Compact Optical Neural Probes With Up to 20 Integrated Thin-Film $\mu$LEDs Applied in Acute Optogenetic Studies

This paper reports on the development, characterization and in vivo validation of compact optical neural probes. These novel intracerebral devices comprise micro light-emitting diodes (μLEDs) integrated along their slender probe shanks with up to 20 μLEDs per device. Blue light with a peak wavelength of 455 nm is emitted from circular apertures 100 μm in diameter. The μLEDs are structured on GaN-on-sapphire wafers and subsequently transferred onto silicon (Si) carrier wafers. The wafer-scale transfer process provides the opportunity to process the functional GaN layer stack from both sides and hence enables maximizing the efficiency of the μLEDs. Combined with standard MEMS fabrication processes for Si, linear μLED arrays with small inter-μLED distances are achieved on thin probe shanks with cross-sections measuring 150 μm × 65 μm. Devices are interconnected using highly flexible polyimide cables in order to mechanically decouple them from the peripheral electronics during in vivo experiments. Assembled probes emit a peak optical radiant flux of 440 μW (emittance 56 mW mm-2) at 5 mA driving current. Thermal characterization of test probes reveals a temperature increase of 1.5K measured using an integrated thermistor. Electrical functionality stress tests have been carried out to evaluate the device passivation against the physiological environment. It is estimated to endure at least 48 h during continuously pulsed μLED operation. A compact driving circuitry enables low-noise μLED operation in in vivo optogenetic experiments. The radiant flux necessary to elicit an acceptable neuronal response is determined between 1.36 μW and 17.5μW. Probe validation successfully demonstrates the layer-specific stimulation in the cortex in multiple in vivo trials.