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Experimental phantom work

We have a lot of experience in the use of experimental models in vascular engineering. In particular, experiments that image and quantify the wall strain and those that capture the rupture location of an experimental model.

Experimental Rupture Tests

We began using the injection-moulding technique to create our experimental models and had good success using this approach. Our 2008 Journal of Biomechanical Engineering paper provides all the necessary detail and can be found here. However, this approach requires a lot of manual, and very technical, CAD/CAM to create acceptable moulds. We performed our first rupture-testing using our well-studied and reported idealised AAA model by creating a series of models using a single material and exploding these in the lab. This is described in our 2009 Journal of Endovascular Therapy paper.

We have also developed new blends of rubbers that mimic the behaviour of the AAA wall and the intraluminal thrombus (ILT). Our 2010 Journal of Biomechanical Engineering paper describes how we make the ILT analogue, whereas our 2009 paper in Medical Engineering & Physics describes the wall material.

Additionally, by measuring the colour of these rubber-blends using spectrophotometry, we can create calibration charts that link colour to certain material properties. This allows us to create experimental models with randomly distributed variations of rubber – measure the colour and relate it to the material properties at any location on the model – and perform rupture test experiments. Details of these patient-specific rupture tests can be found in our 2010 Journal of Biomechanics paper. In the slow-motion video on the right, recorded at 2,000 frames per second using a Photron Fastcam SA1-1 (which is capable of recording at a mind-blowing 675,000 frames per second!), we can see a patient-specific experimental model rupturing on the posterior surface. We used mirrors to visualise the entire model, so the model in the centre is the true model, with reflections either side.



Photoelastic Method

We have also performed experiments using the photoelastic method. This technique has been used in biomedical applications for over 70 years and involves creating an experimental model using epoxy resin and applying a reflective coating to the inside surface. We can image and quantify the wall strain under pressurisation by using the colour bands of isochromatic fringes visualised through a polariscope. Much more detail on the photoelastic method is available at the Vishay website which is an excellent resource for anyone interested. Although these materials do not mimic the AAA wall very well (especially at strains above about 5%), they do provide qualitative agreement with computational models and are useful to add another dimension to the validation and verification process. In the figure below, (A) shows the experimental rupture site, (B) the computational wall strain, (C) the Gaussian surface curvature, (D) a close-up of the wall strain, and (E) the colour fringes obtained using the photoelastic method. Our 2012 Journal of Biomechanics paper that uses the photoelastic method together with the finite element method provides much more detail.



3D Printing

Recently we have begun to harness the advancements in 3D printing which is now at a stage whereby models can be created using a vast number of different materials, each with different material properties, and a layer thickness of 16 micrometers. At this resolution the accuracy of the resulting 3D models are truly remarkable. We used the Objet500 Connex 3D printer at the University of Limerick to create experimental phantoms of our idealised AAA geometry and investigated these 3D models using a series of experimental and numerical techniques, such as:

  • Ultrasound-based pulse wave imaging (PWI)
  • Uniaxial tensile testing
  • Tear testing
  • Dynamic mechanical analysis (DMA)
  • 3D topography analysis (see image below)
  • Scattering analysis with ultrasound
  • Finite element analysis (FEA) simulations
  • Fluid-structure interaction (FSI) simulations

We have recently published our latest work in the new journal 3D Printing and Additive Manufacturing in collaboration with Aidan Cloonan at the University of Limerick (who led this research), Tim McGloughlin at KUSTAR, and Elisa Konofagou at Columbia University.

The image on the right shows 3D topography (top row), height maps (middle row) and PWI and FSI results (bottom row)