Thesis
Mechanical analysis and simulation of the nitinol ring-stent : assessing the radial strength, fatigue safety and compaction strains
- Creator
- Awarding institution
- University of Strathclyde
- Date of award
- 2018
- Thesis identifier
- T14935
- Person Identifier (Local)
- 201178073
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Disease of the vascular system is often treated by ‘minimally invasive’ endovascular methods, which are less traumatic than conventional ‘open surgery’. The endovascular treatments, including implants, are delivered to the target area by catheter devices. The aorta, the largest artery in the body which stems from the heart, is susceptible to aneurysms and dissection of the arterial wall layers, which can both be fatal if left untreated. These specific conditions are often treated with endovascular stent-graft implants: flexible polymer conduits supported by metallic structures. Vascutek, a company based near Glasgow, is unique in designing and producing stent-grafts based on the ‘ringstent’ technology: the structural annular components being formed of multiple turns of a superelastic alloy called Nitinol. Competitors in the stent-graft market mainly implement the more common ‘Z-stent’ structure, also usually with Nitinol. The work of this thesis addresses the requirement of Vascutek to have advanced methods of analysing the mechanical performance of ring-stents in terms of: the loading which they apply to vessels; their fatigue performance in the pulsatile environment of the cardiac cycle, and the ability to compact into small diameter delivery systems without incurring detrimental levels of material strain. Building on previous efforts at The University of Strathclyde, the work presented here represents the first complete development, and thorough validation, of a Finite Element simulation methodology which captures the ‘bundle’ geometry and mechanical response of the multiple-strand ring-stent technology. A unique method has been devised to capture and simulate the non-linear response of human aortic tissue, specific to thoracic or abdominal locations and for a range of patient ages. Currently, there exists no standard approach in the industry to mechanically represent artery, partly due to the variability and difficulty of obtaining data. The bespoke Finite Element Analysis capabilities have provided the first quantification of the ‘radial strength’ of ring-stents, and knowledge of how the radial force is distributed on the artery wall. This includes evidence that if devices are implanted at high ‘oversize’, distribution is less uniform and there is no significant increase in total radial force. Furthermore; the first reliable simulation of the ring-stents interacting with the non-linear arterial response has been executed; the results showing a significant reduction to the vessels’ physiological motion and a corresponding increase in factor of safety when compared to assessments made assuming linear elastic artery. The latter goes beyond current standard practice for fatigue, and will allow justification of design of new products which are in development. An understanding, and verification, of how classic beam theory can be used to inform ring-stent design has also been provided. The methodology devised here has also been used to inform and analyse a component level Nitinol wire fatigue-to-failure test programme required to attain the strain based limits, against which the device simulations are assessed. This combined work (empirical side undertaken by colleagues at Vascutek) is a unique study, which successfully defines the fatigue strength of Nitinol wire at multiple ‘non-zero’ mean-strain states. Regarding the analysis on compaction of ring-stents, geometric mathematical tools and Micro Computed Tomography X-Ray imaging have been implemented in the design process, providing fidelity in measuring strain levels and insight to optimise the designs to allow the highly desirable reduction in catheter profile. To truly optimise the profile of delivery systems, a more complete understanding of the effects of straining Nitinol to high levels is required, which is ongoing in further research collaborations with The University of Strathclyde.
- Advisor / supervisor
- Dempster, William
- Nash, David
- Resource Type
- Note
- This thesis was previously held under moratorium from 10th September 2018 until 10th September 2023.
- DOI
- Date Created
- 2018
- Former identifier
- 9912621391102996
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