Design and MEMS Microfabrication of an Optrode for Combined Optogenetics and Electrophysiology Studies

Optogenetics is a biological technique that uses light to control cells in living tissues, typically neurons, that have been genetically modified to express light-sensitive ion channels. Using this technique, neuroscientists can investigate the neural circuits underlying neurological diseases with a higher spatio-temporal resolution when compared to other known neuromodulation methods. As employed today, optogenetics requires methods for guiding sufficiently strong and precisely timed light to specific brain regions, while the experimental subject carries out behaviors of interest. For this role, miniaturized devices (namely optrodes) shall be properly engineered to hold the required components (e.g. light source, recording electrodes, etc) whilst complying with some surgical and biocompatibility issues. In this work, an optrode was designed and fabricated using an in-house MEMS microfabrication technology. The custom-made device featured (a) low impedance level with TiN-coated microelectrodes, (b) sufficient optical power delivery through on-chip-uLEDs, and (c) miniaturized dimensions with tolerable tissue damage during long-term animal experiments. In addition, different optrodes were fabricated to allow different experiment conditions (i.e. chronic or acute implantation, multi-site or multi-layer studies). A MEMS cavity for the on-chip-uLED was engineered on the optrode's shaft in order to further minimize the induced tissue damage during the surgical implantation. Last but not least, this customized optrode is also compatible with our in-house CMOS technology and can be further upgraded with additional electronic functionalities, as well as with the deposition of novel materials. After the microfabrication and system integration, in-vitro experiments on three different designs were performed to characterize electrically the electrode impedance, the control of uLED's light intensity and pulse frequency.