Design of Biomedical Devices

Analysis and Design of Biomedical Stents and Stent/Grafts

The design process for stents typically involves balancing conflicting mechanical requirements. For example, a compliant design that provides good radial support often leads to significant elastic recoil in deployment, a design that minimizes compaction diameter often sacrifices structural strength, a stiff design that assures secure contact with the vessel may cause damage to the vessel and restenosis. Researchers at the Silicon Valley office of ARA are developing finite element analysis software that can be used by engineers to predict the performance of stent designs.

An example finite element analysis of the response of a stent wire used in a stent/graft is shown in Figure 1. Stresses and strains developed in the wire are calculated throughout the complete process of fabrication, deployment and service. During the interaction with the artery, stresses and deformations in the artery were calculated.

These analyses are well beyond the capabilities of most available commercial finite element codes because of the high level of nonlinearity; the large deformations involved and the contacts that occurs between the wire, balloon and artery. When the wire is compacted to fit into the delivery system it experiences tight curvature due to bending, contacts with itself and twists to accommodate the small diameter of the delivery catheter.

The interaction between the wire and the artery during deployment of the stent is shown in Figure 2. This interaction is difficult to simulate because of the great differences in compliance between the stiff wire and the relatively soft artery.

We are developing interactive graphical user interfaces to allow designers to optimize stent designs without requiring extensive knowledge of finite element techniques. The user inputs parameters for stent geometry and materials in terms of physical quantities. This software will significantly reduce the time to market of new stent designs. It is envisioned that the design tool will incorporate all of the necessary features of a finite element code but will not require the user to have extensive knowledge of running finite element codes.

The above analyses were performed using LS-DYNA developed by the Livermore Software Technology Corporation (LSTC).

http://www.lstc.com/
LS-DYNA - General Purpose Transient Dynamics Finite Element Program.

Dr. Steven Kirkpatrick
Principal Engineer
e-mail: skirkpatrick@ara.com

Figure 1. Calculated response of the stent wire during compaction.

Figure 2. Calculated response of the stent wire during deployment.

Figure 3. Graphical Interface for the stent design tool.