Vertical-junction photodiodes for smaller pixels in retinal prostheses

Abstract

Objective. To restore central vision in patients with atrophic age-related macular degeneration, we replace the lost photoreceptors with photovoltaic pixels, which convert light into current and stimulate the secondary retinal neurons. Clinical trials demonstrated prosthetic acuity closely matching the sampling limit of the 100 μm pixels, and hence smaller pixels are required for improving visual acuity. However, with smaller flat bipolar pixels, the electric field penetration depth and the photodiode responsivity significantly decrease, making the device inefficient. Smaller pixels may be enabled by (a) increasing the diode responsivity using vertical p–n junctions and (b) directing the electric field in tissue vertically. Here, we demonstrate such novel photodiodes and test the retinal stimulation in a vertical electric field. Approach. Arrays of silicon photodiodes of 55, 40, 30, and 20 μm in width, with vertical p–n junctions, were fabricated. The electric field in the retina was directed vertically using a common return electrode at the edge of the device. Optical and electronic performance of the diodes was characterized in-vitro, and retinal stimulation threshold measured by recording the visually evoked potentials in rats with retinal degeneration. Main results. The photodiodes exhibited sufficiently low dark current (<10 pA) and responsivity at 880 nm wavelength as high as 0.51 A W−1, with 85% internal quantum efficiency, independent of pixel size. Field mapping in saline demonstrated uniformity of the pixel performance in the array. The full-field stimulation threshold was as low as 0.057 ± 0.029 mW mm−2 with 10 ms pulses, independent of pixel size. Significance. Photodiodes with vertical p–n junctions demonstrated excellent charge collection efficiency independent of pixel size, down to 20 μm. Vertically oriented electric field provides a stimulation threshold that is independent of pixel size. These results are the first steps in validation of scaling down the photovoltaic pixels for subretinal stimulation.

Ambient Light Rejection Integrated Circuit for Autonomous Adaptation on a Sub-Retinal Prosthetic System

“…Electrical performance of the proposed work is summarized on Table 2 along with other previous works for comparison. The prior literature presented in [ 8 , 9 , 21 , 33 ] only has stimulation pixels. Although Park et al [ 7 ] developed an edge stimulation method to increase contrast sensitivity, it cannot compensate for pixel saturation caused by the ambient light.…”

“…Retinal implants have great promise in restoring vision for the blind, who suffer from retinal diseases such as retinitis pigmentosa and age-related macular degeneration [ 1 , 2 , 3 , 4 ]. The fundamental idea for retinal prosthetics is to electrically stimulate impaired retina cells using a microelectrode array and its driving circuitry [ 5 , 6 , 7 , 8 , 9 ]. This retinal prosthesis can be classified into epi-retinal [ 5 , 6 ] and sub-retinal implants [ 7 , 8 , 9 ], based on the anatomical location.…”

“…The fundamental idea for retinal prosthetics is to electrically stimulate impaired retina cells using a microelectrode array and its driving circuitry [ 5 , 6 , 7 , 8 , 9 ]. This retinal prosthesis can be classified into epi-retinal [ 5 , 6 ] and sub-retinal implants [ 7 , 8 , 9 ], based on the anatomical location. While the epi-retinal implant is placed onto an inner retinal layer, known as the ganglion cells, the subretinal implant is located in the outer retina, called photoreceptor cells.…”

Electronic “photoreceptors” enable prosthetic vision with acuity matching the natural resolution in rats

“…We modeled the electric field in the retina generated by a subretinal 30μm-thick array 1.5 mm in diameter with 40-μm photovoltaic pixels, described in detail in 10 . In this array, each pixel has a SIROF-coated active electrode 18 μm in diameter exposed to electrolyte.…”

“…Here, we present an alternative approach to high-resolution prosthetic vision with a planar monopolar subretinal implant 10 , where the high penetration depth and high contrast are enabled by the spatiotemporal modulation of the pixels, utilizing the exponentially changing conductivity of the photodiodes under bias. In our approach, images captured by a camera are projected onto the subretinal photovoltaic arrays from the augmented-reality glasses using pulsed near-infrared (880nm) light 5,7 .…”