Drug diffusion and structural design criteria for conventional and auxetic drug-eluting stents.

摘要

Most balloon angioplasty procedures include the insertion of tiny cylindrical wire mesh structures, called cardiovascular stents, to prevent the elastic recoil that follows arterial dilatation. The scaffolding characteristics of the stent provide strength to the artery wall. However, recognition of the stent as a foreign material triggers a human immune system response causing closure of the artery. This condition is known as restenosis.; A recent advancement to counteract restenosis is to employ drug-eluting stents to locally deliver immunosuppressant drugs. Currently, drug-eluting stents usually consist of a metal wire structure coated with a microporous polymer matrix or a nanoporous metal matrix structure. Either type of drug-eluting stent may also have a polymer top coat. The drug resides within the pores of the matrix, from which it diffuses into the adjacent arterial tissue. The polymer top coat controls the drug delivery rate to provide a more constant long term dosage.; In this project, Fick's law of diffusion in cylindrical coordinates was used to model the diffusion of immunosuppressant drug molecules from the stent matrix into the adjacent arterial tissue. Extensive parametric analyses were performed to determine the minimum required stent matrix thickness to provide sufficient drug delivery so that the stent retains its efficacy.; In addition to efficient drug delivery, the drug-eluting stent must also exhibit high circumferential strength to support the arterial wall and low flexural rigidity for maneuverability during deployment.; In this project, an analytical procedure was developed to estimate the circumferential and the flexural stiffnesses of stents using the invariant properties of orthotropic materials. These invariant properties were determined from apparent micromechanical stiffnesses using a mechanics of materials approach. Furthermore, a unique auxetic (negative Poisson's ratio) stent structure is proposed that exhibits high circumferential strength in its expanded configuration and low flexural rigidity in its crimped configuration.; Results generated with the analytical diffusion model, developed in this project, compare favorably with previously published clinical data. The circumferential and flexural stiffnesses estimated using the analytical procedure developed in this project compare favorably with the results from rigorous finite element analyses and previously published experimental data.