Development of a Biaxial Testing System for Research of Soft Tissue Biomechanics using Laboratory Models

The rupture of the cap tissue layer of a fibroatheroma in human coronary vessels is considered the key event leading to the formation of a thrombus and myocardial infarction, resulting in more than half a million deaths in the US every year. In this study, we are interested in investigating the biomechanics of different elastomer materials that can be used as laboratory models to replicate coronary arteries’ ultimate tensile stress (0.2 - 2.08 MPa). To this end, we developed a biomechanical testing system that allows us to characterize the material properties of small samples with high accuracy and precision. We built and validated the performance of the instrument’s subsystems (displacement, strain rate, gram-force readings, temperature control, gripping mechanism, and strain measurement) to ensure that the data being collected is accurate. With our developed tensile system, we test several Polydimethylsiloxane (PDMS) elastomers (silicone) to find a suitable laboratory model to replicate the coronary artery. The first uniaxial elastomer tests were performed using Sylgard 184, where we manipulated the curing temperature, mix ratio of elastomer and cross linker, and addition of curing reagents (n=10 / group) to adjust its tensile strength. However, the lowest ultimate stress that Sylgard 184 could achieve was 15.5 ± 2.3 MPa. Sylgard 170 was the following elastomer to be tested, starting with several uniaxial tests using different mix ratios of elastomer/cross linker, followed by the addition of other elastomers and curing reagents (n=10 / group). With our uniaxial test of the mixture Sylgard 170(R1:1) 40% + Sylgard 527(R1:1) 60% + silicone thinner (20% of Sylgard 170 mass), it yielded ultimate tensile stresses of 1.8 ± 0.19. We prepared cruciform samples (n=10) with ASTM D412 Type C dimension (20% of the original) with this formulation, resulting in average ultimate stresses of 0.52 ± 0.09 MPa. The ultimate stress results of the uniaxial and biaxial tests were within the range of the coronary arteries’ ultimate tensile stress. It was concluded that Sylgard 170(R1:1) 40% + Sylgard 527(R1:1) 60% + silicon thinner (20% of Sylgard 170 mass) was the most suitable laboratory model to study soft tissue mechanics among the tested formulations