An integrated computational-experimental approach for the in situ estimation of valve interstitial cell biomechanical state
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Mechanical forces are known to regulate aortic valve interstitial cell (AVIC) functional state by modulating their biosynthetic activity, translating to differences in tissue composition and structure and, potentially, leading to aortic valve (AV) dysfunction. While advances have been made toward the understanding of AVIC behavior ex-situ, the AVIC biomechanical state in its native extracellular matrix (ECM) remains largely unknown. Consequently, changes in AVIC behaviors, such as stiffness and contractility, resulting from pathological cues in-situ remain unidentified. We hypothesize that improved descriptions of AVIC biomechanical state in-situ, obtained using an inverse modeling approach, will provide deeper insight into AVIC interactions with the surrounding ECM, revealing important changes resulting from pathological state, and possibly informing pharmaceutical therapies. To achieve this, a novel integrated numerical-experimental framework to estimate AVIC mechanobiological state in-situ was developed. Flexural deformation of intact AV leaflets was used to quantify the effects of AVIC stiffness and contraction at the tissue level. In addition to being a relevant deformation mode of the cardiac cycle, flexure is highly sensitive to layer-specific changes in AVIC biomechanics. As a first step, a tissue-level bilayer model that accurately captures the bidirectional flexural response of AV intact layers in a passive state was developed. Next, tissue micromorphology was incorporated in a macro-micro scale framework to simulate layer-specific AVIC-ECM interactions. The macro-micro AV model enables the estimation of changes in effective AVIC stiffness and contraction in-situ that are otherwise grossly inaccessible through experimental approaches alone. Finally, microindentation studies examining AVIC activation were run in parallel with in-situ studies to emphasize the necessity of an in-situ approach, and the advantage it affords over existing ex-situ methodology. In conclusion, the developed numerical-experimental methodology can be used to obtain AVIC properties in-situ. Most importantly, it can lead to further understanding of AVIC-ECM mechanical coupling under various pathophysiological conditions and the investigation of possible treatment strategies targeting the myofibroblast phenotype characteristic of early signs of sclerotic valvular disease.