In-vivo pH Imaging System for Hydrogen Ion Dynamics Observation in the Brain of a Freely-Moving Mouse.

To study changes in extracellular ions in the living brain of freely moving animals in both normal and disease states, developing an in-vivo imaging system based on a high spatially-resorbed image sensor device is necessary. Here, we report the in-vivo pH imaging system for monitoring hydrogen ion (H ⁺ ) distribution in the brain of a freely-moving mouse. The developed CMOS ion image sensor has a 256 × 32 array of 5.65 × 4.39 μm ² pixel size, which is improved from the previous sensor device (128 × 32-array of 23.55 × 23.55 μm ² pixel size) and achieves a pH resolution of 0.016 pH with a temporal resolution of 62 fps. A reference electrode without an internal solution adaptable for freely-moving in-vivo experiments was also fabricated. Both the sensor and reference electrode were sufficiently lightweight at 1 g each to be implanted into the experimental animal. We have, for the first time, successfully obtained real-time imaging in awake and unrestrained freely-moving experiments, allowing us to visualize H ⁺ dynamics in the living brain.