(H-301) The Identification of Left Ventricular Directional Dependent Mechanical Stress Relaxation Properties

Introduction: Myocardial infarction (MI) is a leading cause of mortality and morbidity. The major concern of MI is the severe myocardial cell death that could activate the formation of scar tissue and eventually lead to heart failure. Cardiovascular regenerative approaches are seeking to develop physiological cardiac patch to minimize scar tissue growth and generate new cardiac tissues [1,2]. To mimic healthy native cardiac tissues, the patch must possess viscoelastic properties and therefore correspond appropriately to the cardiac cycle in the circulation. However, the viscoelastic properties have not been co-related to the orientation of the myocardial extracellular matrix (ECM) structure. Therefore, this study was focused on determining the viscoelastic properties of the native left ventricle (LV) in different directions that could directly be associated with the ECM microstructure, which could provide insights on designing a physiologically relevant cardiac patch.

Materials and Methods: Healthy porcine hearts were obtained from a local commercial slaughterhouse. The two perpendicular directions of the LV were defined as long axis and short axis. LV samples were prepared for histological and mechanical assessments. For the histological assessment, samples were sectioned at 7 um thickness. Masson’s Trichrome staining was used particularly to examine the collagen and muscle contents. Images (n=12/axis) were processed by color deconvolution. Mechanical assessment was done by performing uniaxial tensile testing (n=4/long axis; n=3/short axis) and nanoindentation (n=48/samples). Averaged linear stiffness, averaged maximum tensile stress, elastic modulus, and stress relaxation were subsequently calculated. An unpaired t-test was performed for statistical analysis with statistical significance determined to have occurred if p< 0.05.

Results, Conclusions, and Discussions: Collagen content was significantly less along the short axis compared to the long axis (Fig. 1(A); p< 0.05). However, there was no significant difference observed in the muscle content between the long and short axes (p >0.05). Tensile properties (linear stiffness and maximum tensile stress) and elastic modulus did not exhibit significant difference between the two axes (p >0.05). The average elastic modulus for long and short axes was 15-20 kPa, like adult human myocardium at a relaxed phase (Fig. 1(B)) [3,4]. The stress relaxation showed a much more rapid decrease in the short axis at the initial rapid linear decay stage (Fig. 1(C)). This phenomenon corresponded to the less collagen content of short axis presented from histological assessment.The main findings of this study indicate that a directional dependence exists in the LV myocardium due to the density difference of the collagen. Having less collagen content, short axis of LV myocardium carried a faster stress relaxation at the initial stage, which was indicative of a higher level of viscous property. On the contrary, long axis possessed a higher collagen content that led to a more elastic property. As the initial stage of stress relaxation is relevant to the LV diastolic filling, our results elucidate the concomitant LV pressure increase and LV volume increase during diastole are due to the differences in the stress relaxation properties between the long axis and short axis, of the LV (Fig. 1C). Hence, an effective design of a cardiac patch for MI requires the patch to incorporate these directionally dependent mechanical properties. We are fortunate that we successfully published these results recently [5]. In our presentation, we would therefore like to summarize these results, as well as describe our preliminary results on our next steps to create a cardiac patch, using engineered tissues derived from dynamic culture of induced pluripotent stem cell (IPSC)-cardiomyocytes.

Acknowledgements (Optional): The authors are grateful to the National Science Foundation for supporting this Engineering Research Centers Program under NSF Cooperative Agreement No. EEC-1647837.

References (Optional): [1] Parmacek & Epstein. (2009). N Engl J Med. [2] Krishna et al. (2011). J Pharm Bioallied Sci. [3] Nagueh et al. (2004). Circulation. [4] Weis el at. (2000). Circ. Res. [5] Lin & Paolino et al (2022).