Mechanical modeling of self-expandable stent fabricated using braiding technology

Intraluminal Flow Diverter Design Primer for Neurointerventionalists

The clinical use of flow diverters for the treatment of intracranial aneurysms has rapidly grown. Consequently, the market and technology for these devices has also grown. Clinical performance characteristics of the flow diverter are well-known to the clinician. However, the engineering design principles behind how these devices achieve ideal clinical performance are less understood. This primer will summarize flow diverter design parameters for neurointerventionalists with the aim of promoting collaboration between clinicians and engineers.

Computational modeling of braided venous stents - Effect of design features and device-tissue interaction on stent performance

Designing venous stents with desired properties is challenging due to the partly conflicting performance criteria, e.g., enhancing flexibility may be at odds with increasing patency. To evaluate the effect of design parameters on the mechanical performance of braided stents, computational simulations are performed using finite element analysis. Model validation is performed through comparison with measurements. Considered design features are stent length, wire diameter, pick rate, number of wires, and stent end-type, being either open-ended or closed looped. Based on the requirements of venous stents, tests are defined to study the effect of design variations with respect to the following key performance criteria: chronic outward force, crush resistance, conformability, and foreshortening. Computational modeling is demonstrated to be a valuable tool in the design process through its ability of assessing sensitivities of various performance metrics to the design parameters. Additionally, it is shown, using computational modeling, that the interaction between a braided stent and its surrounding anatomy has a significant impact on its performance. Therefore, taking into account device-tissue interaction is crucial for the proper assessment of stent performance.

Machine learning and reduced order modelling for the simulation of braided stent deployment

Endoluminal reconstruction using flow diverters represents a novel paradigm for the minimally invasive treatment of intracranial aneurysms. The configuration assumed by these very dense braided stents once deployed within the parent vessel is not easily predictable and medical volumetric images alone may be insufficient to plan the treatment satisfactorily. Therefore, here we propose a fast and accurate machine learning and reduced order modelling framework, based on finite element simulations, to assist practitioners in the planning and interventional stages. It consists of a first classification step to determine a priori whether a simulation will be successful (good conformity between stent and vessel) or not from a clinical perspective, followed by a regression step that provides an approximated solution of the deployed stent configuration. The latter is achieved using a non-intrusive reduced order modelling scheme that combines the proper orthogonal decomposition algorithm and Gaussian process regression. The workflow was validated on an idealized intracranial artery with a saccular aneurysm and the effect of six geometrical and surgical parameters on the outcome of stent deployment was studied. We trained six machine learning models on a dataset of varying size and obtained classifiers with up to 95% accuracy in predicting the deployment outcome. The support vector machine model outperformed the others when considering a small dataset of 50 training cases, with an accuracy of 93% and a specificity of 97%. On the other hand, real-time predictions of the stent deployed configuration were achieved with an average validation error between predicted and high-fidelity results never greater than the spatial resolution of 3D rotational angiography, the imaging technique with the best spatial resolution (0.15 mm). Such accurate predictions can be reached even with a small database of 47 simulations: by increasing the training simulations to 147, the average prediction error is reduced to 0.07 mm. These results are promising as they demonstrate the ability of these techniques to achieve simulations within a few milliseconds while retaining the mechanical realism and predictability of the stent deployed configuration.

An experimental investigation of the mechanical performance of PLLA wire-braided stents

Much of our current understanding of the performance of self-expanding wire-braided stents is based on mechanical testing of Nitinol-based or polymeric non-bioresorbable (e.g. PET, PP etc.) devices. The small amount of data present for bioresorbable devices characterizes stents with big nominal diameters (D>6mm), with a distinct lack of data describing the mechanical performance of small-diameter wire-braided bioresorbable devices (D≤5mm). This study presents a systematic investigation of the mechanical performance of wire-braided bioresorbable Poly-L-Lactic Acid (PLLA) stents having different braiding angles (α=45° , α=30°, and α=20°), wire diameters (d=100μm, and d=150μm), wire count (n=24 and n=48), braiding patterns (1:1-1, 2:2-1 and 1:1-2) and stent diameters (D=5mm, D=4mm, and D=2.5mm). Mechanical characterisation was carried out by evaluating the radial, longitudinal and bending response of the devices. Our results showed that smaller braid angles, larger wire diameters, higher number of wires and smaller stent diameter led to an increase in the stent mechanical properties across each of the three mechanical tests performed. It was found that geometrical features of a polymeric braided stent could be adapted to achieve a similar performance to the one of a metallic device. In particular, substantial increases in stent mechanical properties were found for a low braiding angle and when the braiding pattern followed a one-over-one-under configuration with two wires in parallel (1:1-2). Finally, it was shown that a mathematical model proposed in literature for metal braided stents can provide reasonable predictions also of polymeric stent performance but just in circumstances where wire friction does not have a dominant role. This study presents a wide range of experimental data that can provide an important reference for further development of wire-braided bioresorbable devices.

Mechanical property analysis and design parameter optimization of a novel nitinol nasal stent based on numerical simulation

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.

Evaluation of resistance to radial cyclic loads of poly(L-lactic acid) braided stents with different braiding angles

Poly(L-lactic acid) (PLLA) braided stents have superior biocompatibility and flexibility, substituting metal stents in peripheral blood vessels. However, the radial supporting capacity of PLLA braided stent should be improved to bear the dynamic load from the peripheral artery. This paper evaluated the radial support performance of PLLA braided stents with different braiding angles after the radial cyclic loads test. The results indicate that braiding angle of stents is an important parameter affecting its ability to resist radial cyclic loads. The stent with a smaller braiding angle has better initial radial support but insufficient durability, while the stent with a larger braiding angle could maintain adequate radial support and suitable ability to resist radial cyclic loads. The theoretical analysis, verified by observing the surface morphology of filament crossover points, found that filaments of the stents with smaller braiding angles have more significant axial displacement and axial rotation angle during radial compression, which made the friction phenomenon more intense and led to insufficient ability to resist radial cyclic loads. This study could provide a meaningful idea for preparing biodegradable braided stents with suitable ability to resist radial cyclic loads.

Investigation of changing geometry parameters of nickel-titanium shape memory alloys wire stent in cardiovascular implants

The purpose of this investigation was to assess the effects of changing the Nitinol stent's geometrical characteristics on the superelastic behavior of the stent using the finite element method. Four different geometrical parameters were considered and analyzed in sixteen stents. These geometrical parameters consisted of stent length, number of stent's meanders, inside diameter of the stent and wire diameter. Results showed that decreasing either stent length or the number of stent meanders and increasing either the inside diameter of the stent or wire diameter generally result in more sensible superelastic behavior exposure. The effect of changing stent length is independent of the inside diameter and number of the stent's meanders. Effects of changing inside diameter and number of stent's meanders were exactly dependent together. In addition, increasing wire diameter has a great effect on the maximum force. In conclusion, the results of this study may serve as guidelines for future stent design.

Influence of geometric parameters on partial compressive force and pushing performance of flow diverter

Research on flow diverter (FD) has progressed over the past decades; however, the relationships between parameters such as stent diameter, porosity, and number of wires and the properties of FDs, such as partial compressive force and push resistance, are not well understood. In this study, the partial compressive force and push resistance of braided FDs with varying porosity (61%-75%), diameter (2.5-5.0 mm), and number of wires (48 or 64) were evaluated using finite element analysis (FEA) and bench tests. At a small compression ratio, the 48-wire stents exhibited a larger partial compressive force than 64-wire stents of the same diameter. But when the compression ratio was 50%, the 64-wire stents had better resistance to pressure. The partial compressive force decreased as the stent diameter increased when all other parameters were equal. However, the influence of the diameter decreased as the stent porosity increased. The push resistance decreased as the porosity and diameter increased, and increased with the number of wires. These results provide useful information for FD design. Decreasing the number of wires can reduce the push resistance, while the push resistance is mainly influenced by the porosity and number of wires, and almost has no relationship with the partial compressive force. The FEA model proved very reliable, and corresponded well to the bench test results, which indicates that this model can be utilized to guide the design of FDs.

Computational modeling of braided-stent deployment for interpreting the mechanism of stent flattening

This study develops a computational model of the braided stent for interpreting the mechanism of stent flattening during deployment into curved arteries. Stent wires are expressed using Kirchhoff's rod theory and their mechanical behavior is treated using a corotational beam formulation. The equation of motion of the braided stent is solved in a step-by-step manner using the resultant elastic force and mechanical interactions of wires with friction. Examples of braided-stent deployment into idealized arteries with various curvatures are numerically simulated. In cases of low curvature, the braided stent expands from a catheter by releasing the bending energy stored in stent wires, while incomplete expansion is found at both stent ends (ie, the fish-mouth phenomenon), where relatively little bending energy is stored. In cases of high curvature, much torsional energy is stored in stent wires locally in the midsection of the curvature and the bending energy for stent self-expansion is not fully released even after deployment, leading to stent flattening. These findings suggest that the mechanical state of the braided stent and its transition during deployment play an important role in the underlying mechanism of stent flattening. NOVELTY STATEMENT: This study developed a computational mechanical model of the braided stent for interpreting stent flattening, which is a critical issue observed during deployment into highly curved arteries. Mechanical behaviors of the stent wires are appropriately treated by corotational beam element formulation with considering multiple contacts. We conducted numerical examples of the stent deployment into curved arteries and found that the mechanical state of the braided stent during deployment associated with occurrences of the stent flattening. We believe this finding gives new insight into the mechanism of stent flattening and would advance the design of the stent and its deployment protocol.

Numerical modeling of bare and polymer-covered braided stents using torsional and tensile springs connectors

Computational modeling of braided stents using the finite element (FE) method has become an essential tool in the design and development of these medical devices. One of the most challenging issues in such a task is representing in an accurate manner the interaction between the interlacing wires. With the goal of achieving a compromise between accuracy and computational affordability, we propose a new approach consisting in using 1D FE formulations equipped with torsional springs at the crossover points of the wires. In the case of covered braided stents, the model is enriched with a set of tensile springs (defined in the longitudinal direction), aimed at capturing the stiffening effect of the polymeric membrane. The predictive capabilities of the proposed model are evaluated using data of our own experimental tests, as well as data from other tests in the literature. The simulations demonstrate that the proposed model is able to predict the (markedly nonlinear) behavior of stents when subjected to radial and axial cycle loads, with errors at the end of the compression stage ranging from 0.5% to 10% in all cases.

Deformation mechanics of self-expanding venous stents: Modelling and experiments

Deformation properties of venous stents based on braided design, chevron design, Z design, and diamond design are compared using in vitro experiments coupled with analytical and finite element modelling. Their suitability for deployment in different clinical contexts is assessed based on their deformation characteristics. Self-expanding stainless steel stents possess superior collapse resistance compared to Nitinol stents. Consequently, they may be more reliable to treat diseases like May-Thurner syndrome in which resistance against a concentrated (pinching) force applied on the stent is needed to prevent collapse. Braided design applies a larger radial pressure particularly for vessels of diameter smaller than 75% of its nominal diameter, making it suitable for a long lesion with high recoil. Z design has the least foreshortening, which aids in accurate deployment. Nitinol stents are more compliant than their stainless steel counterparts, which indicates their suitability in veins. The semi-analytical method presented can aid in rapid assessment of topology governed deformation characteristics of stents and their design optimization.

Intraductal Tissue Sampling Device Designed for the Biliary Tract

Clinical sampling of tissue that is read by a pathologist is currently the gold standard for making a disease diagnosis, but the few minimally invasive techniques available for small duct biopsies have low sensitivity, increasing the likelihood of false negative diagnoses. We propose a novel biopsy device designed to accurately sample tissue in a biliary stricture under fluoroscopy or endoscopic guidance. The device consists of thin blades organized around the circumference of a cylinder that are deployed into a cutting annulus capable of comprehensively sampling tissue from a stricture. A parametric study of the device performance was done using finite element analysis; this includes the blade deployment under combined axial compression and torsion followed by an axial 'cutting' step. The clinical feasibility of the device is determined by considering maximum deployment forces, the radial expansion achieved and the cutting stiffness. We find practical parameters for the device operation to be an overall length of 10 mm and a diameter of 3.5 mm for a [Formula: see text] blade thickness, which allow the device to be safely deployed with a force of 10N and achieve an expansion over 3x its original diameter. A model device was fabricated with these parameters and a [Formula: see text] thickness out of a NiTi superalloy and tested to validate the performance. The device showed strong agreement with an equivalent numerical model, reaching a peak force within 2% of that predicted numerically and fully recovering after compression to 20% of its length. Clinical and Translational Impact Statement -This pre-clinical research conceptually demonstrates a novel expandable device to biopsy tissue in narrow strictures during an ERCP procedure. It can greatly improve diagnostic tissue yield compared to existing methods.

A finite element investigation on design parameters of bare and polymer-covered self-expanding wire braided stents

Self-expanding covered braided stents are routinely used across a diverse range of clinical applications, but few computational studies have attempted to replicate their complex behaviour. In this study, a computational framework was developed to predict the functional performance of bare and covered self-expanding wire braided stents, with a systematic evaluation on the effect of various braid and cover parameters presented. Simulated radial force and kink deformation tests show good agreement to experimental data for covered braided stents across a range of braid angles and cover thicknesses. Our results demonstrate that braid angle is a key governing parameter that dictates the radial and kink performance of both bare-metal and covered wire braided stents. It was also demonstrated that addition of a polymeric cover to a wire braided stent causes a stiffer radial response across all braid angles, particularly when thicker and/or stiffer covering systems were considered. This study represents the first experimentally-validated computational model for covered wire braided stent systems and has excellent potential to be used in future design of these devices for a range of applications.

A braided stent becomes flattened inside a curved catheter tube: A micro-CT imaging study

BACKGROUND

The braided stent is a widely accepted endovascular treatment device consisting of woven metal wires. One of the unsolved issues for the braided stent is the stent flattening phenomena when deployed into highly curved arteries. Although a recent computational study highlighted that the mechanical state of the stent inside the catheter before the deployment plays an essential role in causing stent flattening, there is no experimental observation for the stent inside the curved catheter.

OBJECTIVE

We investigated braided stent shapes in curved catheter tubes with various curvatures by micro-computed tomography (CT).

METHODS

A braided stent was deployed into catheter tubes and set in rectangular cases with constant curvature. The three-dimensional shape of the stent was imaged by micro-CT, and its cross-sectional flatness was quantitatively assessed.

RESULTS

Stent flattening occurred in cases of high curvatures of the outer side of the tube curvature, and the degree of flatness increased with increasing tube curvature. This demonstrates that stent flattening can be caused inside the highly curved catheter before deployment.

CONCLUSIONS

This preliminary and first observational report provides new insight into the mechanism of stent flattening and emphasizes the importance of the geometrical and mechanical state of the stent inside the catheter.

Braided composite stent for peripheral vascular applications

Braided composite stent (BCS), woven with nitinol wires and polyethylene terephthalate (PET) strips, provides a hybrid design of stent. The mechanical performance of this novel stent has not been fully investigated yet. In this work, the influence of five main design factors (number of nitinol wires, braiding angle, diameter of nitinol wire, thickness and stiffness of the PET strip) on the surface coverage, radial strength, and flexibility of the BCS were systematically studied using computational models. The orthogonal experimental design was adopted to quantitatively analyze the sensitivity of multiple factors using the minimal number of study cases. Results have shown that the nitinol wire diameter and the braiding angle are two most important factors determining the mechanical performance of the BCS. A larger nitinol wire diameter led to a larger radial strength and less flexibility of the BCS. A larger braiding angle could provide a larger radial strength and better flexibility. In addition, the impact of the braiding angle decreased when the stent underwent a large deformation. At the same time, the impact of the PET strips increased due to the interaction with nitinol wires. Moreover, the number of PET strips played an important role in the surface coverage. This study could help understand the mechanical performance of BCS stent and provides guidance on the optimal design of the stent targeting less complications.

MECHANICAL CHARACTERIZATION OF BRAIDED SELF-EXPANDING STENTS: IMPACT OF DESIGN PARAMETERS

In this paper, the mechanical performance of braided nitinol stents was systematically studied to provide guidelines for optimum stent designs. The influences of braiding patterns, braiding angles, wire diameters, and strand numbers on the mechanical behavior of the stent in terms of crimping strain, radial strength, longitudinal flexibility, and stability were characterized utilizing finite element method. Our results have shown that the two key design factors of braided stents are the braiding angle and wire diameter. A smaller braiding angle can increase radial stiffness and have better longitudinal flexibility and can maintain the stent stability. The wire diameter has less influence on the radial stiffness than the braiding angle, but the longitudinal flexibility is most sensitive to the wire diameter. The strand number is directly proportional to the radial stiffness and inversely proportional to the longitudinal flexibility. Compared to the classical crossing pattern, we have also proposed two patterns. These patterns have a minimal impact on the crimping and radial stiffness of stents, but the stent made from them are more flexible. Among three crossing patterns, it is interesting to see that the classical pattern is the most stable crossing pattern for stand numbers larger than 36, but it became the most unstable pattern at the strand number of 24. This work has shed light on the optimum design of braided stents.

Elastic recovery of polymeric braided stents under cyclic loading: Preliminary assessment

Over the last decades, stents have been largely used to treat vascular diseases such as coronary artery or peripheral vessel stenosis. Among the solutions which are commercially available to treat vascular stenosis, metallic stents represent the gold standard. However, issues such as restenosis, corrosion and fractures have been reported with these devices and are especially due to the material which is used. Braided polymeric stents could present an alternative to replace metallic stents especially in peripheral vessels where flexibility is required. Among polymeric materials, polyethylene terephthalate (PET), could be a good candidate as its biocompatibility has already been widely proven especially in the field of cardiovascular applications. Moreover, braided devices have been already used for the stenting of peripheral zones, providing locally outstanding flexibility due to the discontinuity of these structures. The goal of this work was to evaluate the radial strength and the recovery performances of polymeric braided stents made from PET monofilaments. In particular, the behavior of these stents under repeated cyclic radial compression loading was assessed and compared to results obtained with a metallic braided stent of same diameter. Results show that polymeric braided stents provide 100% elastic recovery after 20% diameter compression over 2000 repetitive loading cycles. However, radial strength goes slightly down with cycling, which points out that friction occurs in the braid. It comes out from the study that a braided polymer stent shows suitable mechanical behavior compared to a metallic stent over cyclic loading up to 2000 cycles. Moreover, it is shown that the mechanical behavior of these stents depend highly on the braid angle.

A HYBRID TUBULAR BRAID WITH IMPROVED LONGITUDINAL STIFFNESS FOR MEDICAL CATHETER

Medical catheters are widely used in various medical procedures, such as diagnostics, biopsies and air change. A desirable catheter needs to be flexible for low discomfort, and stiff in longitudinal direction for easy manipulation. Tubular braid is often employed as reinforcement structure for catheters, which plays an important role in the overall mechanical properties. Current tubular braids adopt identical braiding angles for all the yarns, resulting in limited longitudinal stiffness. In this paper, a novel hybrid braid with different braiding angles for the two sets of yarns is proposed and analyzed. Both experimental and numerical results show that the hybrid braid has a higher longitudinal stiffness than the uniform one due to the geometrical incompatibility generated by the hybrid braiding angles. The effects of design parameters are also investigated through a parametric study, and an increase of 418.3% is achieved in the optimum case. In addition, the bending flexibility of the hybrid braid is found to be comparable with the uniform one. The new structure shows great promise for engineering applications where high longitudinal stiffness is required.

A foldable manipulator with tunable stiffness based on braided structure

Minimally invasive surgery (MIS) has recently seen a surge in clinical applications due to its potential benefits over open surgery. In MIS, a long manipulator is placed through a tortuous human orifice to create a channel for surgical tools and provide support when they are operated. Currently the relative large profile and low stiffness of the manipulators limit the effectiveness and accuracy of MIS. Here we propose a new foldable manipulator with tunable stiffness. The manipulator takes a braided skeleton to enable radial folding, whereas membrane is used to seal the skeleton so as to adjust stiffness through creating negative pressure. We demonstrated experimentally, numerically, and analytically that, a flexible and a rigid state were obtained, and the ratio of bending stiffness in the rigid state to that in the flexible state reached 6.85. In addition, the manipulator achieved a radial folding ratio of 1.95. The proposed manipulator shows great potential in the design of surgical robots for MIS. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B, 2019.

Comparison of Pipeline Embolization Device Sizing Based on Conventional 2D Measurements and Virtual Simulation Using the Sim&Size Software: An Agreement Study

BACKGROUND AND PURPOSE

The Sim&Size software simulates case-specific intraluminal Pipeline Embolization Device behavior, wall apposition, and device length in real-time on the basis of rotational angiography DICOM data. The purpose of this multicenter study was to evaluate whether preimplantation device simulation with the Sim&Size software results in selection of different device dimensions than manual sizing.

MATERIALS AND METHODS

In a multicenter cohort of 74 patients undergoing aneurysm treatment with the Pipeline Embolization Device, we compared apparent optimal device dimensions determined by neurointerventionalists with considerable Pipeline Embolization Device experience based on manual 2D measurements taken from rotational angiography with computed optimal dimensions determined by Sim&Size experts blinded to the neurointerventionalists' decision. Agreement between manually determined and computed optimal dimensions was evaluated with the Cohen κ. The significance of the difference was analyzed with the Wilcoxon signed rank test.

RESULTS

The agreement index between manual selection and computed optimal dimensions was low (κ for diameter = 0.219; κ for length = 0.149, P < .01). Computed optimal device lengths were significantly shorter (median, 14 versus 16 mm, T = 402, r = -0.28, P < .01). No significant difference was observed for device diameters.

CONCLUSIONS

Low agreement between manually determined and computed optimal device dimensions is not proof, per se, that virtual simulation performs better than manual selection. Nevertheless, it ultimately reflects the potential for optimization of the device-sizing process, and use of the Sim&Size software reduces, in particular, device length. Nevertheless, further evaluation is required to clarify the impact of device-dimension modifications on outcome.

Mechanical characterizations of braided composite stents made of helical polyethylene terephthalate strips and NiTi wires

The novel braided composite stent (BCS), woven with both nitinol wires and polyethylene terephthalate (PET) strips, were characterized and compared with the braided nitinol stent in the same weaving pattern. Finite element models simulating the stent compression and bending were developed to quantify its radial strength and longitudinal flexibility. The interaction between the nitinol wires and the PET strips were also delineated. Results showed that the PET strips enforced more constrains on the BCS and thus enhance its radial strength especially at a larger compression load. The longitudinal flexibility of the BCS was less sensitive to the presence of the PET strips. This work suggested that the novel design of the BCS could acquire the advantage of a covered stent without compromising its mechanical performance. The fundamental understanding of the braided composite stent will facilitate a better device design.

NUMERICAL INVESTIGATIONS OF THE FLEXIBILITY OF INTRAVASCULAR BRAIDED STENT

Braided stents are commonly used to treat cerebral aneurysm, but there is little information about the bending characteristic of braided stent used for cerebral aneurysm. This paper investigates how geometrical parameters of braided stent influence its flexibility. Eight groups of braided stent models with different geometries (i.e., nominal diameter, length, braiding angle, number of wires, diameter of wire, frictional coefficient among wires and porosity) were constructed. Parametric analyses of these models were carried out by using Abaqus/Explicit. When the nominal diameter varied from 2[Formula: see text]mm to 5.5[Formula: see text]mm, the forces required for flexural deformation decrease from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N; when the axial length varied from 10[Formula: see text]mm to 40[Formula: see text]mm, the forces required for flexural deformation decrease from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N; when the braiding angle increases from 30[Formula: see text] to 75[Formula: see text] (the number of wires is 48 and the diameter of the wire is 0.026[Formula: see text]mm), the forces required for bending deformation decrease from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N; when the diameter of wires increases from 0.026[Formula: see text]mm to 0.052[Formula: see text]mm (the number of wires is 24 and the braiding angle is 60[Formula: see text]), the forces required for flexural deformation increase from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N; and when the number of wires increases from 14 to 48 (the braiding angle is 75[Formula: see text] and the diameter of the wire is 0.026[Formula: see text]mm), the forces required for flexural deformation increase from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N. From the data above it can be seen that the diameter of wires, the number of wires and braiding angle have a larger impact on bending characteristics of braided stent; and the axial length and nominal diameter have a smaller impact on bending characteristics of braided stent. Results of the present study may provide theoretical guidance for the design of self-expanding braided stent and its clinical practice.

Virtual endovascular treatment of intracranial aneurysms: models and uncertainty

Virtual endovascular treatment models (VETMs) have been developed with the view to aid interventional neuroradiologists and neurosurgeons to pre-operatively analyze the comparative efficacy and safety of endovascular treatments for intracranial aneurysms. Based on the current state of VETMs in aneurysm rupture risk stratification and in patient-specific prediction of treatment outcomes, we argue there is a need to go beyond personalized biomechanical flow modeling assuming deterministic parameters and error-free measurements. The mechanobiological effects associated with blood clot formation are important factors in therapeutic decision making and models of post-treatment intra-aneurysmal biology and biochemistry should be linked to the purely hemodynamic models to improve the predictive power of current VETMs. The influence of model and parameter uncertainties associated to each component of a VETM is, where feasible, quantified via a random-effects meta-analysis of the literature. This allows estimating the pooled effect size of these uncertainties on aneurysmal wall shear stress. From such meta-analyses, two main sources of uncertainty emerge where research efforts have so far been limited: (1) vascular wall distensibility, and (2) intra/intersubject systemic flow variations. In the future, we suggest that current deterministic computational simulations need to be extended with strategies for uncertainty mitigation, uncertainty exploration, and sensitivity reduction techniques. WIREs Syst Biol Med 2017, 9:e1385. doi: 10.1002/wsbm.1385 For further resources related to this article, please visit the WIREs website.

Selection of helical braided flow diverter stents based on hemodynamic performance and mechanical properties

BACKGROUND

Although flow diversion is a promising procedure for the treatment of aneurysms, complications have been reported and it remains poorly understood. The occurrence of adverse outcomes is known to depend on both the mechanical properties and flow reduction effects of the flow diverter stent.

OBJECTIVE

To clarify the possibility of designing a flow diverter stent considering both hemodynamic performance and mechanical properties.

MATERIALS AND METHODS

Computational fluid dynamics (CFD) simulations were conducted based on an ideal aneurysm model with flow diverters. Structural analyses of two flow diverter models exhibiting similar flow reduction effects were performed, and the radial stiffness and longitudinal flexibility were compared.

RESULTS

In CFD simulations, two stents-Pore2-d35 (26.77° weave angle when fully expanded, 35 μm wire thickness) and Pore3-d50 (36.65°, 50 μm respectively)-demonstrated similar flow reduction rates (68.5% spatial-averaged velocity reduction rate, 85.0% area-averaged wall shear stress reduction rate for Pore2-d35, and 68.6%, 85.4%, respectively, for Pore3-d50). However, Pore3-d50 exhibited greater radial stiffness than Pore2-d35 (40.0 vs 21.0 mN/m at a 3.5 mm outer diameter) and less longitudinal flexibility (0.903 vs 0.104 N·mm bending moments at 90°). These measurements indicate that changing the wire thickness and weave angle allows adjustment of the mechanical properties while maintaining the same degree of flow reduction effects.

CONCLUSIONS

The combination of CFD and structural analysis can provide promising solutions for an optimized stent. Stents exhibiting different mechanical properties but the same flow reduction effects could be designed by varying both the weave angle and wire thickness.

Virtual-versus-Real Implantation of Flow Diverters: Clinical Potential and Influence of Vascular Geometry

BACKGROUND AND PURPOSE

Intracranial stents have become extremely important in the endovascular management of complex intracranial aneurysms. Sizing and landing zone predictions are still very challenging steps in the procedure. Virtual stent deployment may help therapeutic planning, device choice, and hemodynamic simulations. We aimed to assess the predictability of our recently developed virtual deployment model by comparing in vivo and virtual stents implanted in a consecutive series of patients presenting with intracranial aneurysms.

MATERIALS AND METHODS

Virtual stents were implanted in patient-specific geometries of intracranial aneurysms treated with the Pipeline Embolization Device. The length and cross-section of virtual and real stents measured with conebeam CT were compared. The influence of vessel geometry modifications occurring during the intervention was analyzed.

RESULTS

The virtual deployment based on pre- and poststent implantation 3D rotational angiography overestimated (underestimated) the device length by 13% ± 11% (-9% ± 5%). These differences were highly correlated (R² = 0.67) with the virtual-versus-real stent radius differences of -6% ± 7% (5% ± 4%) for predictions based on pre- and poststent implantation 3D rotational angiography. These mismatches were due principally to implantation concerns and vessel-shape modifications.

CONCLUSIONS

The recently proposed geometric model was shown to predict accurately the deployment of Pipeline Embolization Devices when the stent radius was well-assessed. However, unpredictable delivery manipulations and variations of vessel geometry occurring during the intervention might impact the stent implantation.

Design of barrier coatings on kink-resistant peripheral nerve conduits

Here, we report on the design of braided peripheral nerve conduits with barrier coatings. Braiding of extruded polymer fibers generates nerve conduits with excellent mechanical properties, high flexibility, and significant kink-resistance. However, braiding also results in variable levels of porosity in the conduit wall, which can lead to the infiltration of fibrous tissue into the interior of the conduit. This problem can be controlled by the application of secondary barrier coatings. Using a critical size defect in a rat sciatic nerve model, the importance of controlling the porosity of the nerve conduit walls was explored. Braided conduits without barrier coatings allowed cellular infiltration that limited nerve recovery. Several types of secondary barrier coatings were tested in animal studies, including (1) electrospinning a layer of polymer fibers onto the surface of the conduit and (2) coating the conduit with a cross-linked hyaluronic acid-based hydrogel. Sixteen weeks after implantation, hyaluronic acid-coated conduits had higher axonal density, displayed higher muscle weight, and better electrophysiological signal recovery than uncoated conduits or conduits having an electrospun layer of polymer fibers. This study indicates that braiding is a promising method of fabrication to improve the mechanical properties of peripheral nerve conduits and demonstrates the need to control the porosity of the conduit wall to optimize functional nerve recovery.

Medical Textiles as Vascular Implants and Their Success to Mimic Natural Arteries

Vascular implants belong to a specialised class of medical textiles. The basic purpose of a vascular implant (graft and stent) is to act as an artificial conduit or substitute for a diseased artery. However, the long-term healing function depends on its ability to mimic the mechanical and biological behaviour of the artery. This requires a thorough understanding of the structure and function of an artery, which can then be translated into a synthetic structure based on the capabilities of the manufacturing method utilised. Common textile manufacturing techniques, such as weaving, knitting, braiding, and electrospinning, are frequently used to design vascular implants for research and commercial purposes for the past decades. However, the ability to match attributes of a vascular substitute to those of a native artery still remains a challenge. The synthetic implants have been found to cause disturbance in biological, biomechanical, and hemodynamic parameters at the implant site, which has been widely attributed to their structural design. In this work, we reviewed the design aspect of textile vascular implants and compared them to the structure of a natural artery as a basis for assessing the level of success as an implant. The outcome of this work is expected to encourage future design strategies for developing improved long lasting vascular implants.

Hemodynamic Changes Caused by Flow Diverters in Rabbit Aneurysm Models: Comparison of Virtual and Realistic FD Deployments Based on Micro-CT Reconstruction

Adjusting hemodynamics via flow diverter (FD) implantation is emerging as a novel method of treating cerebral aneurysms. However, most previous FD-related hemodynamic studies were based on virtual FD deployment, which may produce different hemodynamic outcomes than realistic (in vivo) FD deployment. We compared hemodynamics between virtual FD and realistic FD deployments in rabbit aneurysm models using computational fluid dynamics (CFD) simulations. FDs were implanted for aneurysms in 14 rabbits. Vascular models based on rabbit-specific angiograms were reconstructed for CFD studies. Real FD configurations were reconstructed based on micro-CT scans after sacrifice, while virtual FD configurations were constructed with SolidWorks software. Hemodynamic parameters before and after FD deployment were analyzed. According to the metal coverage (MC) of implanted FDs calculated based on micro-CT reconstruction, 14 rabbits were divided into two groups (A, MC >35%; B, MC <35%). Normalized mean wall shear stress (WSS), relative residence time (RRT), inflow velocity, and inflow volume in Group A were significantly different (P<0.05) from virtual FD deployment, but pressure was not (P>0.05). The normalized mean WSS in Group A after realistic FD implantation was significantly lower than that of Group B. All parameters in Group B exhibited no significant difference between realistic and virtual FDs. This study confirmed MC-correlated differences in hemodynamic parameters between realistic and virtual FD deployment.

The need for stent-lesion matching to optimize outcomes of intracoronary stent implantation

Intracoronary stents have markedly improved the outcomes of catheter-based coronary interventions. Intracoronary stent implantation rates of over 90% during coronary angioplasty are common. Stent implantations are associated with a small but statistically significant number of adverse outcomes including restenosis, thrombosis, strut malapposition, incomplete strut endothelialization, and various types of stenting failure. Better matching of biomechanical properties of stents and lesions could further improve the clinical outcome of intracoronary stenting. Thus, in this article, we assess the need for advanced intracoronary stent-lesion matching. We reviewed the data on biomechanics of coronary stents and lesions to develop knowledge-based rationale for optimum intracoronary stent selection. The available technical information on marketed intracoronary stents and the current understanding of the biomechanical properties of coronary lesions at rest and under stress are limited, preventing the development of knowledge-based rationale for optimum intracoronary stent selection at present. Development of knowledge-based selection of intracoronary stents requires standardization of mechanical stent testing, communication of the nonproprietary technical data on stents by the industry and dedicated research into procedural stent-lesion interactions.

High fidelity virtual stenting (HiFiVS) for intracranial aneurysm flow diversion: in vitro and in silico

A flow diverter (FD) is a flexible, densely braided stent-mesh device placed endoluminally across an intracranial aneurysm to induce its thrombotic occlusion. FD treatment planning using computational virtual stenting and flow simulation requires accurate representation of the expanded FD geometry. We have recently developed a high fidelity virtual stenting (HiFiVS) technique based on finite element analysis to simulate detailed FD deployment processes in patient-specific aneurysms (Ma et al. J. Biomech. 45:2256-2263,(2012)). This study tests if HiFiVS simulation can recapitulate real-life FD implantation. We deployed two identical FDs (Pipeline Embolization Device) into phantoms of a wide-necked segmental aneurysm using a clinical push-pull technique with different delivery wire advancements. We then simulated these deployment processes using HiFiVS and compared results against experimental recording. Stepwise comparison shows that the simulations precisely reproduced the FD deployment processes recorded in vitro. The local metal coverage rate and pore density quantifications demonstrated that simulations reproduced detailed FD mesh geometry. These results provide validation of the HiFiVS technique, highlighting its unique capability of accurately representing stent intervention in silico.

Introduction of a high-throughput double-stent animal model for the evaluation of biodegradable vascular stents

Current stent system efficacy for the treatment of coronary artery disease is hampered by in-stent restenosis (ISR) rates of up to 20% in certain high-risk settings and by the risk of stent thrombosis, which is characterized by a high mortality rate. In theory, biodegradable vascular devices exhibit crucial advantages. Most absorbable implant materials are based on poly-L-lactic acid (PLLA) owing to its mechanical properties; however, PLLA might induce an inflammatory reaction in the vessel wall. Evaluation of biodegradable implant efficacy includes a long-term examination of tissue response; therefore, a simple in vivo tool for thorough biocompatibility and biodegradation evaluation would facilitate future stent system development. Rats have been used for the study of in vivo degradation processes, and stent implantation into the abdominal aorta of rats is a proven model for stent evaluation. Here, we report the transformation of the porcine double-stent animal model into the high-throughput rat abdominal aorta model. As genetic manipulation of rats was introduced recently, this novel method presents a powerful tool for future in vivo biodegradable candidate stent biocompatibility and biodegradation characterization in a reliable simple model of coronary ISR.