Heart-On-a-Chip with Integrated Ultrasoft Mechanosensors for Continuous Measurement of Cell- and Tissue-Scale Contractile Stresses.

Heart-on-a-chip platforms aim to recapitulate cardiac tissue structure and function in vitro. Traditionally, microfabricated pillars are used to estimate contractile forces based on pillar deflection. However, this approach measures only global forces at the pillar interface and lacks the spatial resolution needed to capture local mechanical stresses. In this study, we present a non-destructive optical method for continuous, multi-scale stress mapping using ultrasoft edge-labeled micro-spherical stress gauges (eMSGs). These embedded mechanosensors visibly deform in response to cellular and extracellular matrix (ECM)-generated stresses, enabling real-time measurements at cell and tissue scales. Our platform features dual cell-seeding chambers with flexible polydimethylsiloxane pillars, into which neonatal rat cardiomyocytes are seeded within a fibrin/Geltrex hydrogel containing eMSGs. Over time, tissues compacted, aligned, and exhibited spontaneous contractions and calcium transients. By modulating ECM composition, we found that reduced fibrin concentration enhanced contractile frequency, regularity, and force generation. Analysis of eMSG deformation enabled calculation of lateral and longitudinal stresses, revealing the impact of compaction and contraction on local mechanics. Finally, drug testing was performed using norepinephrine, which enhanced contractile force, and blebbistatin, which inhibited contraction, demonstrating robust pharmacological responsiveness. This platform provides a powerful tool for real-time biomechanical analysis and drug testing in engineered cardiac tissues.