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Research Papers

Mechanical Evaluation of Polyvinyl Alcohol Cryogels for Covered Stents

[+] Author and Article Information
Jason D. Weaver

Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 313 Ferst Drive, Suite 2127, Atlanta, GA 30332jason.weaver@bme.gatech.edu

David N. Ku1

George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 801 Ferst Drive, Atlanta, GA 30332david.ku@me.gatech.edu

1

Corresponding author.

J. Med. Devices 4(3), 031002 (Aug 31, 2010) (6 pages) doi:10.1115/1.4001863 History: Received February 21, 2010; Revised April 30, 2010; Published August 31, 2010; Online August 31, 2010

Covered stents could reduce restenosis rates by preventing cellular migration with a physical barrier and may have reduced thrombotic complications if an appropriate material is selected. Previous Dacron™ or poly(tetrafluoroethylene) (PTFE) covered stents have had mixed clinical results in part because they are too thick and too thrombogenic at small diameters. Ideally, the covering should be as thin as a stent strut, mechanically able to expand as much as a stent, and durable enough to withstand deployment. As an alternative to PTFE, thin polyvinyl alcohol (PVA) cryogel membranes were tested for their ability to stretch with uniaxial tension tests and for puncture strength with a modified ASTM method. Additionally, PVA cryogel covered stents were made by coating expanded bare metal stents. These covered stents were then hand-crimped onto a balloon catheter and expanded. PVA cryogel membranes were made as thin as 100μm—thinner than some stent struts—and stretched to approximately 3.0 times their original diameter (similar to a stent during deployment). PVA cryogel membranes resisted puncture well with an average push-through displacement of 4.77 mm—allowing for safe deployment in vessels of up to 9 mm in diameter. Push-through displacement did not depend on membrane thickness in the range tested—a trait that could reduce stent profile without increased risk of puncture. All the PVA cryogel covered stents tolerated the crimping and expansion process well and there was little to no visible membrane damage. In conclusion, based on the results of these mechanical tests, PVA cryogels are mechanically suitable for covered stent membranes. This work represents a first step toward the creation of a new class of covered stent, which could prevent complications from both restenosis and thrombosis.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Uniaxial tensile test apparatus (left) and puncture test apparatus (right)

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Figure 2

Typical stress-stretch ratio curves during uniaxial tension testing of a 10% and 20% PVA cryogel membrane (both at 0.1 mm/s). The ultimate stretch ratios fall within the lower half of typical BMS expansion range (27).

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Figure 3

Puncture pressure versus probe displacement for two typical PVA cryogels. Puncture pressure was calculated by dividing the load by the cross-sectional area of the drilled hole. Probe displacement is given as a fraction of the test mandrel radius. A fractional displacement of 1.0 is set as the safety requirement.

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Figure 4

Push-through displacement versus membrane thickness for all puncture tests. Push-through displacement does not depend on membrane thickness within the range tested. The dashed line represents the push-through displacement safety requirement.

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Figure 5

Close-up view of an expanded PVA cryogel covered Express (top) and Palmaz Genesis stent (bottom)

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