2011 ANSYS, Inc. May 7, 20121

Finite Element Modeling of Traditional and Innovative Biomedical Stents

Davide Fugazza, ANSYS Belgium

Vinay Carpenter, ANSYS India

Marc Horner, ANSYS, Inc.

2011 ANSYS, Inc. May 7, 20122

1. What is a stent?

2. Motivation

3. Stent geometry

4. Material properties

5. Finite element mesh

6. Boundary/Loading conditions

7. Selected results

8. Solution process

9. Innovative materials

Overview

2011 ANSYS, Inc. May 7, 20123

A stent is a tiny tube placed into an artery, blood vessel, or

other duct, partially occluded by a plaque, to restore the

original width and then re-establish a correct blood flow.

1. What is a stent?

1

2

3

1 stent mounted on a balloon catheter

2 balloon is inflated and stent expands

3 balloon is removed and stent is implanted in the vessel

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2. Motivation

Need of highly reliable numerical tools to simulate

biomedical devices

Numerical analyses allow investigating both

material behavior and structures

Possibility of improving design

Experimental testing is complex and costly

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3. Stent geometry(*)

Outer diameter = 0.058 (~ 1.47 mm)

Radial thickness = 0.0006 (~ 0.15 mm)

OD

RT

(*) courtesy ASTM Endovascular Devices Test Methods Task Group (F04.30.06)

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3. Stent geometry (contd)

234 bodies

Create an hexahedral mesh

Take advantage of symmetries

and/or repetitive patterns

Localize slicing locations

Model decomposition

T

o

o

l

s

S

t

r

a

t

e

g

i

e

s

T

a

s

k

slicing planes

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4. Material properties

Material : stainless steel (E = 200000 MPa, = 0.3)

Constitutive model : elasto-plastic with multilinear hardening

Yielding value,y ~ 300 MPa

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5. Finite element mesh

SOLID 185, 3D 8-Node Structural Solid

SOLID 186, 3D 20-Node Structural Solid

Outer diameter ~ 1.47 mm

Radial thickness ~ 0.15 mm

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6. Boundary/Loading conditions

Deformation Stage Objectives

Load Step 1

Radial expansion of the stentSimulate balloon inflation

Load Step 2

Recoil of the stentSimulate balloon deflation

We focus on the stent behavior only therefore the balloon will not be

modelled. Expansion and recoil are simulated by imposing target

displacements to a rigid cylinder.

Output quantities of interest :

1. Stresses after radial expansion

2. Residual stresses and plastic strains after recoil

2011 ANSYS, Inc. May 7, 201210

6. Boundary/Loading conditions (contd)

Load Step 1

Radial expansion of 0.875 mm

Balloon modelled as rigid

target in contact with stent

Load Step 2

Radial recoil to 0.7 mm

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7. Solution process

Complex behavior :

Non-linear material : elasto-plastic constitutive model

Non-linear BCs : contact between stents and balloon

Geometric non-linearities : large deflections

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8. Selected results

max~ 1.8 y

inflation deflation

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8. Selected results (contd)

inflation deflation

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9. Innovative materials

Question: any alternatives to steel? I believe so

Shape-memory alloys (SMAs) : materials with intrinsic ability to remember

an original shape featuring at the macroscopic level two uncommon

characteristics not present in materials typically used in engineering.

Superelasticity (SE) : upon loading and unloading cycles an SMA can

undergo large deformations without showing residual strains.

Shape-memory effect (SME) : the SMA can recover its original shape

through thermal cycles.

SE SME

New in R14!

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9. Innovative materials (contd)

An example of SMA material: Nitinol (Nickel Titanium Naval Ordinance Lab.)

Nitinol is a compound of Nickel and Titanium. Due to its unique properties,

which depend upon temperature and processing history, it has found

widespread acceptance as a material of choice for medical implants and

other engineering devices.

Biomechanics : orthodontics, orthopedics, eyeglasses, etc.

Mechanics : actuators, thermal valves, connectors, etc.

Structural : vibration control systems, dissipation devices, etc.

Macroscopic effects not available in traditional materials

Innovative and commercially valuable applications

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9. Innovative materials (contd)

Can we use/simulate shape-memory alloy stents? I would feel confident

SMA stents are manufactured with a diameter larger than that of the target

vessel. They are then crimped at or below room temperature and placed in

a delivery system. At the treatment site the stent is released from the

delivery system and expands until it hits the vessel wall and conforms to it.

Then, at body temperature, the stent shows a superelastic behavior.

Nitinol vs. Steel

Nitinol Steel

2011 ANSYS, Inc. May 7, 201217

Thank you!