An experimental and computational investigation on the biomechanics of polymer-covered self-expanding stents

Nitinol self-expanding stents are commonly used to treat peripheral artery disease. However, the femoropopliteal artery presents a challenging environment for stent placement, where large arterial deformations occur many times daily during limb flexion. Compared to other arterial locations, femoropopliteal stenting is associated with relatively low clinical success and high rates of fracture. Recently, novel device platforms have emerged in the form of polymer-covered self-expanding stents that have shown promising clinical outcomes in femoropopliteal applications. For these devices, a traditional metallic stent frame is combined with a flexible polymer covering, whose primary function is to act as mechanical barrier to tissue ingrowth and intimal hyperplasia, thereby reducing the likelihood of long-term restenosis. While this provides opportunities to improve the design of peripheral stents and enhance patient outcomes, there are distinct challenges in understanding their behaviour across existing laser-cut and wire braided stent platforms. In particular, the relative contributions of the stent frame and polymer cover properties towards the functional performance of these composite systems is not known. The objective of this thesis is to investigate the mechanics of polymer-covered self-expanding wire braided and laser-cut stents for femoropopliteal applications through a combined experimental-computational approach. In this thesis, detailed experimental benchtop studies were carried out to investigate the mechanics of polymer-covered self-expanding wire braided and laser-cut stents. In parallel, a finite element-based computational framework was developed, whereby novel strategies were used to implement polymer coverings across wire braided and laser-cut stents and predict mechanical performance. For self-expanding wire braided stents, this combined experimental-computational approach was used to systematically evaluate the effect of braid angle and polymer cover thickness on the response under a number of different loading regimes. Similarly, for laser-cut stent systems, the approach was used to evaluate the role of the characteristic cell design (e.g. open-cell, closed-cell and separated z-ring) and polymer cover properties on functional performance. Finally, the computational frameworks were used to predict the in vivo implant performance of bare-metal and polymer-covered wire braided and laser-cut stent systems, and investigate the influence of severe bending loading on the fatigue response of these devices. It was found that polymer coverings fundamentally alter the deformation mechanisms and functional performance of self-expanding wire braided stents, while laser-cut stents largely retain their characteristic behaviour when compared to their original bare-metal design. It was shown that braid angle is a key governing parameter that dictates the radial and kink performance of both bare-metal and polymer-covered wire braided stents. Despite their complex behaviour, the distinct design flexibility offered by these systems mean that performance could still be tailored for specific applications, in particular with low braid angles and relatively thin cover thicknesses. The predicted implanted performance of each stent type showed that laser-cut stents demonstrated superiority in lumen gain and vessel conformance upon deployment, but promoted vessel kinking and their fatigue life was estimated to be in a low-cycle range even for moderate bending. The results showed that wire braided stents had lower lumen gain and vessel conformity compared to laser-cut systems, with good predicted fatigue performance, but substantial malapposition under bending. In conclusion, the work performed in this thesis enhances current understanding on the functional performance of both wire braided stents and covered stents. The outcomes from the experimental data and computational frameworks developed in this thesis form a benchmark for the future development of covered wire braided and laser-cut stent systems.

Funder

Irish Research Council for Science, Engineering and Technology

Publisher

NUI Galway

Rights

Attribution-NonCommercial-NoDerivs 3.0 Ireland

cbn

Collections

University of Galway Theses (PhD Theses)