Important Equations:

o

o

Thermal-Electric Fracture Detection in Nitinol Stents

During Fatigue Testing CyclesAusten Scudder, Mir Shams, Andrew Hastert, Dr. Ilya Avdeev (Advisor)

MotivationPeripheral artery stents experience high

failure rates, often leading to potentially

fatal restenosis. A method to determine

the specific failure time and location in a

stent during a fatigue testing cycle would

yield better stent designs and could

eventually allow finite element

simulations to replace physical testing

methods altogether.

Thermal-Electric Theory

Figure 1: Peripheral stents located in the

common carotid artery undergo extreme

loading conditions. Fatigue tests

simulate various modes of stress,

including compression, torsion and

bending.

Nitinol is an electrically conductive material. Therefore, the

stent can be modeled as parallel resistance elements in a

series. Moreover, the stent itself is has an effective

resistance as a result of the smaller struts. Any fracture of a

strut or connector affects the resistive structure, thus

altering the overall resistance. Any deviation from the initial

resistance can be observed by incorporating the stent into a

Wheatstone bridge circuit and measuring the voltage

output.

Figure 2: A Wheatstone bridge

circuit is a common technique in

detection of small resistive

changes.

ANSYS Simulation Experimental

References

Future Work Figure 9: The

image to the left

displays the

unfractured stent

with a current

density relative to

it’s maximum. The

image to the right

shows a stent

with a fracture

through the main

connector

(located between

the two markers).

The cold zone is

shown in blue

shows that little to

no current flows

through the

localized area

which contains

the fracture.

Figure 6: ANSYS Multiphysics simulation of a thermal-electric model is a computationally

intensive process. Achieving a steady state temperature requires non-linear calculations

and must be performed iteratively.

[1] M.Z. Shams, Andrew L. Hastert and I.V. Avdeev

“Motion Tracking and Mechanical Analysis of

Peripheral Vascular Stents,” to appear in the

proceedings of IASTED International Conference on

Biomedical Engineering (BioMed 2011), February 16-

18, 2011, Innsbruck, Austria.

[2] I.V. Avdeev and M.Z. Shams “Modeling Vascular

Stents: Coupling Solid with Reduced Order FE

Models,” Proceedings of WCCM/APCOM 2010, July

19-23, 2010, Sydney, Australia.

ANSYS Results

Equation 1: This equation is for a Wheatstone bridge circuit. If all

resistances are properly balanced the output voltage will be zero. A

deviation of resistance in any element will cause an imbalance and a

voltage will be detected.

Figure 3: The stent is composed of a series of rings, each

having an effective resistance due to the struts which compose

it. Any strut disassociation will interrupt the current flow and

result in a different effective resistance for the stent.

Equation 2: This equation shows a unitless factor of change for a

stent with a single fracture, where: n = numbers of rings, m = number of

struts per ring, and r = resistance of each strut.

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

+ V -

i

Resistive elements have a

thermal response due to

current flow. If current is

applied to a fractured stent,

the fracture location can be

identified by using a high

resolution thermal imaging

device and tracing the thermal

gradient to the cold zone.

Boundary conditions:

oCurrent at proximal nodes, i = 15mA

oVoltage at distal nodes, Vout = 0 Volts

oAmbient air temperature, Tinf = 20°C

o Improve analytical model of

lumped stent resistance

o Resolve discrepancies

between experimental and

simulation results

o Conduct thermal

measurements experimentally

o Design DAQ program for real-

time implementation

Figure 8: In an unfractured stent, the gradient

above would be evenly stepped at consistent

intervals. The above stent is fractured at the

connector and displays an inconsistent transition of

electric potential from the proximal to distal end. The

pattern will be constant regardless of input, however

the scale changes proportionally to the applied

current.

o Resistance of an unfractured

15-ring stent was measured in

order to evaluate quantitative

accuracy of computer

simulations, R=2.1±0.1 Ω.

o Thermal measurements have

not yet been obtained.

Figure 7: Resistance was measured across 15 rings of a Protégé

EverFlex nitinol stent using a Cen-Tech 37772 multi-meter. Two plunger

leads were used as the positive and negative affixments. Plunger leads

were positioned counter-radially.

Figure 5: (Left) Boundary

conditions must be applied

evenly and consistently

between simulations to

ensure accurate interpretation

of results. Nodes were

selected at the distal and

proximal ends of the stent.

Using the given model,

resistance had to be indirectly

calculated by determining the

maximum electric potential.

Figure 10: (Left)

Temperature profile of an

unfractured stent. No

change in temperatures

exceeding 0.06°C (ΔT).

Figure 11: (Right)

Temperature profile of a

fractured stent. Hot spots

occurring to the left and

right of the fracture

where current

concentrations occur.

Changes in temperature

approach 0.12°C (ΔT).

Temperature can be

scaled to more

observable levels by

increasing current load.

Figure 4: A nitinol stent

can be modeled as a

resistor to determine

when a fracture occurs.