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Research Papers

Uniform Expansion of a Polymeric Helical Stent

[+] Author and Article Information
Nasim Paryab, Duane Cronin, Pearl Lee-Sullivan

Department of Mechanical and Mechatronics Engineering,  University of Waterloo, 200 University Avenue West Waterloo, Ontario N2L 3G1, Canada

Xiong Ying, Freddy Y. C. Boey, Subbu Venkatraman

 Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore

J. Med. Devices 6(2), 021012 (May 14, 2012) (10 pages) doi:10.1115/1.4005777 History: Received December 18, 2009; Accepted May 27, 2011; Published May 14, 2012; Online May 14, 2012

Helical coil polymeric stents provide an alternative method of stenting compared to traditional metallic stents, but require additional investigation to understand deployment, expansion, and fixation. A bilayer helical coil stent consisting of PLLA and PLGA was investigated using the finite element model to evaluate performance by uniform expansion and subsequent recoiling. In vitro material characterization studies showed that a preinsertion water-soaking step to mimic body implantation conditions provided the required ductility level expansion. In this case, the mechanical contribution of the outer PLGA layer was negligible since it softened significantly under environmental conditions. The viscoelastic response was not considered in this study since the strain rate during expansion was relatively slow and the material response was primarily plastic. The numerical model was validated with available experimental expansion and recoiling data. A parametric study was then undertaken to investigate the effect of stent geometry and coefficient of friction at the stent-cylinder interface on the expansion and recoiling characteristics. The model showed that helical stents exhibit a uniform stress distribution after expansion, which is important for controlled degradation when using biodegradable materials. The results indicated that increasing stent width, pitch value, and coil thickness resulted in a larger diameter after recoiling, which would improve fixation in the artery. It was also noted that a helical stent should have more than five coils to be stable after recoiling. This work is part of a larger research study focused on the performance of a balloon-inflated polymeric helical stent for artery applications.

Copyright © 2012 by American Society of Mechanical Engineers
Topics: stents
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References

Figures

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Figure 1

A typical helical stent [25]

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Figure 2

Stress-strain results for PLLA samples at 25 and 37 °C in air and water bath using a DMA tension clamp

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Figure 3

Stress-strain results of PLGA samples in 25 and 37 °C in air and in water bath using a DMA tension clamp

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Figure 4

Helical stent geometry

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Figure 5

Predicted helical stent expansion using applied radial displacement

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Figure 6

Convergence study, element size quoted as single element volume

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Figure 7

Unexpanded and expanded stent showing foreshortening and uncoiling

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Figure 8

Pressure versus diameter for expansion of an 8.5 coil stent

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Figure 9

Stent recoiling (outside diameter) following expansion and removal of balloon

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Figure 10

Before expansion, after expansion, and after recoiling for an eight-coil stent

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Figure 11

Circumferential stress distribution (in MPa) for an eight-coil stent after expansion

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Figure 12

Circumferential stress distribution (in MPa) for an eight-coil stent after expansion

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Figure 13

Circumferential stress distribution (MPa) for an eight-coil stent after recoiling

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Figure 14

Equivalent plastic strain distribution for an eight-coil stent after recoiling

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Figure 15

Evaluation parameters as a function of number of stent coils

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Figure 16

Evaluation parameters as a function of COF

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Figure 17

Evaluation parameters as a function of stent width

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Figure 18

Evaluation parameters as a function of stent thickness

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Figure 19

Evaluation parameters as a function of stent pitch

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Figure 20

Two-layer stent including the PLLA (inner layer) and the PLGA (outer layer) layers

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Figure 21

One-layer stent model including the PLLA layer

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Figure 22

von misses stress changes during expansion of two stents: one with just PLLA and the other with two layers of PLLA and PLGA

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Figure 23

The FEM model which examined the flexibility of the helical stent

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