Free Recovery Effects of Shape-Memory Polymers for Cardiovascular Stents
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Free Recovery Effects of Shape-Memory Polymers for Cardiovascular Stents Christopher M. Yakacki1, Robin Shandas1,2, Craig Lanning2, and Ken Gall3 1
Department of Mechanical Engineering, University of Colorado at Boulder Boulder, CO 80309-0427, U.S.A. 2 Division of Cardiology, The Children’s Hospital/University of Colorado Health Science Center Denver, CO 80218, U.S.A. 3 School of Materials Science and Engineering/George Woodruff School of Mechanical Engineering, The Georgia Institute of Technology Atlanta, GA 30332, U.S.A. ABSTRACT The shape-memory effect was examined in polymer stents intended for cardiovascular applications. Four polymer networks were synthesized from poly(ethylene glycol) dimethacrylate and tert-butyl acrylate with 10 wt% and 20 wt% crosslinker, and with glass transition temperatures (Tg) of 52°C and 55°C. Solid and 50% porous stents were manufactured and tested for free strain recoverability at temperatures at or just above 37°C. Stents with lower glass transition temperatures and a higher degree of crosslinking recovered faster than their counterparts. Lower deformation (packaging) temperatures and higher recovery temperatures induce more rapid recovery. The presence of geometrical features, such as pores, initiated recovery sooner, but had negligible influence on overall recovery. INTRODUCTION Shape-memory polymers (SMP) have been an increasingly popular proposed biomaterial over the last few years [1-3]. By utilizing the shape-memory effect, a biomedical device could be compacted and implanted at a fraction of its original size via minimally invasive surgery. The device would then return to its original size in-vivo when triggered by heat, photo [4], or chemical stimuli. It is important for this activation to be triggered in a manner that is not harmful to the surrounding biological environment. An ideal mechanism for shape-memory activation is to utilize the body’s own natural heat. However, only a few researchers have developed and tested the shape-memory mechanics of their applications in-vitro [4-5]. One goal of this paper is to explore factors affecting the free recovery response during the shape-memory effect in polymers intended for biomedical applications. The second goal of this paper is to introduce a shape-memory polymer stent for cardiovascular applications. Figure 1 shows the deployment of a representative shape-memory polymer stent at body temperature. It is generally accepted that drug-eluting stents are one of the best new innovations in stent technology [6]. However, even with the increased performance of drug-eluting stents, there are still limiting factors to the technology. Currently, drug-eluting stents are simply metal stents with a thin drug-eluting polymer coating. This small amount of polymer has a correspondingly small amount of therapeutic drugs it can release. Also, metal stents do not possess the ability to gently and continually expand a stenosed artery after angioplasty and implantation. This applies to both balloon expandable stents that are set
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