter equal to the average stent diameter: orange surface in Fig. 4d). The width of the wire footprint on this surface (FP) is determined by the radial coordinate oscillation trend (ΔR) based ...
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... regards the pitch angle at which the first contact among the wires appeared (β 1c in Fig. 4b), the intertwined geometry plays the main role. It is possible to assume that the contact occurs on the mean plane (cylindrical surface with diameter equal to the average stent diameter: orange surface in Fig. 4d). The width of the wire footprint on this surface (FP) is determined by the radial coordinate oscillation trend (ΔR) based on the following ...
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... β 1c corresponds to the arctangent of FP first derivative with respect to the wire length (l 1 /l 2 coordinate in Fig. 4d) at the origin. Considering a sinusoidal intertwining, meaning a radial coordinate oscillation equal ...
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... good estimators of the maximum and minimum diameter, the latter of which affect the system deliverability. However, during the braiding procedure, a tensile load is usually applied at the wire extremities, resulting in a more linear trend with respect to the sinusoidal oscillation and, subsequently, in a narrower diameter variation range (↑β 1c , Fig. 4). Thus, the β 1c obtained with the sinusoidal approximation may be considered as the lower limit to which the actual device tends with decreasing load applied during the manufacturing. A more accurate result could be obtained by modifying the wire's radial oscillation (ΔR , Fig. 4, Eq. (35)) based on the applied ...
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... the β 1c obtained with the sinusoidal approximation may be considered as the lower limit to which the actual device tends with decreasing load applied during the manufacturing. A more accurate result could be obtained by modifying the wire's radial oscillation (ΔR , Fig. 4, Eq. (35)) based on the applied load. ...
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... or compressing the stent, under external radial loads. However, each device presents a specific limit, both considering crimping and expansion deformations, after which, to further deform it, significant forces would be required. This stiffening is due to the activation of contacts among the wires in points distant from the overlapping area (Fig. 4b). If higher forces are applied, the stent may deform further but never exceed the physical limit defined by the wires' diameter (Fig. ...
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... expansion deformations, after which, to further deform it, significant forces would be required. This stiffening is due to the activation of contacts among the wires in points distant from the overlapping area (Fig. 4b). If higher forces are applied, the stent may deform further but never exceed the physical limit defined by the wires' diameter (Fig. ...
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... pitch angle related to the maximum compressed configuration (β min in Fig. 4c) is obtained assuming that between two crossing points (circumferential distance ≈ πD/N), the longitudinal displacement should be equal to the diameter of the ...
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... regards the pitch angle at which the first contact among the wires appeared (β 1c in Fig. 4b), the intertwined geometry plays the main role. It is possible to assume that the contact occurs on the mean plane (cylindrical surface with diameter equal to the average stent diameter: orange surface in Fig. 4d). The width of the wire footprint on this surface (FP) is determined by the radial coordinate oscillation trend (ΔR) based ...
Context 13
... regards the pitch angle at which the first contact among the wires appeared (β 1c in Fig. 4b), the intertwined geometry plays the main role. It is possible to assume that the contact occurs on the mean plane (cylindrical surface with diameter equal to the average stent diameter: orange surface in Fig. 4d). The width of the wire footprint on this surface (FP) is determined by the radial coordinate oscillation trend (ΔR) based on the following ...
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... β 1c corresponds to the arctangent of FP first derivative with respect to the wire length (l 1 /l 2 coordinate in Fig. 4d) at the origin. Considering a sinusoidal intertwining, meaning a radial coordinate oscillation equal ...
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... good estimators of the maximum and minimum diameter, the latter of which affect the system deliverability. However, during the braiding procedure, a tensile load is usually applied at the wire extremities, resulting in a more linear trend with respect to the sinusoidal oscillation and, subsequently, in a narrower diameter variation range (↑β 1c , Fig. 4). Thus, the β 1c obtained with the sinusoidal approximation may be considered as the lower limit to which the actual device tends with decreasing load applied during the manufacturing. A more accurate result could be obtained by modifying the wire's radial oscillation (ΔR , Fig. 4, Eq. (35)) based on the applied ...
Context 16
... the β 1c obtained with the sinusoidal approximation may be considered as the lower limit to which the actual device tends with decreasing load applied during the manufacturing. A more accurate result could be obtained by modifying the wire's radial oscillation (ΔR , Fig. 4, Eq. (35)) based on the applied load. ...
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Citations
... These performance features depend largely on the flow diverters' conformability and the radial force that is exerted at the deployment diameter. Braided stent mechanical performance is intricately tied to stent design parameters such as wire diameter, braiding angle, open vs. closed loop configuration, and wire element friction, all of which have been the target of numerous finite element modeling studies to optimize complex design parameters [16,[22][23][24]. While braiding design greatly affects the device performance, there are material properties that affect the mechanical performance of braided stents such as stiffness (elastic modulus), and elasticity [25]. ...
Flow diverter devices are small stents used to divert blood flow away from aneurysms in the brain, stagnating flow and inducing intra-aneurysmal thrombosis which in time will prevent aneurysm rupture. Current devices are formed from thin (~25 μm) wires which will remain in place long after the aneurysm has been mitigated. As their continued presence could lead to secondary complications, an absorbable flow diverter which dissolves into the body after aneurysm occlusion is desirable. The absorbable metals investigated to date struggle to achieve the necessary combination of strength, elasticity, corrosion rate, fragmentation resistance, radiopacity, and biocompatibility. This work proposes and investigates a new composite wire concept combining absorbable iron alloy (FeMnN) shells with one or more pure molybdenum (Mo) cores. Various wire configurations are produced and drawn to 25-250 μm wires. Tensile testing revealed high and tunable mechanical properties on par with existing flow diverter materials. In vitro degradation testing of 100 μm wire in DMEM to 7 days indicated progressive corrosion and cracking of the FeMnN shell but not of the Mo, confirming the cathodic protection of the Mo by the FeMnN and thus mitigation of premature fragmentation risk. In vivo implantation and subsequent μCT of the same wires in mouse aortas to 6 months showed meaningful corrosion had begun in the FeMnN shell but not yet in the Mo filament cores. In total, these results indicate that these composites may offer an ideal combination of properties for absorbable flow diverters.
... 15,16 The most common method to regulate the radial force is to adjust the braiding parameters and/or structure of the stent, in which the braiding parameters mainly include the diameter of the stent, the diameter of the monofilament, and the braiding angle. [17][18][19][20][21] In addition, structural design can also be optimized to improve supportability. Innovative methods about flow channels, such as adding new flow channels in axial, circumferential, and helical directions, have always been considered to obtain better structural stability. ...
For the design of polymer stents, supportability and flexibility are two important indexes, but they are often difficult to balance. The mixed braiding of monofilaments with different diameters provides a potential solution to balance these two indices. In this study, the influence of mixed braiding on the mechanical properties of the stent was further explored by adjusting the knitting ratio of thick and thin monofilaments. By replacing the monofilaments in the stent with monofilaments with smaller diameters in turn, the changing trend of supportability and flexibility is obtained. The results show that, when the number of thick monofilaments is greater than or equal to that of thin monofilaments, the supportability of the stent can not only be maintained but also the flexibility can be greatly improved. Therefore, mixed braiding has the function of adjusting supportability and flexibility.
... Equation (5) also computes the minimum pitch angle associated with the maximum compressed configuration [20]. (5) ...
... It is assumed that 1) the stent undergoes only elastic deformation, and 2) the stent's ends are fixed against rotating around the longitudinal axis because of the friction between the wires at their crossing points. Assuming the stent is subjected to a tension test, the axial force F exerted on the stent may be calculated using the following equation [20]. (6) Where , , and are constants as follow: (7) and represent the moment of inertia and polar moment of inertia, respectively, and and are the shear modulus and Young's modulus respectively. ...
... As and perform an equal amount of work, the pressure required to achieve the same deflection in a stent can be computed as follows: (9) Therefore, the radial force can be calculated as follows. (10) To calculate the radial pressure stiffness , the following equation is found in [20]. (11) Considering one of the open-coiled helical springs under the action of the load , the bending and twisting moments are given as follows. ...
Cardiovascular stents are indispensable medical devices used to treat vessel-related issues such as atherosclerotic plaque. In the past, stents were mainly made of materials like stainless steel or cobalt-chromium alloy. However, over the last two decades, research has focused on the use of Nitinol (NiTi) due to its superior properties such as super-elasticity, biocompatibility, and strength. The aim of this paper is to optimize the design of the open-ended braided stent, a well-known architecture for endovascular stents, subjected to radial compression, for enhanced performance. The optimization process uses Multi-Objective Particle Swarm Optimization (MOPSO), which explores three design variables, namely wire diameter, number of coils, and pitch angle, to determine the optimal shape that maximizes radial pressure stiffness and radial force exerted on the vessel walls while minimizing foreshortening. The analytical model developed is compared against literature findings, and the optimization results are implemented in a finite element analysis solver and compared with existing references. The study finds that the optimized design using MOPSO improves the mechanical performance of the stent, particularly in terms of radial stiffness. The results demonstrate the feasibility of MOPSO for optimizing braided NiTi stents and the use of FEM for validating optimized designs. The work emphasizes the significance of optimizing the design of cardiovascular stents to enhance their performance and assure their effectiveness in treating artery diseases.
... They are capable of accurately predicting their longitudinal and radial properties also in the case of very intricate structures, e.g. multiple twists and looped ends [54]. These models are useful to optimise at an early development stage the material properties and geometrical parameters of braided stents based on the desired application. ...
... Flow diverter stents have started a breakthrough in the endovascular management of intracranial aneurysms (including many wide-necked and fusiform aneurysms that were previously considered untreatable) but their mechanism of action is not thoroughly understood, as about 5-25% of aneurysms remain with circulation even after multiple-layer implantations 16 . A substantial body of work is ongoing to improve aneurysm treatment outcomes by increasing the flowdiversion effect of the implanted stent 17,18 . The functional performance is largely dependent on implantation (e.g., sizing, landing zone) and geometrical features (e.g., braid angle, wire density, wire diameter) with wire material properties also being an important contributor. ...
Developing new capabilities to predict the risk of intracranial aneurysm rupture and to improve treatment outcomes in the follow-up of endovascular repair is of tremendous medical and societal interest, both to support decision-making and assessment of treatment options by medical doctors, and to improve the life quality and expectancy of patients. This study aims at identifying and characterizing novel flow-deviator stent devices through a high-fidelity computational framework that combines state-of-the-art numerical methods to accurately describe the mechanical exchanges between the blood flow, the aneurysm, and the flow-deviator and deep reinforcement learning algorithms to identify a new stent concepts enabling patient-specific treatment via accurate adjustment of the functional parameters in the implanted state.
... One way to obtain materials with higher strength or stiffness at lower weight (density) was to change their geometry, such as using lattice structures rather than solid structures [1,2]. Tubular lattice structure has attracted more and more attention owing to its lightweight and excellent mechanical properties, and has been widely used in various disciplines, such as aerospace engineering [3,4], automotive industry [5,6] and medical engineering [7,8]. ...
... Numerical simulation is an effective method for analyzing the mechanical performances of stents. It has high accuracy and requires fewer resources (Zaccaria et al., 2021), and it can also be used to design and optimize stents (Balossino et al., 2008). Záhora et al. derived the equations of the physical model for the spiral stent to describe the mutual transformation between the axial force and the radial pressure (Záhora et al., 2007). ...
... Thus far, the influence of geometric features of braided metal stents, including wire diameter and wire density, have been predicted under radial compression (Fu et al., 2018), localized radial compression (Kim et al., 2008), bending (Fu et al., 2018;Zaccaria et al., 2020) and elongation (De Beule et al., 2009). Furthermore, the finite element method (FEM) has been widely used to investigate stent mechanics (Tan et al., 2001;McKenna and Vaughan, 2021;Zaccaria et al., 2021). Yet, there are still no studies on the mechanical properties of the nasal stent, and the influence of geometric parameters on the performance of the stent remains unclear. ...
Background: A novel braided nasal stent is an effective alternative to nasal packing after septoplasty that can be used to manage the mucosal flap after septoplasty and expand the nasal cavity. This study aimed to investigate the influence of design parameters on the mechanical properties of the nasal stent for optimal performance.
Methods: A braided nasal stent modeling method was proposed and 27 stent models with a range of different geometric parameters were built. The compression behavior and bending behavior of these stent models were numerically analyzed using a finite element method (FEM). The orthogonal test was used as an optimization method, and the optimized design variables of the stent with improved performance were obtained based on range analysis and weight grade method.
Results: The reaction force and bending stiffness of the braided stent increased with the wire diameter, braiding density, and external stent diameter, while wire diameter resulted as the most important determining parameter. The external stent diameter had the greatest influence on the elongation deformation. The influence of design parameters on von-Mises stress distribution of bent stent models was visualized. The stent model with geometrical parameters of 25 mm external diameter, 30° braiding angle, and 0.13 mm wire diameter (A3B3C3) had a greater reaction force but a considerably smaller bending stiffness, which was the optimal combination of parameters.
Conclusion: Firstly, among the three design parameters of braided stent models, wire diameter resulted as the most important parameter determining the reaction force and bending stiffness. Secondly, the external stent diameter significantly influenced the elongation deformation during the compression simulation. Finally, 25 mm external diameter, 30° braiding angle, and 0.13 mm wire diameter (A3B3C3) was the optimal combination of stent parameters according to the orthogonal test results.
... The difference might be explained by different deformations under their corresponding loading conditions. During the radial crimping, the straight cylindrical braided stent undergoes a change in the pitch angles, resulting in a diameter reduction as well as a longitudinal elongation of the whole device [25,26]. Applying axial loading, the double-disc braided occluder undergoes obvious bending of the loading disc, besides elongation and change in the pitch angles. ...
Background and objective:
Ventricular septal defects (VSDs) are the most common form of congenital heart defects. The incidence of VSD accounts for 40% of all congenital heart defects (CHDs). With the development of interventional therapy technology, transcatheter VSD closure was introduced as an alternative to open heart surgery. Clinical trials of VSD occluders have yielded promising results, and with the development of new material technologies, biodegradable materials have been introduced into the application of occluders. At present, the research on the mechanical properties of occluders is focused on experimental and clinical trials, and numerical simulation is still a considerable challenge due to the braided nature of the VSD occluder. Finite element analysis (FEA) has proven to be a valid and efficient method to virtually investigate and optimize the mechanical behavior of minimally invasive devices. The objective of this study is to explore the axial resistive performance through experimental and computational testing, and to present the systematic evaluation of the effect of various material and braid parameters by FEA.
Methods:
In this study, an experimental test was used to investigate the axial resistive force (ARF) of VSD Nitinol occluders under axial displacement loading (ADL), then the corresponding numerical simulation was developed and compared with the experimental results to verify the effectiveness. Based on the above validation, numerical simulations of VSD occluders with different materials (polydioxanone (PDO) and Nitinol with different austenite moduli) and braid parameters (wire density, wire diameter, and angle between left and right discs) provided a clear presentation of mechanical behaviors that included the maximal axial resistive force (MARF), maximal axial displacement (MAD) and initial axial stiffness (IAS), the stress distribution and the maximum principal strain distribution of the device under ADL.
Results:
The results showed that: (1) In the experimental testing, the axial resistive force (ARF) of the tested occluder, caused by axial displacement loading (ADL), was recorded and it increased linearly from 0 to 4.91 N before reducing. Subsequent computational testing showed that a similar performance in the ARF was experienced, albeit that the peak value of ARF was smaller. (2) The investigated design parameters of wire density, wire diameter and the angle between the left and right discs demonstrated an effective improvement (7.59%, 9.48%, 1.28%, respectively, for MARF, and 1.28%, 1.80%, 3.07%, respectively, for IAS) for the mechanical performance for Nitinol occluders. (3) The most influencing factor was the material; the performance rose by 30% as the Nitinol austenite modulus (EA) increased by 10,000 MPa. The performance of Nitinol was better than that of PDO for certain wire diameters, and the performance improved more obviously (1.80% for Nitinol and 0.64% for PDO in IAS, 9.48% for Nitinol and 2.00% for PDO in MARF) with the increase in wire diameter. (4) For all of the models, the maximum stresses under ADL were distributed at the edge of the disc on the loaded side of the occluders.
Conclusions:
The experimental testing presented in the study showed that the mechanical performance of the Nitinol occluder and the MARF prove that it has sufficient ability to resist falling out from its intended placement. This study also represents the first experimentally validated computational model of braided occluders, and provides a perception of the influence of geometrical and material parameters in these systems. The results could further provide meaningful suggestions for the design of biodegradable VSD closure devices and to realize a series of applications for biodegradable materials in VSD.
... Stent geometry has previously been limited by constraints of topdown fabrication methods. Due to the growing prevalence of additive manufacturing techniques, more complex tubular structures have been developed in recent years to address stent limitations [23][24][25][26][27][28][29][30]. For example, origami-inspired tubular structures have been studied for their improved producibility and versatility [31]. ...
Coronary artery disease (CAD) is the narrowing or blockage of the coronary arteries, usually caused by atherosclerosis. An interventional procedure using stents is a promising approach for treating CAD because stents can effectively open narrowed coronary arteries to improve blood flow to the heart. However, stents often suffer from catastrophic failures, such as fractures and migration of ligaments, resulting in fatal clinical events. In this work, we report a new type of tubular lattice metamaterial with enhanced mechanical resilience under radial compression, which can be used as an alternative for the current stent design. We begin by comparing the radial mechanical performance of the proposed auxetic tubular lattice (ATL) with the conventional diamond tubular lattice (DTL). Our results show that the ductility of ATL increases by 72.7% compared with that of the DTL structure. The finite element simulations reveal that the stress is more uniformly spread on the sinusoidal ligaments for ATL, while rather concentrated on the joints of straight ligaments for DTL. This phenomenon is intrinsically due to the bending of sinusoidal ligaments along both radial and axial directions, while straight beams bend mainly along the radial direction. We then investigated the effects of the geometrical parameters of the sinusoidal ligament on radial mechanical performance. Experimental results indicate that the beam depth h/l has the most significant effect on the stiffness and peak load. The stiffness and maximum load surge by 789% and 1131%, respectively, when h/l increases from 0.15 to 0.30. In contrast, the beam amplitude A/l has a minor effect on the stiffness and peak load compared to beam depth and beam thickness. However, increasing the amplitude of the sinusoidal ligament can enlarge the ductility strikingly. The ductility can increase by 67.5% if the amplitude is augmented from A/l=0.1 to A/l=0.35. The findings from this work can provide guidance for designing more mechanically robust stents for medical engineering.
... Tubular lattice structures have increasingly gained attention due to their lightweight and exceptional mechanical performance, and have witnessed extensive applications in various disciplines, such as aerospace engineering [1,2], automotive industry [3,4], and medical engineering [5,6]. For example, the crashworthiness behavior of braided tubular lattice under quasi-static axial loading conditions was explored in detail [3]. ...
... These properties are crucial for reopening the narrowed vessels, providing good arterial support post-expansion, and easy maneuverability during the deployment [9]. To meet these demands, numerous stent architectures have been created in the past decades [5,[23][24][25][26]. Notably, by leveraging the auxetic effect, innovative stent designs with highly compliant behavior have been studied [15,27]. ...
Tubular lattice structures have gained tremendous attention due to their lightweight and excellent mechanical properties. In this work, we designed a new type of tubular lattice architecture by rolling up planar lattice structures with a negative Poisson’s ratio, and then fabricated samples using a material jetting 3D printing technique. We investigated their bending behavior under large deformation using a combined experimental and numerical approach. It was found that the proposed auxetic tubular lattice (ATL) structure exhibits a more compliant behavior, and the ductility increased by up to 85.4% compared to that of a conventional diamond tubular lattice (DTL) structure. Meanwhile, the ATL structure is characterized by a local bending behavior due to the auxetic effect, while the DTL structure is featured with global bending mode. Finite element simulations further reveal that the stress on the ATL structure distributes locally around the indenter. In contrast, stress on the DTL structure distributes much more uniformly across the span. As such, the ATL structures exhibit excellent global stability compared to the DTL structure. Our parametric study shows that bending stiffness, maximum load, and ductility of the proposed ATL structure are mainly controlled by beam depth, due to its large exponential contribution to the moment of inertia. Increasing beam depth leads to a more desirable ductility than increasing beam thickness. The beam amplitude and tubular curvature have minor effects on the bending performance compared to other geometric parameters. The findings reported in this work can guide designing and optimizing mechanically robust tubular lattice metamaterials for applications in tissue engineering, biomedical devices, and robotics engineering.
