Engineering silicone rubbers for in vitro studies: creating AAA models and ILT analogues with physiological properties

Abstract

In vitro studies of abdominal aortic aneurysm (AAA) have been widely reported. Frequently mock artery models with intraluminal thrombus (ILT) analogs are used to mimic the in vivo AAA. While the models used may be physiological, their properties are frequently either not reported or investigated. This study is concerned with the testing and characterization of previously used vessel analog materials and the development of new materials for the manufacture of AAA models. These materials were used in conjunction with a previously validated injection molding technique to manufacture AAA models of ideal geometry. To determine the model properties (stiffness (beta) and compliance), the diameter change of each AAA model was investigated under incrementally increasing internal pressures and compared with published in vivo studies to determine if the models behaved physiologically. A FEA study was implemented to determine if the pressure-diameter change behavior of the models could be predicted numerically. ILT analogs were also manufactured and characterized. Ideal models were manufactured with ILT analog internal to the aneurysm region, and the effect of the ILT analog on the model compliance and stiffness was investigated. The wall materials had similar properties (E(init) 2.22 MPa and 1.57 MPa) to aortic tissue at physiological pressures (1.8 MPa (from literature)). ILT analogs had a similar Young's modulus (0.24 MPa and 0.33 MPa) to the medial layer of ILT (0.28 MPa (from literature)). All models had aneurysm sac compliance (2.62-8.01 x 10(-4)/mm Hg) in the physiological range (1.8-9.4 x 10(-4)/mm Hg (from literature)). The necks of the AAA models had similar stiffness (20.44-29.83) to healthy aortas (17.5+/-5.5 (from literature)). Good agreement was seen between the diameter changes due to pressurization in the experimental and FEA wall models with a maximum difference of 7.3% at 120 mm Hg. It was also determined that the inclusion of ILT analog in the sac of the models could have an effect on the compliance of the model neck. Ideal AAA models with physiological properties were manufactured. The behavior of these models due to pressurization was predicted using finite element analysis, validating this technique for the future design of realistic physiological AAA models. Addition of ILT analogs in the aneurysm sac was shown to affect neck behavior. This could have implications for endovascular AAA repair due to the importance of the neck for stent-graft fixation.

Fig. 1

AAA model viewed from front and side, arrows show where measurements were taken during compliance testing.

Fig. 2

Pressure – diameter change measurement setup for investigating model compliance

Fig 3

Quarter model used in finite element analysis

Fig 4

Manufacturing sequence for manufacturing AAAILT models, Left: wax lumen, middle: ILT analogue injected around wax lumen, right: wall material injected + allowed to cure before wax is melted out

Fig. 5

Comparison of experimentally calculated and FEA true stress and strain in a dumb bell sample for wall and ILT materials

Fig. 6

Comparison of mean experimental and FEA predicted pressure – diameter change curves for wall models in the neck (left) and the aneurysm (right) regions

Fig. 7

Mean pressure – diameter change curves for each set of models in the neck (left) and aneurysm (right) regions (n = 3 for all cases)

Fig. 8

Mean model stiffness in the neck (left) and aneurysm (right) for each model set (n = 3 for all cases)