Numerical investigations of the mechanical properties of a braided non-vascular stent design using finite element method

Mechanical behaviors of a new elliptical valve stent in bicuspid aortic valve

BACKGROUND AND OBJECTIVE

The conventional valve stents that are cylindrical in shape will become elliptical when implanted in bicuspid aortic valve, thereby reducing the durability of the artificial valve. In this study, a new design of valve stent is presented where valve stents have elliptical cross-section at the annulus and it is expected to have better expandability and circle shape during the interaction between the stent and bicuspid aortic valve, thereby extending the durability of artificial valve.

METHODS

Finite element method (FEM) is used to study the mechanical behavior of the novel valve stent in the bicuspid aortic valve. The effects of three matching relationship between the ellipticity of the stents and the ellipticity of the annulus (i.e., the ellipticity of the stent is greater than, equal to and less than the annulus ellipticity, respectively) on the mechanical behavior of stent expansion are studied. In addition, the expansion mechanical behavior of the novel valve stent at different implantation depths is also compared.

RESULTS

Results indicate that novel valve stent implantation with elliptical features is superior to conventional circular valve stent. When the novel valve stent ellipticity is less than the annulus ellipticity, the ellipticity of the novel valve stent after implantation is smaller than that of the conventional circular valve stent. This indicated that the novel valve stent has better expandability and post-expansion shape, making artificial valve to have better durability. The risk of paravalvular leak after implantation is lowest when the novel valve stent ellipticity is less than annulus ellipticity. When the novel valve stent ellipticity coincides with annulus ellipticity, the aortic wall is subjected to greatest stress. With the increase of implantation depth, the stress on the novel valve stent decrease.

CONCLUSIONS

This study might provide insights for improving stent design for bicuspid aortic valve.

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.

Virtual and analytical self-expandable braided stent treatment models

The Flow Diverter is a self-expandable braided stent that has helped improve the effectiveness of cerebral aneurysm treatment during the last decade. The Flow Diverter's efficiency heavily relies on proper decision-making during the pre-operative phase, which is currently based on static measurements that fail to account for vessel or tissue deformation. In the context of providing realistic measurements, a biomechanical computational method is designed to aid physicians in predicting patient-specific treatment outcomes. The method integrates virtual and analytical treatment models, validated against experimental mechanical tests, and two patient treatment outcomes. In the case of both patients, deployed stent length was one of the validated result parameters, which displayed an error inferior to 1.5% for the virtual and analytical models. These results indicated both models' accuracy. However, the analytical model provided more accurate results with a 0.3% error while requiring a lower computational cost for length prediction. This computational method can offer designing and testing platforms for predicting possible intervention-related complications, patient-specific medical device designs, and pre-operative planning to automate interventional procedures.

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.

A Finite Element Investigation on Material and Design Parameters of Ventricular Septal Defect Occluder Devices

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.

TORSIONAL BEHAVIOR OF STENTS: THE ROLE OF LINKER AND STENT TAPERING INVESTIGATED WITH NUMERICAL SIMULATION

The torsional performance is a major mechanical property of stent. A stent with good torsional performance is easy to deform along blood vessels without damaging the vascular wall to avoid in-stent restenosis (ISR). Therefore, this study aimed to study the effect of stent parameters on torsional performance. The effect of stent parameters on torsional performance was studied via finite element method (FEM). The twist metric (TM) and stress distribution of various stents were compared. The TM values of stents with I-, S-, M-, C-, and V-shaped linkers were 0.0190, 0.0191, 0.0184, 0.0141, and 0.0201[Formula: see text][Formula: see text], respectively. In addition, the TM value of the stent increased by 35.85 times when the number of linkers was increased from 2 to 8 and the stent was twisted at the same angular displacement in clockwise direction. The TM value of the stent with 1.13∘ tapering was 0.010 [Formula: see text], which was lower by 47.64% compared with that of cylindrical stent. Compared with the shape of the linker, the number of linkers had a more remarkable effect on torsional performance. Torsional performance was observably enhanced with the decrease in the number of linkers. Among the five stents with different linker shapes, the torsional performance of the stent with C-shaped linker was the best. Besides, the torsional performance of the tapered stent was better than that of the cylindrical stent. Moreover, the torsional performance increased by increasing the stent tapering. This work might provide insights into better stent design and clinical decisions.

Finite Element Analysis and Bench Testing of Ventricular Septal Defect Occluder

Complications after transcatheter closure of the ventricular septal defect (VSD) is significantly associated with the mechanical behaviour of the VSD occluder. This study aims to investigate the effect of structural parameters of the VSD occluder. A mechanical model of the VSD occluder was constructed by theoretical modelling. The mechanical properties of the VSD occluders with different braiding angles (30°, 45°, 60°), materials (nitinol (NiTi), polydioxanone (PDO)) and waist-heights (3 mm, 4 mm) were analysed and validated by bench tests. For the 30°NiTi, 45°NiTi, 60°NiTi and 45°PDO occluders, the bending angles at the waist under 1 mm radial shrinkage were 112°, 121°, 155° and 155°, respectively. And the maximum principal strains at the waist were 16.62%, 8.19%, 1.20%, and 0.66%, respectively. The maximum radial deformations with 0.5 rad axial bending at the waist were 1.73, 1.44, 0.41 and 1.68 mm, respectively. When the occluders were implanted into VSD with the mean thickness of 3.5 mm, high stress appeared at the margin and the contact area, and the area with the 3-mm-occluder was much larger. In conclusion, the 60°NiTi occluder showed better ability to fit the deformation of the defect than the other NiTi occluders, and the 45°PDO occluder performed better under compression conditions but poorly under bending conditions than the 45°NiTi occluder. The choice of the appropriate waist-height is beneficial to eliminate associative complication by reducing the contact stress.

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.

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.

Fatigue behavior of stent in tapered arteries: The role of arterial tapering and stent material

Stenting has achieved great success in treating cardiovascular diseases due to its high efficiency and minimal invasiveness. However, fatigue of stents severely limits its long-term outcome. In this article, finite element method was adopted to study the effects of arterial tapering and stent material on the fatigue performance of stents. A series of tapered vessels with different taper levels and two sets of stents with different materials were modeled. The Goodman diagram was used to evaluate the fatigue resistance of stents. Results showed that the fatigue resistance of stents can be extremely improved by simply changing stent material. In addition, the taper of the arteries had an important influence on the fatigue resistance of the stent. The fatigue life of the stent will be shortened with the increase of the arterial taper. The method that predicted stent fatigue life in tapered vessels can help clinicians select stents that are more suitable for tapered vessels and help stent engineers design stents that are more resistant to fatigue.

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.

Multi-Objective Optimization Design of Balloon-Expandable Coronary Stent

PURPOSE

Recent studies suggested that suboptimal delivery and longitudinal stent deformation can result in in-stent restenosis. Therefore, the purpose of this paper was to study the effect of stent geometry on stent flexibility and longitudinal stiffness (LS) and optimize the two metrics simultaneously. Then, the reliable and accurate relationships between metrics and design variables were established.

METHODS

A multi-objective optimization method based on finite element analysis was proposed for the investigation and improvement of stent flexibility and LS. The relative influences of design variables on the two metrics were evaluated on the basis of the main effects. Three surrogate models, namely, the response surface model (RSM), radial basis function neural network (RBF), and Kriging were employed and compared.

RESULTS

The accuracies of the three models in fitting flexibility were nearly similar, although Kriging made more accurate prediction in LS. The link width played important roles in flexibility and LS. Although the flexibility of the optimal stent decreased by 13%, the LS increased by 48.3%.

CONCLUSIONS

The obtained results showed that the multi-objective optimization method is efficient in predicting an optimal stent design. The method presented in this paper can be useful in optimizing stent design and improving the comprehensive mechanical properties of stents.

Numerical research on the biomechanical behaviour of braided stents with different end shapes and stent-oesophagus interaction

Quasi-static and dynamic numerical analyses are carried out by referring to computational models of commercial self-expandable braided stents with 3 commonly used end shapes, to evaluate the influence of different end shapes of stent on the biomechanical interaction between stent and oesophagus. The end shape has no influence on the equivalent stress, but has a great influence on the contact stress in the narrowest zone of the oesophagus-neoplasm system. However, the end shapes have significant effect on the equivalent stress and the contact stress in the healthy area of the oesophagus in contact with the stent ends. The results show that the maximum equivalent stress of the oesophagus occurs in the zone contact with the cup-shaped end and the maximum contact stress occurs in the zone contact with the edge of the trumpet-shaped stent end. Moreover, the stent apposition is almost not affected by the end shapes. Although small zones with an incomplete stent apposition appear in the transition zones of spherical-cup-shaped stent, such occurrence might not contribute to stent malapposition or stent migration. Therefore, these stents with 3 types of end shapes all have good stent apposition. Finally, the numerical simulation results can be used to assess the mechanical performance of stents with different end shapes, the effectiveness of stent expansion therapy, and the possibility of complications after stent implantation.

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.