Biomechanical phenotyping of minuscule soft tissues: An example in the rodent tricuspid valve

The biomechanical phenotype of soft tissues - i.e., the sum of spatially- and directionally-varying mechanical properties - is a critical marker of tissue health and disease. While biomechanical phenotyping is always challenging, it is particularly difficult with miniscule tissues. For example, tissues from small animal models are often only millimeters in size, which prevents the use of traditional test methods, such as uniaxial tensile testing. To overcome this challenge, our current work describes and tests a novel experimental and numerical pipeline. First, we introduce a micro-bulge test device with which we pressurize and inflate miniscule soft tissues. We combine this microbulge device with an optical coherence tomography device to also image the samples during inflation. Based on pressure data and images we then perform inverse finite element simulations to identify our tissues' unknown material parameters. For validation, we identify the material parameters of a thin sheet of latex rubber via both uniaxial tensile testing and via our novel pipeline. Next, we demonstrate our pipeline against anterior tricuspid valve leaflets from rats. The resulting material parameters for these tissues compare excellently with data collected in sheep via standard planar biaxial testing. Additionally, we show that our device is compatible with other imaging modalities such as 2-Photon microscopy. To this end, we image the in-situ microstructural changes of the leaflets during inflation using second harmonic generation imaging. In summary, we introduce a novel pipeline to biomechanically phenotype miniscule soft tissues and demonstrate its value by phenotyping the biomechanics of the anterior tricuspid valve leaflets from rats.