Implantable in Vivo PH Image Sensor with Integrated Reference Electrode for Biological Experiments on Awake Mouse

We report the fabrication of a reference electrode-mounted implantable pH image sensor with 256 × 32 pixels and a pixel pitch of 5.65 × 4.39 μm
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and in vivo imaging in mouse living brain. This sensor incorporates an Ag/AgCl electrode on a complementary metal-oxide semiconductor (CMOS)-based potentiometric array sensor that serves as a reference electrode for free-running experiments in mice. The sensitivity of the sensor with the Ag/AgCl electrode was 47.2 mV/pH. The pixel-to-pixel variation in linearity and sensitivity was small, and the pH measurement characteristics were almost equivalent to those of existing glass reference electrodes. After performing in vivo measurements in restrained mice, we verified the extracellular response in the brains of epilepsy-induced unrestrained mice as well. We realized real-time imaging of extracellular pH changed in a mouse brain under running conditions for the first time.