Smart peripheral stents for the lower extremity: design, manufacturing, and evaluation

Lead Research Organisation: University of Birmingham
Department Name: Metallurgy and Materials

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.

Publications

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Kendall J (2019) In-vitro Study of Effect of the Design of the Stent on the Arterial Waveforms in Procedia Structural Integrity

 
Description We have shown the feasibility of 3D printing customised stents that achieve the required performance for nitinol through the optimisation of 3D printing parameters, alloy chemistry, and post-processing. The results could also be utilised to create preforms for stents that can then finished to reduce the processing time required to manufacture the stents. In theory, over 100 stents can be 3D printed in 1 hour. Laser micromachining, the current fabrication method for stents, produces significant waste, and is an energy intensive process.
Exploitation Route Johnson Matthey, one of the partners in this programme, have connected us to various material suppliers in their organisation (Johnson Matthey medical) to assess the potential for commercialisation. A patent application is currently being drafted.
Sectors Healthcare

 
Description Not yet, as the current is still in progress. The potential for the findings being used will be in the: 1. Medical implant industry 2. Healthcare field
 
Description Centre for Doctoral Training in Topological Design
Amount £5,000,000 (GBP)
Funding ID EP/S02297X/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 09/2019 
End 08/2024
 
Description Collaboration with Johnson-Matthey plc 
Organisation Johnson Matthey
Country United Kingdom 
Sector Private 
PI Contribution Johnson-Matthey agreed to patent some of the findings of this project, in return for a royalty agreement with the University of Birmingham. The scope of the patent is entirely based on the work performed in this project, and will support the development of a specific product for Johnson-Matthey.
Collaborator Contribution The idea was suggested to the research team by Johnson-Matthey, and the research team performed all the underlying scientific work to prove the scientific worthiness of the idea.
Impact The patent application will be filed in March 2020.
Start Year 2017
 
Description Collaboration with Trinity College Dublin on 3D Printing of Paediatric Stents 
Organisation Trinity College
Country Canada 
Sector Academic/University 
PI Contribution Due to our current involvement in the EPSRC funded programme on 3D printing of smart stents for lower extremity (total value £1M), we support the project by Prof. Triona Lally using our expertise on stents 3D printing due to the significant overlap with your proposed investigation. We supply the following in-kind support to this project: • Use of SLM equipment (approximate costs based on commercial rates): £10,000 • Powder: £2,000 • Attendance of 1 review meeting in Dublin: £250 • Powder characterisation (based on commercial rates): £500
Collaborator Contribution Prof. Lally's team provide us with new applications for 3D printing of commercial stents.
Impact None
Start Year 2019
 
Description Interview for 3D Printing Industry website 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact I was interviewed by the 3D printing Industry website as one of the leading experts on additive manufacturing on my current research, and the future direction of research in this field.
Year(s) Of Engagement Activity 2019
URL https://3dprintingindustry.com/news/leading-additive-manufacturing-academics-give-insights-into-2019...