Nanoindentation-Driven Insights into the Micro-Mechanical and Viscoelastic Behaviors of Porcine Atria and Ventricles

An understanding of the micro- and mesoscale mechanical behaviors of myocardial tissue is imperative for advancements in cardiac health. However, a gap exists in systematically studying the microlevel mechanical properties of myocardium across various cardiac regions under a standardized testing methodology. Addressing this gap, utilizing nanoindentation techniques, our study employs a porcine model to investigate the influence of indentation speed and depth on the myocardium's elastic modulus and its viscoelastic properties in ventricles and atria. The results demonstrate that variations in nanoindentation speed significantly affect the myocardium's elastic modulus. In the left ventricle (LV), there is an observable ∼1.26-fold and ∼1.13-fold increase in the elastic modulus when the nanoindentation speed is increased from ±5 to ±10 μm/s and then to ±20 μm/s, respectively. Subsequently, a decrease in rate sensitivity is noted, attributed to the predominance of elastic responses, potentially nearing a strain rate threshold. Conversely, increasing the indentation depth leads to a notable nonlinear decrease in the myocardium's elastic modulus, indicative of a heterogeneous structural composition that adapts to varied pressure and volume conditions. Analyzing the load-decreasing curves for both ventricles and atria, we found them to be well-aligned with a viscoelastic model integrating two Maxwell units with over 80% fitting accuracy. Remarkably, the LA displayed reduced short-term and long-term relaxation time constants, about 3.22-fold and 23.44-fold lower than those of the RA, aligning with their distinct functional roles. Moreover, the time-dependent stress relaxation characteristics are well-represented by a modified Maxwell model, showing fitting accuracy greater than 88%. The ventricles exhibited lower viscosity coefficients compared to the atria, reflecting their unique functional requirements and structural differences. In conclusion, this research sheds light on the intricate variations in the elastic modulus of myocardial tissue as influenced by indentation rate, depth, and specific cardiac regions. It also unveils distinct viscoelastic behaviors within these regions at the microlevel. These insights are invaluable for benchmarking in the creation of engineered cardiac tissues and provide critical data for the development of computational models that simulate the mechanics of cardiac tissue.