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

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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.