Design of Auxetic Stents by Topology Optimization

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As high efficiency and minimal invasive, percutaneous coronary intervention with stents is a popular treatment for coronary artery disease but suffering from the risks of stent thrombosis and in-stent restenosis. Apart from the biological factors, stenting structures have been demonstrated to have strong associations with the incidences of these complications. The impacts of stenting structures mainly concern two aspects, the mechanical failures, and the induced hemodynamic changes. This thesis introduces mechanical metamaterials and topology optimization into stent design to overcome the limitations. As a kind of artificially engineered metamaterials, the auxetic material with unique mechanical properties due to the effective negative poison’s ratio is incorporated into the stent design to overcome mechanical failures of the conventional stenting structures. Based on the auxetics, a multiscale topology optimization method is developed to generate auxetic stenting structures to enhance their mechanical performances. Additionally, a modified fluid permeability is proposed to quantify the stent induced obstructions to the blood flow. It successfully combines hemodynamic considerations into the developed topology optimization design for stents. Chapter 1 provides a brief introduction to this research. Chapter 2 gives the background and a comprehensive literature review, including different coronary stents, the mechanical and hemodynamic factors of the complications, the stenting optimization, topology optimization, multifunctional artificial cellular composites, and auxetic metamaterials. In Chapter 3, an extended parametric level set method (X-PLSM) is proposed to transform the parametric level set method (PLSM) from the cartesian coordinate system to a curvilinear system. Thus, the X-PLSM can implicitly represent the boundaries of shell structures efficiently rather than using the conventional PLSM for 3D models, which can benefit the design of stents. In Chapter 4, a multiscale topology optimization based on X-PLSM is developed to introduce auxetics into the design of self-expanding (SE) stents, aiming to enhance the stenting performances and overcome their mechanical failures. Then, the optimized auxetic SE stent is numerically validated in software ANSYS and prototyped using additive manufacturing techniques. In Chapter 5, a modified fluid permeability (MFP) is proposed to quantify the stent induced obstructions to the blood flow. After that, a multiscale multi-objective topology optimization based on PLSM is developed and performed to introduce the auxetic properties, maximize macroscopic stiffness, and minimize the stent induced obstructions. During the optimization, the impacts of each design objective function on the stenting structures are discussed. In Chapter 6, the deformation mechanism of the optimized stents and the surrounding blood flow is numerically simulated in ANSYS and CFX, respectively. Finally, conclusions and prospects are provided in Chapter 7.
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