<p>We present an electrically addressable optrode array capable of delivering light to 181 sites in the brain, each providing sufficient light to optogenetically excite thousands of neurons <italic>in vivo</italic>, developed with the aim to allow behavioral studies in large mammals. The device is a glass microneedle array directly integrated with a custom fabricated microLED device, which delivers light to 100 needle tips and 81 interstitial surface sites, giving two-level optogenetic excitation of neurons <italic>in vivo</italic>. Light delivery and thermal properties are evaluated, with the device capable of peak irradiances >80 mW / mm<sup>2</sup> per needle site. The device consists of an array of 181 80 μm × 80 μm<sup>2</sup> microLEDs, fabricated on a 150-μm-thick GaN-on-sapphire wafer, coupled to a glass needle array on a 150-μm thick backplane. A pinhole layer is patterned on the sapphire side of the microLED array to reduce stray light. Future designs are explored through optical and thermal modeling and benchmarked against the current device.</p>
Optogenetics is a powerful tool for neural control, but controlled light delivery beyond the superficial structures of the brain remains a challenge. For this, we have developed an optrode array, which can be used for optogenetic stimulation of the deep layers of the cortex. The device consists of a 10×10 array of penetrating optical waveguides, which are predefined using BOROFLOAT® wafer dicing. A wet etch step is then used to achieve the desired final optrode dimensions, followed by heat treatment to smoothen the edges and the surface. The major challenge that we have addressed is delivering light through individual waveguides in a controlled and efficient fashion. Simply coupling the waveguides in the optrode array to a separately-fabricated μLED array leads to low coupling efficiency and significant light scattering in the optrode backplane and crosstalk to adjacent optrodes due to the large mismatch between the μLED and waveguide numerical aperture and the working distance between them. We mitigate stray light by reducing the thickness of the glass backplane and adding a silicon interposer layer with optical vias connecting the μLEDs to the optrodes. The interposer additionally provides mechanical stability required by very thin backplanes, while restricting the unwanted spread of light. Initial testing of light output from the optrodes confirms intensity levels sufficient for optogenetic neural activation. These results pave the way for future work, which will focus on optimization of light coupling and adding recording electrodes to each optrode shank to create a bidirectional optoelectronic interface.