Disposable MEMS optrode array for multi-wavelength neurostimulation

Abstract

This dissertation presents an optical neural probe for an implantable multi-wavelength photostimulation system based on a newly-proposed MEMS optrode array (MOA) with improved light delivery efficiency and capability. The proposed system delivered the highest total light power from a single commercially available LED to the distal tip of the waveguide among conventional LED-waveguide-based optical neural probes by expanding the waveguide into an array. To improve light delivery, a novel MEMS microlens array was adopted as the optical collimator between the light source and the waveguide. This research also proposes and validates a simple and practical method for suppressing surface crystallization of borosilicate glass during high temperature annealing as well as a method of modeling and fabricating a through-substrate square-shaped glass microlens array using thermal reflow process, both for the first time in this field of research. The probe proposed in this study comprises of a disposable MOA and a reusable unit, where the MOA is fabricated by manually assembling a 4×4 array of 6 mm-long optical fibers with a 4×4 array of thermally-reflowed square-shaped glass microlenses. The reusable unit includes a domed-top LED and driving circuitry. The MOA, developed for implantation on a nerve system, is separable from the LED light source to reduce the risk of infection from probe reuse while minimizing the waste of the probe after animal experiments. The optimal geometry of the microlens was derived through finite element analysis on computational fluid dynamics and geometrical optics. Surface crystallization on borosilicate glass, which is a well-known and fatal limitation that can occur when fabricating a three-dimensional glass structure via thermal reflow process, was effectively prevented on a commercially available glass wafer by applying a simple surface treatment prior to the annealing. Quantitative analysis of the crystallization and surface ion concentration verified that 30 seconds or more of fluorine plasma treatment can significantly suppress the nucleation of cristobalites. Average surface roughness and optical transparency were enhanced 15 times and 3 times, respectively, compared to untreated samples, suggesting that the proposed method is practical for vitrification in borosilicate glass during the thermal reflow process. Square-shaped glass microlenses with a footprint of 300×300 µm2 were uniformly fabricated in good agreement with the finite analysis modeling. Vitrified surfaces with an average surface roughness of 47 nm was achieved through the proposed procedure. The measurement results fully validated the fabrication procedure and the capability of the microlens array as light collimating optics in the proposed neural probe. The fabrication of the MEMS optrode array including fiber manipulation and assembly was carried out with customized aid apparatus. Optical characterization of the MEMS optrode array is presented, followed by an analysis of loss factors. The capabilities and limitations of the present device is discussed in terms of the multi-wavelength illumination and disposable functionality. The total light delivery efficiency of the probe was measured to be −10.63 dB, of which the use of microlenses attributed to the improvement of 3.15 dB. Light loss was mainly caused by the coupling loss between the LED and the microlens, which is estimated to be −6.04 dB, and the rough facets of the diced optical fibers. A more precise and robust design for the assembly sheath will be also helpful in significantly preventing the degradation of light delivery efficiency caused by misalignment between the disposable MOA and the reusable LED unit.