Abstract Body: Coordinated electrical propagation is essential for normal cardiac function. While cardiac implantable electronic devices can pace the heart and terminate life-threatening arrhythmias, no therapy exists to repair disrupted conduction pathways caused by injury, disease, or congenital defects. To address this unmet need, we have developed a ‘bioelectric thread’ designed to electrically couple cardiac tissues and optimized its delivery in vivo to re-engineer conduction pathways in the heart. High purity ventricular cardiomyocytes (vCMs) differentiated from human-induced pluripotent stem cells (hiPSCs) were incorporated into bioelectric threads with 5% human cardiac fibroblasts in a fibrinogen-gelatin hydrogel before extruding the mixture into 3.6 cm x 1 mm PDMS wells. Stretched threads were cultured in a maturation medium for 2 weeks under field electrical stimulation, following a ramp pacing protocol. Bioelectric threads (n = 6 differentiation batches) are electromechanically active with action potential duration at 80% repolarization (APD₈₀) of 305 ± 29 ms, calcium duration at 80% repolarization (CaD₈₀) of 595 ± 62 ms, and conduction velocity (CV) 49.6 ± 6.7 mm/s at 0.5 Hz. Threads positioned onto two separate engineered heart tissues (formed in collagen hydrogel) fused together and electrically coupled in vitro as early as day 1, with directed electrical propagation from one tissue to the other improving over time. Bioelectric threads injected into the free wall of immunosuppressed rat hearts remained viable after 1 day in vivo, as confirmed via ex vivo optical mapping of Langendorff perfused hearts. Point stimulation showed bioelectric threads following 2 Hz pacing with normal electrical propagation in the host myocardium. Immunohistochemistry revealed a continuous path of human Ku80⁺ cells along the myocardial injection tract with robust connexin-43 expression, suggesting that thread electrical syncytium is preserved post-delivery. While host-graft coupling was not yet evident at day 1, ongoing studies up to 4 weeks are designed to assess the temporal emergence of electrical integration. Taken together, these findings support bioelectric threads as a promising strategy for restoring disrupted cardiac conduction pathways.