A recoil resilient luminal support.

Abstract

Cardiovascular disease (CVD) refers to a class of diseases affecting normal function of cardiovascular system and its momentous role to carry oxygenated blood to the entire body. Taking lives of more than 17 million people in 2008, CVD has yet remained as the primary cause of deaths around the world. Statistics from World Health Organisation in 2002 associated CVD with 10% of the disability-adjusted life years lost in low/middle-income countries and 18% in high-income countries. Atherosclerosis, as one of the primary causes of CVD, refers to the thickening of vascular walls due to deposition of fatty materials wherein it can lead to impeded or completely occluded blood flow. Obstruction of coronary arteries, referred to as coronary heart dis-ease (CHD), is estimated to become the single leading health problem by 2020. Occurrence and further development of CHD is associated with a number of biological and environmental factors such as an individual’s genetic predisposition, lifestyle, climate conditions, exercise habits and emotions to name a few. Treatment and management of vascular constrictions consists of a combination of non-surgical and surgical methods. The former includes approaches such as healthy lifestyle changes and pharmacological interventions while the later includes bypass grafting, balloon angioplasty with or without deployment of a stent and atherectomy. Stents are mechanical devices that provide a chronic support against internal walls of occluded vessels to restore their normal luminal patency. In the past decade, stenting has prevailed as the conventional treatment option in management of CVD, exceeding current number of by-pass grafting procedures, owing this success to its proven efficacy in short and long-term treatment of occluded vessels. Common stent structures are simply made of a metal mesh, e.g. stainless steel, and deployed in a blood vessel such as an artery during a per-cutaneous coronary intervention procedure, also known as angioplasty. Several attempts to meet the often self-competing objectives of stents such as high radial strength, low elastic recoil, axial flexibility, trackability in tortuous paths, biocompatibility and radio-pacity gave birth to a multitude of different stent design and improvement iterations to date. The acute luminal gain after stenting is often compromised by the two most common post intervention complications, namely in-stent thrombosis (formation of blood clot) and restenosis (re-narrowing of the lesion). Induced trauma during stent deployment is proven to play a key role in the occurrence of these complications. Elastic recoil of stents after deployment due to the intrinsic material properties and the compressive forces from a vessel accounts for both acute and chronic luminal loss after stent deployment. Mitigation measures such as over-expansion in balloon-expandable (BE) stents and use of self-expandable (SE) stents so far have proven to aggravate vascular trauma leading to thrombosis and restenosis. Pharmacological approaches such as systemic administration of blood-thinners or localized drug release in drug-eluting (DE) stents aim to control these complications by inhibition of an accentuated inflammatory response from the body. Despite the promising results in reduction of restenosis after use of DE stents, increased rate of late thrombosis raised concerns about efficacy of these stents in comparison with bare-metal (BM) stents. Moreover, it is important to note that in these approaches the mechanical aspect of the problem still stands. As a result, to meet the often-competing aforementioned imperatives of stents, a new design paradigm is called for. To address the issue of recoil and extend capability of current stents to controlled and incremental expansion steps with alternative expansion mechanisms, in this thesis a novel recoil resilient stent is proposed and developed. The proposed luminal support called a recoil resilient ring (RRR) is an open ring with overlapping ends and asymmetrical sawtooth structures from the two ends that are intermeshed. Utilized as a standalone support or integrated with other stent structures, upon expansion of the RRR, the teeth from opposite ends can slide on top of each other, yet interlock step-by-step in the opposite direction so to keep the final expanded state against compressive forces that normally cause recoil. Design, fabrication and compatibility of the proposed stent with current state-of-the-art stent deployment procedures and its superior radial strength in comparison with commercial stents are extensively studied in this thesis through finite element modelling (FEM) and experimental studies. The RRR is fabricated from Nitinol sheets with trans-formation temperatures well above typical body temperature ensuring martensite mode of operation of the device after deployment. Fabrication is carried out by linear patterning of Nitinol sheets of 200-μm thickness utilizing μEDM technology. Superior radial strength of the RRR in comparison with a commercial stent composed of the stiffer material, stainless steel, is demonstrated via experimental and numerical studies. Hemodynamic risk assessment of the proposed design as a standalone and integrated support compared with a typical commercial stent is then carried out by transient computational fluid dynamics (CFD). Subject to a realistic pulsatile blood flow, spatial and temporal restenosis risk indicators of three luminal supports are extensively studied utilizing CFD. These luminal supports include a standalone RRR, a nominal BE stent and an RRR-integrated stent. Risk factors including extension of areas subject to low wall shear stress as the primary risk factor of restenosis after deployment, tendency of sup-ports to migrate in response to fluid drag forces as well as flow supply changes to side branches are extensively investigated. Furthermore, sensitivity of the results to the dimensional assumptions of the deployment domain, branching vessels and patency of the supports is studied. Our results indicate superior hemodynamic performance of the standalone RRR compared with the others. In addition, close correspondence of the performance indicators of RRR-integrated stent and the standalone stent demonstrates minimal hemodynamic footprint of the proposed RRR highlighting its merit as a viable luminal support given its superior radial strength. Attractive attributes such as shape memory effect of Nitinol, the thermally trained expanded shape of the RRR, its unique incremental slide and lock expansion mechanism and its higher transformation temperature compared to the body temperature, bring new potential for alternative controlled and incremental actuation of the RRR. These alternative expansion methods, by application of direct or electrically-induced heat are further explored through extensive analytical, multi-field numerical and experimental studies. The knowledge and contributions made in the current work, in addition to the design, development, experimental and multi-field numerical results provide a general engineering framework applicable to other biomedical luminal supports in the future.