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

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.

Additive Manufacturing (AM) of microscale devices, with load bearing capabilities, is hugely challenging due to the limitation in resolution of current AM technologies as well as the cross-discipline nature of the problem. The outcomes of this research will lead to breakthroughs in manufacturing of Nitinol load-bearing smart implants by additive processes. Uptake of the research outcomes by manufacturing industries will help grow the UK markets of biomedical devices and materials, and minimise the usage of valuable materials considerably. The consortium will particularly use Abbott, Johnson-Matthey, Lucideon and MTC's established international marketing and distribution channels to disseminate the techniques and results achieved in the project. This project will develop lesion-specific implants, and the outcomes will serve as driving forces to boost the technology development of personalised therapies, especially for complex and critical life-threatening cases in vulnerable patients such as the ageing population. With the support of NIHR Trauma Management Healthcare Technology Co-operative, we will endeavour to make a significant impact on health service sectors.

Two impact case studies will be carried out in the third year of this project. Case study 1, via collaboration with Abbott (USA), will involve designing and manufacturing of carrier stents for interventional heart valve surgery, which currently is an exploding market. For heart valves (such as aortic, mitral and tricuspid valves), the need for patient-specific cuff geometry is urgent as a few standard sizes do not fit for all patients. Case study 2, via collaboration with A-STENT (Germany), involves designing stents for coronary artery bifurcation stenosis. The designed and fabricated stent must fit the anatomy of bifurcation stenosis and also accommodate the variable load-bearing nature of the stent. It is very likely that new techniques for material processing and stent manufacturing will be put forward for patents according to UK regulations.

To be globally visible and recognised, four international symposiums will be organized by UoB, LU and MMU, respectively, within the conferences and meetings of International Professional Bodies and Societies in relevant fields. The consortium will take active roles in presenting at high-level International Conferences, particularly through invited, keynote and plenary speeches. The original results will be published in prestigious international journals, with at least 6 Gold open-access publications in high-ranked journals. In particular, a Project Website will be built and dedicated to reporting progress. All investigators have established academic, industrial and clinical contacts across the globe, which will be fully utilised to promote the overall impact of the research. Towards the final quarter of this project, a healthcare-engineering-oriented workshop will be organised at LU to further promote the impact of our research.

Three institutions will fully utilise Open Days, school visits, showcase and festival events to engage with school pupils, visitors, university students and general public. The team will publish web-based documents and video-clips about significant developments and outcomes through institutional websites, blogs, YouTube, Twitter and Facebook. All investigators will actively seek opportunities to deliver interviews, public speeches and articles for radio, TV and newspapers, regarding the project achievements and their connection with public audiences. Project investigators, Drs Willcock (LU), Feng (MMU), Cox (UoB) and Jamshidi (UoB), will set role models to attract women into technology, engineering and science, promoting Equal Challenge Unit's Athena SWAN Charter.