Design and Fabrication of a Thin Film Optogenetic ECoG Array with Individually Addressable micro-LEDs

Abstract

Optogenetics is a neuromodulation technique that uses light to control genetically modified cells to express light sensitive ion channels. Optogenetics allows stimulation of only the specific cells in the region that have been genetically modified and thus results in a high resolution of stimulation.
Currently optogenetic implants are used to stimulate specific regions in the brain, either deep in the cortex or on certain regions on the surface of the cortex. An implant with a larger surface area would potentially allow stimulation of the entire cortex simultaneously, if required. By also including recording sites on this implant, it is possible to record responses at one end of the brain produced due to optogenetic stimulation on the other end of the brain. Thus, the underlying neural circuit can be mapped for investigation.
Thin film technology (TFT) so far has had a huge impact in the field of large flexible displays. The flexible substrates and processes employed for the fabrication of flexible displays can be used for the realisation of an optogenetic array that covers the cortex of the brain while being flexible and conformal to the shape of the brain.
This work explores the implementation of TFT in fabricating a flexible large area high-density optogenetic ECoG array. The fabricated array features multi-stacked alternate layers of thin film Au/Ti (for electrodes and interconnects) and thin film SiN (insulation and passivation) on a flexible polyimide substrate to provide a high-density array for improved resolution. Commercial micro-LEDs were bonded to the surface of the array using ICA (Isotropic Conducting Adhesive) to provide on-site stimulation. The resulting flexible implant was characterised to determine the electrode impedance, behaviour of the passivation layer in phosphate buffered saline and thermal characteristics of the micro-LEDs. The final device was implanted on the cortex of a mouse.