Finite Element Modellisation Of Balloon Catheter

Chapter 1

Introduction

1.1

Introduction

The Company.

Boston Scientific (NYSE: BSX) is a worldwide developer, manufacturer and marketer of medical devices with approximately 16,000 employees and revenue of $5.6 billion in 2004.

Boston Scientific's mission is to improve the quality of patient care and the productivity of health care delivery through the development and advocacy of less invasive medical devices and procedures.

Boston Scientific's history began in the late 1960s, when co-founder John Abele acquired an equity interest in Medi-tech, Inc., a research and development company focused on developing alternatives to traditional surgery.

(Online: Boston Scientific)

The Product

The Extractor RX Retrieval Balloon is indicated for use endoscopically to remove stones from the biliary system, or to facilitate or contrast medium while occluding the duct with the balloon

The Extractor RX Retrieval Balloon is compatible with the RX Biliary System, which is designed to provide secure guidewire access during device advancement, manipulation and exchange.

(Online: Extractor ™ RX Retrieval Balloon Spec Sheet)

The Problem

In testing, the balloon sometimes separates from the tube upon inflation. The separation is caused by failure in peeling of the adhesive.

In order to develop a more suitable adhesive, we must first define the forces it will be under. These forces are complex due to the hyperelastic behaviour of the latex balloon. We will rely on a finite element analysis of the catheter to solve for these forces.

Before a finite element analysis can be attempted the hyperelastic properties of the latex must be found.

To successfully address this problem the following tasks must be accomplished;

1. Research hyperelastic behaviour to the point of being able to test hyperelastic properties.

2. Design and build equipment suitable to test hyperelastic properties using available resources.

3. Input properties to Ansys and model catheter

4. Obtain values of stress on point of adhesion

1.2

Objectives

Research

Firstly it is necessary for me to gain an understanding in the field of hyperelasticity.

Not only to overcome the difficulties of performing a Finite Element Analysis on a hyperelastic material, but also because I intend to model the material myself.

Analytical material models do not describe the structural properties of plastics and elastomers under all conditions. Most models describe a particular stress-strain relationship that exists under specific conditions (Miller 2001:2) and are not suitable for the large-scale deformation involved in the inflation of a balloon catheter.

In using Ansys for stress analysis on hyperelastic materials, many convergence difficulties are experienced mainly because a hyperelastic material can undergo several orders of magnitude higher strain than a traditional material. (Stuparu 2002:2)

This means I will need to familiarize myself with Ansys more than is necessary to create a conventional, linear model.

For theoretical simplicity, hyperelastic materials are assumed to be incompressible. An incompressible material will cause volumetric locking that requires special treatment in element formulation. This limits the choice of element type for a stress analysis. (Stuparu 2002:3)

Analysis

1) Material

The material will be analysed using a equibiaxial tension test and a simple tension test to calculate the Mooney-Rivlin hyperelastic material constants.

The stress/strain (engineering strain) curves for both equibiaxial and uniaxial tension tests will be recorded and entered into the *MOONEY subprogram in ANSYS. This will automatically calculate the Mooney-Rivlin hyperelastic constants.

The constants will be outputted to a text file for future reference and also fitted to the material model of the Latex.

2) Model

ANSYS will output displacement component data for each node after inflation. It will output the stress/strains at the corner nodes of the elements. It will give a PowererGraphics Summary displaying the average magnitude of the forces. It will also output the direction of the forces at the desired points. I hope to record the magnitude and direction of the forces on the adhesive to suggest an appropriate peel test.

Application

Hopefully the results of my analysis will be used by Boston Scientific to gain a better understanding of the stresses the adhesive will be expected to sustain. This knowledge is vital in designing a more effective adhesive. I believe the model will be quite useful in simulating different stresses on the catheter, for example the catheter in free air or in the body. It will also help them predict the effects of redesigning the catheter, in preliminary stages of development.

Test/Validation

The main Validation of this project is how the behaviour of the model compares to that in real life. Because this is a mainly theoretical project, there is little validation or testing that can be implemented. However if the model reacts to inflation in a similar manner to the catheter then I believe the model is validated by its result.

Chapter