This paper discusses some issues regarding the structural behavior of a Balloon-Expandable (BE) stent made of stainless steel material (AISI316L). BE stent is a tubular, often mesh-like, structure which is expanded inside a diseased (stenosed) artery segment to restore blood flow and keep the vessel open following angioplasty. Most of BE stent designs have two fundamental constituents: expandable ring elements, and connecting elements “bridges” which connect adjacent rings together. The stent design is a major factor which determines its reliability during insertion into the blocked artery and throughout the long term in vivo service. The objective of this paper is to study the structural behavior of the stent in order to show the effect of the stent design, especially the geometry of the “bridges” in terms of flexibility, torsion and expansion. The numerical investigation is based on Finite Element Analysis (FEA) using Abaqus© finite element code. It has been demonstrated by FEA that the geometry of the connecting elements “bridges” has a significant impact on the structural behavior of the stent.
Cardiovascular BE stents are tiny mesh-like devices placed into an artery, blood vessel, or other duct to hold the structure open. They are commonly used to treat conditions that result from blocked or damaged blood vessels in the body. Most of BE stents are made from stainless steel material that can be plastically deformed through the inflation of a balloon, upon deployment, BE stents can undergo as much as 20–30% plastic strain. After the balloon is deflated the stent remains in its expanded shape, except for a slight recoil caused by the elastic portion of the deformation. Numerous stents design are used in clinical practice today [1], they may be made from cylindrical braided wire meshes, coiled strip, laser-cut metal tubes or etched sheet. The majority of stents are made from laser cut tubing. In any case, all stents contain stress-concentration features at which the stress is locally high, and it is at these locations that failure may potentially occur. The stent design is a major factor which determines its reliability during insertion into the blocked artery and throughout the long term in vivo service. Some design requirements should be considered when designing BE stents such as: high radial strength, good flexibility, good fatigue properties, low elastic radial and longitudinal recoil and optimum scaffolding.
FEA is an extremely useful complement and has proven to be effective and capable of providing a better and a more detailed understanding for fatigue and design [2], [3], [4], [5], [6], [7], [8] and [9] of BE stents. The mechanical and structural behavior of some BE stents have been investigated recently by few researchers such as De Beule et al. [10], Wu et al. [11], Gervaso et al. [12], Walke et al. [13], Barrera et al. [14], and Park et al. [15]. Indeed, these contributions are very useful and provide valuable information about the structural integrity of cardiovascular implants. However, a little attention has been focused on determining the influence of the connecting element “bridges” on the structural behavior of BE stents. Therefore, this present paper discusses using FEA the importance of the bridges geometry and their location when designing BE stents. The objective is to perform numerical investigations that will provide quantitative data of stresses over the bridges of the stent that are generated by mechanical loadings: bending, torsion and expansion. These numerical data could be used to show how the stent behaves and to study the factors that have an influence on the fatigue lifetime of the cardiovascular BE stent. At first, some details regarding the stent design and FE Modeling methodology are presented. Then, some results obtained by numerical simulations are illustrated and followed by a general discussion about BE stent design. Finally, some concluding remarks are presented in Section 5.
A multitude of prerequisites are present in the quest for the ‘ideal’ stent, such as the stent flexibility, foreshortening, recoil and radial strength. Therefore, FEA of stent design is performed here which combines parametric stent design with dedicated virtual bench-mark tests. This powerful combination of virtual variation in stent geometry shortens the design process significantly and allows easy evaluation of the original design and its variations. The FEA provides a qualitative validation of a widely used balloon-expandable stents design. Numerical investigations of the structural behavior were focused on the mechanical response of the stent during the deployment process. It has been demonstrated that the geometry of the bridges that connect two adjacent rings together, play an important role in the improvement of the stent behavior, especially in terms of flexibility. The unsymmetrical N-shaped Flex-connectors have proven better mechanical properties regarding elastic recoil, flexibility, torsion and radial strength. Further investigation may be conducted by combining FEA with optimization strategies in order to improve the structural behavior of the stent.
