Compact silicon-based optrode with integrated laser diode chips, SU-8 waveguides and platinum electrodes for optogenetic applications

We report on a compact optrode, i.e. a MEMS-based, invasive, bidirectional neural interface allowing to control neural activity using light while neural signals are recorded nearby. The optrode consists of a silicon (Si) base carrying two pairs of bare laser diodes (LDs) emitting at 650 nm and of two 8 mm-long, 250 mu m-wide and down to 50 mu m-thick shanks extending from the base. Each LD is efficiently coupled to one of four 15 or 20 mu m-wide and 13 mu m-high SU-8 waveguides (WGs) running in pairs along the shanks. In addition, each shank comprises four 20 mu m-diameter platinum electrodes for neural recording near the WG end facets. After encapsulation of the LDs with a Si cover chip blocking stray light and protecting the LDs from the harsh environment to which the probe is destined, the compact base measures only 4 x 4 x 0.43 mm(3). The time averaged radiant emittance at the WG end facet is 96.9 mW mm(-2) for an LD current of 35 mA at a duty cycle of 5%. The absolute electrode impedance at 1 kHz is 1.54 +/- 0.06 M Omega. Using infrared thermography, the temperature increase of the probe during LD operation was determined to be about 1 K under neuroscientifically relevant operating conditions.