An Implantable CMOS Image Sensor With Self-Reset Pixels for Functional Brain Imaging

In this paper, we propose and demonstrate an implantable CMOS image sensor with self-resetting pixels. The self-resetting function is implemented using a four-transistor Schmitt trigger inverter. The pixel has no counter for the number of self-resets, because the application does not require radiometric (linear) response. The pixel is fabricated using the 0.35- $\mu \text{m}$ 2-poly 4-metal standard CMOS technology, which results in the pixel size of $15~\mu \text {m} \times 15 ~\mu \text{m}$ and a fill factor of 31%. The effective peak signal-to-noise ratio is >59 dB. An image sensor prototype comprising a $60\times 134$ pixel array is designed, and an implantable device is fabricated. As an example imaging experiment, we demonstrate blood-flow imaging of a rat-brain surface using the sensor. Intensity-change images are successfully obtained from the self-resetting pixel outputs with the image processing.