Numerous applications enabled by optogenetic tools require the delivery of different light profiles within tissue. In some instances, wide-field illumination is needed to blanket a region of a specific brain layer, while cell body-size beams are necessary in other cases to achieve highly-selective localized stimulation; in addition, spatiotemporal patterning or multi-level access of light may be incorporated. In a recent paper published in Biomedical Optics Express, Abaya, et al. presented a novel device structure that facilitates three-dimensional deep-tissue light penetration with capabilities for simultaneous spatiotemporal modulation with different wavelengths. This device consists of > 1 mm length needle-shaped waveguides that circumvent intrinsic tissue absorption and scattering. This array can be adapted to a wide variety of illumination systems (e.g., microscope, μLED arrays, optical fibers, collimated beams, spatial light modulators) and can potentially facilitate the advancement of a broad range of applications utilizing optical neural stimulation.
The waveguide array is micromachined from fused silica/quartz wafers, which are transparent to visible and near-infrared wavelengths. First, bevel blades are used with a dicing saw to create arrays of pyramids as tips for the array. Next, the shanks are formed by dicing down material in between the pyramids to produce waveguides having a pyramidal tip atop a rectangular shank. The waveguides are then thinned down to the desired width by etching in 49% hydrofluoric acid solution. The arrays are singulated and annealing is performed to smooth the rough sidewall surfaces resulting from the dicing and etching steps. The array size and waveguide length, width and tip angle may be varied independently from wafer to wafer to control the stimulation area, depth of access, output beam spot size and output beam divergence, respectively. The waveguide length and tip angle does not affect the normalized power out of each waveguide tip, but the width needs to be larger than the optical source for maximum power coupling in order to maintain the same transmission efficiency of ~71% in air. This value is expected to increase when the waveguides are implanted in tissue. The dominant loss in the waveguide is due to total internal reflection within the pyramidal tips that prevents some of the light from projecting into the tissue.