Abstract
This paper reports the fabrication and characterization of an optical neuro-stimulator array that consists of 32-channel microscale light-emitting diodes (µ-LEDs) coupled with microscale reflectors for intensity enhancement. The hemi-spherical micro-reflector is able to collect the rear side emission of LED while also acting as a collimator to focus the diverged LED light, aiming towards driving power minimization through light intensity increase, for wireless neuro-stimulator applications. The micro-reflector was constructed by wet etching of silicon followed by aluminum coating as the reflective mirror. The reflective cavity was filled with polydimethylsiloxane (PDMS) that acts as the planarization polymer to facilitate device integration with the µ-LED chip. Deviation of hemi-spherical geometry cavities due to the uneven lateral and vertical etching rate was shown, and the surface morphology was characterized experimentally. Optical intensity enhancement was studied in both simulation and experiments, demonstrating that the micro-reflector enables 65% intensity enhancement. The reflector-coupled LED had an operating temperature increase of <1oC, well within the ANSI/AAMI safety limit for biomedical implants. The potential of the stimulator for use in optogenetics was validated by in-vitro experiments.
Highlights
Optogenetics is the technology of delivering light to tissues of interest while collecting readouts from the cells using targeted control tools (Deisseroth, 2011)
To address the need for intensity boost from μ-LEDs while achieving a better spatial resolution toward multi-site stimulation, we propose a reflector-coupled μ-LED array to act as a surface light source for minimally invasive, epidural optogenetic neuromodulation
Based on the solution recipe reported by Albero et al (2009), in this paper, we studied several different combinations of masking materials to determine the best protocol for wet silicon etching, including plasma enhanced chemical vapor deposition (PECVD) silicon oxide (SiO), PECVD nitride and low-pressure chemical vapor deposited (LPCVD) nitride
Summary
Optogenetics is the technology of delivering light to tissues of interest while collecting readouts from the cells using targeted control tools (Deisseroth, 2011). The targeted cells, after being transfected by the appropriate opsin, requires optical stimulation of sufficient intensity [1 mW/mm or 7 mW/mm for excitatory and inhibitory opsins (Aravanis et al, 2007), respectively] to activate or silence the cell. Micro-Reflector Integrated μLED Optogenetic Neurostimulator express high levels of the opsins due to the relatively small optical current mediated by each opsin molecule (Aravanis et al, 2007; Deisseroth, 2011). A successful optogenetic tool requires a structured, timevarying light stimulus of a certain minimum intensity that could be automatically modulated based on the difference between desired and measured outputs (Grosenick et al, 2015). Examples include microscope focused light delivery (Ayling et al, 2009), SiON 3D waveguides (Zorzos et al, 2012), polymer waveguides (Kwon and Li, 2013; Wu et al, 2013), glass optrode-array (Abaya et al, 2012), and fiber-optic probes (Stark et al, 2012; Chen et al, 2013; Nussinovitch and Gepstein, 2015)
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