Development of thermally efficient, ultrathin, diamond-based µLED neural implants for versatile optogenetics

  • Antoine BOUDET

Student thesis: Doctoral Thesis

Abstract

Optogenetics helps revolutionise the study of neural networks by allowing localised, genetically targeted, millisecond-timescale optical stimulations of neurons. Optogenetic implants embedded with LED sources hold great potential for experimental neuroscience, as they dispense with external optical coupling and enable site-specific light delivery to deep-brain regions. Although this technology has attracted firm interest in recent years, a long-standing question is whether these devices can attain optical powers or pulse rates suited for high-illumination protocols or circuit-level stimulation (e.g P > 50mW.mm2), while keeping average or peak Joule heating of tissue within commonly accepted â€safeâ€� boundaries (e.g <1°C increase). Typical LED substrates are restricted by their thermal and optical properties, and heterogeneously integrated devices have thus far suffered from poor optical output or exceedingly invasive footprints resulting from design or manufacturing trade-offs. As a result, none of the LED-based implant demonstrated in the literature has ever been shown to safely output high optical power while exhibiting dimensions comparable to more mature guided-light devices (e.g width ~<100µm). This work presents novel diamond-based LED optrodes which, for the first time, simultaneously demonstrate minimal invasiveness, optical performance and outstanding thermal efficiency. The devices harness the superior thermal conductivity, corrosion resistance and increasing availability of synthetic diamond which make it a promising candidate for the next generation of hybrid bioimplants.Our single-crystal diamond optrodes have penetrating shanks, each integrating 4 transfer-printed AlInGan µLEDs (50x50µm2, λ = 455nm) and 8 electrodes. As key milestones, specific techniques were developed for a) the processing of ultrathin diamond membranes, b) the optimised manufacturing of high-efficiency transfer-printing LEDs (EQE >4% at irradiance ILED = 100mW.mm-2) and c) their reliable transfer onto highly textured substrates, yielding an advanced printing method via adhesive picodroplets. For the first time, the probe dimensions (shank LxWxT = 5.5mm x 150µm x 25µm) and large irradiance range (up to ~300mW/mm2 per LED at 3mA driving current) approach those of state-of-the-art monolithic Silicon-based optrodes, with a thermal performance improved by more than an order of magnitude. This is predicted to allow a uniquely wide set of optogenetic protocols including extended/high-power optical pulses at high duty cycles, capable of stimulating thousands of neurons while keeping peak tissue temperature increase below 1°C. Process scalability on commercial wafers is demonstrated on a polycrystalline diamond membrane (20mm diameter, 50µm thickness), opening the way for novel, inexpensive diamond-based tools for neurophotonics and biomedical applications.
Date of Award26 Sep 2019
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council) & University of Strathclyde
SupervisorKeith Mathieson (Supervisor) & Martin Dawson (Supervisor)

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