J. D. Weiland

High-Density Flexible Parylene-Based Multielectrode Arrays for Retinal and Spinal Cord Stimulation

Novel flexible parylene-based high-density electrode arrays have been developed for functional electrical stimulation in retinal and spinal cord applications. These electrode arrays are microfabricated according to single-metal-layer and, most recently, dual-metal-layer processes. A new heat-molding process has been implemented to conform electrode arrays to approximate the curvature of canine retinas, and chronic implantation studies have been undertaken to study the mechanical effects of parylene-based prostheses on the retina, with excellent results to date. Electrode arrays have also been implanted and tested on the spinal cords of murine models, with the ultimate goal of facilitation of locomotion after spinal cord injury; these arrays provide a higher density and better spatial control of stimulation and recording than is typically possible using traditional fine-wire electrodes. Spinal cord stimulation typically elicited three muscle responses, an early (direct), a middle (monosynaptic), and a late (polysynaptic) response, classified based on latency after stimulation. Stimulation at different rostrocaudal levels of the cord yielded markedly different muscle responses, highlighting the need for such high-density arrays.

In vivo study of response threshold in retinal degenerate model at different degenerate stages

Retinal prostheses are being developed to apply electrical stimulation to the retina in order to restore vision of individuals who suffer from diseases such as retinitis pigmentosa (RP) and aged related macular degeneration (AMD). Various electrical stimulus parameters have been extensively studied in both experimental and clinical settings. Both electrophysiological and psychophysical results have shown that outer retina disease exhibit higher stimulus threshold in one degenerate group versus the control group. Fewer studies have been conducted to investigate the change in threshold currents as a function of different degenerate stages. We propose to study the electrophysiological change in degenerate rat retinas by using an in vivo recording method. We recorded retinal-driven superior colliculus cells response in two control groups and four degenerate groups. Current pulses of seven different stimulus pulse durations were applied to the retinas to obtain strength duration curve per group. Preliminary results showed that for the postnatal (P) day 90 and 180 degenerate groups, threshold currents were not significantly different from the normal control group (P90 and P230). For P300 degenerate group, the threshold currents progressively increased. For P760 degenerate group, threshold currents were significantly elevated across all the stimulus pulse durations tested. Charge densities calculated for P760 degenerate group exceeded the safe limit of the stimulating electrode. Cell morphology in all control and degenerate groups is still under investigation for a correlation study.

Clinical results with the model 1IRP implant

This paper discuss the feasibility study of implanting an epiretinal prosthesis in humans with bare or no light perception vision from retinitis pigmentosa. Patients were implanted with a Second Sight intraocular epiretinal prosthesis in the eye with worse vision. The implant consisted of an extraocular microelectronic device and an intraocular electrode array, connected by a multiwire cable. At the end of the procedure, it was concluded that all the subjects could see visual perceptions that correlate to electrical stimulus from a chronically implanted retinal prosthesis. Patients used a camera driven stimulus to perform simple tasks.

Injury and Repair: Prostheses

Sight restoration in the event of retinal degenerative diseases or even loss of the eyes has been an aim of researchers for over 50 years. Cortical prostheses, where visual signals are transmitted directly from a camera to the visual cortex, were the first to be studied, although technical difficulties and tissue complexities have slowed progress in this area. Research on retinal electronic prosthetic devices has been more successful, with many groups around the world working on a technology that effectively replaces retinal photoreceptor cells. Proof of principle has been established in that initial results from prosthesis clinical trials have demonstrated restoration of light perception and some form of vision in subjects originally having no or bare light perception.