To evaluate in vivo physiological functions, electrophysiological signals must be monitored with high precision and high spatial or temporal resolution. Ultraflexible, multielectrode arrays (MEAs) were recently fabricated to establish conformal contact on the surfaces of organs and to measure electrophysiological signal propagation at high spatial-temporal resolution. However, plastic substrates with a high Young’s modulus incorporated in the process caused difficulties during implantation due to dynamic movement-based hemodynamics at the surface of the heart. In a new study published in Science Advances, Wonryung Lee and colleagues have developed an active MEA fabricated to demonstrate nonthrombogenicity, stretchability and stability. The arrays allowed long-term electrocardiographic (ECG) monitoring in the beating hearts of rats, even with capillary bleeding. The measured ECG signals exhibited a high signal-to-noise (SNR) ratio of 52 dB as a result of the active data reading.