and demonstrate that elicited RGC responses can be modulated by both light intensity and pulse width. The described optical system uses a liquid crystal display (LCD) illuminated by a laser beam to form patterns of NIR light (see description in Methods section), enabling only intensity modulation within each video frame. However, since the retinal response can also be modulated by varying the pulse width, DLP™ technology based on an array of high-speed actuated micro-mirrors, can also be employed31
. Such a device would allow both the duration and timing of exposure to be precisely controlled on the scale of individual pixels. In addition to higher throughput compared to an LCD, this high-speed control would allow the sequential activation of nearby pixels to further reduce pixel crosstalk.
The full-field stimulation thresholds were more than two orders of magnitude below ocular safety limit and were determined using cathodal-first stimulation pulses. Thresholds for single pixel stimulation were more than one order of magnitude below the ocular safety limits. This could perhaps be lowered, by up to 2-7 times, if anodal-first pulses are used32
. Our fabrication process can be easily altered to accommodate the opposite polarity.
As stimulation thresholds rapidly increase with distance to the target cells12,33
, high resolution retinal stimulation requires close electrode-neuron proximity – apposition on the order of the size of the stimulating electrode12
. To assess the proximity of the inner retinal neurons to subretinal implants we fabricated 30 μm-thick (matching our active devices) polymer (Microchem SU8) implants that can be sectioned together with tissue for histology. These devices were coated with SIROF to match our electrode material and implanted in the subretinal space of RCS rats for 6 weeks34
. The histological section of an RCS rat retina shown in demonstrates that the inner nuclear layer (INL) is separated from the implant surface by 5 – 25μm. Electric current must penetrate at least this depth to ensure effective neural stimulation.
Figure 6 (a) Retinal histology of a flat polymer implant in the subretinal space of an RCS rat, with the numerically calculated current distribution from a 115 μm pixel (pixel schematics overlaid). (b) Retinal histology of a pillar array implant, overlaid (more ...)
We have computed the current density distribution from single pixels of various dimensions with the COMSOL Conductive Media DC package, where the retina is modeled as a resistive medium. presents the current density in front of a 115μm pixel with a 40 μm active electrode and circumferential returns, superimposed upon a histological section of a flat implant in the subretinal space of an RCS rat. The current penetrates sufficiently deep to provide targeted stimulation of many neurons in the INL. However, smaller, 62μm pixels with 20μm active electrodes can affect fewer such neurons, and will likely require either increased light intensity or improved proximity for effective stimulation.
We have previously shown that retinal cells migrate inside and around three-dimensional sub-retinal implants, an effect that may be utilized to attain intimate neuron-electrode proximity35
. The histological section shown in depicts an array of 15μm diameter, 65μm tall pillars six weeks after implantation. The bipolar cells surround the pillar tops, placing them in cellular-scale proximity to the tips of the pillar electrodes. The computed current distribution for this geometry overlays the histological section, demonstrating that such pillar arrays can effectively deliver stimulation currents to the inner nuclear layer, and could provide a mechanism for reducing the thresholds and improving stimulation localization in future devices.
Finally, the need to conform to eye curvature inherently limits the sizes of rigid implants to at most a few millimeters. The use of a flexible silicon substrate36
can overcome this limitation and allow larger arrays to deform elastically to the curvature of the eye. Trenches etched between neighboring pixels leave 0.5 μm thin silicon “springs” (Supplemental Figure 4a,b
). Optical coherence tomography shows a 6 mm-wide sample array with 75μm pixels conforming to the curvature of a pig retina (Supplemental Figure 4c
) despite its very large size, a feat made possible by the inherent 2-dimensional deformability of the flexible silicon mesh.
Since the photovoltaic implant is thin and wireless, the surgical procedure is much simpler than in other retinal prosthetic approaches. As in conventional subretinal surgery, the procedure involves a partial vitrectomy followed by a subretinal injection to create a retinal bleb. The implant is then inserted into the subretinal space through a retinal incision, and the retina is reattached19
. As large incisions can complicate retinal re-attachment, retinotomies should be as small as practically possible. Since all pixels in the implant function independently several smaller arrays may be inserted through the same retinotomy to tile a large area.
In conclusion, we demonstrate that NIR light-induced photovoltaic stimulation using a subretinal photodiode array elicits bursts of RGC spikes in both healthy and degenerate rat retinas at irradiances substantially below ocular safety limits. The response can be modulated by either pulse duration or irradiance in each pixel. Such a fully integrated wireless implant promises the restoration of useful vision to patients blinded by degenerative retinal diseases.