Localized Temperature Monitoring in Mouse Brain during Light Delivery via a Non‐Planar Tapered Fiber‐Integrated µRTD Sensor

ABSTRACT Monitoring local brain temperature with high spatial precision is essential to understanding neurophysiological processes and managing the side effects of optical neuromodulation techniques. We present a novel multifunctional neural interface integrating a microscale resistance temperature detector (µRTD) onto the curved surface of a tapered optical fiber (TF), enabling co‐localized light delivery and thermal sensing with minimal footprint. The µRTD, patterned via an unconventional two‐photon polymerization (TPP)‐based process on the fiber surface, exhibits thermal sensitivity <0.1°C and low self‐heating under physiologically‐safe bias conditions. We demonstrate the system's capacity to resolve subtle temperature changes induced by optogenetic stimulation/inhibition protocols (the latter requiring illumination periods of hundreds of milliseconds up to several seconds), revealing significant thermal accumulation only under long, high‐intensity illumination. This integration resolves the spatial mismatch of multimodal probes and reduces implant cross‐section compared to coaxial or side‐by‐side configurations. Furthermore, the TPP approach is modular, allowing integration with additional functionalities (i.e., electrophysiological recording or thermoplasmonics). By uniting photonic and thermal readout into a minimally invasive probe, our technology offers a powerful tool for studying thermally mediated neural processes, enhancing the safety and interpretability of optical neurotechnologies. Its integration potential positions this platform as a complementary technology for next‐generation multifunctional neural interfaces.