Stent Geometry Research Articles

A comparative invitro study of distinct and novel stent geometries on mechanical performances of poly-L-lactic acid cardiovascular stents.

Poly-L-lactic acid (PLLA) is one of the representative polymeric materials serving as bioresorbable stents (BRS) for cardiovascular disease due to its proper biodegradation, high biocompatibility, and adequate mechanical properties among polymer candidates for BRS. However, PLLA BRS as cardiovascular stents also have limitations because their mechanical properties including low radial strength and high elastic recoil are inferior to those of metallic-based BRS stents. In the study, we developed and manufactured distinct and novel types of stent geometries for investigating mechanical properties of thin-walled PLLA BRS (110 μm) for cardiovascular applications. Five key mechanical tests, including radial strength, crimping profile, flexibility, elastic recoil, and foreshortening were performed through a comprehensive analysis. In addition, we applied the finite element method for further validation and insight of mechanical behaviors of the PLLA BRS. Results revealed that Model 2 had advantages in high flexibility as well as radial strengths, which would be a proper option for complex and acutely curved lesions. Model 3 would be an optimum selection for stent placement in mild target site due to its strength in minimum elastic recoil. Even though Model 4 showed the highest radial strength, finite element simulation showed that the geometry caused higher maximum stress than that of Model 2 and Model 3 during the crimping process. Model 1 showed the most vulnerable geometry among the tested models in both invitro and finite element analysis. Such data may suggest potential guidance in regard to understanding the mechanical behaviors of PLLA BRS as not only applicable cardiovascular but also peripheral and intracranial stents.

Comprehensive Geometric Parameterization and Computationally Eficient 3D Shape Matching Optimization of Realistic Stents

Abstract Flexible and compact shape representation schemes are essential for design optimization problems. Current shape representation schemes for coronary stent designs concern predominantly idealized or Independent Ring (IR) designs, which are outdated and only consider a small number of core design variables (such as strut width, height, and thickness) and ignore clinically critical characteristics such as the number of connectors. No reports exist on the geometry parameterization of the latest Helical Stents (HS) that have more complex designs than IR stents. Here, we present two new shape parameterization schemes to fully capture the 3D designs of contemporary IR and double-helix HS stents. We developed a 3D stent geometry builder based on 17 (IR) and 18 (HS) design variables, including strut width, thickness, height, number of connectors and rings, stent length, and strut centerline shape. The shape of the strut centerline was derived via a combination of NURBS, PARSEC, quarter circle, and straight line segments. Shape matching for complex 3D geometries, such as the contemporary stents within limited function evaluations, is not trivial and requires efficient parameterization and optimization algorithms. We used shape matching optimization with a limited function evaluation budget to test the proposed parameterization and two surrogate-assisted optimization algorithms relying on predictor believer and an expected improvement maximization formulation. The performance of these algorithms is objectively compared with a gradient-based optimization method to highlight their strengths. Our work paves the way for more realistic, full-fledged stent design optimization with structural and hemodynamic objectives in the future.

Easy-to-use formulations based on the homogenization theory for vascular stent design and mechanical characterization

Background and objectivesVascular stents are scaffolding structures implanted in the vessels of patients with obstructive disease. Stents are typically designed as cylindrical lattice structures characterized by the periodic repetition of unit cells. Their design, including geometry and material characteristics, influences their mechanical performance and, consequently, the clinical outcomes. Computational optimization frameworks have proven to be effective in assisting the design phase of vascular stents, facilitating the achievement of enhanced mechanical performances. However, the reliance on time-consuming simulations and the challenge of automating the design process limit the number of design evaluations and reduce optimization efficiency. In this context, a rapid and automated method for the mechanical characterization of vascular stents is presented, taking the stent geometry, conceived as the periodic repetition of a unit cell, and material as input and providing the mechanical response of the stent as output. MethodsVascular stents were assumed to be thin-walled hollow cylinders sharing the same macroscopic geometrical characteristics as the cylindrical lattice structure but composed of an anisotropic homogenized material. Homogenization theory was applied to average the microscopic inhomogeneities at the stent unit cell level into a homogenized material at the macro-scale, enabling the calculation of the associated homogenized material tensor. Analytical formulations were derived to relate the stent mechanical behavior to the homogenized stiffness tensor, considering linear elastic theory for thin-walled hollow cylinders and three loading scenarios of relevance for vascular stents: radial crimping; axial traction; torsion. Validation was conducted by comparing the derived analytical formulations with results obtained from finite element analyses on typical stent designs. ResultsHomogenized stiffness tensors were computed for the unit cells of three stent designs, revealing insights into their mechanical performance, including whether they exhibit auxetic behavior. The derived analytical formulations were successfully validated with finite element analyses, yielding low relative differences in the computed values of foreshortening, radial, axial and torsional stiffnesses for all three stents. ConclusionsThe proposed method offers a rapid, fully automated procedure that facilitates the assessment of the mechanical behavior of vascular stents and is suitable for effective integration into computational optimization frameworks.

In Silico Investigation of the Interlaminar and Mechanical Fracture of Arteries with Atheromatic Plaque during Angioplasty Treatment.

The reduction in the inner diameter of the artery due to the creation of atheromatic plaque on the artery lumen, known as artery stenosis, disrupts the blood flow, leading to medical complications, which can be fatal. The angioplasty procedure aims to reopen the artery and uses a stent to keep it open. In this study, an effort is made to determine the point of the stent, the plaque and the artery during the expansion phase of the angioplasty using the in silico Finite Element Analysis method. A literature-based design was chosen for the stent geometry, whereas simplified shapes of the balloon and the two artery layers were used. Additionally, two plaque designs were the benchmark for the eight distinct artery stenosis models within the Abaqus environment. In the context of stent angioplasty simulations, failure patterns were investigated. An inverse relationship was observed between artery stenosis and pressure at the artery failure point, while an increased danger of interlaminar failure was detected in models with larger artery stenosis. This study verifies the necessity for the inclusion of interlaminar failure in future angioplasty research.

Bicuspid valve CT registry of balloon-expandable TAVR: BETTER TAVR registry.

The anatomic substrate of bicuspid valves may lead to suboptimal TAVR stent expansion and geometry. We evaluated determinants of stent geometry in bicuspid valves treated with Sapien transcatheter aortic valve replacement (TAVR) valves. A multicenter retrospective registry of patients (February 2019 to August 2022) who underwent post-TAVR computed tomography to determine stent area (vs. nominal valve area) and stent ellipticity (maximum diameter/minimum diameter). Predictors of relative stent expansion (minimum area/average of inflow + outflow area) and stent ellipticity were evaluated in a multivariable regression model, including valve calcium volume (indexed by annular area), presence of raphe calcium, sinus diameters indexed by area-derived annular diameter, and performance of pre-dilation and post-dilation. The registry enrolled 101 patients from four centers. The minimum stent area (vs. nominal area) was 88.1%, and the maximum ellipticity was 1.10, with both observed near the midframe of the valve in all cases. Relative stent expansion ≥90% was observed in 64/101 patients. The only significant predictor of relative stent expansion ≥90% was the performance of post-dilation (OR: 4.79, p = 0.018). Relative stent expansion ≥90% was seen in 86% of patients with post-dilation compared to 57% without (p < 0.001). The stent ellipticity ≥1.1 was observed in 47/101 patients. The significant predictors of stent ellipticity ≥1.1 were the indexed maximum sinus diameter (OR: 0.582, p = 0.021) and indexed intercommisural diameter at 4 mm (OR: 2.42, p = 0.001). Stent expansion has a weak negative correlation with post-TAVR mean gradient (r = -0.324, p < 0.001). Relative stent expansion ≥90% was associated with the performance of post-dilation, and stent ellipticity ≥1.1 was associated with indexed intercommisural diameter and indexed maximum sinus diameter. Further studies to determine optimal deployment strategies in bicuspid valves are needed.

Three-Dimensional Geometric Analysis of Viabahn VBX Bridging Stent Grafts in Fenestrated Endovascular Aortic Repair: A Multicenter, Retrospective Cohort Study.

The primary aim of this study was to assess the 3-dimensional flare geometry of the Gore Viabahn VBX balloon-expandable covered stent (BECS) after fenestrated endovascular aortic repair (FEVAR) and to determine and visualize BECS-associated complications. This multicenter retrospective study included patients who underwent FEVAR between 2018 and 2022 in 3 vascular centers participating in the VBX Expand Registry. Patients with at least one visceral artery treated with the VBX and with availability of 2 post-FEVAR computed tomography angiography (CTA) scans (follow-up [FU] 1: 0-6 months; FU2: 9-24 months) were included. The flare geometry of the VBX, including flare-to-fenestration distance, flare-to-fenestration diameter ratio, flare angle, and apposition with the target artery were assessed using a vascular workstation and dedicated CTA applied software. In total, 90 VBX BECS were analyzed in 43 FEVAR patients. The median CTA FU for FU1 and FU2 was 35 days (interquartile range [IQR], 29-51 days) and 14 months (IQR, 13-15 months), respectively. The mean flare-to-fenestration distance was 5.6±2.0 mm on FU1 and remained unchanged at 5.7±2.0 mm on FU2 (p=.417). The flare-to-fenestration diameter ratio was 1.19±0.17 on FU1 and remained unchanged at 1.21±0.19 (p=.206). The mean apposition length was 18.6±5.3 mm on FU1 and remained 18.6±5.3 mm (p=.550). The flare angle was 31°±15° on FU1 and changed to 33°±16° (p=.009). On FU1, the BECS-associated complication rate was 1%, and the BECS-associated reintervention rate was 0%. On FU2, the BECS-associated complication rate was 3%, and the BECS-associated reintervention rate was 1%. The flare geometry of the VBX bridging stent did not change significantly during 14 months follow-up in this study. Three-dimensional geometric analysis of the flare may contribute to identify the origin of endoleaks and occlusions, but this should be confirmed in a larger study including enough patients and BECS to compare complicated and uncomplicated cases. The three-dimensional flare geometry of the Gore Viabahn VBX BECS was assessed on the first and second postoperative CTA scans, and geometrical changes during this period were identified. For BECS that were diagnosed with a type 3c endoleak or occlusion, the BECS geometry was analyzed to detect geometrical components that were related to the complication. Geometric analysis of the flare may help to better detect and identify the cause of such complications.

Feasibility assessment of polyvinyl alcohol-based bioresorbable cardiovascular stents manufactured via solvent-cast direct writing extrusion

BackgroundBioresorbable stents represent a promising approach in cardiovascular interventions compared to drug-eluting stents. Additive manufacturing, particularly ink deposition, offers customization and versatility. This study delves into the potential of solvent cast direct-write 3D printing to fabricate cardiovascular stents using environmentally friendly solvents. MethodologyPolyvinyl alcohol, a biocompatible synthetic polymer that dissolves in water, was investigated as a suitable material for stent fabrication. The polymer was deposited on a rotating mandrel and subsequently crosslinked to establish a pseudostable state. Test specimens and stents were fabricated for characterization of both the material and stent dynamics. ResultsThis outcome is potentially suitable for deployment in the human body environment and adaptable to various biomedical applications, such as drug delivery patches or implants. The research optimized the fabrication of various stent geometries using polyvinyl alcohol and evaluated the kinetics of the working environment of these stents. Specifically, the 8-cell diamond stent showed remarkable characteristics, such as a high overexpansion of more than 0.5 mm, a compression force of 0.02 N and an elastic recovery of 88.85 %, with a strut thickness of 50.25 μm. Additionally, the study discusses the possibility of sterilizing polyvinyl alcohol with different methods, ethanol and autoclave were selected. ConclusionsThe results indicate that autoclaving leads to an increase in crystallinity. This yields a decrease in water absorption and an increase in mechanical properties. These results suggest that polyvinyl alcohol-based stents fabricated by solvent-cast direct writing are potential candidates for bioresorbable stent design.

The Impact of Target Vessel Anatomy and Bridging Stent Geometry on Branched Endovascular Aortic Repair Outcomes

This study aimed to evaluate the impact of target vessel anatomy and bridging stent geometry on target vessel instability in branched endovascular aortic repair (B-EVAR). This retrospective, single centre cohort study included all consecutive B-EVARs performed between September 2018 and December 2022 for thoraco-abdominal aortic aneurysm (TAAA) or complex abdominal aortic aneurysm (CAAA). The primary endpoints were target vessel instability and related re-interventions at 12 months. Secondary endpoints were 30 day results, including target vessel instability and re-interventions. Target vessel instability analysis consisted of assessment of target vessel anatomy, including diameter, aortic trunk to branch angle, and tortuosity. Post-operative parameters included change of clock position/horizontal misalignment, bridging length (gap), sealing length, tortuosity, post-stenting angle, and oversizing ratio. A total of 69 patients (TAAA: n= 56, 81%; CAAA: n= 13, 19%) and 271 (133 visceral and 138 renal) target vessels were included. The cumulative incidence of target vessel instability was 4.8%, 6.4%, and 7.9% at one, two, and three years, respectively. In the renal target vessel group, vessel diameter ≤ 4 mm (hazard ratio [HR] 1.28, 95% confidence interval [CI] 1.116 - 2.54; p= .022) and a bridging length ≥ 25 mm (HR 1.320, 95% CI 1.066 - 1.636; p= .011) were associated with increased target vessel instability. In visceral vessels, a change in clock position/horizontal misalignment ≥ 70 minutes (HR 1.072, 95% CI 1.026 - 1.121; p= .002) showed a significant association with target vessel instability. Target vessel diameter, bridging length (gap), and horizontal misalignment seemed to be associated with adverse target vessel outcomes. This may be solved with more customised endograft solutions to reduce the negative impact of the latter parameter.

Clinical and radiological outcomes of extracranial carotid artery stent placement: A single-center study

Objectives: Carotid artery stenting (CAS) and carotid endarterectomy are established treatments for carotid artery stenosis. We evaluated the early and mid- to late-term clinical and radiological outcomes of patients who underwent CAS. Materials and Methods: This retrospective study included 98 patients (112 arteries), who underwent CAS. Baseline demographics, stent types, embolic protection devices, and procedural complication rates within 30-day and 3-year post-CAS, including transient ischemic attack (TIA), stroke, death, and stent restenosis, were analyzed. Results: The 30-day complication rates included TIA (5.1%), ipsilateral stroke (4.1%), and death (4.1%). At three-year follow-up, TIA (8.5%), ipsilateral stroke (2.1%), restenosis (1.1%), and death (6.4%) were observed. Contralateral carotid artery angiography revealed neointimal hyperplasia in two vessels (1.9%) and 70–99% restenosis 1 (1%). Notably, a significant association was observed between neointimal hyperplasia and stent geometry, with a higher incidence observed in open-cell stents compared to closed-cell stents (P = 0.03). Conclusion: Our study demonstrated comparable early-term and lower mid- to late-term complication rates compared to prior studies. A multidisciplinary approach with meticulous technique, appropriate materials, and careful patient selection can optimize CAS outcomes.

A hemodynamic study of blood flow models on various stent graft configurations during aorto-iliac reconstruction.

To compare the hemodynamic performance of three (Bottom Up non-ballet, Top-Down non-ballet, Top Down ballet) idealized stent graft configurations used during endovascular repair of abdominal aortic aneurysms, under the influence of various rheological models. Ten rheological models are assumed and a commercial finite volume solver is employed for the simulation of blood flow under realistic boundary conditions. An appropriate mesh convergence study is performed and five hemodynamic variables are computed: the time average wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), endothelial cell activation potential and displacement force (DF) for all three configurations. The choice of blood flow model may affect results, but does not constitute a significant determinant on the overall performance of the assumed stent grafts. On the contrary, stent graft geometry has a major effect. Specifically, the Bottom Up non-ballet type is characterized by the least favorable performance presenting the lowest TAWSS and the highest OSI, RRT and ECAP values. On the other hand, the Top Down ballet type presents hemodynamic advantages yielding the highest TAWSS and lowest OSI, RRT and ECAP average values. Furthermore, the ballet type is characterized by the lowest DF, although differences observed are small and their clinical relevance uncertain. The effect of the assumed rheological model on the overall performance of the grafts is not significant. It is thus relatively safe to claim that it is the type of stent graft that determines its overall performance rather than the adopted blood flow model.

Geometric optimization of vascular stents modeled as networks of 1D rods

We develop a novel mathematical and computational framework for geometric optimization of mesh-like devices such as stents, based on modeling mesh-like structures as networks of one-dimensional curved rods. To simplify calculations, the curved rods are approximated by piecewise straight rods. Constrained optimization problems for different cost functionals are stated and mathematically analyzed. The cost functionals considered include: (1) stents' compliance, (2) L2 norm of displacement, (3) L2 norm of contact moment (which is related to fatigue), and (4) multicriteria optimization in which stents are optimized to achieve maximal radial stiffness and minimal bending rigidity. The optimization parameters are stent's vertices, namely, the location of points where the stent struts meet. Existence of solutions to the mathematically posed optimization problems is obtained, and a numerical method based on the gradient descent algorithm is proposed to find the solutions. Three representative stents' geometries are numerically analyzed to show that the optimization algorithms provide tangible solutions. The stent geometries considered are those of Palmaz type stents, single zig-zag stent rings, and Express type stents. Interesting findings are obtained, including several new stent designs. Several optimized stents are presented, including an optimized Palmaz stent with a reduction in contact moment of 30%, and optimized Express and Palmaz stents with a reduction in compliance by more than 70%.

Multi-objective design optimization of bioresorbable braided stents

Background and objectivesBioresorbable braided stents, typically made of bioresorbable polymers such as poly-l-lactide (PLLA), have great potential in the treatment of critical limb ischemia, particularly in cases of long-segment occlusions and lesions with high angulation. However, the successful adoption of these devices is limited by their low radial stiffness and reduced elastic modulus of bioresorbable polymers. This study proposes a computational optimization procedure to enhance the mechanical performance of bioresorbable braided stents and consequently improve the treatment of critical limb ischemia. MethodsFinite element analyses were performed to replicate the radial crimping test and investigate the implantation procedure of PLLA braided stents. The stent geometry was characterized by four design parameters: number of wires, wire diameter, initial stent diameter, and braiding angle. Manufacturing constraints were considered to establish the design space. The mechanical performance of the stent was evaluated by defining the radial force, foreshortening, and peak maximum principal stress of the stent as objectives and constraint functions in the optimization problem. An approximate relationship between the objectives, constraint, and the design parameters was defined using design of experiment coupled with surrogate modelling. Surrogate models were then interrogated within the design space, and a multi-objective design optimization was conducted. ResultsThe simulation of radial crimping was successfully validated against experimental data. The radial force was found to be primarily influenced by the number of wires, wire diameter, and braiding angle, with the wire diameter having the most significant impact. Foreshortening was predominantly affected by the braiding angle. The peak maximum principal stress exhibited contrasting behaviour compared to the radial force for all parameters, with the exception of the number of wires. Among the Pareto-optimal design candidates, feasible peak maximum principal stress values were observed, with the braiding angle identified as the differentiating factor among these candidates. ConclusionsThe exploration of the design space enabled both the understanding of the impact of design parameters on the mechanical performance of bioresorbable braided stents and the successful identification of optimal design candidates. The optimization framework contributes to the advancement of innovative bioresorbable braided stents for the effective treatment of critical limb ischemia.

Bioabsorbable Polymeric Stent for the Treatment of Coarctation of the Aorta (CoA) in Children: A Methodology to Evaluate the Design and Mechanical Properties of PLA Polymer.

This study presents a methodology that combines experimental tests and the finite element method, which is able to analyse the influence of the geometry on the mechanical behaviour of stents made of bioabsorbable polymer PLA (PolyLactic Acid) during their expansion in the treatment of coarctation of the aorta (CoA). Tensile tests with standardized specimen samples were conducted to determine the properties of a 3D-printed PLA. A finite element model of a new stent prototype was generated from CAD files. A rigid cylinder simulating the expansion balloon was also created to simulate the stent opening performance. A tensile test with 3D-printed customized stent specimens was performed to validate the FE stent model. Stent performance was evaluated in terms of elastic return, recoil, and stress levels. The 3D-printed PLA presented an elastic modulus of 1.5 GPa and a yield strength of 30.6 MPa, lower than non-3D-printed PLA. It can also be inferred that crimping had little effect on stent circular recoil performance, as the difference between the two scenarios was on average 1.81%. For an expansion of diameters ranging from 12 mm to 15 mm, as the maximum opening diameter increases, the recoil levels decrease, ranging from 10 to 16.75% within the reported range. These results point out the importance of testing the 3D-printed PLA under the conditions of using it to access its material properties; the results also indicate that the crimping process could be disregarded in simulations to obtain fast results with lower computational cost and that new proposed stent geometry made of PLA might be suitable for use in CoA treatments-the approach that has not been applied before. The next steps will be to simulate the opening of an aorta vessel using this geometry.

An All-in-One Tool for 2D Atherosclerotic Disease Assessment and 3D Coronary Artery Reconstruction.

Diagnosis of coronary artery disease is mainly based on invasive imaging modalities such as X-ray angiography, intravascular ultrasound (IVUS) and optical coherence tomography (OCT). Computed tomography coronary angiography (CTCA) is also used as a non-invasive imaging alternative. In this work, we present a novel and unique tool for 3D coronary artery reconstruction and plaque characterization using the abovementioned imaging modalities or their combination. In particular, image processing and deep learning algorithms were employed and validated for the lumen and adventitia borders and plaque characterization at the IVUS and OCT frames. Strut detection is also achieved from the OCT images. Quantitative analysis of the X-ray angiography enables the 3D reconstruction of the lumen geometry and arterial centerline extraction. The fusion of the generated centerline with the results of the OCT or IVUS analysis enables hybrid coronary artery 3D reconstruction, including the plaques and the stent geometry. CTCA image processing using a 3D level set approach allows the reconstruction of the coronary arterial tree, the calcified and non-calcified plaques as well as the detection of the stent location. The modules of the tool were evaluated for efficiency with over 90% agreement of the 3D models with the manual annotations, while a usability assessment using external evaluators demonstrated high usability resulting in a mean System Usability Scale (SUS) score equal to 0.89, classifying the tool as "excellent".

Abstract Number ‐ 130: Comparison of the Stents Length by Virtual Stent Simulation with the Actual Stent Length

Introduction The selection of the size of a braided stent, such as the flow diverter stent, currently depends on the surgeon’s experience. However, the suitable selection is difficult even for experienced surgeons due to shortening during the deployment. The unsuitable choice of the stent size may result in poor stent apposition or dislodgement of the stent, which may lead to inadequate treatment and increase the risk of complications. To overcome the selection problem, we developed a virtual stent simulation that virtually reproduces the stent geometry preoperatively. However, the lengths of the deployed stents in the virtual stent simulation have not been compared with those in the actual stents. In this study, we evaluated the error between the length of the virtual stent and the actual stent to investigate the clinical usefulness of the virtual stent simulation. Methods We identified 48 internal carotid artery cerebral aneurysms treated with a single flow diverter stent (PipelineTM) between October 2016 and March 2022. The vessel geometries were reconstructed from the preoperative angiographic images, and the virtual stent geometries were reproduced using virtual stent simulation with our in‐house code. The actual stent geometries were reconstructed from the postoperative angiographic images and overlaid on the preoperative vessel geometries using affine transformation. The lengths of the virtual and actual stents were measured, respectively. The differences between the lengths of the virtual and actual stents were calculated by subtracting the actual stent length from the virtual stent length. The errors between the virtual and actual stents were estimated to validate the virtual stent simulation. Results The average maximum diameter in the 48 aneurysms was 16.4±5.91 mm. The average difference in length between the virtual stents and the actual stents was +1.70 ± 4.01 mm, with the average error based on virtual stent length of +3.24 ± 11.3%. The virtual stents of 16 aneurysms were shorter than the actual stents, and the average difference in length was ‐2.97 ± 1.82 mm. On the other hand, the virtual stents of the other 32 aneurysms were longer than the actual stents, with the average difference in length of +4.04 ± 2.47 mm. One possible cause of these differences is that the actual stent may slightly change the geometry of the parent vessel due to the restoring force of the stent. Another reason is that the actual length of the stent may change slightly as the surgeon pushes and pulls the catheter during the procedure. Conclusions Comparing the lengths of virtual and actual stents, it was confirmed that there was some error between the stent length reproduced by the virtual stent simulation and the actual stent length. In particular, the virtual stents tend to be slightly longer than the actual stents. Although our virtual stent may be useful for identifying the appropriate size, it is necessary to investigate the causes of such errors and to realize virtual stent simulations with fewer errors.

A thermodynamic framework for additive manufacturing of crystallizing polymers, Part II: Simulation of the printing of a stent

We use the theory developed by [43] to simulate in Abaqus the printing of a drug-eluting bioresorbable stent (BRS), which is used to treat a stenosed renal artery, by fused deposition modeling (FDM). The stent is assumed to be printed using a semi-crystalline polylactic acid (PLA). We control the process parameters to architect the microstructure of the stent such that the stent has an amorphous glassy phase on a crystalline backbone, which would ensure sufficient strength along with the regulated release of the drug into the bloodstream. Through this simulation, we highlight those regions in the stent geometry that should adhere to strict quality control requirements during the fabrication process. In this regard, it is observed that a preexisting crack at the core of the third layer has a very high tendency to propagate along the ‘r’ or ‘z’ direction, whereas a preexisting crack at the core of the second layer has a very high tendency to propagate along the ‘r’ direction, thus increasing the probability of an intra-layer delamination process during service. Similarly, the stent has a high chance of inter-layer delamination between the second and third layers by the mode-I mechanism for cracking. Moreover, the final cup-like warped geometry of the stent could injure the arterial wall during deployment by balloon expansion and also during service, thus increasing the risk of neointimal hyperplasia.

Lasers in the manufacturing of cardiovascular metallic stents: Subtractive and additive processes with a digital tool

Laser beams can be manipulated to achieve different types of interaction mechanisms with metals allowing them to heat, melt, vaporize, or ablate them. Today's laser sources are robust, fast-addressable optoelectronic devices, easily integrated into automation systems along with sophisticated CAD/CAM solutions. Being a photonic digital tool, the laser beam is a fundamental tool for Industry 4.0 and is already widely exploited in the manufacturing of metallic stents. The conventional manufacturing method of laser cutting employs a subtractive method to cut the stent mesh on tubular feedstock. On the other hand, laser beams can be exploited to melt metallic powders to produce stent geometries in a layer-by-layer fashion. The present work provides a short state of the art review concerning the works focusing on the two laser-based manufacturing processes underlining the evolution of the laser source types and used materials. The work provides insights into the future opportunities and challenges that should be faced by the manufacturing research communities in the light of improving the biomedical device performance by exploiting the possibilities provided by the digital tool.

PERFORMANCE ANALYSIS OF HEXAGONAL UNIT CELL STENT DESIGN AND SINGLE-LAYERED MANUFACTURABILITY WITH FDM

İlaç salınımlı stentlerden sonra geliştirilen biyobozunur polimer esaslı stentler için yapılan ön klinik çalışmalardan elde edilen sonuçlara göre polimer stentlerin kullanılabilirliği sorgulanmaktadır. Polimer stentlerde strat kalınlığının yüksek olması ve mekanik özelliklerin zayıflığı nedeniyle tasarım ve malzeme iyileştirmesine gidilerek yeni modellerin ortaya çıkarılması üzerine araştırmalara devam edilmektedir. Balonla genişleyen stentlerde oluşan uç açılması arter içerisinde intimal ve medial katmanlara zarar vermektedir. Bu zarar, erken dönemde neointimal hiperplazi ve restenoza sebep olmaktadır. Metalik stent geometrilerinden biri olan Palmaz-Schatz birim hücre modelinde balonla genişleme esnasında uç açılması ve kısalma oranı meydana gelmektedir. Bu çalışmada, Palmaz-Schatz stent geometrisinin genişlemesiyle oluşturduğu geometri dikkate alınarak PLLA malzemeden altıgen birim stent geometrisinde yeni tasarım gerçekleştirilmiştir. Sonlu elemanlar simülasyonuyla, ince strat kalınlığında tasarlanan altıgen stendin genişlemesinde oluşan uç açılması ve kısalma oranı belirlenmiştir. Ayrıca, stent genişlemesinde üç katmanlı arter ve kalsifik plak yapısında meydana gelen doku hasarı ölçülmüştür. Altıgen geometrideki stentte oluşan uç açılması ve kısalma oranı Palmaz-Schatz modeline göre azaltılmıştır. Bununla birlikte, PLA/PHA karışımı malzemeden eriyik yığma modelleme ile altıgen stent geometrisinin 3B plaka baskısı tek katmanlı olarak üretilmesi sonrası ısıtıcı bir tubular tabla üzerinde sarılarak stent formuna getirilmiştir

Effects of stent shape on focal hemodynamics in intracranial atherosclerotic stenosis: A simulation study with computational fluid dynamics modeling.

The shape of a stent could influence focal hemodynamics and subsequently plaque growth or in-stent restenosis in intracranial atherosclerotic stenosis (ICAS). In this preliminary study, we aim to investigate the associations between stent shapes and focal hemodynamics in ICAS, using computational fluid dynamics (CFD) simulations with manually manipulated stents of different shapes. We built an idealized artery model, and reconstructed four patient-specific models of ICAS. In each model, three variations of stent geometry (i.e., enlarged, inner-narrowed, and outer-narrowed) were developed. We performed static CFD simulation on the idealized model and three patient-specific models, and transient CFD simulation of three cardiac cycles on one patient-specific model. Pressure, wall shear stress (WSS), and low-density lipoprotein (LDL) filtration rate were quantified in the CFD models, and compared between models with an inner- or outer-narrowed stent vs. an enlarged stent. The absolute difference in each hemodynamic parameter was obtained by subtracting values from two models; a normalized difference (ND) was calculated as the ratio of the absolute difference and the value in the enlarged stent model, both area-averaged throughout the arterial wall. The differences in focal pressure in models with different stent geometry were negligible (ND<1% for all cases). However, there were significant differences in the WSS and LDL filtration rate with different stent geometry, with ND >20% in a static model. Observable differences in WSS and LDL filtration rate mainly appeared in area adjacent to and immediately distal to the stent. In the transient simulation, the LDL filtration rate had milder temporal fluctuations than WSS. The stent geometry might influence the focal WSS and LDL filtration rate in ICAS, with negligible effect on pressure. Future studies are warranted to verify the relevance of the changes in these hemodynamic parameters in governing plaque growth and possibly in-stent restenosis in ICAS.

Fluid-Structure Interaction Study of The Effect of Stent on Local Hemodynamics Parameters at The Stented Carotid Artery Bifurcation

Previous fluid-structure interaction (FSI) studies on carotid bifurcation focused on the effect of wall compliance on the predicted hemodynamic features in normal or atherosclerotic carotid arteries. However, FSI study on patient-specific post-stent carotid model is still lacking, and to the authors knowledge no such a study has been reported so far. This study attempts to simulate the full-scale patient-specific post- stent carotid bifurcation geometry with the hope to understand the effect of wall compliance on hemodynamic quantities. The FSI model was based on patient-specific geometry consisting of three components i.e., the carotid artery wall and stent as the solid domain, and blood in the fluid domain. Full FSI simulations incorporating patient-specific boundary conditions at the inlet and outlets, and realistic homogenous incompressible carotid wall and stent were completed to evaluate the flow patterns and wall shear stress. The FSI simulation results were compared with the corresponding rigid-wall model. The quantitative difference in time-averaged wall shear stress (TAWSS) distribution between the FSI and rigid-wall models shows that FSI model predicted 8% less of the area in the low TAWSS band (&lt;0.4%) compared 0.4 - 1.2 Pa. The results suggest that although the effect of wall compliance on flow patterns in the patient-specific post-stent carotid model is negligible, its quantitative effect on wall shear stress may not be trivial and should be considered in future studies