Thermal and optical characterization of a micro-LED probe

Over the last few years, there has been a significant drive to improve light sources for in vivo optogenetic control of neuronal activity. In particular, recent work has focused on the design and microfabrication of compact devices that exhibit multiple optical stimulation sites in order to gain control over closely spaced regions within the brain (see here a list of posts related to this). In a recent Optics Letters paper, McAlinden, Massoubre and colleagues presented a novel microprobe device with integrated light sources. The probe produces sufficient light for optogenetic stimulation without causing significant heating in local brain tissue. The device (see figure below) consists of a 100 µm wide, 50-100 µm thick probe with 5 individually addressable microLEDs. Each LED has a diameter of 40 µm, but can be reduced to 10 µm if higher density probes are required. The LED probes are p-n diodes made from quantum well structures using GaN on Sapphire material. The spacing between LEDs is fixed to allow 1mm of neural tissue to be addressed.

a) Layout of LED probe, showing the single n-contact and bonding pads linked with tracks to the p-contacts for each LED. (b) Schematic cross section of the fabricated devices, showing the quantum well (QW) LED structure. (c) Tip of the five-site LED probe, before final thinning, with a single LED switched on, emission is through the sapphire substrate. Scale bar is 200 μm.

a) Layout of LED probe, showing the single n-contact and bonding pads linked with tracks to the p-contacts for each LED. (b) Schematic cross section of the fabricated devices, showing the quantum well (QW) LED structure. (c) Tip of the five-site LED probe, before final thinning, with a single LED switched on, emission is through the sapphire substrate. Scale bar is 200 μm.

The light output from the LEDs was measured, with scattering and absorption accounted for by experimentally measuring the light transmitted through varying thicknesses of brain tissue. The recorded light levels are sufficient to stimulate a region up to 150 µm away from the LED surface. A concern with LED devices of this type is heat dissipation into the neural tissue. The paper details measurements and modelling, showing that the probe can be used to provide pulses suitable for optogenetic studies without exceeding a 0.5 degrees increase in the local temperature of the brain tissue.

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