Background

Design of new coronary stents involves consideration of various performance criteria including radial stiffness, durability, crimpability, material, and manufacturing choices among others. As it is time consuming and expensive to evaluate and optimize the stent designs by means of prototyping and physical testing, computer simulation offers powerful techniques to conduct virtual optimization of the stent designs early during device development. In this proof-of-concept work, a process for optimizing a representative coronary stent model through parametric and nonparametric approaches is demonstrated, and the evolution of design and performance measures of the stent through the optimization steps are discussed.

Methods

A stainless steel, balloon deployed coronary stent of dimensions 8.5 mm length, 1.5 mm outer diameter, and 0.15 mm thickness is considered. The base geometry of the stent is modeled as a parameterized 2D surface developed in SolidWorks. The 2D geometry is then meshed, extruded, and wrapped to create the final 3D cylindrical stent with 41,688 reduced integration hexahedral elements and 316 elastic–plastic stainless steel material using abaqus/cae. A finite element (FE) model comprising of the 3D stent, crimping and expansion devices, blood vessel represented as a straight hollow cylinder meshed with 22,400 hybrid hexahedral elements with Holzapfel–Gasser–Ogden strain energy potential based hyperelastic material [1] with two local material directions, along with appropriate contact, boundary, and loading conditions, is developed in abaqus/cae. A FE analysis is then run using a nonlinear static procedure in abaqus/Standard involving two steps; the stent is first crimped to catheter dimensions by radially contracting the crimping device using boundary conditions and is then balloon-expanded and deployed on to the blood vessel by radially expanding the balloon device using boundary conditions, followed by being subject to blood pressure loading by applying a cyclical pressure loading on the interior surface of the blood vessel. Snapshots of FE analysis results are shown in Fig. 1. The FE analysis is then followed with a FE-safe fatigue analysis to estimate the durability of the stent using the Brown–Miller critical plane search algorithm [2] and Gerber fatigue reserve factor (FRF) calculation with an infinite life design to meet approximately 10 years’ use (1× 10 9 loading cycles).