A research team in the Department of Electrical and Electronic Information Engineering at the Toyohashi University of Technology has developed a MicroLED neural probe for neuroscience. This MicroLED tool can optogenetically control and observe neural activity in the brain. The work "Development of a neural probe integrated with high-efficiency MicroLEDs for in vivo application" was published in the Japanese Journal of Applied Physics.
An LED probe with 16 MicroLEDs was fabricated in one shank and it was reported to effectively activate neural activity in the depth direction in the cerebral cortex in vivo. This design allowed for the realization of complex neural activity manipulation; however, electrophysiological experiments require the use of neural recording electrodes to capture the manipulated neural activity.
Because the LED probe and neural recording electrode probe are different, it is necessary to precisely control the position of each probe and insert both devices for observation. Thus, the development of an integrated MicroLED and neural recording electrode probe is essential.
New optogenetic technology that enables the manipulation and recording of single neurons in the deep regions has been realized; however, there are a few reports of such integrated probes. Furthermore, among those that have been reported, they are not suitable for neuroscience research because the light output of MicroLEDs is as low as several μW because of their small size. Although an increase in the MicroLED size leads to an increase in light output, this size increase also lowers the spatial resolution and increases the amount of stimulation-induced heat, resulting in more heat damage. As such, it is necessary to optimize the trade-off between light output and device size; in addition, the installation of highly efficient LED devices that suppress Joule heat is required.
In this study, a neural probe with six micro-light-emitting diodes (MicroLEDs) and 15 neural electrodes was fabricated for optogenetic application. Local field potentials, which provide information about the neural activity, were successfully recorded using the neural probe, indicating the effectiveness of the neural electrodes. The MicroLEDs on the probe exhibited highly consistent current-voltage characteristics and sufficient light output of 20 mW mm−2 at 1 mA to manipulate neural activity. The light distribution in brain tissue was simulated to estimate the optical stimulation area and a number of optically stimulated neurons. The increase in LED temperature, i.e. ΔT, was investigated because high temperatures can damage brain tissue. A curve illustrating the relationship between ΔT and the wall-plug efficiency was derived. The wall-plug efficiency was increased 1.8 times by installing an Ag mirror on the back of a MicroLED. These results suggest that the MicroLED neural probe would significantly contribute to the development of neuroscience research-purposed optogenetic technology.
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