Experimental modelling of aortic aneurysms: Novel applications of silicone rubbers

Abstract

A range of silicone rubbers were created based on existing commercially available materials. These silicones were designed to be visually different from one another and have distinct material properties, in particular, ultimate tensile strengths and tear strengths. In total, eleven silicone rubbers were manufactured, with the materials designed to have a range of increasing tensile strengths from approximately 2-4MPa, and increasing tear strengths from approximately 0.45-0.7N/mm. The variations in silicones were detected using a standard colour analysis technique. Calibration curves were then created relating colour intensity to individual material properties. All eleven materials were characterised and a 1st order Ogden strain energy function applied. Material coefficients were determined and examined for effectiveness. Six idealised abdominal aortic aneurysm models were also created using the two base materials of the study, with a further model created using a new mixing technique to create a rubber model with randomly assigned material properties. These models were then examined using videoextensometry and compared to numerical results. Colour analysis revealed a statistically significant linear relationship (p<0.0009) with both tensile strength and tear strength, allowing material strength to be determined using a non-destructive experimental technique. The effectiveness of this technique was assessed by comparing predicted material properties to experimentally measured methods, with good agreement in the results. Videoextensometry and numerical modelling revealed minor percentage differences, with all results achieving significance (p<0.0009). This study has successfully designed and developed a range of silicone rubbers that have unique colour intensities and material strengths. Strengths can be readily determined using a nondestructive analysis technique with proven effectiveness. These silicones may further aid towards an improved understanding of the biomechanical behaviour of aneurysms using experimental techniques.

New Approaches to Abdominal Aortic Aneurysm Rupture Risk Assessment

“…36 To develop accurate representations of the AAA wall, a technique has been reported that can manufacture silicone AAA models with random or controlled material properties, with methods in place to nondestructively assess these material properties. 35 It has also recently been demonstrated that the intraluminal thrombus, which may significantly influence the rupture site, can be included in experimental models. 39 These benchtop AAA analogues can be used to determine rupture locations and validate numerically predicted burst sites.…”

“…13,17,35 It is now possible to create patient-specific rubber AAA models 36 that are nonuniform in wall strength 35 similar to the in vivo situation. 31 These models can then be studied using the rupture technique reported by the authors.…”

Use of the photoelastic method and finite element analysis in the assessment of wall strain in abdominal aortic aneurysm models

“…Custom-made aluminium moulds were designed with CAD/CAM and then machined using a computer numerical control (CNC) 3-axis milling machine. The use of moulds eliminates the influence of edge concentrations due to cutting samples from flat sheets (Doyle et al, 2009a;Feger, 1998). Each sample was 2 mm thick, 4 mm wide, with a gauge length of 20 mm.…”

“…Samples were first subjected to a pre-conditioning regime of 10 cycles to an extension of 20% of the gauge length to overcome the Mullins effect (Mullins, 1969) and stabilise the stress-strain response of the material. It is known from previous work (Doyle et al, 2009a) that although failure testing exceeds the strain range of pre-conditioning, eliminating the Mullins effect when testing to failure enables reproducible stress-strain results, as compared to using a single extension to failure. Therefore, on the 11th cycle each sample was extended until failure.…”

“…However, the edge effect from the cutting process can yield poor results and an inaccurate stress-strain response (Doyle et al, 2009a;Feger, 1998). In addition, the experimental models can vary slightly in wall thickness (Doyle et al, 2008 and therefore, if dogbone samples with varying wall thicknesses were used to develop a constitutive material model, where the principles of that model rely on the assumption of a constant wall thickness, significantly more errors may be introduced into the system.…”