Effect of shape and coating of a subretinal prosthesis on its integration with the retina.

Retinal stimulation with high spatial resolution requires close proximity of electrodes to target cells. This study examines the effects of material coatings and 3-dimensional geometries of subretinal prostheses on their integration with the retina. A trans-scleral implantation technique was developed to place microfabricated structures in the subretinal space of RCS rats. The effect of three coatings (silicon oxide, iridium oxide and parylene) and three geometries (flat, pillars and chambers) on the retinal integration was compared using passive implants. Retinal morphology was evaluated histologically 6 weeks after implantation. For 3-dimensional implants the retinal cell phenotype was also evaluated using Computational Molecular Phenotyping. Flat implants coated with parylene and iridium oxide were generally well tolerated in the subretinal space, inducing only a mild gliotic response. However, silicon-oxide coatings induced the formation of a significant fibrotic seal around the implants. Glial proliferation was observed at the base of the pillar electrode arrays and inside the chambers. The non-traumatic penetration of pillar tips into the retina provided uniform and stable proximity to the inner nuclear layer. Retinal cells migrated into chambers with apertures larger than 10 mum. Both pillars and chambers achieved better proximity to the inner retinal cells than flat implants. However, isolation of retinal cells inside the chamber arrays is likely to affect their long-term viability. Pillars demonstrated minimal alteration of the inner retinal architecture, and thus appear to be the most promising approach for maintaining close proximity between the retinal prosthetic electrodes and target neurons.

Tissue damage by pulsed electrical stimulation.

Repeated pulsed electrical stimulation is used in a multitude of neural interfaces; damage resulting from such stimulation was studied as a function of pulse duration, electrode size, and number of pulses using a fluorescent assay on chick chorioallontoic membrane (CAM) in vivo and chick retina in vitro. Data from the chick model were verified by repeating some measurements on porcine retina in-vitro. The electrode size varied from 100 microm to 1 mm, pulse duration from 6 micros to 6 ms, and the number of pulses from 1 to 7500. The threshold current density for damage was independent of electrode size for diameters greater than 300 microm, and scaled as 1/r2 for electrodes smaller than 200 microm. Damage threshold decreased with the number of pulses, dropping by a factor of 14 on the CAM and 7 on the retina as the number of pulses increased from 1 to 50, and remained constant for a higher numbers of pulses. The damage threshold current density on large electrodes scaled with pulse duration as approximately 1/t0.5, characteristic of electroporation. The threshold current density for repeated exposure on the retina varied between 0.061 A/cm2 at 6 ms to 1.3 A/cm2 at 6 micros. The highest ratio of the damage threshold to the stimulation threshold in retinal ganglion cells occurred at pulse durations near chronaxie-around 1.3 ms.

Optoelectronic retinal prosthesis: system design and performance.

The design of high-resolution retinal prostheses presents many unique engineering and biological challenges. Ever smaller electrodes must inject enough charge to stimulate nerve cells, within electrochemically safe voltage limits. Stimulation sites should be placed within an electrode diameter from the target cells to prevent 'blurring' and minimize current. Signals must be delivered wirelessly from an external source to a large number of electrodes, and visual information should, ideally, maintain its natural link to eye movements. Finally, a good system must have a wide range of stimulation currents, external control of image processing and the option of either anodic-first or cathodic-first pulses. This paper discusses these challenges and presents solutions to them for a system based on a photodiode array implant. Video frames are processed and imaged onto the retinal implant by a head-mounted near-to-eye projection system operating at near-infrared wavelengths. Photodiodes convert light into pulsed electric current, with charge injection maximized by applying a common biphasic bias waveform. The resulting prosthesis will provide stimulation with a frame rate of up to 50 Hz in a central 10 degrees visual field, with a full 30 degrees field accessible via eye movements. Pixel sizes are scalable from 100 to 25 microm, corresponding to 640-10,000 pixels on an implant 3 mm in diameter.