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Finite Element Modeling of Traditional and Innovative Biomedical Stents

Davide Fugazza, ANSYS Belgium Vinay Carpenter, ANSYS India Marc Horner, ANSYS, Inc. 1

© 2011 ANSYS, Inc.

May 7, 2012

Overview 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

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© 2011 ANSYS, Inc.

May 7, 2012

1. What is a stent? 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.

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stent mounted on a balloon catheter

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balloon is inflated and stent expands

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balloon is removed and stent is implanted in the vessel

© 2011 ANSYS, Inc.

May 7, 2012

2. Motivation Experimental testing is complex and costly

Need of highly reliable numerical tools to simulate biomedical devices

Numerical analyses allow investigating both material behavior and “structures”

Possibility of improving design 4

© 2011 ANSYS, Inc.

May 7, 2012

3. Stent geometry(*) Outer diameter = 0.058’’

(~ 1.47 mm)

Radial thickness = 0.0006’’ (~ 0.15 mm)

RT OD

(*) courtesy ASTM Endovascular Devices Test Methods Task Group (F04.30.06) 5

© 2011 ANSYS, Inc.

May 7, 2012

Task

3. Stent geometry (cont’d) “slicing” planes

Create an hexahedral mesh

Strategies

Take advantage of symmetries and/or repetitive patterns

Localize “slicing” locations

Tools

Model decomposition

234 bodies 6

© 2011 ANSYS, Inc.

May 7, 2012

4. Material properties Material : stainless steel (E = 200000 MPa, ν = 0.3) Constitutive model : elasto-plastic with multilinear hardening

Yielding value,σy ~ 300 MPa

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© 2011 ANSYS, Inc.

May 7, 2012

5. Finite element mesh

Outer diameter ~ 1.47 mm Radial thickness ~ 0.15 mm

SOLID 185, 3D 8-Node Structural Solid SOLID 186, 3D 20-Node Structural Solid 8

© 2011 ANSYS, Inc.

May 7, 2012

6. Boundary/Loading conditions Deformation Stage

Objectives

Load Step 1 Radial expansion of the stent

Simulate balloon inflation

Load Step 2 Recoil of the stent

Simulate 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 :

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1.

Stresses after radial expansion

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Residual stresses and plastic strains after recoil

© 2011 ANSYS, Inc.

May 7, 2012

6. Boundary/Loading conditions (cont’d) Balloon modelled as rigid target in contact with stent

Load Step 1 Radial expansion of 0.875 mm

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© 2011 ANSYS, Inc.

May 7, 2012

Load Step 2 Radial recoil to 0.7 mm

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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© 2011 ANSYS, Inc.

May 7, 2012

8. Selected results

σmax~ 1.8 · σy

inflation

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© 2011 ANSYS, Inc.

May 7, 2012

deflation

8. Selected results (cont’d)

inflation

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© 2011 ANSYS, Inc.

May 7, 2012

deflation

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! 14

© 2011 ANSYS, Inc.

May 7, 2012

9. Innovative materials (cont’d) 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. Macroscopic effects not available in traditional materials

Innovative and commercially valuable applications

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Biomechanics

: orthodontics, orthopedics, eyeglasses, etc.

Mechanics

: actuators, thermal valves, connectors, etc.

Structural

: vibration control systems, dissipation devices, etc.

© 2011 ANSYS, Inc.

May 7, 2012

9. Innovative materials (cont’d) 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

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© 2011 ANSYS, Inc.

May 7, 2012

Steel

Thank you!

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© 2011 ANSYS, Inc.

May 7, 2012