Blue-Green and Ultraviolet micro-LEDs in Neural Imaging and Stimulation
1. Blue-Green InGaN and Ultraviolet AlGaN Multiple Quantum Well LED Device Fabrication
The Blue-Green LED devices were fabricated from standard epitaxially grown p-n junction GaN heterostructure wafers (on sapphire), with an InGaN/GaN active multiple quantum well region. The 340 nm wavelength LEDs were fabricated from AlGaInN quantum-well p-n junction heterostructures, according to a material design typical for these compact solid state light emitters. Mesa-type LEDs were etched using chlorine-based RIE, photolithography and electron beam deposition were used to make metalic contacts and followed by annealing processes with specific conditions according to materials. The completed devices were flip-chipped for light extraction through the polished sapphire backside. Using an epoxy die bonder, electrically conductive epoxy dots were deposited onto a suitably patterned silicon submount. For heat management and mechanical stability, thermally conductive, electrically insulating, epoxy was deposited in the area surrounding the optical apertures
Each chip containing three 3×2 arrays of devices was mounted onto a commercial DIP package for easy connection to the control circuitry and mounting under the microscope.
2. Blue-Green micro-LEDs for Dynamical Imaging of Neuronal Circuitry
Imaging of biological structures at cellular level by photonic techniques is an advanced science and craft, most commonly applied in the context of fluorescently labeled probes that are excited by lasers or incoherent lamps within an integrated optical microscope-based imaging system. Micro-LEDs and their arrays enable greater flexibility of the illumination arrangements and with the aim towards compact, portable instrumentation.
We can envision the use of these micro LED arrays with pixel-size matched/imaged photodiode arrays for dynamical optically recording the activity of multiple neural cells, eventually extending to an entire neuronal circuitry on a chip-scale device. By rapid electrically directed scanning of the LED arrays, neurons in their microcircuitry will be illuminated sequentially with fluorescent signals being detected by the corresponding photodiode elements
(a) Microscope image of hippocampal neural cells in culture (differential contrast mode). (b) Two individual elements of the micro-LED array elements located under two neighboring cells. (c) Fluorescent images when both illuminating LEDs are turned ON, and one is ON at a given gated time, respectively. (d) A concept schematic of the chip-scale optical recording system.
3. UV micro-LEDs for Activation of Electrical Response in Neural Cells by Photochemical Flash Photolysis
The capability of UV LEDs to uncage neurotransmitter and activate proximate individual hippocampal cultured cells was demonstrated. The entire LED assembly with the corresponding focusing optics was mounted on a 3D stage that allowed for precise positioning of the LED element below the region of interest. Whole-cell recordings were made on a given neuron.
(a) Image of the neuron with corresponding puffer (left) and patch (right) pipettes, (b) Control experiment with the puffer: red trace indicating the duration of puffer pulse, black trace shows the cell’s response, (c) control experiment with the LED; blue trace indicating the duration for which LED is ON, black trace shows the cell’s response, and (d and e) response recorded from two different cells when both the puffer (red) and LED (blue) were ON.
However, with both the puffer and the LED turned ON (ON times indicated by the red and blue insets), we could readily observe neural cell electrical switching, as shown from the recordings from two cells in figure 9(d) and (e). The ability to trigger action potentials in the neuron indicates that the UV LED effectively uncaged the glutamate that was delivered in the immediate vicinity of the neuron, providing the neurotransmitter excitation action. (It is also clear from figure (d) and (e) that by carefully controlling the picospritzer conditions, it is possible to achieve shorter latencies between beginning of depolarization and the firing of action potential). Experiments such as these hence show graphically the utility of the nitride UV micro-LEDs as efficient and effective sources for flash photolysis experiments simplifying and reducing the cost and size of conventional setups that require expensive and complicated instrumentation.
4. Combining Multicore Imaging Fiber with Matrix Addressable Blue/Green LED Arrays for Spatiotemporal Photonic Excitation in the research of visual system development
A. Blue/green LED array fabrication and image fiber coupling.
B. Photostimulation of retinal ganglion cells for visual system development studies.
10x10 matrix addressable LED array coupled to image fiber has been used to deliver patterned visual stimuli directly onto the retina of developing (Nieuwkoop and Faber, developmental stage 48) Xenopus laevis tadpoles. The retinal sensitivity is especially pronounced in this animal in the blue-green region of the spectrum. The response of optic tectal neurons in the tadpole brain was probed electrically via a standard electrophysiological patch clamp microelectrode technique, under different light excitation patterns from the microscale flexible LED-array driven device, in order to map the so-called visual receptive field (i.e. a single neuron’s response to stimulations from different retinal areas) of tectal neurons. We could measure the degree of convergence of retinal inputs into a given tectal cell. This was done at different developmental stages to study the formation of the retinotopic map during development of the visual system. Additionally, we note that we have been able to apply direct optical stimulation of the brain as well (with finite optical absorption by neural cells in the blue), to elicit some neural response. In sum, that these experiments show the utility of the blue (and green) LED microarrays, integrated with the imaging optical fiber, as a flexible and compact optoelectronic tool for all types of photoexcitation schemes where portability and compactness have premium value.