High-efficiency ultrasound-powered micro-LEDs for optogenetic applications

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Mondal, Tanmay
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University College Cork
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Gallium nitride (GaN) light-emitting diodes (LEDs) play the most important role in visible light applications, and more than 87% LEDs are used in indoor/outdoor lighting, automotive lighting, etc. Recent advancements in neuroscience and technology opened a new opportunity for LEDs to be used for optogenetic applications. Optogenetics is an emerging field that combines genetic engineering and light for precise control or monitoring of biological functions of cells, neurons, or organs, and µLEDs (size ≤ 100 µm) are considered the best candidate as a light source for in-vivo optogenetic applications. This work focuses on realizing a wireless ultrasound (US) powered implant-compatible device (also called neural DUSTs) enabling in-vivo electrophysiology, optogenetics, and ultra-localized drug delivery in freely moving animals. The device requires a minimum of three components: a PZT (Lead Zirconate Titanate piezoelectric material) cube to harvest the US energy, a rectifier chip for AC-DC conversion, and a µLED to deliver the optical power. The µLED should be capable of delivering a minimum of 5 mW/mm2 optical power at ~ 1 mA for activating light-sensitive proteins (Channelrhodopsin-2) while the temperature rise is < 1 oC, have an operating voltage below 3.3 V (limit of rectifier chip) and custom-made to match physical layout with rectifier. This requires the need for a highly efficient µLED. It is fortuitous that the optimum wavelength of the cation-selective membrane channel Channelrhodopsin-2 is around 470 nm where GaN material has the highest efficiency. Indium Gallium Nitride (InGaN) quantum well-based efficient blue (λpeak= 483 nm) and UV (λpeak= 371 nm) µLEDs grown on a patterned sapphire substrate (PSS) were specifically designed and fabricated with a low turn-on voltage of 2.3 V (blue) and 3.1 V (UV) which was lower than the corresponding band-gap voltage. A high wall-plug efficiency of 40.7% and 25.4% was measured from the 100 µm mesa blue and UV µLEDs, respectively. Investigation with Ag-based p-contact showed a further 20% improvement in extraction efficiency. A localized drug delivery system (DDS) based on photochromic spiropyran (SP) was characterized and developed, which activates upon exposure with 370 nm UV. The activation of DDS by ultrasound-powered and the drug release by electrically powered UV µLED were successfully demonstrated. A new dual-colour µLEDs (combinations of red, blue, and UV) on a single chip was designed and fabricated targeting dual-colour optogenetic applications. The characterization results presented here highlight challenges, issues resolved, and further improvement scope. Finally, two US-powered implant-compatible DUSTs were developed by integrating a PZT, rectifier chip, and µLED with a total volume of 2.85 mm3 (basic-DUST) and 0.33 mm3 (compact-DUST) respectively and demonstrated. To the best of our knowledge, the compact-DUST demo is the smallest implant-compatible US-powered DUST with integrated µLED. Future work should be done to achieve more efficient µLEDs, integrate DUSTs with dual colour µLEDs and develop a wafer-scale integration process.
Semiconductor , Micro-LED , LED , Optogenetics , High efficiency , EQE , WPE , Blue LED , UV LED , Drug delivery , Dual wavelength , Pachading , DUSTs
Mondal, T. 2023. High-efficiency ultrasound-powered micro-LEDs for optogenetic applications. PhD Thesis, University College Cork.