Elsevier

Journal of Biomechanics

Volume 41, Issue 15, 14 November 2008, Pages 3202-3212
Journal of Biomechanics

Mechanical modeling of self-expandable stent fabricated using braiding technology

https://doi.org/10.1016/j.jbiomech.2008.08.005Get rights and content

Abstract

The mechanical behavior of a stent is one of the important factors involved in ensuring its opening within arterial conduits. This study aimed to develop a mechanical model for designing self-expandable stents fabricated using braiding technology. For this purpose, a finite element model was constructed by developing a preprocessing program for the three-dimensional geometrical modeling of the braiding structure inside stents, and validated for various stents with different braiding structures. The constituent wires (Nitinol) in the braided stents were assumed to be superelastic material and their mechanical behavior was incorporated into the finite element software through a user material subroutine (VUMAT in ABAQUS) employing a one-dimensional superelastic model. For the verification of the model, several braided stents were manufactured using an automated braiding machine and characterized focusing on their compressive behavior. It was observed that the braided stents showed a hysteresis between their loading and unloading behavior when a compressive load was applied to the braided tube. Through the finite element analysis, it was concluded that the current mechanical model can appropriately predict the mechanical behavior of braided stents including such hysteretic behavior, and that the hysteresis was caused by the slippage between the constituent wires and their superelastic property.

Introduction

Though stents in their current form date back to the intravascular device used by Charles Dotter when he inserted a plastic tube into the peripheral artery of a dog (Dotter, 1969), they were not implanted into the human body until the 1980s, when the balloon angioplasty was first used and found to have some limitations, such as restenosis and abrupt closure. There have been various type of stents, which can be categorized according to their deployment (balloon-expandable stents and self-expandable ones) and fabrication (wire type and slotted tube type) methods (Jung et al., 2003). The mechanical behavior of a stent is an important factor to ensure its opening within the arterial conduits. Since the technical success of a stent for many types of obstructive lesion depends directly on its mechanical behavior, a mechanical model is required to design the stent structure through predictive modeling.

Several computational studies have been carried out to investigate the mechanical behavior of stents, including their compression, foreshortening, and dogboning behavior using the finite element method (Etave et al., 2001; Migliavacca et al., 2002; Petrini et al., 2004; Thériault et al., 2006; Wang et al., 2006). Employing finite element analysis, Lally et al. (2005) demonstrated that the arterial wall stress, which is related to the restenosis rate, can be changed by adjusting the stent design. Gu et al. (2005) investigated the effects of the covering on the mechanical behavior of the covered microstent. Most of these studies have focused on balloon-expandable and slotted tube stents made of stainless steel.

Recently, self-expandable stents have been increasingly used due to their advantages over balloon-expandable stents, such as their good radial and bending compliance and the prevention of balloon trauma (Duerig and Wholey, 2002; Rapp, 2004). Schillinger et al. (2006) reported that self-expandable Nitinol stents were superior to balloon angioplasty for the superficial femoral artery disease. Self-expandable stents have been made using shape memory alloys such as Nitinol as a base material, because they exhibit good biocompatibility (Duerig et al., 1999) and superelastic behavior that endows them with self-expandable properties. Nitinols have been drawn into one-dimensional wires which are subsequently fabricated into tubular stents. Since manual fabrication has several disadvantages such as limited output, high cost, nonuniformity, etc., an automated fabrication method such as the braiding method is quite useful (Jung et al., 2004).

In this study, an automated braiding machine was built in the laboratory to fabricate several sample stents and the mechanical behavior of self-expandable braided stents was characterized. A Nitinol wire was used as the constituent material to impart self-expandable properties to the stents and a one-dimensional superelastic constitutive law (Auricchio, 1995) was employed to analyze their mechanical behavior. Finally, the compression and bending behavior of the stents were simulated using the finite element method to determine their optimum structure.

Section snippets

Fabrication of braided stents

To fabricate the sample stents, a braiding machine was built in the laboratory (see Fig. 1). In the braiding process, two or more braiding wires are interlaced to form a three-dimensional textile structure. The carriers carrying the braiding wires are moved in the radial or along the circumferential directions in the braiding bed (see Fig. 2 for schematic four-step braiding motion). Several parameters are involved in designing the braided structural product, among which the number of wires per

Superelasticity of Nitinol wire

Nitinol, which is a shape memory alloy, has a special property called superelasticity, which is attributed to the crystalline phase changes associated with its reversible martensitic transformation. This superelasticity features a closed hysteresis between loading and unloading due to the phase transformation, though no permanent strain is present (Auricchio, 1995). The hysteresis can be observed from the stress–strain curve when the material is subjected to tensile loading and subsequent

Results

The mechanical behavior of the stents fabricated using the braiding technology is discussed in conjunction with the experimental results and the mechanical model incorporating the constitutive modeling of the Nitinol wires (in Section 3.1) and the geometrical modeling (in Section 3.2) into the finite element analysis.

Discussion

In general, stents are delivered into abnormally narrowed or closed conduit through a catheter in which they are compressed. If a stent with a large hysteresis between loading and unloading is installed into a conduit, the stent may not provide enough pressure to open the conduit because it will lose the compressive force due to the hysteresis upon being released from the catheter. In this aspect, braided stents may be problematic because they showed appreciable hysteresis in the compression

Conclusions

A mechanical modeling was performed to simulate the mechanical performance of self-expandable braided stents using a beam-based geometrical model of the braid structure and a superelastic constitutive equation for the shape memory Nitinol alloy wires. The model was validated by performing a finite element analysis incorporating the superelasticity and comparing the simulation results with the experimental ones. Through the finite element analysis, the hysteretic behavior of the braided stents

Conflicts of interest

All authors declare that there are no conflicts of interest.

Acknowledgement

The authors would like to thank the Korea Science and Engineering Foundation (KOSEF) for sponsoring this research through the SRC/ERC program of MOST/KOSEF (R11-2005-065).

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