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

Lead Research Organisation: Loughborough University
Department Name: Wolfson Sch of Mech, Elec & Manufac Eng

Abstract

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.

Planned Impact

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.

Publications

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Abdulsalam M (2021) The composition of vulnerable plaque and its effect on arterial waveforms. in Journal of the mechanical behavior of biomedical materials

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Abdulsalam M (2019) Distinguish the Stable and Unstable Plaques Based on Arterial Waveform Analysis in Procedia Structural Integrity

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He R (2020) Patient-specific modelling of stent overlap: Lumen gain, tissue damage and in-stent restenosis. in Journal of the mechanical behavior of biomedical materials

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He R (2021) Pioneering personalised design of femoropopliteal nitinol stents. in Materials science & engineering. C, Materials for biological applications

 
Description This project investigated the potential of additive manufacturing in fabricating metallic stents (nitinol, Co-Cr and stainless steel) via selective laser melting, with a focus on design of per-sonalised stents. Firstly, constitutive models have been developed and calibrated to describe the superelastic behaviour of shape-memory alloy nitinol and the hyperelastic deformation of artery layers and plaque with the incorporation of tissue damage and growth. This has been used to understand the biomechanical interaction between stent and native artery and guide the process of stent design. Secondly, a series of computer models have been created to simulate the process of stent crimping and self-expansion inside a diseased peripheral artery. This is essential for iterative design of "personalised" or "lesion-specific" design of stents. Thirdly, characterisations of additively manufactured stents (316L stainless steel, Co-Cr alloy & nitinol) have been carried out using SEM, EBSD and spherical nanoindentation, in compar-ison with commercial ones. This includes their crimping and expansion behaviours. The data serve as important benchmarks for further development of innovative additive manufacturing process in stent manufacturing. Finally, a highly effective computer approach has been de-veloped for designing personalised nitinol stents, with lumen gain, stress reduction and desir-able lumen shape set as key design objectives. This is the first time for a complete design of "lesion-specific" or "personalised" stent. Personalised stents show outstanding performance compared to a commercial stent based on computer simulations. The method can be used to develop a wide range of personalised medical implants.
Exploitation Route The data and models produced out of this project are of high value to research communities working in the field of biomedical engineering, especially additive manufacturing of medical materials and implants. The computer approach for designing personalised nitinol stents will be of high interest to biomedical research communities worldwide, as the method can be used to design a wide range of personalised medical implants. We have exchanged the pro-ject outcomes with leading research groups in the UK, Italy, Germany, USA, China and Ire-land through invited visits and seminars. The impact of our findings has already been gener-ated through our high-quality publications in leading international journals and nation-al/international conferences. These achievements are highly valuable to further research and development of additive manufacturing technologies in manufacturing "personalized" medical implants which can bring significantly improved outcomes to patients. The research out-comes have also been shared with our collaborators in healthcare technology sector, includ-ing NHS hospitals, for potential commercial exploitation in the future.
Sectors Healthcare

 
Description According to RWTH Aachen University hospital (Germany), the advanced models developed in this pro-ject contribute substantially to accuracy of computed prediction of in-stent restenosis caused by tissue damage during stenting, one of the major clinical concerns for percutaneous coro-nary treatment of myocardial infarction. Moreover, the modelling approach contributes to the vision of full-scale in silico artery-stent network reconstruction for precision medicine. The project outcomes are also highly valuable to Abbott Vascular (a world-leading stent manufacturer) in their continuous research and development of coronary and peripheral artery stents. According to Abbott, the work at Loughborough University is especially useful in their development of new-generation stent design having optimised strut geometry. Before the long-term performance for new genera-tion design can be tried out in humans, the computational modelling techniques created by Loughborough University offer a way to anticipate how the new product would compare to the previous generation that already had years of clinical data generated. The work per-formed by Loughborough University can also be translated to characterize how stents will interact with anatomy, an important indicator in designing medical implants.
Sector Healthcare
Impact Types Economic

 
Title Supplementary Information files for A computational study of fatigue resistance of nitinol stents subjected to walk-induced femoropopliteal artery motion 
Description Supplementary Information files for A computational study of fatigue resistance of nitinol stents subjected to walk-induced femoropopliteal artery motionFatigue resistance of nitinol stents implanted in femoropopliteal arteries is a critical issue because of their harsh biomechanical environment. Limb flexions due to daily walk expose the femoropopliteal arteries and, subsequently, the implanted stents to large cyclic deformations, which may lead to fatigue failure of the smart self-expandable stents. For the first time, this paper utilised the up-to-date measurements of walk-induced motion of a human femoropopliteal artery to investigate the fatigue behaviour of nitinol stent after implantation. The study was carried out by modelling the processes of angioplasty, stent crimping, self-expansion and deformation under diastolic-systolic blood pressure, repetitive bending, torsion and axial compression as well as their combination. The highest risk of fatigue failure of the nitinol stent occurs under a combined loading condition, with the bending contributing the most, followed by compression and torsion. The pulsatile blood pressure alone hardly causes any fatigue failure of the stent. The work is significant for understanding and improving the fatigue performance of nitinol stents through innovative design and procedural optimisation. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
URL https://repository.lboro.ac.uk/articles/dataset/Supplementary_Information_files_for_A_computational_...
 
Title Supplementary Information files for A computational study of fatigue resistance of nitinol stents subjected to walk-induced femoropopliteal artery motion 
Description Supplementary Information files for A computational study of fatigue resistance of nitinol stents subjected to walk-induced femoropopliteal artery motionFatigue resistance of nitinol stents implanted in femoropopliteal arteries is a critical issue because of their harsh biomechanical environment. Limb flexions due to daily walk expose the femoropopliteal arteries and, subsequently, the implanted stents to large cyclic deformations, which may lead to fatigue failure of the smart self-expandable stents. For the first time, this paper utilised the up-to-date measurements of walk-induced motion of a human femoropopliteal artery to investigate the fatigue behaviour of nitinol stent after implantation. The study was carried out by modelling the processes of angioplasty, stent crimping, self-expansion and deformation under diastolic-systolic blood pressure, repetitive bending, torsion and axial compression as well as their combination. The highest risk of fatigue failure of the nitinol stent occurs under a combined loading condition, with the bending contributing the most, followed by compression and torsion. The pulsatile blood pressure alone hardly causes any fatigue failure of the stent. The work is significant for understanding and improving the fatigue performance of nitinol stents through innovative design and procedural optimisation. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
URL https://repository.lboro.ac.uk/articles/dataset/Supplementary_Information_files_for_A_computational_...
 
Description Abbott 
Organisation Abbott
Department Abbott Vascular
Country Spain 
Sector Private 
PI Contribution Contribute to a better understanding of mechanical performance of bioresorbable polymeric stents
Collaborator Contribution Supply of ABSORB stents for benchmark studies and provision of technical advice and feedbacks.
Impact DOI: 10.1016/j.jbiomech.2016.05.035
Start Year 2015
 
Description Johnson Matthey 
Organisation Johnson Matthey
Country United Kingdom 
Sector Private 
PI Contribution Provide insights into additive manufacturing of nitinol materials and devices.
Collaborator Contribution Supply of nitinol powder, commercial nitinol tubes and access to facilities as well as technical advices to the project
Impact Project ongoing and will report outcomes at later stage.
Start Year 2017
 
Description Lucideon 
Organisation Lucideon
Country United Kingdom 
Sector Private 
PI Contribution Study of performance of novel biodegradable PLLA.
Collaborator Contribution Technical advice and supply of materials
Impact N/A
Start Year 2016
 
Description NHS hospital 
Organisation University Hospitals Birmingham NHS Foundation Trust
Country United Kingdom 
Sector Public 
PI Contribution Provide an understanding of new stent technology.
Collaborator Contribution Provide technical advice from a medical point of view
Impact Project ongoing and will report outcomes at later stage.
Start Year 2017
 
Description The MTC 
Organisation Manufacturing Technology Centre (MTC)
Country United Kingdom 
Sector Private 
PI Contribution Techniques related to additive manufacturing of medical devices; diversify the topics of their academic engagement into the biomedical sector and ensure that their technology road map reflects emerging technologies and market sectors.
Collaborator Contribution Provide access to their powder characterisation and NDT & metrology facilities, as well as helping to maximise the impact of the project.
Impact Project ongoing and will report the relevant outputs when they are generated.
Start Year 2017
 
Description International Conference on Stents 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact ICS3M 2019 - International Conference on Stents Materials, Mechanics and Manufacturing, London, UK; 15-17 July 2019

The International Conference on Stents is organised by Technical Committee 14 "Integrity of Biomedical and Biological Materials" of the European Structural Integrity Society (ESIS).

The aim of this Conference is to bring together specialists in biomedical engineering, biomechanics, materials science, experimental mechanics, modelling and various aspects of processing and manufacturing as well as medical practitioners to discuss advances in stent technologies.
Year(s) Of Engagement Activity 2019
URL https://moamrg.co.uk/?p=ICS3M2019&p2=main