Investigation of novel Mg-Li-Y alloys for bioresorbable vascular scaffolds

MacLeod, Kenneth John

Thesis

Investigation of novel Mg-Li-Y alloys for bioresorbable vascular scaffolds

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Creator

MacLeod, Kenneth John

Rights statement

Strathclyde Thesis Copyright

Awarding institution

University of Strathclyde

Date of award

2023

Thesis identifier

T16699

Person Identifier (Local)

201981739

Qualification Level

Doctoral (Postgraduate)

Qualification Name

Doctor of Philosophy (PhD)

Department, School or Faculty

Department of Mechanical and Aerospace Engineering

Abstract

Magnesium alloys are promising candidates for application in the development of new bioresorbable medical devices. Magnesium alloys are known to be safe in vivo and degrade within suitable time frames for a range of bioresorbable medical devices. Through selective alloying and processing the mechanical properties of magnesium can be tailored to suit specific device requirements. Bioresorbable vascular scaffolds (BVS) are one such device where magnesium alloys are being applied. BVS offer the potential to revolutionise the treatment of arterial disease through the removal of the long-term risks associated with current treatments. Recent developments in BVS technology have considered the use of a range of materials with magnesium alloys being one of the most promising material candidates for use in this new technology. One of the limiting factors of magnesium alloys for application in BVS technology is their relative lack of ductility, meaning new alloys must be developed. In this work two Mg-4Li-xY (x = 0.5 and 2%) alloy wires are investigated for application in a new wire form balloon expandable BVS device. The two alloys are supplied cold drawn to a diameter of 125μm. Firstly, a series of thermal treatments are applied to maximise the ductility of each alloy wire. It is found that ductility is maximised soon after complete recrystallisation and deteriorates during prolonged annealing through grain coarsening in the Mg-4Li-0.5Y alloy wire and increased precipitation of a Mg24Y5 phase in both alloys. Both alloy wires are shown to be capable of achieving tensile elongation to failure of ≈20% and survive high idealised bending strains (>40%). Microstructural investigation revealed that recrystallisation is initiated, first, from regions closest to the surface of the wire and progresses inwards as the annealing time is prolonged. Following annealing for maximum elongation to failure both alloy wires developed a transverse basal texture. However, the increased Y content in the Mg-4Li-2Y alloy wire resulted in a weaker basal texture developing during annealing. Both alloy wires were shown to exhibit relatively high ductility, indicating they are both suitable candidates for application in BVS technology. Both alloy wires were applied in the design of the novel wire form BVS under investigation and the processing routes applied during manufacture were investigated to optimise the mechanical performance of the device. Devices manufactured from the Mg-4Li-0.5Y alloy wire could survive over expansion of 1mm beyond its nominal diameter of 3mm and had a radial force at its nominal diameter >1N/mm. Devices manufactured from the Mg-4Li-2Y alloy wire had a radial force >1N/mm at its nominal diameter but inconsistent failure of the device occurred during 1mm over expansion. These failures initiate from the internal diameter of the new device’s novel wave form geometry. An increase in number of secondary phase particles in the Mg-4Li-2Y device is expected to contribute to these fractures with evidence found of cracks initiating adjacent to these particles. Further, the forming route and subsequent annealing process applied during the manufacture of the novel device, results in a split in texture developing through the device. Along the inner diameter of the wave form, deformation is dominated by tensile twinning, resulting in the recrystallised microstructure being dominated by new grains nucleated from these twins. Consequently, along the inner diameter of the wave form, the grains are aligned with their c-axis parallel to the expected loading direction during device expansion. This will limit the activation of prismatic slip which likely contributes to the failure of the devices investigated. Application of the processing routes developed within this work have resulted in enhanced mechanical performance of the novel BVS device under investigation. Devices manufactured from the Mg-4Li-0.5Y alloy exhibited high radial force (>1N/mm) and repeatable over expansion, up to 1mm beyond its nominal diameter. These advancements have resulted in the device having comparable performance to competitors, allowing it to progress to the next stages of development. The methodologies developed as part of this work to characterise and investigate the devices identified several potential avenues for continued device/alloy development. Consequently, several routes for continued research are identified that will aid in improving the performance of the Mg-4Li-0.5Y devices. Further, routes to address the sources of failure in the Mg-4Li-2Y are identified. If resolved, it may lead to these devices outperforming the Mg-4Li-0.5Y devices, further advancing this technology towards being applied in the clinical setting to improve the quality of life of patients.

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