A Self‐Supporting Flexible Electrode for Tracking and Reversible Quantification of Mg2+ and Ca2+ in the Brains of Freely Behaving Animal

Monitoring dynamic neurochemical signals in the brain of free‐moving animals remains great challenging in biocompatibility and direct implantation capability of current electrodes. Here we created a self‐supporting polymer‐based flexible microelectrode (rGPF) with sufficient bending stiffness for direct brain implantation without extra devices, but demonstrating low Young's modulus with remarkable biocompatibility and minimal position shifts. Meanwhile, screening by density functional theory (DFT) calculation, we designed and synthesized specific ligands targeting Mg2+ and Ca2+, and constructed Mg‐E and Ca‐E sensors with high selectivity, good reversibility, and fast response time, successfully monitoring Mg2+ and Ca2+ in vivo up to 90 days. Using this powerful tool, we discovered for the first time that, during the 4‐aminopyridine‐induced seizure in the live brain, extracellular Mg2+ inhibited Ca2+ influx. Moreover, the timing of initial changes in Mg2+ and Ca2+ levels during seizures aligned with neural pathways, which had not been previously reported.