Smart Peripheral Stents for the Lower Extremity - Design, Manufacturing and Evaluation

外周动脉疾病是指由于脂肪沉积在血管壁上而导致肢体动脉的部分或全部阻塞。由于腿部肌肉的血液供应受到限制，这种疾病对患者的健康和福祉造成了逐渐的损害。典型的症状包括走路时疼痛和腿部组织死亡。血管支架可以有效地治疗这种疾病，这种支架本质上是由合成材料制成的网状物，用于重新打开堵塞的血管。然而，考虑到疾病的复杂性和持续暴露于严重的生物力学力量，外周动脉支架植入已被证明是有问题的。因此，它需要定制的设计，以改善通畅时间和减少并发症的介入治疗。此外，目前的支架制造(如激光切割和光刻)是一种浪费材料和耗时的过程。选择性激光熔化(SLM)的添加制造(AM)提供了一种最有前途的方法来制造具有定制设计和广泛节省原材料的支架。本研究旨在开发智能支架，用于治疗复杂的下肢外周动脉狭窄。这项研究将使用超弹性形状记忆合金镍钛合金，因为这种材料非常灵活，即使在非常大的变形后也能自动恢复其原始形状(智能)。由镍钛合金制成的支架对疾病区域复杂的血管几何形状表现出高度的符合性。为了实现这一目标，路易斯安那大学的先进材料力学小组、大华大学的先进材料与加工实验室和密歇根大学的生物工程小组共同参与了这一项目。大华大学将专注于使SLM用于制造结构(样品和原型)，具有更小的特征尺寸(小于200微米)，由镍醇粉末制成。特别是，UOB将应用铂族金属的微掺杂来改善SLMed NiTiol的生物兼容性和辐射不透明度，并开发技术来防止在SLM过程中发生的镍蒸发，这种蒸发可能会导致超弹性行为的显著损失。由大华大学交付的样品和支架的机械性能将在路易斯安那大学使用支架卷曲器和配备环境浴池的微型测试仪进行测试。样品和支架，无论是收到的还是测试的，都将经过扫描电子显微镜/电子显微镜/EBSD表征，以进一步了解SLMed Nitinol的行为。MMU的体外装置将用于研究支架原型在弯曲和径向压缩等可选生物力学力作用下的体外性能，包括血液动力学。这些实验研究将为大华大学优化SLM关键参数提供进一步的指导。此外，UOB将开发一个中尺度计算机模型来模拟AM过程，包括微掺杂和镍蒸发，以支持微SLM工艺的适应和优化。支架变形的有限元模拟将由LU(固体力学)和MMU(流体力学)联合进行，包括支架-动脉系统的局部变形和血流动力学的体外和体内模拟。将仿真结果与实验结果进行比较。LU的研究人员还将向UOB提供针对病变特定支架的设计，以供AM定制支架使用。将特别考虑最适合SLM过程的设计。该设计将基于MMU提供的实际患者的3D病变成像和LU的迭代有限元分析，并在MMU进行体外性能评估。这一成果将推动个性化治疗的发展，特别是针对脆弱患者的复杂和危重疾病，如人口老龄化。

Peripheral arterial disease refers to partial or total block of limb arteries due to the accumulation of fatty deposits on the vessel wall. The disease imposes a progressive damage to patients' health and wellbeing due to the restriction of blood supply to leg muscles. Typical symptoms include pain when walking and dying of leg tissue. The disease can be effectively treated by vascular stents which are essentially meshes of synthetic materials used to reopen the blocked blood vessels. However, stenting in peripheral arteries has proved problematic, given the complexity of the disease and constant exposure to severe biomechanical forces. Consequently, it requires customised design in order to improve patency times and reduce complications in interventional therapy. In addition, current stent manufacturing (such as laser cutting and photo etching) is a material wasteful and time consuming process. Additive manufacturing (AM) via Selective Laser Melting (SLM) offers the most promising approach to generate stents with customized designs and extensive saving of raw materials. This research aims to develop smart stents for treatment of complex periphery artery stenosis in the lower limbs. Superelastic shape memory alloy, Nitinol, will be used in this study, as the material is extremely flexible and can automatically recover its original shape even after very large deformation (smart nature). Stents made of Nitinol demonstrate high conformability to the complex vessel geometry in diseased regions.To achieve the aim, the Mechanics of Advanced Materials group at LU, the Advanced Materials & Processing Lab at UoB and the Bioengineering group at MMU are brought together to collaboratively work on the project. UoB will focus on adapting SLM for manufacturing structures (samples and prototypes), with smaller feature sizes (less than 200 microns), out of Nitinol powders. In particular, UoB will apply micro-doping of platinum group metals to improve the biocompatibility and radiopacity of SLMed Nitinol, as well as develop techniques to prevent Ni evaporation which occurs during SLM and can result in significant loss of superelastic behaviour. Mechanical behaviour of the samples and stents, delivered by UoB, will be tested at LU using a stent crimper and a microtester fitted with an environmental bath. Samples and stents, both as-received and tested, will undergo SEM/TEM/EBSD characterisation to gain further insights of the SLMed Nitinol behaviour. An in-vitro setup at MMU will be used to study the in-vitro performance, including haemodynamics, of stent prototypes subjected to optional biomechanical forces such as bending and radial compression. These experimental studies will provide further guidance to UoB for optimisation of key SLM parameters. In addition, a mesoscale computer model will be developed at UoB to simulate the AM process, including micro-doping and Ni evaporation, to support the adaption and optimisation of the micro-SLM process. Finite element simulations of stent deformation will be carried out jointly by LU (solid mechanics) and MMU (fluid mechanics), including in-vitro and in-silico modelling of local deformation and haemodynamics of the stent-artery system. Simulation results will be compared with experimental results. The researchers at LU will also deliver the design of lesion-specific stents to UoB for AM of customised stents. Particular considerations will be given to designs which best suits the SLM process. The design will be based on 3D lesion imaging of actual patients provided by MMU and iterative finite element analyses at LU, with in-vitro performance assessment at MMU. The outcome will serve as a driving force to boost the development of personalised therapies, especially for complex and critical diseases in vulnerable patients such as ageing populations.