Dr. James Loudin Profile

Dr. James Loudin

at Stanford Univ

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Publications (4)

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Proceedings Article | 7 February 2012 Paper

Photovoltaic retinal prosthesis for restoring sight to the blind: implant design and fabrication

Lele Wang, Keith Mathieson, Theodore Kamins, James Loudin, Ludwig Galambos, James Harris, Daniel Palanker

Proceedings Volume 8248, 824805 (2012) https://doi.org/10.1117/12.909104

KEYWORDS: Photodiodes, Silicon, Oxides, Etching, Semiconducting wafers, Doping, Image processing, Photovoltaics, Electrodes, Deep reactive ion etching

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We have designed and fabricated a silicon photodiode array for use as a subretinal prosthesis aimed at restoring sight to patients who lost photoreceptors due to retinal degeneration. The device operates in photovoltaic mode. Each pixel in the two-dimensional array independently converts pulsed infrared light into biphasic electric current to stimulate remaining retinal neurons without a wired power connection. To enhance the maximum voltage and charge injection levels, each pixel contains three photodiodes connected in series. An active and return electrode in each pixel ensure localized current flow and are sputter coated with iridium oxide to provide high charge injection. The fabrication process consists of eight mask layers and includes deep reactive ion etching, oxidation, and a polysilicon trench refill for in-pixel photodiode separation and isolation of adjacent pixels. Simulation of design parameters included TSUPREM4 computation of doping profiles for n⁺ and p⁺ doped regions and MATLAB computation of the anti-reflection coating layers thicknesses. The main process steps are illustrated in detail, and problems encountered are discussed. The IV characterization of the device shows that the dark reverse current is on the order of 10-100 pA-negligible compared to the stimulation current; the reverse breakdown voltage is higher than 20 V. The measured photo-responsivity per photodiode is about 0.33A/W at 880 nm.

Proceedings Article | 12 February 2011 Paper

Photovoltaic retinal prosthesis

James Loudin, Keith Mathieson, Ted Kamins, Lele Wang, Ludwig Galambos, Philip Huie, Alexander Sher, James Harris, Daniel Palanker

Proceedings Volume 7885, 788513 (2011) https://doi.org/10.1117/12.876560

KEYWORDS: Electrodes, Retina, Photodiodes, Diodes, Photovoltaics, Neurons, Visualization, Projection systems, Video, Goggles

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Electronic retinal prostheses seek to restore sight to patients suffering from retinal degenerative disorders. Implanted electrode arrays apply patterned electrical stimulation to surviving retinal neurons, producing visual sensations. All current designs employ inductively coupled coils to transmit power and/or data to the implant. We present here the design and initial testing of a photovoltaic retinal prosthesis fabricated with a pixel density of up to 177 pixels/mm². Photodiodes within each pixel of the subretinal array directly convert light to stimulation current, avoiding the use of bulky coil implants, decoding electronics, and wiring, and thereby reducing surgical complexity. A goggles-mounted camera captures the visual scene and transmits the data stream to a pocket processor. The resulting images are projected into the eyes by video goggles using pulsed, near infrared (~900 nm) light. Prostheses with three pixel densities (15, 55, and 177 pix/mm²) are being fabricated, and tests indicate a charge injection limit of 1.62 mC/cm² at 25Hz. In vitro tests of the photovoltaic retinal stimulation using a 512-element microelectrode array have recorded stimulated spikes from the ganglion cells, with latencies in the 1-100ms range, and with peak irradiance stimulation thresholds varying from 0.1 to 1 mW/mm². With 1ms pulses at 25Hz the average irradiance is more than 100 times below the IR retinal safety limit. Elicited retinal response disappeared upon the addition of synaptic blockers, indicating that the inner retina is stimulated rather than the ganglion cells directly, and raising hopes that the prosthesis will preserve some of the retina's natural signal processing.

Proceedings Article | 18 February 2009 Paper

High resolution optoelectronic retinal prosthesis

Jim Loudin, Rostam Dinyari, Phil Huie, Alex Butterwick, Peter Peumans, Daniel Palanker

Proceedings Volume 7163, 71631B (2009) https://doi.org/10.1117/12.807668

KEYWORDS: Electrodes, Photodiodes, Photovoltaics, Platinum, Diodes, Video processing, Tissues, Iridium, Video, Information visualization

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Electronic retinal prostheses seek to restore sight in patients with retinal degeneration by delivering pulsed electric currents to retinal neurons via an array of microelectrodes. Most implants use inductive or optical transmission of information and power to an intraocular receiver, with decoded signals subsequently distributed to retinal electrodes through an intraocular cable. Surgical complexity could be minimized by an "integrated" prosthesis, in which both power and data are delivered directly to the stimulating array without any discrete components or cables. We present here an integrated retinal prosthesis system based on a photodiode array implant. Video frames are processed and imaged onto the retinal implant by a video goggle projection system operating at near-infrared wavelengths (~ 900 nm). Photodiodes convert light into pulsed electric current, with charge injection maximized by specially optimized series photodiode circuits. Prostheses of three different pixel densities (16 pix/mm², 64 pix/mm², and 256 pix/mm²) have been designed, simulated, and prototyped. Retinal tissue response to subretinal implants made of various materials has been investigated in RCS rats. The resulting prosthesis can provide sufficient charge injection for high resolution retinal stimulation without the need for implantation of any bulky discrete elements such as coils or tethers. In addition, since every pixel functions independently, pixel arrays may be placed separately in the subretinal space, providing visual stimulation to a larger field of view.

Proceedings Article | 5 March 2007 Paper

Progress toward a high-resolution retinal prosthesis

Alex Butterwick, Alex Vankov, Phil Huie, Karthik Vijayraghavan, Jim Loudin, Daniel Palanker

Proceedings Volume 6426, 64260R (2007) https://doi.org/10.1117/12.701787

KEYWORDS: Electrodes, Photodiodes, Visualization, Retina, Eye, LCDs, Neurons, Goggles, Information visualization, Video

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Electronic retinal prostheses represent a potentially effective approach for restoring some degree of sight in blind patients with retinal degeneration. Functional restoration of sight would require hundreds to thousands of electrodes effectively stimulating remaining neurons in the retina. We present a design of an optoelectronic retinal prosthetic system having 3mm diameter retinal implant with pixel sizes down to 25 micrometers, which allows for natural eye scanning for observing a large field of view, as well as spatial and temporal processing of the visual scene to optimize the patient experience. Information from a head mounted video camera is processed in a portable computer and delivered to the implanted photodiode array by projection from the LCD goggles using pulsed IR (810 nm) light. Each photodiode converts pulsed light (0.5 ms in duration) into electric current with efficiency of 0.3 A/W using common bi-phasic power line. Power is provided by the inductively-coupled RF link from the coil on the goggles into a miniature power supply implanted between the sclera and the conjuctiva, and connected to subretinal implant with a thin 2-wire trans-scleral cable. 3-dimensional structures in the subretinal prosthesis induce retinal migration and thus ensure close proximity between stimulating electrodes and the target retinal neurons. Subretinal implantations of the 3-dimentional pillar and chamber arrays in RCS rats with 2 and 6 week follow-up demonstrate achievement of intimate proximity between the stimulation cites and the inner nuclear layer. In some instances formation of a fibrotic seal has been observed.

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