Mechanical modeling of self-expandable stent fabricated using braiding technology
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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2021, Journal of BiomechanicsCitation Excerpt :This limitation of analytical models may be explained by the fact that they do not consider inter-wire and stent-machine frictional interactions. To overcome these deficiencies, various Finite Element beam models have been proposed in the literature (see, e.g. Kelly et al., 2019; Kim et al., 2008; Shanahan et al., 2017b; Zhao et al., 2013 and references therein). The most intricate aspect in these FE approaches is how to accurately model the contact between the interlacing wires.